HUMAN BIOLOGY

CirCulation: the heart and blood vessels 123

What is the cardiovascular system?

• The cardiovascular system consists of the heart and blood
  vessels. The system transports blood to and from all living
  cells in the body.


• Some fluid and dissolved substances in blood move from blood
  vessels to the extracellular fluid.


• The lymphatic system collects excess extracellular fluid and
  returns it—along with useful substances it contains—to the
  cardiovascular system.

taKe-hoMe Message

arteriole Blood vessel

that connects arteries with

capillaries.

artery Large-diameter

blood vessel that carries

blood away from the heart

at high pressure.

capillary Narrow blood

vessel that functions in the

exchange of substances

between blood and the

extracellular fluid.

cardiovascular

system The heart and

blood vessels, which

together move blood

throughout the body.

heart Muscular, blood-

pumping organ of the cardio-

vascular system. It generates

pressure required to move

blood through the system.

vein Large-diameter vessel

that returns blood to the

heart.

venule Vessel that receives

blood from capillaries; the

smallest-diameter vessel in

the venous system.

cardiovascular system. This vessel

network is part of the lymphatic

system, which includes organs with

major roles in body defenses. We

consider it more fully in Chapter 9.

blood circulation is

essential for maintaining

homeostasis

The heart and blood vessels

are sometimes referred to as the

“circulatory system.” This name is

apt because blood circulates through

the system, bringing body cells such

essentials as oxygen and nutrients

from food. Circulating blood also

takes away the wastes produced

by our metabolism, along with

excess heat. In fact, cells depend

on blood to constantly pick up and

deliver an extremely diverse range

of substances, including those that

move into or out of the digestive

system, the urinary system, and the

respiratory system (Figure 7.3).

7 Circulation: The Heart and Blood Vessels

the cardiovascular system

“Cardiovascular” comes from the Greek kardia (heart) and
the Latin vasculum (vessel). As you can see in Figure 7.1 the
cardiovascular system has two main elements, the heart
and blood vessels.

• The heart is a muscular pump that generates the
  pressure required to move blood throughout the
  body.


• Blood vessels are tubes of different diameters that
  transport blood (Figure 7.2).
  The heart pumps blood at high pressure into arteries,
  which have a large diameter. From there blood flows into
  smaller and narrower vessels called arterioles, which
  branch into even narrower capillaries. Fluids and solutes
  diffuse out of the capillaries into the extracellular fluid
  and eventually enter body cells. In the reverse route, fluid
  and solutes move from body cells into the extracellular
  fluid and from there into the blood in capillaries. Blood
  flows from capillaries into small venules, then into large-
  diameter veins that return blood to the heart.
  As you will read later on, the volume of blood flowing
  to a particular part of the body is adjustable. So is the rate
  at which it flows. This flexibility permits the cardiovascular
  system to deliver blood in ways that suit conditions in
  different parts of the body. For example, blood flows
  rapidly through arteries, but in capillaries it must flow
  slowly so that there is time for substances moving to and
  from cells to diffuse into and out of the extracellular fluid.

the cardiovascular system is linked
to the lymphatic system

The heart’s pumping action puts pressure on blood flow-
ing through the cardiovascular system. Partly because
of this pressure, small amounts of water and some pro-
teins dissolved in blood are forced out and become part
of extracellular fluid (the fluid outside cells). A network
of drainage vessels picks up excess extracellular fluid
and usable substances in it and returns them to the

Figure 7.2 The cardiovascular system includes networks of
branching blood vessels.

Urinary

System

Cardiovascular

System

food, water intake

elimination

of carbon

dioxide

elimination of

excess water,

salts, wastes

water,

solutes

carbon

dioxide

oxygen

nutrients,

water,

salts

elimination

of food

residues

rapid transport

to and from all

living cells

oxygen intake

Respiratory

System

Digestive

System

Figure 7.3 Together with the other organ systems shown

here, the cardiovascular system helps maintain stable

chemical and physical conditions in the extracellular fluid.

Biophoto Associates/Science Source

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