Write concisely on the cardiopulmonary changes in man in response to exercise.

Outline:

·        Changes in oxygen and carbon dioxide levels

·        Tissue blood flow

·        Nervous control

·        Circulatory response

·        Stimulation of ventilation

Essay:

            Exercise is associated with very extensive alterations in the circulatory and respiratory systems to meet the increased oxygen demand for muscle contraction. The amount of oxygen is increased by increased blood flow and ventilation rate. At the same time, the amount of carbon dioxide produced increases due to increased tissue respiration and this too must be removed.

            There is an increase in blood flow and ventilation before the onset of exercise and this is probably due to psychic stimuli (thought of exercise) and afferent impulses from proprioceptors in muscles, tendons, and joints which relay the information to the vasomotor center. The vasoconstrictor area is strongly stimulated while the activity of the cardiac inhibitory center is highly attentuated. The heart is stimulated greatly to increase heart rate and increasing pumping strength as a result of sympathetic discharge and inhibition of the tonic parasympathetic discharge leading to an increase in stroke volume. Most of the arterioles of the peripheral circulation are strongly constricted except the arterioles in the active muscles, which are vasodilated by both the actions of epinephrine on b₂ receptors and local vasodilators. Thus, the heart is stimulated to supply the increased blood flow required by the muscles and blood flow through most nonmuscular areas of the body, with the exception of the heart and brain, is temporarily reduced. This also leads to a pressure difference between the two regions with the net result of blood flowing to the skeletal muscles. At the same time, it prevents pooling of blood in the veins and pulmonary circulation, shifting these ‘reservoirs’ of blood to the muscles.

            Local mechanisms maintaining a high blood flow in exercising muscle include a fall in tissue P_O2, a rise in tissue P_CO2, and accumulation of K⁺ and other vasodilator metabolites. Dilation of the arterioles and precapillary sphincters causes a 10-100 fold increase in the number of open capillaries. The average distance between the blood and the active cells – and the distance oxygen and metabolic products must diffuse is thus greatly decreased. The decreased pH and increased temperature shift the dissociation curve for hemoglobin to the right, so that more oxygen is given up by the blood. Increased concentration of 2,3-DPG in the red blood cells further decrease the oxygen affinity of hemoglobin and facilitates the extraction of oxygen by the tissues. The net result is an up to 3-fold increase in the arteriovenous oxygen difference, and the transport of carbon dioxide out of the tissue is enhanced too. All these changes combine to make it possible for the oxygen consumption of skeletal muscle to increase 100-fold during exercise.

            The increase in the extraction of oxygen from the blood in exercising muscles decreases 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.