12th Zoology

Mechanism of Breathing

Mechanism of Breathing

The process of inspiration and expiration happens due to pressure
changes in the thoracic cavity. The thorax is an airtight compartment bounded
by the sternum in front, the vertebral column at the back, the ribs encircling
the sides and the diaphragm found below.

The rib bones are provided with the two sets of muscles namely external and internal intercostal muscles. By the contraction and expansion of these muscles the volume of the thoracic cavity is reduced or increased. The floor of the thoracic cavity is completely closed by the diaphragm. The act of breathing is performed by expansion and contracton of the thoracic cavity. (Mechanism of Breathing)


Inspiration is the process by which fresh air is drawn into the
lungs. It is an active process. The muscles participating in the inspiration
process are external – intercostal muscles present in between the ribs and
the diaphragm.

During quiet respiration, contraction of external intercostal muscle
causes the ribs to move anteriorly and outwardly. This movement enlarges
the cavity of the thorax by increasing it side to side and in dorso-ventral
dimensions. (Mechanism of Breathing)

The contraction of radial muscles of the diaphragm leads to
flattening of inelastic, dome shaped central part of the diaphragm. As a result
of these muscular movements, the volume of the thoracic cavity is increased.
This causes the air pressure within the lungs to fall below the atmospheric
pressure. So air (tidal air) from outside passes through the air passage into
the lungs to equalize the pressure.


Inspiration is followed by expiration. It is a passive process.
Expiration is the process by which air is exhaled or blown out from the lungs.
The expiration results when the volume of the thoracic cavity is decreased
and air pressure in the lung is increased. The expiratory process involves the
following movements.
1. The diaphragm relaxes and rises to resume the original dome shape.
2. The ribs take their original position as a result of contraction of the internal
intercostal muscles.

Gaseous exchange in the alveoli

Once the air is within the lungs the process of gaseous exchange
begins. Capillaries of the pulmonary artery remains close to the wall of the
alveloli. This enhances the exchange of gases.
Oxygen carbon-di-oxide water vapour
Inhaled % 21.00 0.04 variable
Exhaled % 15.7 3.6 saturated

Oxygen and carbon-di-oxide are exchanged across the alveolar
membrane by diffusion from the site of higher to low partial pressure until the
partial pressure of the two regions are equal. This process is a simple
physical one which does not involve any secretary or active transport

In the atmospheric air there is a high concentration of oxygen
20-95% (PO2 140mm Hg) while the proportion of carbon dioxide is low

The alveolar PO2 is about 100mm Hg and the PO2 of venous blood
is about 40mm Hg. This pressure gradient is sufficient for the transfer of O2.
The PCO2 of venous blood is 46mm.Hg and that of alveolar air is only 6mm.Hg
(1/10th of O2), it is adequate for CO2 transfer by diffusion. CO2 diffuses 20
times faster than O2.

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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