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    HUMAN RESPIRATORY SYSTEM The system which brings about inspiration, expiration, exchange of gases in lungs and transport of gases betweenthe lungs and tissues is known as Respiratory system.   The human respiratory system consists of a pair of  nostrils, nasal cavity, nasopharynx, larynx, trachea,bronchi,   bronchioles and alveoli (air sacs) forming the lungs.The nostrils lead into nasal cavity, which opens into the upper part of the pharynx called nasopharynx.It continues into larynx or voice box or adam’s apple that connects the pharynx to the trachea.The opening of larynx (glottis is guardred by a leaf like epiglottis).The trachea or wind pipe is connected to the larynx at the posterior and is 11 cm long. It is guarded by 16-20incomplete ring of hyaline cartilages (C- shaped) which prevent it from collapsing.The trachea divides into two bronchi at the lower end. The right bronchus is wider.The bronchi are divided at the posterior into bronchioles. Which enter into the lungs.The respiratory tract from the nose to the bronchioles is lined by ciliated epithelium.The bronchioles divide into may alveolar duct each of which terminates in an alveolus (air chamber), the twolungs contain about 300 million alveoli .The lungs of man is spongy. The two lungs are enclosed in a double layered membrane, the pleura .The right lung is divided into 3 lobes and the left lung into two lobes . Inside the lungs the bronchioles divideinto alveolar ducts, which finally open into alveoli (air spaces).The lungs occupy most of the chest cavity. This cavity is lined with a serous membrane, the pleura.There is a small amount of serous fluid between the lungs and the pleura. The fluid lessens the friction betweenthe membrane and the lung.Internally, the cavity of the lung has very small (microscopic) air spaces, the alveoli. Each alveolus is lined by alayer of flattened polygonal squamous cells.The human lungs contain about 700 million alveoli, with a total surface area available 100 times that of the body.This makes a large surface area available to the lungs so that sufficient oxygen taken up by haemoglobin of theblood and CO 2 is given off.    MECHANISM OF RESPIRATION   The main purpose of respiration is to provide oxygen to the tissues and to remove CO 2 from them.The entire process is accomplished in three steps. (i) Breathing or pulmonary ventilation. (ii) Exchange of oxygen and carbon dioxide. (iii) Transport of gases in blood. Breathing and Pulmonary Ventilation : Breathing is a mechanical process and is completed in two phases, inspiration and expiration.In inspiration the ribs are elevated and the diaphragm contracted and flattened, the chest cavity is enlarged.This increase in the volume of the chest cavity and lungs causes the air pressure in the lungs to fall below theatmospheric pressure and air passes through the air passage ways to the lungs to equalize the pressure. In Expiration the ribs and diaphragm return to their srcinal position so the volume of chest cavity decreases.The distended elastic lungs then contract and the air is forced out.Changes in the intrapleural pressure also responsible for air entering and leaving the lungs.In inspiration, expansion of the thorax, aided by descent of the diaphragm, decreases into thoracic pressure from4 to 10 mm Hg, and air pushes into the lungs.Thus, in inspiration the lungs are extending passively in response to the various mechanisms that result in anincrease in thoracic volume.In expiration, the size of the thorax is decreased, the intrathoracic pressure is raised to-2mm Hg. and air is forcedout of the lungs.    The diaphragm is the main muscle of inspiration. If the diaphragm descends 10 mm, it will increase the thoraciccavity volume by 250 ml.When it relaxes, passive expiration results.The contraction and relaxation of the diaphragm is controlled by the phrenic nerves arising in the neck from the3rd 4th and 5th cervical nerves and passing down through the thorax to the diaphragm.Besides diaphragm, the external intercostals are the muscles mainly responsible for the elevation of the ribs ininspiration.They are inserted between two neighboring ribs, sloping forward and downward and their relaxation brings aboutpassive expiration.The internal intercostals form a deeper layer of muscle between the ribs with the fibers running in the oppositedirection, from above downward and backward.On Contraction, these muscle depress the ribs aiding in expiration during very deep breathing (active expiration). (a) Eupnea – Normal respiration (b) Hypernea– Increase in respiratory rate and depth. (c) Dyspnea – Irregularities of respiration. (d) Apnea – Cessation of respirationThe normal rate of respiration in the adult is 14 breaths/minute, but in children it may be up to 30/minute. Inexercise it is further increased.Each inspiration admits about 350 ml of new air to mix with the 2500 ml of old air present in the lungs.The quantity of new air that enters the lungs per minute is known as the minute volume, which in the averageadult is about 4900 ml (350 × 14).During exercise, the rate of breathing increases due to the increased demand for oxygen. The demand of extraoxygen is fulfilled by the expansion of rib cage. Tidal Volume : (TV) The volume of air inspired and expired by the lungs during normal effortless breathing, iscalled tidal volume . (TV is about 500 ml of air) Inspiratory Reserve volume (IRV) : The extra volume of air that can be inspired beyond the normal tidalvolume is called inspiratory reserve volume. (1RV, is about 2500 - 3000 ml of air)    Expiratory reserve volume (ERV) : The extra volume of air that can be expired beyond the normal tidalvolume is called expiratory reserve volume (ERV, is about 1000 ml of air). Residual Volume (RV) : The volume of air that remains in the lungs even after maximum forceful expiration iscalled residual volume (RV is about 1500 ml of air) Pulmonary Capacities : When any two or more of the above mentioned pulmonary volumes are consideredtogether, such combinations are called pulmonary capacities. Inspiratory Capacity : is the total amount of air a person can inspire by maximum distension of his lung. It isequal to tidal volume and inspiratory reserve volume. It is about 3500 ml of air. Functional residual capacity (RV + ERV) : is the amount of air that remains in lungs after normal expiration.It is about 2500 ml of air. Vital capacity (IRV + TV + ERV) is the maximum amount of air which can be expelled forcefully from lungsafter first filling with a maximum deep inspiration. It is about 4600 ml. Exchange of gases In both external as well as internal respiration, exchange of respiratory gases occurs. In external respiration,there is exchange of CO 2 of blood and O 2 of air or water while in internal respiration, there is exchange of O 2 of blood and CO 2 of the body cells.These gas exchanges are physical process and depends upon the principle of diffusion. The kinetic motion of the molecules provides the energy required for this diffusion of gaseousmolecule itself. Diffusion of any molecule takes place from high to low concentration.The process of diffusion is directly proportional to the pressure a used by the gas alone. The pressure exerted byan individual gas is called partial pressure. It is is represented as PO 2 , PCO 2 , PN 2 for oxygen, carbon dioxideand nitrogen respectively. Partial Pressure of a gas is the pressure exerted by the gas individually. Which is calculated as follows.Partial pressure of gas =Total pressure of the mixture of gasesPercentage of a gas in the mixture The partial pressure of a gas is directly proportional to its concentration in the mixture.Total pressure of the air at the sea level = 760 mm Hg.The inspired air ultimately reaches the alveoli of the lung which in turn receives the blood supply of thepulmonary circulation.At this place, the oxygen of the inspired air is taken in by the blood, and carbon dioxide is released into thealveoli for expiration.For efficient gaseous exchange, the organ must have the following characteristics : (i) It should have a large surface area ? (ii) It must be highly vascular, thin, moist, direct or indirect contact with source of oxygen (air or water),permeable to the respiratory gases (O 2 & CO 2 ).The respiratory membrane has a limit of gaseous exchange between alveoli and pulmonary blood. It iscalled diffusing capacity and is defined as the volume of gas, that diffuse through the membrane per minute fora pressure difference of 1mm Hg.At a particular pressure difference, the diffusion of carbon dioxide is 20 times faster than oxygen, and that of oxygen is two times faster that nitrogen.Due to the existing pressure difference of oxygen and carbon dioxide between the alveoli & the blood capillary,oxygen diffuses from alveolar air to the capillary blood, whereas carbon dioxide diffuses from capillary blood tothe alveolar air.
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