Introduction to Blood Flow
The blood (vehicle for transporting material) is driven through the blood vessels
(conduits) by a variety of motive forces.
Arterial Blood Flow
Blood is driven through the arteries by a combination of forces as follows:
• the force produced by the contraction of the ventricular walls
• the elastic recoil of the arterial walls
Systole - When the left ventricle contracts (systole), it forces the blood into the aortic arch.
Above the base cylinder, the wall of the aortic arch is mainly elastic FCT.
As the blood fills the aortic arch, the walls are stretched.
Diastole - When the ventricle relaxes (diastole), the wall of the arch recoils and presses against the blood.
With the closing of the aortic semilunar valve, the blood is forced to move out along the arteries in a pressure pulse.
Since the elasticity of the arterial walls produces a continuous pressure, the blood moves continuously throughout the system.
The highest pressure is called the systolic pressure, and the lowest pressure is the diastolic pressure.
Vasoconstriction and Gravity
Vasoconstriction is the actual contraction of the arterial walls. Vasoconstriction can further increase the pressure on the blood in the arteries.
Gravity helps to move blood to the trunk and lower members.
However, it is a hindrance in moving blood to the head and neck.
Venous Blood Flow
There is usually a low level of pressure in the veins.
There are valves in the veins that ensure that blood flows continuously toward the heart.
Therefore, as pressure is applied to a vein, there will be a pump effect.
Pressure from Arteries
The muscular compartments of the upper and lower limbs tend to be full in healthy persons.
Therefore, as blood enters the arteries within these compartments, a volume of blood must leave through the veins.
Pressure from Muscular Contractions
During muscular activity, additional forces press against the veins and produce a "milking action."
Again, blood moves through the veins back toward the heart.
In the head and neck, gravity helps to move the blood down through the veins.
In the trunk and lower limbs, the valves help to prevent a backward flow of blood in the veins.
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