This document summarizes the structure and function of blood vessels. It describes the three main types of blood vessels - arteries, which carry blood away from the heart; veins, which carry blood back towards the heart; and capillaries, which allow for the exchange of gases, nutrients, and waste between blood and tissues. Each vessel type has distinct layers in its walls to facilitate blood flow and transport. Arterioles branch from arteries and connect to capillaries, while venules branch from capillaries and merge to form veins. The document highlights key differences between arteries and veins like wall composition and the presence of valves in veins.

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Blood vessels
Section 4, Chapter 15

2. Veins
Arteries
•Returns blood towards the
heart
•Convey blood away from the
heart
Venules
Arterioles
•Receives blood from
capillaries
•Thinner vessels that convey
blood towards capillaries
Capillaries
•Site of exchange between blood
and body tissues

3. Walls of the blood vessels – 3 Layers
Tunica Interna (inner)
Endothelium
•A layer of smooth simple squamous
epithelium
•Secretes biochemicals with a wide
variety of functions.
Basement membrane
•Bed of connective tissue with elastic &
collagenous fibers

4. Walls of the blood vessels – 3 Layers
Tunica Media (middle)
Smooth Muscles
•Vasoconstriction – muscles contract,
decreasing diameter of vessel
•Vasodilation – muscles relax, allowing
vessel diameter to increase
Elastic Connective tissue
• Recoil of elastic fibers helps propel
blood through vessels

5. Walls of the blood vessels – 3 Layers
Tunica Externa (outer)
Fibrous Connective Tissue
• Elastic and collagenous fibers
• Attaches blood vessel to organs
Vasa Vasorum “vessels of the vessels”
• Provide blood supply to walls of thicker
arteries

6. Arterioles
•Arterioles are smaller divisions of arteries.
•metarterioles – small arterioles that join capillaries
•Arteriovenous shunt – connects an arteriole directly to a venule
Shunt allows blood to bypass a capillary bed.
Figure 15.27
An arteriovenous shunt
provided by a metarteriole.

7. Capillaries
•Capillaries - smallest diameter blood vessels
•Consists of a single layer of endothelial cells
•Site of gas, nutrient, and waste exchange
Slits
• Spaces between endothelia that
facilitate diffusion across vessel wall
Figure 15.28 Substances are exchanged through
openings (slits) separating endothelial cells.

8. Capillaries
Figure 15.26 A precapillary
sphincter at the base of a
capillary.
Precapillary sphincters
• Smooth muscles that regulate the flow
of blood through a capillary
• Closes a capillary bed when oxygen
demand to an organ is low

9. Capillaries
Sinusoids
• large cavities within discontinuous capillaries
• Allows a rapid exchange of nutrients, debris,
proteins, and even cells.
• located throughout the liver and spleen.
Artificially colored electron micrograph
depicts sinusoids throughout the liver.

10. •Venules
•Continue from capillaries and merge to form veins
•Veins
•Convey blood from body back to the atria of heart
•Veins follow a pathway roughly parallel to arteries
•Vessel wall of veins has 3 layers (tunics) similar to arteries

11. Differences between veins and arteries
– Veins have poorly developed tunica media
• Thinner walls, and a larger luman than arteries
– Tunica Interna of veins contain valves
• Valves prevent blood from flowing backwards towards capillaries.
– Veins act as blood reservoirs
• Most blood (60-70%) is in the veins and venules.
Figure 15.31. Venous valves (a) open as blood moves towards
the heart, but (b) close to prevent blood from moving
backward away from the heart.

12. Differences between
veins and arteries
Figure 15.25 Blood vessels (a) the wall of
an artery. (b) The wall of a vein. (c) cross
section of an arteriole (bottom) and a
venule (top).

13. End of Section 4, Chapter 15