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Section 4, Chapter 15
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 15.27
An 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 venule Shunt 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
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