BREATHING AND EXCHANGE OF GASES

 

EXCHANGE OF GASES :

• Alveoli are the primary site of exchange of respiratory gases.
• Exchange of gases also takes place between blood and tissues.
• Exchange of O₂ and CO₂ take place in the pressure gradient, by simple diffusion.
• Pressure contributed by an individual gas in a mixture of gases is called the partial pressure and is represented by pO₂ for oxygen and pCO₂ for carbon dioxide.
• Diffusion of O₂
  • pO₂ in alveolar air = 104 mm Hg.
  • pO₂ in venous blood = 40 mm Hg.
  • O₂ diffuses from alveoli to venous blood.
  
  
• Diffusion of CO₂
  • pCO₂ is venous blood = 45 mm Hg.
  • pCO₂ is alveolar air = 40 mm Hg
  • CO₂ diffuses from venous blood to alveoli.
  
  
• Solubility of CO₂ is 20-25 times higher than that of O₂; the amount of CO₂ that can diffuse through the diffusion membrane per unit difference in partial pressure is much higher compared to that of O₂.
• Respiratory membrane is formed by;
  • Thin Squamous epithelium of the alveoli.
  • Endothelium of alveolar capillaries
  • Basement membrane between them.

TRANSPORT OF GASES :

• Blood is the medium of transport of O₂ and CO₂.
• About 97 per cent of O₂ is transported by RBCs in the blood.
• 3 per cent of O₂ is transported in the plasma in dissolved state.
• 20-25 per cent of CO₂ transported in the RBC in the form of carbamino-haemoglobin.
• 70 percent CO₂ carried as bicarbonate ion in plasma.
• 7 percent CO₂ transported in dissolved state in plasm.

Transport of Oxygen :

• Haemoglobin is red coloured pigment present in the RBC.
• O₂binds with hemoglobin reversibly to form oxy-hemoglobin.
• Each haemoglobin can binds maximum with four O₂ molecules.
• Binding of Oxygen with haemoglobin is primarily related with partial pressure of O₂.
• Partial pressure of CO₂, hydrogen ion concentration (pH) and temperature are the factors that influence this binding.
• A sigmoid curve is obtained when percentage of saturation of hemoglobin with O₂ is plotted against the partial pressure of O₂ (pO₂). This curve is called oxygen dissociation curve.
• Condition favourable for binding of Hemoglobin with O₂ at alveolar level;
  • High pO₂
  • Low H⁺ ion concentration.
  • Low temperature.
  
  
• Condition favourable for dissociation of HbO₂ into Hb and O₂ at tissue level;
  • Low pO₂
  • High H⁺ ion concentration.
  • High temperature.
  
  
• Every 100 ml of oxygenated blood can deliver around 5 ml of O₂ to the tissues under normal physiological conditions.

Transport of Carbon dioxide:

• 20-25 percent of CO₂ is carried out in the RBC by binding with the free amino group of haemoblobin by formation of carbamino-haemoglobin.


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• When pCO₂ is high and pO₂ is low as in the tissues, more binding of CO₂ occurs whereas, when the pCO₂ is low and pO₂ is high as in the alveoli, dissociation of CO₂ from carbamino-haemoglobin takes place.
• 70 per cent of CO₂ transported in the form of HCO₃- in the plasma.
• CO₂ from the tissue diffused into the plasma and along with the water it forms carbonic acid which dissociated into HCO₃- and H⁺. This reaction is catalysed by an enzyme called carbonic anhydrase present in the plasma membrane of RBC and plasma.


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REGULATION OF RESPIRATION :

• Specialized centre present in the medulla region of the brain called respiratory rhythm centre is primarily responsible for regulation of breathing.
• Pneumotaxis centre of pons region of brain has moderate regulation.
• Neural signal from this centre can reduce the duration of inspiration and alter the rate of respiration.
• Chemosensitive area adjacent to rhythm centre is sensitive to CO₂ and H⁺ ion.
• Receptors associated with aortic arch and carotid artery also can recognize changes in the CO₂ and H⁺ concentration and send necessary signals to the rhythm centre for remedial actions.

DISORDERS OF REPIRATORY SYSTEM :

• Asthma : is a difficulty in breathing causing wheezing due to inflammation of bronchi and bronchioles.
• Emphysema : a chronic disorder in which alveolar walls are damaged due to which respiratory surface is decreased. It caused due to smoking.

CBSE Biology (Chapter Wise) Class XI ( By Mr. Hare Krushna Giri ) 
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