Assertion: Aldosterone is a steroid hormone and is important in the control of sodium and potassium ion concentration in mammals. 
Reason: It upgrades sodium ioh concentration in the ECF by promoting reabsorption of sodium ions from renal tubules and excretion of potassium ions in urine

Options

A. If both assertion and reason are true reason is the correct explanation of assertion
B. If both assertion and reason are true but reason is not the correct explanation of assertion
C. If assertion is true but reason is false
D. If both assertion and reason are false.

Answer

A

Explanation

Both assertion and reason are true and reason is the correct explanation of assertion .
Aldosterone is a steroid hormone (mineralocorticoid) produced by the outer-section ( zona glomerulosa ) of the adrenal cortex in the adrenal gland to regulate sodium and potassium balance in the blood. At the late distal tubule & collecting duct, aldosterone has two main actions :
● Acting on mineralocorticoid receptors (MR) on principal cells in the distal tubule of the kidney nephron, it increases the permeability of their apical (luminal) membrane to potassium and sodium and activates their basolateral `Na^(+)//K^(+)` pumps, stimulating ATP hydrolysis leading to phosphorylation of the pump and a conformational change in the pump exposes the `Na^(+)` ions to the outside. The phosphorylated form of the pump has a low affinity for `Na^(+)` ions, hence reabsorbing sodium `(Na^(+))` ions and water into the blood, and secreating potaasium `(K^(+))` ions into the urine. (Chlorine anions are also reabsorbed in conjuction with sodium cations to maintain the system's electrochemical balance.)
● Aldosterone stimulates `H^(+)` secretion by intercalated cells in the collecting duct, regulating plasma bicarbonate `(HCO_(3)^(+))` levels and its acid/base balance. Aldosterone may act on the central nervous system viathe posterior pituitary gland to release vasopressin (ADH) which serves to conserve water by direct actions on renal tubular resporation.
Aldosterone is responsible for the reabsortion of about `2%` of filtered sodium inthe kidneys, which is nearly equal to the entire sodium content in human blood under normal GFR (glomerular filtration rate.)