The kidney's role in homeostasis

The kidney's role
in homeostasis
Blood pressure control
Macular densa in the wall of distal tubule
make contact with cells in the endothelium
of the arterioles which release a hormone
called renin, this is called juxtaglomerular
apparatus which maintains a constant blood
flow through the glomerulus through tubule
feedback mechanism.
Renin acts via the renin angiotensin system
to produce both local vasoconstriction of
the efferent arteriole increasing GFR ad
also peripheral vasoconstriction to increase
arterial blood pressure.
For example: when there is dehydration,
sodium deficiency, or haemorrhage, there
will be decrease in blood volume and
blood pressure.
This will stimulate kidney to produce renin
which produce angiotensin I which is
converted to angiotensin II.
Release of angiotensin II stimulates adrenal
cortex to produce aldosterone which
reabsorbed more sodium and water and
eliminates potassium which increase blood
volume and blood pressure.
Erythropoietin production
This hormone is produced by
erythropoietinproducing cells of the
kidney to maintain normal red cell count
and prevention of anemia.
Erythropoietin production is stimulated by
hypoxia and stopped when hypoxia is
corrected, thus, its production is controlled
by the negativefeedback principle.
pH balance
pH scale is a logarithmic scale (range 114)
that measures the concentration of free
hydrogen ions in a fluid. The scale is
reciprocal, when pH is low, hydrogen ions is
high. Normal pH is between 7.357.45.
Kidney controls the acid base balance by
controlling the bicarbonate ions
concentration in the body.
When plasma bicarbonate concentration is
normal (25mmol/l) then all of the filtered
bicarbonate is reabsorbed in the proximal
tubule and distal tubule.
When the plasma bicarbonate
concentration is higher than normal, then
excess bicarbonate is lost in the urine.
Or, undergoes bicarbonate ‘trapping’
where bicarbonate ion is reserved in the
blood and excretes excess hydrogen ions.
The rise in hydrogen ion concentration (acidosis) is corrected by
kidneys by secreting the excess hydrogen ion into the urine and in the
process generating further bicarbonate for reabsorption into the
plasma. Fall in hydrogen ion concentration (alkalosis) in the body fluids
is corrected by kidney by reducing the amount of hydrogen ion
secretion in the urine.
Waste excretion in urine
Excess ions sodium, potassium, calcium,
magnesium, chloride, bicarbonate,
phosphate and ammonium.
Metabolic wastes urea, creatinine, uric
acid, nitrogenous waste.
Drug metabolitesmost pharmacological
agents are detoxified in liver and then
excreted through the kidney.
Metabolites of hormonefor example
human chorionic gonadotrophin in the
urine in early pregnancies.
Electrolytes balance
Potassium Aldosterone is the only hormone
involved in the control of potassium content in the
body.
Small increase of extracellular
potassium directly stimulates adrenal
cortex to produce aldosterone which
affects the distal tubule of the
nephron to secrete potassium into
the urine.
When the aldosterone level falls, the
reverse occurs and less potassium is
secreted and is controlled by negative
feedback system.
Calcium is important in the extracellular fluid in
controlling nerve and muscle conduction.
Calcium reabsorption occurs depending
on the level of circulating parathyroid
hormone (PTH).
Phosphateis important in buffer systems to
maintain the plasma pH and exists in
equilibrium with calcium.
When plasma phosphate level is low, the
filtered phosphate is reabsorbed in the
early proximal tubule and if it is high, it is
excreted in the urine.
Magnesiumalso an important intracellular
cation involved in energy storage and
production.
Similar to calcium, increase in PTH,
increase tubular reabsorption of
magnesium.
Fluid volume balance
ADH is secreted by the posterior pituitary
gland and ADH receptors are found in
collecting ducts of kidney tubules.
When there is rise in plasma osmolality, water is
reabsorbed into the blood, diluting the plasma
and reducing osmolality to normal.
A fall in plasma osmolality leads to decrease in
ADH secretion by negative feedback. ADH
receptors are found in collecting ducts of
kidney tubules.
Aldosterone is a steroid hormone secreted
from the adrenal cortex of the kidney and
effects the distal tubule of the nephron.
Secretion of aldosterone is regulated by
peptide, angiotensin II.
When more aldosterone is secreted, the
more sodium is reabsorbed and therefore
water is reabsorbed.
Atrial natriuretic peptide (ANP) is released
from cardiac atrial cells in response to
stretch in atrium from fluid overload.
ANP inhibits aldosterone secretion by adrenal
cortex, reduce renin release by kidney and
reduction of ADH from the posterior pituitary
to reduce the fluid volume by excreting sodium
and water.
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