Henry Family Blog

Cardiac Cycle Phases:

-Blood goes into Ventricle, AV valves open, SL valves closed

-Ventricles contract, AV valves close, pressure increases, begins to open SL valves

-SL valves open, blood is ejected to pulmonary and systemic circulations

-All 4 valves closed, atria full with blood, intraventricular pressure decreases

-Pressure in atria increases, AV valves open and ventricles fill

There are three types of volumes associated with the cardiac cycle; end diastolic volume (EDV), end systolic volume (ESV), stroke volume (SV).  The EDV is the volume in the ventricle at the end of diastole.  The ESV is the volume in the ventricle at end of systole.  The SV is the volume ejected per beat from each ventricle.

There are three factors that affect stroke volume; contractility, preload, afterload.  Contractility is the forcefulness of a contraction of the ventricle fibers.  It depends on the calcium kinetics and agents that increase or decrease the availability of calcium in the sarcolemma.  Preload is the degree of stretch of the heart before a contraction, preload is proportional to EDV.  Afterload is the pressure that must be overcome before a semilunar valve opens and blood can be sent to pulmonary and systemic circulation.

Hemodynamics:

Hemodynamics is the regulation of blood flow.  It involves blood pressure, mean arterial pressure, total peripheral resistance, venous return and velocity of flow.  These are factors that affect blood flow.  Blood flows from areas of higher pressure to areas of lower pressure.  It is influenced by pressure and resistance.  The greater the difference, the greater the blood flow.  The greater the resistance, less blood flow.

Blood Pressure:

Blood pressure is pressure that is exerted by blood on the walls of a vessel.  It is caused by contractions of the ventricles.  It is highest in the aorta-120mm Hg during systole and 80 during diastole.  Pressure falls steadily in systemic circulation with distance from left ventricle.  If heart rate increases cardiac output then blood pressure rises.  If there is a decrease in blood volume that is less than 10%, blood pressure drops.  Water retention increases blood pressure.

Mean Arterial Pressure:

MAP=DBP + (0.33 x [SBP - DBP])

Represents the average pressure in the blood vessels.  When we talk about blood flow from an area of high pressure to an area of low pressure in the vascular bed-this is the pressure we are interested in knowing. 

Total Peripheral Resistance:

Blood flows from high to low pressure areas, but is influenced by resistance to flow.  Total peripheral (TPR) influences the magnitude of blood flow through the vascular bed.  TPR is the sum of all friction between blood and the walls of the vessels.  It is influenced by the size of the lumen, blood viscosity and total blood vessel length.  The smaller the diameter of the vessel, the greater resistance it offers to blood flow.  The smaller the diameter, increased resistance, decreased blood flow, increased bloop pressure=vasoconstriction.  The larger the diameter, decreased resistance, increased blood flow, decreased blood pressure=vasodialation.

Blood viscosity mostly depends on the ratio of red blood cells to plasma.  It can be increased by dehydration and polycythemia (many red blood cells).  If blood viscosity increases, the resistance to blood flow increases which increases blood pressure.

Blood vessels, the longer the vessel, the greater the resistance to flow.  The greater the resistance to flow, the greater the blood pressure.  There are 200 miles of blood vessels for every pound of fat.  This is what may cause high blood pressure in obese people. 

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