Question

2. (a) State and explain the changes in the lung volume, the alveolar pressure (Palv) and airflow between the atmosphere and the lungs during passive exhalation.  

(b) What happens to the ventilation rate during exercise? Briefly describe and explain how the chemoreceptors work during exercise.

Answer 1

When we inhale air, it enters in to lungs and the volume of lungs increases. When we exhale air, the volume of lungs will decrease. When a person is not doing any strenuous work and passively inhaling andf exhaling, only small volume of air is inhaled and exhaled. This volume is only 500 ml and it is called the tidal volume.

Lungs are never empty. They are filled with residual volume of air which is around 1500 ml. Expiratory reserve volume is also not exhaled during normal exhalation which is also 1500 ml. So together, ERV and RV is called functional residual capacity with which the lungs are filled with in normal passive breathing. So lung volume is 3000 ml when exhaled. When we inhale we take in 500 ml and henc ethe lung volume increases by 500 ml to become 3500 ml. So the change in lung volume is 3500ml to 3000 ml

Alveolar pressure is the pressure with in the alveoli. This alveolar pressure changes as we inhale and exhale air. When the glotis is closed and no air is entering or leaving, the pressure in alveoli is same as the atmospheric pressure. It is considered 0 cm H2O. But during inhalation when the alveoli are relaxed and expanded , the pressure falls to -1 cm H2O due to which the tidal volume of air enters. So the alveolar pressure difference is enough to draw 500 ml of air in to lungs during normal breathing. Again when the ribcage and diaphragm come to their normal position, the alveolar air pressure increases to +1 cm H2O. This is enough to expell 500 ml of air from lungs.

b.

During exercise muscles contract more strenuously. This requires more O2 supply to muscles. Muscles are using more glucose to generate energy and liberating CO2 in to the blood.Increased Co2 levels will make the blood more acidic. This happens because CO2 increases the H+ ions in blood and hence makes it more acidic than normal. There are chemoreceptors in aortic body and carotid body. These detect changes in blood gases and also changes in blood pH. There are chemoreceptors present in medulla which can also detect the changes in blood CO2 levels and blood pH. These chemoreceptors then stimulate the respiratory centers in the brain to increase the rate and depth of the breathing which is essential to supply sufficient O2 to the tissues. So the rate of ventilation will increase during exercise for increased delivery of O2 to strenuously working muscles.