Excretion

Excretion — removing metabolic waste

Excretion is the removal of waste products made by the body's own metabolism (not the same as egestion, which removes undigested food). Several organs share this job — explore each one in the Explore tab.

The urinary system and the kidney

The urinary system consists of two kidneys, two ureters, a bladder, and a urethra. Each kidney has an outer cortex, an inner medulla (arranged in pyramids), and a central pelvis that connects to the ureter at the hilum.

The nephron — the kidney's filtering unit

Each kidney contains around a million tiny filtering units called nephrons. Each nephron:

• Performs ultrafiltration — filtering blood under pressure to remove small molecules (water, salts, glucose, and urea) from the blood into the nephron.
• Carries out reabsorption — reclaiming useful substances (glucose, most water and salts) back into the blood.
• Excretes waste — the remaining fluid (containing urea and excess water and salts) becomes urine.

Kidney disease and treatment

Kidney function can be damaged by conditions such as kidney stones, or kidney failure (sometimes linked to overuse of certain painkillers) or infections such as bilharzia. Severe kidney failure is treated with dialysis (artificially filtering the blood) or a kidney transplant.