Topic 13: Excretion in humans

Every cell in your body produces waste, and some of it is toxic. Excretion is how the body removes these harmful by-products of metabolism before they build up to dangerous levels. The star organs here are the kidneys, two bean-shaped filters that clean roughly your entire blood volume many times each day, deciding with remarkable precision what to keep and what to flush away as urine.

What excretion is and why it matters

Excretion is the removal from the body of the toxic waste products of metabolism and substances in excess of requirements. It is different from egestion, which is the removal of undigested food in faeces – that material was never part of the body’s chemistry.

• Carbon dioxide from respiration is excreted by the lungs.
• Excess water, ions and urea are excreted by the kidneys in urine.
• Urea is made in the liver when excess amino acids are broken down, a process called deamination.

Because the body cannot store protein, surplus amino acids are not wasted as protein but are converted: the nitrogen-containing part forms urea, which is toxic and must be removed by the kidneys.

The kidney and the nephron

Blood enters each kidney through the renal artery and leaves, cleaned, through the renal vein. Inside, around a million tiny tubules called nephrons do the work. Each nephron carries out two key stages.

• Filtration: in the glomerulus, high blood pressure forces small molecules – water, glucose, ions and urea – out of the blood and into the tubule. Large items like blood cells and proteins stay behind.
• Selective reabsorption: as the filtrate flows along the tubule, all the glucose, plus useful amounts of water and ions, are reabsorbed back into the blood. Urea and excess water and salts remain to form urine.

Urine drains down each ureter to the bladder, where it is stored before leaving through the urethra.

Kidney failure: dialysis and transplants

If the kidneys fail, urea and excess water are not removed and the patient becomes very ill. Two treatments are used.

• Dialysis: the patient’s blood flows on one side of a partially permeable membrane, with dialysis fluid on the other. The fluid contains normal blood levels of glucose and ions but no urea, so urea diffuses out of the blood down its concentration gradient while useful substances are kept.
• Transplant: a healthy donor kidney replaces the failed ones. It offers a long-term cure but requires a good tissue match and drugs to prevent rejection by the immune system.

Dialysis is regular and time-consuming; a transplant frees the patient from this but depends on donor availability.