K325 / 2027

Excretion in humans

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Which is an example of excretion rather than egestion?

  1. Removing carbon dioxide made in respiration, through the lungs
  2. Moving swallowed food from the mouth to the stomach
  3. Passing undigested fibre, which the body never absorbed, out of the anus
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Removing carbon dioxide made in respiration, through the lungs

The carbon dioxide is a metabolic waste produced by cells, so its removal is excretion.

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Which structure carries urine from the bladder to the outside?

  1. Ureter
  2. Urethra
  3. Renal vein
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Urethra

The urethra is the exit route from the bladder.

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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
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Water, glucose, ions and urea

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

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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
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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.

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Glucose is present in initial filtrate but normally absent from final urine. Which process explains this?

  1. Glucose becomes too large to fit in the ureter.
  2. Glucose is selectively reabsorbed from the tubule fluid into blood.
  3. Glucose is broken down to carbon dioxide and water in the bladder wall.
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Glucose is selectively reabsorbed from the tubule fluid into blood.

It can be filtered first and then returned to blood through the tubule wall.

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Invented values in g per 100 cm3: plasma has glucose 0.10, urea 0.03, protein 8.0. Which values are expected in the filtrate at the start of the tubule?

  1. Glucose 0, urea 2.0, protein 0
  2. Glucose 0.10, urea 0.03, protein 0
  3. Glucose 0.10, urea 0.03, protein 8.0
  4. Glucose 0.20, urea 0.06, protein 4.0
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Glucose 0.10, urea 0.03, protein 0

Small molecules are filtered at plasma levels; proteins are too large to pass through.

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Why do red blood cells normally stay on the blood side of a dialyser?

  1. They are too large to cross the partially permeable membrane.
  2. They move by active transport away from the membrane.
  3. The dialysis fluid has exactly the same red-cell concentration.
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They are too large to cross the partially permeable membrane.

Small solutes such as urea can cross while cells and most large proteins remain in blood.

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Why does dialysis fluid contain glucose at the same concentration as normal blood plasma?

  1. So that the glucose kills bacteria in the dialysis tubing
  2. So that urea is pushed out of the blood more quickly
  3. So that glucose diffuses into the blood to feed the patient
  4. So that there is no net loss of glucose from the blood
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So that there is no net loss of glucose from the blood

With no concentration gradient, glucose does not diffuse out of the blood into the fluid.

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Question 1 of 6

Follow nitrogen to the outside

4 marks

Explain why excess nitrogen from amino acids must be removed. Trace urea from its formation to the outside, naming the kidney, ureter, bladder and urethra roles. Explain why this is different from egestion. [4]

Separate waste formation, blood transport, urine formation, storage and release.

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