Excretory system, water–salt balance and homeostasis
Organs of excretion
Kidneys remove urea, uric acid, creatinine and excess salt and water; lungs remove carbon dioxide and water vapour; skin removes water, salt and a little urea in sweat; the liver excretes bile pigments. Each kidney holds about a million nephrons.
Urine formation
- Ultrafiltration in the glomerulus - about 125 mL per minute, roughly 180 L of filtrate daily, because the efferent arteriole is narrower than the afferent.
- Selective reabsorption in the proximal tubule - all glucose and amino acids, most water and sodium.
- The loop of Henle builds a salt gradient by counter-current flow, so water is reabsorbed from the collecting duct. Daily urine is 1–1.5 L.
Hormonal control
ADH from the posterior pituitary makes collecting ducts permeable to water, so more ADH means less urine. Aldosterone from the adrenal cortex retains sodium and water. Renin–angiotensin responds to low blood pressure.
Homeostasis
Kidneys steady water, salt, pH and blood pressure. Dialysis clears wastes by diffusion across a semi-permeable membrane but does not restore reabsorption or hormone production such as erythropoietin.
Worked example
A mason works all day in May heat, sweats heavily and passes little dark urine. Water loss raises blood osmolarity, ADH rises and urine concentrates. Give oral rehydration solution rather than plain water alone, because salt has been lost too.
Marks-losing traps
Urea is made in the liver, not the kidney; ADH is made in the hypothalamus and released by the posterior pituitary; a dialysis patient still needs erythropoietin and iron care; drinking excessive plain water can dangerously dilute sodium.