Excretion in Humans

Excretion and egestion are not the same thing

You get rid of waste all day, but not all of it counts as excretion. Excretion is the removal from the body of the waste products of metabolism, together with substances that are present in excess of the body's requirements. The key word is metabolism: excretion deals with waste that the body's own cells have made, or with useful substances that are simply surplus to need.

Egestion is a completely different process. It is the removal of undigested food from the alimentary canal as faeces. That material was never absorbed into the body's cells, so it was never part of metabolism at all. This is why egestion is not excretion, and why the examiner marks the two words separately.

The three waste products you must know are carbon dioxide, urea, and excess salts and water. Carbon dioxide is made by every respiring cell, urea is made in the liver, and excess salts and water are taken in with food and drink and are simply not needed.

The lungs excrete carbon dioxide

Aerobic respiration produces carbon dioxide as a waste product in every cell of the body: glucose + oxygen -> carbon dioxide + water. Carbon dioxide dissolves in the blood to form an acid, so if it were allowed to build up it would make the blood dangerously acidic and would poison the enzymes in the cells.

The lungs are the organs that remove it. Carbon dioxide diffuses out of the blood in the capillaries around the alveoli and into the air inside the alveoli, and it leaves the body in the air you breathe out. That is why exhaled air contains far more carbon dioxide than inhaled air.

Notice that the lungs are an excretory organ as well as a gas exchange surface. The same diffusion that brings oxygen in carries the carbon dioxide waste out - one process doing two jobs.

The urinary system: kidneys, ureters, bladder and urethra

The kidneys are two bean-shaped organs at the back of the abdomen, one on each side of the spine. They filter the blood and excrete urea, excess ions such as sodium, and excess water. The fluid that leaves a kidney carrying these wastes is urine.

Urine passes out of each kidney down a tube called a ureter to the bladder, a muscular bag that stores urine until it is convenient to release it. From the bladder, urine leaves the body through a single tube called the urethra.

Learn the order - kidney -> ureter -> bladder -> urethra - because identifying these four structures on a diagram is a Core skill. The commonest error is to confuse the ureter with the urethra: the ureter carries urine from the kidney to the bladder, while the urethra carries it from the bladder out of the body.

Urea is made in the liver from excess amino acids

Protein in your food is digested to amino acids, which are absorbed into the blood and carried to the liver. The body cannot store amino acids and cannot excrete them unchanged, so any amino acid that is not needed must be dealt with.

The liver first uses the amino acids it needs, assimilating them by converting them into the body's own proteins. The surplus amino acids are then broken down by deamination. Deamination is the removal of the nitrogen-containing part of the amino acid; that nitrogen-containing part is converted into urea, and the rest of the molecule is respired to release energy.

Urea is toxic, so it must not be allowed to accumulate. The liver releases it into the blood, and the kidneys remove it in the urine. This is why the importance of excretion is usually summarised as the removal of the toxic waste urea.

Inside the kidney: cortex, medulla and the nephron

A kidney has an outer region called the cortex and an inner region called the medulla, and you must be able to identify both on a diagram. The working units that actually filter the blood are the nephrons - about a million of them in each kidney.

Each nephron starts with a knot of capillaries called the glomerulus, which sits inside a cup-shaped structure called Bowman's capsule. Blood arrives at the glomerulus under high pressure, because the vessel bringing blood in is wider than the vessel taking blood out. That high pressure forces water, glucose, urea and ions out of the blood into Bowman's capsule - a process called ultrafiltration. Large molecules such as proteins, and whole blood cells, are too big to pass through, so they stay in the blood.

As the fluid flows along the tubule, selective reabsorption happens: all of the glucose, most of the water and some of the ions are reabsorbed back into the blood. Glucose is taken back completely because the body cannot afford to lose it, and water is reabsorbed by osmosis. What is left behind is urine - urea, excess water and excess ions - which flows down the collecting duct to the ureter.

ADH and the control of the water content of the blood

The body must keep the water content of the blood constant, or cells will lose or gain too much water by osmosis. The kidneys control this, and the control is supervised by a hormone.

ADH (antidiuretic hormone, also called vasopressin) is released from the pituitary gland. It travels in the blood to the kidneys and makes the walls of the collecting ducts more permeable to water, so more water is reabsorbed into the blood. The result is a smaller volume of more concentrated urine.

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. On a hot day, or if you drink very little, you lose water in sweat and in the air you breathe out, so the blood becomes more concentrated. More ADH is released, more water is reabsorbed, and you produce a small volume of concentrated urine.

When the kidneys fail: dialysis and transplant

If the kidneys stop working, urea and excess ions build up in the blood and the water balance of the body is lost, so the person will die without treatment. There are two treatments: dialysis and a kidney transplant.

Dialysis does the kidney's job outside the body. Blood is taken from the patient and flows through a dialysis machine, separated from a dialysis fluid by a partially permeable membrane. The fluid contains the correct concentration of glucose and salts and no urea. Because there is no urea in the fluid, urea diffuses out of the blood down its concentration gradient. Because the glucose and salt concentrations match those in the blood, there is no net diffusion of glucose or salts, so the body keeps what it needs. Excess ions and water also move out, and the cleaned blood is returned to the patient.

The glucose concentration matters. If the fluid were more concentrated than the blood, water would be drawn out of the blood by osmosis; if it were more dilute, water would move into the blood and the patient would become overhydrated. Matching the concentrations keeps the water potential of the fluid close to that of the blood. Dialysis must be repeated several times a week, because between sessions it cannot respond to the body's changing needs.

A kidney transplant places a healthy kidney from a donor into the patient. It restores full kidney function continuously, so the patient no longer needs dialysis and can eat a more normal diet. The main risk is rejection by the patient's immune system, so the patient must take immunosuppressant drugs, and a donor kidney that matches the patient's tissue type is much more likely to succeed. Dialysis can start straight away and needs no donor, but a transplant that works gives a far better quality of life.