Mechanism of Breathing

 MECHANISM OF BREATHING

Respiratory movements in man are carried out with the help of intercostal muscles and diaphragm.

There are two phases of each breathing movement, inspiration and expiration.

Mechanism of breathing showing: (a) inspiration, (b) expiration

(1) Inspiration (Inhalation).

It involves intake of fresh air in the alveoli of the lungs. It includes an active process and consumes the mechanical energy.

There are two types of inspiratory muscles:

(a) Phrenic muscles: These muscles extend from diaphragm to ribs and vertebral column.

(b) External intercostal muscles. These are 11 pairs of muscles present between 12 pairs of ribs dorsoventrally and laterally. When phrenic muscles contract, the diaphragm is flattened whereas when extemal intercostal muscles contract, the ribs are pulled forward, upward and outward. The thoracic cavity increases in all directions which results in increase in volume and decrease in pressure. So, air moves into the lungs.

(2) Expiration.

It involves the expelling of air of high pCO₂ out of the body.

During rest, the expiration is a passive process and simply involves the relaxation of the inspiratory muscles (phrenic and external intercostal muscles).

These decrease the volume of thoracic cavity. But during forceful expiration, two expiratory muscles also help in expiration:

(a) Abdominal muscles. These extend from the ribs to the abdominal organs. When these contract, the abdominal visceral organs are pulled upward towards the diaphragm. So the diaphragm becomes more convex and thoracic cavity decreases anteroposteriorly.

(b) Internal intercostal muscles. These are also 11 pairs of muscles present between the ribs. When these contract, ribs are pulled backward, downward and inward, so thoracic cavity decreases dorsoventrally and laterally. Due to the above changes larger amount of air is passed out.

Pulmonary Volumes

(i) Tidal volume (TV): The volume of air inspired or expired involuntarily in each normal breath. It is about 500 ml of air in average young adult man.

(ii) Inspiratory reserve volume (IRV) : The maximum volume of air which a person can inhale over and above tidal volume by deepest possible voluntary inspiration. It is about 2500-3000 ml.

(iii) Expiratory reserve volume (ERV) : The volume of air which can be expired over and above the tidal volume with maximum effort. It is about 1000-1100 ml (according to NCERT).

(iv) Residual volume (RV) : The volume of air left in the lungs even after maximum forceful expiration. It is about 1100-1200 ml (according to NCERT).

Pulmonary Capacities

The combination of two or more pulmonary volumes. These include:

(i) Inspiratory capacity (IC) : Tidal volume + Inspiratory reserve volume. It is about 3000 -3500 ml.

(ii) Functional Residual Capacity (FRC) : Expiratory reserve volume + Residual volume. It is about 2500 ml.

(iii) Vital capacity (VC) : Inspiratory reserve volume + Tidal volume + Expiratory reserve volume. It varies from 3500 ml to 4500 ml depending upon age, sex and height of person.

(iv) Total lung capacity (TLC) : It is tidal volume + inspiratory reserve volume + Residual volume + Expiratory reserve volume. It is about 5800 ml.

With the exception of FRC and RV and TLC, all other lung values and lung capacities can be measured with the help of a simple spirometer.

Various pulmonary air volumes

All pulmonary volumes and capacities are about 20% to 25% less in women than in men, more in athletic people than asthmatics.