Questions on osmoregulation and kidney function

Question 1

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.

Accepted Answer

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 B1 sodium ions in the filtrate then diffuse into the cell down that gradient through a co-transporter protein B1 glucose is carried in with the sodium, against its own concentration gradient, and then leaves the cell into the blood by facilitated diffusion B1 the surface facing the filtrate carries microvilli, giving a large surface area for the carrier and channel proteins the membranes are packed with A1 the cytoplasm contains many mitochondria, supplying the ATP that the sodium pumps consume

Question 2

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 itself concentrates the urine.

Accepted Answer

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 B1 this makes the tissue fluid of the medulla more concentrated than the ascending limb at every level, so the medulla has a lower water potential 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 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 A1 the fluid leaving the ascending limb is more dilute than plasma; the urine is concentrated later, in the collecting duct, as water leaves it down the gradient the loop has built

Question 3

A person has a glomerular filtration rate of 120 cm³ min⁻¹ and produces 1.8 dm³ of urine in a day. Calculate the volume of filtrate produced in a day, calculate the volume reabsorbed, and calculate the percentage of the filtrate that is reabsorbed.

Accepted Answer

M1 convert the glomerular filtration rate to a daily volume: 120 × 60 × 24 = 172 800 cm³ per day A1 172.8 dm³ of filtrate is produced per day M1 reabsorbed volume = filtered volume − urine volume = 172.8 − 1.8 = 171.0 dm³ M1 percentage reabsorbed = reabsorbed volume ÷ filtered volume × 100 = 171.0 ÷ 172.8 × 100 A1 99.0 per cent of the filtrate is reabsorbed

Question 4

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

Accepted Answer

B1 the efferent arteriole leaving the glomerulus is narrower than the afferent arteriole bringing blood in, so the blood is squeezed into a bottleneck 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 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 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

Question 5

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.

Accepted Answer

B1 water is lost in sweat, so the water potential of the blood falls and becomes more negative B1 osmoreceptors in the hypothalamus lose water by osmosis and shrink, and send impulses along neurones to the posterior pituitary B1 the posterior pituitary releases more antidiuretic hormone into the blood, and it binds to receptors on the cells of the collecting duct wall A1 vesicles carrying aquaporins fuse with the membrane facing the filtrate, so the wall becomes more permeable and more water leaves by osmosis down the gradient in the medulla, giving a small volume of concentrated urine

Question 6

Explain why a haemodialysis fluid contains no urea but contains glucose at the same concentration as normal blood plasma.

Accepted Answer

B1 a substance diffuses across the partially permeable membrane only where a concentration gradient exists between the blood and the fluid B1 urea is deliberately absent from the fluid so that a steep gradient exists from blood to fluid, and urea that has built up in the patient's blood diffuses out and is removed B1 glucose is included at the normal blood concentration so that no gradient exists for it A1 without a gradient, glucose already at the normal concentration does not diffuse out of the blood, so useful glucose is not lost during dialysis

Question 7

Describe two common causes of kidney failure, and describe the resulting changes in blood urea concentration, ion concentration and blood pressure.

Accepted Answer

B1 diabetes and high blood pressure are the two most common causes of kidney failure B1 with fewer working nephrons, urea is filtered and excreted more slowly, so its concentration in the blood rises B1 ion concentrations in the blood drift away from normal, because the kidney's control of their reabsorption and excretion is lost A1 water that should have been excreted is retained instead, which raises blood volume and in turn raises blood pressure

Question 8

Compare how the proximal convoluted tubule and the loop of Henle each affect the water content of the filtrate, making clear which one is responsible for the concentration gradient in the medulla.

Accepted Answer

B1 at the proximal convoluted tubule water leaves the filtrate by osmosis continuously, following the solutes reabsorbed there, so a large proportion of the filtered water is reclaimed at this point B1 the loop of Henle works differently: its descending limb does let water leave by osmosis, but only because the ascending limb has actively pumped ions into the surrounding tissue, not because the loop itself is reabsorbing water for its own sake B1 the ascending limb is impermeable to water altogether, so no water crosses there regardless of the ion pumping taking place A1 the loop's real function is to build the medullary concentration gradient that the collecting duct will later use, rather than to reabsorb a large fraction of the filtrate's water itself

Question 9

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.

Accepted Answer

B1 a thick medulla holds unusually long loops of Henle B1 sodium and chloride ions are pumped out of the ascending limb over a greater depth of tissue, so the single effect is multiplied over more levels and the gradient in the medulla is steeper A1 the collecting duct passes down through that steeper gradient, so water leaves it by osmosis all the way down and a smaller volume of more concentrated urine is produced

Question 10

A kidney is cut in half lengthways. Name the tough outer layer enclosing the organ, name the region in which the renal capsules lie, and name the funnel-shaped space that the collecting ducts drain into.

Accepted Answer

B1 the fibrous capsule encloses the kidney B1 the renal capsules and convoluted tubules lie in the cortex, the outer region B1 the collecting ducts drain into the renal pelvis, which narrows into the ureter

Question 11

Explain why blood and dialysis fluid are made to flow in opposite directions through a dialysis machine, rather than in the same direction.

Accepted Answer

B1 opposite, counter-current flow maintains a concentration difference between blood and fluid along the whole length of the exchange surface B1 if the two fluids flowed in the same direction, the difference would be largest at the start and would shrink to almost nothing by the far end, as the two fluids approached the same composition A1 with counter-current flow, fresh dialysis fluid is always meeting blood that has already lost some urea further back, so diffusion of urea out of the blood can continue effectively along the entire membrane

Question 12

Drinking alcohol is followed by producing an unusually large volume of dilute urine, even though no extra water has been drunk. Suggest why, given that alcohol inhibits the release of ADH.

Accepted Answer

B1 with ADH release inhibited, less ADH reaches the collecting duct, so fewer aquaporins are inserted into the membrane facing the filtrate B1 the collecting duct wall is therefore less permeable to water than usual, so less water leaves it by osmosis down the gradient the loop of Henle has built A1 more water than usual stays in the filtrate and is passed as urine, so a large volume of dilute urine is produced despite no extra water having been taken in, which is why drinking alcohol can cause dehydration

Question 13

Explain why the blood vessels running through the renal medulla, the vasa recta, are shaped as hairpin loops rather than running straight through like an ordinary capillary.

Accepted Answer

B1 a straight vessel would carry blood in one direction only, continuously washing solute out of the medulla and into the general circulation B1 that would remove the concentration gradient the loop of Henle has built, since diffusion between the vessel and the surrounding tissue would gradually even out the difference A1 because the vasa recta run as hairpins alongside the loop of Henle, blood descending into the medulla takes up solute and blood ascending back out returns most of it, so the gradient is maintained rather than washed away

Question 14

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.

Accepted Answer

B1 the basement membrane, a mesh of collagen and glycoproteins, is the filter B1 it holds back plasma proteins, such as albumin at about 66 000 relative molecular mass

Question 15

State what is meant by osmoregulation, and state what is meant by ultrafiltration.

Accepted Answer

B1 osmoregulation is the control of the water potential of the blood and body fluids B1 ultrafiltration is filtration under pressure through a partially permeable barrier, separating small molecules from large ones