Excretion in humans

Excretion is the process of removing toxic materials, the waste products of metabolism, and substances in excess of requirements from the body. It is vital for maintaining a constant internal environment, a process called homeostasis. Without excretion, toxic substances like urea and carbon dioxide would build up to poisonous levels, denaturing enzymes and disrupting cell function. It's important not to confuse excretion with egestion, which is the removal of undigested food (faeces) from the gut.

The urinary system is the body's primary excretory system for removing nitrogenous waste (urea) and maintaining water balance. It consists of two kidneys, which filter the blood. Blood enters each kidney at high pressure through a renal artery and leaves, cleaned, through a renal vein. The waste fluid, urine, passes from each kidney down a tube called the ureter to the bladder. The bladder is a muscular bag that stores urine. When it is full, a sphincter muscle relaxes, allowing urine to be expelled from the body through the urethra.

Each kidney contains over a million microscopic filtering units called nephrons. The nephron is where the real work of the kidney happens: filtering blood and producing urine. It starts with the Bowman's capsule, a cup-shaped structure surrounding a knot of capillaries called the glomerulus. Blood arrives at the glomerulus under high pressure. This pressure forces water, urea, salts, glucose, and amino acids out of the blood and into the Bowman's capsule, forming the glomerular filtrate. This process is called ultrafiltration. Large molecules like proteins and all blood cells are too big to pass through, so they remain in the blood.

After ultrafiltration, the glomerular filtrate contains useful substances that the body cannot afford to lose. Selective reabsorption is the process where these useful substances are taken back into the blood from the nephron tubule. This happens primarily in the proximal convoluted tubule. All glucose and amino acids, and most of the water and some salts, are reabsorbed. Glucose and amino acids are reabsorbed by active transport, which requires energy, to ensure every last bit is recovered. The remaining fluid, now consisting mainly of water, urea, and excess salts, continues down the nephron to become urine.

Osmoregulation is the control of the water potential of the blood. The kidneys play a crucial role in this by adjusting the volume and concentration of urine. This process is controlled by a hormone called ADH (Anti-diuretic Hormone), which is released from the pituitary gland in the brain. If the blood becomes too concentrated (e.g., due to dehydration or sweating), the pituitary gland releases more ADH. ADH travels in the blood to the kidneys and makes the walls of the collecting ducts and distal convoluted 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 the blood is too dilute, less ADH is released, the collecting ducts become less permeable, and a large volume of dilute urine is produced. This is a classic 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 an uncontrolled balance of water and salts in the body, which can be fatal. One treatment is dialysis, where a machine artificially filters the blood. The patient's blood flows on one side of a partially permeable membrane, while a special dialysis fluid flows on the other side. The membrane allows small waste molecules like urea to diffuse out of the blood and into the dialysis fluid, down a steep concentration gradient. The dialysis fluid has the same concentration of glucose and essential salts as healthy blood, so these useful substances are not lost. It contains no urea, maximising the diffusion gradient for urea out of the blood. Large molecules like proteins and blood cells cannot pass through the membrane.