The Importance of the Kidneys in the Excretory System

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The excretory system involves a process which removes unwanted products from the body's chemistry. In general, excretion is the process of separating wastes from the body fluids and eliminating them. Examples 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 getting rid of undigested foods from the body... via the colon.)

There are four (4) organ systems which are responsible for excretion:

• 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 the 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 fourth excretory system is the urinary system. We will place most of our emphasis on this organ system. It excretes a wide variety of metabolic wastes, toxins, drugs, salts, water, hormones, and hydrogen irons.

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

DIAGRAM OF THE URINARY EXCRETORY SYSTEM

BLOOD SUPPLY OF THE URINARY EXCRETORY SYSTEM

The kidney's unique anatomy allows it to efficiently discharge its functions. The kidneys must have a good blood supply in order to perform their functions properly. For this reason the kidneys contain a lot of blood vessels. Both kidneys together 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 your entire volume of blood gets filtered 20 to 25 times each day!

THE FILTERING PROCESS OF THE URINARY EXCRETORY SYSTEM

Once the blood enters the kidneys the filtering process begins. Water and other substances, such as amino acids, glucose and waste products (urea and uric acid) are removed from the blood. This process is performed by microscopic structured within the kidneys known as nephrons. Each kidney contains over one million nephrons.

The filtered blood moves through tiny blood vessels called capillaries, in the nephrons and exits the kidneys through the Renal Vein.

The substances which 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, via the urethra.

The urinary excretory system performs one of the most important operations in which the kidneys play a significant role. Should the kidney not be able to perform this critical function, the results would be contamination of the blood... leading to serious life-threatening diseases.

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