IGCSE Biology Excretion in humans notes

Excretion is the removal of waste products of metabolism — substances made by body cells that the body no longer needs and cannot store. At Core, two products matter: carbon dioxide, a waste of respiration, is excreted through the lungs when you breathe out; and urea, with excess water and excess ions, is excreted by the kidneys as urine.

A labelled diagram to picture: the body with two exits marked — arrows carrying carbon dioxide from respiring cells in the blood to the lungs, and arrows carrying urea, water and ions dissolved in blood plasma to the kidneys, where they leave as urine. Note the distinction from egestion: undigested material leaving through the gut was never in the cells, so egestion (faeces) is not excretion.

A labelled diagram to picture: two bean-shaped kidneys at the back of the abdomen, one each side of the spine; a ureter running down from each kidney to the bladder, a muscular storage sac; and the urethra carrying urine from the bladder out of the body. Blood arrives at each kidney through the renal artery and leaves through the renal vein — the kidney is not a simple pipe: blood is processed, and the waste leaves separately as urine down the ureter.

Supplement: in section, a kidney shows an outer dark cortex and an inner medulla; the ureter opens into a pale space (the pelvis) where urine collects before running down. Identifying cortex and medulla in a section or image is a one-mark skill — cortex is the outer rim, medulla the darker inner region.

Supplement. Each kidney contains thousands of microscopic tubules called nephrons, each with its own blood supply. The routine is filter, then reclaim: at the glomerulus, high pressure forces water, glucose, urea and ions out of the blood into the nephron (the filtrate — cells and large proteins stay in the blood); as the filtrate flows along the nephron, all of the glucose, some of the ions and most of the water are reabsorbed back into the blood.

What is left reaches the collecting duct as urine, containing urea, excess water and excess ions. The quantities are worth memorising exactly: all the glucose (none in healthy urine), some of the ions and most of the water are reabsorbed — the details of how are not required, only the outline of what goes where.

Supplement. The liver is where absorbed amino acids are processed. Its constructive role is assimilation: the liver converts amino acids into proteins (such as plasma proteins) — using them to build the body’s own molecules. But the body cannot store excess amino acids, so the surplus is dealt with destructively: urea is formed in the liver from excess amino acids.

Deamination is the removal of the nitrogen-containing part of amino acids to form urea. The nitrogen-containing amino group is stripped off and converted to urea — which the blood then carries to the kidneys for excretion — while the rest of the amino acid can be used in respiration or converted to carbohydrate or fat. Sequence to remember: excess amino acid → deamination in the liver → urea → blood → kidney → urine.

Supplement. The syllabus limits the explanation to one idea: the toxicity of urea. Urea is poisonous — if it accumulated in the blood, it would damage cells and organs. Excretion keeps the blood concentration of urea low and safe: kidneys remove it as fast as the liver makes it. The same logic applies to carbon dioxide (it lowers blood pH) and to water and ions (their balance affects cells), but exam marks sit on the phrase urea is toxic.