712
UNIT 4
Maintenance of the Body
19
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-
mens, their
combined
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
2
but the combined cross-sectional area of all the capillaries is
4500 cm
2
. 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
2
, 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-
swer is
autoregulation
, 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
organs declines
(Figure 19.13)
.
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
inversely
related to cross-sectional area.
Te same thing happens with blood flow inside our blood vessels.
As shown in
Figure 19.14
, 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).
Brain
Heart
Skeletal
muscles
Skin
Kidneys
Abdomen
Other
750
250
1200
500
1100
1400
600
750
750
12,500
600
600
400
1900
Total blood flow during
strenuous exercise
17,500 ml/min
Total blood
flow at rest
5800 ml/min
Figure 19.13
Distribution of blood flow at rest and during
strenuous exercise.
50
40
30
20
10
0
5000
Aorta
Arteries
Arterioles
Capillaries
Venules
Veins
Venae cavae
Relative cross-
sectional area of
different vessels
of the vascular bed
4000
3000
2000
1000
0
Total area
(cm
2
) of the
vascular
bed
Velocity of
blood flow
(cm/s)
Figure 19.14
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.
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