The Cardiovascular System

Stroke Volume

Stroke volume is the amount of blood that is ejected from the heart during VEP and is measured in ml.  The amount of blood ejected from the heart is determined by:

1. Preload -- Volume of blood returned to the heart

2. At rest, the heart ejects 50% - 60% of the preload (Ejection Fraction:EF)

3.  

4. Contractility -- Force of contraction

5. Afterload -- The resistance of the aortic and pulmonary vein pressure during ventricular contraction

Stroke volume is determined by the difference between left ventricular end diastolic volume (LVEDV) and left ventricular end systolic volume (LVESV):

                            SV = LVEDV - LVESV

Ejection Fraction is that portion of blood ejected (SV) divided by the preload (LVEDV) or:

                            EF = (SV/LVEDV) * 100

The Effect of Exercise on Stroke Volume

As mentioned before, SV is dependent on preload, contractility, and afterload.  When we exercise SV increases, however, it is limited by the fact that the heart cannot eject all of the blood within the ventricle. A residual volume is always maintained in the ventricle.   As the volume of blood returned to the heart (LVEDV) increases, stroke volume will increase.  The increase in SV due to increased LVEDV is know as the Frank-Starling law.  Contractility is controlled by the sympathetic nervous system and is independent of the preload.  Afterload increases during exercise because blood pressure increases, therefore afterload is detrimental to SV during exercise

Cardiac Output

Cardiac output (Q) represents the total blood volume that passes through the heart during a given time frame (usually a minute).  Cardiac output also represents the body's ability to meet changing metabolic needs based on aerobic energy expenditure of the musculature. Q is the product of heart rate (HR) and stroke volume:

            Q = HR * SV

If we expand on the formula:

            Q = HR * (LVEDV -LVESV)

Since SV is rather hard to measure, we use the FICK equation to estimate Q:

            Q = VO₂/a-vO_2diff

_{And to measure VO2max:}

            _{VO2max = Q * a-vO2diff}

Myocardial Oxygen Consumption

The amount of O₂ usage in muscle is usually determined by the a-vO₂diff.  However unlike muscle, the demand of the cardiac muscle is much higher.  The myocardium uses approximately 60%-70% of the available oxygen at rest, and this usage increases with exercise.  To estimate the myocardial oxygen consumption, we use rate pressure product (RPP) which is a function of heart rate and systolic blood pressure.  Because both heart rate and systolic blood pressure (SBP) are farely linear and increase with exercise, we can assume that they are a good estimate of cardiac oxygen consumption.

                RPP = (SBP  * HR)/100

Mean Arterial Pressure

As mentioned above, one of the limiting factors for stroke volume is afterload, or the pressure in the aorta that decreases ejection of blood from the left ventricle.   A way to estimate afterload is by measuring the mean arterial pressure (MAP).   To estimate MAP you need to know the pulse pressure(PP) and the diastolic blood pressure.:

                PP = SBT - DBT

                MAP =  (PP/3) + DBT

                MAP = Q * TPR: where TPR is the resistance of the vessels to blood flow.

 

 

 

                TPR = (Length * viscosity)/(radius)⁴

                TPR = MAP/Q