Excretion

First, two words that sound alike. Egestion is getting rid of undigested food (it never entered your cells). Excretion is removing harmful wastes made by the body's own cells, mainly nitrogen wastes such as urea and uric acid.

The human excretory system: two kidneys, two ureters, a urinary bladder and a urethra. The kidneys make urine; the ureters carry it to the bladder, where it is stored until it is passed out.

Each kidney is made of a huge number of tiny filtering units called nephrons. In each one:

• a cluster of very thin blood capillaries sits inside a cup-shaped Bowman's capsule;
• blood is filtered into the cup: water, glucose, amino acids, salts and urea all go through;
• as this liquid runs along the long tubule, the useful things (glucose, amino acids, salts and most of the water) are taken back into the blood;
• what is left is urine.

The numbers show how much is taken back: about 180 litres are filtered every day, but only one or two litres leave the body as urine.

If the kidneys fail, an artificial kidney cleans the blood by dialysis: blood flows through tubes in a special fluid, and the wastes pass out. Unlike a real kidney, it takes nothing back.

Organ donation, from the book: a person can donate an organ, with their own and their family's consent, to someone whose organ has failed. Anyone, of any age or gender, can become a donor. Most donations happen after death, but a kidney, part of the liver or a lung can be given while alive. A transplant can save or transform a person's life.

Plants get rid of waste differently. Extra water leaves by transpiration. Some wastes are stored in vacuoles, in leaves that later fall, or as resins and gums in old xylem; some are released into the soil.