The Cardiovascular System: Blood Vessels
Elastic arteries are pressure reservoirs, expanding and re-
coiling as the heart ejects blood. Consequently, blood ﬂows
fairly continuously rather than starting and stopping with the
pulsating rhythm of the heartbeat. If the blood vessels become
hard and unyielding, as in atherosclerosis, blood ﬂows more
intermittently, similar to the way water ﬂows through a hard
rubber garden hose attached to a faucet. When the faucet is
on, the high pressure makes the water gush out of the hose.
But when the faucet is shut oﬀ, the water ﬂow abruptly be-
comes a trickle and then stops, because the hose walls can-
not recoil to keep the water under pressure. Also, without the
pressure-smoothing eﬀect of the elastic arteries, the walls of
arteries throughout the body experience higher pressures.
Battered by high pressures, the arteries eventually weaken and
may balloon out or even burst. (Tese problems are discussed
A Closer Look
on pp. 700–701.)
The relationship of blood vessels to each other and to lymphatic vessels.
Lymphatic vessels recover excess tissue ﬂuid and return it to the blood.
are the thick-walled arteries near the heart—the
aorta and its major branches. Tese arteries are the largest in
diameter, ranging from 2.5 cm to 1 cm, and the most elastic (±a-
ble 19.1). Because their large lumens make them low-resistance
pathways that conduct blood from the heart to medium-sized
arteries, elastic arteries are sometimes called
Elastic arteries contain more elastin than any other vessel
type. It is present in all three tunics, but the tunica media con-
tains the most. Tere the elastin constructs concentric “holey”
sheets of elastic connective tissue that look like slices of Swiss
cheese sandwiched between layers of smooth muscle cells.
Although elastic arteries also contain substantial amounts of
smooth muscle, they are relatively inactive in vasoconstriction.
Tus, in terms of function, they can be visualized as simple elas-