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 alveoli. lungs contain about 300 million alveoli. pleura.. 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. 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.
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 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 volume. (TV is about 500 ml of air) called tidal volume. 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. 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 O 2 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 PO 2, PCO2, PN 2 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 (O 2 & 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.
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 O 2 in water is rather low, but this shortcoming is overcome by the fact that the O 2 is bound to carrier substances in the blood. In human blood, 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 O 2 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. Hb4 + 4O2 Hb4O8 or Hb4 (O2)4 (oxyhaemoglobin) 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, therefore, contains 19.6 ml of O 2 (combined to haemoglobin) per 100 ml of blood. In addition to this there is about 0.2 to 0.3 ml of O 2 which is dissolved in the plasma. The arterial blood and the alveoli have the same O 2 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 O 2 and haemoglobin in the red blood cells to form oxyhaemoglobin is a reversible one.
The liberation of O 2 from the blood to the tissue is is just as important as its its rapid absorption by the blood during its passage through the lungs. Hb 4 Oxyhaemoglobin
⎯⎯ → 4 Hb Reduced haemoglo h aemoglobin bin
4O2
+
The reduced haemoglobin is further transported via blood to the lungs and the cycle is repeated while the O 2 that has diffused into the cells is utilized in the oxidation of carbohydrates, resulting in the release of CO 2 & energy. At high O2 pressure, the haemoglobin combines with O 2 to form oxyhaemoglobin. Each iron atom can bind one O 2 molecule, and when all sites are occupied, the haemoglobin cannot take on any more, since it is fully loaded or saturated. At low O2 pressure O2 dissociate from its binding, and the haemoglobin will eventually give up all its O 2. At any given O2 concentration there is a definite proportion between the amount of haemoglobin & oxyhaemoglobin. In this way the actual relationship between the partial pressure of O 2 and the degree of saturation of the haemoglobin with O 2 is shown by the remarkable oxygen haemoglobin dissociation curve.
Note : Increased CO2 concentration shifts the curve to the right. The curve shows that the haemoglobin is almost completely oxygenated (saturated) with O 2, at the O2 partial pressure of about 100 mm Hg). At higher O 2 pressure, no more O 2 is taken up by the haemoglobin. At lower O2 pressure, O2 is given off and at 30 mm Hg, O 2 pressure, half the haemoglobin is present as oxyhaemoglobin. As the O2 pressure decreases further, more oxygen is given off, and all is given up when the O 2 pressure reaches zero.
Thus, the degree of haemoglobin saturation is lowered with the fall in the partial pressure of O 2. In the passage of blood through the tissue where the O 2 tension is low, rapid dissociation of oxyhaemoglobin occurs, yielding a comparatively large quantity of O 2 to the surrounding surrounding tissues and cells where it is is most needed. lungs (PO2 = 100 mm Hg) Hb + O2 → HbO2 (Oxyhaemoglobin) Tissues
(PO2 = 30 to 40 mm Hg ) HbO2 → Hb + O2
During Exercise : There is a fall in tissue PO 2, an increase in PCO 2 and an increase in pH, local temperature and 2, 3- diphosploglycerate concentration. All these factors promote the release of oxygen from oxyhaemoglobin (shifting the oxygen-haemoglobin dissociation curve to the right) and thus increasing the efficiency of oxygen delivery to the active tissues. Factors Affecting Oxygen Dissociation Curve of haemoglobin Following four factors influence the dissociation curve. 1.
H+ concentration.
2.
Carbon dioxide tension
3.
Temperature
4.
Erythrocyte concentration of 2 ,3 diphosphoglycerate (DPG). Increase in these factors bring right word shift of the curve thereby decreasing the affinity of haemoglobin haemoglobin for oxygen.
Carbondioxide is evolved in the b ody as a result of various metabolic activities of the cells & diffuses into blood. The total amount of CO 2 in the various blood is about 60 ml per 100 ml blood, and the arterial blood contains about 50 ml total CO 2 per 100 ml. Thus, a relatively small amount of CO 2 is given off in the lungs. Carbon dioxide that diffuses into into the blood is transported in the following following three ways : (i)
Transport of CO2 in physical solution : As CO2 enters the blood from the tissues, tissues, it combines with water of the plasma plasma to form carbonic acid (H2CO3). Thus, about 7% of CO 2 is carried in solution in the plasma as carbonic acid. CO2 + H2O
H2CO3
HCO3– + H+
These ions then combine with the buffers of the blood. (ii) Transport of CO 2 as carbamino compounds : About 20 to 25% of CO2 is transported as carbamino compounds. In the red blood cells it combines directly. With the amino groups (– NH 2) of the haemoglobin to form the So- called carbaminohaemoglobin. CO2 + Hb. NH2
Hb. NH. COOH (carbaminohaemogl (carbaminohaemoglobin) obin)
(iii)
Transport of CO2 as bicarbonates : The rest, or about 70% of the total CO 2 is carried in the form of bicarbonates in both the plasma and red blood cells. As CO2 enters the blood cells from the tissues, it combine with water to form carbonic acid (H 2CO3), which dissociates to hydrogen ions (H +) and bicarbonate ions (HCO 3–). The latter diffuse into the plasma and with sodium or potassium ions is the plasma form sodium or potassium bicarbonate. Carbonic (Zn - containing enzyme) anhydrage
CO2 + H2O
+|
H Hydro Hydrogen gen Ion Ion
Na+ + HCO3
−
K+ + HCO3–
⎯ →
⎯⎯ →
+
−
HCO3 Bicarbonate ions
Na. HCO3 (sodium bicarbonate) KHCO3 (Potassium bicarbonate)
A small amount of bicarbonate ions is transported in the RBC. Whereas most of them diffuse into the plasma to be carried by it. The majority of bicarbonate ions (HCO 3–) formed within the erythrocytes erythrocytes diffuse out into into the plasma along a concentration gradient. Hydrogen ions combine with haemoglobin to form the haemoglobinic acid (H.Hb) Carbonic anlydrase CO2 + H2O H2CO3 H2CO3
H+ + HCO3–
KHbO2
KHb + O2
H+ + HCO3 + KHb
Haemoglobinic acid
H.Hb + KHCO3
The dissociation of H 2CO3 increases the number of bicarbonate ions in the red blood cells, & therefore the ions tend to diffuse away into the plasma. For each bicarbonate ion that comes out from the red cells, one negatively charged chloride ion present in the plasma moves into the red cells in order to maintain acid-base equilibrium for the blood & the electrical neutrality of the red blood corpuscles. Thus, chloride shift involves the passage of chloride ions from the plasma into the red blood corpuscles to balance, the bicarbonate ions that have passed from the red blood corpuscles into the plasma.
The CO2 capacity of the red blood corpuscles is further increased by the chloride shift because the removal of bicarbonates, in this way, enhances their formation from the carbonic acid. In the red blood corpuscles most of the buffering is provided by the haemoglobin (Hb) itself. The latter is a negatively charged blood protein & combine with positively charged hydrogen ions (formed in the dissociation of H2CO3 forming the haemoglobinic acid. This reaction is shown as follows :BHb– + H+
HHb– + B+ (haemoglobinic acid)
Liberation of CO 2 in the Lungs The CO2 is carried by the blood stream to the lungs in the form of carbonic acid, bicarbonates, and carbamino compounds. Due to a higher tension of CO 2 in the lung capillaries (than the tension of CO 2 in the lung alveoli), CO 2 is given off from the blood and the reaction taking place are as follows : (i)
H2CO3
(ii) NaHCO3 or
Carbonic anhydrase
⎯⎯⎯ ⎯⎯ ⎯⎯ ⎯⎯ →
H2O + CO2
acidity acidity of oxyhaemogl oxyhaemoglobin obin
⎯ ⎯⎯ ⎯⎯ ⎯⎯ ⎯⎯ ⎯⎯ →
2NaHCO3
(iii) HbNHCOOH
Na+ + HCO3–
acidity acidity of oxyhaemogl oxyhaemoglobin obin
⎯⎯ ⎯⎯ ⎯⎯ ⎯⎯ ⎯⎯ ⎯⎯ →
Increasing oxyhaemoglobin
⎯⎯⎯ ⎯⎯⎯ ⎯⎯⎯ ⎯⎯ →
Na2CO3 + H2O + CO2
HbNH2 + CO2
The reciprocal effect of oxygenation on acid strength of the haemoglobin, the so called Haldane effect. accounts for the CO2 exchange. Haldane effect : Binding of oxygen with haemoglobin tends to displace carbon dioxide from the blood. effect. This phenomenon is called Haldane effect. In the lungs, chloride ions move out of the red blood corpuscles & bicarbonate ions move back in. The enzyme carbonic anhydrase then promotes the rapid reformation of free CO 2, & this gas diffuses from the blood into the lung alveoli. Bohr Effect Carbon dioxide reacts with water to form carbonic acid that lower the pH in active tissue and induces oxyhaemoglobin to give up more of it O 2. This phenomenon is called Bohr effect. effect.
Regulation of Respiration of Respiration The respiratory rhythm is controlled by the nervous system. The rate of respiration can be enhanced as per demand of the body during strenuous physical exercises. A number of groups of neurons located bilaterally in the medulla oblongata control bilaterally in the medulla oblongata control the respiration. These are called respiratory centres. centres. Three groups of respiratory centres have been identified namely : dorsal respiratory group, ventral respiratory group and pneumotaxic centre. The dorsal respiratory group is present in the dorsal portion of medulla oblongata. The signals from these neurons generate the basic respiratory rhythm. rhythm. The nervous signals released from this group is transmitted to the diaphragm, which is the primary inspiratory muscle. The ventral respiratory group of neurons are located anterolateral to the dorsal respiratory group. During normal respiration, this remains inactive and even does not play any role in the basic respiratory drive, the respiratory signal of this group contributes to fulfil the d emand by regulating both inspiration and expiration. The pneumotaxic centre is located dorsally in the upper pons. It transmits signals to the inspiratory area. Primarily, it controls the switch off point of inspiration. When this signal is strong, the inspiration lasts only for 0.5 seconds or more, resulting into complete filling of lungs. The strong signal causes increased rate of breathing because inspiration, as well as expiration, is shortened. The concentration of CO 2 and H+ cause increased strength of inspiratory, as well as expiratory signal. However, oxygen has no such direct effect.
(i)
Bronchitis
It is inflammation of bronchi and is characterised by hypertrophy and hyperplasia of sero-mucous gland and goblet - cells lining the bronchi. It may be caused by cigarette smoking or exposure to pollutants.
(ii)
Bronchial Asthma It is caused by hyper sensitivity of the bronchioles to any foreign substance and is characterised by the spasm of the smooth muscles of the w alls of bronchiole. The excess mucous secreted by its wall may clog the bronchi & bronchioles. (iii) Emphysema It is the inflammation or abnormal distension of bronchiole or alveoli that results in the loss of elasticity of these parts. Cigarette smoking and chronic bronchitis are the two reasons for this. (iv) Pneumonia It is an acute inflammation of the alveoli caused mainly by Diplococcus infection. Most of the air space is occupied by the fluid with dead WBC. Uptake of oxygen becomes adversely affected. (v) Occupational lung diseases Silicosis & asbestosis are two common occupational diseases. They are characterised by fibrosis of upper part of lung.
(i)
Minimising the exposure of harmful dust at the work place.
(ii) Workers should be well informed about the harm of the exposure to such dusts. (iii) Use of protective gears & clothing by the workers at the work place. (iv) Regular health check up. (v) Holiday from duty at short intervals for the workers in such areas. The patient may be provided p rovided with symptomatic treatment like bronchodilators & antibiotics, to remove underlying secondary infection.
