Kidney & Osmoregulation

Ultrafiltration of blood plasma occurs in the renal (Bowman's) capsule largely because:

• Athe efferent arteriole is narrower than the afferent arteriole, raising hydrostatic pressure in the glomerulus✓
• Bactive transport pumps plasma into the capsule
• Cthe efferent arteriole is wider than the afferent arteriole
• Dthe basement membrane actively secretes fluid

Answer: The efferent arteriole has a smaller diameter than the afferent arteriole, creating a high hydrostatic (blood) pressure within the glomerular capillaries. This pressure forces small molecules across the filtration barrier into the capsular space.

Which component of blood plasma would normally NOT be present in the glomerular filtrate of a healthy person?

• Aglucose
• Burea
• Csodium ions
• Dplasma proteins such as albumin✓

Answer: The basement membrane of the glomerulus acts as a molecular sieve. Large plasma proteins such as albumin are too big to pass through, so they remain in the blood. Glucose, urea and ions are small enough to be filtered.

The epithelial cells lining the proximal convoluted tubule are adapted for selective reabsorption. Which feature most directly increases the surface area for reabsorption of glucose and amino acids?

• Aa thick, impermeable cell wall
• Bthe absence of mitochondria
• Cmicrovilli on the cell-surface membrane facing the lumen✓
• Dtight junctions that prevent all movement of ions

Answer: Cells of the proximal convoluted tubule have a brush border of microvilli facing the lumen, greatly increasing the surface area for reabsorption. They also contain many mitochondria and co-transporter proteins to drive uptake of glucose and amino acids.

Glucose is reabsorbed from the proximal convoluted tubule into the epithelial cells mainly by:

• Aco-transport with sodium ions, which are then actively pumped out into the blood✓
• Bsimple diffusion down its concentration gradient
• Cactive transport directly using ATP at the luminal membrane
• Dosmosis through aquaporin channels

Answer: Na⁺ is actively pumped from the epithelial cell into the blood at the basal membrane, lowering intracellular Na⁺. Na⁺ then re-enters from the lumen via a co-transporter protein, dragging glucose with it (secondary active transport / facilitated co-transport).

The counter-current multiplier of the loop of Henle establishes a high solute concentration in the medulla. This is achieved because:

• Athe descending limb actively pumps out water
• Bthe descending limb actively pumps Na⁺ and Cl⁻ out of the filtrate
• Cthe collecting duct secretes urea into the descending limb
• Dthe ascending limb actively transports Na⁺ and Cl⁻ out into the medullary interstitium while being impermeable to water✓

Answer: The thick ascending limb is impermeable to water but actively transports Na⁺ and Cl⁻ into the surrounding medulla, raising its solute concentration. This high concentration draws water osmotically out of the descending limb and the collecting duct.

A desert mammal that must conserve water would be expected to have, compared with a mammal from a wet habitat:

• Ashorter loops of Henle and a thinner medulla
• Blonger loops of Henle and a thicker medulla, producing more concentrated urine✓
• Cno loops of Henle at all
• Da wider afferent arteriole to increase filtration

Answer: Longer loops of Henle (and a correspondingly thicker medulla) create a steeper, longer concentration gradient in the medulla. This allows more water to be reabsorbed from the collecting duct, producing small volumes of highly concentrated urine to conserve water.

Osmoreceptors that monitor the water potential of the blood and initiate the response to dehydration are located in the:

• Aposterior pituitary gland
• Badrenal cortex
• Chypothalamus✓
• Ddistal convoluted tubule

Answer: Osmoreceptors in the hypothalamus detect a fall in the water potential of the blood. They stimulate the posterior pituitary to release ADH and also generate the sensation of thirst, both of which act to restore normal blood water potential.

When the water potential of the blood falls below normal, the secretion of antidiuretic hormone (ADH) increases. The direct effect of ADH on the collecting duct is to:

• Adecrease the permeability of the collecting duct to water
• Bincrease the number of aquaporin channels in the collecting-duct membrane, increasing water reabsorption✓
• Cactively pump water out of the filtrate using ATP
• Dincrease active transport of glucose out of the collecting duct

Answer: ADH binds to receptors on collecting-duct cells, causing vesicles containing aquaporins to fuse with the membrane. This raises water permeability, so more water is reabsorbed by osmosis into the medulla, producing a smaller volume of concentrated urine.

After drinking a large volume of water, the resulting change in urine would be caused by:

• Aincreased ADH release, giving a large volume of dilute urine
• Bincreased ADH release, giving a small volume of concentrated urine
• Cdecreased ADH release, giving a large volume of dilute urine✓
• Ddecreased ADH release, giving a small volume of concentrated urine

Answer: A high water intake raises the blood water potential. Osmoreceptors detect this and ADH secretion falls, so the collecting duct becomes less permeable to water. Less water is reabsorbed, producing a large volume of dilute urine.

In haemodialysis, the dialysis fluid is designed so that it contains:

• Aa normal plasma concentration of glucose and mineral ions, but no urea✓
• Bno glucose and no mineral ions, to remove them all from the blood
• Ca high concentration of urea to prevent its loss
• Da higher concentration of glucose than the blood to add energy

Answer: Dialysis fluid contains glucose and ions at normal plasma concentrations so there is no net diffusion gradient and these substances are retained. It contains no urea, so urea diffuses down its concentration gradient from blood into the dialysis fluid and is removed.

Which of the following is a genuine advantage of a successful kidney transplant over long-term haemodialysis?

• AThere is no risk of rejection or need for medication
• BDiet no longer needs any consideration whatsoever
• CIt is always cheaper in every respect and carries no surgical risk
• DIt provides continuous filtration and frees the patient from regular dialysis sessions, generally improving quality of life✓

Answer: A transplanted kidney filters the blood continuously, so the patient does not depend on frequent dialysis sessions and usually has a better quality of life and fewer dietary restrictions. However, transplants carry surgical risk and require immunosuppressant drugs to limit rejection.

The presence of protein in the urine (proteinuria) of a patient most likely indicates damage to the:

• Acollecting duct
• Bglomerular basement membrane / filtration barrier✓
• Cloop of Henle
• Dureter

Answer: Plasma proteins are normally too large to cross the glomerular filtration barrier. Their appearance in urine suggests that the basement membrane or podocyte filtration slits have been damaged, allowing proteins to leak into the filtrate.