The main site of exchange of gases is alveoli. The gases are exchanged between tissues and blood. Carbon dioxide and oxygen are exchanged in these site by simple diffusion on the basis of pressure or concentration gradients. Two important factors that influence the rate of diffusion are:

• The solubility of the gases
• The thickness of the membranes

The diagram shows the partial pressures of the two gases in the atmosphere and the sites of diffusion.

A pressure gradient also occurs for carbon dioxide in the opposite direction, i.e., from blood to alveoli and tissues to blood. The solubility of carbon dioxide is 20-25 times higher than that of oxygen, the amount of carbon dioxide that can diffuse through the membrane per unit difference in partial pressure is very high in comparison to that of oxygen.

The diffusion membrane consists of three major layers:

• Thin squamous epithelium of alveoli
• Endothelium of alveolar capillaries
• A thin basement material

All the factors in our body are favourable for the diffusion of oxygen from alveoli to tissue and that of carbon dioxide from the tissue to alveoli.

Transport of gases

Transport of oxygen and carbon dioxide occurs in the blood. About 97 percent of oxygen is transported by red blood cells in the blood. The remaining 3% of oxygen is carried in a soluble state through plasma. About 20-25% of carbon dioxide is transported by red blood cells whereas 70 percent of it is carried in the form of bicarbonate. Around 7% of carbon dioxide is carried in a soluble state through plasma.

Transport of Oxygen

• Haemoglobin is red in colour due to the presence of iron in it and makes our blood look red in colour. Oxygen can attach with haemoglobin in a reversible reaction to form oxyhaemoglobin.
• All the haemoglobin molecules have the capacity to carry four molecules of oxygen.
• Partial pressure created by carbon dioxide, temperature and hydrogen ion are some other factors that can interfere with this binding.
• When the percentage saturation of haemoglobin with oxygen is plotted on a graph against the pO₂, it gives a sigmoid curve. The curve is known as the Oxygen dissociation curve.

• This curve is very useful in studying the effect of factors like hydrogen ion concentration, partial pressure of carbon dioxide on the binding of oxygen with haemoglobin.
• Alveoli region has high pO₂, low pCO₂ low temperature and lesser hydrogen ion concentration, which favours the dissociation of oxygen from the oxyhaemoglobin. This signifies that oxygen gets attached to haemoglobin in the lung and gets dissociated in the tissues.
• Around 5m of oxygen is delivered to tissue in every 100 ml of oxygenated blood under normal physiological conditions.

Transport of Carbon Dioxide

• Carbon dioxide is carried by haemoglobin in the form of carbamino-haemoglobin (20-25%)
• The partial pressure of carbon dioxide works in the binding.
• When the partial pressure of carbon dioxide is the low and partial pressure of oxygen is high, it leads to dissociation of carbon dioxide from carbamino-haemoglobin, which mean the carbon dioxide which is attached to haemoglobin from the tissue is transported at the alveoli.
• Carbonic anhydrase, the enzyme present in high concentration in red blood cells and in low concentration in plasma facilitates the reaction in both directions.

• At the tissue site where partial pressure of carbon dioxide is high due to the catabolic reaction, carbon dioxide diffuses into the blood and forms Hydrogen carbonate (HCO₃) and Hydrogen ions.
• At the alveoli where partial pressure is low, the reaction moves in the opposite direction resulting in the formation of carbon dioxide and water. Thus, carbon dioxide gets trapped as bicarbonate at the tissue and is transported to the alveoli where carbon dioxide is released.
• 4 ml of carbon dioxide is delivered to the alveoli by every 100 ml of deoxygenated blood.

How exercise affects our breathing?

The increase in physical activity and muscle cells respire vigorously when we exercise. The heart rate increases because the body needs more oxygen and releases more carbon dioxide.

Breathing rate is calculated by counting the number of times a person breathe in a minute. The tool used to measure the depth of breathing is spirometer.

Blood pH

• It is from 7.35 to 7.45
• The concentration of carbon dioxide in the blood and tissues increases during exercise. This makes the blood more acidic.
• To prevent the over-acidification of blood:
  • The material in blood plasma reacts with the excess CO
  • The rate and depth of breathing are increased so that it increases the rate of removal of carbon dioxide from the blood.

How smoking affects our respiratory system?

Smoking can be disastrous for our lungs and can cause many respiratory-related issues and lung cancers.

Tobacco in cigarettes contains nicotine which has the capacity to affect the brain and is addictive.

• Effect on the passage of air
  The work of mucus present in the lungs can trap pathogens. This mucus is pushed out of the lungs through the air like structures called cilia which are present on the epithelial cells of the trachea, bronchi, and bronchioles.
  The smoke from the cigarette release harmful chemicals that damage these epithelial cells and thus mucus is not pushed out of lungs leading to accumulation of mucus. Smoke also creates irritation in bronchi leading respiratory disorder called bronchitis.

• Effects on the alveoli
  The smoke also has a deteriorating effect on alveoli. The walls of alveoli tend to break and join again to form large air space than normal conditions. This decrease the efficiency of exchange of gases leading to the lung disorder called emphysema. It is a kind chronic obstructive pulmonary disease in which the oxygen decreases in the blood and the person find it difficult to breathe.