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Excretion

10 min read

Getting rid of metabolic waste

Excretion is the removal of the waste products of metabolism from the body. Body cells make carbon dioxide, water, urea and salts that must not build up. The main organs of excretion are the lungs (carbon dioxide and water vapour), the kidneys (urea, excess water and salts), and the skin (sweat: water, salts and a little urea).

A nephronThe filtering unit of the kidney. Blood enters the glomerulus, a knot of capillaries inside Bowman’s capsule, where water, glucose, salts and urea are filtered out under pressure. In the proximal tubule all the glucose and much water are reabsorbed. The loop of Henle and distal tubule adjust water and salts, and the collecting duct carries urine to the ureter.Bowman's capsuleglomerulusproximal tubuleloop of Henledistal tubulecollecting duct → urineblood inFiltered: water, glucose,salts, ureaReabsorbed: all glucose,most water and salts

The kidney has about a million tiny filtering tubes called nephrons. Blood is filtered under pressure, useful substances such as all the glucose are taken back, and the remaining water, urea and salts form urine. Urine passes down the ureters to the bladder and out through the urethra.

The skin

Structure of human skinA section of skin. The epidermis on top has a dead outer layer. Beneath is the dermis with sweat glands, hair follicles, blood vessels, sensory receptors and nerve endings. Below is a layer of fat for insulation. Sweat evaporates to cool the body, and blood vessels widen or narrow to lose or keep heat.epidermis (dead cells on top)dermissweat ductsweat glandfat layerhairhair follicletouch receptorarteryvein

Sweat glands remove water, salts and a little urea. Sweating also cools the body, because evaporating sweat takes heat from the skin. This links excretion to temperature control.

Water balance

The amount of water we lose must equal the amount we take in. If you drink a lot or sweat little, the kidneys make large amounts of dilute urine. If you sweat a lot, they make a small amount of concentrated urine.

Try itDaily water balance

Water gained from drinks and food, and lost by sweating, breathing and faeces. Find the urine needed to keep the balance.

Check yourself Which waste does the kidney remove?
Check yourself Which substance is completely reabsorbed in a healthy nephron?
Check yourself Why do you make less urine on a hot day?

Key terms

Excretion
Removal of metabolic waste from the body.
Nephron
The filtering unit of the kidney.
Urea
A waste made from the breakdown of excess amino acids.
Reabsorption
Taking useful substances back into the blood.

Summary

  • Excretory organs are the lungs, kidneys and skin.
  • The nephron filters blood and reabsorbs useful substances; what remains is urine.
  • Sweat removes water and salts and cools the body.
  • Kidneys keep the water balance of the body.
Coordination Regulation