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MHT-CET : Biology Entrance Exam

MHT - CET : Biology - Excretion and Osmoregulation Page 1

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Excretory System


  1. The removal of metabolic waste products from the body of the animals is called excretion.
  2. Osmoregulation is the process by which the osmotic pressure is regulated or kept constant in the blood and the tissue fluids.
  3. Osmoregulation and excretion together help the body to maintain its physiological equilibrium also called as homeostasis.
  4. The process of breaking down of amino acids into forms in which they can be excreted is called deamination. In vertebrates the process of deamination takes place in the liver.
  5. The elimination of nitrogenous wastes in the form of ammonia is called as ammonotelism. Animals eliminating nitrogenous wastes in the form of ammonia are called ammonotelic. Animals like aquatic invertebrates, bony fishes and aquatic amphibians like salamanders and tadpoles are ammonotelic.
  6. The elimination of nitrogenous wastes in the form of urea is called ureotelism. Animals excreting nitrogenous wastes in the form of urea are called ureotelic. Mammals such as man, whale, seal, camel, kangaroo, rat, etc; amphibians like toad and frog, fishes like shark and sting-ray and reptiles like terrapin and turtle are ureotelic.
  7. The elimination of nitrogenous wastes in the form of uric acid is called uricotelism. Animals that excrete uric acid are called uricotelic. Birds, land reptiles, insects, land snails, etc., are called uricotelic.
  8. The excretory organs (excretory system) of man consists of a pair of kidneys, a pair of ureters, a urinary bladder and urethra.
  9. Kidneys
    • Kidneys are a pair of purplish brown, bean-shaped organs lying on the dorsal side of the abdominal cavity outside the peritoneum, one on either side of the vertebral column.
    • Between the right and left kidneys run two main blood vessels namely dorsal aorta and inferior vena cava.
    • Each kidney bears a longitudinal opening on its concave side called hilium.
    • Renal arteries, renal veins and nerves enter or leave the kidney at the hilius.
    • The ureter also originates from the hilius.
    • The human kidney is enclosed in a capsule of fibrous connective tissue, called renal capsule.
    • The kidney tissue can be divided into two regions; cortex - the outer region and medulla - the inner region.
    • The hilium leads to a funnel-shaped space, the renal pelvis. The ureter is connected to the pelvis.
    • The urine in the natural state is filled in the hollow regions of the kidney.
    • Nephrons are the structural and functional units of kidney. Each kidney has around million nephrons.
    • A nephron along with the connecting tubule is called uriniferous tubule.
  10. Ureters
    • Ureters are a pair of narrow, muscular tubes, which originate from the renal pelvis and open into the urinary bladder.
  11. Urinary Bladder
    • Urinary bladder is median and unpaired.
    • It is a muscular sack with smooth muscles. It lies behind the pubis bone of the pelvic girdle.
    • Ureters from both the kidneys open into the urinary bladder.
    • The urinary bladder stores the urine till it is expelled through the urethra.
  12. Urethra:
    • The urethra is a median tube arising from the lower end of the bladder.
    • The opening of the bladder is guarded by a sphincter muscle called urethral sphincter.
    • The opening of this sphincter, which is under voluntary control, results in micturition(urination) and in turn emptying of the bladder.
    • In females, the urethra is relatively shorter than in males.
  13. The Structure of Uriniferous Tubule Nephron:
    • The nephron is the structural and functional unit of kidney.
    • The nephron is a thin, coiled duct, lined by a single layer of epithelial cells.
    • The proximal end of nephron is blind (ie., it has no opening anywhere). The distal end of the nephron opens into a collecting tubule.
    • The proximal and distal ends of nephron both lie in the cortex of the kidney whereas the middle region lies in the medulla.
    • The nephron consists of two parts, viz. Malpighian corpuscle and Renal tubule.
    • Malpighian Corpuscle
    • The malpighian corpuscle is the anterior part of the nephron. It consists of double walled cup-shaped structure called Bowman's capsule. The Bowman's capsule is the widest part of the nephron. It is made up of two layers, the outer parietal and the inner visceral. Both the layers are continuous along the rim of the cup. The Bowman's capsule encloses a knot of blood capillaries called glomerulus. The glomerulus and the Bowman's capsule are intimately connected with each other both structurally and functionally. Together, they are called renal corpuscle or malpighian corpuscle.
    • Renal Tubule:
      The renal tubule arises from the base of the Bowman's capsule and has an outer diameter of about 60 micrometers. The connection between the renal tubule and the Bowman's capsule is called the neck. The renal tubule is differentiated into four regions, viz. the proximal convoluted tubule, Henle's loop, the distal convoluted tubule and the collecting tubule.
  14. Circulation of Blood Within the Kidney
    The circulation of blood within the kidney can be represented thus,
    Renal artery Arterial branches Afferent arteriole Glomerulus Efferent arteriole Peritubular capillary network Venule Renal vein.
  15. Urine Formation
    The separation of urine from the blood occurs in the following three steps:
    • Ultrafiltration
      This is the first stage of urine formation and it takes place in the renal corpuscle. The main structures involved in the filtration are the glomerulus and the visceral layer of the Bowman's capsule. There are present pores of about 0.1 micrometre in diameter on the endothelium of glomerular capillaries. These pores are large enough to let through all constituents of blood plasma except large proteins. The blood cells are bigger in size and hence do not pass through these openings. Endothelial cells rest on the thin basement membrane. On the other side of this thin basement membrane are situated the epithelial cells of the visceral layer of Bowman's capsule. The membrane is made up of a layer of fine filaments called glomerular slit diaphragm. The substances pass through the endothelium and the slit diaphragm of the basement membrane. The filtrate can thus reach the urinary space of the Bowman's capsule through the gaps that are present in the visceral epithelial layer. Along with it the substances pass from the blood into the lumen of nephron. This process is called ultrafiltration.
    • Reabsorption:
      Two processes are involved in the reabsorption process, namely, osmosis and active transport.
    • Secretion
      From the capillaries, the cells of the tubules absorb substances like potassium ions, ammonia ions, hydrogen ions, creatinine, etc and secrete them into the lumen of the tubule. This process is called tubular secretion. The resulting fluid in the tubule forms the urine.
  16. Urine
    • The amount of urine produced varies depending on the time of the day (lesser during night) and the food and fluids consumed.
    • Normal urine is slightly acidic with a pH range of 5 to 7.
  17. Kidney and Osmoregulation
    • The main function of reabsorption is to maintain the water balance of the blood and other body fluids. According to the needs of the body the urine produced may be hypotonic or hypertonic in comparison with the body fluids. This is mainly due to the hormonal control of reabsorption of water in the renal tubules.
    • Antidiuretic hormone (ADH), a hormone secreted by the pituitary gland is produced when the water content of the body fluids is less than normal. The effect of ADH is mainly on the permeability of the distal tubule and the collecting duct.
  18. The Skin as the Excretory Organ
    The two glands present in the skin which help the skin to perform as an excretory organ are:
    • sweat glands
    • sebaceous glands
  19. Lung as an Excretory Organ
    It throws carbon dioxide out of the body. Water is lost from the lung surfaces by evaporation, thus helping in osmoregulation.



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