Exchange of Gases in the lungs

A brief cessation of  2-3 minutes of gaseous exchange can lead to choking and the death of an organism, so, breathing is a continuous process. The average rate of respiration is 16-20 breaths per minute in normal adults and in the case of children, it maybe 30-50. In order to perform aerobic respiration; organisms need a continuous supply of oxygen. The carbon dioxide generated after the metabolism must be expired out regularly. Organisms have different methods for the intake of oxygen and the exhaling of carbon dioxide. Unicellular organisms use diffusion, some plants also use diffusion for transfer of gases. In mammals including human beings, the respiratory system does the job of exchange of gases into the lungs and out of lungs. Fish use gills for the respiratory function. Insects have a system of spiracles and tracheae which is used for taking in oxygen.


What is the gaseous exchange 


Gaseous exchange is the process of a combination of external respiration and internal respiration. During the process, the lungs absorb oxygen into the blood and send the carbon dioxide out of the blood. External respiration occurs in the lungs where oxygen gets diffused into the blood and carbon dioxide diffuses out of the blood to reach into the alveolar space. Internal respiration occurs at the cell level where oxygen diffuses out of the blood and carbon dioxide diffuses out of the cells.  Scientifically, when we speak about gas exchange, we consider the one that is taking place at the alveolar spaces where there is close contact between the lung tissues and the circulatory system. To assist the gas exchange, the layers of cells lining the alveoli and the surrounding capillaries are piled in a very close, thick, single-layered and transparent fashion so that the lung tissues and circulatory system are proximal to each other. The barrier between the lungs and blood capillaries in the pulmonary barrier having measuring about 1 micron (1/10,000 of a centimetre).In order for the gases to exchange they must cross in and out of this barrier. Gas exchange takes place based on the gradient (difference) in the partial pressure of oxygen and carbon dioxide between the lungs and the circulating blood. In an inspired gas there is more oxygen content hence the partial pressure of oxygen is higher than that of the blood. This difference will push the oxygen from high pressure to low pressure area, I.e. from lungs to blood. Similarly, while expiration carbon dioxide pressure is higher in blood than the lungs which will push the carbon dioxide from blood towards the lungs. The processes that assist the inspiration and expiration mediated by respiratory muscles are the ventilation, diffusion, and perfusion. Ventilation is the process of air movement in and out of the lungs. Diffusion is the spontaneous spread of gases, without any active energy or effort by the body. The movement takes place between the gas in the alveoli and the blood in the capillaries in the lungs. Perfusion is help in pumping the blood throughout the lungs.


Oxygenation and deoxygenation of blood

The blood after the utilization by the body is deoxygenated because there is more carbon dioxide in this content. The carbon-rich blood enters the heart through superior and inferior vena cava. From the IVC and SVC, blood spontaneously pushed to right atrium and ventricles. From the RV it is pumped to lungs through pulmonary arteries and here comes the role of gas exchange to convert this deoxygenated blood into oxygen-rich blood with the help of pulmonary alveoli and blood capillaries. After the purification (oxygenation ) blood is pushed back to hear through pulmonary veins into the left atrium., From the left atrium blood moves to the left ventricle. The left ventricle will push the blood to aorta thereby helping to reach the systemic circulation. To support the absorption of oxygen and release of carbon dioxide, about 5 -8 litres of air per minute is brought in and out of the lungs, and about 3/10th  of a litre (of oxygen is transferred from the alveoli into the blood each minute.  At the same time, a similar volume of co2 moves from the blood to the alveoli and is exhaled.




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