12th Zoology

Mechanism of urine formation

Mechanism of urine formation

Urine is continually formed by each nephron and the processes
involved in the formation of urine are

Glomerular ultra filtration, Tubular Reabsorption, Tubular Secretion

Glomerular ultra filtration (Mechanism of urine formation)

Ultrafiltration of blood takes place in the malpighian body which acts as a biological filter. A malpighian body comprises Bowmann’s capsule and glomeruli.

Dynamics of filtration (Mechanism of urine formation)

The kidneys normally receive an abundant blood supply of about
1200ml/min or about 20 to 25 percent of the cardiac output. It flows through
the capillaries of glomerulus where the blood pressure is comparatively high.
The high blood pressure brings about effective filtration. The hydrostatic
pressure (forward pressure 75mm/Hg.) of the blood in the afferent
glomerular capillaries and the cumulative effect of the opposition pressures
and renal intratubular pressure (10mm/Hg.) play an important role in
producing the glomerular filtrate. The hydrostatic pressure of the blood is
always greater than the opposing pressures existing in the plasma protein
and renal capillaries. Thus the available net filtering force (75-50mm/Hg. =
25mm/Hg.) is chiefly responsible for glomerular filtration. The fluid in the
capsule which is obtained by the process is termed glomerular filtrate. The
volume of the glomerular filtrate produced each minute is called glomerular
filtration rate (GFR). In man it is about 125ml/min. In 24 hours the total
volume of glomerular filtrate is 170 to 180 liters.

Tubular Reabsorption (Mechanism of urine formation)

This is the second step in the urine formation. The glomerular filtrate
contains many useful substances such as glucose, amino acids, mineral salts
and vitamins dissolved in large amount of water. Reabsorption takes place in
the uriniferous tubules. Reabsorption of useful substances is a differential or
selective process. Substances such as glucose, sodium and calcium, are called
“high threshold substance”. They are actively reabsorbed in
considerable quantities. Substances like urea and uric acid etc which are
called “low threshold substances” are reabsorbed in small quantities by a
simple diffusion process or passive reabsorption. Substances like creatinine
are not reabsorbed. They are completely eliminated.

Reabsorption in Proximal Convoluted Tubule (Mechanism of urine formation)

Proximal convoluted tubule is responsible for the reabsorption of
water, glucose, sodium phosphate and bicarbonates. The urine is found to

be isotonic in the proximal convoluted tubule. Isotonic condition of a
solution indicates no passage of water across the membrane separating two
such solutes.

Reabsorption in Henle’s loop

Urine becomes more and more hypertonic as it passes through the
descending limb of the loop of Henle’s. This is due to the fact that the thin
descending portions of the Henles loop are freely permeable to sodium. As
the urine slowly passes through the thick ascending limb of the loop of Henle,
it becomes less hypertonic since the sodium is actively transported from the
ascending limb to the descending limb through the interstitial tissue space.

Reabsorption in distal convoluted tubule

On entering the distal convoluted tubule, the urine becomes nearly
isotonic to the surrounding tissue fluid due to the active transport of sodium
and passive transport of water. Summary of Renal Filtration and
Reabsorption in 24 hours

Reabsorption in collection tubule

As urine (isotonic) passes into the collecting tubule it becomes once
more hypertonic by the osmotic reabsorption of water under the
influence of the hormone ADH. The release of ADH is controlled by the
osmoreceptors in the hypothalamus in response to changes in the osmotic
pressure of the plasma circulating through the collecting tubule. Thus the urine
formed contains 96% of water, 2% urea and 2% of the metabolic products.

Tubular Secretion

This is the final step in the formation of urine during which the walls
of tubule actively remove additional waste substances which are harmful to
the body from the blood that have escaped filtration.

Related Topics in Zoology:

Bio Zoology All Important Topics

  1. Human Physiology Introduction

  2. Nutrition

  3. Carbohydrates Poly hydroxyaldehydes (or) ketones

  4. Proteins (Polypeptides)

  5. Lipids

  6. Vitamins – Functions Of Vitamins

  7. Deficiency of Vitamin

  8. Minerals – Water – Role of water

  9. Balanced diet

  10. Obesity

  11. Digestive System

  12. Dental Caries (Tooth decay)

  13. Root Canal Treatment

  14. Peptic ulcer

  15. Hernia and Types

  16. Appendicitis (Appendix)

  17. Gall Stones

  18. Hepatitis

  19. Fractures – Types of fractures

  20. Mechanism of fracture

  21. Dislocation of joints

  22. Arthiritis

  23. Rickets and Osteomalacia – Orthopedics

  24. Muscles

  25. Mechanism of muscle contraction

  26. Types of muscle contraction

  27. Myasthenia Gravis

  28. Respiration – Process of pulmonary respiration

  29. Mechanism of Breathing

  30. Regulation of Respiration

  31. Pneumonia Tuberculosis Symptoms Treatment

  32. Bronchitis – Acute bronchitis, Chronic Bronchitis Causes

  33. Circulatory System – Functioning of Human heart

  34. Cardiac Cycle

  35. Coronary blood vessel and its significance

  36. Myocardial infarction

  37. Angina pectoris

  38. Angiogram – Angioplasty

  39. Atherosclerosis

  40. Heart block Echo cardiography Heart Valves

  41. Rheumatic Heart Disease (RHD), ICCU – (Intensive Coronary Care Unit)

  42. Blood Pressure

  43. Heart transplantation

  44. Pulse rate

  45. Cardio Pulmonary Resuscitation (CPR)

  46. Blood – Composition of plasma – Blood cells

  47. Clotting of Blood or Haemostasis

  48. Thrombosis

  49. Nervous system Co-ordination systems

  50. The Brain – Fore Brain, Midbrain, Hindbrain

  51. Memory

  52. Sleep – Types of sleep

  53. Stroke – Brain haemorrhage

  54. Alzheimer – Meningitis (Brain fever)

  55. Conditioned reflex

  56. Electroencephalography EEG

  57. Right and Left brain concept

  58. Spinal cord functioning

  59. Chemical co-ordination – Functions of Endocrine glands

  60. Hypothalamus

  61. Pituitary gland – hormone

  62. Hormones of Neurohypophysis – vasopressin

  63. Thyroid gland

  64. Parathyroid Gland

  65. Pancreas

  66. Adrenal gland

  67. Gonads

  68. Receptor Organs – Eye

  69. Photochemistry of Retinal visual Pigments

  70. Errors of refraction

  71. Optometry – Retinopathy

  72. Cataract – Lens Replacement – Glaucoma – Nyctalopia

  73. Eye Infections and Eye Care

  74. Ear

  75. Mechanism of hearing

  76. Defects of the ear

  77. Hearing Aid – Noise pollution

  78. Skin and functions of skin

  79. Melanin functions

  80. Effects of solar radiation / UV radiation – Skin grafting

  81. Dermatitis

  82. Tongue – Mechanism of Stimulation

  83. Excretion Ureotelism Nephron

  84. Mechanism of urine formation

  85. Renal Failure, Dialysis, Kidney Machines

  86. Kidney stone – Kidney transplantation

  87. Diabetes mellitus

  88. Functioning of male reproductive system

  89. Functioning of female reproductive system

  90. Ovulation and fate of the ovum – Menstrual cycle

  91. Fertilization

  92. Birth control

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