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Renal Role in Homeostasis (II-Electrolytes Balance (Calcium (Ca++ (Calcium…
Renal Role in Homeostasis
II-Electrolytes Balance
Calcium
Ca++
Calcium is the main mineral constituents of bones with phosphate and magnesium
-The ionized form of calcium is the active form playing important role in blood clotting and controlling nerve, muscle conduction.
-Calcium is filtered and reabsorbed is very similar to sodium through different parts of nephron.
-At distal convoluted tubules, the last 10-15% of the calcium filtered is reabsorbed under the control of parathormone.
-Decrease of ionized serum Ca++ will stimulate parathyroid gland to secrete parathormone which in turn, increase bone resorption, activate vit. D at kidney to increase gut absorption of calcium, and increase renal tubular reabsorption of calcium.
Magnesium
Mag+
-It is the second major intracellular cation that used to produce and storage of intracellular energy.
-55% of body magnesium is stored in bone.
-Under normal condition, the gut reabsorption is equal to urinary excretion.
Sodium
Na+
Sodium (Na+) is the major extracellular cation and contribute to
95% extracellular osmolality with chloride anions.
-Sodium is freely filtered at the glomerulus.
-At proximal convoluted tubules, 65% of Na+ is reabsorbed obligatory by active transport back to blood circulation.
-At thick ascending loop of Henle, 20 - 25 of %Na+ is more reabsorbed obligatory back to circulation by by active transport.
-At the distal convoluted tubules, 10 -15% of Na+ is regulatory absorbed by active transport under control of aldosterone hormone.
Potassium
K+
-Potassium (K+) forms 98% of the intracellular cations.
-Potassium is freely filtered at the glomerulus.
-At proximal convoluted tubules, 65% of K+ is obligatory reabsorbed by active transport back to blood circulation
-At thick ascending loop of Henle, K+ is more reabsorbed obligatory back to circulation by by active transport.
-At the distal convoluted tubules, K+ is regulatory absorbed by active transport under control of aldosterone hormone in exchanging of sodium excretion based on body need.
Phosphate
acidic: PH2O4- basic: PHO4-
-Phosphate is also one of the main mineral constituents of the bone and one of the buffering system that maintaining pH of the plasma.
-There are two ionized forms of phosphate, acid phosphate (H2PO4-) and alkaline phosphate ( HPO4-).
-when plasma phosphate is less than 1 mmol/1, most of the filtered phosphate is completely reabsorbed in proximal convoluted tubules and more reabsorbed at distal convoluted tubules under control of parathormone.
-when it is more than 1 mmol/1, the extra phosphate amount excreted in urine.
I-Fluid Volume Balance
atrial natriuretic peptide hormone
ANP
-The the increase in the extracellular fluid volume will stimulate receptors in the wall of the right atrium which in turn release ANP
-ANP will inhibit aldosterone secretion, reduce renin release, and reduce ADH release
The result is increase renal sodium excretion and water secretion follow
Once the extracellular volume decrease the ANP secretion will be inhibited
antidiuretic hormone
ADH
-The increase plasma osmolality will stimulate osmoreceptors in hypothalamus which in turn sends signals to posterior pituitary gland to secrete ADH
-ADH open the water channels of the renal collecting ducts to reabsorb water back to the blood circulation by osmosis and hence decrease plasma osmolality back to normal
-Once the plasma osmolality back to normal the water reabsorption stops from the renal collecting ducts
Aldosterone
-Aldosterone is a steroid hormone secreted from adrenal gland cortex in response of increasing level of angiotensin II not by extracellular osmolality.
-The actions of aldosterone secretion are more sodium reabsorption (by active transport) associated by water reabsorption (by osmosis) and finally potassium
secretion in distal convoluted tubules.
-In chronic pathological secretion of aldosterone as in cushing syndrome there are increase in serum sodium, increase in extracellular fluids (edema), and finally, hypokalemia.
III-Acid-Base Balance
acidosis
increase H+ concentration
-The acidosis is due to increasing in serum acidic hydrogen ions concentration and/or decreasing in serum basic bicarbonate ions concentration.
-The initial body response to the increase in the hydrogen ions concentration in the body fluids is corrected by the healthy lung via blowing off carbon dioxide by increasing respiratory rate to return pH back to normal (7.35 to 7.45).
-The long term body response to the increase in the hydrogen ions concentration in the body fluid is done by the healthy kidneys via increasing hydrogen ions secretion by the distal convoluted tubules coupled by increasing the basic bicarbonate ions reabsorption from renal tubules to return pH back to normal (7.35 to 7.45).
alkalosis
decrease H+ concentration
-The alkalosis is due to increasing in serum basic bicarbonate ions concentration and/or decreasing in serum acidic hydrogen ions concentration.
-The healthy kidney plays the main role in correcting alkalosis via reducing hydrogen ions secretion at renal distal tubules forming less acidic urine coupled by decreasing bicarbonate reabsorption by renal tubules and hence, more bicarbonate excreted in urine
forming more alkaline urine to return pH back to normal.
-The lung will reduce ventilation rate to accumulate carbon dioxide in the serum which will combine with water to form carbonic acid and in turn the acid dissociate into acidic hydrogen ion increasing its serum concentration to return pH back to normal.
IV-Erythropoietin Production
-By negative feedback mechanism, once the RBC and hypoxia are corrected the erythropoietin hormone secretion is inhibited
-Erythropoietin hormone is released from the kidney in response to decrease in RBC count and hypoxia stimulating the precursors of RBC cells in bone marrow to produce more RBC for more oxygen to be carried.
V-Waste Excretion in Urine
-All the filtered creatinine plus the secreted creatinine in proximal and distal convoluted tubules is excreted in urine without reabsorption.
-50% of the filtered uric acid is reabsorbed in proximal convoluted tubules by passive diffusion and some secreted in distal convoluted tubules.
-All the ammonia is excreted combining with hydrogen ions to form ammonium preventing H+ reabsorption back to circulation to keep pH within normal range.
-All drug metabolites are eventually filtered and secreted to be excreted via urine.
VI-Blood Pressure Control
-Juxtaglomerular apparatus is responsible for maintaining constant blood flow through glomerulus despite fluctuations in arterial pressure.
-In cases of fall in glomerular blood flow due to hypotension caused by hemorrhage, dehydration, and decreasing extracellular body fluid the apparatus will release renin causing local vasoconstriction of efferent arterioles to increase glomerular blood pressure and hence, the glomerular blood flow.
-Renin in the blood circulation will cause peripheral vasoconstriction leading to increase of the peripheral blood pressure too.
-Also, renin is the precursor of angiotensin II which is responsible for increase sodium, water reabsorption and potassium secretion at the distal convoluted tubules restoring extracellular body fluid back to normal.