The Cardiovascular System: Blood Vessels
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Blood Vessel Structure
Blood is transported throughout the body via a continuous
system of blood vessels. Arteries transport blood away from the
heart; veins carry blood back to the heart. Capillaries carry blood
to tissue cells and are exchange sites.
Structure of Blood Vessel Walls
All blood vessels except capillaries have three layers: tunica
intima, tunica media, and tunica externa. Capillary walls are
composed of the tunica intima only.
Elastic (conducting) arteries are the large arteries close to the
heart that expand during systole, acting as pressure reservoirs,
and then recoil during diastole to keep blood moving. Muscular
(distributing) arteries carry blood to speciﬁc organs; they are less
stretchy and more active in vasoconstriction. Arterioles regulate
blood ﬂow into capillary beds.
Atherosclerosis is a degenerative vascular disease that decreases
the elasticity of arteries.
Capillaries are microscopic vessels with very thin walls. Most
exhibit intercellular cleFs, which aid in the exchange between
blood and interstitial ﬂuid.
Te most permeable capillaries are sinusoid capillaries (wide, tortuous
channels). ±enestrated capillaries with pores are next most permeable.
Least permeable are continuous capillaries, which lack pores.
Vascular shunts (metarteriole–thoroughfare channels) connect
the terminal arteriole and postcapillary venule at opposite ends
of a capillary bed. Most true capillaries arise from and rejoin the
shunt channels. Precapillary sphincters regulate the amount of
blood ﬂowing into the true capillaries.
Veins have comparatively larger lumens than arteries, and a
system of valves prevents backﬂow of blood.
Normally most veins are only partially ﬁlled; for this reason, they
can serve as blood reservoirs.
Te joining together of blood vessels to provide alternate
channels in the same organ is called an anastomosis. Vascular
anastomoses form between arteries, between veins, and between
arterioles and venules.
Cardiovascular System; Topic: Anatomy Review: Blood Vessel
Structure and Function, pp. 1–27.
Physiology of Circulation
Introduction to Blood Flow, Blood Pressure,
Blood ﬂow is the amount of blood ﬂowing through a vessel, an
organ, or the entire circulation in a given period of time. Blood
pressure (BP) is the force per unit area exerted on a vessel wall
by the contained blood. Resistance is opposition to blood ﬂow;
blood viscosity and blood vessel length and diameter contribute
Blood ﬂow is directly proportional to blood pressure and
inversely proportional to resistance.
Cardiovascular System; Topic: Factors that Affect Blood Pressure,
Systemic Blood Pressure
Systemic blood pressure is highest in the aorta and lowest in the
venae cavae. Te steepest drop in BP occurs in the arterioles,
where resistance is greatest.
Arterial BP depends on compliance of the elastic arteries and on
how much blood is forced into them. Arterial blood pressure is
pulsatile, and peaks during systole; this is measured as systolic
pressure. During diastole, as blood is forced distally in the
circulation by the rebound of elastic arteries, arterial BP drops to
its lowest value, called the diastolic pressure.
Pulse pressure is systolic pressure minus diastolic pressure. Te
mean arterial pressure (MAP)
diastolic pressure plus one-third
of pulse pressure and is the pressure that keeps blood moving
throughout the cardiac cycle.
Low capillary pressure (35 to 17 mm Hg) protects the delicate
capillaries from rupture while still allowing adequate exchange
across the capillary walls.
Venous pressure is nonpulsatile and low (declining to zero)
because of the cumulative eﬀects of resistance. Venous valves,
large lumens, functional adaptations (muscular and respiratory
pumps), and sympathetic nervous system activity promote
Maintaining Blood Pressure
Blood pressure varies directly with CO, peripheral resistance
), and blood volume. Vessel diameter is the major factor
determining resistance, and small changes in the diameter of
vessels (chieﬂy arterioles) signiﬁcantly aﬀect blood pressure.
Cardiovascular System; Topic: Measuring Blood Pressure, pp. 1–13.
BP is regulated by autonomic neural reﬂexes involving barorecep-
tors or chemoreceptors, the cardiovascular center (a medullary
center that includes the cardiac and vasomotor centers), and au-
tonomic ﬁbers to the heart and vascular smooth muscle.
Activation of the receptors by falling BP (and to a lesser extent by
a rise in blood CO
, or falling blood pH or O
the vasomotor center to increase vasoconstriction and the
cardioacceleratory center to increase heart rate and contractility.
Rising BP inhibits the vasomotor center (permitting vasodilation)
and activates the cardioinhibitory center.