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6.4.10Organisms Respond to Changes in their Environments (A Level only)

Nephron Function — AQA A Level Biology 7402 Revision Notes & Topical Questions

5 min read 7 parts Lesson 41 of 44 1 quiz

📚 AQA A Level Biology 7402 Hub / Topical Revision
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6.4.10 Nephron Function — AQA A Level Biology 7402 notes: Explain how the nephron produces urine by ultrafiltration and selective reabsorption, and how the loop of Henle creates a concentration gradient.The functions of the kidney are to remove nitrogenous waste (mainly urea) and…

  • concentration of the filtrate
  • capillary endothelium
  • basement membrane
  • glomerular filtrate
  • co-transport
  • ascending limb

Section 1 of 7Overview of urine formation

The functions of the kidney are to remove nitrogenous waste (mainly urea) and to regulate the water potential and ion concentration of the blood.

Ultrafiltration
Urine is made in the nephron by three processes: ultrafiltration in the Bowman's capsule, selective reabsorption in the proximal convoluted tubule,
  • and concentration of the filtrate by the loop of Henle, distal tubule and collecting duct.
Blood enters the glomerulus in the afferent arteriole and leaves in the narrower efferent arteriole.

Section 2 of 7Ultrafiltration

The efferent arteriole is narrower than the afferent arteriole, so there is a high hydrostatic pressure in the glomerular capillaries.

This forces water and small solutes out of the blood: water, glucose, amino acids, urea and ions.
Capillary endothelium
Filtration occurs across three layers: the capillary endothelium (with fenestrations,
  • or pores), the basement membrane and the podocytes of the capsule epithelium with gaps between their foot-like processes.
    • (a gel of collagen and glycoproteins which acts as the main filter and prevents molecules larger than about 69 000 relative molecular mass from passing)
Blood cells and large proteins stay in the blood.
Glomerular filtrate
The fluid collected in the capsule is the glomerular filtrate.
The filtrate has the same composition as plasma without large proteins.

Section 3 of 7Selective reabsorption in the proximal convoluted tubule

About 85% of the filtrate is reabsorbed here.

The cells of the proximal tubule have microvilli on the surface facing the lumen to increase surface area, and many mitochondria to provide ATP.
Sodium–potassium pumps in the basal membranes actively move Na+ out of the cell into the tissue fluid, so the cell has a low Na+ concentration.
Co-transport
Na+ then moves from the lumen into the cell by co-transport with glucose or amino acids through carrier proteins.
Glucose and amino acids move out into the blood by facilitated diffusion.
As the solute concentration in the tissue fluid rises, water moves by osmosis into the blood.
Normally all glucose is reabsorbed; if the concentration of glucose in the filtrate is too high, as in diabetes, some remains in urine.
Small proteins are reabsorbed by endocytosis and urea is partly reabsorbed by diffusion.

Section 4 of 7Loop of Henle and the countercurrent multiplier

The loop of Henle creates a region of very negative water potential in the medulla so water can be reabsorbed from the filtrate in the collecting duct.

Ascending limb
In the ascending limb, which is impermeable to water, Na+ and Cl- ions are actively transported out into the medulla.
This lowers the water potential of the tissue fluid in the medulla.
Descending limb
In the descending limb,
  • which is permeable to water, water leaves the filtrate by osmosis into the medulla and is carried away by the blood capillaries (vasa recta).
As a result the filtrate becomes more concentrated as it passes down the descending limb, and more dilute as it passes up the ascending limb.
The arrangement of the two limbs running in opposite directions multiplies the concentration gradient,
  • so the medulla has a very low water potential at its deepest point.
Animals with longer loops of Henle, such as desert mammals, can produce more concentrated urine and conserve water.

The filtrate then passes through the distal convoluted tubule, where ion concentration is adjusted, and into the collecting duct, which passes through the medulla; its permeability to water is controlled by ADH.

Section 5 of 7Exam-style questions

  1. Describe how glucose is reabsorbed from the proximal convoluted tubule into the blood. [4]4 marks

  2. Explain how ultrafiltration happens in the glomerulus. [4]4 marks

  3. Explain why the filtrate in the descending limb becomes more concentrated. [2]2 marks

Section 6 of 7Key terms

TermMeaning
UltrafiltrationFiltration under pressure through the glomerulus and capsule
Glomerular filtrateFluid filtered from blood into the capsule
PodocyteCell of the capsule with finger-like projections
Selective reabsorptionReturn of useful substances from filtrate to blood
Co-transportMovement of two substances together through a carrier protein
Loop of HenlePart of the nephron creating the concentration gradient in the medulla
Basement membraneLayer of collagen and glycoproteins acting as a filter

Section 7 of 7Exam technique

1
Saying that protein is filtered; large proteins stay in the blood.
2
Saying that reabsorption of glucose is by active transport directly; it is co-transport, with Na+ pumped actively.
3
Confusing the ascending and descending limbs.
4
Forgetting that the ascending limb is impermeable to water.
5
Writing that filtration is by osmosis; it is by pressure.

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