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| Gaseous Exchange |
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| Exchange of Respiratory Gases |
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| The exchange of respiratory gases is through membranes. In lungs, the gases are exchanged between the lungs and the blood capillaries and in the tissues, the gases are exchanged between the cells and the blood surrounding them. |
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| The gases move across the membranes by the process of diffusion. Diffusion depends on the partial pressure of the gas concerned. The gases move from the region of higher partial pressure to the region of lower partial pressure. Further, the partial pressure of one gas is independent of the partial pressure of another gas that is mixed with it. Thus the partial pressure of oxygen and carbon dioxide are independent of each other. |
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| Composition of the air that we breathe in is: |
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| Nitrogen - 78%, Oxygen - 21%, Carbon dioxide - .03 - .04%, Hydrogen - traces, noble gases - traces. |
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| In terms of partial pressures, the PO2 is 158mmof Hg and PCO2 is 0.3mm of Hg in the atmospheric air. |
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| The venous capillaries lining the alveoli have impure blood that has low concentration of oxygen and thus lower partial pressure (PO2 in venous blood is 40mm of Hg). The oxygen of the air in the lungs has higher partial pressure (100mm of Hg) and therefore, easily diffuses into the blood through the thin barrier of the alveolus wall. Similarly since the concentration of carbon dioxide is quite high (46mm of Hg) in the venous blood, the gas easily diffuses out into the alveolar space where the partial pressure of carbon dioxide (PCO2 in venous blood is 40mmof Hg) is lower. |
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| From here, the air leaves the lungs. This air has comparatively more concentration of carbon dioxide than the air that entered the lungs. This air cannot be completely expelled from the lungs. Thus the air that is inhaled mixes with the carbon dioxide - rich air that is remaining from the previous expiration. This results in a change in the partial pressure of the respiratory gases. |
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| Will the partial pressure of oxygen in the atmospheric air increase or decrease on entering the lungs? |
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| How will the partial pressure of carbon dioxide be affected? (see the table given below for answers) |
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| Oxygen transport |
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| Most of the oxygen is transported in a combined state. Some of the oxygen is present in solution in the blood. For each decilitre of blood, 4.6ml oxygen enters the blood in the lungs. Of this only 0.17ml remains in the plasma in the solution form. Remaining enters the red blood cells and combines with the haemoglobin. |
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| The haemoglobin pigment (Hb) has an affinity for oxygen. Oxygen combines with the iron ions (Fe2+) of the Hb molecule. In the arteries lining the lungs, it combines with oxygen and forms HbO2, oxyhaemoglobin. |
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| Each haemoglobin molecule contains four Fe2+ ions. So, each haemoglobin molecule can carry a maximum of four oxygen molecules. The saturation of the Hb molecule depends on the amount of oxygen in the alveolar air. |
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| It is found that haemoglobin has more affinity for oxygen under high partial pressure of oxygen, low temperatures and low acidity conditions. |
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| Oxygen - Haemoglobin Dissociation Curve |
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| However, in the tissues the partial pressure of oxygen is low as it is constantly being used up. Temperature is higher because of metabolic reactions and the acidity is also high. Under these conditions, the oxyhaemoglobin gives up its oxygen and forms haemoglobin again. More active the tissue is, lesser is the partial pressure of oxygen. Thus more oxygen is released by the oxyhaemoglobin in the more active tissues. |
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| Carbon dioxide transport |
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| Carbon dioxide is transported in three different forms - dissolved gas, bicarbonates and carbaminohaemoglobin molecules. About 3.7ml of carbon dioxide enters each 100ml of blood. A small amount in the plasma is transported in the dissolved form. Most of the carbon dioxide enters the red blood cells. Of the gas entering the RBCs, 70% is converted to bicarbonate ions and remaining 30% forms carbaminohaemoglobin. |
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| Formation of bicarbonates |
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| Carbon dioxide first combines with water to form carboxylic acid in the presence of zinc and an enzyme called the carbonic anhydrase. The carboxylic acid is then split into hydrogen ions and bicarbonate ions. Most of the bicarbonate ions come out of the RBCs and are transported by the blood plasma. The hydrogen ions are absorbed by the haemoglobin. |
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| In the lungs, the reverse reaction takes place and the haemoglobin gives up the hydrogen ions which combine with the bicarbonate to form the carboxylic acid. This then forms carbon dioxide and water. |
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| Both the reactions are catalyzed by an enzyme called the carbonic anhydrase. |
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| After the formation of haemoglobinic acid (carbonate ions + haemoglobin), the chloride ions (Cl-) diffuse from plasma into the erhthrocytes to maintain the ionic balance. The neutrality is maintained electrochemically. This is called the chloride shift. The chloride ions combine with the potassium ions to form KCl (potassium chloride) in the RBC. HCO3- (hydrogen carbonate ions) in the plasma combine with Na+ to form sodium hydrogen carbonate (NaHCO3). Most of the CO2 (almost 70%) is transported from tissues to the lungs in the above way. |
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| Formation of carbaminohaemoglobin |
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| Some of the carbon dioxide entering the erythrocytes combines with the globin (protein part) of haemoglobin to form carbaminohaemoglobin. |
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| The blood transports the oxygen to the different tissues where it is used to break down the food and release energy. In the tissues also the gaseous exchange takes place by diffusion. The tissue cells are surrounded by the tissue fluid. The arteries with oxygen-rich blood branch into fine capillaries. The PO2 of the arterial blood in the arterial capillaries is higher than that of the tissue fluid. This is because the cells use up oxygen for respiration. Due to the higher arterial PO2, the oxygen diffuses from the blood into the tissue fluid from where the cells take it up. |
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| The cells release carbon dioxide into the tissue fluid which increases the PCO2 of the tissue fluid more than that of arterial blood. Consequently, carbon dioxide enters the blood capillaries from the tissue fluid. |
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| As the capillaries reach the tissue fluid, the oxyhaemoglobin dissociates and only the oxygen enters the tissue fluid. The carbon dioxide also forms bicarbonates and carbaminohaemoglobin only in the blood. Thus in the tissue fluid, the respiratory gases are present in solution. |
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| As a person climbs up a mountain, the atmospheric pressure falls along with which, the partial pressures of the gases also fall. This reduces the partial pressure of oxygen in the lungs. Reduction of PO2 in lungs lowers the amount of oxygen in blood. This causes various symptoms like breathlessness, nausea, dizziness, headache, irritability, mental fatigue and bluish tinge on the skin, nails and lips. This is called mountain sickness. |
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| An opposite condition occurs in the deep-sea divers. In deep sea, the partial pressure of all gases is more in the lungs. This forces nitrogen to mix with the blood and other body fluids. When the diver is pulled up the partial pressure falls and also the solubility of nitrogen. Nitrogen then evolves from the body fluid and forms bubbles in the blood stream. These bubbles can block the important blood vessels of the body leading to lungs or brain resulting in serious complications. This is called decompression sickness. To avoid this, the diver should be lifted slowly out of the water. |
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