Osmoregulation & Excretion

Excretion vs egestion

Excretion is the removal of metabolic waste products made by the body's own cells (e.g. carbon dioxide, urea, excess water and salts). This is different from egestion — the removal of undigested food (faeces) that never entered the body's cells at all. The main excretory organs are the lungs (CO₂), skin (some urea, salts and water in sweat), and the kidneys (urea, excess water and salts, as urine).

The nephron — the kidney's functional unit

Each kidney contains roughly a million tiny filtering units called nephrons. Urine formation happens in three stages as fluid moves through each nephron:

Stage 1: Ultrafiltration

Blood enters the glomerulus — a tight knot of capillaries inside the cup-shaped Bowman's capsule — under high pressure. This pressure forces water, glucose, amino acids, urea and salts (small molecules) out of the blood and into the capsule as filtrate. Blood cells and large plasma proteins are too big to be forced through and remain in the blood.

Stage 2: Selective reabsorption

As the filtrate flows through the PCT, loop of Henle, and DCT, useful substances are reabsorbed back into the blood via surrounding capillaries:

• Glucose and amino acids — normally reabsorbed completely in the PCT. (Glucose appearing in urine is a warning sign of diabetes mellitus, where blood glucose is so high that the PCT's reabsorption capacity is exceeded.)
• Most water and many salts — reabsorbed throughout the tubule, with the AMOUNT of water reabsorbed in the DCT and collecting duct fine-tuned by the hormone ADH (see below).

Stage 3: Tubular secretion

Some additional waste substances (e.g. excess hydrogen and potassium ions, and some drugs) are actively secreted directly from the blood into the tubule fluid, mainly in the DCT. What remains after all three stages — water, urea, excess salts and other wastes — is urine, which passes to the collecting duct, then the renal pelvis, ureter, bladder, and out through the urethra.

Osmoregulation — controlling water balance

Osmoregulation is the regulation of the water and salt balance (osmotic concentration) of the blood, and it works through the same negative feedback pattern as other homeostatic mechanisms:

Kidney failure & dialysis

If the kidneys fail, urea and other wastes build up to toxic levels in the blood. Dialysis is a medical treatment that artificially performs the kidney's filtering role: the patient's blood is passed alongside a dialysis fluid across a selectively permeable membrane, allowing urea and excess substances to diffuse out of the blood, while useful substances like glucose are kept at the correct concentration and are not lost.