The Excretory System
High-Yield Summary
- Excretory system regulates blood pressure, blood osmolarity, acid-base balance, and nitrogenous waste removal. Urine flow: kidneys → ureters → bladder → urethra.
- Nephron portal blood supply: afferent arteriole → glomerulus → efferent arteriole → vasa recta (2 capillary beds in series, unusual arrangement).
- Nephron segments in order: Bowman's capsule → PCT (bulk reabsorption/secretion) → loop of Henle (descending = water-permeable, ascending = salt-permeable; countercurrent multiplier) → DCT (hormone-responsive) → collecting duct (final concentration, ADH/aldosterone-responsive).
- 3 kidney processes: filtration (pressure-driven, glomerulus, Starling forces), secretion (active, blood → tubule), reabsorption (selective, tubule → blood).
- RAAS: kidney releases renin → angiotensinogen → angiotensin I → (ACE in lungs) → angiotensin II → aldosterone → Na+ reabsorption (water follows) → raises blood volume/pressure WITHOUT changing osmolarity.
- ADH: released by posterior pituitary, triggered by low blood volume/high osmolarity, acts on collecting duct → increases water reabsorption → raises blood volume/pressure WHILE decreasing osmolarity.
Key Terms
- Nephron
- Functional unit of the kidney (~1 million per kidney) that performs filtration, reabsorption, and secretion.
- Glomerulus
- Dense, high-pressure capillary network in the nephron specialized for filtration.
- Countercurrent multiplier
- Opposite-direction flow of filtrate (loop of Henle) and blood (vasa recta) that removes water without collapsing the osmotic gradient.
- Glomerular hydrostatic pressure
- Pressure of blood inside glomerular capillaries; the dominant force driving filtration.
- Renin
- Enzyme released by the kidney when blood pressure/volume is low, converting angiotensinogen to angiotensin I — kicks off RAAS.
RAAS (Aldosterone) vs. ADH
| RAAS / Aldosterone | ADH |
|---|---|
| Reabsorbs Na+ AND water together | Reabsorbs water WITHOUT matched Na+ increase |
| Raises blood volume/pressure | Raises blood volume/pressure |
| Does NOT significantly change osmolarity | DECREASES osmolarity |
RAAS Cascade
- 1Kidney releases renin (triggered by low blood pressure/volume).
- 2Renin converts angiotensinogen (from liver) → angiotensin I.
- 3ACE (in lungs) converts angiotensin I → angiotensin II.
- 4Angiotensin II stimulates adrenal cortex → releases aldosterone.
- 5Aldosterone acts on DCT/collecting duct → increases Na+ reabsorption (water follows) → raises blood volume/pressure; also promotes K+/H+ excretion.
Common MCAT Trap
- Filtration is NONSELECTIVE (pressure-driven); secretion and reabsorption are SELECTIVE (active/targeted). Don't call filtration 'selective.'
- Descending limb of loop of Henle is permeable to WATER, impermeable to salt; ascending limb is the OPPOSITE (permeable to salt, impermeable to water) — commonly reversed on test questions.
- Aldosterone doesn't change osmolarity (Na+ and water move together); ADH DOES change osmolarity (only water moves) — this distinction is a favorite MCAT discriminator.
Quick Recall
What's the nephron's blood supply order?
What are the 3 kidney processes and their directions?
Why doesn't aldosterone change blood osmolarity but ADH does?
What's the difference in water/salt permeability between the descending and ascending limbs of the loop of Henle?