Mechanism of Breathing

The process of inspiration and expiration happens due to pressure
changes in the thoracic cavity. The thorax is an airtight compartment bounded
by the sternum in front, the vertebral column at the back, the ribs encircling
the sides and the diaphragm found below.

The rib bones are provided with the two sets of muscles namely external and internal intercostal muscles. By the contraction and expansion of these muscles the volume of the thoracic cavity is reduced or increased. The floor of the thoracic cavity is completely closed by the diaphragm. The act of breathing is performed by expansion and contracton of the thoracic cavity. (Mechanism of Breathing)

Inspiration

Inspiration is the process by which fresh air is drawn into the
lungs. It is an active process. The muscles participating in the inspiration
process are external – intercostal muscles present in between the ribs and
the diaphragm.

During quiet respiration, contraction of external intercostal muscle
causes the ribs to move anteriorly and outwardly. This movement enlarges
the cavity of the thorax by increasing it side to side and in dorso-ventral
dimensions. (Mechanism of Breathing)

The contraction of radial muscles of the diaphragm leads to
flattening of inelastic, dome shaped central part of the diaphragm. As a result
of these muscular movements, the volume of the thoracic cavity is increased.
This causes the air pressure within the lungs to fall below the atmospheric
pressure. So air (tidal air) from outside passes through the air passage into
the lungs to equalize the pressure.

Expiration

Inspiration is followed by expiration. It is a passive process.
Expiration is the process by which air is exhaled or blown out from the lungs.
The expiration results when the volume of the thoracic cavity is decreased
and air pressure in the lung is increased. The expiratory process involves the
following movements.
1. The diaphragm relaxes and rises to resume the original dome shape.
2. The ribs take their original position as a result of contraction of the internal
intercostal muscles.

Gaseous exchange in the alveoli

Once the air is within the lungs the process of gaseous exchange
begins. Capillaries of the pulmonary artery remains close to the wall of the
alveloli. This enhances the exchange of gases.
Oxygen carbon-di-oxide water vapour
Inhaled % 21.00 0.04 variable
Exhaled % 15.7 3.6 saturated

Oxygen and carbon-di-oxide are exchanged across the alveolar
membrane by diffusion from the site of higher to low partial pressure until the
partial pressure of the two regions are equal. This process is a simple
physical one which does not involve any secretary or active transport
mechanism.

In the atmospheric air there is a high concentration of oxygen
20-95% (PO2 140mm Hg) while the proportion of carbon dioxide is low
(0.04%).

The alveolar PO2 is about 100mm Hg and the PO2 of venous blood
is about 40mm Hg. This pressure gradient is sufficient for the transfer of O2.
The PCO2 of venous blood is 46mm.Hg and that of alveolar air is only 6mm.Hg
(1/10th of O2), it is adequate for CO2 transfer by diffusion. CO2 diffuses 20
times faster than O2.

Mechanism of Breathing

Inspiration

Expiration

Gaseous exchange in the alveoli

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