IB Biology Higher Level Human and Health Physiology Notes

7/27/2019 IB Biology Higher Level Human and Health Physiology Notes

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Defence Against Infectious Disease

This article follows on from some of the SL syllabus focusing on Human Health and Physiology. Our

defence system is also known as our immune system and as mentioned in Topic 6, the immune

system protects us from harmful pathogens that cause disease. Can you explain the basics of antibodyproduction? Hopefully yes if youve read topic 6, but can you describe what this has to do with

immunity and vaccination? Do you know how antibodies can be used as bullets or the mechanism

behind blood clotting? What can you say about the MMR/autism controversy and a failed clinical trial?

Read on to find out...

Antibody productionWhen a pathogen enters the body, it is recognised due to the presence of an antigen attached to it. The antigen

is what triggers an immune response that eventually leads to the production of antibody that will lead to the

destruction of that pathogen (antibody that is specific for that antigen). The immune response is achieved by a

fascinating yet complex series of reaction as outlined in topic 6.3

Examiner Tip

Before going any further it would be a good idea to check out the SL material on antibody production. You

should be able to explain the above diagram; if you cant, go back to Topic 6.3 and make sure that you

understand the process of antibody production.

http://www.youtube.com/watch?feature=player_embedded&v=37zJFVsKlKQ

In addition to the details you learnt at SL, you must know the following:

The last part of the production of antibody (step 6 in the diagram above) is the change in B cells to memory

cells. This is important because it is the memory cells that act as the basis for immunity. This will be

discussed in greater detail below.


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The process of antibody production is governed by challenge and response. Challenge and response is the

way that the body responds to a threat. When the immune system recognises a pathogen, the pathogens

antigen challenges the immune mechanisms. In turn, the immune system begins a reaction, which primarily

involves the production of antibody to destroy that pathogen and its antigen.

Antibody is produced via clonal selection. Clonal selection refers to the specific part of antibody production,where only one B cell is activated by an activated helper T cell. This B cell then proliferates and creates

many clones of itself. In this way, many identical B cells have been created that will produce the same

antibody, that is effective against the same antigen. In other words, clonal selection is the way that a large

amount of one antibody is produced.

Important

Challenge and response is the theory underlying the production of antibody; antigen challenges the immune

response and it responds by producing antibody. Clonal selection is how a specific antigen causes one type of B

cell to be activated and then replicate many times via mitosis. This ensures that the correct antibody is produced

in vast quantities.

Monoclonal antibodiesThe antibody that is produced in a normal immune response is described as being polyclonal antibody.

Polyclonal, as the name suggests, means that several clones of a B cell have made this antibody. In contrast to

this, monoclonal antibodies are produced by only one clone of a B cell. This is a very arbitrary difference and is

only really seen in the production of monoclonal antibody as outlined below. A natural response always

produced polyclonal antibodies; monoclonal antibodies can only be produced artificially.

Important

Monoclonal antibody is antibody that is produced by only one plasma B cell.

The following technique is used to make large quantities of monoclonal antibody:


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1. An animal (often a rat or mouse) is inoculated with a form of a pathogen. This means that at least the

antigen of that pathogen is injected into the animal. The antigen used will determine the antibody that will be

produced. For example, Hepatitis B vaccine contains Hepatitis B Virus Surface Antigen (HBsAg) rather than

the whole pathogen, i.e. the whole virus.

2. The animal then has an immune response that results in the production of B cells producing the desired

antibody. One of these B cells is extracted from the animal.

3. A tumour cell, which has the capacity to grow and divide endlessly is fused with this B cell via placing the

two cells in close proximity and running an electric current through them. The result of this combination is

a hybridoma cellthat is able to divide endlessly and produce the desired antibody.

4. This hybridoma cell is then allowed to divide until there is a large number of cells and then the antibody that

they produce is extracted and purified.

Be Aware

Do not confuse monoclonal and polyclonal antibodies. In normal immune reactions there are polyclonal

antibodies as several different B cells can be activated. It is only artificially that monoclonal antibodies are

produced.

Important

The production of monoclonal antibodies requires an animal to be exposed to an antigen. Following inoculation,

one of the animals cells is removed and fused with a tumour cell to form a hybridoma that will produce large

quantities of the correct antibody.

Once purified, monoclonal antibody has many uses, particularly in medicine, for example in the treatment anddiagnosis of many diseases.


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Monoclonal antibodies are being used more and more in the treatment of cancer. In short, a specific type of

monoclonal antibody is produced. This antibody will look for, recognise and then bind to cancer cells. The trick is

that attached to the monoclonal antibody is a variety of substances that are used to destroy the cancer cells. In

some instances the antibody has a flag attached to it that attracts the bodys immune system and causes the

cancer cell to be destroyed, as with Rituxan in non-Hodgkin lymphoma. In other cases, the attached molecule

can act to directly stop further production of cancer cells such as the effects of Herceptin against

HER2/neu. Check out the following link to see a comprehensive overview of this topic:

Pregnancy tests

Monoclonal antibodies are also frequently used as diagnostic tools; they are used to check whether an individual

has a certain disease or condition. One specific example of this is with pregnancy testing:

A pregnancy test stick is dipped into the womans urine and stick will recognise a specific molecule that is only

produced in pregnancy called human chorionic gonadotrophin (hCG). If hCG is present it will bind to monoclonal

anti-hCG antibodies that are present on the pregnancy test stick to form a hCG-anti-hCG antibody complex.

The hCG-anti-hCG antibody complex then moves down the stick towards the reaction strip. The binding of the

hCG-anti-hCG antibody complex with another type of antibody that recognises the complex causes a reaction

with a dye present in the reaction strip. As such a colour change is seen.

There is also a control strip on the stick that will react if the test has been carried out properly. In this strip, there

are different antibodies that recognise the anti-hCG antibodies, even if there is no hCG attached to it and causes

a colour change. This strip will always have a colour change unless the test is broken so it acts as confirmation

of the test.

A pregnant woman will have hCG in her urine, so this will trigger the reaction strip and the confirmation strip to

change colour. A woman who is not pregnant will have no hCG so there will be no binding of hCG to the anti-

hCG antibodies. As such, there will be no binding of the hCG-anti-hCG antibody complexes to the antibodies in


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the reaction strip (no colour change), but there will still be binding of the anti-hCG antibodies to the antibodies in

the confirmation strip (colour change).

This is explained in detail, with a good animation, in the following link:

Important

Monoclonal antibodies are used in the diagnosis and treatment of many medical conditions, as well as in

pregnancy testing.

Examiner Tip

Dont worry if youre not comfortable with the exact details of the uses of monoclonal antibodies; as long as you

can briefly outline their applications in medical treatment and pregnancy testing you should be fine in the exam!

ImmunityOur immune system is made up of different types of cells including phagocytes (macrophages), B cells, Helper

T-cells, memory cells, and plasma cells. Immunity is the term used to describe when our immune system has

already met a certain disease or pathogen before and thus is able to respond to it more effectively. Humans gain

natural and active immunity by catching a disease and recovering from it. It is important that you know the

different classifications of immunity, outlined below.

Important

Immunity is the way in which the body is able to more effectively to respond to a pathogen having encountered it

once before.

There are two types of immunity: active immunity and passive immunity. Active immunity involves the body

being forced to produce antibodies because it has been exposed to a pathogen followed by the normal immune

response of T helper cells, B cells and plasma cells. Specifically, it is the final stage in the production of antibody

that leads to immunity.

Adaptive immunity


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After the activated B cell has been stimulated to proliferate and form many plasma cells (clones), these clones

produce antibody and destroy the antigen/pathogen. Once the disease has been combated, a few of the

plasma cells become memory cells. Memory cells are dormant cells, until they are once again stimulated by

their specific antigen. Upon stimulation they revert to plasma cells and are able to produce the necessary

antibody immediately and in greater quantities than normally. This is because they do not have to go through the

process of being selected by the helper T-cells before taking action. In this way, the immune response is

generated before the pathogen has time to actually cause disease and there is thus immunity to that pathogen!

In contrast to this, there is passive immunity, which is where an individual acquires antibody without producing

any itself. In passive immunity there is no challenge and response in the individual because the antibodies are

made externally and then transferred across. The main ways in antibody can be transferred are via the placenta,

via the breast milk (as a substance known as colostrum) and via direct injection of antibodies. Once these

antibodies have been transferred into the individuals they act in exactly the same way as naturally produced

antibodies.

Important

Active immunity is immunity due to the production of antibodies by the organism itself after the bodys defence

mechanisms have been stimulated by antigens.

Important

Passive immunity is immunity due to the acquisition of antibodies from another organism in which active

immunity has been stimulated, including via the placenta, colostrum, or by injection of antibodies.

Examiner Tip

During the first six months after birth, a baby is relatively well-protected from infectious diseases thanks to the

fact that it carries the same antibodies as its mother, transferred through the placenta. After this, the baby must

rely on its own immune defence system which is much weaker. It is thus not a coincidence that many national

vaccination programmes against for example tetanus and diphtheria start at such an early age as 3 months.

Vaccination

Vaccinations are used to induce immunity artificially. A vaccine contains either dead, weakened (attenuated) or

subunit forms of the pathogen. After the pathogen/antigen has entered the body, it is recognised by

phagocytes/macrophages and a normal immune response occurs. This leads to antibodies being produced to


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destroy the pathogen/antigen and importantly leads to the production of plasma cells and then memory cells.

This initial response to a pathogen/antigen is known as the primary immune response.

Memory cells are vital to immunity as they are re-activated when the pathogen/antigen is encountered for real.

They facilitate the production of large amounts of antibody quickly and this means that the pathogen is destroyed

before it has chance to cause disease. As the pathogen is no longer able to cause the disease, that individual issaid to be immune to that pathogen or have immunity to that disease. The re-activation of memory cells and

production of antibody is known as the secondary immune response.

Most vaccines are administered by injection, with some need to be ingested (swallowed). Often a booster shot

may be needed years after the first vaccination to complete the immunisation process. This is the case for

example with tetanus. Immunity lasts for life or for a very long period (decades).

Important

Vaccination is an artificial way of providing long lasting immunity to a pathogen/disease, usually via injection or

ingestion of that pathogen.

Important

Vaccination involves the introduction of an attenuated pathogen into an individual so that an immune response is

triggered; because the pathogen has been weakened, it is easier for the immune response to destroy it. Once

the pathogen has been cleared, plasma B cells become memory cells capable of mounting a strong immune

response very quickly.

The benefits and dangers of vaccinationVaccination is a great health policy that is designed to prevent disease. There are numerous advantages to

vaccination that cover more than just the immediate healthcare implications. However, vaccination is not without

risk (even though these risks are very small) so there are both advantages and disadvantages as follows:

Benefits Complete elimination of disease

Prevention of epidemics/pandemics

Prevention of effects/symptoms of disease

Reduction in healthcare costs

Important

The benefits of vaccination include prevention of, elimination of and reduced spread of a disease, as well as

reduced cost of treating diseases.

Dangers Possible toxic effects of mercury (a part of most vaccines)


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Possible overload of the immune system

Past concerns over links to autism (although now found to be unfounded concerns)

Important

The dangers of vaccination include health concerns over parts of vaccines and the immune system being

overwhelmed.

The concerns of a possible link to autism were particularly with the MMR (mumps, measles and rubella) vaccine

in the UK in the early 1990s. The following link outlines how a team of doctors made a huge mistake in one

study that led thousands of British children not receiving the MMR vaccine.

Process of blood clottingThe constituents of blood were described in Topic 6, where it was stated that blood consists of red cells, white

cells, platelets, and plasma. In the process of clotting, it is platelets (or thrombocytes) that play a pivotal role.

The start point for clotting is when a cell is damaged and the classic example of this is a break to the skin such

as a cut/gash. When cells are damaged or die, there is a large amount of clotting factor released from platelets

and it is this release of clotting factor that initiates the clotting cascade.

The clotting cascade is a series of interconnected reactions that lead to the formation of a scab. The first

reaction is the conversion of pro-thrombin into thrombin and it is this reaction that is directly caused by the

presence of clotting factors.


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The next reaction is the conversion of fibrinogen to fibrin. Fibrinogen is a soluble protein, however fibrin is an

insoluble protein that is found as a series of strands. These strands are adherent and form a sticky mesh that

traps parts of blood in it. In particular, red blood cells and platelets become stuck in the fibrin mesh and form the

initial blood clot.

The process of blood clotting is very important. Without it a small cut could lead to organisms bleeding to death.

Also, the formation of scabs acts as a barrier to prevent microorganisms from entering the body, which helps

prevent infection and illness.

Whilst clotting is very useful, excess clotting can be very dangerous. The clotting cascade, with its interlinked

reactions is designed to prevent from blood clotting unnecessarily.

Important

Blood clotting involves the activation of pro-thrombin via tissue factor, which causes fibrinogen to become

insoluble fibrin and form a mesh that is the initial blood clot.

Important

Clotting prevents excess blood loss and pathogens entering the body. As clotting occurs as a reaction cascade,

it cannot easily be stimulated so it is unlikely to occur unnecessarily, which can lead to various health problems.

What you should know

Challenge and response is the theory underlying the production of antibody; antigen challenges the immune

response and it responds by producing antibody.

Clonal selection is how a specific antigen causes one type of B cell to be activated and then replicate many

times via mitosis. This ensures that the correct antibody is produced in vast quantities.

Monoclonal antibody is antibody that is produced by only one plasma B cell.

Monoclonal antibodies are used in the diagnosis and treatment of many medical conditions, as well as in

pregnancy testing.

Immunity is the way in which the body is able to more effectively to respond to a pathogen having encountered

it once before.

Active immunity is immunity due to the production of antibodies by the organism itself after the bodys defence

mechanisms have been stimulated by antigens. Passive immunity is immunity due to the acquisition of antibodies from another organism in which active

immunity has been stimulated, including via the placenta, colostrum, or by injection of antibodies.

Vaccination is an artificial way of providing long lasting immunity to a pathogen/disease, usually via injection or

ingestion of that pathogen.


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Vaccination involves the introduction of an attenuated pathogen into an individual so that an immune response

is triggered; because the pathogen has been weakened, it is easier for the immune response to destroy it.

Once the pathogen has been cleared, plasma B cells become memory cells capable of mounting a strong

immune response very quickly.

The benefits of vaccination include prevention of, elimination of and reduced spread of a disease, as well as

reduced cost of treating diseases.

The dangers of vaccination include health concerns over parts of vaccines and the immune system beingoverwhelmed.





Clotting prevents excess blood loss and pathogens entering the body. As clotting occurs as a reaction

cascade, it cannot easily be stimulated so it is unlikely to occur unnecessarily, which can lead to various health

problems.


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Muscles and Movement

In Topic 6 the nervous system was introduced, explaining how signals from the central nervous system

are sent to effector structures such as muscles in order to bring about movement. In this article, the

specific details of how muscles contract is examined and you will see the details of this complexprocess. Did you know that ATP is needed to form myosin actin cross-bridges? Can you explain the

cross-bridge cycle?

Structures required for movement

The BodyAnimals have a complex arrangement of systems and structures that enable them to move. It is the interaction of

the muscular and skeletal systems that allows movement, but there are other important structures that are

required.

BonesBone is the main support of the body of an animal. In humans, we have 206 bones and these provide a

framework for our bodies, as well as acting as protection for internal organs, such as the skull around the brain

and the ribs around the lungs. Specifically, bones act as a rigid platform that muscles can be anchored to.

Important

Bones are the basic support structures of the human body, which act to support the rest of the body and protect

internal organs.


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MusclesMuscles are the driving force behind movement. It is the contraction and relaxation of muscle groups (described

as antagonistic pairs) that physically move and reposition limbs as well as giving structural support to the rest ofthe body. For example, the muscles in the back enable us to remain upright and not slump forward. Importantly,

it is muscles that contract to bring about the movement of joints.

Important

Muscles facilitate the movement of limbs and work in antagonistic pairs.

LigamentsLigaments are strong filament-like structures that hold joints together. Ligaments play a crucial role in holding

two bones together in a way that means they have both strength and a range of movements. Without ligaments,

there would be no concept of joints as bones would all be fused together!

Important

Ligaments are the structures that hold joints together; they link one bone to another.


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TendonsTendons are similar to ligaments as they are again strong filament-like structures, but their function is to attach

muscles to bones. It is this attachment that allows muscles to create forces by contracting; muscles pull againstthe resistance of the bone they are attached to and thus move the joint/limb/structure.

Important

Tendons attach muscles to bones so that muscle contraction causes movement.

NervesNerves are also filament-like structures, but they are very delicate and much more like fine thread than strongrope. Their role is the innervation of muscles, which means transmitting nerve impulses from the brain and spinal

cord (CNS) to that muscle or group of muscle. In other words, it is nerves that convey the message to move to

muscles so that they can contract and then bring about movement.

Important

Nerves are the structures that allow the CNS to control muscles and cause contraction.

The elbow jointhttp://www.youtube.com/watch?feature=player_embedded&v=mPdhHMDueiY


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You need to appreciate the general roles of the structures required for movement (as above), but you must also

have a specific understanding of the structures of the elbow joint. You may also be asked to draw a diagram of

the human elbow and should include the following parts with their correct functions:

Comparing the movement of different jointsThe hip joint and knee joint carry out different movements. Both are referred to as synovial joints, meaning that

they are formed from bone and ligaments, but they differ in their range and type of movements.


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The Hip JointThe hip joint is said to be a ball and socket joint and it is capable of moving in all directions. It can flex, extend,

abduct, adduct and rotations. In other words it can move forwards, backwards, from side to side and can turn

inwards and outwards.

Important

The hip joint is a ball and socket joint, which can move in all directions/planes.

The Knee JointThe knee joint is not as flexible as it is a hinge joint that can only move in one plane; it can only flex and extend,

so it can only bend backwards and then straighten.

Important

The knee joint is a hinge joint, which can only move in one plane/direction.

Examiner Tip


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When thinking about the differences between ball and socket joints and hinge joints, try to relate them to every

day structures. Ball and socket joints are like a joystick, whereas hinge joints are like the spine on a book.

Check out this link to see the knee moving under MRI.

Muscles, myofibrils and sarcomeresMuscles are made up of many smaller components structures. One large muscle contains many smaller muscle

fibres. Each muscle fibres is self contained by its own sarcolemma (a specific type of cell membrane) and

contains several myofibrilsthat consist of repeating units called sarcomeres. Sarcoplasmic reticulum(a specific

type of endoplasmic reticulum) surrounds the myofibrils, and mitochondria are situated between the myofibrils.

The sarcomeres that make up myofibrils have dark and light bands formed from two types of protein called actin

and myosin. The dark bands of myofibrils are where actin and myosin overlap, whereas the light bands arewhere there is only actin or myosin (no overlap). The overlap of action and myosin leads to the formation of

cross-bridges and it is there cross-bridges that produce the contraction of muscle. At either end of a sarcomere,

there are structures called Z lines. It is these lines that anchor the actin and myosin filaments in place. You

should be able to label a diagram of a sarcomere with the structures includes below:

Actin filaments (thin)

Myosin filaments (thick)

Light and dark bands

Z lines


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Important

A muscle is made up of many muscle fibres, each of which is composed of myofibrils that contain thousands of

individual sarcomeres. A sarcomere consists of interlinked actin and myosin filaments.

Muscle contractionThe process of muscle contraction ultimately involves the shortening of the sarcomere. In order for muscles to

contract in the first place they need energy in the form of ATP, but the actual mechanism of muscle contraction

involves the formation of cross-bridges between the actin and myosin filaments.

1. As an action potential arrives at the muscle (from a motor neuron) it causes the release of Ca2+ from the

sarcoplasmic reticulum.

2. Calcium allows the actin and myosin fibres to join together as it acts to expose a binding site on actin that

myosin heads can attach to. This is the formation of actin-myosin cross-bridges.

3. ATP then binds to the cross bridges, which allows the myosin heads to alter their shape and prepares the

two filaments for movement.

4. ATP is hydrolysed to ADP and phosphate and this releases energy that is used to shorten the sarcomere

(move actin closer to the centre).

5. A new ATP then binds and causes the cross-bridge to break and the myosin head to reset to its original

position so that it can form a new cross-bridge.


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6. This is referred to the cross-bridge cycle of attaching, sliding, detaching and resetting and it is how muscle

contracts.

Important

Muscle contraction occurs as a cycle. When an action potential reaches a muscle, calcium ions are released,

which leads to the binding of myosin heads onto actin filaments. With the addition and hydrolysis of ATP, myosin

pulls actin towards the centre of the sarcomere and contraction occurs as the sarcomere shortens.


Bones are the basic support structures of the human body, which act to support the rest of the body and

protect internal organs.

Muscles facilitate the movement of limbs and work in antagonistic pairs.

Ligaments are the structures that hold joints together; they link one bone to another.

Tendons attach muscles to bones so that muscle contraction causes movement.

Nerves are the structures that allow the CNS to control muscles and cause contraction.

The hip joint is a ball and socket joint, which can move in all directions/planes.

The knee joint is a hinge joint, which can only move in one plane/direction.

A muscle is made up of many muscle fibres, each of which is composed of myofibrils that contain

thousands of individual sarcomeres.

A sarcomere consists of interlinked actin and myosin filaments.

Muscle contraction occurs as a cycle. When an action potential reaches a muscle, calcium ions are

released, which leads to the binding of myosin heads onto actin filaments. With the addition and

hydrolysis of ATP, myosin pulls actin towards the centre of the sarcomere and contraction occurs as the

sarcomere shortens.


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The Kidney

Humans have two kidneys located just under the ribs at the back of the abdomen. The kidneys are

responsible for excretion, which is the way in which the body can remove any waste products or toxic

metabolites. In particular, excretion removes the waste products of many of the chemical reactions thatnaturally occur inside the body (metabolic reactions).Are you able to explain the mechanisms involved

in excretion? Do you know why glucose is found in the urine of patients with poorly controlled

diabetes? Or why drinking too much coffee or coke makes you need the toilet? Read on for the

answers...

Excretion occurs in the form of the kidney producing urine and filtering the blood to determine which substances

and how much of each substance the body wants to keep or get rid of. Excretion involves two processes known

as ultrafiltration and selective reabsorption. The kidney also has various homeostatic functions such as

regulating the water concentration of the body, which is called osmoregulation.

Important

The kidneys play vital roles in excretion and water balance.

Excretion is the removal from the body of the waste products of metabolic pathways.

Excretion consists of ultrafiltration followed by selective reabsorption.

Diagram of the structure of the human kidney


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You need to be able to draw the macrostructure of a kidney for the exams. When drawing the human kidney,

make sure to include the following:

Cortex (outer edge of the kidney)

Medulla (middle of the kidney, shown with pyramids)

Pelvis on the concave side of the kidney

Ureter shown connected to the pelvis on the concave side

Renal artery and vein shown originating from the concave side

Diagram of a nephronIn addition to the macrostructure of the kidney, you must also be able to draw the microstructure of the kidney

(including all of the following structures). Each kidney is made up of hundreds of thousands of nephrons and it is

the nephron that you need to be able to draw. A nephron is the functional unit of the kidney and each of its

structures has specific functions that are important in its role of excretion.

Branches of renal artery

Glomerulus

Bowman's capsule

Proximal convoluted tubule (PCT)

Loop of Henl (ascending and descending limbs)

Distal convoluted tubule (DCT)


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Collecting duct

UltrafiltrationUltrafiltration is the first step in the process of excretion. In order for ultrafiltration to occur, the kidney needs to

have direct access to the blood, so the blood supply for the kidney is a very good one via the renal artery. Once

the renal artery has entered the kidney, it splits into thousands of smaller branches that are called arterioles.

Each nephron has its own arteriole and every arteriole is divided into two parts; the afferent and efferentarteriole.

The afferent arteriole branches many times to form the glomerulus, which is a complex system of blood vessels

and it is here that ultrafiltration actively occurs. The efferent arteriole comes back off the glomerulus and leads to

the renal vein. Think of the afferent and efferent arterioles as being on either side of the glomerulus; afferent

enters the glomerulus and efferent exits the glomerulus.

As blood passes through the glomerulus, it is under very high pressure for two reasons. Firstly, it is because the

afferent arteriole has a larger diameter than the efferent arteriole, so blood is squeezed into the efferent arteriole.

Secondly, the diameter of the branches of the glomerulus is very small so blood is being forced through a narrow

space. Due to high pressure, a lot of the substances that are contained in blood are forced out of the blood and

into the Bowmans capsule. However, there are elements of control over what is filtered.


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The wall of the glomerulus is formed by what are described as fenestrated capillaries. Fenestrated capillaries

have spaces or pores, which can act as filters. In this instance, the fenestrations act to prevent large molecules

(such as proteins) from leaving the blood plasma. The basement membrane of the Bowmans capsule also

contributes to filtration of substances because it is made up of podocytes. Podocytes are a type of specialised

cells that have foot-like projections that moderate what is transported across the capsule. As such, only small

molecules are able to pass from the blood into the Bowmans capsule.

Examiner Tip

To remember which arteriole is afferent and efferent think about how the Efferent arterioleExits the glomerulus.

Important

Ultrafiltration occurs in the glomerulus of each nephron; it is the movement of small substances from the blood

into the Bowmans capsule.

Ultrafiltration occurs due to the high pressure of blood in the glomerulus (because of the difference in size of

afferent and efferent arterioles), the fenestrated nature of capillaries and the podocytes of the Bowmans

capsule.

Selective ReabsorptionSelective reabsorption follows on from ultrafiltration and it is how the kidney determines how much of a

substance is moved back into the blood and how much is allowed to pass into urine and be excreted. Once

substances have been ultrafiltered into the Bowmans capsule, they are said to be in the glomerular filtrate and itis the job of the proximal convoluted tubule (PCT) to reabsorb these substances if need be. Selective

reabsorption is so called because the PCT can actively select what and how much it wants to reabsorb. For

example, very little urea is selectively reabsorbed, but all glucose should be (see more on this below).

In the PCT, there is a layer of microvilli that is similar to the microvilli found in the small intestine of the digestive

tract. These microvilli act to increase the surface area of the PCT and thus increase the area over which

selective reabsorption can occur. Glucose, amino acids and various sodium/potassium salts are actively

transported out of the glomerular filtrate and into the surrounding blood of the efferent arteriole. In addition to

active transport, there is also osmosis. Osmosis occurs because of the movement of glucose/amino acids/salts

into the blood of the efferent arteriole. As such, the blood has a high concentration and water is able to across

via osmosis.

Be Aware

Do not get confused about where the blood vessels surrounding each nephron come from. The efferent arteriole

that leaves the glomerulus/Bowmans capsule does not go directly to the renal vein. It follows the path of the

nephron, wrapping around various tubules and ducts.

Important

Selective reabsorption occurs in the proximal convoluted tubule (PCT) of the nephron; it is the re-uptake of

specific substance found in the glomerular filtrate.


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Selective reabsorption involves active transport of certain substances such as glucose and amino acids and the

osmosis of water.

The PCT has microvilli that increase its surface area so increase the area over which reabsorption can occur.

Blood, glomerular filtrate and urineDue to the processes of ultrafiltration and selective reabsorption, there are differences in the concentration of

proteins, glucose and urea between blood plasma, glomerular filtrate and urine. The differences are outline in

the chart below:

Protein is found solely in the blood, as it is too large to travel through the fenestrated capillaries of the

glomerulus or in between the podocytes of the basement membrane of the Bowmans capsule.

Glucose is found in roughly equal concentrations in the blood and glomerular filtrate, as it is almost entirely

ultrafiltered. However, no glucose is meant to be found in urine because it is an important substance used in a

huge number of metabolic processes. As such, glucose should be completely reabsorbed by the PCT of the

nephron.

Important

Protein should not be found in either the glomerular filtrate or urine.

Glucose should be found in the glomerular filtrate, but not urine.

Urea should be found in both the glomerular filtrate and in urine where it will be at a higher concentration.

Glycosuria and diabetesGlycosuria is the medical terms for the presence of glucose in urine. Glucose is not normally found in the urine

since as it is actively transported back into the blood in selective reabsorption. However, glucose is sometimes

found in the in the urine of people with untreated or poorly controlled diabetes.


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Diabetes is a condition in which there is a deficiency of insulin and consequently a rise is the concentration of

glucose in the blood (as insulin acts to lower blood glucose). When there is a raised blood glucose

concentration, the transporters in the proximal convoluted tubule for glucose can become overwhelmed and

struggle to cope. As such, some glucose will not be reabsorbed and will pass into urine. It is important to note

that the appearance of glucose in the urine of diabetic patients is not due to the failure of the kidney; the

transporters in the PCT are perfectly functional, but they cannot handle the amount of glucose in the glomerular

filtrate.

Important

Glycosuria is glucose in the urine and indicates diabetes; this occurs because blood glucose is elevated in

diabetes, and thus glucose transporters in the PCT cannot handle the amount of glucose in the glomerular

filtrate and some passes into the urine.

Examiner Tip

Before other diagnostic tools were developed, doctors used to taste the urine to determine whether the patient

had diabetes. Because of the glycosuria, the urine produced by diabetes patients has a sweet taste.

OsmoregulationOsmoregulation is the regulation of the concentration of water within an individual and it is another of the

important functions of the kidney. Water balance is controlled by the collecting ducts and the Loop of Henl, with

the hormone Anti-diuretic hormone (ADH) playing an important role in this process.

Collecting ducts and ADH


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The main site of the control of water regulation is the collecting duct. When the body senses that there has been

a decrease in the amount/concentration of water in the body (actually, an increase in blood concentration ofsodium chloride) it acts by releasing anti-diuretic hormone (ADH). ADH is also known as Vasopressin and is

released when there is a stimulus registered by osmoreceptors in the hypothalamus. The two main stimuli for

osmoreceptors are high blood plasma concentration of sodium chloride and reduced blood pressure. When ADH

is released by the pituitary gland (part of the brain) there is a huge increase in the permeability of the collecting

ducts as ADH creates pores for water to move out of the collecting ducts and back into the blood.

Simply put, if ADH is present, water is able to leave the collecting duct and there will be less water moving into

urine, so less water is lost from the body.

In addition to anti-diuretics, there are also diuretics, which make you urinate more as they prevent the formation

of water channels. One such natural diuretic is caffeine, which is found in coffee and cola amongst other

substances. This link discusses this in further detail.

Loop of Henl

The Loop of Henl controls water balance by altering the amount of water that can be reabsorbed via the

collecting ducts. The role of the loop of Henl is to create an osmotic gradient within the medulla of the kidney

(the area surrounding each nephron) and it does this by reabsorbing water and ions.


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In the descending limb of the Loop of Henl, water moves into the medulla, which increases the concentration of

the glomerular filtrate. As the medulla is now less concentrated than the glomerular filtrate, sodium and

potassium ions diffuse into the medulla. This shift of sodium and potassium means there is now an osmotic

gradient between the medulla and the fluid in the collecting ducts, which draws water out of the collecting ducts

and into the medulla via osmosis. Water is then reabsorbed from the medulla by the efferent arteriole and this

cycle begins again.

It is important that water in the medulla is regularly reabsorbed by the body via the efferent arteriole. This regular

reabsorption means that when the glomerular filtrate first reaches the descending limb, it is less concentrated

than the medulla and so water will move into the medulla and the Loop of Henl cycle can begin.

Osmoregulation occurs in a cyclic fashion because:

1. The descending limb of the Loop of Henl is permeable to water and impermeable to salts, so water can

move into the medulla, but sodium/potassium cannot.

2. The ascending limb of the Loop of Henl is impermeable to water and permeable to salts, so

sodium/potassium can move into the medulla, but water cannot.

Important

Osmoregulation is the control of the water balance of the blood, tissue or cytoplasm of a living organism.

Osmoregulation is largely down to the effects of ADH on the collecting ducts and the action of the Loop of

Henl.


The kidneys play vital roles in excretion and water balance.

Excretion is the removal from the body of the waste products of metabolic pathways.

Excretion consists of ultrafiltration followed by selective reabsorption.

Ultrafiltration occurs in the glomerulus of each nephron; it is the movement of small substances from the

blood into the Bowmans capsule.

Ultrafiltration occurs due to the high pressure of blood in the glomerulus (because of the difference in size

of afferent and efferent arterioles), the fenestrated nature of capillaries and the podocytes of the

Bowmans capsule.

Selective reabsorption occurs in the proximal convoluted tubule (PCT) of the nephron; it is the re-uptake

of specific substance found in the glomerular filtrate.

Selective reabsorption involves active transport of certain substances such as glucose and amino acids

and the osmosis of water.

The PCT has microvilli that increase its surface area so increase the area over which reabsorption can

occur.


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Protein should not be found in either the glomerular filtrate or urine.

Glucose should be found in the glomerular filtrate, but not urine.

Urea should be found in both the glomerular filtrate and in urine where it will be at a higher concentration.

Glycosuria is glucose in the urine and indicates diabetes; this occurs because blood glucose is elevated

in diabetes, and thus glucose transporters in the PCT cannot handle the amount of glucose in the

glomerular filtrate and some passes into the urine.

Osmoregulation is the control of the water balance of the blood, tissue or cytoplasm of a living organism.

Osmoregulation is largely down to the effects of ADH on the collecting ducts and the action of the Loop of

Henl.

The Reproductive Systems

In topic 6, the reproductive system was introduced. Here we will zoom in on the structure of the

reproductive systems in males and females and have a closer look at the function of structures that are

important in order for reproduction to occur. Furthermore you will learn about the process of birth and

how fertilisation can take place outside of the body (IVF).

Testis

You must be able to recognise certain structures when looking at a light micrograph of testis tissue including:

Interstitial cells (Leydig cells)

Germinal epithelium cells

Developing spermatozoa

Sertoli cells

Examiner Tip


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For more information and to gain some understanding of what a light micrograph looks like,watch this video, but

remember that this goes into more detail than you need to know.

Spermatogenesis

http://www.youtube.com/watch?feature=player_embedded&v=MBe8DD_7r30

Spermatogenesis is the male form of gametogenesis (the formation of gametes); it is the production of

spermatozoa, also known as sperm, and it takes place in the seminiferous tubules of the testes. There are

several stages in spermatogenesis:

1. Spermatogenesis begins with a single spermatogonium. Spermatogonia are diploid cells (2n) so they have a

full amount of genetic material. Cell division by mitosis takes place so that there are many spermatogonia.

2. The spermatogonia then grow (there is the cell growth) and form larger cells that are called primary

spermatocytes.

3. Primary spermatocytes then divide by meiosis. The first half of meiosis (meiosis I) results in the production of

a haploid cell (n), which is called a secondary spermatocyte. The second division of meiosis occurs (meiosis II)

and the secondary spermatocytes become known as spermatids.

4. Spermatids are then matured in the Sertoli cells of the testis and undergo differentiation to form mature

spermatozoa (sperm).

Important

Spermatogenesis is the formation of gametes, specifically the formation of sperm cells.


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Spermatogenesis occurs in the testes; initially in seminiferous tubules and then later in Sertoli cells.

Spermatogenesis involves mitosis, cell growth, meiosis and then differentiation.

Hormones in spermatogenesisOne role of Luteinising Hormone and Follicle Stimulating Hormone is in the regulation of the menstrual cycle andin particular the preparation of the ovaries for the release of an egg. As such, it is no surprise that these two

hormone two hormones also play an important role in spermatogenesis. Testosterone, the male sex hormone, is

also required for the production of mature and functional sperm.

LH is released from the anterior pituitary gland and stimulates the secretion of testosterone by the interstitial

cells, which are also known as Leydig cells.

FSH is also released from the anterior pituitary gland and triggers meiosis I to start; FSH causes the

reduction division of meiosis that turns primary spermatocytes into secondary spermatocytes.

Testosterone is produced in the testes and has a similar effect to FSH in that it prompts the second division

of meiosis and thus prompts the development of secondary spermatocytes into spermatids. Testosterone

also generally helps with the maturation of sperm (from primary spermatocyte onwards).

Important

LH stimulates the secretion of testosterone. FSH stimulates meiosis I. Testosterone stimulates meiosis II.

The production of semen

When males ejaculate, they do not produce sperm alone; the fluid produced is known as semen and is a mixture

of sperm and various useful substances. Upon ejaculation, sperm exit the testicle and pass through the

epididymis, the seminal vesicle and then the prostate gland before being propelled along and then out of the

urethra. The substances that make up semen are produced by the seminal vesicle and prostate. Whilst the

epididymis does not directly produce a component of semen, it plays a vital role in the development of sperm.

The image to the right shows a semen sample, with four sperm in it (stained dark purple).

Important


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Semen consists of sperm and various fluids, which are required for sperm to survive the hostile conditions of the

female reproductive system and reach the ovum.

The epididymis is attached to the bottom of the testicle and it responsible for the concentration of sperm, which it

does by removing unnecessary testicle fluid. It is also where sperm learn how to swim, a vital process in order

for sperm to be functional.

The seminal vesicle produces various nutrients that will allow the sperm to survive as they travel towards an

egg. In particular, fructose is made as this can be used in cellular respiration to provide the sperm with energy.

Mucus is also added, which acts to protect the sperm.

The prostate gland releases a fluid that is alkaline, which is crucial as this alkaline fluid neutralises the acids

produced by the vagina. There is also the addition of several useful minerals that will nourish the sperm.

Important

The epididymis is where sperm mature and learn how to swim.

The seminal vesicle produces fructose and mucus.

The prostate gland produces alkaline fluids containing useful minerals.

Diagram of ovary

In the same way that you might be asked to analyse an image of testis tissue, you must also be familiar with and

able to label a diagram of an ovary with the following structures:

Germinal epithelium

Primary follicles

The mature follicle(Graafian follicle)

Secondary oocyte

You could include the following for the sake of completeness:

Primary oocyte

Corpus luteum


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Corpus albicans

Outer layer of germ cells medulla

Stroma

Region where blood vessels enter and leave

Examiner Tip

For more information and to gain some understanding of what a light micrograph looks like,watch this video, but

remember that this goes into more detail than you need to know.

OogenesisOogenesis is the production of an ovum, also known as an egg, and occurs in the ovaries. It is the female formof gametogenesis and has many similarities to spermatogenesis.

1. An individual oogonium divides by mitosis (cell division) to form many oogonia.

2. Each oogonium then develops and grows within an individual follicle of the ovaries. Developed oogonia are

called primary oocytes.

3. Primary oocytes are diploid and divide via the reduction division of meiosis I to form two haploid nuclei as the

pairs of homologous chromosomes are separated. However, the division of the primaryoocyte is not equal. More

cytoplasm goes to one half of the chromosomes to form the secondary oocytes. The other set of chromosomes

is left with very little cytoplasm and is called the first polar body, which will degenerate and breakdown.

4. Secondary oocytes then undergo the second division of meiosis to form a mature ovum. Again, there is an

unequal distribution of cytoplasm with the ovum taking most of it leaving a small structure called the second

polar body, which will degenerate as the first polar body did.


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{definitionbox_start} Oogenesis is the formation of gametes, specifically the formation of ova.

Oogenesisoccurs within the follicles of the ovaries.

Oogenesis involves mitosis, cell growth, and then meiosis. {definitionbox_end}

The timing of oogenesis is not as simple as spermatogenesis. In spermatogenesis one process follows the other

without interruption, but in oogenesis there are two pauses:

Firstly, primary oocytes begin meiosis I, but stop in metaphase, which is described as arrested development.

Anaphase does not occur until a female reaches puberty, at which stage the primary oocyte is finally able to

divide to form the secondary oocyte and first polar body. The secondary oocyte then begins meiosis II, but is

also stopped in metaphase. It is not until a sperm has penetrated the outer layer of an egg that the secondary

oocyte progresses through meiosis II to produce the mature ovum and second polar body.

Another difference between oogenesis and spermatogenesis is the timing of the two. Spermatogenesis does not

occur at all until a boy reaches puberty. After puberty has begun, the production of sperm will continue

throughout virtually all of the mans life. On the other hand, the production of ova begins in a female foetus,

which means it occurs even before that female has been born. It is also important to note that by the time a

female is born she is not able to increase the number of eggs that she has i.e. the number of oocytes a womanhas at birth is the maximum number she will ever have and this number decreases with every menstrual cycle.

Important


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Oogenesis has various pauses in it; oogenesis pauses in metaphase I until puberty and then again in

metaphase II until a sperm has penetrated the Zona Pellucida of the egg cell.


Spermatogenesis is the formation of gametes, specifically the formation of sperm cells.

Spermatogenesis occurs in the testes; initially in seminiferous tubules and then later in Sertoli cells.

Spermatogenesis involves mitosis, cell growth, meiosis and then differentiation.

LH stimulates the secretion of testosterone.

FSH stimulates meiosis I.

Testosterone stimulates meiosis II.

Semen consists of sperm and various fluids, which are required for sperm to survive the hostile

conditions of the female reproductive system and reach the ovum.

The epididymis is where sperm mature and learn how to swim.

The seminal vesicle produces fructose and mucus.

The prostate gland produces alkaline fluids containing useful minerals.

Oogenesis is the formation of gametes, specifically the formation of ova.

Oogenesisoccurs within the follicles of the ovaries.

Oogenesis involves mitosis, cell growth, and then meiosis.

Oogenesis has various pauses in it; oogenesis pauses in metaphase I until puberty and then again in

metaphase II until a sperm has penetrated the Zona Pellucida of the egg cell.

Reproduction

This article is a HL extension of the SL reproduction topics. In particular, this article focuses on how

reproduction occurs and takes you from gamete to zygote; sperm and egg to baby. Can you explain

the acrosome reaction? Are you able to describe the function of the umbilical cord? Do you know what

processes control birth? The answers are below...

Spermatogenesis compared to oogenesis


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In the previous article the processes of spermatogenesis and oogenesis were outlined. You need to be able to

outline both processes and a common way of examining this is to ask you to compare the two. Drawing a table

such as the one below clearly shows the differences between the two processes. Notice how there are many

more differences than similarities:


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Mature sperm and egg

The process of fertilisation is the joining of the two gametes; the sperm and the egg. In order to understand this

process, you must be aware of the structure of each of the gametes. You might be asked to draw a diagram of

each in the exam and if you are able to draw a diagram of the gametes your understanding of the processes

involved in fertilisation should improve! Include the following structures for the sperm and egg respectively:

Acrosome

Plasma membrane

Haploid nucleus

Centriole


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Mitochondria

Note the ratios of size between head, mid piece and tail.

Important

Fertilisation is the joining of two haploid nuclei (sperm and egg) to form a diploid nucleus (zygote).

Haploid nucleus

Cytoplasm (must show large volume relative to nucleus four to one ratio of diameter at a minimum)

First polar body (needs to be drawn on the outside of the cell)

Plasma membrane

Cortical granules (need to be drawn in vicinity of plasma membrane)

Zona Pellucida

The process of fertilization

Fertilisation is the way in which two haploid nuclei fuse to form a diploid nucleus. It is how a sperm and an egg

create a zygote and it is the basis of sexual reproduction! Fertilisation occurs as a series of steps, with several

specific reactions that you need to be familiar with:

Step 1Firstly, there is the acrosome reaction. As the sperm reaches the ovum in the uterus it attaches to glycoproteins

in the Zona Pellucida. This attachment leads to an influx of calcium ions into the sperm, which in turn, causes


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the release of the enzymes contained with the acrosome vesicle. Enzymes are released, which digests a part of

the Zona Pellucida to allow the sperm entry.

Step 2

The sperm head burrows into the egg membrane through the hole created via the acrosome reaction. Once the

head of the sperm has passed inside the eggs plasma membrane, the secondary oocyte then recommences

meiosis and there is the production of the ovum and the second polar body.

The image to the right shows an electron micrograph of the process of the acrosome reaction. A is the initial

contact of the sperm and egg. B is a close up showing how glycoproteins of the Zona Pellucida interact with thesperm. C and D show the sperm head moving further inside the egg,

Step 3Lastly, there is the cortical reaction, which involves the exocytosis of the enzymes contained in the cortical

granules. As these enzymes react with the glycoproteins of the Zona Pellucida, they cause a series of cross-

links to be formed. These cross-links create an impenetrable barrier as the Zona Pellucida thickens and prevents

other sperm from entering.

Important

Fertilisation involves the acrosome reaction, penetration of the egg cell membrane by the sperm and the cortical

reaction.

http://www.youtube.com/watch?feature=player_embedded&v

=MBe8DD_7r30

Embryonic development


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Once fertilisation has been completed i.e. once the two nuclei of the sperm and ovum have fused, the result is

described as a zygote. Fertilisation usually occurs in the fallopian tube and by the time the zygote has floated

down to the uterus, it will have begun to divide via mitosis. The first mitotic division usually occurs approximately

twenty four hours after fertilisation and mitosis will then continue throughout the rest of the pregnancy.

The diagram to the right shows the basics of early embryonic development and where it occurs in relation to the

female reproduction system.

With every mitotic division the zygote grows in size and a progressively larger ball of cells is created. The zygote

becomes two cells, then four, then eight and so on and eventually forms a structure called the blastocyst.

Approximately six days following fertilisation the blastocyst will consist of around thirty two cells and will have

reached the uterus. The blastocyst is a hollow ball of cells, which consists of a collection of cells called the inner

cell mass that will form the actual foetus and then cells that surround the inner cell mass called chorionic villi,

which will form the placenta. Chorionic villi burrow into the endometrium (the lining of the uterus) in the process

of implantation, which allows the blastocyst to attach firmly to the uterus.

Important

Fertilisation usually occurs in one of the fallopian tubes.

After fertilisation, the zygote repeatedly divides via mitosis until it reaches a point where it has enough

cells to form a blastocyst.

The blastocyst consists of an external chorion and an internal inner cell mass.

The chorion of the blastocyst is the part of the zygote that actively implants into the lining of the uterus,

the endometrium.

The role of hCG in early pregnancyhCG (human chorionic gonadotrophin) is a vital hormone for the development of a pregnancy. It is secreted by

the placenta (formed once a blastocyst has implanted into the endometrium) and stimulates the ovaries. In

particular, hCG targets the corpus luteum and turns into a structure called the corpus luteum of pregnancy,

which releases large amounts of oestrogen and progesterone.


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Oestrogen and progesterone have two important actions. Firstly, they maintain the endometrium so that the

placenta has access to a good blood supply, which is required for the blastocyst to develop properly. Secondly,

oestrogen and progesterone have negative feedback effects of FSH and LH so pregnancy will lead to the

prevention of another menstrual cycle and the release of another egg.

hCG can be detected in both the mothers blood and urine and withhelp of monoclonal antibodies, hCG levels

can be detected in urine and this is the basis of the standard pregnancy test.

Important

hCG causes the corpus luteum to develop and produce enough oestrogen and progesterone to maintain the

endometrium and inhibit the menstrual cycle.

The placentaThe placenta is the structure that links mother and foetus; it is the route by which all substances enter and exit

the baby. It is quite literally the foetus life line and has important roles in pregnancy. As is often the case in

biology the placenta has specific adaptations that allow it to carry out its functions more effectively.

Running between mother and baby is the umbilical cord, which is attached to the placenta (mother) and directly

to the foetus. The umbilical cord has two umbilical arteries and one umbilical vein. The umbilical vein is the

structure that brings oxygenated blood and other important nutrients to the foetus and strangely, it is the

umbilical arteries that transports deoxygenate blood away from the foetus (arteries normally carry oxygenated

blood).

Importantly, the placenta is the point at which maternal and foetal blood come into close contact. Blood from the

two cannot mix as they may be different types of blood and mixing would lead to considerable problems and

likely termination of the pregnancy. Instead, maternal blood flows very close to foetal cells so that there can bethe vital exchange of materials between mother and baby.

Important

The placenta (umbilical cord) is the link between mother and baby that allows for the exchange of nutrients and

important substances such as oxygen.

In addition to supporting the foetus with everything that it needs in terms of oxygen and nutrients, the placenta

also plays a role in hormone release and the more general maintenance of pregnancy. After around 40 days, the

ovaries and the corpus luteum no longer play an important role in the maintenance of pregnancy. After roughly 6

weeks it is the placenta that produces progesterone and oestrogen to maintain the endometrium and prevent

menstruation/ovulation (via FSH and LH negative feedback).


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Important

The placenta takes over the production of oestrogen and progesterone approximately 40 days after fertilisation.

An important part of the placenta is the amniotic sac, which is a derivation of cells of the external most part of the

placenta (the chorion). The amniotic sac protects the foetus and also produces amniotic fluid, which also

protects the foetus. Amniotic fluid and its surrounding sac act to cushion the foetus within the uterus.

The image to the right shows a foetus at eight weeks, with the amniotic sac surrounding it.

Important

The amniotic sac contains amniotic fluid, which protects the foetus.

BirthThe process of birth has an incredibly complex control mechanism involving dozens of different enzymes and

stimulants with aspect of both negative feedback and positive feedback. Thankfully, you do not need to know the

full detail of birth, only the shortened version that is summarised below:

1. The process of birth (labour) begins when progesterone levels fall.

2. The hormone oxytocin is then secreted, which causes the the uterus to contract. As well as the endometrium

of the uterus, there is also the myometrium, which is the muscular part of the uterus that actively contracts.

3. Oxytocin acts in a positive feedback loop, which means that oxytocin stimulates the process of more

oxytocin and so forth. As such, the concentration of oxytocin rapidly increases accompanied by stronger,

more powerful contractions.


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4. Before the baby is able to leave the uterus, it has to push through the opening of the uterus, which is the

cervix. The cervix is a disc-like structure that relaxes and widens once oxytocin has reached a certain

concentration and it is this dilation of the cervix that means labour is almost complete.

5. Once the cervix has dilated, the baby pushes through the cervix and out of the vagina (the birth canal) and

the umbilical cord is cut. At this stage, the baby begins to breathe for itself. Contractions continue after thebaby has been born so that the placenta is removed from the uterus too.

Important

Labour is triggered by a fall in progesterone levels, which causes the release of oxytocin. In turn, Oxytocincauses contractions in the myometrium of the uterus leading to the birth of the baby.


Fertilisation is the joining of two haploid nuclei (sperm and egg) to form a diploid nucleus (zygote).

Fertilisation involves the acrosome reaction, penetration of the egg cell membrane by the sperm and the

cortical reaction.

Fertilisation usually occurs in one of the fallopian tubes.

After fertilisation, the zygote repeatedly divides via mitosis until it reaches a point where it has enough cells to

form a blastocyst.

The blastocyst consists of an external chorion and an internal inner cell mass.

The chorion of the blastocyst is the part of the zygote that actively implants into the lining of the uterus, the

endometrium.

hCG causes the corpus luteum to develop and produce enough oestrogen and progesterone to maintain the

endometrium and inhibit the menstrual cycle.

The placenta (umbilical cord) is the link between mother and baby that allows for the exchange of nutrients

and important substances such as oxygen.

The placenta takes over the production of oestrogen and progesterone approximately 40 days after

fertilisation.

The amniotic sac contains amniotic fluid, which protects the foetus.

Labour is triggered by a fall in progesterone levels which causes the release of oxytocin In turn Oxytocin

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IB Biology Higher Level Human and Health Physiology Notes

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