K325 / 2027

Lesson 3 of 5 / Excretion in humans

Ultrafiltration in the kidney

Why does glucose enter the filtrate while red blood cells normally stay in blood?

I can explain pressure-driven ultrafiltration at the glomerulus and Bowman's capsule.

  • I can explain filtration and reabsorption in urine formation.Syllabus

    Syllabus K325, Topic 7(c). Explain filtration and reabsorption in urine formation.

About 5 min

Make a guess. You are not marked.

Glucose is useful to the body. Is it filtered out of the blood into the kidney tubule?

  1. It is filtered with urea, salts and water.
  2. The filter lets just the wastes through.
  3. Glucose molecules are too large to be filtered.
Show the answer

It is filtered with urea, salts and water.

Ultrafiltration lets all small molecules through, useful or not. Glucose is taken back later.

The key idea

High blood pressure filters water and small dissolved substances into the capsule, while cells and most large proteins remain in blood.

A nephron is a microscopic kidney unit. At its beginning, a cup called Bowman's capsule surrounds a knot of capillaries called the glomerulus.

glucose starts in glomerular bloodBlood inBlood outGlomerulusBowman's capsuleTo tubuleHigh pressure drives filtration
Predict whether this component can cross the normal filtration barrier.

The incoming arteriole is shown wider than the outgoing arteriole: resistance to outflow helps maintain filtration pressure. Blood and capsule fluid remain separate. Symbol sizes are illustrative, and the diagram does not model every property of the barrier.

Explanation

Blood enters the glomerulus through an incoming, or afferent, arteriole and leaves through an outgoing, or efferent, arteriole. The narrower outgoing vessel offers resistance to outflow, helping maintain high pressure in the glomerular capillaries. This pressure pushes some water and small dissolved substances across the filtration barrier into Bowman's capsule.

This process is called ultrafiltration. Small substances such as glucose, amino acids, dissolved ions and urea enter the filtrate along with water. Useful molecules as well as waste molecules are filtered.

Blood cells and most large plasma proteins are retained by the normal filtration barrier. Size is a useful distinction in this course model, although the real barrier has additional properties. Red cells do not leave blood to become part of normal urine.

The fluid in the capsule is filtrate, not final urine. It moves into the nephron tubule, where its composition changes through selective reabsorption. Blood continues along vessels outside the tubule.

Pressure-driven filtration is not the same process as diffusion down a concentration gradient or active transport using cellular energy. Keep the driving force clear when explaining this first stage.

Step by step
  1. 1

    Locate the two spaces

    Blood is in glomerular capillaries; filtrate collects in Bowman's capsule.

  2. 2

    State the driving force

    Relatively high blood pressure pushes fluid across the filtration barrier.

  3. 3

    Compare what passes

    Small dissolved molecules can pass; cells and most large plasma proteins remain in the blood.

Worked example

Useful does not mean unfiltered

A model sample of initial filtrate contains glucose but no red blood cells. Explain this observation.

One way to explain it

Glucose molecules are small enough to pass across the filtration barrier with water under pressure. Red blood cells are too large to pass through the normal barrier, so they remain in glomerular blood.

Why this answer works
  • Use properties of the barrier, not whether the substance is useful.
  • Include pressure as the driving force.
  • Later reabsorption explains why glucose is normally absent from final urine.

Watch out for this

A student says: "Ultrafiltration allows only waste into the nephron." What is wrong with this?

Show the answer

It also filters useful small substances, including glucose. Selective reabsorption returns useful materials afterwards.

Check your understanding

Which set normally enters Bowman's capsule during ultrafiltration?

  1. Water, glucose, ions and urea
  2. Red blood cells, platelets and large plasma proteins
  3. Urea only, because the kidney recognises every waste molecule
Show the answer

Water, glucose, ions and urea

These small substances can cross the filtration barrier. The filtrate still contains useful materials.

Exam-style questions

Check your understanding

Which feature makes ultrafiltration in the glomerulus possible?

  1. High blood pressure, because the vessel leaving is narrower than the one entering
  2. Active transport of urea into the capsule, using energy from respiration
  3. Osmosis of plasma proteins into the capsule, carrying the water with them
  4. Low blood pressure, so that red blood cells can pass slowly into the capsule
Show the answer

High blood pressure, because the vessel leaving is narrower than the one entering

The narrower efferent vessel keeps pressure high in the glomerulus, forcing small molecules out.

Quick recall

Card 1 of 3. Answer in your head, then check.

Why does glucose enter the filtrate while red blood cells normally stay in blood?

You can return to this lesson any time.