Excretion — Edexcel Linear - OL Revision Notes & Topical Questions

Metabolic reactions in cells produce waste products that must be removed: excretion is the removal of the waste products of metabolism.

Many metabolic reactions produce waste. Excess amino acids are broken down in the liver (deamination) to form urea, which is carried in the blood to the kidneys. Carbon dioxide is a waste product of respiration.

Urea is toxic in high amounts, so it must be removed. Egestion (removal of undigested food as faeces) is not excretion.

The kidneys are located at the back of the abdomen and have two important functions: excretion (removing waste such as urea) and osmoregulation (controlling the water content of the body). If body cells lose or gain too much water by osmosis, they shrink or may burst; the water content of the blood must therefore stay within limits.

The kidney has three regions

• the cortex (outer, where filtration occurs), the medulla (inner) and the renal pelvis (where urine collects before the ureter).
• Each kidney contains about a million microscopic units called nephrons.

A nephron consists of

• the glomerulus (a knot of capillaries) surrounded by the Bowman’s capsule (together the renal corpuscle in the cortex);
  • the proximal convoluted tubule;
  • the loop of Henle dipping into the medulla;
  • the distal convoluted tubule;
  • and the collecting duct, which carries urine to the renal pelvis.
• A network of capillaries wraps around the tubules to take up the reabsorbed substances.

The kidneys filter the entire blood volume many times a day and receive about a fifth of the cardiac output.

Be ready to state the difference between the terms: the kidneys filter blood, the ureters carry urine to the bladder, the bladder stores it, and the urethra carries it out. Exam diagrams of the urinary system often ask for these labels and the direction of blood flow in the renal artery and vein.