Biology › Hormonal communication, plant responses and homeostasis › The kidney: filter everything, then take almost all of it back

The kidney: filter everything, then take almost all of it back

Your kidneys push about 180 dm3 of fluid out of your blood every day and hand back all but a litre and a half of it. Filtering indiscriminately and then reclaiming selectively looks like a strange way to build an organ, and it is the only way to build one that can excrete something it has never met.

Before this Water potential, osmosis and the movement of water down a gradient · Co-transport, carrier proteins and the sodium-potassium pump

What you should be able to do

• Label a nephron and name the parts in the order the filtrate meets them.
• Explain ultrafiltration in terms of hydrostatic pressure and the three layers of the barrier.
• Describe selective reabsorption at the proximal convoluted tubule, including co-transport.
• Explain how the loop of Henle builds a concentration gradient in the medulla.
• Describe the control of water potential by osmoreceptors, ADH and aquaporins.
• State what a dialysis fluid must contain, and justify each component.

A hundred thousand filters in each kidney

Blood arrives at each kidney down a renal artery, and about a fifth of everything your heart pumps goes through the pair of them. Inside, the work is done by roughly a million nephrons, each a tube about 3 cm long with its own blood supply.

State the design before the detail, because it explains every structure in the organ. A nephron does not select what to excrete: it pushes a large volume of plasma out through a filter that stops only large molecules, then reclaims what is worth keeping as the filtrate runs down the tube. About 180 dm3 of filtrate is produced a day and 1.5 dm3 leaves as urine, so more than 99 per cent is taken back. The advantage is that a substance the body has never encountered — a drug, a toxin, a novel metabolite — is filtered out automatically and simply not reclaimed.

Nephron

The functional unit of the kidney: a renal capsule and its tubule, running from the cortex into the medulla and back.

Ultrafiltration

Filtration under pressure through a partially permeable barrier, separating small molecules from large ones.

Selective reabsorption

The return of useful substances from the filtrate to the blood, by transport proteins in the tubule wall.

Osmoregulation

The control of the water potential of the blood and body fluids.

Boards frame this material differently and it is worth knowing which you are sitting. OCR A treats it as excretion, so the liver and the making of urea come with it. AQA treats it as control of the water potential of the blood, and stops at ADH. CAIE asks for the cells themselves — the microvilli, the mitochondria, the basement membrane — in more detail than either.

Ultrafiltration: pressure, and a sieve made of protein

The glomerulus is a knot of capillaries inside the cup of the renal capsule. Blood reaches it along an afferent arteriole and leaves along an efferent arteriole that is narrower. That one detail is the reason the whole thing works: the blood is being squeezed into a bottleneck, so the hydrostatic pressure inside the glomerular capillaries is far higher than in an ordinary capillary bed, and fluid is forced out through the capillary wall.

Three layers lie between the blood and the capsule space. The capillary endothelium is fenestrated, with pores between and through its cells that are far too wide to sort molecules. The podocytes lining the capsule have foot processes with filtration slits between them, also wide. Between the two lies the basement membrane, a mesh of collagen and glycoproteins, and this is the filter. It holds back anything above about 69 000 relative molecular mass, which means plasma proteins stay in the blood while water, glucose, amino acids, mineral ions and urea pass through.

Not all of that pressure ends up as filtration. Two things oppose it: the hydrostatic pressure of the fluid already in the capsule, and, more importantly, the water potential of the plasma, made more negative by the proteins left behind. Filtration continues only while the outward pressure exceeds their sum, which is why it stops before the blood leaves the glomerulus.

Selective reabsorption, and the cells built for it

The proximal convoluted tubule is where the bulk of the reclamation happens. All of the glucose and all of the amino acids are reabsorbed here, along with most of the water and sodium — textbooks quote figures between two thirds and 85 per cent, and no mark scheme turns on which.

Glucose is taken back by co-transport, and the sequence is one you have met in the ileum. Sodium ions are actively pumped out of the tubule cell at the far side, into the blood, which lowers the sodium concentration inside the cell. Sodium in the filtrate then diffuses into the cell down that gradient through a co-transporter protein, and glucose is dragged in with it, against its own concentration gradient. Glucose then leaves the cell into the blood by facilitated diffusion. Note where the ATP is actually spent: on the sodium pump, not on the glucose.

The cells lining this part of the tubule are built for the job in three ways worth quoting. Their surface facing the filtrate carries microvilli, which give a large surface area for carrier and channel proteins. Their membranes are packed with those transport proteins. And their cytoplasm is full of mitochondria, supplying the ATP the sodium pumps consume. A question showing you an electron micrograph of a tubule cell is asking for those three, each linked to its function.

Water follows by osmosis all the way along. As solutes are reabsorbed, the water potential of the filtrate rises above that of the surrounding tissue and blood, and water moves out. Nothing pumps water anywhere in the kidney, or anywhere else in biology.

The loop of Henle: building a gradient, not reabsorbing water

The loop dips from the cortex into the medulla and comes back, and the two limbs have different properties. The descending limb is permeable to water; the ascending limb is not, and its thick upper section actively transports sodium and chloride ions out into the surrounding tissue.

Those two facts, plus the fact that fluid keeps flowing, are enough to build a gradient far steeper than the pump could manage on its own. The pump can only ever create a difference of about 200 arbitrary units between the ascending limb and the tissue around it — the single effect. Because the two limbs run alongside each other with fluid flowing in opposite directions, that small difference is applied again and again at successive levels, and the effect is multiplied down the loop.

Run through it once in words. Ions are pumped out of the ascending limb, so the tissue becomes concentrated and the ascending limb dilute. Water then leaves the descending limb by osmosis, since it is permeable and the tissue outside it is now more concentrated, and the fluid in the descending limb becomes as concentrated as the tissue. That concentrated fluid flows round the hairpin into the ascending limb, where the pump acts on it again — starting this time from a higher concentration. Repeat, and the tip of the medulla reaches about four times the concentration at the top.

A straight blood vessel running through the medulla would wash the gradient away. The vasa recta are hairpins instead, running alongside the loop, so blood descending picks up solutes and blood ascending gives them back, and it leaves with the gradient intact.

Now the payoff, and the answer to the lie at the top. The collecting duct carries filtrate from the distal tubule back down through the whole depth of the medulla. Every millimetre of that descent, the tissue outside is more concentrated than the fluid inside, so water leaves by osmosis wherever the duct wall will let it. The loop built the gradient; the collecting duct spends it.

One prediction follows immediately, and questions love it. Longer loops build a steeper gradient, so more water leaves the collecting ducts and the urine is more concentrated. Desert mammals such as the kangaroo rat have exceptionally long loops and thick medullas; a beaver has short ones. Given an unfamiliar species and a measurement of medulla thickness, that is the reasoning to use.

ADH: changing what the duct will let through

How much water actually leaves the collecting duct is regulated, and the loop is where the negative feedback of the first lesson gets its most examinable example.

Osmoreceptors in the hypothalamus monitor the water potential of the blood. When you sweat, or eat salt, or drink too little, the water potential of the blood falls, water leaves the osmoreceptor cells by osmosis and they shrink, and they send impulses along neurones to the posterior pituitary. The pituitary releases antidiuretic hormone into the blood.

ADH binds to receptors on the cells of the collecting duct wall and acts through cyclic AMP, exactly as adrenaline does at the liver. The cascade causes vesicles carrying aquaporins — water channel proteins — to fuse with the membrane facing the filtrate. More aquaporins means the wall is more permeable to water, so more water leaves down the gradient the loop has built, and a small volume of concentrated urine is produced. The reabsorbed water raises the water potential of the blood, the osmoreceptors are no longer stimulated, and ADH secretion falls.

Drink a litre of water and the reverse runs: water potential rises, less ADH is released, aquaporins are withdrawn into vesicles, the duct becomes less permeable and a large volume of dilute urine is produced. Alcohol inhibits ADH release, which is why it dehydrates you despite being drunk in quantity.

When the kidney fails, and what dialysis has to replace

Diabetes and high blood pressure cause most kidney failure, with infections, autoimmune disease and inherited polycystic disease behind them, and what follows is the same whatever the cause: urea accumulates, ion concentrations drift, water is retained and blood pressure rises. Untreated it is fatal. OCR A examines this section directly; AQA's specification stops with ADH, and CAIE is likelier to test urine analysis.

In haemodialysis, blood is taken from an artery, passed through a machine on one side of a partially permeable membrane and returned to a vein, while dialysis fluid flows on the other side of that membrane in the opposite direction. Everything about the fluid's composition follows from one idea: a substance will diffuse across only if there is a concentration gradient, so you create a gradient for the things you want removed and abolish it for the things you want kept.

Two design points earn marks on their own. The fluid is replaced continuously, so the gradient for urea never runs down. And blood and fluid flow in opposite directions, which keeps a difference in concentration along the whole length of the exchange surface — the same counter-current principle as the fish gill and the loop of Henle, working here for the third time in this course.

Haemodialysis takes several hours, three times a week, and blood composition drifts between sessions. Peritoneal dialysis uses the patient's own peritoneum as the membrane and can be done at home. A transplant restores continuous function and a normal diet, at the cost of a shortage of donors and a lifetime of immunosuppressants, which leave the recipient open to infection.

In the exam

• Ultrafiltration questions want the narrower efferent arteriole and the high hydrostatic pressure it causes. Naming the basement membrane as the filter is the second mark, and 'the pores' on its own is not enough.
• Say that water moves by osmosis down a water potential gradient. Nothing in the kidney pumps water, and an answer that says it does loses marks wherever it appears.
• For the proximal tubule, describe co-transport in the right order: sodium pumped out into the blood first, then sodium and glucose in together from the filtrate.
• Never write that the loop of Henle reabsorbs water. It creates the gradient in the medulla; the water leaves the collecting duct.
• ADH changes the permeability of the collecting duct by inserting aquaporins. It does not pump water, and it does not act on the loop.
• In a dialysis question, justify each component of the fluid by the gradient it creates or abolishes. Marks come in pairs: what the concentration is, and what that achieves.

Questions

Written to the command words the boards use. Try them on paper before opening a scheme: the marks go to points made, not to length.

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