Identify the following structures.

Renal artery	dropdown
Interlobar vein	dropdown
Cortical radiate vein	dropdown

A. Semipermeable walls of the renal tubule: Reabsorption and secretion occur across the tubular walls, but the initial barrier to protein entry is located within the renal corpuscle. By the time fluid reaches the tubules, it should already be an ultrafiltrate. The tubules do not perform primary filtration.

B. Absence of blood supply in Bowman's capsule: Bowman's capsule is intimately associated with the glomerulus, which is a high-density capillary network. It receives a massive blood supply specifically to facilitate filtration. The absence of blood would mean no filtrate or urine could ever be produced.

C. Active protein pumps: The filtration of blood at the glomerulus is a passive process driven by hydrostatic and oncotic pressures. There are no active metabolic pumps that move proteins against a gradient in the filtration barrier. The exclusion of protein is based on size and charge.

D. Podocyte processes and capillary endothelial fenestrations: The three-layered barrier consists of fenestrated endothelium, the glomerular basement membrane, and podocyte slit diaphragms. These structures provide a mechanical sieve and a negative charge. They prevent the passage of large cells and anionic plasma proteins like albumin.

A. It increases water reabsorption by directly stimulating ADH release: ADH release is triggered by hypothalamic osmoreceptors sensing sodium concentrations and blood volume. Urea itself is not the primary physiological trigger for the neurohypophyseal release of ADH. It is a metabolic byproduct that aids the osmotic gradient.

B. It promotes active sodium reabsorption in the distal tubule: Urea transport is largely passive or facilitated and does not drive the active transport of sodium. Sodium reabsorption in the distal tubule is primarily regulated by aldosterone. Urea's role is restricted to the osmotic environment of the inner medulla.

C. It contributes to the medullary osmotic gradient through recycling: Urea is reabsorbed from the inner medullary collecting ducts and moves into the thin limbs of the loop of Henle. This recycling traps urea in the medulla, accounting for nearly 50% of its hypertonicity. This high osmolarity facilitates maximal water reabsorption.

D. It reduces the permeability of the collecting ducts to water: Urea does not alter the intrinsic water permeability of the ductal epithelium. Water permeability is strictly controlled by the presence of ADH-induced aquaporins. Urea provides the osmotic "pull" that makes that permeability effective for water recovery.