The urinary system
The urinary system has four parts. Two kidneys, each about the size of a fist, sit against the back wall of the abdomen just below the ribs. They make urine, which flows down two ureters to theurinary bladder, a muscular bag that stores it. Urine leaves the body through theurethra, controlled by ring-shaped sphincter muscles. The kidneys receive about a fifth of all the blood pumped by the heart through the renal arteries.
Inside the kidney
A kidney has an outer cortex and an inner medulla, and a funnel-shapedrenal pelvis that collects urine. About a million microscopic nephrons do the work. Each nephron starts in the cortex with a glomerulus, a knot of capillaries inside a cup calledBowman’s capsule, and continues as a long tubule that dips into the medulla and back before draining into a collecting duct.
The nephron, step by step
- Filtration: blood pressure forces water and small molecules (salts, glucose, amino acids, urea) out of the glomerulus into Bowman’s capsule. Blood cells and large proteins stay in the blood.
- Selective reabsorption: in the proximal tubule, all the glucose and amino acids and about two thirds of the water and salt are taken back into the surrounding capillaries, often by active transport.
- The loop of Henle builds up a very salty medulla: the descending limb loses water, the ascending limb pumps out salt.
- Secretion: in the proximal and distal tubules, some substances such as hydrogen ions, potassium and many drugs are actively moved from the blood into the tubule.
- The collecting duct decides the final amount of water in urine, depending on ADH.
Water balance and ADH
The brain constantly monitors how concentrated the blood is. When you are short of water, the hypothalamus detects it and the pituitary gland releases antidiuretic hormone (ADH). ADH inserts water channels (aquaporins) into the collecting duct, so water flows out into the salty medulla and back into the blood: you produce a small volume of dark, concentrated urine. When you drink a lot, ADH falls, the ducts stay watertight and you produce lots of pale, dilute urine. Human urine can range from about 50 to 1,200 mOsm/kg, but the kidneys must always excrete at least about half a litre a day to remove wastes. This is an example of homeostasis by negative feedback.
Salt, acid and blood pressure
The kidneys also control the levels of sodium, potassium and calcium in the blood, and its acidity, by adjusting reabsorption and secretion. The hormone aldosterone from the adrenal glands increases salt reabsorption. Because water follows salt, the kidneys help control blood volume and therefore blood pressure; they even release a hormone, renin, when pressure falls. They also make erythropoietin, which stimulates red blood cell production, and activate vitamin D.
Kidneys and health
Doctors test urine because it reveals what is happening in the blood: glucose in urine can indicate diabetes, protein can indicate damaged filters. The filtration rate (GFR) is used to measure kidney function. When kidneys fail, wastes and fluid build up, and patients may need dialysis, where a machine filters the blood, or akidney transplant. A person can live a healthy life with only one working kidney.
Key terms
- Filtrate
- The fluid filtered from the blood into the nephron.
- Reabsorption / secretion
- Moving substances from the tubule back into the blood / from the blood into the tubule.
- Osmoregulation
- Controlling the water and salt balance of the body.
- Urea
- The main nitrogen-containing waste, made in the liver from broken-down proteins.
Common misconceptions
- “Kidneys only remove waste.” They also control water, salt, acid, blood pressure and red blood cell production.
- “Urine is made in the bladder.” The bladder only stores it; the kidneys make it.
- “Drinking more water always means healthier kidneys.” Healthy kidneys adjust to a wide range of intakes; extreme amounts can be harmful.
Educational model only. This simulator uses simplified textbook values and cannot assess kidney function, hydration or blood sugar in a real person. It is not medical advice or clinical guidance. If you have concerns about your health, speak to a qualified healthcare professional. See our educational and medical disclaimer.
Key takeaways
- Kidneys → ureters → bladder → urethra.
- Nephrons filter blood, then reabsorb what the body needs and secrete extra wastes.
- Over 99 % of filtered water is reabsorbed; ADH controls how much in the collecting duct.
- The kidneys maintain homeostasis of water, salts, acidity and blood pressure.
Model note: whole-body values use simplified textbook relationships. The nephron animation shows the share of each substance reabsorbed in each region; real nephrons have many more transport processes.