Excretion in humans, at a glance

Excretion removes metabolic wastes, toxic materials and substances in excess of the body's requirements.

• Urea is formed in the liver and removed mainly by the kidneys.
• Carbon dioxide from respiration is excreted through the lungs.
• Egestion removes undigested material from the alimentary canal; it is not the removal of a metabolic waste made by cells.

Keep in mind: The liver forms urea during amino-acid metabolism. Kidneys remove it from blood. Undigested food is removed by egestion.

Kidneys form urine; ureters deliver it to the bladder; the urethra carries it out of the body.

• Urine: kidneys -> ureters -> bladder -> urethra -> outside.
• Blood: renal artery -> kidney capillaries -> renal vein.
• The bladder stores urine; it does not filter blood.

Keep in mind: The kidneys process blood and form urine. The bladder mainly stores the urine that arrives through the ureters.

High blood pressure filters water and small dissolved substances into the capsule, while cells and most large proteins remain in blood.

• Glomerulus: capillary knot. Bowman's capsule: receives filtrate.
• Water, glucose, amino acids, ions and urea can enter the filtrate.
• Ultrafiltration is driven by pressure; it does not select only waste substances.

Keep in mind: It also filters useful small substances, including glucose. Selective reabsorption returns useful materials afterwards.

Selective reabsorption moves useful substances from the nephron fluid back into the blood; the remaining fluid becomes urine.

• Glucose and amino acids are normally reabsorbed completely at usual blood concentrations.
• Much of the water and needed ions return to blood; water moves by osmosis.
• Urine contains urea, water and ions; it is not simply unchanged filtrate.

Keep in mind: The liver produces most urea. Reabsorbing water reduces fluid volume and can concentrate the urea that remains.

Small wastes diffuse from blood into dialysis fluid; the membrane retains blood cells and large proteins.

• Fresh dialysis fluid has little or no urea, maintaining net diffusion from blood.
• Controlled concentrations of useful solutes help avoid unnecessary net losses.
• Large surface area, a thin membrane and renewed fluid support exchange.

Keep in mind: Useful small solutes can also cross. Carefully controlled fluid composition removes wastes while avoiding unnecessary losses or unsuitable water movement.