CHAPTER – 17: Breathing and Exchange of Gases
Every
living cell requires continuous expenditure of energy for various life
processes like growth, development and multiplication. This energy is
derived from the oxidation of organic compounds. The biological
oxidation of these compounds constitutes the process of respiration.
Respiration is thus defined as, “the biochemical oxidation of organic compounds like glucose to yield energy”.
Organs for respiratory exchange in various animals:· In simple animals like Amoeba, Paramecium, the body organisation is very simple, so gases can diffuse in and out from the general surface of the body. The air diffuses across the membrane from the side where its partial pressure is more to the side where its partial pressure is less. However, there are no special organs of respiration.
· There are no special organs for respiration in Hydra as the body organisation is very simple and the cells are more or less directly exposed to the environment. Dissolved oxygen enters into the cells of Hydra through the general body surface, as there is less oxygen concentration within the cells. Carbon dioxide produced after respiration also comes out in a similar way. This process is termed as diffusion.
· There are no special respiratory organs in earthworms and leeches but the exchange of gases occurs through the skin (cutaneous respiration). The skin is always kept moist by the secretions of mucous glands, and is richly supplied with blood capillaries. Oxygen from the atmosphere dissolves into mucous and diffuses in. It is then transported to the body tissues by hemoglobin of the blood. In them, hemoglobin is dissolved in plasma and not present in the corpuscles unlike other animals.
· In insects, gas exchange occurs through a tracheal system because in them the integument has become impermeable to gases to reduce the water loss. Trachea are fine tubes that open to the outside by spiracles. Each trachea branches into tracheoles that again branch extensively in the tissues and finally end into air sacs.
Inspiration and expiration occur through the spiracles. When the abdominal muscles relax, the air is drawn into the spiracles, trachea and tracheoles. This then diffuses through the body fluids to reach the cells. When the abdominal muscles contract, the air is driven out through the tracheal system via the spiracles. Thus in insects expiration is an active process but inspiration is passive.
· In the marine annelid Nereis respiration occurs by the whole body surface, but more specially by thin, flattened lobes of parapodia, which possess extensive capillary network. They are richly supplied with blood capillaries and are highly permeable to respiratory gases.
· Aquatic animals like prawns, fishes and tadpoles (of frog) respire with the help of gills. Gills are richly supplied with blood and can readily absorb oxygen dissolved in water. The surface of the gills is increased by the presence of gill plates. Each gill plate has many flat and parallel membranes like gill lamellae. Water moves over these gills in single direction only. The oxygen absorbed by the gills from the water is taken by blood and carbon dioxide is given out into the water.
· In amphibians like frogs and toads, some cutaneous respiration takes place across their moist and highly vascular skin, particularly during hibernation. However, they mainly respire through the lungs and the moist mucus membrane of the buccal cavity.Toads have less of cutaneous respiration than frogs.
Human Respiratory System:
All mammals have lungs for the purpose of respiration. This is known as pulmonary respiration. The mammalian respiratory system consists of the nasal cavity, nasopharynx, larynx, trachea, bronchi, bronchioles and lungs.
1. Nasal cavity: It is a large cavity lying dorsal to the mouth and is lined by mucous secreting epithelium. The nasal cavity opens outside through a pair of external nostrils or nares. Bones and cartilages support the nasal cavity. The nasal cavity is divided into two parts by a nasal septum. The cavity opens inside into pharynx through two internal nostrils. Air while passing through the nasal cavity is filtered, and only the clean air free from dust particles and foreign substances enters the pharynx. The air also gets warmed and moistened in this chamber. It is important to note that air can also be inhaled through mouth directly, but this is not advisable because the air will not be filtered, warmed and moistened. This gradually will harm the respiratory system.
2. Nasopharynx: It is a chamber situated behind the nasal cavity. At the level of soft palate, it becomes continuous with the mouth cavity or oral pharynx. It also receives the openings of eustachian tubes on its lateral sides and is thus connected to the middle ear.
3. Larynx: It is a chamber situated in the region of neck. It is supported by four cartilages : thyroid is the largest and in the form of a broad ring incomplete dorsally, cricoid is a complete ring lying at the base of thyroid, a pair of arytenoids lying above the thyroid but in front of cricoid, and epiglottis situated behind the tongue that serves to cover the entrance to the trachea so that food particles may not enter into it. Larynx is also known as voice box since it helps in the production of sound.
4. Trachea: It is a tube starting from larynx running through the neck and the thoracic cavity. The trachea runs through the neck in front of the oesophagus. The trachea or windpipe is about 12 cm long and divided into two bronchi in the thoracic region.
5. Bronchi and bronchioles: The two bronchi enter into right and left lungs of either side. Inside the lungs they further branch into many smaller bronchioles with a diameter of about 1 mm. These bronchioles further divide into terminal and then into respiratory bronchioles. Each respiratory bronchiole divides into a number of alveolar ducts that further divide into atria, which swell up into air sacs or alveoli.
6. Lungs: A pair of conical shaped lungs is situated in the double walled sacs called pleural cavities. They are spongy and richly supplied with blood vessels and capillaries. They have about 300-400 millions of alveoli through which exchange of gases occur. Lungs have various bronchioles ending into alveoli where exchange of gases occurs. The alveoli are thin walled pouches the walls of which have epithelial linings supported by basement membrane.
Mechanism of breathing or pulmonary respiration:
· Respiration involves the following steps:
· Breathing or pulmanory ventilation by which atmospheric air is drawn in and Carbon dioxide air released out.
· Diffusion of gses of oxygen and carbon dioxide across alveolar membrane.
· Transport of gases by the blood.
· Diffusion of oxygen and carbon dioxide between blood and tissues.
· Utilisation of oxygen by the cells for catabolic reactions and release of carbon dioxide.
Mechanism of Breathing:
Inspiration: During this process, some intercostal muscles contract thus pulling the ribs upwards and outwards. Lateral thoracic walls also move outwards and upwards. At the same time the diaphragm becomes flattened as it moves down towards the abdomen. This results in the increase in the volume of thoracic cavity thus lowering the pressure in the lungs. To fill up this gap, air from outside rushes in to bring about inhalation or inspiration. Hence, inspiration is brought about by contraction of the diaphragm and some intercostal muscles; these muscles are known as inspiratory muscles.
Expiration: During this process, the ribs return back to their original position, inwards and backwards, by the relaxation of intercostal muscles and also the diaphragm becomes dome-shaped again. Lateral thoracic walls also move inwards and downwards. This decreases the volume of the thoracic cavity thus increasing the pressure inside the lungs. So the air from the lungs rushes out through the respiratory passage bringing about expiration or exhalation.
A person breathes about 12 to 16 times per minute while at rest. However, this breathing rate is higher at the time of muscular exercise and in small children.
In forceful expiration, a different group of intercostal muscles and some abdominal muscles contract to reduce the volume of the thorax more than that in ordinary expiration. So more air is expelled out. Such muscles are known as expiratory muscles.
Pulmonary air volumes: Air flows into and out of the lungs because of the pressure gradient. Spirometer is an instrument used to measure the amount of air exchanged during breathing. Some terms regarding pulmonary air volumes are as follows:
1. Tidal volume: It is the volume of air that is breathed in and breathed out while sitting at rest (effortless respiration) or “quiet breathing”. It is about 500 ml in an adult person.
2. Vital capacity: It is the volume of air that can be maximum expelled out after a maximum inspiratory effort. It is about 4,500 ml in males; and 3,000 ml in females. The higher the vital capacity, the greater will be the capacity for increasing the ventilation of lungs for exchange of gases. It is more in athletes and mountain dwellers.
3. Residual volume: It is the volume of air that remains inside the lungs and respiratory passage( about 1.5 litres ) after a maximum forced exhalation.
4. Inspiratory reserve volume (IRV): It is the volume of air that can be taken in by forced inspiration over and above the normal inspiration or tidal volume. It is about 2,000 ml to 3,500 ml.
5. Expiratory reserve volume (ERV): It is the volume of air that can still be given out by forced expiration over and above the normal inspiration or tidal volume. It is about 1,000 ml.
6. Total lung capacity: It is the volume of air in the lungs and respiratory passage after a maximum inhalation effort. It is equivalent to vital capacity plus residual volume. It is about 5,000 to 6,000 ml in adult males.
Pulmonary exchange of gases:
In most of higher animals including man, the air from outside reaches up to the alveoli of lungs in the process of breathing. This inspired air contains about 21 per cent oxygen, 0.04 per cent carbon dioxide, 78.6 per cent nitrogen and small amounts of other gases and atmospheric moisture. In this inspired air the partial pressure of oxygen (Po2) is 158 mm Hg; and that of carbon dioxide (Pco2) is 0.3 mm Hg. The lungs and alveoli also contain some air even after expiration. But this air has more of carbon dioxide and less of oxygen than the inspired air. So when this air mixes with the inspired air the partial pressure of oxygen in alveolar air now becomes 100 mm Hg and that of carbon dioxide becomes 40 mm Hg. However, the percentage of oxygen now becomes 13.1% and that of carbon dioxide 5.3%.
The pulmonary artery contains deoxygenated blood and this has Po2 much less (40 mm Hg) than that of alveolar Po2. So oxygen from the alveolar air diffuses into the blood capillaries (oxygenation). This oxygenated blood is collected from alveoli of lungs by the pulmonary veins. It has a Po2 of about 95 mm Hg and at this partial pressure, the oxygenated blood has 19.8 per cent oxygen. Further, the deoxygenated blood in the pulmonary artery has a Pco2 of 46 mm Hg and Pco2 of alveolar air is 40 mm Hg. So the blood while passing through the alveoli of lungs also unloads carbon dioxide. The pulmonary vein carrying oxygenated blood, thus, has carbon dioxide at the partial pressure of 40 mm Hg. At these partial pressures the carbon dioxide contents of the blood decreases from 52.7 per cent to 49 per cent.
Gas transport in blood:
Oxygen transport :
The hemoglobin pigment of blood mainly transports oxygen. From alveoli of lungs, oxygen can readily diffuse into erythrocytes and combines loosely with hemoglobin (Hb) to form a reversible compound oxyhemoglobin (HbO2). Combining of oxygen with hemoglobin to form oxyhemoglobin is a physical process. There is no change in the valency of iron atom; it is ferrous in oxyhemoglobin and also in hemoglobin. This reaction, therefore, is an “oxygenation” process and not oxidation. When fully oxygenated, hemoglobin has about 97 per cent of oxygen. Hemoglobin is dark red in colour; whereas oxyhemoglobin is bright red in colour.
Inside the tissues, as the partial pressure of oxygen is less, oxyhemoglobin gets dissociated into oxygen and hemoglobin. Further, as Po2 is much lower and Pco2 is much higher in active tissues than in passive tissues, so much of oxygen is released from oxyhemoglobin in active tissues. High tension of oxygen favours the formation of oxyhemoglobin while low tension of oxygen favours its dissociation. However, very little of oxygen is found in the blood plasma. Each decilitre of blood releases up to 4.6 ml, of oxygen in the tissues, 4.4 ml from oxyhemoglobin and 0.17 ml from the dissolved oxygen in the plasma.
Carbon dioxide transport :
Carbon dioxide is produced in the tissues as an end product of tissue respiration. For its elimination, it gets dissolved in tissue fluid and passes into the blood. In the tissues, 100 ml of blood receives about 3.7 ml of carbon dioxide. It is transported both by the plasma and hemoglobin of blood. From the tissues, carbon dioxide diffuses into the blood plasma and forms carbonic acid (H2CO3-) in the presence of an enzyme carbonic anhydrase. Inside the erythrocytes, some of the carbonic acid forms bicarbonates and is thus transported. As carbonic acid, carbon dioxide is transported by blood plasma.
Carbonic anhydrase
CO2 + H2O \================\ H2CO3 (carbonic acid)
H2CO3 \================\ H+ + HCO3- (bicarbonate)
If all the carbon dioxide produced by the tissues is carried by blood plasma in this way, then pH of the blood will be lowered to about 4.5. This would immediately cause death. So only about 10% of the CO2 produced by the tissue is actually transported as carbonic acid.
About 20% of the total CO2 produced is transported by the hemoglobin of blood as carbaminohemoglobin.
CO2 + Hb.NH2 ---------------------à Hb.NH.COOH
About 70 % of the total CO2 produced is transported as bicarbonate ions of the blood. Bicarbonates are formed both in the erythrocytes and in the plasma of blood.
In erythrocytes. CO2 from the plasma enters the erythrocytes and combines with water to form carbonic acid in the presence of the enzyme carbonic anhydrase. Carbonic acid soon dissociates to form H+ and HCO3- ions.
CO2 + H2O ------------à H2CO3 \==============\ H+ + HCO3-
Hence, carbon dioxide is carried in the blood in three major forms; bicarbonates in plasma and erythrocytes, carbaminohemoglobin in erythrocytes, and small amounts of dissolved carbon dioxide in plasma.
On reaching the lungs, blood is oxygenated. Oxyhemoglobin is a stronger acid than deoxyhemoglobin. So it donates H+ ion, which joins bicarbonate (HCO3-) to form carbonic acid and this carbonic acid is cleaved into water and carbon dioxide by an enzyme carbonic anhydrase. Oxygenation of hemoglobin releases carbon dioxide from carbaminohemoglobin. By this way, every decilitre of blood releases about 3.7 ml of carbon dioxide in the lungs. Then this carbon dioxide is removed from the lungs by exhalation.
Gas exchange in tissues:
In the tissues, gases are exchanged by diffusion (as in the lungs). In tissues, as the partial pressure of oxygen is very low (about 40 mm Hg), so the oxygen gets unloaded here. When the blood leaves the tissues it has Po2 of 40 mm Hg. However, for carbon dioxide it is just the reverse. The blood entering into tissues has Pco2 of 40 mm Hg; while Pco2 of tissues is 46 mm Hg. So some of carbon dioxide from tissues gets loaded into the blood.
Disorders of Respiratory Systerm:
1. Asthma: difficulty in breathing causing wheezing due to inflammationof bronchi and bronchioles.
2. Emphysema: alveolar walls are damaged diue to which respiratiory surface is decreased, causes of this is cigarette smoking.
3. Occupational Respiratory Disorders: long exposure to the dust of industries like stone breaking, etc cause an inflammation on lung tissues and leads to lung damage.
Distinguish between:
1. Inspiratory muscles and expiratory muscles :
Inspiratory muscles are a group of intercostal muscles, the contraction and relaxation of which bring about inspiration and expiration respectively. Expiratory muscles are a group of different intercostal muscles and some abdominal muscles which contract to reduce the thoracic cavity more than that in ordinary expiration as in forceful respiration.
2. Tracheoles and bronchioles :
Tracheoles are the finer branches of tracheal tubes present in insects that ramify into the tissues. Bronchioles are the finer branches of bronchus that branch further to open into alveoli of lungs in mammals.
3. Carbaminohemoglobin and oxyhemoglobin :
Carbaminohemoglobin is a reversible compound formed when hemoglobin combines with carbon dioxide; oxyhemoglobin is a reversible compound formed when hemoglobin combines with oxygen.
In carbon monoxide poisoning, Hb combines irreversibly with CO to form carboxyhemoglobin.
4. Inspired air and alveolar air :
Inspired air is the air taken inside the lungs during inspiration. It contains about 21% oxygen and 0.03% carbon dioxide. This air now mixes up with the air already present inside the lungs, which has more of carbon dioxide and less of oxygen. This mixed air is now called as alveolar air and it has 13.1% oxygen and 5.3% carbon dioxide.
Explain why the following things happen:
1. Far more oxygen is released from oxyhemoglobin in a more active tissue than in a less active one.
The dissociation of oxyhemoglobin to oxygen and deoxyhemoglobin depends upon the partial pressures of oxygen and carbon dioxide in the tissues. In a more active tissue, the Po2 is lower and Pco2 is higher as compared to that of a less active tissue. So far more oxygen is released from oxyhemoglobin in a more active tissue than in a less active one.
2. Oxygenation of blood promotes the release of carbon dioxide from the blood in the lungs.
The oxygenation of blood in the lungs depends on the partial pressures of oxygen in the pulmonary artery and in the alveolar air. Further, the oxygen affinity of hemoglobin is enhanced with the fall in partial pressures of carbon dioxide that results from the elimination of carbon dioxide from the blood into the lungs. In the lung alveoli, hemoglobin is exposed to high Po2 and less Pco2.
3. Oxygen leaves the blood from tissue capillaries, but carbon dioxide enters the blood in tissue capillaries.
Inside the tissue capillaries, there is more of Pco2 and less of Po2. The blood coming to tissues has more of Po2 and less of Pco2. So oxygen is unloaded from the blood and carbon dioxide is loaded to the blood in the tissue capillaries.
4. Erythrocytes can carry out anaerobic metabolism only.
Erythrocytes can carry out anaerobic metabolism only because they lack mitochondria.
5. Gaseous exchanges continue in the lungs without interruption during expiration.
Gaseous exchanges continue in the lungs uninterrupted because some air is always present inside the lung alveoli even during expiration.
6. Contraction of inspiratory muscles causes inspiration while relaxation causes expiration.
Contraction of inspiratory muscles increases the volume of pleural cavities; while expiration is brought about passively by the relaxation of those muscles. As the muscles relax, the diaphragm moves up towards the thorax and the intercostal muscles move the lateral thoracic walls inwards and downwards. This decreases the volume of pleural cavities and the air rushes out.
7. Oxygen enters the blood from the alveolar air but carbon dioxide leaves the blood to enter the alveolar air.
Inside the alveoli of lungs, there is more of Po2 and less of Pco2 .The blood coming to lung alveoli has more of Pco2 and less of Po2. So oxygen is loaded to the blood and carbon dioxide is unloaded from the blood in the alveoli of lungs.