Excretory system

The excretory system is a passive biological system that removes excess, unnecessary or dangerous materials from an organism, so as to help maintain homeostasis within the organism and prevent damage to the body. It is responsible for the elimination of the waste products of metabolism as well as other liquid and gaseous wastes. As most healthy functioning organs produce metabolic and other wastes, the entire organism depends on the function of the system; however, only the organs specifically for the excretion process are considered a part of the excretory system. The excretory system gets rid of waste called urine or "pee".

As it involves several functions that are only superficially related, it is not usually used in more formal classifications of anatomy or function.

Excretory functions

Removes metabolic and liquid toxic wastes as well as excess water from the organism.

Within each kidney there are an estimated one million microscopic nephrons, where blood filtration takes place. Each nephron contains a cluster of capillaries called a glomerulus. A cup-shaped sac called a bowmans capsule surrounds each glomerulus. The blood that flows through the glomerulus is under great pressure. This causes water, glucose and urea to enter the bowmans capsule. White blood cells, red blood cells and proteins remain in the blood. As the blood continues in the excretory system, it passes through the renal tubule. During this time, reabsorption occurs: glucose and chemicals such as potassium, sodium, hydrogen, magnesium and calcium are reabsorbed into the blood. Almost all the water removed during filtration returns to the blood during the reabsorption phase. The kidneys control the amount of liquid in our bodies. Now only wastes are in the nephron. These wastes are called urine and include urea, water and inorganic salts. The cleansed blood goes into veins that carry the blood from the kidneys and back to the heart.

Component organs

Skin

Excretion by definition is passive and deals with metabolic wastes as filtered by the kidneys. Though the sweat may contain a trace amount of metabolic wastes, sweating is an active process of secretion not excretion, specifically for temperature control and pheromone release. Therefore, its role as a part of the excretory system is minimal at best. Specifically, the skin secretes a fluid waste called sweat, or perspiration.

Lungs

Main article: Lungs

The lungs and gills of organisms constantly secrete gaseous wastes from the bloodstream as a regular part of respiration.

Kidneys

Main article: Kidneys

In some cases, excess wastes crystallize as kidney stones. They grow and can become a painful irritant that may require surgery or ultrasound treatments. Some stones are small enough to be forced into the urethra.

Kidneys perform several homeostatic functions:

1. Maintain volume of extracellular fluid
2. Maintain ionic balance in extracellular fluid
3. Maintain pH and osmotic concentration of the extracellular fluid.
4. Excrete toxic metabolic by-products such as urea, ammonia, and uric acid.

Defecation

Main article: Defecation

Organisms eliminate solid, semisolid or liquid waste material (feces) from the digestive tract via the anus during the process of defecation. Waves of muscular contraction known as peristalsis in the walls of the colon move fecal matter through the digestive tract towards the rectum. Undigested food may also be expelled this way; this process is called egestion.

Ureter

In human anatomy, the ureters are muscular ducts that propel urine from the kidneys to the urinary bladder. In the adult, the ureters are usually 25–30 cm (10–12 in) long. In humans, the ureters arise from the renal pelvis on the medial aspect of each kidney before descending towards the bladder on the front of the psoas major muscle. The ureters cross the pelvic brim near the bifurcation of the iliac arteries (which they run over). This "pelviureteric junction" is a common site for the impaction of kidney stones (the other being the uteterovesical valve). The ureters run posteriorly on the lateral walls of the pelvis. They then curve anteriormedially to enter the bladder through the back, at the vesicoureteric junction, running within the wall of the bladder for a few centimeters. The backflow of urine is prevented by valves known as ureterovesical valves. In the female, the ureters pass through the mesometrium on the way to the urinary bladder.

Urinary bladder

The urinary bladder is the organ that collects urine excreted by the kidneys prior to disposal by urination. It is a hollow muscular, and distensible (or elastic) organ, and sits on the pelvic floor. Urine enters the bladder via the ureters and exits via the urethra.

Embryologically, the bladder is derived from the urogenital sinus and, it is initially continuous with the allantois. In males, the base of the bladder lies between the rectum and the pubic symphysis. It is superior to the prostate, and separated from the rectum by the rectovesical excavation. In females, the bladder sits inferior to the uterus and anterior to the vagina. It is separated from the uterus by the vesicouterine excavation. In infants and young children, the urinary bladder is in the abdomen even when empty.

Urethra

In anatomy, the urethra (from Greek οὐρήθρα - ourethra) is a tube which connects the urinary bladder to the outside of the body. The urethra has an excretory function in both sexes to pass urine to the outside, and also a reproductive function in the male, as a passage for semen during sexual activity.

Urine Formation

First, the blood goes through the afferent artery, to the capillaries called glomerulus, to the Bowman's capsule. The Bowman's capsule squeezes the blood from its contents-primarily food and wastes. After the squeezing process, the blood will then come back to get the food nutrients it need. The wastes will then go to the collecting duct, to the renal pelvis, and to the ureter, which will be then secreted out of the body.

External links

• CLC Biology: Excretory/Urinary System
• The Franklin Institute: Excretory System