Where pee comes from
Your kidneys clean your blood all day long. The waste they take out becomes pee.
Meet the kidneys
You have two kidneys. Each one is shaped like a bean and is about as big as your fist. They sit high up at the back of your tummy, one on each side of your backbone, tucked under your ribs. Their job is to clean your blood. The waste they take out becomes pee. It runs down two thin tubes into a stretchy bag, the bladder, and leaves your body through one more tube.
The urinary system is two kidneys, two ureters, the bladder and the urethra. The kidneys lie against the back wall of the abdomen, behind the peritoneum (the lining of the belly, so they are called retroperitoneal), at about the level of the 12th thoracic to the 3rd lumbar vertebra, their upper ends sheltered by the lowest ribs; the right one usually sits a little lower because the liver is above it. Each is about 10 to 12 cm long and weighs about 150 g. The ureters are muscular tubes about 25 to 30 cm long that run down to the back of the bladder. The urethra carries urine out: in men it is about 20 cm long and runs through the prostate and the penis (the body shown here); in women it is only about 4 cm. The kidneys in this model come from the Human Reference Atlas and are fitted to this body.
Dirty in, clean out
Blood rushes into each kidney through a short, wide tube that comes straight off the biggest pipe in your body. Inside, the kidney sieves out the waste, and any water and salt your body does not need. The yellow specks are the waste. Cleaner blood goes back out, through a vein. The waste and extra water drip down the thin tubes as pee. Your kidneys get about a fifth of all the blood your heart pumps.
Each renal artery branches off the abdominal aorta at about the first lumbar vertebra; the right one passes behind the inferior vena cava. Together the kidneys receive about a fifth of the heart’s output, roughly a litre of blood a minute, far more than their size alone would need. They remove urea (left over from breaking down protein), creatinine, extra salts and acid, and set how much water the body keeps. The renal veins drain into the inferior vena cava; the left one is longer and crosses in front of the aorta, as you can see here. The kidneys also make hormones that help control blood pressure, tell the bone marrow to make red blood cells, and switch on vitamin D for strong bones.
The tank
Pee trickles down the two tubes, a little squirt at a time, day and night. It collects in the bladder, a stretchy bag of muscle that swells slowly as it fills, like a balloon. As it fills, it sends messages to your brain, and when it is getting full you feel it: time to find a toilet! When you go, the bladder squeezes and the pee rushes out. A grown-up makes about 1.5 litres of pee a day.
Waves of muscle (peristalsis) push urine down the ureters in small spurts, a few times a minute. The ureters enter the bladder wall at a slant, so a full or squeezing bladder presses them shut, which helps stop urine flowing back up. The bladder wall is a muscle, the detrusor, which stays relaxed while it fills; an adult bladder holds about 1.5 to 2 cups. Stretch sensors in the wall tell the brain how full it is, and the urge comes well before it is full. To urinate, the brain relaxes the sphincter muscles at the bladder neck and around the urethra, and the detrusor contracts. Adults pass about 1 to 2 litres of urine a day, depending mostly on how much they drink. The filling here is sped up a lot: really it takes hours.
A million tiny filters
Each kidney is packed with about a million tiny filters, far too small to see. Each one is a little ball of blood vessels with a long, twisty tube attached. Water and small bits leak out of the blood into the tube: about a bathtub full every day! Then the tube takes back almost all of it, the water, sugar and salt your body needs. What is left is a little water with the waste and extra salt in it. That is pee. It drips into little cups inside the kidney, then into a funnel, and down to the bladder.
The working unit is the nephron, about a million per kidney. Blood is filtered in the glomerulus, a knot of capillaries in the outer layer, the cortex: water, salts, glucose and waste pass into the tubule, while blood cells and most proteins stay in the blood. Together the two kidneys filter about 125 mL a minute, roughly 180 litres a day, yet only 1 to 2 litres leave as urine: the tubule takes back about 99% of the water, nearly all the glucose and most of the salt. Its loop of Henle dips into the medulla, whose cone-shaped pyramids are shown here, and helps make the tissue there salty, so water can be drawn out of the ducts that pass through it. How much is drawn out is set by ADH, a hormone made in the hypothalamus and released by the back of the pituitary: it makes the collecting ducts let more water through. Collecting ducts run down each pyramid and open at its tip, the papilla, into a minor calyx. The calyces join into major calyces and the renal pelvis, the funnel that leads into the ureter.