Cardiovascular System Cardiovascular Dynamics during Exercise Mc Ardle

Cardiovascular System Cardiovascular Dynamics during Exercise Mc. Ardle, Katch and Katch, 4 th ed.

Cardiovascular Dynamics During Exercise Cardiac Output (Q): amount of blood pumped per minute. • Q = Heart Rate x Stroke Volume. • Fick Equation. VO 2 = HR x SV x a-v O 2 diff

Resting Cardiac Output • Cardiac Output = 5 L/min trained & untrained • Untrained = 70 bpm x 71 ml = 5000 ml/min • Trained = 50 bpm x 100 ml = 5000 ml/min • Larger stroke volumes after training due to increased vagal tone & strengthen heart.

Exercise Cardiac Output • Blood flow from heart increases in direct proportion to exercise intensity. • Increases @ higher intensity mainly due to increases in heart rate. • Untrained max 22 L/min • Trained max 35 L/min

Increased Cardiac Output • Venous return must increase • Venoconstriction - reduces capacity to hold large volume of blood • Muscle pump - active muscles squeeze veins forcing blood back towards heart • Respiratory pump - inspiration lowers thoracic pressure

Stroke Volume: amount of blood pumped each cardiac cycle. • Increased diastolic filling before systole occurs through any factor that increases venous return (preload). Left Ventricular End Diastolic Volume Minus Left Ventricular End Systolic Volume

Stroke Volume & VO 2 max • SV increases progressively with intensity up to about 50% max VO 2 • After reach 50% max VO 2, Q increases because of heart rate • Well trained endurance athletes’ SV rises to maximal levels

Stroke Volume Increases SV increases due to • Enhanced filling increases EDV (preload) • Greater contractility from neurohormonal influencegreater systolic emptying • Expanded blood volume and decreased afterload

Stroke Volume Increases • Increased EDV • fuller ventricle = greater stroke volume • Frank-Starling’s mechanism • Decreased ESV • catecholamines increase contractility via increased Ca 2+ • Afterload - pressure required to open the aortic semilunar valve • decreases during exercise due to vasodilation

Cardiovascular Drift • Prolonged exercise in warm environment causes dehydration • Dehydration reduces blood volume • Reduced blood volume decreases stroke volume • Heart rate rises to maintain required cardiac output.

Exercise Heart Rate • Heart rate and VO 2 are linearly related in trained and untrained throughout major portion of exercise range. • Endurance training reduces HR at any given submaximal workload due to ↑ SV.

Heart Rate and Oxygen Consumption • In healthy individuals, heart rate increases linearly with exercise load or oxygen uptake and plateaus just before maximal oxygen consumption. • If exercise load is held constant, below lactate threshold, steady state is reached in about 2 - 3 minutes.

Distribution of Cardiac Output • Blood flow to tissues is proportional to metabolic activity • Muscle tissue receives about same amount blood as kidneys at rest • During intense exercise, significant blood is shunted from kidneys & splanchnic regions (areas that temporarily tolerate reduced flow)

Shunting of blood via constricting arterioles and closing precapillary sphincters.

Distribution during Exercise • Blood flow to skin increases during light and moderate exercise • During intense exercise, nearly 85% blood shunted to muscles. Cutaneous blood flow reduced even when hot.

Cardiac Output and Oxygen Transport • Maximal cardiac output relates to maximal oxygen uptake in 6: 1 ratio. • Females have a larger cardiac output compared to males at any level of submaximal VO 2 – most likely due to 10% lower [hemoglobin]. • Children have small SV

Oxygen Extraction VO 2 SV HR a-v O 2 (ml/min) (L/min) (bpm) Untrained Rest 300 ml . 075 82 48. 8 Max 3100 ml . 112 200 138 Trained Rest 300 ml . 105 58 49. 3 Max 3440 ml . 126 192 140. 5 (ml/L) • Increased arterio-venous oxygen extraction with increased work intensity • Fick Equation: • VO 2 max = maximum cardiac output x maximum a-v O 2 diff • arterial O 2 - venous O 2 = extraction

Increasing Oxygen Consumption During Exercise • O 2 extraction depends upon O 2 content of blood & removal rate by tissues • O 2 removal depends upon: • • capillary density; improves with aerobic training. myoglobin content; improves with aerobic training. mitochondria number; improves with aerobic trg. oxidative capacity of mitochondria; improves with aerobic training. • muscle fiber type • PO 2 gradient from capillaries to tissue

Upper-Body Exercise • Highest VO 2 attained during upper body exercise ranges between 70%-80% of VO 2 max in lower body exercise. • Max HR and pulmonary ventilation probably less because smaller muscle mass. • Produces greater physiological strain (SBP) for any level VO 2 than lower-body exercise.

Illustration References • Mc. Ardle, William D. , Frank I. Katch, and Victor L. Katch. 2003. Essentials of Exercise Physiology 3 rd ed. Image Collection. Lippincott Williams & Wilkins. • Plowman, Sharon A. and Denise L. Smith. 1998. Digital Image Archive for Exercise Physiology. Allyn & Bacon.