Mechanism of Breathing and Its Regulation in Humans

The stage of respiration during which air is inhaled into the lungs through the mouth or nose due to muscle contraction and then exhaled due to muscle relaxation is called breathing. 

1.0Mechanism of Breathing

Lungs cannot expand or contract on their own. The contraction and expansion of lungs is brought about by diaphragm muscles and external intercostal muscles. 

Inhalation (Inspiration)

Inhalation is intake of fresh air from outside into the alveoli of the lungs. 

It occurs by expansion of lungs which is brought about by enlargement of thoracic cavity. Inhalation involves the following steps:-

The diaphragm (a dome shaped muscular structure that separates thoracic cavity from abdominal chamber) contracts so that the diaphragm lowers down and becomes flat.

External intercostal muscles contract so that the ribs and sternum are pulled upward and outward. This causes enlargement of the thoracic cavity resulting in the expansion of lungs.

Expansion of lungs reduces the pressure of air inside so that the fresh air is pulled from outside into the lungs passing through nostrils, trachea and bronchi.

Fresh air has a rich supply of O₂ which goes into the blood passing through thin membranes of alveoli and blood capillaries. As a result the blood in the capillaries becomes loaded with oxygen and expels carbon dioxide into the alveoli for exhalation.

Exhalation [Expiration]

The mechanism of breathing out of carbon dioxide is called exhalation.

During exhalation, muscle of the diaphragm relaxes so that the diaphragm becomes dome shape.

The external intercostal muscles also relax by which ribs and sternum comes downward and inward, by which the volume of thoracic cavity is reduced. It increases the air pressure so that the foul air moves out.

Residual volume (RV) is the amount of air remaining in the lungs at the end of a maximum expiration.

Cellular Respiration

Once inside the tissues, oxygen acts upon the digested food (glucose) which has reached the cells of the tissues. As a result, energy and carbon dioxide are released. This occurs in the mitochondria of the cells and is called cellular respiration.

Differences Between Cellular Respiration and Breathing

2.0Lungs Capacity and Breathing Rate

The total volume of the lungs when fully inflated is about 5 litres in an adult. However, in quiet breathing, when asleep or at rest, we normally exchange only about 500 cm³ (1/2 litres). During exercise we can take in and expel an extra 3 litres. There is a residual volume of 1.5 litres which cannot be expelled, no matter how hard we breathe out.

At rest, we normally inhale and exhale about 16 times per minute. During exercise, the breathing rate may rise to 20 or 30 breaths per minute. The increased rate of breathing during exercise allows more oxygen to dissolve in the blood and supply the active muscles. 

3.0Role of Blood During Gaseous Exchange

One of the most important functions of the blood is to transport oxygen to different organs of the body and collect carbon dioxide from them. A respiratory pigment-haemoglobin (Hb) present in the red blood corpuscles (RBCs) acts as a respiratory carrier in our body.

Haemoglobin + Oxygen → Oxyhaemoglobin (In the blood capillaries of lung alveoli)

Oxyhaemoglobin → Oxygen + Haemoglobin (At tissue level)

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