HUMAN RESPIRATORY SYSTEM
The system which brings about inspiration, expiration, exchange of gases in lungs and transport of gases between the 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 16-20 incomplete 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 two lungs 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 divide into 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 between the membrane and the lung. Internally, the cavity of the lung has very small (microscopic) air spaces, the alveoli. Each alveolus is lined by a layer 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 the blood and CO2 is given off.
MECHANISM OF RESPIRATION
The main purpose of respiration is to provide oxygen to the tissues and to remove CO2 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 the atmospheric 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 original 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 from 4 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 an increase in thoracic volume. In expiration, the size of the thorax is decreased, the intrathoracic pressure is raised to-2mm Hg. and air is forced out of the lungs.
The diaphragm is the main muscle of inspiration. If the diaphragm descends 10 mm, it will increase the thoracic cavity 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 the 3rd 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 in inspiration. They are inserted between two neighboring ribs, sloping forward and downward and their relaxation brings about passive expiration. The internal intercostals form a deeper layer of muscle between the ribs with the fibers running in the opposite direction, 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 respiration The normal rate of respiration in the adult is 14 breaths/minute, but in children it may be up to 30/minute. In exercise 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 average adult is about 4900 ml (350 × 14). During exercise, the rate of breathing increases due to the increased demand for oxygen. The demand of extra oxygen 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, is called 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 tidal volume 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 tidal volume 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 is called residual volume (RV is about 1500 ml of air) Pulmonary Capacities : When any two or more of the above mentioned pulmonary volumes are considered together, 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 is equal 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 lungs after 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 CO2 of blood and O2 of air or water while in internal respiration, there is exchange of O2 of blood and CO2 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 gaseous molecule 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 by an individual gas is called partial pressure. It is is represented as PO2, PCO2, PN2 for oxygen, carbon dioxide and 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 gases Percentage 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 the pulmonary circulation. At this place, the oxygen of the inspired air is taken in by the blood, and carbon dioxide is released into the alveoli 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 (O2 & CO2). The respiratory membrane has a limit of gaseous exchange between alveoli and pulmonary blood. It is called diffusing capacity and is defined as the volume of gas, that diffuse through the membrane per minute for a 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 to the alveolar air.
TRANSPORT OF GASES IN BLOOD :
Blood is the medium for the transport of oxygen from the respiratory organ to the different tissues, and carbon dioxide from tissue to the respiratory organ. Transport of Oxygen : The solubility of O2 in water is rather low, but this shortcoming is overcome by the fact that the O2 is bound to carrier substances in the blood. In human blood, the O2 carrier respiratory pigment is haemoglobin (a conjugated protein made up of haem, a prosthetic group containing iron, and globin the protein portion). The maximum amount of O2 which the normal human blood can absorb is 20 ml per 100 ml of blood. When O2 passes from the lung alveoli into the lung capillaries, it diffuses into the blood and unite with haemoglobin to form oxyhaemoglobin. Hb4O8 or Hb4 (O2)4 (oxyhaemoglobin) Hb4 + 4O2 Under the normal conditions the arterial blood which has been exposed to the alveoli of the lungs is not quite completely oxygenated. With an O2 tension of 100 mm of Hg, it is usually 98% saturated and therefore, contains 19.6 ml of O2 (combined to haemoglobin) per 100 ml of blood. In addition to this there is about 0.2 to 0.3 ml of O2 which is dissolved in the plasma. The arterial blood and the alveoli have the same O2 pressure (100 mm of Hg). But the cells and the tissues of the body the O2 tension is considerably low (1 to 40 mm of Hg). The O2 is accordingly liberated from the oxyhaemoglobin and diffuses out from the blood through the thin capillary walls into the cells. This is made possible by the important fact that the combination between O2 and haemoglobin in the red blood cells to form oxyhaemoglobin is a reversible one.
The liberation of O2 from the