Exchange of gases occurs primarily between the alveoli, blood and tissues. It takes place on the basis of pressure and concentration gradients using simple diffusion. Partial pressure is the pressure contributed by an individual gas in a mixture of gases. It is represented as pO₂ for oxygen and pCO₂ for carbon dioxide. The pO₂ in alveoli is higher than in tissues, so, O₂ diffuses into the tissues through blood. The pCO₂ is higher in tissues than in alveoli, so. CO₂ diffuses into the alveoli from where it is exhaled.
Carbon dioxide is 25 times more soluble than oxygen, so, the amount of carbon dioxide that can diffuse through the diffusion membrane is much more than that of oxygen.
About 97 per cent of oxygen is transported by red blood cells, whereas the remaining three per cent is carried in a dissolved state through the plasma. Oxygen combines with haemoglobin on the surface of the lungs to form oxyhaemoglobin and gets dissociated in the tissues. Each haemoglobin molecule can carry four molecules of oxygen. The percentage saturation of haemoglobin with oxygen, if plotted against the partial pressure of oxygen, results in a sigmoid curve called the oxygen dissociation curve.
93 per cent of carbon dioxide from tissues is transported to the alveoli through RBC’s. A major part of it is trapped as bicarbonate ions by carbonic anhydrase in red blood cells and a minor part is transported by binding to haemoglobin in RBC as carbamino-haemoglobin. The rest seven per cent is carried in a dissolved state through the plasma. Finally, it reaches the alveoli and releases as carbon dioxide.
The respiratory rhythm is maintained by the human brain. This is regulated by three respiratory centres in the pons and medulla of the brain, called the respiratory rhythm centre. Many disorders of the respiratory system such as asthma, emphysema and fibrosis are caused by environmental factors.