Function: NaCl is the main determinant of ECF osmolality and volume.
· 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 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
Function: K+ is critical for many cell functions.
· 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)
· 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.