Endocrine System, Part 2

Ashlee BlackKelsey HunterMelanie O’Bar

Homeostasis! There are simple and complex pathways in the

endocrine system. The 4 named in your text are: simple endocrine, simple neurohormone, simple neuroendocrine, and complex neuroendocrine. Simple endocrine: stimulus > receptor protein on endocrine

cell > secretion goes to blood vessel > to target effectors > response.

Simple neurohormone: stimulus > sensory neuron connected to hypothalamus/posterior pituitary > secretion through neurosecretory cell > blood vessel > target effectors > response.

Simple neuroendocrine: stimulus > sensory neuron on hypothalamus > neurosecretory cell secretes hormone-releasing hormone > blood vessel > receptor site on endocrine cell > blood vessel > target effectors > response.

Homeostasis, cont. Complex neuroendocrine pathways are those in which a hormone

secreted by one endocrine gland produces a response in another endocrine gland. (Pituitary gland) Seems very similar to simple neuroendocrine…

In endocrine and neuroendocrine pathways, the outgoing signal is called an efferent signal and is either a hormone or neurohormone.

Feedback Loops: connect the response to the initial stimulus. Negative feedback: the effector response reduces the stimulus

and the response eventually stops. Prevents overreaction of the system and “wild” fluctuations in the environment. Contributes to the hormonal control of blood glucose and calcium levels.

Positive feedback: Reinforces the stimulus and initiates greater response. Release of milk is an example (neurohormone pathway).

Regulation of Body Temperature Body temperature is the balance between the

heat produced by the body and the heat lost from the body. In humans, as other mammals, the core temperature of the body remains constant despite the temperature of the surrounding environment.

For the body to function optimally, the temperature must be maintained within narrow limits. There are two kinds of body temperature: core temperature and surface temperature. Core temperature is the temperature of the deep tissues of the body. It normally remains constant at about 98.6 degrees Fahrenheit (37.0 degrees Celsius). However, body temperature varies from person to person and is affected by factors such as exercise, sleep, eating and drinking, and time of day.

Something We Floridians Are All Too Familiar With…

The sweat pores allow loss of fluid as part of temperature control of the body. As a rise in external temperature is sensed by the nerve endings in the skin, the message is relayed to the hypothalamus, the temperature-regulating area of the brain. The brain then sends nerve impulses to the sweat (eccrine) glands stimulating them to release sweat until the skin receptors detect that the skin's temperature is back to normal. The brain then sends messages to stop the release of sweat. The human body has between two and three million eccrine glands that secrete moisture on the skin primarily to cool the body through evaporation.

Body heat is transferred from skin and blood to the sweat, The sweat evaporates transferring heat away and in doing so cools the body

The Hypothalamus Is Back… The body's surface temperature rises and falls in

response to the environment. Body temperature is maintained by the hypothalamus, which constantly monitors blood temperature and activates mechanisms to compensate for changes.

When the body's surface temperature falls, the hypothalamus sends nerve impulses to the skin to stimulate shivering, which generates heat by muscle activity, and to restrict the blood vessels in the skin (skin arterioles), which limits heat loss. When the surface temperature rises, the hypothalamus stimulates the sweat glands in the skin to produce sweat and dilates the skin arterioles to increase heat loss.

The message that is responsible for the restriction and dilation of skin arterioles is hormonal, and is sent through the bloodstream from the pituitary gland.

Primary Methods of Temperature Regulation

Hairs with the erector pili muscle (controlled by nerves)

Sweat glands (controlled by nerves and hormones)

Blood arterioles (controlled by hormones) Shivering: When core temperature falls an

uncontrolled phase of rapid muscular contraction occurs which generates heat that is used to raise the core temperature (controlled by nerves)

When the Body Temperature Falls:

Blood flow to the surface via arterioles in reduced.

Notice that (a) is dilated but (b) & (c) are vasoconstricted: Less heat is lost to the environment by radiation, heat is retained in the core.

When the Body Temperature Rises:

Blood flow to the surface is increased.

(a) is closed and (b) and (c) arterioles are open: vasodilation

Blood flows closer to the surface

More heat is lost to the external environment

Core temperature is reduced

Nerves vs. Hormones Nerves are faster (one ten thousandth of a

second) while hormones are slower-acting. Nerves have a shorter effect while

hormones last longer in the bloodstream. Nerves are electrical impulses while

hormones are chemical. Nervous system performs short term crisis

management while Endocrine system regulates long term ongoing metabolic processes

Paracrine communication involves chemical messengers between cells within one tissue while Endocrine communication is carried out by endocrine cells releasing hormones

Further Comparison Together, the nervous and endocrine systems

coordinate the functions of all body systems.  How does the nervous system achieve this? The nervous system achieves this through the use of

nerve impulses and the secretion of neurotransmitter substances that either excite or inhibit the effector.

            How does the endocrine system achieve this control? The endocrine system, in contrast, regulates body

functions by releasing chemical messengers called hormones (“to urge on” or “to set in motion”) into the bloodstream for delivery throughout the body.

And more… The nervous system causes muscles to

contract and glands to secrete.  The endocrine system regulates metabolic activities, growth and development, and reproduction.

The nervous system tends to act in milliseconds.  The endocrine system acts in seconds, minutes, hours, weeks, months, even years, depending upon the hormone.

Each nerve affects one part of your body. On the other hand, hormones can affect lots of parts of the body at the same time

Remember… One way of keeping hormones and nerves straight in

your mind is to compare them to radio and telegraph communication systems. Whereas nerves are relatively hard-wired from one spot to another, hormones are broadcast into the circulatory system. Carried in the blood, they come into contact with every cell in the body, just as we are bathed in radio waves all of the time. But only those cells with the correct receptor cells will bind, and thus respond to, any give hormone. Hormones can therefore be highly specific, just as nerves; they can also be very general, eliciting responses from all the cells in an organ or organ system without having to directly innervate each one.

Blood Glucose Concentration Insulin and glucagon regulate the sugar level in the

body. These two hormones are manufactured in the pancreas and through circulation are carried to the liver where they perform their functions. Enzymes that convert glucose to glycogen though a condensation reaction are stimulated by Insulin. Enzymes that hydrolyze glycogen to glucose are stimulated by glucagon. Receptors in the pancreas are sensitive to the changes in sugar level, thus releasing the necessary requirements of insulin and glucagon depending on the needs of the body. The beta cells found in the islets of the pancreas make insulin and the alpha cells make glucagon.

The Pacreas is Back, Too… If there is too much glucose in the blood, then receptors in the

pancreas detect this. They send a message to the cerebrum, inducing feelings of satiety (so that intake of food is decreased). They also send messages to the Islets of Langerhans (the B-cells) to produce insulin. This insulin is released into the bloodstream via capillaries, and has various effects. It increases the intake of glucose by all cells, and stimulates the conversion of glucose into glycogen. This reduces the amount of glucose in the blood, so that they return to equilibrium.

If there is too little glucose in the blood, then the same receptors in the pancreas detect this. They send a message to the cerebrum, inducing feelings of hunger (so that intake of food is increased). They also send a message to the A-cells in the islets of langerhans to produce glucagon.

A-cells make glucagon and affect mainly liver cells, to break down glycogen.

B-cells make insulin and affect all cells, to take in glucose.

Return of the Pancreas, cont. Glucagon is released into the bloodstream via capillaries and

stimulates the conversion of glycogen into glucose in liver. The liver is also stimulated to convert amino acids into glucose. Thus, the levels of glucose in the blood increase so that equilibrium is reached.

To elevate glucose levels, glucagon binds to receptors on hepatocytes (liver cells) and other cells (e.g. muscle cells). This activates an enzyme, glycogen phosphorylase, inside the hepatocyte to hydrolyse glycogen to glucose. This process is called glycogenolysis. In rodents, alpha cells are located in the periphery of the islets, however in humans the islet architecture is generally less organized and alpha cells are frequently observed inside the islets as well. When being viewed by an electron microscope, alpha cells can be identified by their characteristic granules with a large dense core and a small white halo.

Alpha Cells! endocrine cells in the islets of Langerhans of

the pancreas. They make up 15-20% of the cells in the islets. They are responsible for synthesizing and secreting the peptide hormone glucagon, which elevates the glucose levels in the blood.

Beta Cells! are a type of cell in the pancreas in areas called the islets of Langerhans.

They make up 65-80% of the cells in the islets. Beta cells make and release insulin, a hormone that controls the level of

glucose in the blood. There is a baseline level of glucose maintained by the liver, but it can respond quickly to spikes in blood glucose by releasing stored insulin while simultaneously producing more. The response time is fairly quick, taking approximately 10 minutes.

Apart from insulin, beta cells release C-peptide, a byproduct of insulin production, into the bloodstream in equimolar quantities. C-peptide helps to prevent neuropathy, and other symptoms of diabetes related to vascular deterioration[1]. Measuring the levels of C-peptide can give a practitioner an idea of the viable beta cell mass.

β-cells also produce amylin, also known as IAPP, islet amyloid polypeptide. Amylin functions as part of the endocrine pancreas and contributes to glycemic control. Amylin's metabolic function is now somewhat well characterized as an inhibitor of the appearance of nutrient [especially glucose] in the plasma. It thus functions as a synergistic partner to insulin. Whereas insulin regulates long term food intake, increased amylin decreases food intake in the short term.

Diabetes One in 20 of the world's adult population now

suffers from diabetes. Diabetes currently affects 246 million people worldwide and is expected to affect 380 million by 2025. It is the fourth leading cause of global death by disease.

The westernisation of people's lifestyles is leading to a rapid spread of type 2 diabetes, particularly in developing countries.

In 2007, the five countries with the largest numbers of people with diabetes were India (40.9 million), China (39.8 million), the United States (19.2 million), Russia (9.6 million) and Germany (7.4 million).

Diabetes, cont. Type 2 or adult onset diabetes is associated with obesity,

poor eating habits and a sedentary lifestyle. It is caused by lack of insulin in the body and resistance to insulin that is naturally produced. The main risk factors for Type 2 diabetes are being

overweight, aged over 40, having a family history of diabetes and being Asian or Afro-Caribbean.

Up to 80% of Type 2 diabetes is preventable by adopting a healthy diet and exercise.

Prof. Alberti, who is president of the Royal College of Physicians said: "Diabetes causes an enormous burden to people and economies world-wide.

Common complications include circulatory disease, including strokes and amputations as well as kidney problems and blindness.

Kidney Failure Kidneys remove waste from the blood. Failing kidneys lose their ability to filter out

waste products, resulting in kidney disease, also called nephropathy.

In the kidneys, capillaries with even smaller holes in them act as filters. As blood flows through the blood vessels, small molecules such as waste products go through the holes. These become part of the urine. Useful substances, such as protein and red blood cells, are too big to pass through the holes in the filter and stay in the blood.

Kidneys, cont. High levels of blood sugar, as in diabetes, make

the kidneys filter too much blood. After having to work so hard, the kidneys start to leak. Useful protein is lost in the urine. Having small amounts of protein in the urine is called microalbuminuria. When kidney disease is diagnosed early, treatments may keep kidney disease from getting worse. Having larger amounts is called macroalbuminuria. When kidney disease is caught later, end-stage renal disease, or ESRD, usually follows.

Waste builds up in the blood.

Kidneys, Kidneys, Kidneys Treatment: centers around tight control of

blood glucose and blood pressure, exercise, losing weight, eating less salt, avoiding alcohol and tobacco

Once kidneys fail, dialysis is necessary. (Hemodialysis or Peritoneal dialysis)

A kidney transplant may also be an option.

Ta Da!

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