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.
- 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.
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.
