Maintenance of the Body
Just as in our analogy of the river and lake, blood ﬂows fast-
est where the total cross-sectional area is least. As the arterial
system branches, the total cross-sectional area of the vascular
bed increases, and the velocity of blood ﬂow declines propor-
tionately. Even though the individual branches have smaller lu-
cross-sectional areas and thus the volume
of blood they can hold are much greater than that of the aorta.
For example, the cross-sectional area of the aorta is 2.5 cm
but the combined cross-sectional area of all the capillaries is
. Tis diﬀerence results in fast blood ﬂow in the aorta
(40–50 cm/s) and slow blood ﬂow in the capillaries (about
0.03 cm/s). Slow capillary ﬂow is beneﬁcial because it allows
adequate time for exchanges between the blood and tissue cells.
As capillaries combine to form ﬁrst venules and then veins,
total cross-sectional area declines and velocity increases. Te
cross-sectional area of the venae cavae is 8 cm
, and the velocity
of blood ﬂow varies from 10 to 30 cm/s in those vessels, depend-
ing on the activity of the skeletal muscle pump.
Autoregulation: Local Regulation
of Blood Flow
As our activities change throughout the day, how does each or-
gan or tissue manage to get the blood ﬂow it needs? Te an-
, the automatic adjustment of blood ﬂow
to each tissue in proportion to the tissue’s requirements at any
instant. Local conditions regulate this process independent of
control by nerves or hormones. MAP is the same everywhere
in the body and homeostatic mechanisms adjust cardiac output
urine in the kidneys. Te rate of blood ﬂow to each tissue and
organ is almost exactly the right amount to provide for proper
function—no more, no less.
When the body is at rest, the brain receives about 13% of total
blood ﬂow, the heart 4%, kidneys 20%, and abdominal organs
24%. Skeletal muscles, which make up almost half of body mass,
normally receive about 20% of total blood ﬂow. During exercise,
however, nearly all of the increased cardiac output ﬂushes into
the skeletal muscles and blood ﬂow to the kidneys and digestive
Velocity of Blood Flow
Have you ever watched a swi± river emptying into a large lake?
Te water’s speed decreases as it enters the lake until its ﬂow
becomes almost imperceptible. Tis is because the total cross-
sectional area of the lake is much larger than that of the river.
Velocity in this case is
related to cross-sectional area.
Te same thing happens with blood ﬂow inside our blood vessels.
As shown in
, the speed or velocity of blood
ﬂow changes as blood travels through the systemic circulation.
It is fastest in the aorta and other large arteries (the river), slow-
est in the capillaries (whose large total cross-sectional area make
them analogous to the lake), and then picks up speed again in
the veins (the river again).
Total blood flow during
flow at rest
Distribution of blood ﬂow at rest and during
sectional area of
of the vascular bed
) of the
Blood ﬂow velocity and total cross-sectional
area of vessels.
Various blood vessels of the systemic circulation differ
in their total cross-sectional area (e.g., the cross section of all systemic
capillaries combined versus the cross section of all systemic arteries
combined), which affects the velocity of blood ﬂow through them.