Why does glomerular filtration rate (GFR) remain relatively stable during exercise or rest?

A. Aldosterone: This steroid hormone primarily stimulates the reabsorption of sodium and the secretion of potassium in the distal tubule. While water follows sodium osmotically, aldosterone does not directly regulate the water permeability of the collecting duct. Its primary target is the monovalent cation.

B. Antidiuretic hormone (ADH): Also known as vasopressin, this peptide binds to V2 receptors on principal cells to induce the apical insertion of aquaporin-2 channels. This directly increases the water permeability of the collecting ducts. It is the most potent regulator of free water recovery.

C. Atrial natriuretic peptide (ANP): This hormone is secreted by the atria in response to high blood volume and functions as a diuretic. It inhibits sodium reabsorption and suppresses ADH release to increase urine output. It decreases water reabsorption to lower systemic blood pressure.

D. Cortisol: While glucocorticoids have some weak mineralocorticoid activity, their primary metabolic role is unrelated to the regulation of water channels in the kidney. Excessive cortisol can interfere with water balance, but it is not a physiological promoter of collecting duct water recovery.

A. It uses the Na+/K+ pump: This pump is an example of primary active transport, which requires ATP to move ions against their concentration gradients. Simple diffusion is a passive process that occurs spontaneously without the use of molecular pumps. It relies on kinetic energy rather than metabolic energy.

B. It moves molecules down their concentration gradient without energy: This is the fundamental definition of passive transport across a semi-permeable membrane. Molecules naturally migrate from areas of high concentration to areas of low concentration until equilibrium is reached. No cellular energy or ATP is consumed during this process.

C. It is the main mechanism for glucose absorption: Glucose is a large, polar molecule that cannot easily pass through the lipid bilayer via simple diffusion. It requires secondary active transport (SGLT) and facilitated diffusion (GLUT) to enter and exit renal cells. Its absorption is strictly carrier-mediated in the nephron.

D. It requires a specific receptor for each solute: Receptors and carrier proteins are characteristics of facilitated diffusion and active transport. Simple diffusion involves the movement of small, non-polar, or lipid-soluble substances directly through the phospholipid bilayer. It is characterized by a lack of specificity and saturation.