Lecture 7-9 Blood Vessels.docx - Blood Vessels Lecture 7...

Blood Vessels
Lecture 7
Types of blood vessels (3 main types of blood vessels)
 Arteries: 3 main types. Following types of arteries go from biggest to smallest
- Elastic : largest of arteries. They leave the heart (aorta, pulmonary trunk, and slightly
off the branches of those)
o More stretching ability than other arteries
- Muscular : majority of our arteries. Contain thick smooth muscle walls which are able
to vasodilate or vasoconstrict to change the direction the blood flows in body
- Arterioles : these are the arteries before they reach capillaries. Smallest arteries
 Capillaries: form network/beds where gas, nutrient, & waste exchange occurs. Only
point in system where things leave/enter the blood.
 Veins: these go from smallest to largest
- Venules : smallest of veins. On other side of capillaries
- Small veins :
- Medium or large veins
NOTE : Great vein entering the heart is known as the superior vena cava
Blood vessel structure
 Has 3 layers:
1. Tunica interna (or tunica intima)
- Innermost layer. Closest to the blood (or luma/tube of the blood vessel)
- Largest feature is a layer of simple squamous epithelial cells which line the
lumen (or inside of our blood vessels) called endothelial cells
2. Tunica media
- Surrounds the tunica interna. This is the middle layer
- We have smooth muscle and elastic fibers
- This layer changes dramatically when we go from arteries to veins (the
thickness and # of elastic fibers change)
 this layer disappears in capillaries
3. Tunica externa
- Outermost layer. It contains CT which attaches blood vessels onto other
structures
- CT varies as we go through the 3 different types of vessels
- Nerve supply to our vessels are here
-
Comparing blood vessel structure
Tunica interna:
 Endothelial cells line everything ! (artery, veins and capillaries)
- The layer is similar in all vessels except veins

- In veins we have extensions of the endothelial layer which moves into lumen
o The extensions make flaps which create a valve system , why? in veins we
have low pressure, so it stops blood going in wrong direction
 Basement membrane: attaches all the epithelial cells together. It’s a network of collagen
fibers
- If we have damaged endothelial cells, the basement membrane of collagen fibers get
exposed causing platelets to stick on to it to start forming a clot
- This causes plaque to build up inside the blood vessels  leading to heart stroke
 In arteries only , the next layer is the internal elastic lamina: elastic fibers which surround
the tunica interna
- it contains holes to allow NT to reach the smooth muscle in tunica media to activate
it
NOTE : capillaries are only made of endothelial cells and basement membrane, why? it’s kept
thin to allow efficient gas exchange
Tunica media:
 Smooth muscle: runs in a circle around the lumen so when they contract they make the
diameter of the blood vessel smaller
- In arteries : smooth muscle can be very thick (40 layers of cells thick)
- In veins : smooth muscle is only 1-2 layers thick, why? they don’t vasodilate or
vasocontrict
 External elastic lamina: only present in arteries .