Exercise physiology

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      • Cardiovascular responses to exercise
        •  involve a combination of
          • central nervous system (CNS) mechanisms
          • local mechanisms
        • Central nervous system (CNS) mechanisms
          • produces increased sympathetic outflow to the heart and blood vessels
          • decreased parasympathetic outflow to the heart.
          • increase in cardiac output.
          • There is concomitant increase in venous return is accomplished by two mechanisms
            • contraction of skeletal muscle around the veins has a mechanical (squeezing) action(MCQ)
            •  activation of the sympathetic nervous system produces venoconstriction. (MCQ)
          • increased sympathetic outflow causes selective arteriolar vasoconstriction
            • In the circulation of the skin, splanchnic regions, kidney, and inactive muscles, vaso- constriction occurs via a1 receptors(MCQ)
            • results in increased resistance and decreased blood flow to those organs
            • In the exercising skeletal muscle, however, local metabolic effects override any sympathetic vasoconstricting effects, and arteriolar vasodilation (MCQ)
          • Other locations where vasoconstriction does not occur (MCQ)
            • coronary circulation
            • cerebral circulation.
          • In the cutaneous circulation, there is a biphasic response
            • Initially, vasoconstriction occurs (due to increased sympathetic outflow)
            • later, however, as body temperature increases, there is selective inhibition of sympathetic cutaneous vasoconstriction
        • Local Responses in Muscle
          • Active hyperemia
            • production of vasodilator metabolites such as lactate, potassium, and adenosine also increases. (MCQ)
            • Vasodilation of the arterioles results in increased blood flow to meet the increased metabolic demand of the muscle.
            • This vasodilation in the exercising muscle also produces an overall decrease in TPR.

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      • Exercise and Potassium
        • Exercise causes a K+ shift out of cells
          • depletion of cellular ATP stores opens K+ channels in the muscle cell membranes (MCQ)
          • K+ shift is reversed during a subsequent period of rest.
        • strenuous exercise can result in hyperkalemia. (MCQ)
          • in a person treated with a Beta2- adrenergic antagonist
          • in those with impaired renal function
        • K+ shift out of cells assists in the local control of blood flow to exercising skeletal muscle.
          • K+ is a  vasodilator metabolites(MCQ)
      • Response of the respiratory system to exercise
        • more O2 is supplied by increasing the ventilation rate:
        • when a trained athlete is exercising,
          • his O2 consumption may increase from its resting value of 250 mL/min to 4000 mL/min, his ventilation rate may increase from 7.5 L/min to 120 L/min. (MCQ)
          • Both O2 consumption and ventilation rate increase more than 15 times the resting level!
        • Arterial PO2 and PCO2
          • Remarkably, mean values for arterial PO2 and PCO2 do not change during exercise.
          • arterial pH may decrease, because the exercising muscle produces lactic acid
        • Venous Pco2
          • The PCO2 of mixed venous blood must increase during exercise because skeletal muscle is adding more CO2 than usual to venous blood.
        • Muscle and Joint Receptors
          • Muscle and joint receptors send information to the medullary inspiratory center and participate in the coordinated response to exercise.
          • inspiratory center is commanded to increase the ventilation rate.
        • Cardiac Output and Pulmonary Blood Flow
          • Cardiac output increases during exercise to meet the tis- sues’ demand for O2
          • Since pulmonary blood flow is the cardiac output of the right heart, pulmonary blood flow increases
          • There is a decrease in pulmonary resistance associated with perfusion of more pulmonary capillary beds, which also improves gas exchange.
          • V/Q  ratio becomes more “even,” (MCQ)
          • There is decrease in the physiologic dead space. (MCQ)
        • O2-Hemoglobin Dissociation Curve
          • shifts to the right (MCQ)
          • reasons for this shift,
            • increased tissue PCO2
            • decreased tissue pH
            • increased temperature.
          • The shift to the right is advantageous is associated with (MCQ)
            • an increase in P50
            • decreased affinity of hemoglobin for O2
            • easier to unload O2 in the exercising skeletal muscle

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