Excretory Products and Their Elimination

As the body carries out its activities, it produces wastes that would be harmful if they built up — especially nitrogen-containing wastes from breaking down proteins. Excretion is the removal of these harmful metabolic wastes from the body. The main nitrogenous waste in humans is urea, which is made in the liver from ammonia and removed by the kidneys. (Animals that excrete mainly urea, like humans, are called ureotelic; those excreting ammonia are ammonotelic, and those excreting uric acid, like birds, are uricotelic.)

The main organ system for excretion in humans is the excretory (urinary) system, which consists of:

• A pair of bean-shaped kidneys — which filter the blood and make urine.
• Two ureters — tubes that carry urine from the kidneys to the bladder.
• The urinary bladder — a muscular bag that stores urine.
• The urethra — the tube through which urine leaves the body.

Each kidney is made of about a million tiny filtering units called nephrons — the functional units of the kidney. A nephron has a cup-shaped Bowman's capsule enclosing a knot of capillaries (the glomerulus), followed by a long coiled tubule. Besides the kidneys, the lungs remove CO₂ and water vapour, the skin removes salts and water as sweat, and the liver removes some wastes through bile.

The kidneys make urine from the blood in three steps that take place along the nephron:

• Glomerular filtration — blood under pressure is filtered in the glomerulus and Bowman's capsule. Water and small molecules (glucose, salts, urea) pass through into the tubule, while blood cells and large proteins are held back. The filtered liquid is called the glomerular filtrate.
• Reabsorption (tubular reabsorption) — as the filtrate flows along the tubule, the useful substances the body needs — all the glucose, most of the water, and many salts — are reabsorbed back into the blood. This is why we do not lose valuable nutrients.
• Secretion (tubular secretion) — some extra wastes and ions (like H⁺ and K⁺) are actively added from the blood into the tubule. What remains is urine.

Out of about 180 litres of filtrate formed each day, only about 1.5 litres become urine — the rest is reabsorbed. The urine then passes to the bladder and is released.

The kidneys also keep the body's water and salt balance steady — a process called osmoregulation. When the body is short of water, a hormone (ADH, the anti-diuretic hormone) makes the kidney reabsorb more water, so the urine is concentrated and small in amount. When there is plenty of water, less ADH is released, so more dilute urine is made. In this way the kidney constantly adjusts how much water to keep and how much to remove, keeping the internal environment stable (homeostasis).

Because the kidneys filter the blood and control its composition, kidney problems can make harmful wastes build up in the body. Some important kidney disorders:

• Kidney (renal) failure — the kidneys stop filtering the blood properly, so wastes like urea build up in the blood. It can be sudden (acute) or long-term (chronic).
• Kidney stones (renal calculi) — hard crystals of salts that form in the kidney, causing severe pain and blocking the flow of urine.
• Infections of the urinary tract.
• Presence of glucose or protein in the urine, which can signal diabetes or kidney damage.

When the kidneys fail badly, the blood must be cleaned artificially by a process called dialysis. In haemodialysis, the patient's blood is passed through a machine (an artificial kidney) where it flows past a selectively permeable membrane; wastes such as urea diffuse out into a special dialysing fluid while useful substances are kept, and the cleaned blood is returned to the body. Dialysis keeps a patient alive but must be repeated regularly. The long-term cure for permanent kidney failure is a kidney transplant, in which a healthy kidney from a matched donor replaces the failed one. To keep the kidneys healthy, drink enough water, eat a balanced diet that is not too high in salt or protein, and treat infections promptly.