MDCAT Homeostasis & Excretion 2026

Nephron Structure

Bowman's capsule + glomerulus → proximal convoluted tubule → Loop of Henle (descending + ascending) → distal convoluted tubule → collecting duct.

Urine Formation

• Filtration — glomerulus; high pressure; ~180 L/day filtered
• Reabsorption — PCT: glucose, amino acids, ~65% water
• Loop of Henle — creates osmotic gradient; descending permeable to water, ascending to ions
• DCT — fine-tuning under aldosterone (Na⁺)
• Collecting duct — ADH increases water reabsorption

Feedback Loops

Low blood water → hypothalamus releases ADH from posterior pituitary → collecting duct more permeable → concentrated urine.

Pair with Coordination & Control.