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Cardiovascular Dynamics (Exercise Responses) deals with the function of the cardiovascular system and how it adapts to the demands placed on it factors to consider: cardiac output, blood pressure, distribution of blood flow, and oxygen consumption Cardiac Output defined: it is the volume of blood that is pumped out of the left ventricle in one minute; measured in litres per minute (L/min) readings: at rest 5-6 L/min; during exercise readings can reach greater than 30L/min other factors that contribute to cardiac output are stroke volume and heart rate Stroke Volume (SV) defined: it is the amount of blood that is ejected from the left ventricle in a single beat; measured in milliliters (mL) it is calculated by subtracting the left ventricular end-systolic volume (LVESV) from the left ventricular end-diastolic volume (LVEDV) LVESV – is the amount of blood remaining in the left ventricle after the contraction of the ventricle LVEDV – is the amount of blood in the left ventricle after the contraction of the left atrium therefore, SV(mL) = LVEDV(mL) – LVESV(mL) it is regulated by 3 main factors both at rest and during exercise a) LVEDV b) aortic blood pressure c) strength of the ventricular contraction LVEDV is the amount of blood that is returned to the ventricle before it contracts the ventricle has the capacity to stretch to accommodate increases in LVEDV – this stretching results in a more forced contraction of the cardiac muscle and an increase in the amount of blood ejected – referred to as the Frank-Starling Law therefore, the most important factor that regulates SV is the amount of blood that is returned to the heart (venous return) during exercise, venous return increases as a result of: a) constriction of the veins (venoconstriction) b) skeletal muscle pumps (with each contraction of skeletal muscle, blood is pushed or messaged back to the heart) c) thoracic pump (return of blood in the veins to the heart) d) nervous stimulation of the heart the efficiency of SV is measured by the ejection fraction (EF) – which is the proportion of blood that is ejected from the left ventricle during a single heart beat EF(%) = SV(mL)/LVEDV(mL) x 100 the average EF at rest is ~50-60%; it increases during exercise as the intensity of the exercise increases; during maximal exercise EF can increase to ~85% heart rate (HR) is the number of times the heart contracts in a minute (beats per minute; beats/min) cardiac output (Q) can be calculated as the product of stroke volume (SV) and heart rate (HR) Q(L/min) = SV(mL) x HR (beats/min) during exercise, Q can increase to 15-25 L/min depending on the intensity of exercise the increase in Q occurs very early in exercise, then becomes constant at a new higher level – these increases are from an increase in SV and HR increase in SV occurs very early in exercise then plateaus prolonged exercise shows a slight decline in SV due to excessive fluid loss (through sweating) increase in Q is related to the intensity of exercise – an increase in Q with an increase in exercise intensity with prolonged exercise Q is maintained but changes occur with HR and SV – this is called cardiovascular drift which is characterized by a slow steady rise in HR and decrease in SV cardiovascular drift results from physiological changes associated with an increase in body temperature that occurs during exercise changes include a decrease in plasma volume, redistribution of blood flow to the skin and dehydration all of these changes result in a decrease in venous return of blood to the heart and with a decrease in SV, therefore, the body compensates through an increase in HR and Q being maintained Blood Pressure defined: is the forced exerted by the blood against the walls of the arteries during exercise changes in blood pressure can occur depending on type, duration, and intensity of the exercise examples: 1) aerobic or endurance exercise – increase in systolic blood pressure, no change in diastolic blood pressure during the activity 2) resistance exercise – short but large increases in systolic and diastolic blood pressure the greater the exercise the greater the rise in systolic blood pressure post-exercise hypotension occurs with low intensity exercise – blood pressure drops below normal resting values hypertension occurs when blood pressure readings are greater than 140/90 mmHg risk of cardiovascular disease aerobic exercise training leads to improvement in resting blood pressure diet too can influence resting blood pressure – by decreasing amounts of saturated fats and cholesterol, increasing fibre and complex carbohydrates Blood Flow Distribution during exercise, skeletal muscle has an increased need for oxygen and the cardiovascular system attempts to match oxygen to meet the need of blood flow increase in delivery of oxygen is achieved by an increase in Q and redistribution of blood flow the system increases the amount of blood flow directed to the working muscle while decreasing blood flow to less active organs (table 7.1, p.118)