Aldosterone directly increases Na reabsorption by increasing the activities of the ENaC and NaKATPase in the principal cells in the tubule. The urinary lumen then says- aldosterone , you have taken up all my positive Na. Can I please have something positive in return. Aldosterone then helps the tubular principal cell to kick out K into the lumen. Remember K is the predominant positive charge intracellularly. But I think aldosterone helps to reabsorb more Na then it helps to excrete K. So the urinary lumen says hey I need more positive charge please. The neighbouring alfa intercalated cell being the good neighbor now excretes H. Thus aldosterone directly reabsorbs Na and excretes K and also indirectly excretes H.

Renin is secreted in response to low urinary flow in the renal tubules which in turn is due to hypovolemia e.g. gastroenteritis, excessive sweating, shock etc. Renin leads to splitting of the large angiotensinogen molecule made by the liver and thus produces angiotensin I. Angiotensin I is carried by blood to the lung where Angiotensin Converting Enzyme (ACE) splits off two more amino acids from it to lead to the creation of angiotensin II. Angiotensin II apart from being a potent vasoconstrictor ( and thus help to bring up the BP) also leads to aldosterone secretion by the adrenal medulla. Aldosterone leads directly to Na reabsorption by the Principal cells in the distal parts of the tubule along with K excretion. As the amount of Na reabsorbed is more than the K excreted, the tubular fluids asks for more positively charged ions and the neighbouring alpha-intercalated cells try and help by secreting H ions. Thus aldosterone leads directly to Na absorption and K secretion; and indirectly leads to H secretion and water reabsorption ( as water follows Na passively).Hope that makes sense?

@@DrSurjitTarafdarThank you, but when would the body know when to stop, won't this eventually lead to a hypokalemic crisis leading to serious cardiac issues ?