Kidney & Osmoregulation

In the human kidney, where does ultrafiltration of the blood occur, and what mainly prevents plasma proteins from entering the filtrate?

• AIn the loop of Henle; the plasma proteins are actively reabsorbed back into the blood by carrier proteins
• BIn the collecting duct; the plasma proteins are removed by the action of ADH
• CIn the proximal convoluted tubule; the plasma proteins are broken down by digestive enzymes in the filtrate
• DIn the glomerulus and Bowman's capsule; proteins are too large to pass through the basement membrane✓

Answer: Ultrafiltration occurs at the glomerulus, driven by high hydrostatic blood pressure. The basement membrane acts as a molecular filter: small molecules pass into Bowman's capsule, but large plasma proteins are retained in the blood.

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✓
• Bpodocytes actively pump plasma across the basement membrane into the capsular space using ATP from their mitochondria
• Cthe efferent arteriole is wider than the afferent arteriole, so blood leaves the glomerulus faster than it enters it
• Dthe basement membrane actively secretes water, ions and glucose into the capsule, using a sodium gradient set up in the tubule

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 between neighbouring cells that block the movement of ions and water between them

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 a concentration gradient that is maintained by continuous filtration at the glomerulus
• Cactive transport directly using ATP hydrolysed by a glucose pump in the luminal membrane of the cell
• Dosmosis through aquaporin (water) 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 into the medulla, while remaining freely permeable to water
• 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, so a steeper solute gradient can be built up in a smaller space
• Blonger loops of Henle and a thicker medulla, producing more concentrated urine✓
• Cno loops of Henle at all
• Da wider afferent arteriole to raise the glomerular filtration rate (GFR), so that more filtrate is produced each minute

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 wall to water, so that a larger volume of dilute urine is produced
• Bincrease the number of aquaporin channels in the collecting-duct membrane, increasing water reabsorption✓
• Cactively pump water out of the filtrate, which requires ATP from the mitochondria of the duct epithelium (the lining cells)
• Dincrease the active transport of glucose, urea and salts out of the collecting duct, raising the solute concentration of the blood

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, no mineral ions and no bicarbonate buffer, so that these solutes diffuse out of the blood
• Ca concentration of urea higher than that of the blood, so that urea diffuses into the patient rather than out
• Da concentration of glucose well above that of the blood, so that the patient gains glucose at every session

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, where water permeability is controlled by ADH
• Bglomerular basement membrane / filtration barrier✓
• Cloop of Henle, which sets up the solute gradient in the medulla
• 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.

What happens during ultrafiltration in the glomerulus?

• ASmall molecules are forced out by high hydrostatic pressure✓
• BSmall molecules are drawn out by low hydrostatic pressure
• CLarge proteins are forced out by high hydrostatic pressure
• DWater alone is forced out by high hydrostatic pressure

Answer: The efferent arteriole is narrower than the afferent, raising pressure so water, glucose, salts and urea pass through the basement membrane. Blood cells and large plasma proteins are too big to cross, which is why they are absent from normal urine.

Where is glucose reabsorbed in the nephron?

• AThe proximal convoluted tubule, by co-transport with sodium✓
• BThe distal convoluted tubule, by co-transport with sodium
• CThe loop of Henle, by simple diffusion down its gradient
• DThe collecting duct, by osmosis alongside water movement

Answer: Essentially all glucose is reclaimed in the proximal convoluted tubule, whose cells have microvilli and many mitochondria to power sodium-linked co-transport. Glucose appearing in urine means that mechanism has been overwhelmed, as in untreated diabetes.

How does the loop of Henle enable concentrated urine to be produced?

• AIt creates a salt gradient in the cortex by counter-current multiplication
• BIt actively pumps water out of the filtrate along its whole length
• CIt creates a salt gradient in the medulla by counter-current multiplication✓
• DIt reabsorbs all the urea from the filtrate into the blood

Answer: The ascending limb pumps sodium and chloride into the medulla while the descending limb is permeable to water, building a steep osmotic gradient deeper into the medulla. Water then leaves the collecting duct passing through it. Water is never pumped directly.

What is the effect of ADH on the collecting duct?

• AIt decreases permeability to water, so less is reabsorbed
• BIt increases permeability to salts, so more are reabsorbed
• CIt increases permeability to water, so more is reabsorbed✓
• DIt decreases permeability to urea, so less is reabsorbed

Answer: ADH causes aquaporins to be inserted into the collecting duct membrane, so more water leaves by osmosis into the concentrated medulla and urine becomes more concentrated. Low water potential in the blood triggers its release.

Which structures detect a fall in the water potential of the blood?

• AOsmoreceptors in the posterior pituitary
• BBaroreceptors in the carotid arteries
• COsmoreceptors in the hypothalamus✓
• DChemoreceptors in the medulla oblongata

Answer: Hypothalamic osmoreceptors shrink as blood water potential falls, stimulating ADH production. The hormone is stored and released by the posterior pituitary, so the two are easily confused — detection and release happen in different places.

What is the main nitrogenous waste excreted by mammals?

• AUrea, formed in the kidney from ammonia
• BUrea, formed in the liver from ammonia✓
• CAmmonia, excreted directly without conversion
• DUric acid, formed in the liver from ammonia

Answer: Deamination of excess amino acids yields toxic ammonia, which the liver converts to far less toxic urea in the ornithine cycle. The kidney excretes urea but does not make it; uric acid is the bird and reptile route.

Why do desert mammals typically have a long loop of Henle?

• AA longer loop builds a steeper medullary gradient, conserving water✓
• BA longer loop builds a shallower medullary gradient, conserving water
• CA longer loop increases the total volume of filtrate produced
• DA longer loop removes more glucose from the filtrate

Answer: A longer loop extends further into the medulla and produces a more concentrated interstitial fluid, so more water can be drawn out of the collecting duct and very concentrated urine is produced. Glucose reabsorption happens earlier and is unrelated.

Why is glucose normally absent from urine?

• AIt is too large to pass through the basement membrane
• BIt is broken down by enzymes within the nephron
• CIt is never filtered out of the blood in the first place
• DIt is entirely reabsorbed in the proximal convoluted tubule✓

Answer: Glucose is small enough to be filtered freely, but co-transport in the proximal convoluted tubule reclaims essentially all of it. Its presence in urine indicates blood glucose has exceeded the reabsorption capacity.

What does the term selective reabsorption mean?

• AWaste substances are returned to the blood from the filtrate
• BAll substances are returned to the blood from the filtrate
• CUseful substances are returned to the blood from the filtrate✓
• DSubstances are secreted from the blood into the filtrate

Answer: Ultrafiltration is indiscriminate about small molecules, so the nephron must reclaim what the body needs — glucose, amino acids, most salts and much of the water — while leaving urea to be excreted.