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The kidney: filter everything, then take almost all of it back questions

The structure of a nephron, ultrafiltration and the basement membrane, selective reabsorption at the proximal convoluted tubule, the loop of Henle as a counter-current multiplier, osmoreceptors, ADH and aquaporins in the collecting duct, and kidney failure with what a dialysis fluid must contain and why.

6 original questions · 23 marks · the kidney: filter everything, then take almost all of it back notes · Hormonal communication, plant responses and homeostasis

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• 5 marks · Describe

  Describe how glucose in the filtrate is reabsorbed at the proximal convoluted tubule, and describe two features of the tubule cells that suit them to the job.

  Mark scheme

  1. B1 sodium ions are actively pumped out of the tubule cell at the side facing the blood, which lowers the sodium concentration inside the cell
  2. B1 sodium ions in the filtrate then diffuse into the cell down that gradient through a co-transporter protein
  3. B1 glucose is carried in with the sodium, against its own concentration gradient, and then leaves the cell into the blood by facilitated diffusion
  4. B1 the surface facing the filtrate carries microvilli, giving a large surface area for the carrier and channel proteins the membranes are packed with
  5. A1 the cytoplasm contains many mitochondria, supplying the ATP that the sodium pumps consume; the ATP is spent on the sodium pump rather than on the glucose

• 5 marks · Explain

  Explain how the loop of Henle produces a concentration gradient in the tissue of the medulla, and explain why it is wrong to say that the loop reabsorbs the water.

  Mark scheme

  1. B1 the thick section of the ascending limb actively transports sodium and chloride ions out into the surrounding tissue, and that limb is not permeable to water
  2. B1 the pump can only ever create a difference of about 200 arbitrary units between the ascending limb and the tissue around it, which is the single effect
  3. B1 the descending limb is permeable to water, so water leaves it by osmosis into the now more concentrated tissue and the fluid inside becomes more concentrated
  4. B1 because the two limbs run alongside each other with fluid flowing in opposite directions, that concentrated fluid is delivered to the pump again at a higher starting concentration, so the small difference is multiplied down the loop
  5. A1 the water itself is reabsorbed later and elsewhere, from the collecting duct running back down through the gradient the loop has built; the loop spends ATP building the gradient rather than reclaiming the water

• 4 marks · Explain

  Explain how ultrafiltration produces a filtrate in the renal capsule that contains glucose and urea but no plasma protein.

  Mark scheme

  1. B1 the efferent arteriole leaving the glomerulus is narrower than the afferent arteriole bringing blood in, so the blood is squeezed into a bottleneck
  2. B1 the hydrostatic pressure in the glomerular capillaries is therefore much higher than in an ordinary capillary bed, and fluid is forced out through the capillary wall
  3. B1 the pores of the fenestrated endothelium and the filtration slits between the podocytes are both far too wide to sort molecules, so the basement membrane between them does the sorting
  4. A1 water, glucose, amino acids, mineral ions and urea are small enough to pass through it, while plasma proteins are too large and stay in the blood

• 4 marks · Describe

  A person sweats heavily during a long run. Describe the sequence of events that raises the concentration of their urine, naming the receptors, the hormone and the part of the nephron involved.

• 3 marks · Suggest

  A desert rodent produces urine four times as concentrated as human urine, and the medulla of its kidney is unusually thick. Suggest how the structure of its nephrons accounts for this.

• 2 marks · State

  State which of the three layers between the blood and the capsule space acts as the filter during ultrafiltration, and state one component of the blood that it holds back.

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