The Kidneys and Osmoregulatory Organs

Core Concepts

Gross structure of the kidney

Each kidney has three regions: the outer renal cortex, the inner renal medulla (organized into pyramids), and the central renal pelvis, which funnels urine into the ureter. Urine travels down each ureter to the urinary bladder and out through the urethra. Blood enters via the renal artery and leaves via the renal vein. A commonly taught figure is about one million nephrons per human kidney — verify against current texts. The kidney also secretes renin (blood pressure), erythropoietin (red blood cell production), and activates vitamin D.

The nephron: the functional unit

Each nephron is one long tubule with a blood-filtering capsule at one end. In order: the renal corpuscle (the glomerulus, a knot of capillaries, inside Bowman's capsule), the proximal convoluted tubule (PCT), the loop of Henle, the distal convoluted tubule (DCT), and the collecting duct. Blood supply: an afferent arteriole feeds the glomerulus; the efferent arteriole branches into peritubular capillaries (and, for long-looped nephrons, the vasa recta) that wrap the tubules.

Filtration

In the renal corpuscle, blood pressure forces water and small solutes — ions, glucose, amino acids, urea — through the filtration slits of Bowman's capsule, while blood cells and large proteins stay behind. The filtrate is essentially protein-free plasma. Filtration is pressure-driven, costs no energy, and is nonselective for small solutes; selectivity comes later. A commonly taught estimate: an adult filters about 180 liters per day, nearly all reabsorbed — confirm against current references.

Reabsorption and secretion

Reabsorption moves substances from tubule fluid back into the blood. The PCT is the workhorse: it actively transports glucose, amino acids, and Na⁺ out of the filtrate, with Cl⁻ and water following, so most filtered glucose, amino acids, and water are recovered early. Glucose transporters can saturate — very high blood glucose leaves some in the tubule fluid, and it appears in urine.

Secretion moves substances from blood into the tubule fluid: the PCT and DCT secrete H⁺ (helping regulate blood pH), K⁺, and foreign molecules such as some drugs. The kidney's bookkeeping:

Excretion = Filtration − Reabsorption + Secretion

The loop of Henle and the medullary gradient

The loop of Henle is the kidney's countercurrent multiplier. The descending limb is water-permeable, so water leaves as the fluid descends into the saltier medulla. The ascending limb is impermeable to water but actively pumps Na⁺ and Cl⁻ out, so the medulla grows progressively saltier toward the loop's tip. The vasa recta act as a countercurrent exchanger, picking up reabsorbed water and solutes without washing the gradient away.

When the collecting duct crosses the salty medulla, water can be drawn out of its fluid — but only if the duct is water-permeable, which is exactly what ADH controls. More ADH → more water reabsorbed → concentrated urine; little ADH → relatively impermeable duct → dilute urine.

The juxtaglomerular apparatus

Where the DCT passes between the arterioles, the macula densa cells sense NaCl in the tubule fluid and the juxtaglomerular (JG) cells of the afferent arteriole secrete renin when blood pressure or NaCl delivery is low. Renin starts the RAAS, raising blood pressure and stimulating aldosterone to increase Na⁺ reabsorption — a direct link between kidney and blood pressure.

Common Confusions