Urinary system

All sorts of wastes are constantly being formed as a result of the processes which take place in the body. In contrast to unicellular organisms, which pass their wastes directly into their surroundings, human beings require special organs for this. The liver and the kidneys are specialized in the excretion function, but the skin and the lungs also play a part by excreting wastes in sweat and breath. Through the liver and the kidneys we also get rid of other things which have entered the body from outside, such as drugs. Another quality which distinguishes us from unicellular organisms is our ability to maintain an internal environment which differs from the external environment. Unicellular organisms adapt to the external environment; they are not able to close themselves off sufficiently. Humans can. The kidneys are the organs which enable us to keep our internal environment in equilibrium, a process known as homeostasis. The quantities of certain substance, such as salts and acids, in our bodies is accurately regulated, but blood pressure is also regulated via the kidneys by controlling the salt and water in the body. Because of the kidneys’ purification function the urine can be used to obtain information about the state of the body and possible treatment. In sports, for example, the urine is examined to detect doping, and women’s urine is examined to determine whether they are pregnant. Urine is also frequently checked for sugar, because sugar in the urine is one of the clearest signs of diabetes.


Excretion is the elimination of waste materials from the body. This vital function is mainly performed by the kidneys, but the intestines, the liver, the skin and the lungs also play a part in this process. It is an essential requirement of all organisms that their internal chemical composition is maintained within certain limits. In the human body the pH (degree of acidity), osmolarity (concentration of dissolved chemicals) and water content of the blood must be finely controlled. Toxic substances, such as accidentally ingested poisons, drugs or certain products of metabolism potentially damaging to the tissues, must also be promptly excreted. To remain healthy both the quantity and quality of the blood, and consequently the tissue fluids, must be maintained within narrow limits.

Organs of excretion

The liver can be thought of as the chemical factory of the body, and one of its major roles is to convert drugs, hormones and waste products of metabolism into a form that can be excreted by the kidneys. This often involves the conversion of fat soluble substances into water soluble ones. The liver can also act directly as an organ of excretion by expelling substances into the bile, a liquid that is stored in the gall bladder and released into the duodenum, the first part of the small intestine, during the digestion of foods. Whereas the intestine is primarily an organ of absorption, it also temporarily stores the solid residue of food or roughage, millions of dead bacteria and ‘discarded’ epithelial cells. These contents are mixed with bile, which is responsible for the characteristic brown colour of faeces, and excreted from the body via the rectum and the anus.

Another major organ involved with excretion is the skin. This is the protective covering of the body, but it is not simply a tough membrane separating the inside of the body from the outside world, but a complex organ in its own right. It contains sweat glands, which are especially numerous in the hair-bearing areas of the body: the scalp, armpits and pubic region. In hot conditions the skin helps the body to maintain a stable temperature by sweating. Water and salt are lost together with excess heat. Some drugs and food substances such as garlic and curry are also excreted in the sweat.

As well as breathing air in and out, the lungs remove carbon dioxide produced in the tissues, and this directly influences the chemical composition of the blood. This process is another form of excretion. Some water is also excreted from the lungs in the form of water vapour, which is why you can see your breath as a white cloud of water droplets on a frosty morning. The most important organ of excretion, though, is the kidney. It functions by filtering out from the blood all its components except protein and circulating cells. From this filtrate, the kidney then selectively reab-sorbes most of the water and electrolytes (such as sodium salts). Waste materials and excess water are left behind. A special concentrating mechanism extracts more water from the filtrate, and some substances are actively secreted into the final concentrated fluid – urine. By these means the kidney removes unwanted substances and retains just the correct amount of water and dissolved chemcicals to maintain the internal chemical balance of the body. The composition of the urine – the presence or absence in it of certain chemical substances – can therefore indicate the body’s state of health. For this reason, analysis of a sample of urine is often used as an aid to diagnosis. For example, the presence in the urine of certain hormones indicates that a woman is pregnant; the presence of sugar (glucose) may be indicative of diabetes mellitus; and albumin or other proteins in the urine (proteinuria) may be only a temporary, harmless condition or it can be a symptom of a kidney disorder. Bacteria in the urine may be caused by cystitis or some other inflammatory infec- tion of the urinary tract, whereas blood in the urine (haematuria) generally indicates a kidney disorder.