Essentials of Anatomy and Physiology

and pulmonary circulation becomes fully functional
within a few days.

VELOCITY OF BLOOD FLOW

The velocity, or speed, with which blood flows differs
in the various parts of the vascular system. Velocity is
inversely related (meaning as one value goes up, the

other goes down) to the cross-sectional area of the

particular segment of the vascular system. Refer to

Fig. 13–9 as you read the following. The aorta receives

all the blood from the left ventricle, its cross-sectional

area is small, about 3 cm^2 (1 sq. inch), and the blood

moves very rapidly, at least 30 cm per second (about 12

inches). Each time the aorta or any artery branches,

the total cross-sectional area becomes larger, and the

speed of blood flow decreases. Think of a river that

306 The Vascular System

4,000

3,000

2,000

1,000

30

20

10

120

100

80

60

40

20

0

Blood pressure

(mm Hg)

Pulsepressure

Blood velocity

(cm/sec)

C

ross-sectionalarea (cm

2 )

Diastolic pressure

Systolic pressure

Aort

a

Arte

ries

Arte

rio

les

Capilla

rie

s

Ven

ules

Vei

ns

Ven

a

Cava

e

A

B

C

D

Figure 13–9. Characteristics of the vas-

cular system. (A) Schematic of the branch-

ing of vessels. (B) Cross-sectional area in

square centimeters. (C) Blood velocity in

centimeters per second. (D) Systemic

blood pressure changes. Notice that sys-

tolic and diastolic pressures become one

pressure in the capillaries.

QUESTION: Look at the cross-sectional

area and blood velocity. As area increases,

what happens to velocity? Where is veloc-

ity slowest?