Renal System

Nephron Anatomy and Function

• Flow: glomerulus to proximal convoluted tubule to loop of Henle to distal tubule to collecting duct. The PCT reclaims 65-70 percent of filtrate including all glucose and amino acids; the descending limb is water-permeable, the ascending limb pumps NaCl but excludes water, building the medullary gradient.
• GFR is about 180 L/day yet urine is only about 1.5 L — 99 percent of filtrate is reabsorbed.

Filtration and Clearance

• Net filtration pressure: $P_{net} = (P_{GC} - P_{BS}) - (\pi_{GC} - \pi_{BS})$ Efferent arteriole constriction (angiotensin II) raises glomerular pressure and GFR; a ureteral stone raises Bowman's pressure and lowers it.
• Clearance: $C_x = \frac{U_x \cdot V}{P_x}$ Inulin clearance equals GFR; clearance above GFR means net secretion, below means net reabsorption.

Hormones and Concentration

• ADH inserts aquaporins in the collecting duct (its absence causes diabetes insipidus with dilute polyuria); aldosterone drives $Na^+$ reabsorption and $K^+$ secretion; ANP opposes both.
• The countercurrent multiplier (NKCC pumping in the thick ascending limb) creates a gradient up to about 1200 mOsm; loop diuretics collapse it, which is why they are the most powerful class.

Key Takeaways

• Descending limb loses water; ascending limb loses salt.
• Arteriolar tone is the main GFR control knob.
• Compare any clearance to inulin to classify tubular handling.
• ADH concentrates urine only because the medullary gradient exists.