Wednesday, March 6, 2013

Renal physiology (Part 3): regulation of sodium and potassium excretion

Renal physiology: regulation of sodium and potassium excretion

Sodium regulation
Function: NaCl is the main determinant of ECF osmolality and volume.
Sodium regulation
·         Low-pressure receptors in the pulmonary vasculature and cardiac atria, and high-pressure baroreceptors in the aortic arch and carotid sinus, recognize changes in the circulating volume.
·         Decreased blood volume causes increase sympathetic activity and ADH secretion, causing decrease in NaCl excretion.
·         Increased blood volume causes increase sympathetic activity and decrease ADH secretion causing increase in NaCl excretion.
Renin-Angiotensin-Aldosterone system
Renin is an enzyme made and stored in the juxta-glomerular cells found in the walls of the afferent arteriole.
Factors increasing renin secretion:
·         Reduced perfusion of afferent arteriole
·         Sympathetic nerve activity
·         Reduced Na+ delivery to the macula densa
Renin converts angiotensinogen to Angiotensin I.
Angiotensin I is converted to angiotensin II in the lungs by angiotensin-converting enzyme (ACE).
Angiotensin II result in the retention of salt and water by:
·         Stimulates aldosterone secretion (resulting in NaCl reabsorption)
·         Vasoconstriction of arterioles
·         Stimulates ADH secretion and thirst
·         Enhances NaCl reabsorbtion by the proximal tubule

Potassium regulation
Function: K+ is critical for many cell functions.
Potassium regulation
·         A large concentration gradient across cell membranes is maintained by Na+-K+-ATPase pump. Insulin and adrenaline also promotes cellular uptake of K+.
·         The kidney excretes up to 95% of K+ ingested in the diet.
·         The DCT and CD are able to both reabsorb and secrete K+.
Factors increasing K+ secretion:
·         Increased dietary K+ (driven by the electrochemical gradient)
·         Aldosterone
·         Increased rate of flow of tubular fluid
·         Metabolic alkalosis (acidosis exerts the opposite effect)

Osmolality = moles per kg water. Osmolarity = moles per litre of solution.



atef soliman said...


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