Mechanism of Pulmonary Respiration

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Respiration through lungs is called pulmonary respiration. The mechanism of pulmonary respiration includes breathing movement, exchange of gases in lungs, transport of gases by blood and exchange of gase by tissue.

Breathing or pulmonary ventilation

Pulmonary ventilation is commonly referred to as breathing. It is the process of air flowing into the lungs during inspiration (inhalation) and out of the lungs during expiration (exhalation).

Inspiration (the intake of air)

It is an active phase of ventilation because it is the result of muscle contraction. During inspiration, the diaphragm contracts and the thoracic cavity increases in volume. This decreases the intraalveolar pressure so that air flows into the lungs. Inspiration draws air into the lungs.

Expiration (releasing air)

Expiration is the process of letting air out of the lungs during the breathing cycle. During expiration, the relaxation of the diaphragm and elastic recoil of tissue decreases the thoracic volume and increases the intraalveolar pressure. Expiration pushes air out of the lungs.

Image showing Breathing movements.

Image Showing Breathing Movements.

Image showing Breathing movements.

Exchange of gases at the alveolar surface

Gas exchange between air and blood occurs at the alveoli. Gaseous exchange occurs by the process of simple diffusion between the alveolar air and the deoxygenated blood in capillaries. Due to the existing pressure difference of oxygen and carbon dioxide between the alveoli and the blood capillaries, oxygen diffuses from alveolar air to the capillary blood, whereas carbon dioxide diffuses from capillary blood to the alveolar air. Oxygenated blood is taken from the lungs to the heart by pulmonary vein.

Volume exchanged during breathing:

Table showing Air volume exchanged during breathing.
Tbale showing Air volume exchanged during breathing.

Tidal volume (TV)

Volume of air inhaled and exhaled without any noticeable effort (normal breathing)


Vital capacity (VC)

Volume of air that can be maximally breathed out after a maximum inspiration (VC = IRV + TV + ERV)

3400 – 4800 mL

Inspiratory reserve volume (IRV)

Volume of air that can be taken in by forced inspiration over and above the normal inspiration

2000 – 3000 mL

Expiratory reserve volume (ERV)

Volume of air that can be expelled by forced expiration over and above the normal expiration.

1000 mL

Residual volume (RV)

Volume of air that control be forced out even on forced expiration. This is the air that remains in the lungs and in the air passage.

1000 – 1500 mL

Total lung capacity

Sum of all lung volumes (maximum air that remains in the lungs after a maximum inhalation)

5500 – 6000 mL

Vital capacity may be highly reduced in smokers and people suffering from tuberculosis. Athletes and singers on the other hand have higher vital capacity.

Transport of oxygen by blood from lungs to tissues

Oxygen is carried in the blood in two forms; as dissolved oxygen and as oxyhaemoglobin. (I)The amount of dissolved oxygen in the blood is very small. Only 3 percent of oxygen is transported in dissolved condition by plasma to the body cells. (II) In oxyhaemoglobin, about 97 percent of the oxygen is transported from the lungs to the tissues in combination with haemoglobin.

Oxygen and haemoglobin combine in an easily reversible reaction to form oxyhaemoglobin .

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