The kidney's role in homeostasis
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The kidney's role in homeostasis
Blood pressure control
Macular densa in the wall of distal tubulemake contact with cells in the endotheliumof the arterioles which release a hormonecalled renin, this is called juxtaglomerularapparatus which maintains a constant bloodflow through the glomerulus through tubulefeedback mechanism.
Renin acts via the renin angiotensin systemto produce both local vasoconstriction ofthe efferent arteriole increasing GFR adalso peripheral vasoconstriction to increasearterial blood pressure.
For example: when there is dehydration,sodium deficiency, or haemorrhage, therewill be decrease in blood volume andblood pressure.
This will stimulate kidney to produce reninwhich produce angiotensin I which isconverted to angiotensin II.
Release of angiotensin II stimulates adrenalcortex to produce aldosterone whichreabsorbed more sodium and water andeliminates potassium which increase bloodvolume and blood pressure.
This hormone is produced byerythropoietinproducing cells of thekidney to maintain normal red cell countand prevention of anemia.
Erythropoietin production is stimulated byhypoxia and stopped when hypoxia iscorrected, thus, its production is controlledby the negativefeedback principle.
pH scale is a logarithmic scale (range 114)that measures the concentration of freehydrogen ions in a fluid. The scale isreciprocal, when pH is low, hydrogen ions ishigh. Normal pH is between 7.357.45.
Kidney controls the acid base balance bycontrolling the bicarbonate ionsconcentration in the body.
When plasma bicarbonate concentration isnormal (25mmol/l) then all of the filteredbicarbonate is reabsorbed in the proximaltubule and distal tubule.
When the plasma bicarbonateconcentration is higher than normal, thenexcess bicarbonate is lost in the urine.
Or, undergoes bicarbonate ‘trapping’where bicarbonate ion is reserved in theblood and excretes excess hydrogen ions.
The rise in hydrogen ion concentration (acidosis) is corrected bykidneys by secreting the excess hydrogen ion into the urine and in theprocess generating further bicarbonate for reabsorption into theplasma. Fall in hydrogen ion concentration (alkalosis) in the body fluidsis corrected by kidney by reducing the amount of hydrogen ionsecretion in the urine.
Waste excretion in urine
Excess ions sodium, potassium, calcium,magnesium, chloride, bicarbonate,phosphate and ammonium.
Metabolic wastes urea, creatinine, uricacid, nitrogenous waste.
Drug metabolitesmost pharmacologicalagents are detoxified in liver and thenexcreted through the kidney.
Metabolites of hormonefor examplehuman chorionic gonadotrophin in theurine in early pregnancies.
Potassium Aldosterone is the only hormoneinvolved in the control of potassium content in thebody.
Small increase of extracellularpotassium directly stimulates adrenalcortex to produce aldosterone whichaffects the distal tubule of thenephron to secrete potassium intothe urine.
When the aldosterone level falls, thereverse occurs and less potassium issecreted and is controlled by negativefeedback system.
Calcium is important in the extracellular fluid incontrolling nerve and muscle conduction.
Calcium reabsorption occurs dependingon the level of circulating parathyroidhormone (PTH).
Phosphateis important in buffer systems tomaintain the plasma pH and exists inequilibrium with calcium.
When plasma phosphate level is low, thefiltered phosphate is reabsorbed in theearly proximal tubule and if it is high, it isexcreted in the urine.
Magnesiumalso an important intracellularcation involved in energy storage andproduction.
Similar to calcium, increase in PTH,increase tubular reabsorption ofmagnesium.
Fluid volume balance
ADH is secreted by the posterior pituitarygland and ADH receptors are found incollecting ducts of kidney tubules.
When there is rise in plasma osmolality, water isreabsorbed into the blood, diluting the plasmaand reducing osmolality to normal.
A fall in plasma osmolality leads to decrease inADH secretion by negative feedback. ADHreceptors are found in collecting ducts ofkidney tubules.
Aldosterone is a steroid hormone secretedfrom the adrenal cortex of the kidney andeffects the distal tubule of the nephron.Secretion of aldosterone is regulated bypeptide, angiotensin II.
When more aldosterone is secreted, themore sodium is reabsorbed and thereforewater is reabsorbed.
Atrial natriuretic peptide (ANP) is releasedfrom cardiac atrial cells in response tostretch in atrium from fluid overload.
ANP inhibits aldosterone secretion by adrenalcortex, reduce renin release by kidney andreduction of ADH from the posterior pituitaryto reduce the fluid volume by excreting sodiumand water.