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.