Excretion in humans

Excretion is the process of removing waste products that have been made by the body's cells during metabolic reactions. These wastes, if allowed to build up, can be toxic. The main metabolic wastes in humans are carbon dioxide (from aerobic respiration) and urea (from the breakdown of excess amino acids in the liver). It's also the removal of substances in excess of the body's requirements, like excess water and salts. This is different from egestion, which is the removal of undigested food (faeces) that has never been part of the body's cells.

The urinary system is responsible for producing, storing, and eliminating urine. Blood enters the two kidneys via the renal artery. The kidneys filter the blood and produce urine. This urine travels down two tubes called the ureters to the bladder, which is a muscular sac that stores urine. When the bladder is full, a ring of muscle called a sphincter relaxes, and urine is expelled from the body through the urethra. The filtered, 'cleaner' blood leaves each kidney through the renal vein. Therefore, blood in the renal artery has a high concentration of urea, while blood in the renal vein has a very low concentration of urea.

Each kidney contains about a million microscopic filtering units called nephrons. Blood from the renal artery enters a tiny knot of capillaries called the glomerulus, which is surrounded by the Bowman's capsule. The blood vessel leading into the glomerulus (afferent arteriole) is wider than the one leading out (efferent arteriole). This difference creates very high pressure in the glomerulus. This high pressure forces water, urea, glucose, amino acids, and mineral ions out of the blood and into the Bowman's capsule. This process is called ultrafiltration. Large molecules like proteins and all blood cells are too big to pass through the filter, so they remain in the blood.

After ultrafiltration, the glomerular filtrate contains useful substances like glucose, amino acids, and some water and salts that the body needs to keep. If these were lost in urine, we would die quickly. Selective reabsorption is the process of taking these useful substances back into the blood. All of the glucose and amino acids are reabsorbed from the first part of the tubule (the proximal convoluted tubule) back into the blood capillaries. This process requires energy and is done by active transport. The amount of water and mineral ions reabsorbed is variable and depends on the body's needs. This reabsorption happens along the rest of the nephron (loop of Henle, distal tubule, and collecting duct) and is controlled by hormones. The substances left behind (urea, excess water, excess salts) form urine.

Osmoregulation is the control of the water potential of the body's fluids, like blood plasma. This is a key part of homeostasis. When the blood becomes too concentrated (e.g., you are dehydrated), this is detected by the hypothalamus in the brain. The hypothalamus then signals the pituitary gland to release more Anti-Diuretic Hormone (ADH). ADH travels in the blood to the kidneys and makes the walls of the collecting ducts and distal tubules more permeable to water. This causes more water to be reabsorbed from the filtrate back into the blood, producing a small volume of concentrated urine. Conversely, if you drink a lot of water, the blood becomes too dilute, less ADH is released, the collecting ducts become less permeable, less water is reabsorbed, and you produce a large volume of dilute urine. This is an example of negative feedback.

Kidney failure occurs when the kidneys can no longer filter waste products from the blood effectively. This leads to a build-up of toxic urea and uncontrolled levels of water and salts in the body, which can be fatal. The two main treatments are dialysis and kidney transplant.

Dialysis: A patient's blood is passed through a machine containing a partially permeable membrane. On the other side of the membrane is dialysis fluid. This fluid contains the same concentration of useful substances (like glucose and some salts) as healthy blood, but no urea. This allows urea and excess salts to diffuse out of the blood and into the dialysis fluid down a steep concentration gradient, while preventing the loss of useful substances. Patients need this treatment several times a week.

Kidney Transplant: This involves surgically placing a healthy kidney from a donor into the patient. The donor is often a person who has recently died or a living person with a compatible tissue type. The main risk is rejection, where the recipient's immune system attacks the new kidney as a foreign object. To prevent this, patients must take immunosuppressant drugs for the rest of their lives.