Exercise 21: Anatomy of Blood
Vessels
3 Tunics (t. intima, t. media, t. externa)
1
Very thick
tunica media
in arteries
2
3
Only tunica
intima in
capillaries
Figure 11.9b
Blood Vessel Tunic
Description
Tunica intima
• Most internal tunic;
• Its smooth surface
decreases friction
Tunica media
Bulky middle tunic contains
smooth muscle and elastin;
Tunica externa (adventitia)
Most superficial tunic
*Also called
“Tunica adventia”
Tunic/
Vessel
Tunica
Intima
Tunica
media
Tunica
externa*
Artery
X
X
X
Vein
X
X
X
Capillary
X
Only tunica intima in capillaries
Capillaries
 Walls of capillaries are only one cell layer thick to allow
for exchanges between blood and tissue
Only capillaries directly serve the needs
of the body’s cells. It is through the capillary
Walls that exchanges are made between tissue
cells and blood.
Respiratory gases, nutrients, and wastes move
along diffusion gradients.
Artery
Veins
VALVE
Blood Vessels: The Vascular System
Figure 11.9a
Elastic recoil of artery propels blood along
They have the greatest amount of elastin,
enabling them to expand. When the heart
relaxes, the recoil propels blood onward.
Muscular Pumps aids blood flow in Veins
As skeletal muscles surrounding the veins contract
and relax, blood is milked through the veins
toward the heart.
Respiratory Pump aids blood flow
in veins!
Pressure changes
that occur in the
thorax during
breathing also aid
blood return to the
heart through the
veins.
Arteries are thicker than veins
Because arteries are closer to the
pumping action of the heart, they
must be able to expand as blood is
propelled into them and then recoil
passively as the blood flows off into
the circulation during diastole.
Arterial walls must be strong and
resilient to withstand blood pressure
fluctuations.