EXCRETORY SYSTEM

EXCRETORY SYSTEM

Excretory System Definition

The excretory system consists of the organs that remove metabolic wastes from the body. In humans, this includes the removal of liquid nitrogenous waste in the form of urine, and solid wastes especially from the breakdown of hemoglobin. The removal of urine is accomplished by the urinary tract, while solid wastes are expelled through egestion from the large intestine.

FUNCTIONS

The excretory system functions as the bulwark and balance to the digestive system. While we consume food and drink to nourish the body and provide energy, the excretory system ensures that homeostasis is maintained, irrespective of changes to the nutritive value of food.

It regulates the fluid balance of the body, maintaining adequate salt and water levels. When there is excess water, it is removed through the production of hypotonic urine. When we consume salty food, or lose water through perspiration, the concentration of urine is increased, to preserve the osmolarity of body fluids.

Excretory System Organs

The primary excretory organs in the human body are the kidneys, ureters and urinary bladder, involved with the creation and expulsion of urine. Through these organs, much of the nitrogenous waste of the body, especially urea, is expelled. Other organs such as the liver, large intestine and skin are also necessary for the excretion of specific metabolic wastes.

Kidneys

The kidneys are paired, bean-shaped organs located in the abdomen, on either side of the spine, under the diaphragm. They are made of a large number of structural and functional subunits called nephrons. These nephrons perform the primary task of filtering blood and removing waste products. Each nephron snakes between the outer cortex of the kidney and the inner medulla, with different activities occurring at each site.

A process of ultrafiltration creates the glomerular filtrate from blood, which is remarkably similar in composition to blood plasma. Water, small molecules, and proteins smaller than 30 kilodaltons in size can pass freely into the lumen of the Bowman’s capsule.

Urinary BladderFUNCTIONS OF KIDNEYS

• Kidneys remove nitrogenous waste urea from blood.
• Kidneys remove excess of water from body.
• Kidneys maintain proper pH of blood by removing the excess of acids and bases from blood.
• Kidneys maintain proper balance of concentration of salts in blood.
• Kidneys remove other toxic substances from blood like urea, uric acid and ammonia.
• It maintains volume of extracellular fluid.

URINARY BLADDER

The urinary bladder is a sac-like structure with muscular walls that holds urine until it is expelled from the body during micturition. The bladder receives urine through two ureters – one from each kidney –that enter through openings called ureteric orifices. These orifices are located at the convex fundus of the organ. Urine exits the bladder through the urethra.

The walls of the bladder are made of smooth muscle and the inner epithelial lining of this organ consists of a remarkable tissue called transitional epithelium. The cells of this stratified tissue change shape based on whether the bladder is empty or full, allowing it to remain elastic, accommodating up to half a liter of urine.

Liver

The liver is the main detoxifying organ of the body, especially for nitrogenous wastes. The cells of the liver play host to biochemical processes that create ammonia from amino acids. Since ammonia is extremely toxic, it is quickly converted to urea before being transported in the blood towards the kidney.

Most animals make the choice between ammonia, urea and uric acid as the preferred mode for nitrogenous waste excretion, based on the availability of water. While ammonia is toxic, it can be quickly diluted and removed from the body, and therefore remains the chemical used by aquatic animals. Terrestrial animals with regular access to water tend to use urea, which has lower toxicity. Birds and other animals that have minimal water intake expend energy to convert urea into uric acid.

Large Intestine

The liver is also necessary for the removal of the decomposed hemoglobin, some drugs, excess vitamins, sterols and other lipophilic substances. These are secreted along with bile and finally removed from the body through feces. The large intestine therefore plays a role in excretion, especially for hydrophobic particles.

Skin

The skin is a secondary excretory organ, since sweat glands in the dermis can remove salts and some excess water. The skin also has sebaceous glands that can secrete waxy lipids.

Excretory System Facts

• *The urinary bladder can hold up to 600 ml of liquid. During early pregnancy, the uterus presses on to the bladder, creating a greater frequency of micturition.
• *Most of the amniotic fluid surrounding the growing fetus is fetal urine, though its composition is very different from normal urine. The bladder of the fetus begins to empty around the 10th week of gestation.
• *This fetal urine and the amniotic fluid are actually important for the development of fetal lungs.
• *The white parts in bird excreta are composed mostly of uric acid.
• *The brownish pigmentation of feces mostly derives from bile salts