What vessel's smaller diameter contributes to the high glomerular hydrostatic pressure?

A. Sodium and potassium: These cells represent the primary site for aldosterone-mediated electrolyte regulation in the distal nephron. They utilize apical sodium channels and potassium channels to facilitate sodium reabsorption and potassium secretion. This mechanism is critical for maintaining systemic fluid balance and normokalemia.

B. Glucose and urea: Glucose reabsorption occurs almost exclusively in the proximal convoluted tubule via specialized sodium-glucose cotransporters. While the medullary collecting ducts are permeable to urea under certain conditions, principal cells do not regulate its transport. Their metabolic machinery is specialized for ion and water homeostasis.

C. Bicarbonate and chloride: Acid-base balance and bicarbonate transport are the primary functions of intercalated cells, which are distinct from principal cells. While chloride often follows sodium passively, principal cells do not actively regulate its concentrations. Intercalated cells manage the secretion of hydrogen and bicarbonate ions.

D. Calcium and phosphate: The regulation of these minerals occurs primarily in the proximal tubule and the distal convoluted tubule under parathyroid hormone influence. Principal cells lack the specific receptors and transporters required for significant calcium or phosphate handling. Their role is restricted to water and monovalent cation transport.

A. It causes water to move out of cells, shrinking them: This occurs in a hyper-osmolar environment, where the high concentration of extracellular solutes draws water out via osmosis. Hypo-osmolarity involves a lower solute concentration outside the cell. The osmotic pressure gradient would favor influx, not efflux.

B. It increases vascular resistance, raising blood pressure: Hypo-osmolarity is often associated with fluid overload, but it does not directly cause vasoconstriction. In many cases, it is linked to low sodium levels which can impair vascular tone. It primarily affects fluid distribution rather than active vessel resistance.

C. It causes water to move into cells, swelling and possible rupture: In a hypo-osmolar state, the intracellular fluid has a higher solute concentration than the surrounding plasma. Water moves down its osmotic gradient into the cells to achieve equilibrium. This cellular edema can lead to lysis and organ dysfunction.

D. It enhances sodium retention, preventing edema: Hypo-osmolarity typically triggers mechanisms to excrete water or retain sodium to restore balance. However, if the condition persists, the low osmotic pressure in the blood allows fluid to leak into the interstitial spaces. This results in the formation of edema.