Respiratory System in Animals
Respiratory System in Animals
It is also called external respiration. In this gaseous exchange oxygen passes from alveoli to pulmonary capillary blood and CO2. Comes to alveoli from pulmonary capillary. In order to be exchange the gases have to pass through alveolocapillary membrane or respiratory membrane. Composition of alveolocapillary membrane is epithelium lining of alveolar wall, epithelial basement membrane, a thin interstitial space, capillary basement membrane and capillary endothelial membrane.
Thickness of respiratory membrane is 0.5 m. Respiratory membrane has a limit of gaseous exchange between alveoli and pulmonary blood. It is called diffusion capacity. Diffusion capacity is defined as volume of gas that diffuse through membrane per minute for a pressure difference of 1 mm Hg. Exchange of gases through alveolocapillary membrane is a purely physical diffusion phenomenon. No chemical reaction is involved. Diffusion of a gas depends upon pressure gradient across the membrane and solubility of gas.
More pressure gradient quickly diffusion. Diffusion of CO2 is 20 times faster than oxygen. Diffusion is also directly proportional to thickness of membrane, surface area of membrane, permeability of membrane. As already mentioned, we inhale 500 ml. of air (tidal volume) in each breath, i.e., 6000 ml. each minute (at a normal rate of 12 breaths per minute). About 150 ml. of inspired or expired air in each breath is retained in the respiratory passageways. Since this air is not involved in gaseous exchange, the space enclosed by respiratory passageways is called “dead space”. Obviously, only 350 ml of inhaled air in each breath (4200 ml per minute) reaches into the lung alveoli, mixes with the functional residual air of alveoli and, thus, takes part in gaseous exchange. It brings with it about 69 ml of O2. When it is expired, it takes back only about 48 ml of O2. With each breath, thus, about 21 ml of CO2 becomes available to pulmonary blood for absorption from alveolar air. Thus, our normal intake of amounts to about 250 ml per minute. Similarly, the inspired air in each breath brings only about 0.14 ml of into alveolar air, but when expired, it contains about 18.55 ml of CO2. Thus, with each breath, our blood gets rid of about 18.4 ml of CO2 (about 220 ml each minute).
Since gaseous exchange occurs continuously between the 2300 ml of alveolar air and pulmonary blood, the 350 ml of atmospheric air, reaching into and leaving lung alveoli in each breath, in effect, merely serves to slowly renew the alveolar air. This slow renewal of alveolar air prevents sharp and sudden changes in , and pH-concentrations in blood.
Partial pressure : Partial pressure of a gas is the pressure it exerts in a mixture of gases, and is equal to the total pressure of the mixture divided by percentage of that gas in the mixture. For instance, if the pressure of atmospheric air at sea level is 760 mm. Of mercury (Hg) and oxygen forms 21% of the air, the partial pressure of oxygen will be 21% of 760, or 159 mm. Hg. In other words, the partial pressure of a gas is proportional to its concentration in the mixture. Only about 0.3 ml. of O2 can dissolve in 100 ml. of plasma, about 20 ml. O2 of is carried by haemoglobin in 100 ml. of blood. In atmospheric air except these gases some traces of helium, argon and neon are also found.
A part of the inspired air is left in the respiratory tract, the so called “dead space”, where no gaseous exchange occurs. This “dead space” air is expelled at the next expiration. The expired air, thus, contains fresh air from the “dead space” and foul air from the lungs. Therefore, the alveolar air has less oxygen and more carbon dioxide than the expired air. A part of the expired air is also left in the dead space. This air enters the lungs at the next inspiration. Some air is also left in the lungs after expiration as residual air. The fresh inspired air is mixed up in the lungs with the foul air from the “dead space” and the stale residual air. Therefore, the alveolar air has less O2 and more CO2 than the inspired air also. The inspired air has the composition of the atmospheric air. Exchange of gases in lungs can be divided into two steps:
Uptake of O2by blood in lung : The (partial pressure of oxygen) of the alveolar air is higher than the of blood in alveolar capillaries. Due to a difference between air and blood, oxygen diffuses rapidly from the alveolar air into the blood of alveolar capillaries. It may be remembered that gases always diffuse from a region of higher partial pressure (concentration) to a region of lower partial pressure or concentration. At rest RBC stay for only one second in a pulmonary capillary. Hb becomes saturated within about 0.3 sec. During exercise, circulation speed is high RBC stays for 0.3 sec in a capillary, again it sufficient duration for oxygenation.
Release of CO2 by the blood : The (partial pressure of carbon dioxide) of blood reaching the alveolar capillaries is higher than the of alveolar air. Therefore, carbon dioxide diffuses from the blood of alveolar capillaries into the alveolar air. The exchange of gases in the alveoli that raises the of blood and lowers its is the external respiration. The blood oxygenated by this respiration is returned from the lungs by pulmonary veins to the left side of the heart. The heart supplies the oxygenated blood to the body tissues. In alveolar air partial pressure of O2 is 100 mm Hg and in pulmonary capillary blood is 40 mm Hg. Thus O2 from alveolar air transfer to blood. in alveolar air is 40mm Hg and is pulmonary capillary blood CO2 is 46mm Hg. Thus CO2 flows from blood to alveolar air.
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