The Excretory System

The excretory system is responsible for removing metabolic waste products from the body's chemistry. The process of excretion involves the separation of wastes from the body's tissues and fluids, and eliminating them.

These waste products are generated as by-products of metabolism.

Metabolism is a process of chemical changes within cells and tissues that is necessary for maintaining life and growth. It consists of two (2) metabolic operations: anabolism and catabolism.

Anabolism is a process of chemical exchanges in which molecules are synthesized or constructed from simpler units to more complex forms. Examples are the building of muscles or bone growth.

Catabolism works in the opposite manner to anabolism. Instead of building up molecules this process breaks them down from complex forms into simpler units, usually resulting in the release of energy. Protein digestion is an example of catabolism.

The lowest or minimum level of cell chemistry that is required to maintain the body's vital processes (such as respiration and heartbeat) is referred to as Basal Metabolism.

The many chemical exchanges that take place during the metabolic process will inevitably produce chemical wastes. If these waste products are allowed to accumulate in the body, they can become toxic and harm the body's organs and tissues. Consequently, excretion is necessary in order to separate these wastes from the tissues and eliminate them from the body.

Examples of some waste products eliminated by the excretory system include the removal of carbon dioxide in the air we exhale, and unwanted nitrogen in sweat, nails and hair.

Don't confuse excretion with defecation. Defecation is a separate waste elimination process and involves getting rid of undigested foods from the body... via the colon.

The excretory system consists of four (4) main organ systems that are responsible for eliminating metabolic wastes form the body. They are:

  • The respiratory system - helps to excrete carbon dioxide, water, and other gases when we breathe.
  • The integumentary system - also excretes some water in addition to inorganic salts, lactic acid, and urea in sweat.
  • The digestive system - not only eliminates food residue (which is not a process of excretion) but also excretes water, salts, carbon dioxide, lipids, bile pigments, and cholesterol.
  • The urinary excretory system - excretes a wide variety of metabolic wastes, toxins, drugs, salts, water, hormones, and hydrogen irons, via the urine. The kidneys are the main organs of this category of the body's excretory system.

The following diagram gives an illustration of the structure of the urinary excretory system.

Urinary Excretory System

Blood Supply of the Urinary Excretory System 

The kidney's unique anatomy allows it to effectively discharge its many functions. In order to function efficiently, the kidneys must have a very good blood supply. For this reason the kidneys contain lots of blood vessels. Together, both kidneys contain over 160 km of blood vessels.

Blood is supplied to the kidneys by the Renal Artery. On average, the kidneys account for only 0.4% of body weight, however, they receive approximately 21% of cardiac output. Over 180 liters (50 gal) of blood is supplied to the kidneys each day. Considering that a human adult body contains 7 to 8 liters of blood, this means that the entire volume of blood gets filtered 20 to 25 times each day!

The Excretion Process of the Urinary System

Once blood enters the kidneys the filtering process begins. Through a process of chemical exchanges, excess salt, water, amino acids, glucose and other waste products such as urea and uric acid are removed from the blood. Substances that are useful to the body are returned to the blood.


This filtering process is performed by microscopic structures within the kidneys known as nephrons. Each kidney contains over one million nephrons.

The filtered blood moves through tiny blood vessels in the nephrons called capillaries. It then exits the kidneys through the Renal Vein and is transported throughout the rest of the body.

The substances that are extracted from the blood during the filtering process move through a tube in the nephron, called the renal tubule. (The two kidneys contain about 16 km of tubules.) Those substances which are useful to the body, such as water, salt, glucose and amino acids are reabsorbed into the blood, through the capillaries. Just enough water and salt are reabsorbed to give the blood its correct composition.

The unwanted substances, such as urea, uric acid, excess water and salt remain in the renal tubule and eventually pass into the ureters. (The ureters are approximately 25 to 30 cm (10 to 12 in) long and about 0.5 cm (0.2 in) in diameter.) The waste products form what is known as urine. It moves along the ureters into the bladder where it is eventually expelled from the body, through the urethra.

The urinary system is an essential element of the body's excretory system. It plays a vital role in eliminating dangerous toxin and other waste substances, and helps to maintain homeostasis in the body.

Homeostasis (ho-me-oh-stay-sis) is the ability of the body to maintain internal stability, even when the environment around it changes. In a state of homeostasis, the body is able to detect harmful changes in its environment and activate mechanisms that counteract them.

If the urinary excretory system becomes damaged or is unable to perform its critical function, the results would be contamination of the blood... leading to serious life-threatening diseases. If the process of excretion is significantly inhibited, toxins begin to accumulate in the body, resulting in serious consequences to the body's organs and tissues.

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