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Urea production and ultrafiltration

Respiration, Muscles and the Internal Environment · Homeostasis, hormones and the kidney · note 5 of 7

Spec 7.19
A Level only (what this means)A Level only: only in the full A Level, not the AS Level. AS Level students can skip it. What the labels mean
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Urea production and ultrafiltrationSpec 7.19

In short

Excess amino acids cannot be stored, so in the liver the amino group is removed by deamination and converted to urea, which is less toxic. Urea is carried in the blood to the kidneys, where ultrafiltration at the glomerulus forces small molecules, including urea, through the capillary wall, basement membrane and podocytes into Bowman's capsule.

Making urea in the liver

  1. Amino acids not needed for protein synthesis cannot be stored. In the liver, the amino group is removed from each excess amino acid (deamination), forming ammonia and a keto acid.
  2. The keto acid can be respired or converted to carbohydrate or fat.
  3. Ammonia is toxic and very soluble, so it is combined with carbon dioxide in the ornithine cycle (details are not required) to make urea, using ATP.
  4. Urea is soluble and much less toxic. It is carried in the blood plasma to the kidneys.

Ultrafiltration

Ultrafiltration is filtration at the molecular level. Blood in the glomerulus is at high hydrostatic pressure, because the afferent arteriole is wider than the efferent arteriole. This forces water and small solutes out of the capillaries into the Bowman's capsule, through three layers:

  • the fenestrated capillary endothelium, whose cells have many small pores (fenestrations) through them;
  • the basement membrane, a mesh of collagen and glycoproteins that acts as the filter and stops large proteins;
  • the podocytes of the Bowman's capsule, which have filtration slits between their feet-like extensions.

The glomerular filtrate contains water, glucose, amino acids, urea and inorganic ions. It does not contain blood cells or large plasma proteins, which are too large to pass through. The pressure is opposed by the water potential of the blood (which is lowered by the plasma proteins) and the pressure of fluid in the capsule. The volume filtered is about 125 cm³ per minute in humans.

Exam tip:

State all three layers and the reason for the high pressure (wide afferent, narrow efferent arteriole). 'Ultrafiltration' needs a high pressure, not just a membrane.

Glomerulus inside Bowman's capsule with a wider afferent arteriole bringing blood in and a narrower efferent arteriole taking it away, and filtrate leaving to the proximal convoluted tubule. (opens full size in a new tab)
The wide afferent and narrow efferent arteriole keep the pressure high in the glomerulus.
The filtration barrier: capillary endothelium with fenestrations (pores through the cells), basement membrane, and podocytes with foot processes and filtration slits; water, glucose, amino acids, urea and ions pass into the capsule space while blood cells and large proteins stay in the blood. (opens full size in a new tab)
Three layers filter the blood: endothelium, basement membrane and podocytes.

Written and checked against the Edexcel International A Level Biology (YBI11) specification · Updated October 2026

Frequently asked questions

What is the difference between negative and positive feedback?

In negative feedback the response reverses the change, so a factor such as temperature stays near its set point. In positive feedback the response increases the change and moves the factor further away, as with oxytocin in childbirth. Homeostasis relies on negative feedback.

How does the body regulate temperature during exercise?

Respiration in muscles releases heat, raising the temperature of the blood. Thermoreceptors in the hypothalamus detect this and trigger vasodilation of skin arterioles and sweating. These transfer heat to the surroundings, so core temperature returns to the set point by negative feedback.

How does the kidney filter blood in ultrafiltration?

The afferent arteriole is wider than the efferent arteriole, so blood in the glomerulus is at high pressure. Water and small solutes are forced through pores in the capillary endothelium, the basement membrane and the podocytes into the Bowman's capsule, while blood cells and large proteins stay in the blood.

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