Osmoregulation and kidney function

A million 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.

The gross structure comes first, since a kidney in section is labelled from these regions. Each one is wrapped in a tough fibrous capsule that holds its shape and protects it. Inside, the outer band is the cortex, which looks granular because it is packed with renal capsules and convoluted tubules. Beneath it the medulla looks striped, because what runs through it are loops of Henle, collecting ducts and blood vessels lying parallel to one another, and it is gathered into cone-shaped renal pyramids whose tips point inwards. Those tips drain into the renal pelvis, the funnel-shaped space at the centre of the organ, and the pelvis narrows into the ureter that carries urine to the bladder. Artery, vein and ureter all meet the kidney at the same notch on its inner curve.

The overall design accounts for the structure of the organ. A nephron does not select what to excrete: it forces a large volume of plasma through a barrier that retains blood cells and plasma proteins, then selectively reabsorbs useful substances as the filtrate passes along the tubule. About 180 dm³ of filtrate is produced a day and 1.5 dm³ leaves as urine, so more than 99 per cent is reabsorbed. A consequence is that a substance the body has no reabsorption mechanism for, such as a drug or a novel metabolite, is filtered out and not reclaimed.

Describe the drawing

A single nephron. At the top left a glomerulus sits inside a Bowman's capsule, fed by one arteriole and drained by another. The filtrate passes into a coiled proximal convoluted tubule, down a straight descending limb into the medulla, round a hairpin, back up the ascending limb into the cortex, through a coiled distal convoluted tubule, and into a thick collecting duct running down through the medulla to the ureter. A dashed line separates cortex from medulla.

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.

Fibrous capsule

The tough outer layer enclosing a kidney, which protects it and holds its shape.

Renal pelvis

The funnel-shaped space at the centre of a kidney, into which the collecting ducts drain and which narrows into the ureter.

Exam-board note. OCR A treats this material as excretion, so the liver and the formation of urea are examined with it. AQA treats it as control of the water potential of the blood and ends at ADH. Cambridge International 9700 requires more detail of the cells themselves: the microvilli, the mitochondria and the basement membrane.

Ultrafiltration: pressure, and a sieve made of protein

The glomerulus is a knot of capillaries inside the cup of the renal capsule, also called Bowman's capsule. Blood reaches it along an afferent arteriole and leaves along an efferent arteriole that is narrower. Because the outflow vessel is narrower than the inflow vessel, 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.

Describe the drawing

The filtration barrier drawn as three stacked bands with blood above and capsule space below. The top band is the capillary endothelium, broken by wide pores; the middle band is the basement membrane, drawn unbroken; the bottom band is the layer of podocytes, broken by narrow slits. One arrow passes right through all three, while a second stops dead at the basement membrane. Lists beneath name what crosses and what does not.

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, each slit bridged by a thin protein sheet called the slit diaphragm. Between the two lies the basement membrane, a mesh of collagen and glycoproteins, and at A-level this is the layer named as the filter. In full, all three layers contribute, and the selectivity is graded by size and charge rather than a single sharp cut-off: albumin, at about 66 000 relative molecular mass, is almost entirely held back, 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; published figures for the proportion of water range from about two thirds to 85 per cent.

Glucose is taken back by co-transport. Prerequisite: co-transport of glucose 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. The ATP is spent on the sodium pump, not on the movement of glucose.

The cells lining this part of the tubule are adapted in three ways. 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. In an electron micrograph of a tubule cell, link each of those three features to its function.

Water follows by osmosis wherever the tubule wall is permeable to it. As solutes are reabsorbed, the water potential of the filtrate rises above that of the surrounding tissue and blood, and water moves out. Water is not actively transported at any point in the nephron.

The loop of Henle: building a gradient for the collecting duct

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, which is 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.

Describe the drawing

The loop of Henle building its gradient in five steps, with concentrations in arbitrary units shown in the descending limb, the ascending limb and the surrounding medulla. First every column reads 300 and there is no gradient. Then the ascending limb pumps sodium and chloride out, so the tissue reads 400 while the ascending limb reads 200: a difference of 200, the same at every level. Water then leaves the descending limb by osmosis until it too reads 400. Flow carries that fluid deeper and the pump repeats, so the difference multiplies down the loop until the tip reads 1200 and the top still reads 300. Finally the collecting duct is drawn running back down through the gradient, with water leaving it along its whole length.

The sequence 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 remove the gradient. The vasa recta are hairpin vessels running alongside the loop, so blood descending takes up solutes and blood ascending returns them, and the gradient is maintained.

The gradient is used by the collecting duct. The collecting duct carries filtrate from the distal tubule back down through the whole depth of the medulla. At every millimetre of that descent, the tissue outside is more concentrated than the fluid inside, so water leaves by osmosis wherever the duct wall is permeable to water. The loop establishes the gradient; water leaves the collecting duct down it.

One prediction follows from that. 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. Apply the same reasoning to an unfamiliar species given a measurement of medulla thickness.

ADH: changing what the duct will let through

The volume of water leaving the collecting duct is regulated by a negative feedback loop acting on the permeability of the duct wall.

Osmoreceptors in the hypothalamus monitor the water potential of the blood. After sweating, a salt intake or a low water intake, 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 on those of the last part of the distal convoluted tubule, and acts through cyclic AMP, exactly as adrenaline does at the liver. The cascade causes vesicles carrying aquaporins, which are 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.

Describe the drawing

Two drawings of the same collecting duct side by side. On the left, with little antidiuretic hormone, a single aquaporin sits in the wall and one arrow of water leaves, so a large volume of dilute urine passes on. On the right, with a great deal of the hormone, five aquaporins sit in the same length of wall and five arrows of water leave, so a small volume of concentrated urine passes on.

After a large intake of water the sequence reverses: the water potential of the blood 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, so it increases urine volume and can cause dehydration despite the volume of liquid consumed.

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. Exam-board note: OCR A examines renal failure and dialysis; AQA's specification ends with ADH; Cambridge International 9700 examines urine analysis.

Kidney function is judged by the glomerular filtration rate (GFR), the volume of plasma filtered each minute, about 125 cm³ min⁻¹ in a healthy adult. It is estimated from the concentration of creatinine in the blood: creatinine is a waste product of muscle that is filtered and not reabsorbed, so as nephrons are lost the GFR falls and blood creatinine rises. A GFR below about 15 cm³ min⁻¹ is taken as kidney failure. By then the kidney cannot adjust how much sodium and potassium it excretes, and a high blood potassium disturbs the rhythm of the heart.

During haemodialysis, blood leaves and returns through vascular access, usually an arteriovenous fistula or graft, or a venous catheter. It passes along one side of a partially permeable membrane while dialysis fluid flows on the other side of that membrane, in the opposite direction. The composition of the fluid follows from a single principle: a substance diffuses across only where a concentration gradient exists, so the fluid establishes a gradient for substances to be removed and abolishes it for substances to be retained.

Two design features should be stated explicitly. The fluid is replaced continuously, so the gradient for urea is maintained. And blood and fluid flow in opposite directions, which maintains a difference in concentration along the whole length of the exchange surface. This is the same counter-current exchange as in a fish gill and in the vasa recta. The loop of Henle differs: it is a counter-current multiplier, because its ascending limb pumps ions actively.

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.

Urine is also used in diagnosis, because substances in the plasma, or the products the liver turns them into, are filtered into it. A pregnancy test uses monoclonal antibodies to detect hCG, a hormone made by the placenta that passes into the urine, and many drug tests use test strips carrying monoclonal antibodies in the same way. Anabolic steroids, and any positive drug screen, are identified by gas chromatography and mass spectrometry, which separates the substances in the sample and identifies each by the masses of its fragments.