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?
In this lesson: Explain pressure-driven ultrafiltration at the glomerulus and Bowman's capsule.
About 5 min
The key ideaHigh 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.
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
Locate the two spaces
Blood is in glomerular capillaries; filtrate collects in Bowman's capsule.
- 2
State the driving force
Relatively high blood pressure pushes fluid across the filtration barrier.
- 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.
Is this true? "Ultrafiltration allows only waste into the nephron."
It also filters useful small substances, including glucose. Selective reabsorption returns useful materials afterwards.