The Excretory System
The excretory system, also known as the urinary system, is the group of organs by which the body disposes of wastes. Excretion is a process which involves many systems of the body, such as digestive and circulatory systems. However, the main organs which make up this system are the kidneys, bladder, and ureters and urethra.
The Kidneys
The kidneys act as filters for all of the body's waste materials. Contrary to a common misconception, the kidneys are not really located in the lower back. They are farther up in the abdominal cavity, so that they are partially protected by the ribs. Each kidney is surrounded by layers of adipose tissue (fat), to cushion them against blows and to keep them in their proper place within the body. Internally, each kidney is divided into three distinct regions, the renal cortex, the renal medulla, and the renal columns. The filtering system of a kidney works through the actions of tiny structures called nephrons, which are basically a knot of capillaries surrounding a thin renal tubule. Each kidney contains approximately one million nephrons. Capillaries have a very thin wall, and thus molecules are able to diffuse through this wall, either entering or leaving a capillary. This is the way by which oxygen is transferred to our blood in the lungs. In the kidneys, however, wastes and blood plasma are drawn from the blood and passed into the renal tubules. Structures called microvilli in the tubule remove any remaining substances which could be useful to the body. The remaining wastes are then collected and expelled from the kidney as urine. A tube called a ureter connects each kidney to the bladder. The wall of the ureter is composed of smooth muscle, so that the tube itself can transport the urine by peristalsis, rather than depending entirely on gravity.
The Bladder
The urinary bladder is an elastic sac, composed of three layers of smooth muscle tissue. Its single function is to store and remove the body's liquid waste. A single tube called the urethra allows the bladder to expel the waste from the body. The opening from the bladder into the urethra is sealed off by two sphicters (rings of muscle), one voluntary and one involuntary. Like the ureters, the urethra is made of smooth muscle to allow for efficient disposal of waste.
written by Lyle Mullican
