Section 4, Chapter 15

Blood vessels

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Arteries• Convey blood away from

the heart

Arterioles• Thinner vessels that

convey blood towards capillaries

Capillaries• Site of exchange between

blood and body tissues

Venules• Receives blood from

capillaries

Veins• Returns blood towards

the heart

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

Tunica Interna (inner)

Walls of the blood vessels – 3 Layers

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

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

Walls of the blood vessels – 3 Layers

Figure 15.27An arteriovenous shunt provided by a metarteriole.

•Arterioles are smaller divisions of arteries.

• metarterioles – small arterioles that join capillaries

• Arteriovenous shunt – connects an arteriole directly to a venuleShunt allows blood to bypass a capillary bed.

Arterioles

Figure 15.28 Substances are exchanged through openings (slits) separating endothelial cells.

•Capillaries - smallest diameter blood vessels• Consists of a single layer of endothelial cells

• Site of gas, nutrient, and waste exchange

Capillaries

Slits • Spaces between endothelia that

facilitate diffusion across vessel wall

Figure 15.26 A precapillary sphincter at the base of a capillary.

Capillaries

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

Artificially colored electron micrograph depicts sinusoids throughout the liver.

Capillaries

Sinusoids• large cavities within discontinuous

capillaries

• Allows a rapid exchange of nutrients, debris, proteins, and even cells.

• located throughout the liver and spleen.

•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

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.

– 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.

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).

Differences between veins and

arteries

End of Section 4, Chapter 15