A. EXCRETION. Cellular activities, such as respiration, produce wastes. Some of these wastes are harmful to the body and must be removed. An upset in the balance of body chemicals can lead to serious health problems or death. Excretion is the removal of the wastes of cell activities from the body. Do not confuse excretion with egestion. The wastes of excretion are liquids and gases. Solid wastes are removed during egestion.

B. METABOLIC WASTES. Wastes produced by life activities (metabolism) are called metabolic wastes. Carbon dioxide, water, urea, and mineral salts are metabolic wastes. Carbon dioxide is a gaseous waste formed during aerobic respiration. Water is produced by aerobic respiration and other cellular activities. Urea is a nitrogen waste resulting from the breakdown of the amino acids produced during protein digestion. The breakdown of various compounds in the cell form mineral salts.


carbon dioxide                      aerobic respiration

water                                    aerobic respiration and other cellular activities

urea                              breakdown of amino acids from protein digestion

mineral salts                      breakdown of various cell compounds

C. THE ROLE OF BLOOD IN EXCRETION. The wastes of excretion leave body cells and are secreted into intercellular fluid. Wastes pass from intercellular fluid into the blood plasma by the process of diffusion. The blood plasma transports these excretions to excretory organs that remove them from the body.

D. THE EXCRETORY SYSTEM. The excretory system removes cellular wastes from the body. By getting rid of wastes, the excretory system helps maintain a proper balance of body chemicals. The organs of the excretory system are the skin, lungs, urinary system and liver.

E. SKIN. The skin is made up of an outer layer called the epidermis and an inner layer called the dermis (Figure 11-1). The skin contains sweat glands that excrete perspiration through skin pores. Perspiration is made up of nitrogenous wastes (urea), salts, and water. The skin is also involved in controlling body temperature. When perspiration evaporates from the skin, heat is absorbed from skin cells. This absorption of heat lowers body temperature, which results in cooling the body. You might have experienced this yourself when, on a hot summer day, you fanned yourself to get cool.

F. LUNGS. The function of the lungs in respiration was discussed in Chapter 10. The lungs also play a role in excretion. Lungs excrete carbon dioxide and water vapor by diffusion and exhalation.

 HUMAN URINARY SYSTEM. Some of the body’s excretions leave by way of the urinary system. The urinary system consists of the kidneys, ureters, urinary bladder, and the urethra (Figure 11-2). The kidneys are bean-shaped organs that lie along the back wall of the abdomen. Each kidney is made up of masses of microscopic subunits called nephrons. The kidneys act as filters in the removal of urea and excess water and salts from the blood. Besides wastes, useful substances diffuse out of the blood into the kidney. The useful substances are returned to the blood before the blood leaves the kidney. Each kidney sends excretions, called urine, into a tube called the ureter. Urine, made up of urea and water, flows through ureters to the sac-like urinary bladder. The bladder stores urine temporarily. The urinary bladder opens into a single tube, the urethra, through which the urine leaves the body.


H. LIVER. The liver is the largest internal organ in the body . It produces urea, a nitrogenous waste, by breaking down excess amino acids. Other functions of the liver include the removal of poisons, such as alcohol, from the blood and the storage of extra sugar in the form of animal starch (glycogen). The liver changes glycogen into glucose and secretes it back into the bloodstream when the body needs energy.

L EXCRETORY SYSTEM DISORDERS. Disorders of the excretory system can involve the kidneys. skin, liver, as well as other parts of the body.

Excretory System Disorders

• Kidney Stones. Kidney stones are collections of solid material that may block the kidneys, ureters. or bladder.

• Gout. Gout is a disease associated with the production and deposition of uric acid crystals in joints. It is a very painful condition that produces symptoms similar to arthritis.

• Uremia. During uremia urea and other wastes are not filtered out of the blood. The body cells become poisoned and there is urine in the blood.

• Skin Problems. One cause of skin problems, such as blackheads and acne, is the clogging of skin pores.

• Cirrhosis of the Liver. cirrhosis of the liver is a disease of the liver caused by damage to its cells. This leads to a type of high blood pressure. which can cause serious complications. The most common cause is drinking large amounts of alcoholic beverages. Hepatitis. an inflammation of the liver, can also lead to this disease. There may be no symptoms in the early stages of the disease. It is usually discovered during routine medical examinations or blood tests. Later symptoms are mild jaundice (yellow skin), edema (fluid collection in tissues), mental confusion and vomiting of blood. th men. enlaigeinent of breasts and lo of body hair can occur. The treatment is to slow down the disease. This is done by eliminating alcoholic beverages. Liver transplants are the only chance of long-term cures.