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In this article we will discuss about the functions of respiratory system of humans and its mechanisms.
Functions of respiratory system can be broadly classified into:
a. Respiratory
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b. Non-respiratory.
a. Respiratory Function:
It is to provide adequate volume of oxygen to the tissues. Normal person at rest needs about 250 ml of oxygen per minute. Atmospheric air enters the lungs during inspiration. Oxygen from the air diffuses through the alveoli into pulmonary capillary blood. The oxygenated blood reaches the left ventricle and from there it gets pumped to reach all parts of the body.
About 200 ml of carbon dioxide is produced in the body every minute because of tissue metabolism. From the tissues, carbon dioxide enters the blood to reach the lungs for the process of excretion. When deoxygenated blood reaches the lungs, carbon dioxide gets diffused from the pulmonary capillaries into the alveoli. The air from the alveoli is expelled out from the lungs by the process of expiration.
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b. Non-Respiratory Functions:
1. Regulation of acid-base (pH) balance.
2. Mast cells present in the lungs produce heparin, which acts as an anticoagulant.
3. Macrophages in the alveoli have phagocytic function.
4. Converting enzyme present in the lungs play a role in converting angiotensin I to angiotensin II, which is a powerful vasoconstrictor.
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5. The passage of the air through the larynx is essential for vocalization and has role in communication by speech.
6. Plays a minor role in body temperature regulation.
Mechanisms of Respiration:
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Respiration has two phases namely inspiration and expiration. During normal quiet inspiration due to contraction of muscles of inspiration, the chest and lungs expand. The pressure inside the alveoli (intra- alveolar pressure) falls below the atmospheric pressure.
Due to the pressure gradient developed in the direction of the alveolus, air moves from the atmosphere into the lungs. Because inspiration is brought about by the contraction of the muscles, the process of inspiration is an active one.
However, the process of expiration is normally a passive process. The relaxation of the muscles of inspiration and the recoiling of the elastic fibers present in the lungs is more than enough to bring about the expiration.
During expiration, since the alveoli are trying to recoil, the intra-alveolar pressure becomes more than the atmospheric pressure and hence air can be driven out of the lungs into the atmosphere. In forced expiratory states, expiration needs the active contraction of certain muscles. Hence in such states even expiration becomes an active process.
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a. Muscles of inspiration:
Diaphragm and external intercostals are the muscles of inspiration during a normal quiet breathing. However, during forced inspiration, contraction of sternocleidomastoid, scalene, serratus anterior and platysma muscles is very much required. These muscles are known as accessory muscles of inspiration.
b. Muscles of expiration:
The normal quiet expiration is a passive process. However, in forced expiration, even this phase becomes an active process and requires active contraction of certain muscles. The muscles that are involved in forced expiration are known as accessory muscles of expiration and they are internal intercostals and muscles of the anterior abdominal wall.
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Thorax is separated from the abdominal cavity by the diaphragm, a dome-shaped muscle. The thorax has three different diameters namely vertical, transverse and anteroposterior. During inspiration, the thoracic volume gets increased because of increase in the diameters of chest. The increase in the thoracic volume decreases the intra-alveolar pressure.
The most important muscle of inspiration is diaphragm supplied by the phrenic nerve. This is responsible for about 70% increase in the thoracic volume and the rest volume increase in thorax is contributed by the contraction of external intercostals supplied by the intercostal nerves.
This type of respiration is called as abdominothoracic type. In case the external intercostals play a major role in expansion of thorax, the type of respiration is known as thoracoabdominal type.
Contraction of the diaphragm alters the vertical diameter whereas the contraction of the external intercostals increases the anteroposterior and transverse diameters of the thoracic cavity.
