by Tenebris

Created on: December 20, 2011   Last Updated: December 21, 2011

Breathing is the mechanical process through which air is moved in and out of the lungs. Physiological respiration is the actual exchange of gases inside the lungs at the cellular level. Biochemical respiration refers to the metabolic process which combines oxygen and glucose to produce water, carbon dioxide, and adenosine triphosphate (ATP), the basic molecular unit of cellular energy transfer. As it is a purely biochemical reaction, the ATP energy equation will not be covered further in this article.

Breathing

In human beings, breathing is governed by the diaphragm and intercostal muscles. When the diaphragm contracts, it increases the space and thus volume of the thorax. When the diaphragm is relaxed, it reduces the thoracic volume. Since the lungs reside in the thoracic cavity, their volume expands and contracts together with the rest of the cavity.

The intercostal muscles connected to the rib cage also work in conjunction with the diaphragm to further expand or contract thoracic volume. When the diaphragm contracts, the intercostal muscles also contract, lifting the rib cage to expand thoracic volume. When the diaphragm is relaxed, the intercostal muscles also relax.

In accordance with the ideal gas law, air pressure increases as volume decreases. Compression of the lungs thus forces air out of the alveoli of the lungs through the bronchial passages and into the trachea and mouth/nose, while expansion of the lungs draws air in.

This process is known as negative pressure breathing, because incoming air is following its natural pressure gradient to the partial vacuum created in the lungs. Amphibians and some other creatures use positive pressure breathing instead: in which the air is first drawn into the mouth, then forced down into the lungs.

Physiological respiration

Now that fresh air is inside the lungs, oxygen must be selectively exchanged for carbon dioxide at the cellular level. In human beings, the physiological respiratory process consists of four steps: ventilation, pulmonary gas exchange, gas transport, and peripheral gas exchange.

Ventilation has already been covered under physical breathing. As it is controlled by voluntary muscles, it is the only part of the respiratory cycle which can be voluntarily controlled to a limited extent.

Pulmonary gas exchange is governed by changing partial pressures for oxygen and carbon dioxide, such that the oxygen tends to flow into the alveoli along the natural pressure gradient and the carbon dioxide

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