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Question
Explain how gaseous exchange takes place at alveoli and explain the regulation of respiration by brain.
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Solution
Exchange of gases in the alveoli:Once air is within the lungs the process of gaseous exchange begins. Capillaries of the pulmonary artery remain closer to the wall of the alveoli. This enhances the exchange of gases. Oxygen and carbon dioxide are exchanged across the alveolar member by diffusion from the site of higher to lower partial pressure until the partial pressure of the two regions is equal. This process is a simple physical one, which does not involve any secretory or active transport mechanism.In atmospheric air, there is a high concentration of oxygen (20-25%) whereas the proportion of carbon dioxide if low (0.04%). The alveolar PO2 is about 100 mm Hg and PO2 of venous blood is about 40 mm Hg. This pressure gradient is sufficient for the transfer of O2. The PCO2 of venous blood is 46 mm Hg and that of alveolar air is only 6 mm Hg (1/10th of O2), it is adequate for CO2 transfer by diffusion.
Regulation of respiration:In the brain, the medulla oblongata contains a respiratory center. This controls breathing. The respiratory center consists of an inspiratory center and an expiratory center. Axons from the nerve cells of these centers lead to the intercostal muscles through the intercostal nerves and the diaphragm via the phrenic nerves. These nerve fibres transmit impulses to the external intercostal and internal intercostal muscles alternately. The walls of the alveoli have sense endings, which are stimulated by changes in the tension of alveolar walls.
When alveoli are stretched at the height of inspiration the receptors send stimuli to the expiratory center of the medulla through the vagus nerve which inhibits further inspiration. This sequence of events is called Herring-Bruer reflex. In addition, the medulla contains a pneumotaxic center, which s connected to the breathing center and helps to ensure rhythmic breathing. During inspiration, the inspiratory part of the respiratory center sends impulses to the pneumotaxic center, which responds by sending impulses to the expiratory part of the respiratory center. The expiratory center is then activated and so the inspiratory center is inhibited reflexly. Thus the respiratory rhythm is controlled by these centers in the brain.
Exchange of gases in the alveoli:
Once air is within the lungs the process of gaseous exchange begins. Capillaries of the pulmonary artery remain closer to the wall of the alveoli. This enhances the exchange of gases. Oxygen and carbon dioxide are exchanged across the alveolar member by diffusion from the site of higher to lower partial pressure until the partial pressure of the two regions is equal. This process is a simple physical one, which does not involve any secretory or active transport mechanism.In atmospheric air, there is a high concentration of oxygen (20-25%) whereas the proportion of carbon dioxide if low (0.04%). The alveolar PO2 is about 100 mm Hg and PO2 of venous blood is about 40 mm Hg. This pressure gradient is sufficient for the transfer of O2. The PCO2 of venous blood is 46 mm Hg and that of alveolar air is only 6 mm Hg (1/10th of O2), it is adequate for CO2 transfer by diffusion.
Regulation of respiration:
In the brain, the medulla oblongata contains a respiratory center. This controls breathing. The respiratory center consists of an inspiratory center and an expiratory center. Axons from the nerve cells of these centers lead to the intercostal muscles through the intercostal nerves and the diaphragm via the phrenic nerves. These nerve fibres transmit impulses to the external intercostal and internal intercostal muscles alternately. The walls of the alveoli have sense endings, which are stimulated by changes in the tension of alveolar walls.
When alveoli are stretched at the height of inspiration the receptors send stimuli to the expiratory center of the medulla through the vagus nerve which inhibits further inspiration. This sequence of events is called Herring-Bruer reflex. In addition, the medulla contains a pneumotaxic center, which s connected to the breathing center and helps to ensure rhythmic breathing. During inspiration, the inspiratory part of the respiratory center sends impulses to the pneumotaxic center, which responds by sending impulses to the expiratory part of the respiratory center. The expiratory center is then activated and so the inspiratory center is inhibited reflexly. Thus the respiratory rhythm is controlled by these centers in the brain.
When alveoli are stretched at the height of inspiration the receptors send stimuli to the expiratory center of the medulla through the vagus nerve which inhibits further inspiration. This sequence of events is called Herring-Bruer reflex. In addition, the medulla contains a pneumotaxic center, which s connected to the breathing center and helps to ensure rhythmic breathing. During inspiration, the inspiratory part of the respiratory center sends impulses to the pneumotaxic center, which responds by sending impulses to the expiratory part of the respiratory center. The expiratory center is then activated and so the inspiratory center is inhibited reflexly. Thus the respiratory rhythm is controlled by these centers in the brain.
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