1. 13.4 – Blood Vessels Arteries Arterioles Capillaries Venules Veins
away from ventricles of heart
Arterioles
receive blood from the arteries
carry blood to capillaries
Capillaries
sites of substance exchange between blood and body cells
Venules
receive blood from capillaries
Veins
Carry blood to atria of the heart

2. (elastic & collagenous fibers)
Artery
Lumen
(a)
(b)
Vein
Valve
Endothelium of
tunica interna
Connective tissue
(elastic & collagenous fibers)
Tunica media
Tunica externa

3. Arteries & Arterioles Arteries Arterioles Thick strong wall
Three layers (tunics)
Endothelial lining
Middle layer of smooth muscle and elastic tissue
Outer layer of CT
Blood is under high pressure
Thinner wall, but still three layers
Endothelial lining
Middle and outer layers are thinner
Some smooth muscle tissue
A small amount of CT
Controls blood flow into a capillary

4. Arteriole
Capillary
Endothelium
Smooth
muscle cell
Precapillary
sphincter

5. Capillaries Smallest blood vessels in diameter
Connect the smallest arterioles with the smallest venule
Capillaries are extensions of the inner lining of arterioles
Walls are endothelium only
Semi-permeable walls

6. Arteriole
Artery
Blood flow
Capillaries
Metarteriole forming arteriovenous shunt)
Venule
Vein
Precapillary
sphincter

7. Endothelial cell Slit Tissue fluid Capillary
Net force at arteriolar end
Outward force, including hydrostatic pressure = 35 mm Hg
Inward force of osmotic pressure = 24 mm Hg
Net outward pressure = 11 mm Hg
Net force at venular end
Outward force, including hydrostatic pressure = 16 mm Hg
Net inward pressure = 8 mm Hg
Capillary
Blood
flow
from
arteriole
Outward force,
including
hydrostatic
pressure
35 mm Hg
Inward force
of osmotic
24 mm Hg
Net outward
11 mm Hg
Lymphatic
capillary
Tissue
cells
16 mm Hg
Inward force of
osmotic pressure
Net inward
8 mm Hg
flow to
venule

8. Veins & Venules Veins Venules
Thinner walls than arteries, but still three tunics
Middle wall is poorly developed
Flap-like valves can be present
Blood is under lower pressure
Function as blood reservoirs
Microscopic vessels that continue from the capillaries
They merge to form veins
Thinner walls than arterioles
Less smooth muscle and elastic tissue than in arterioles

9. (a)
(b)
Toward
heart
100 90 80 70 60 50 40 30 20 10
Percent distribution
Large
veins
Small
and
venules
Systemic
60–70%
Lungs
10–12%
Heart
8–11%
arteries
Capillaries
4–5%
Venous valves allow blood to move toward the heart (a), but prevent blood from moving backward away from the heart (b).