Blood Flow in Arteries and Veins - Biofluidmechanics: An Introduction to Fluid Mechanics Macrocirculation and Microcirculation - page 131

FIGURE 5.2 A schematic of the
major arteries within the systemic cir-
culation. After passing from the heart,
into the aorta, oxygenated blood can
enter the carotid arteries (feed the
head), the subclavian arteries (feed
the arms), the renal arteries (feed the
kidneys), the iliac arteries (feed the
pelvic region), or the femoral arteries
(feed the lower limbs). There are other
vessels that branch off the aorta that
are not included within this figure.

Left carotid artery

Right subclavian artery

Left subclavian artery

Aortic arch

Descending aorta

Right renal artery
Iliac artery

Left femoral artery

Right femoral artery

maintain the integrity of the blood vessel under high pressure; they also regulate blood
flow to the capillary beds. As a side note, the muscles in the larger arteries do not regulate
blood flow; they only act to maintain the blood vessel integrity. When the arteriolar mus-
cles contract, blood flow can be halted in that vessel and shunted to another region that
has a larger blood demand. Under a dilatory relaxation, the vessels diameter can be
increased by 3 to 4 times, which significantly increases the blood flow to its downstream
capillary bed (see Section 5.5 for details on flow resistance). To some extent, these vessels
are the controllers/regulators of blood flow throughout the entire body.
Blood velocity through the arterial circulation is directly proportional to the cross-
sectional area of the particular vascular bed. This is a direct application of the
Conservation of Mass formulation. Notice that we stated that it is proportional to the
cross-sectional area of the vascular bed and not a cross-sectional area of a representative
blood vessel. Under Conservation of Mass, if the cross-sectional area decreases, then the
flow velocity increases, and vice versa. However, we know that the blood flow is slower
in smaller vessels. This is because one must account for the total cross-sectional area of all
similar blood vessels. For instance, there is one aorta with an average cross-sectional area
of approximately 3 cm 2 . Combining all of the small arteries in the body (diameter range
from 0.5 cm to 1.5 cm), there is an average total cross-sectional area of approximately
25 cm 2 . Combining all of the arterioles in the body (diameter range from 20 m),

there is an average total cross-sectional area of approximately 50 cm 2 . For comparison,
capillaries account for a combined cross-sectional area of greater than 2000 cm 2 , with the
total combined venules accounting for a cross-sectional area of approximately 250 cm 2 and
the large veins (vena cava) accounting for a cross-sectional area of approximately 10 cm 2 .
For quantity comparisons, one aorta branches into approximately 200 arteries (large and

μ m to 500 μ