Write a note on pleura.

Pleura : Pleura is the double membrane structure that surrounds the lungs. The space inbetween the two membranes is filled with pleural fluid. 

The outer pleural membrane is in close contact with the thoracic lining whereas the inner pleural membrane is in contact with the lung surface. 

The pleural membrane protects the lungs and the pleural fluid reduces friction on the lung-surface.

The rate of diffusion depends on the following factors:

i. Partial pressure gradients of O₂ (pO₂) and CO₂ (pCO₂),

ii. Solubility of the gases.

iii. Thickness of the diffusion surface.

Haemoglobin is a red coloured iron containing pigment present in the RBCs. O₂ binds with haemoglobin in a reversible manner and forms oxyhaemoglobin.

Each haemoglobin molecule can carry a maximum of four molecules of O₂. Binding of oxygen with haemoglobin is primarily related to partial pressure of O₂. 

The high pO2, low pCO2, lesser H⁺ concentration and lower temperature in the alveoli are favourable for the formation of
oxyhaemoglobin, whereas in the tissues,
low pO2, high pCO2, high H⁺ concentration and higher temperature exist which favours dissociation of the oxygen from the haemoglobin. Thus, O₂ gets bound to haemoglobin in the lung surface and gets dissociated at the tissues. 

(a) Far more oxygen is released from oxyhaemoglobin in a more active tissue than in a less active one.- This is because in more active tissue PO₂ is lower and PCO₂ is higher. The PO₂ is much lower and the PCO₂ is much higher in an active tissue than less active. Both these favour dissociation of oxyhaemoglobin to deoxyhaemoglobin and molecular oxygen.

(b) Oxygenation of blood promotes the release of carbondioxide from blood in the lungs -  because oxyhaemoglobin is a stronger acid than deoxyhaemoglobin. So, it denotes H⁺ ion which combine into water and CO₂ by carbonic anhydrase. Bicarbonates release CO₂. Affinity of haemoglobin for oxygen is enhanced due to the release of CO₂ from blood in lungs which causes fall in PCO₂ of blood.

Conditions necessary for skin to act as respiratory organ are:

1. Skin must be smooth and without exoskeleton.

2. Skin must have lining of mucous cells to keep it moist.

3. Skin must be thin.

4. Skin must have a rich supply of blood.