The nephron and urea
Inside the kidney (Supplement)
- A kidney has an outer cortex and an inner medulla.
- It holds millions of tiny tubes called nephrons.
- Each nephron cleans the blood in two steps.
How a nephron works
- Filtration at the glomerulus (a knot of capillaries): high pressure forces water, glucose, urea and ions out of the blood into the nephron.
- Reabsorption along the tube: the blood takes back all the glucose, some ions, and most of the water.
- What is left — urea, excess water and excess ions — becomes the urine.
Practice
Filtration in the nephron happens at the:
At the glomerulus, high pressure forces water, glucose, urea and ions out of the blood into the nephron.
Practice
During reabsorption the blood takes back:
All glucose, some ions and most water are reabsorbed; urea and the excess are left as urine.
Urea from the liver (Supplement)
- The liver carries out assimilation: joining amino acids into the body's proteins.
- Excess amino acids can't be stored, so the liver breaks them down by deamination — removing the nitrogen part.
- That part is turned into urea, which must be excreted because it is toxic.
Practice
Urea is made in the liver by deamination, which:
Deamination removes the nitrogen-containing part of excess amino acids; this becomes urea.
Practice
Urea must be excreted because it is toxic if it builds up in the blood.
Urea is poisonous, so the kidneys remove it from the blood.
You've got it
Key idea
- nephron: filtration at the glomerulus, then reabsorption of all glucose, some ions, most water
- what's left (urea, excess water + ions) = urine
- the liver makes urea from excess amino acids by deamination; urea is toxic so it must be excreted