How do the kidneys work?

You have two kidneys, each shaped like a bean and about the size of your fist: about 11 to 12 cm long and about 150 g. They are not in your belly at the front but high up against your back, either side of the spine, partly tucked under the lowest ribs. We are facing the patient, so the patient's left is on your right. The right kidney sits a little lower than the left, because the big liver is above it.

Each kidney gets blood straight from the aorta, the body's main artery, through a short, wide renal artery, and sends it back through a renal vein to the inferior vena cava. The flow is huge: together the kidneys get about 1.1 to 1.2 litres a minute, about a fifth to a quarter of everything the heart pumps (see HeartClear), though they are less than half a percent of your weight.

What they make, urine, collects in the renal pelvis and is squeezed down the ureters, two muscular tubes about 25 to 30 cm long, into the bladder, a stretchy bag that holds about 400 to 600 mL. It leaves through the urethra. Sitting on top of each kidney is an adrenal gland, which makes hormones such as adrenaline and aldosterone.

Cut a kidney open and you see an outer layer, the cortex, and an inner medulla made of cone-shaped pyramids. Their tips drip urine into cups called calyces, which join into the pelvis.

A kidney is not one big filter. It is packed with about a million tiny units called nephrons. The real number varies a lot from person to person, from about 200,000 to more than 2.5 million per kidney. Your body can’t grow new ones.

Each nephron starts with a glomerulus, a knot of tiny blood vessels about 0.2 mm across, sitting in a cup called Bowman’s capsule. Blood arrives through the afferent arteriole and leaves through the narrower efferent arteriole. On the way, blood pressure pushes some of the liquid part of blood, the plasma, into the capsule. This liquid is the filtrate.

The filtrate then runs down a long, twisting tube. First the proximal tubule, then the hairpin-shaped loop of Henle, which dips deep into the medulla, then the distal tubule, which touches its own glomerulus again, and finally a collecting duct shared with many other nephrons. All along it, blood vessels called peritubular capillaries wrap the tube and take back what the body needs. Of every 100 drops filtered, about 99 go back into the blood.

Filtering starts in the glomerulus. The wall of its capillaries is a sieve with three layers. The inner lining, the endothelium, is full of holes about 70 to 100 nanometres across. Next is a mesh of protein, the basement membrane, which carries negative charge. Outside it, cells called podocytes wrap the capillary with interlocking “feet”, leaving thin slits covered by a very fine filter, the slit diaphragm.

What gets through? Water, salts, glucose, urea and other small molecules pass almost freely. Blood cells are far too big. Most proteins, like albumin, are held back by their size and their negative charge. Protein in the urine is one of the first signs that the filter is damaged.

The push comes from blood pressure (see HeartClear). About 55 mmHg pushes out, the fluid already in the capsule pushes back with about 15, and the proteins left in the blood pull water back with about 30. The net push is only about 10 mmHg, but the filter is so leaky and so large that it makes about 125 mL a minute: about 180 litres a day. Your kidneys filter all of your blood plasma about 60 times a day.

A reverse osmosis purifier also filters with pressure (see ROClear), but it is very different. It pushes about 375 times harder and holds salts back. The glomerulus lets salts through, then the tubule takes back what the body needs. That way the kidney can get rid of any waste, even ones it has never met, as long as they are small.

Every day your kidneys make about 180 litres of filtrate, but you only pass about 1 to 2 litres of urine. The rest, about 99%, is taken back into the blood. This is reabsorption.

The proximal tubule does most of it. It takes back about two thirds of the salt and water, and all of the glucose, as long as blood sugar is normal. If blood glucose climbs above about 180 mg/dL, as in untreated diabetes, the pumps can’t keep up and sugar spills into the urine, dragging water with it. That is why diabetes can make people thirsty and need the toilet often (see PancreasClear).

The loop of Henle is a clever trick called a countercurrent multiplier. Its thick rising side pumps salt out but won’t let water follow. That makes the medulla around it salty, up to about 1,200 mOsm/kg, four times saltier than blood. Water then leaves the falling side, and later the collecting duct, by osmosis (see OsmosisClear), pulled towards the salt.

How much water the collecting duct takes back is set by a hormone, ADH (vasopressin), from the brain. Short of water? ADH rises, water channels open, and you make a little dark urine, down to about 0.5 litres a day. Drunk plenty? ADH falls and you make lots of pale urine. The concentration can range from about 50 to 1,200 mOsm/kg. Most of the water you drink is absorbed by your gut first (see IntestineClear).

Your kidneys don’t just clean blood. They decide how much water, salt, potassium and acid stay in your body, and they send out hormones. Because they see every drop of blood many times a day, they are in the perfect place to keep things steady.

Blood pressure. When pressure or blood volume falls, say after bleeding or losing water, cells in the kidney’s arterioles release renin. Renin starts a chain: it turns angiotensinogen from the liver into angiotensin I, and an enzyme in the lungs (ACE) makes angiotensin II. That squeezes blood vessels, makes you thirsty and tells the adrenal glands to release aldosterone, which makes the kidneys keep salt, and water follows. Pressure rises again. This is negative feedback. (Blood pressure itself is in HeartClear.)

Water. When your blood gets too concentrated, after a salty meal or a hot day, the brain’s pituitary releases ADH and you feel thirsty. The kidneys keep water until things are back to normal.

Other jobs. When blood carries too little oxygen, as at high altitude, the kidneys release erythropoietin (EPO), which tells the bone marrow to make more red blood cells. They turn vitamin D into its active form so you can absorb calcium, keep potassium between about 3.5 and 5 mmol/L, and get rid of acid to hold blood pH near 7.4. Your liver makes urea from spare protein, and the kidneys are how it leaves (see LiverClear).

Kidney problems are common, and many can be prevented or slowed if they are found early. This chapter shares general facts only: for anything about your own health, talk to a doctor.

Kidney stones. If urine holds more of a substance than it can keep dissolved, crystals form, most often calcium oxalate, and can grow into a stone. About 1 in 10 people get one. A stone hurts when it moves into the ureter, especially at its three narrow points. Many small stones, under about 5 mm, pass on their own; bigger ones may need treatment, such as shock waves that break them up. Dilute urine makes crystals less likely, which is why doctors tell people who have had stones to drink plenty of water.

Chronic kidney disease (CKD) means the kidneys slowly lose nephrons over months or years. The most common causes are diabetes and high blood pressure. It usually causes no symptoms until it is advanced, so people at risk get a simple blood test (eGFR) and a urine test for protein. About 788 million adults worldwide have CKD, about 138 million of them in India. Taking painkillers such as ibuprofen (NSAIDs) often, or in large doses, can also harm the kidneys.

Dialysis takes over when the kidneys fail. In haemodialysis, blood flows through a machine filter made of thousands of hollow fibres, usually about 4 hours, 3 times a week. Wastes like urea and potassium diffuse into a cleaning fluid flowing the other way. Peritoneal dialysis uses the lining of your own belly as the filter. A kidney transplant puts a donated kidney low in the belly. You can live well with one healthy kidney, so a living person can donate one.

See a doctor for blood in the urine, pain in the side or back, swelling of the legs or face, foamy urine, or big changes in how often you pass urine. If you have diabetes or high blood pressure, ask about kidney tests.