Section 4, Chapter 15
•Returns blood towards the heart
•Convey blood away from the heart
•Receives blood from capillaries
•Thinner vessels that convey blood towards capillaries
Capillaries •Site of exchange between blood and body tissues
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
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
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
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.
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.
Figure 15.26 A precapillary sphincter at the base of a capillary.
Precapillary sphincters â€˘ Smooth muscles that regulate the flow of blood through a capillary â€˘ Closes a capillary bed when oxygen demand to an organ is low
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.
•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
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.
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).
End of Section 4, Chapter 15