Exchange of Gases in the lungs


Gaseous exchange is the fundamental reason why we all exist. Breathing is the most primary process required for all of us ,as each and every cell in our body requires oxygen. Gaseous exchange is one of the mechanical process involving a series of steps. It is very important to understand the way each and every part of the respiratory system responds to the entry and exit of gases. The process is continuous and can never be stopped until the death A brief cessation of  2-3 minutes of gaseous exchange can lead to choking and the death of an organism, so, one can understand the importance of this process. The average respiratory rate in an  adult is 16-20 breaths per minute. In the case of children, it may go to the range 30-50 BPM. In order to perform aerobic respiration; organisms need a continuous supply of oxygen. Similarly, the carbon dioxide generated after the utilization by the body must be exhaled out regularly therefore it is a balanced process.


Pulmonary respiration and the cellular respiration

It is very important to understand the difference between the cellular respiration and the pulmonary respiration. Pulmonary respiration is more about how the gases enter in and exit out of the lungs as well as the kind of mechanisms really control this process. It doesn't talk about the chemistry of metabolism, such as the ATP cycle. On the other hand, cellular respiration focus on the process of how each cell in our body utilizes the oxygen to release energy. It is also important to note that the cellular respiration of plants differ from the animals. Plants use different methods for the intake of carbon dioxide and release oxygen. Furthermore, unicellular organisms use diffusion for the transfer of gases. The respiratory mechanism  between the mammals and aquatic animals are extremely different. This is because in mammals, lungs are predominantly used for the respiratory function whereas the creatures like fish, use gills for the respiratory function. On the other hand, insects use a system of spiracles and tracheae for taking the oxygen in.



Process of gas exchange 

Gaseous exchange is the mechanical process of entry and exit of air from in and out of the lungs. A typical respiratory cycle is divided into Inhalation and exhalation. During the process, the lungs absorb oxygen to transfer it to the blood, and blood in response, release the carbon dioxide to the atmosphere. External respiration occurs in the lungs by which the oxygen is diffused into the blood and carbon dioxide diffuses out of the blood. The carbon dioxide released from each cell reaches the lungs to excrete it through expiration.  Respiration that occurs at the level of lungs is external respiration but the exchange of gases between the alveoli and the blood is called internal respiration. The layers of cells lining the alveoli and the surrounding capillaries are piled in a very compact, thick, and single-layered manner so that the gas exchange becomes easy. Alveolar cells are very transparent to the oxygen and carbon dioxide but not to the toxic gases. This is because, lungs present a barrier called pulmonary barrier measuring about 1 micron (1/10,000 of a centimetre). The barrier strongly resist the entry of toxic and unnecessary materials so ,in order for the gases to exchange the barrier should allow them. 


Mechanism of breathing

Gas exchange takes place by 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 in the lungs than that of the blood; therefore, the partial pressure of oxygen is higher in the lungs. Similarly, the partial pressure of carbon dioxide is much higher in exhaled air, this will create a pressure gradient so that the carbon dioxide will be pushed out of the lungs. The inspiration and expiration are mediated by the respiratory muscles. On the other hand, in the internal respiration, the gas movement is aided by the osmosis,  diffusion and perfusion. Ventilation is the process of air movement in and out of the lungs by the help of respiratory muscles whereas the diffusion is the spontaneous spread of movement of gases, without the involvement of active energy. 



Oxygenation and deoxygenation of blood

Oxygenation and deoxygenation are the 2 processes help in the exchange of gases from in and out of the alveolar cells. The blood once pumped into the body reaches each and every cell. Our body cells utilize the oxygen for their cellular metabolism and release the cabin dioxide to the systemic circulation (venous circulation). The venous blood drained into the systemic veins is now deoxygenated or impure blood. The impure blood is collected from each corner of the body with the help of peripheral veins. From the peripheral veins, carbon dioxide reach the right atrium through superior vena cava and the inferior vena cava.  The systemic veins drain the deoxygenated blood into the IVC (inferior vena cava) and the SVC(superior vena cava), ultimately the impure blood reaches the right atrium. RA drains the blood to the RV instantly. From the RV, the venous blood 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 the left atrium through the pulmonary veins. These are the only veins carrying pure blood from the lungs to the heart. From the left atrium, the 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 resulting in the exhalation. The screenshot below depicts the course of movement of blood during the gas exchange.


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