ExcretionEdexcel IGCSE Biology: Revision notes

Excretion is the removal of waste products of metabolism from the body. Carbon dioxide and oxygen are produced/used as waste products of respiration and photosynthesis; in plants, excess of these gases diffuses out mainly through the stomata of leaves.

Humans excrete waste from several organs:

• Lungs — excrete carbon dioxide and water vapour (from respiration)
• Kidneys — excrete urea, excess water and excess ions (as urine)
• Skin — excretes water, ions and a small amount of urea (in sweat)

The urinary system removes urea, excess water and excess ions from the blood and expels them as urine. Its structures are:

• Kidneys — a pair of organs that filter the blood and produce urine
• Ureters — tubes that carry urine from each kidney to the bladder
• Bladder — a muscular sac that stores urine until it is released
• Urethra — the tube through which urine leaves the body from the bladder

Each kidney contains thousands of tiny filtering units called nephrons. A nephron consists of:

• Bowman's capsule — a cup-shaped structure surrounding a knot of capillaries, where filtration begins
• Glomerulus — the knot of capillaries inside the Bowman's capsule, where blood pressure forces small molecules out of the blood
• Convoluted tubules — sections of the nephron (proximal and distal) where useful substances are reabsorbed back into the blood
• Loop of Henle — a long loop of the tubule that helps concentrate the urine
• Collecting duct — carries urine from several nephrons to the ureter, and is where final water reabsorption occurs

Ultrafiltration occurs in the Bowman's capsule: high blood pressure in the glomerulus forces water, glucose, urea and ions out of the blood and into the Bowman's capsule, forming the glomerular filtrate. Blood cells and large proteins are too big to pass through and remain in the blood.

As the filtrate flows along the nephron, selective reabsorption returns useful substances to the blood:

• Glucose is completely reabsorbed at the proximal convoluted tubule, since the body needs to retain all its glucose for respiration
• Water is reabsorbed along the nephron, with the final adjustment happening at the collecting duct, where the amount reabsorbed depends on the body's water needs

Urea and excess water/ions that are not reabsorbed remain in the tubule and are excreted as urine.

The kidney carries out osmoregulation — controlling the water content of the blood — as well as excretion. This is regulated by the hormone ADH (antidiuretic hormone), released by the pituitary gland:

• If blood water content is too low, more ADH is released, increasing the permeability of the collecting duct so more water is reabsorbed into the blood, producing a small volume of concentrated urine
• If blood water content is too high, less ADH is released, so less water is reabsorbed, producing a large volume of dilute urine

Urine normally contains water, urea and ions (but not glucose or protein, since these are either fully reabsorbed or too large to filter).