WBCS Preliminary (Biology):Excretion

In living organisms energy is produced by metabolically burning food. But this is accompanied by the formation of a variety of by-products which are harmful to the organism. These wastes have to be eliminated. The removal of harmful and unwanted toxic waste products of metabolism is known as excretion. Different organisms use varied strategies to do this. Many unicellular organisms remove these wastes by simple diffusion from the body surface into the surrounding water and complex multi-cellular organisms use specialised organs to perform the same function.

• Green plants in darkness or plants that do not contain chlorophyll produce carbon dioxide and water as respiratory waste products. Carbon dioxide released during respiration gets utilized during photosynthesis. Oxygen itself can be thought of as a waste product generated during photosynthesis. Plants can get rid of excess water by transpiration. For other wastes, plants use the fact that many of their tissues consist of dead cells, and that they can even lose some parts such as leaves. Many plant waste products are stored in cellular vacuoles. Waste products may be stored in leaves that fall off. Other waste products are stored as resins and gums, especially in old xylem. Plants also excrete some waste substances into the soil around them.
• In lower animals carbon dioxide is eliminated directly into the environment through the body surface. In higher animals it is excreted out along with the exhaled air through the lungs. Excess water is excreted in the form of urine and sweat. Ammonia, Urea, Uric Acid , Amino Acids are the nitrogenous waste products produced by animals.
• The excretory system of human beings includes a pair of kidneys, a pair of ureters, a urinary bladder and a urethra.
• Kidneys : Kidneys are located in the abdomen, one on either side of the backbone. Urine produced in the kidneys passes through the ureters into the urinary bladder where it is stored until it is released through the urethra.
  • The kidneys are reddish brown, bean-shaped organs situated in the abdominal cavity, one on either side of the vertebral column in the lumbar region of the body. They lie asymmetrically, the right kidney being lower than the left as the right side of the abdominal cavity is occupied by the liver. Each kidney is 10 cm long, 6 cm wide and 4 cm thick and weighs 200 - 250 g in adults. A thin, tough, fibrous whitish capsule envelops each kidney. The outer surface of each kidney is convex while the inner surface is concave.
  • The kidneys function as a pair of filters through which about one litre of blood circulates each minute. The entire blood in the body passes through them in 5 - 6 minutes. In a day it filters 1800 litres of blood which is 400 times the blood volume.
  • A normal adult excretes 1 - 1.8 litres of urine per day.
  • The nephron is the kidneys functional unit. Each kidney contains from one to two million nephrons. Each nephron has a network of capillaries called glomerulus which fits into a cup called the Bowman's capsule and a long tubule consisting of proximal convoluted tubule, Henle's loop, distal convoluted tubule and connecting tubule.
  • How is urine produced? :The purpose of making urine is to filter out waste products from the blood. Just as CO₂ is removed from the blood in the lungs, nitrogenous waste such as urea or uric acid are removed from blood in the kidneys.
  • Hormones of the Kidneys: The human kidney is also an endocrine gland secreting two hormones: 1) Erythropoietin (EPO), Calcitriol, the active form of vitamin D as well as the enzyme renin.

• Diseases of Kidney :
  • Acute Renal Failure: No urine is formed by kidney and the wastes and water accumulates in the body. It can be treated by dialysis, by an artificial filtration of blood through semi permeable membaranes.
  • Kidney stone: It is also known as renal calculi, kidney stones are the result of crystallization of certain substances found in urine, including calcium, phosphate, oxalic acid, and uric acid. Stones may form in the urine collecting area (pelvis) of the kidney, as well as the ureters (narrow tubes connecting the kidney to the urinary bladder).
  • In case of kidney failure, an artificial kidney can be used. An artificial kidney is a device to remove nitrogenous waste products from the blood through dialysis. A matching kidney from another person may also be transplanted.
  • Principle of dialysis Blood is made to flow into the dialysis machine made of long cellulose tubes coiled in a tank having a dialyzing solution. Waste substances diffuse out of lood into tank. The cleansed blood is pumped back into patient.
  • Urinary Bladder:This is a large muscular storage sac that collects urine from both the kidneys through the ureters. As the urine gets drained into the bladder its volume increases. The mouth of the bladder is guarded by a tight ring of muscle called the sphincter which regulates the opening or closing of the bladder. When the sphincter relaxes, urine is released out through the urethra.
  • Urethra:This is a short muscular tube that carries urine at intervals from the urinary bladder to the outside. The base of the urethra is also guarded by a sphincter which keeps the urethra closed except while passing urine.