Download Cardiovascular Dynamics (Exercise Responses)

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)