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BIO 304 . Human Anatomy & Physiology . American River College

Tubular Function and Urine Concentration

Module 15 . Urinary System

How the tubules reshape the filtrate into a final urine, and how ADH and aldosterone fine-tune it. Watch the video, then complete the retrieval check below to unlock your spaced-recall cards.

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Open spaced recall
By the end
  1. For each nephron segment, name what is reabsorbed, what is secreted, and which transporters do the work.
  2. Explain how the loop of Henle builds the medullary concentration gradient.
  3. Describe how ADH and aldosterone fine-tune water and sodium at the collecting duct.

PCT and Loop of Henle

  • Proximal convoluted tubule (PCT)Bulk reabsorption: about 65% of filtered Na+, water, glucose, amino acids reabsorbed here. · SGLT2 cotransporter reabsorbs glucose with sodium. (SGLT2 inhibitors are diabetes drugs.) · Brush border maximizes surface area.
  • Descending limb of HenlePermeable to water, not solute. Water exits into the increasingly hypertonic medulla. Filtrate becomes more concentrated as it descends.
  • Thick ascending limbPermeable to solute, not water. NKCC2 cotransporter pumps Na+, K+, and 2 Cl- out into the interstitium. Filtrate becomes more dilute. Builds the medullary gradient. · This is where loop diuretics (furosemide) act.

DCT and collecting duct

  • Early DCTNCC (Na-Cl cotransporter) reabsorbs sodium and chloride. Thiazide diuretics block this.
  • Late DCT and collecting ductPrincipal cells: ENaC channels for Na+ reabsorption (under aldosterone control); aquaporin-2 channels for water (under ADH control). · Intercalated cells: handle acid-base by secreting H+ or HCO3-.

Hormonal control

  • ADH (antidiuretic hormone)Released from posterior pituitary when plasma osmolarity rises. · Inserts aquaporin-2 into collecting duct cells. Water reabsorbed, urine concentrated.
  • AldosteroneReleased from adrenal cortex (zona glomerulosa) when blood pressure or Na+ drops. · Acts on principal cells: Na+ in, K+ out, water follows Na+. Blood volume and pressure rise.
  • ANPReleased from atrial myocytes when stretched. Opposes aldosterone: promotes Na+ excretion and lowers blood pressure.
Dr. Sharilyn Rennie BIO 304 . Module 15 . Urinary System