homeostasis in humans

8/7/2019 homeostasis in humans

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3.4 Homeostasis in Humans


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The concept of homeostasis

Process of regulating physical and

chemical factors in the internal

environment. Physical factors- body temperature and

blood pressure

Chemical factors- osmotic pressure, sugar

level and pH level.


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Homeostasis occur through a ve

feedback system. Mechanism of ve feedback acts as

corrective measure to restore the factor

that deviates from the normal range backto its normal condition.

A ve feedback system usually involve:

a) Condition that cause the level of a

particular factor to deviate from a normal

level.


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b) These change are detect by receptors.

c) Receptors communicate the changes tothe effectors by means of hormones or

the nervous system.

d) A response is triggered which oppose

the changes by corrective mechanism torestore the factor back to its normal level.

e) The return of the factor to the normal

level is detected by the receptor and thecorrective response is turned off.


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Normal level

Normal level

Fall to

normal level

Corrective

mechanismby ve

feedback

Rise tonormal level

Fall tonormal level

Corrective

mechanismby ve

feedback

Rise to

normal level

A generalised negative feedback system


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The Regulation of Water Balance by

Kidney

Kidney involve in regulating the waterbalance in the blood (the blood osmoticpressure).

When large of water quantity is consumed,the blood osmotic pressure is lowered.

To regulate the water balance and

maintain osmotic pressure in blood, kidneywill produce a large volume of urine to beexcreted.


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Structure of kidney

Human kidneys are bean-shaped. Kidney enclosed within a protective fibrous

capsule. There are three tubes arising fromeach kidney.

a) Renal artery- carry oxygenated blood tokidney. The blood also rich in excretoryproducts such as urea and mineral salts.

b) Renal vein- carry deoxygenated blood away

from the kidney. The blood has less urea andsalts.

c) Ureter- carry urine from the kidney to theurinary bladder.


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Longitudinal section of kidney shows

three distinct region:

a) Cortex (outer region)

b) Medulla (inner region)

c) Pelvis (central cavity in which the ureter

directly connect to)

Each kidney consists of millions of

microscopic tubules- nephrons


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Each nephron can be up to 14mm long,

consist of:

a) Bowmans capsule

b) Proximal convoluted tubule

c) Loop of Henle

d) Distal convoluted tubule,

which open into collecting

duct

Uriniferous

tubule

(kidneytubule)


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Bowmans capsule is a cup shaped

structure and is located in the cortex of

kidney.

Bowmans capsule has a network of

capillaries called glomerulus.

Glomerulus is supplied with blood via an

afferent arteriole which arise from a

branch of the renal artery. Blood is carried

away from the glomerulus through theefferent arteriole.


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The efferent arteriole branches into a

network of capillaries to surround the

tubule before it enter a branch of renal

vein.

Wall of Bowmans capsule and the

capillaries of glomerulus are only one-cellthick and permeable to small molecules.

The uriniferous tubule which continues

with the Bowmans capsule consist ofthree main parts:


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a) Proximal convoluted tubule (in cortex)

b) Loop of Henle (U-shaped and located in

the medulla)c) Distal convoluted tubule (in cortex and

joined to the collecting duct)

Direction of blood flow in kidney:renal artery afferent arterioleglomerulus efferent arteriole bloodcapillaries surrounding the uriniferous

tubule venule renal vein. Hence, there are two capillary network in

kidney: glomerulus and capillaries

surround uriniferous tubule.


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All the constituents of blood plasma exceptplasma proteins and RBC are filtered out

of the glomerulus into the cavity ofBowmans capsule.

The flow direction of the filtrate in thekidney as:

Bowmans capsule proximal convolutedtubule loop of Henle distal convolutedtubule collecting duct pelvis ureter

urinary bladder urethra. The filtrate that is drained into the pelvis of

kidney is called urine.


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Process of Urine Formation

The urine formation involve three mainprocesses:

a) Ultrafiltration

b) Reabsorption

c) Secretion


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Ultrafiltration process (Bowmans

capsule and glomerulus)

Blood enters glomerulus through afferent

arteriole and leaves through the efferent

arteriole.

Blood pressure in the afferent arteriole is

high because it is derived from the renal

artery which branches from aorta.

The diameter of efferent arteriole is

smaller than the afferent arteriole. So,

there is a high resistance in blood flow.


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This produce a high hydrostatic pressure

in glomerulus.

This will cause most of the constituents of

plasma to be filtered out the glomerulus

into the cavity of the Bowmans capsule.

The process where all the constituents of

blood plasma (exp: RBC, platelets, protein

plasma) are filtered under high hydrostatic

pressure into Bowmans capsule isultrafiltration.


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The fluid filtered into Bowmans capsule

is called glomerulus filtrate.

The glomerulus filtrate consists of mainly

dissolved small molecules such as

inorganic ions (sodium ions, glucose,

amino acids and urea) The ultrafiltration process occur at a

higher rate in the glomerulus compared

with other capillaries because:a) The hydrostatic pressure in glomerulus is

higher than other capillaries. ?


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b) The capillaries of glomerulus are longer

than the other capillaries in the body.

c) The capillary walls of the glomerulus

have pores which cause its higher

permeability than other capillaries.


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Reabsorption process

From the Bowmans capsule, theglomerular filtrate flows into the uriniferous

tubule.

Reabsorption process occurs along thewhole uriniferous tubule. Essential solutes

and water in the filtrate are reabsorbed

into the blood capillaries that surround the

tubule.


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At the proximal convoluted tubule:

a) About 75-80% of water is reabsorbedback into the blood capillaries by

osmosis. This occurs because the

glomerular filtrate is hypotonic to the

blood plasma.

b) All glucose, amino acids and some

minerals ions (Na+ and Cl-) in the tubule

are reabsorbed into the bloodstream byactive transport.


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At the loop of Henle:

a) About 15% of water is reabsorbed

through osmosis on the descending limb

which is permeable to water but not to

other solutes.

b) Sodium ions and chloride ions areactively transported out of the filtrate on

the ascending limb which is impermeable

to water.


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At the distal convoluted tubule and thecollecting duct:

a) The amount of water and inorganic ions thatwill be reabsorbed from the filtrate depends onthe bodys needs and is controlled byendocrine system.

b) The rate of reabsorption of water and salts isaffected by the quantity of water and saltsconsumed.

c) It is controlled by hormone as the walls of the

distal convoluted tubule and collecting duct aremore permeable to water if antidiuretichormone (ADH) is present and morepermeable to salts if aldosterone hormone ispresent.


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c) Urea is not reabsorbed throughout the

nephron, excreted in urine.

The remaining filtrate in the tubule which

is channeled into pelvis of the kidney is

called urine.

If plenty of water from the filtrate is

reabsorbed in the distal convoluted

tubule and the collecting duct, then the

amount of urine produced is little andconcentrated.


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If less water is reabsorbed from the filtrate,

a large amount of diluted urine is

produced.

Urine consist of 96% water, 2.5%

nitrogenous waste products such as urea,

uric acid and creatinine, 1.5% inorganicions and traces of bile pigments.

Urine is carried by the ureter from the

kidney to the urinary bladder to be storedtemporarily and excreted through the

urethra.


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Secretion process

Process where unwanted substances(urea, uric acid, ammonia, drugs, alcohol,excess salts & water) in the blood aretransported from the capillaries

surrounding nephron into the kidneytubule.

Secretion process helps to regulate the pHlevel of the blood.

For example: when blood is too acidic, theH+ are secreted. If blood to alkaline, HCO3

-

are secreted.


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Secretion plays an important role in

adjusting urine composition as it passes

through the kidney tubule.


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Osmoregulation

Process of regulating the blood pressureby regulating the water content and theconcentration of salts in the body.

Kidney carry out osmoregulation bycoordinating the rate of reabsorption ofwater and salts during the formation ofurine.

The amount of water and salts in the bloodwill determine the osmotic pressure in theblood.


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Reabsorption of water is controlled by

antidiuretic hormone (ADH) which is

released by pituitary gland.

Reabsorption of salts is controlled by

aldosterone hormone which is produced

by adrenal gland.


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Mechanism of Osmoregulation

a) When blood osmotic pressure is high: Blood osmotic pressure is raised when

water is lost excessively through

sweating or after a salty meal. The high osmotic pressure is detected byosmoreceptors in hypothalamus.

The pituitary gland is stimulated to

release the ADH. Adrenal gland is less stimulated and thus

less aldosterone hormone is released.


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ADH increased the permeability of the wall

of distal convoluted tubule and the

collecting duct toward the water.

So, more water and less salt are

reabsorbed from the tubule in to the blood

capillaries.

This lowers the blood osmotic pressure to

its optimum level.

A small amount of concentrated urine isproduce


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b) When the blood osmotic pressure is low:

Blood osmotic pressure is lowered when

excessive amount of water is consumed. Detected by osmoreceptors in

hypothalamus

Adrenal gland is stimulated to releasealdosterone hormone.

Pituitary gland less stimulated and lessADH released.

Aldosterone hormone cause the distalconvoluted tubule to more permeable tosalts.


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So, more salts are reabsorbed from the

tubule into blood capillaries.

This increase the blood osmotic pressureto its optimum level.

A large amount of dilute urine is produced.


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Consequences of Impaired Kidney

Function

For patient with impaired kidney function, the

kidney cant remove the excess water, mineral

salts or urea. Hence these substances remain in

blood. Kidney that are damage by disease or injury fail

to carry out ultrafiltration at glomerulus. Thus

unable to regulate the blood osmotic pressure,

filter the blood and remove the unwanted wasteproducts.

These problem can be overcome through

haemodialysis.


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Haemodialysis is the treatment for patient

with impaired kidney function to filter the

blood by dialysis using an artificial kidney

machine.

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