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Describe how the structure of the lower respiratory system facilitates gas exchange with reference to the physical principles which regulate this process.

The Function of the Lower Respiratory System

The lower respiratory system or tract comprises of trachea, bronchus and bronchioles, alveoli that comprises of lungs. This system pulls the air from the upper respiratory system in absorbing oxygen and releasing the carbon dioxide outside the body in exchange. There are other supplementary structures like rib or thoracic cage, diaphragm provides support and protect the above mentioned structures. The upper respiratory tract has the function to take in the air and pass it to lower respiratory tract towards trachea or wind pipe. There are alveolar and bronchial structures and the anatomy is such that there is drawing in of air and its processing by the structures of lower respiratory tract that facilitate gas exchange. Therefore, the following essay involves the discussion of structure and function of lower respiratory system facilitating gaseous exchange in reference to physical principles that regulate this process of gas exchange.

Physical principles of gaseous exchange involve diffusion of oxygen and carbon dioxide through the respiratory membrane, ventilation, pulmonary perfusion and exchange of gases. The lower respiratory tract facilitates gaseous exchange working in reference to the above physical principles of gas exchange. Firstly, the trachea covered by cartilaginous rings is an inch in diameter and tubeless. It extends from bottom of larynx below and behind the sternum where it starts branching into smaller tubes called bronchi. At the time of inhalation, air is warm and filtered by upper respiratory tract passing from pharynx to larynx into trachea going down to bronchi and lungs (Cunningham et al. 2014). The cartilaginous rings around the trachea tube support it and help to prevent it from over-expansion or collapse when air is sucked too hard. These rings are C-shaped with gap on posterior side that aid trachea in bending when oesophagus presses hard against swallowed food.  The deoxygenated air from lungs passes back up to trachea during the process of exhalation.

The next structure is bronchi being the passageways that bring air inside and out of lungs. The tubes of the primary bronchus branch off from bottom of trachea subdividing into further secondary and tertiary bronchi finally into bronchioles. These small airways serve the purpose of delivering oxygen-rich air to the lungs from trachea. During the process of exhalation, deoxygenated blood having rich carbon dioxide leaves the lungs through reverse route (Ionescu 2013). Another mechanism takes place in bronchioles where smooth muscle relaxation of bronchioles causes dilation allowing greater ventilation and bronchoconstriction that causes the opposite effect in bronchioles.

Physical Principles of Gaseous Exchange

Lungs are the main and essential organs for the gaseous exchange in the respiratory system. The main gaseous exchange takes place in this organ and responsible for the exchange between oxygen and carbon dioxide. The organ is protected inside the thoracic cage divided into left and right lung. The left lung comprises of two lobes and has slight small volume as compared to right lung. There is a curve at the cardiac notch accommodating heart. Right lung comprises of three lobes being slightly shorter as the diaphragm muscle sits higher below where the liver is accommodated. Oxygen taken from the air is absorbed into the bloodstream that passes through the microscopic sacs called alveoli into surrounding capillaries (Albertine 2016). The deoxygenated air or carbon dioxide waste diffuses the opposite way from capillaries to alveoli. After this, lungs expel the deoxygenated air during the exhalation process. In this organ, physical principle of gaseous exchange takes place called diffusion. Blood present in the pulmonary capillaries takes oxygen and takes out carbon dioxide through the process of diffusion. In this, the exchange of carbon dioxide and oxygen takes place between alveoli in lungs and blood (Protti et al. 2015).  From alveoli oxygen diffuses into the bloodstream and oxygen from blood enters the alveoli. This is carried out through diffusion requiring a concentration gradient where partial pressure or concentration of oxygen in alveoli is kept at a high level or gradient than blood and similarly, partial pressure or concentration of carbon dioxide in alveoli should be less as compared to in blood. This would facilitate gaseous exchange through diffusion in alveoli in lungs and bloodstream (Mercer and Crapo 2015).


External respiration also takes place in alveoli, as these microscopic air sacs served by bronchioles exist inside lungs facilitates gaseous exchange. The terminal ends of respiratory tract where external respiration takes place called alveoli are filled with air during inhalation from the bronchioles. The oxygen diffuses through the pulmonary networks in alveoli surrounding it and pumped into bloodstream. From the deoxygenated blood, carbon dioxide diffuses into alveoli from capillaries expelled through exhalation (Leong and Leong 2016).

Diaphragm is the organ that provides muscle for breathing forming the floor for thoracic cavity. This organ provides the physical process of breathing during inhalation and exhalation. During the process of inhalation, contraction of diaphragm takes place and movement takes inferiorly towards abdominal cavity. This mechanism allows thoracic cavity and lungs’ volume to increase a it takes place during a deep breath (Weibel 2015). During the process of normal exhalation, relaxation of diaphragm takes place along with external intercostals muscles and lungs and thoracic cavity decrease takes place as air is expelled.

Trachea: Tube of Cartilaginous Rings

In all the above organ’s structure and functions, physical properties of gaseous exchange are involved. The primary function of respiratory system in humans is the exchange of gases; carbon dioxide and oxygen. Three main principles or processes are involved in the exchange of gases (oxygen and carbon dioxide) from the external environment to the lungs flowing in the bloodstream, ventilation, perfusion and diffusion. Ventilation mechanism takes place in which the air moves inside and outside of the lungs. Another mechanism, diffusion is the process in which there is spontaneous movement of gases without any effort or energy between the blood in capillaries of lungs and gas in alveoli (Morrell 2015). Perfusion is also involved in the gaseous exchange process where the heart pumps oxygenated blood throughout the lungs.

The partial pressure is important for the exchange of carbon dioxide and oxygen in external respiration between external environment and cells. There are efficient thin walls in alveoli and capillaries that are abundant providing total surface area of 75 square metres. Similarly, in internal respiration, there is intracellular oxygen sue for producing ATP and by the process of simple diffusion along the partial pressure gradients.  Pulmonary perfusion is the process where actual blood flow takes place through pulmonary circulation. The exchange of gases takes place at the air-blood interface facilitated by alveolar-capillary membrane due to large surface area and thinness. The blood pumped into lungs by right ventricle via pulmonary artery. This artery is divided into right and left branches supplying blood to both lungs. These divided parts branch out and supply to each lung with 2% blood pumped by right ventricle that does not perfuse into the alveolar capillaries. This blood is called shunted blood draining into left side heart without the participation of alveolar gaseous exchange (Gilbert-Barness, Spicer and Steffensen 2014).

From the above discussion, it can be concluded that lower respiratory system structure is made in such a way where it facilitates efficient exchange of gases; carbon dioxide and oxygen. The gaseous exchange that takes place in lower respiratory tract involves the physical principles of this regular mechanism. Diffusion takes place in alveoli of lungs due to diffusion towards the concentration gradient. Trachea warms the air and filters it down from pharynx to larynx into trachea finally to lungs. Bronchi act as passageways bringing oxygen rich air from trachea to lungs. Lungs being the essential organs comprises of alveoli are important for the gaseous exchange where oxygen diffuses into alveoli and surrounding pulmonary capillaries to the bloodstream. Similarly, carbon dioxide from deoxygenated blood diffuses into alveoli from capillaries via diffusion and is expelled through exhalation. This is the way lower respiratory tract facilitates gaseous exchange efficiently aligning with the physical principles of gaseous exchange.

References

Albertine, K.H., 2016. Anatomy of the lungs. In Murray and Nadel's Textbook of Respiratory Medicine (Sixth Edition) (pp. 3-21).

Cunningham, F., Leveno, K., Bloom, S., Spong, C.Y. and Dashe, J., 2014. Williams Obstetrics, 24e. Mcgraw-hill.

Gilbert-Barness, E., Spicer, D.E. and Steffensen, T.S., 2014. Respiratory system. In Handbook of Pediatric Autopsy Pathology (pp. 329-354). Springer, New York, NY.

Ionescu, C.M., 2013. The human respiratory system. In The Human Respiratory System (pp. 13-22). Springer London.

Leong, F.J.W.M. and Leong, A.S.Y., 2016. Anatomy and Histology of the Human Lung. A Color Atlas of Comparative Pathology of Pulmonary Tuberculosis, p.31.

Mercer, R.R. and Crapo, J.D., 2015. Architecture of the gas exchange region of the lungs. In Comparative Biology of the Normal Lung (Second Edition) (pp. 93-104).

Morrell, M.J., 2015. One hundred years of pulmonary function testing: a perspective on ‘The diffusion of gases through the lungs of man’by Marie Krogh. The Journal of physiology, 593(2), pp.351-352.

Protti, A., Andreis, D.T., Milesi, M., Iapichino, G.E., Monti, M., Comini, B., Pugni, P., Melis, V., Santini, A., Dondossola, D. and Gatti, S., 2015. Lung anatomy, energy load, and ventilator-induced lung injury. Intensive care medicine experimental, 3(1), p.34.

Weibel, E.R., 2015. On the tricks alveolar epithelial cells play to make a good lung. American journal of respiratory and critical care medicine, 191(5), pp.504-513.

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