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Role of Excretion in Other Organs

Role of Lungs in Excretion
Human lungs eliminate around 18 L of CO2 per hour and about 400 ml of water per day in normal resting condition. Water loss via the lungs is small in hot humid climate and large in cold dry climates. The rate of ventilation and ventilation pattern (i.e. breathing through mouth or nose) also affect the water loss through the lungs. Different volatile materials are also readily eliminated though the lungs.

Role of Skin in Excretion
Human skin possesses glands for secreting two fluids on its surface, viz. sweat from sweat glands, and sebum from sebaceous glands. Sweat is an aqueous fluid (around 99.5% water) containing NaCl, lactic acid, urea, amino acids and glucose. The volume of sweat varies from negligible to 14L a day, rising with activity and temperature. However, the principal function of sweat is the evaporative cooling of the body surface. Sebum is a waxy protective secretion to keep the skin oily and this secretion eliminates some lipids, such as waxes, sterols, other hydrocarbons and fatty acids. Integument in many aquatic animals is excretes ammonia into the surrounding medium by diffusion.

Role of Liver in Excretion
Liver is the main site or elimination of cholesterol, bile pigments (bilrubin and biliviridin), inactivated products of steroid hormones, some vitamins and many drugs. Liver secretes these substances in bile. Bile in turn, carries these materials to the intestine which are ultimately eliminated with the faeces.

Excretory Organs Products Eliminated

Excretory Organs

Products Eliminated

Primary Secondary


Water, nitrogenous wastes from protein catabolism and inorganic salts

Heat and carbon dioxide


Carbon dioxide

Heat and water


Carbon dioxide, water, slats and urea


Alimentary canal

Solid wastes and secretions

Carbon dioxide, water, salts and heat

Abnormal Constituents of Urine
If the body's chemical processes are not operating efficiently, traces of substances, not normally found may appear in the urine, or normal constituents may appear in abnormal amounts. An analysis of the physical, chemical and microscopic properties of urine often provides information about the condition of the body. Such an analysis is called urinalysis.

Albumin is a normal constituent of plasma; it usually appears only in small amounts in the urine because the particles are too large to pass through the capillary network of the glomerulus. The presence of excessive amounts of albumin in the urine is called albuminuria which indicates an increase in the permeability of the glomerular membrane.

The presence of glucose in the urine is called glycosuria. Normal urine contains very small amounts of glucose and clinically it may be considered absent. The most common cause of glycosuria is a high blood sugar level. If more than the required carbohydrate is ingested and is filtered into the glomerular filtrate, kidney tubules fail to reabsorb all of this excess quantity of glucose from the glomerular filtrate and some of it is excreted along with urine. This condition is called temporary (alimentary) glycosuria which is not pathological. A pathological glycosuria results from diabetes mellitus. In this case there is a frequent and continuous elimination of glucose because the pancreas fails to produce sufficient amounts of insulin, required to break down the glucose.

The appearance of red blood cells in the urine is called haematuria. Haematuria generally indicates a pathological condition. One cause of haematuria is acute inflammation of the urinary organs as a result of disease or irritation from kidney stones.

The presence of leucocytes and other components of pus in the urine, referred to as pyuria indicates infection in the kidney or other urinary organs.

Ketone Bodies
Ketone (acetone) bodies appear in normal urine in small amounts. Their appearance in high quantities is a condition called ketosis or acetonuria. Ketosis may indicate abnormalities. It may be caused by diabetes mellitus, starvation or simply too little carbohydrates in the diet. Whatever may be the cause, the excessive amounts of fatty acids are oxidised in the liver and the ketone bodies are filtered from plasma into the glomerular filtrate and urine.

Microscopic examination of urine may reveal casts, which are tiny masses of material that have hardened and assumed the shape of the lumens of tubules and were then flushed out of the tubules by a build up of filtrate behind them. Casts are named according to the material which forms them. Thus, there are epithelial casts, white-blood cell casts and red-blood cell casts.

In a properly collected and processed specimen of urine the finding of bacteria may be of considerable importance. If bacteria are seen in a centrifuged specimen, but not in the sample, it suggests a bacterial count of less than 10,000/ml. The various bacteria in the urine are identified by different microbiological tests. The information regarding the number and the type of bacteria in the urine give clues to certain diseases.

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