1. The thin squamous epithelial cells of the alveoli are composed of Type I and Type II cells.
2. Type II cells are thicker, synthesize and secrete a substance called Surfactant.
3. It lowers the surface tension in the alveoli and prevents the lung from collapsing.

Molecular oxygen is carried in blood in two ways: bound to haemoglobin within the red blood cells and dissolved in plasma. Oxygen is poorly soluble in water, so only 3% of the oxygen is transported in the dissolved form. 97% of oxygen binds with haemoglobin in a reversible manner to form oxyhaemoglobin (HbO₂). The rate at which haemoglobin binds with O₂ is regulated by the partial pressure of O₂. Each haemoglobin carries maximum of four molecules of oxygen. In the alveoli high pO₂, low pCO₂, low temperature and less H+ concentration, favours the formation of oxyhaemoglobin, whereas in the tissues low pO₂, high pCO₂, high H+ and high temperature favours the dissociation of oxygen from oxyhaemoglobin.

When a person travels quickly from sea level to elevations above 8000ft, where the atmospheric pressure and partial pressure of oxygen are lowered, the individual responds with symptoms ' of acute mountain sickness (AMS) headache, shortness of breath, nausea and dizziness due to poor binding of O₂ with haemoglobin.

Haemoglobin belongs to the class of conjugated protein.
1. The iron containing pigment portion haem constitutes 4%.
2. The colourless protein of the histone class is called globin.
3. Each molecule of Haemoglobin contains four atoms of iron, each of which can combine with a molecule of oxygen.

(i) The abnormal state of haem called ferric does not accept or bind with O₂.
(ii) It occurs if RBCs contain less than 1% methaemoglobin.
(iii) Normal state of haem in RBCs is ferrous.
(iv) If ferric state is found to be more, haemoglobin is oxidized.
(v) Only under ferrous state, haemoglobin is oxygenated.