Give a brief account of the effects of exercise on the diffusion of oxygen from the lungs to the blood.

Outline:

·        Increase in oxygen demand during exercise

·        Cardiovascular adaptations

·        Increase oxygen extraction due to greater diffusion gradient

·        Increase in ventilation due to:

- psychic stimuli

- increase in H⁺

·        Clearing oxygen debt

Essay:

            During exercise, the muscles require more oxygen to meet its requirement and it releases large amounts of carbon dioxide. Many cardiovascular and respiratory mechanisms must operate in an integrated fashion if the oxygen needs of the active tissue are to be met and the extra carbon dioxide and heat removed from the body during exercise.

            To meet the increased metabolic demands of the respiring tissues and muscles during exercise, heart rate increases which increases the cardiac output. Circulatory changes increase muscle blood flow while maintaining adequate circulation in the rest of the body. There is an increase in the extraction of oxygen from the blood in exercising muscles and this decrease the amount of oxygen in venous blood going to the lungs in the pulmonary arteries. Therefore, there is a greater alveolar-capillary PO₂ gradient is increased and this enhances the diffusion of oxygen from the alveoli into the blood. At the same time, blood flow per minute is increased from 5.5L/min to as much as 20-35L/min. In less than one second, blood flowing through the pulmonary capillaries becomes oxygenated. The total amount of oxygen entering the blood therefore increases from 250 mL/min to as much as 8000 mL/min.

            There is an abrupt increase in ventilation with the onset of exercise, followed after a brief pause by a further, more gradual increase. The abrupt increase at the start of exercise is presumably due to psychic stimuli (thought of exercise) and afferent impulses from proprioceptors in muscles, tendons, and joints. The arterial pH, PCO₂ and PO₂ remain constant during moderate exercise and therefore there may be other factors involved in stimulating ventilation. Exercise increases plasma K⁺ level, which increases the discharge rate in chemoreceptor afferents, thereby increasing ventilation. In addition, it may be that the sensitivity of the respiratory center to carbon dioxide is increased or that the respiratory fluctuations in arterial PCO₂ increase so that, even though the mean arterial PCO₂ does not rise, it is carbon dioxide that is responsible for the increase in ventilation.  As ventilation is increased, alveolar PO₂ is increased, facilitating the diffusion of oxygen into pulmonary blood.

            When exercise becomes more vigorous, buffering of the increased amounts of lactic acid that are produced liberates more carbon dioxide, and this further increases ventilation. With further accumulation of lactic acid, the increase in ventilation outstrips carbon dioxide production and alveolar and arterial PCO₂ falls. The decline in arterial PCO₂ provides respiratory compensation for the metabolic acidosis produced by the additional lactic acid. The respiratory rate after exercise does not reach basal levels until the oxygen debt is repaid. This may take as long as 90 minutes.