Excretion flashcards

Topic 13: Excretion

Excretion (O Level definition)

The removal of toxic materials and the waste products of metabolism from organisms.

Excretory products: where they are made and how they are excreted

Carbon dioxide: made by respiration in every cell; excreted by the lungs. Urea: made in the liver by deamination; excreted by the kidneys in urine (with excess salts and water).

Deamination

Removal of the nitrogen-containing part of excess amino acids, in the liver, forming urea.

Why must urea be excreted? (O Level)

Urea is toxic. The liver keeps making it, so without excretion its concentration in the blood would rise and damage cells.

Kidney, ureter, bladder and urethra: number and function

Kidneys (2): remove urea, excess salts and water as urine. Ureters (2): kidney to bladder. Bladder (1): stores urine. Urethra (1): bladder to outside.

Route of fluid through a nephron

Bowman's capsule → first coiled tubule → loop of Henle → second coiled tubule → collecting duct → ureter. The glomerulus is the knot of capillaries inside the capsule.

What happens in the nephron (outline)

Filtration at the glomerulus: water, glucose, urea and ions enter the Bowman's capsule; blood cells and large proteins stay in the blood. Reabsorption: all the glucose, some ions and most water return to the blood. Urea, excess water and ions form urine.

Why is urea more concentrated in urine than in plasma?

Urea is filtered but not reabsorbed, while most of the water is reabsorbed, so the urea is left in a much smaller volume of water.

What happens to amino acids absorbed from the small intestine?

They reach the liver in the hepatic portal vein. Those needed are assimilated into proteins (e.g. plasma proteins); the excess cannot be stored, so they are deaminated.