The Nephron

Explain the contribution of each process, which means saying what it does and what it achieves. The question also names three structures, so each process must be located in the right one.

6 marks: roughly two for filtration, two for selective reabsorption, one for secretion and one for tying them to waste removal. Take them in the order the filtrate travels.

Blood enters the glomerulus, a knot of capillaries held under high pressure. That pressure forces water, glucose, salts, amino acids and urea out of the blood and into the Bowman's capsule, the cup-shaped structure surrounding the glomerulus, which collects this fluid — the filtrate — and funnels it into the tubule. Blood cells and large proteins are too big to pass through and remain in the blood.

Filtration sorts by size alone, not by usefulness. Everything small enough is forced out, including glucose and much of the body's water, which the body has no intention of losing. At this stage the filtrate still contains valuable substances; nothing has yet been decided about what to keep.

As the filtrate flows along the tubules, selective reabsorption returns useful substances to the blood in the surrounding capillaries. All the glucose is normally reabsorbed, along with most of the water, amino acids and the salts the body needs, much of it by active transport, which is why the process is selective rather than automatic. Urea is not reabsorbed and so remains in the tubule.

Secretion works in the opposite direction to reabsorption: further wastes and excess ions are actively transported out of the surrounding blood and added into the tubule. This fine-tunes the composition of the blood and removes substances that were not fully cleared by filtration alone.

What remains in the tubule after reabsorption and secretion is urine: urea, excess salts and excess water, which passes to the bladder and is removed from the body. The three processes therefore achieve waste removal between them — filtration gets everything small out of the blood, reabsorption takes back what the body needs, and secretion adds anything the filter missed. Because the amount reabsorbed can be varied, the same mechanism also regulates the body's water and salt balance, so the nephron performs excretion and homeostatic regulation at once.

Explain twice over: first the normal mechanism, then the abnormal finding. The second half depends entirely on a property of the first, so build the mechanism carefully before interpreting the test.

5 marks. Account for glucose being in the filtrate at all, explain how it is recovered, identify the property of that recovery that can fail, then give what a positive test indicates.

Glomerular filtration is not selective. High pressure in the glomerulus forces out everything small enough to pass, sorting by size rather than by usefulness, so glucose is pushed out of the blood into the Bowman's capsule alongside water, salts, amino acids and urea. Its presence in the filtrate is not a mistake; it is an unavoidable consequence of using a broad filter.

Glucose is returned to the blood by selective reabsorption along the tubule. Transport proteins in the tubule wall move glucose out of the filtrate and back into the surrounding capillaries, and in a healthy person with normal blood glucose this reclaims all of it, so none remains by the time the filtrate becomes urine.

Reabsorption of glucose uses active transport, so it depends on a finite number of transport proteins and therefore has a maximum rate. Below that maximum every glucose molecule is recovered; above it the transporters are saturated and the excess glucose cannot be reabsorbed, so it stays in the filtrate and passes into the urine.

Glucose in the urine therefore indicates one of two things. Most commonly, blood glucose concentration is so high that the amount filtered exceeds the tubule's maximum reabsorption rate, and the excess spills over — the classic sign of uncontrolled diabetes mellitus, whose name means "sweet siphon" because it was once diagnosed by the sugar in the urine. Alternatively, the reabsorption mechanism itself may be damaged, so that even normal blood glucose cannot be fully recovered.

A urine glucose test is therefore not a direct measurement of blood glucose at all. It reports that reabsorption has been overwhelmed or has failed, which is why it works as a quick, cheap screen but not as a precise measure, and why a positive result calls for a blood test to establish what is actually happening.