Excretion

Excretion is the process of removing toxic materials, the waste products of metabolism (chemical reactions in cells), and substances in excess of requirements from the body. It's crucial because these waste products, like urea and carbon dioxide, can be poisonous if they build up. Excretion is different from egestion, which is the removal of undigested food (faeces) from the gut. The main human excretory organs are the kidneys (excreting urea, water, salts), lungs (excreting carbon dioxide), and skin (excreting sweat, which contains water, salts, and a little urea).

The urinary system is responsible for producing, storing, and eliminating urine. It consists of four main parts:

1. Kidneys (two): Bean-shaped organs that filter waste from the blood to produce urine.
2. Ureters (two): Tubes that carry urine from each kidney to the bladder.
3. Bladder (one): A muscular sac that stores urine, allowing urination to be infrequent and voluntary.
4. Urethra (one): A tube through which urine passes out of the body.

Blood enters the kidney via the renal artery (bringing waste) and leaves via the renal vein (with waste removed).

While the kidneys excrete urea, it's the liver that actually produces it. Humans cannot store excess amino acids from protein digestion. Instead, these excess amino acids are transported to the liver. Here, they undergo a process called deamination. The nitrogen-containing part of the amino acid is removed and converted into ammonia, which is highly toxic. The liver immediately converts this ammonia into urea, which is much less toxic. The urea is then released into the bloodstream to be transported to the kidneys for excretion. The remaining part of the amino acid (a carbohydrate) can be used for respiration or stored as glycogen.

Inside each kidney are about a million tiny filtering units called nephrons. The first step of urine formation happens in the top part of the nephron, a process called ultrafiltration. Blood arrives at high pressure in a knot of capillaries called the glomerulus. This high pressure forces small molecules out of the blood and into a cup-shaped structure called the Bowman's capsule. This filtered fluid is called the glomerular filtrate. It contains water, urea, glucose, salts, and other small molecules. Large components like red blood cells, white blood cells, platelets, and large protein molecules are too big to pass through the filter and remain in the blood.

After ultrafiltration, the glomerular filtrate contains useful substances (glucose, water, salts) that the body cannot afford to lose. The second stage, selective reabsorption, is the process of taking these useful substances back into the blood. As the filtrate flows along the nephron tubule:

• All glucose is reabsorbed by active transport. This requires energy.
• Some salts are reabsorbed, also by active transport, depending on the body's needs.
• Some water is reabsorbed by osmosis, following the salts and glucose that were moved back into the blood. The amount of water reabsorbed is controlled by the hormone ADH to maintain the correct water balance (osmoregulation).

The substances left behind—urea, excess salts, and excess water—continue down the tubule to form urine.

If both kidneys fail, urea and other toxins build up in the blood, which is fatal if untreated. There are two main treatments: dialysis and kidney transplant. Dialysis: The patient's blood is passed through a machine containing a partially permeable membrane. The blood flows on one side of the membrane, and dialysis fluid flows on the other. The dialysis fluid has no urea, so urea diffuses from the blood into the fluid. It has the same concentration of glucose and salts as normal blood, so these useful substances are not lost. This process must be repeated for several hours, multiple times a week. Kidney Transplant: This is a surgical procedure to place a healthy kidney from a donor into the patient's body. The donor can be living or recently deceased. The donor kidney must be a close tissue match to prevent rejection by the patient's immune system, and the patient must take immunosuppressant drugs for life.