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Mechanism of Respiration

The process of respiration involves the breathing-in of the oxygen-rich air from outside (inspiration) and breathing-out of the air loaded with carbon dioxide (expiration).

Mechanism of Inspiration

During inspiration the intercostal muscles in between the ribs pull them upwards and outward. The muscular diaphragm contracts and flattens. Because of these movements of the ribs and diaphragm the chest expands and the volume of the thoracic cavity is increased. The simultaneous expansion of the pleural cavities surrounding the lungs creates a partial vacuum. The result is the rushing in of external air through the trachea into the lungs. The process of inspiration is also called inhalation.

Mechanism of Respiration

Mechanism of Expiration

The expelling of air or exhalation from the lungs takes place when the size and pressure of the thoracic cavity are reduced. During this process the ribs are lowered by the intercostal muscles and the diaphragm becomes raised to resume its original position. As a result of these movements the size of the thoracic cavity is reduced and the lungs become compressed. The pressure in the lungs is higher than the atmospheric pressure and air rushes out of the lungs through the trachea and nose.

Even after maximum forced exhalation, some air always remains in the lungs. This is called residual volume.

The maximum volume of inspirable air is called inspiratory capacity. Similarly vital capacity is the amount of air that can be forcibly exhaled after a maximum inspiration. Vital capacity is taken as measurement of pulmonary (lung) function.

The volume of air normally inhaled which is equal to the volume of exhaled air, is called tidal volume.

Gaseous Exchange in the Lungs

The movement of air in and out of the lungs and the distribution of air within the lungs are referred to as ventilation of lungs are referred to as ventilation of lungs. The exchange of oxygen (O2) and carbon dioxide (CO2) between alveolar air and lung capillaries, takes place by simple diffusion. The diffusion of gases is caused due to the differential partial pressure of the respiratory gases. Gases move from a high pressure area to low partial pressure area. Venous blood enter the lung capillaries carrying oxygen at a pressure of 40 mm (at rest) and carbon dioxide at a pressure of 46 mm. The oxygen pressure of the alveolar air is 100 mm and the carbon dioxide pressure is 40 mm. Therefore oxygen passes from the alveolar air into blood and carbon dioxide from blood to air sacs.  Arterial blood leaves the lungs, carrying oxygen at a pressure of 100mm and carbon dioxide at a pressure of 40mm. The average tissue fluid pressure of oxygen is 40 mm and carbon dioxide 46 mm. Therefore, oxygen passes from the blood to tissue fluid and carbon dioxide from tissue fluid into blood.

Mechanism of Exchange of Oxygen and Carbon dioxide in the Lungs

Transport of Respiratory Gases

Oxygen Transport
In mammals, the respiratory gases (O2 and CO2) are transported through blood. Blood itself is not the carrier of the respiratory gases. It contains an iron-containing respiratory pigment called haemoglobin which is confined to the red blood cells. The haemoglobin has a special affinity for the respiratory gases. When the blood which is low in O2 reaches the lungs, the heamoglobin of the red blood corpuscles becomes loaded up with O2 to form oxyhaemoglobin.

Oxyhaemoglobin on reaching the cellular level, where partial pressure of oxygen is very low, dissociates into free oxygen and reduced haemoglobin. A small amount of oxygen also gets dissolved in the blood plasma to form a physical solution.

Carbon dioxide Transport
The production of CO2 due to the oxidation of digested food by the cells results in high tissue carbon dioxide partial pressure. This causes the diffusion of CO2 from the cells into the blood through the capillary walls. The CO2 in the blood is transported in two ways, through the plasma and through the blood corpuscles. A small amount of CO2 combines with haemoglobin to form carboxyhaemoglobin inside the red blood corpuscles and is transported to the lungs in this form. A greater portion of CO2 reacts with water to form carbonic acid. Bicarbonates are formed from this carbonic acid and these are transported both in the plasma and red blood corpuscles. From these complexes, CO2 is released when blood is oxygenated in the lungs. Nearly 80% of the CO2 in the plasma is transported in the form of bicarbonates of sodium and potassium and the remaining 20% is transported in the form of carboxyhaemoglobin in the red blood cells.
Besides haemoglobin there are other pigments for the transportation of respiratory gases in animals. All vertebrates have haemoglobin. However, other animals possess pigments like haemocyanin and haemerythrin.

Regulation of Respiration

The regulatory centre for respiration is situated in the medulla of the brain. This respiratory centre is extremely sensitive to the concentration of CO2 in the arterial blood. A decrease or increase in the concentration of CO2 in the arterial blood results in slowing or in the acceleration of respiration respectively. This is reflected in changes in the rate and depth of breathing. In case the breath is held by closing the nose with fingers, CO2 builds up in the blood. When the nose opens again, the breathing becomes accelerated.

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