Q.A person who is on a long hunger strike and is surviving only on water, will have
a. Less amino acids in his urine
b. Macula densa cells
c. Less urea in his urine
d. More sodium in his urine

Recovering what the body needs, and adding what it doesn't

Reabsorption

Of the roughly 180 L of filtrate produced daily, about 99% is reclaimed by selective reabsorption, segment by segment:

• Proximal convoluted tubule (PCT): reabsorbs glucose, amino acids, lactate, most water, and (via active Na+-K+ pumping) most sodium, with urea and uric acid partly reabsorbed too.
• Descending limb of the loop of Henle: permeable to water (via aquaporins) but not salts - water leaves, so Na+/Cl- become concentrated in the remaining fluid.
• Ascending limb: impermeable to water but actively pumps out Na+, Cl- and K+ - this dilutes the tubular fluid while raising the salt concentration of the surrounding medulla.
• Distal convoluted tubule (DCT): reabsorbs more water and bicarbonate (regulating blood pH) while secreting potassium; reabsorption here is need-based and hormonally controlled.
• Collecting duct: reabsorbs sodium and, when ADH raises its aquaporin count, reabsorbs enough water to concentrate the final urine.

Secretion

Tubular secretion moves substances the other way - from peritubular blood into the tubular fluid - to clear them faster and to fine-tune blood pH. Secreted substances include H+, K+, NH4+, creatinine and various organic acids/drugs. For every H+ secreted, a Na+ is reabsorbed in exchange, and the H+ binds buffers (bicarbonate, phosphate, ammonia) in the fluid so it stays trapped rather than being reabsorbed - the mechanism by which the kidney permanently excretes excess acid.

Concentrating the urine (counter-current mechanism) …