Physiology of respiration-1

Physiology of Respiration

The major function of the respiratory system is to supply the body with oxygen and dispose the carbon dioxide. To accomplish this function, three distinct processes must happen. These three processes which are collectively called respiration include the following:

• Pulmonary Ventilation,
• External Respiration,
• Internal Respiration.

Pulmonary Ventilation

Pulmonary ventilation or breathing is the movement of air into and out of the lungs. In this process, the air is exchanged between the lungs and the environment. As the air enters in the lungs, it is distributed throughout the alveoli. There are two phases of ventilation:

• Inspiration (Inhalation): Is the breathing in of air or drawing of air into the lungs.
• Expiration (Exhalation): Is the breathing out of air or expulsion of air from the lungs.

Inspiration

Since the process of inspiration or inhalation requires energy for the contraction of the muscles, it is an active activity. The diaphragm and intercostal muscles flex during this procedure to widen the thoracic cavity. The thoracic cavity lengthens when the diaphragm contracts because it flattens and lowers itself.

In addition, as the intercostals contract, the ribs are pulled upward and outward, widening the thoracic cavity anteriorly, posteriorly, and laterally. The pleura moves up and out with the chest wall, increasing the volume of the thorax's interior. This time frame is 2 seconds long.

The pressure inside the lungs must be lower than the ambient pressure in order for air to be pulled into the lungs. The pressure that the air (gases) surrounding the body exerts is known as atmospheric pressure. The air pressure is 760mmHg at sea level. The intra-thoracic pressure falls (754mmHg) below atmospheric pressure as the thoracic cavity capacity rises. As the thoracic cavity enlarges, the lungs fill it. Air is drawn into the lungs as a result of the lungs expanding and lowering their internal pressure below atmospheric levels. As a result, inspiration happens.

Expiration

Expiration or exhalation is a passive process because no muscular contraction are involved and so it does not require the expenditure of energy. During this process, the muscles of respiration Expiration or exhalation is a passive process because no muscular contractions are involved and relax, allowing the ribs and diaphragm to return to their original positions. The diaphragm resumes its original dome shape. The internal intercostal muscle brings the ribs to their previous position i.e. the chest wall moves downwards and inwards and the volume of the thorax decreases. The elastic lung tissue (alveoli) which were stretched during inspiration now recoils too. This reduces the intrapulmonary space and increases the pressure inside the lungs. As the pressure inside the lungs exceeds (762mmHg) than in the atmosphere, air is expelled from the lungs and the respiratory tract. Thus expiration occurs. The lungs still contain some air and are prevented from complete collapse by the intact pleura.

Exchange of Gases

Gases exchange occurs by the two processes: external respiration, and internal respiration. Gases exchange occurs through the process of diffusion. Gases move from the region of their higher concentration to the region of lower concentration through the diffusion process. As there is higher concentration of O2 in alveoli and higher concentration CO2 in the blood, O2, diffuses from the alveoli into the blood and CO2 diffuses from the blood into the alveoli. Thus, gases exchange occurs between the alveoli and the blood capillaries. The oxygenated blood then return to the heart and pumped throughout the body.

External Respiration

External respiration is the exchange of gases between the blood in pulmonary capillaries and the alveoli of the lungs. In this process, O2 is absorbed from the air and taken to the blood and CO2 is excreted from the blood and taken out to the air. External respiration converts unoxygenated blood from the right side of the heart into oxygenated blood that returns to the left side of the heart.

Internal Respiration

Internal respiration is the exchange of gases between the blood and body cells (tissues). That is why it is also called "tissue respiration". In this process, O2 diffuses from the bloodstream through the capillary wall into the body tissues. At the same time, CO2 diffuses from the tissues into the extracellular fluid, then into the bloodstream towards the venous end of the capillary. Any defect in the circulatory system greatly affects tissue respiration. Internal respiration occurs in tissues throughout the body. When the blood reaches the capillaries of the systemic circulation, its O2 concentration is higher than the tissue cells. Likewise, the concentration of CO2 is higher in cells than that of the capillary blood. Due to this concentration difference, O2 diffuses out of the capillaries into the tissue cells, and CO2 diffuses out of the tissue cells into the blood capillaries through the process of diffusion. Thus, the high CO2 contained deoxygenated blood then returned to the heart and is pumped to the lungs for another cycle of external respiration.

Regulation of Respiration

The term regulation of respiration refers to the adjustment of respiration according to the internal and/ or external environment of the body in order to maintain and restore normal health. E.g. the rate and depth increases when some exercises are done. Regulation of respiration is a complex that must keep pace with moment-to-moment changes in cellular oxygen requirements and carbon dioxide production. Respiration is regulated by nervous impulses and chemical composition of blood.

Nervous Control of Respiration

Inspiration and expiration are dependent upon alternate contraction and relaxation of respiratory muscles. The contractions and relaxations are regulated by nerve impulses primarily from the respiratory control centers located in the medulla and pons of the brain stem. Nerve impulses from the medulla are modified by the pneumotaxic center in the pons. Respiration is regulated so that the levels of O2, CO2, and acid are kept within certain limits.

Nerve endings on the walls of the arch of aorta and the carotid artery are sensitive to O2 deficit in blood. So when there is a shortage of O2 in blood, these nerves become stimulated and transmit impulses to the respiratory centers. From the respiratory center, nerve fibers are again transmitted by the phrenic nerves to diaphragm and the intercostal nerves to the intercostal muscles. These nerve fibers regulate the rate, depth, and rhythm of respiration.

Chemical Control of Respiration

The chemical regulatory mechanisms kick in when the blood's chemical composition changes. The concentrations of CO2 and H+ ions in blood are the two chemical factors that have the greatest impact on controlling the respiratory center's activity. Chemoreceptors are the sensory neurons that respond to chemicals. Both the aortic bodies and the carotid bodies, which are found in the walls of the aorta and the carotid arteries, contain these.

An rise in PCO2, an increase in the concentration of H+ ions in blood, and to a lesser degree, a reduction in O2 all excite these chemoreceptors. The inspiratory neurons of the respiratory center receive signals from the chemoreceptors when they are triggered. Increased rate and depth of respiration result from the stimulation of the respiratory center, which increases the amount of CO2 released into the exhaled air. The chemoreceptors are not stimulated and no stimulatory impulses are sent to the respiratory center if PCO2 or H+ ions levels fall in the blood. Reduced respiratory activity is the result.

Increased CO2, H+ ions in the blood

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Stimulation of chemoreceptors

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Transmission of nerve impulses to the respiratory centers

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Increased rate and depth of respiration

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CO2 excretion

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Decreased CO2 or H+ ions level in blood

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Depression of respiratory centers

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Decrease in respiratory activity