1. Blood Velocities

2. Blood Velocities
Speed / velocity of blood flow changes as blood travels through the systemic circulation.
It is fasted in the aorta and other large arteries.
Slowest in the capillaries.
Picks up speed in the veins leading up to the heart.

3. Blood Velocities
Velocity is inversely related to cross-sectional area of the blood vessels to be filled.
Blood flows fastest where the total cross-sectional area is the least.
Which has largest total cross sectional area? – Capillaries
Demonstrate with rolled up paper

4. Velocity of Blood Flow
As the arterial system continues to branch, the total cross-sectional area of the vascular bed increases, and the velocity of the blood flow decreases proportionally.
Even though the individual braches have smaller lumens, their combined cross-sectional area, and thus the volume of blood they can hold is much greater than the aorta.
EXAMPLE
The cross-sectional area of the aorta is 2.5cm2 and the average velocity of blood flow in the aorta is cm/s. However, the total cross-sectional area of the capillaries is 4500cm2, so capillary blood is very slow (about 0.03cm/s).

5. Velocity of Blood Flow Example
The cross-sectional area of the aorta is 2.5cm2 and the average velocity of blood flow in the aorta is cm/s. However, the total cross-sectional area of the capillaries is 4500cm2, so capillary blood is very slow (about 0.03cm/s).
Slow capillary blood flow is beneficial because it allows adequate time for exchange to be made between the blood and tissue cells.

6. Veins and Velocity of Blood
As capillaries combine to form venules and then vein, total cross-sectional area declines and velocity increases.
The cross sectional area of the venae cavae is 8cm2, and the velocity of blood flow varies from cm/s in those vessels, depending on the activity of the skeletal pump.