BLOOD VESSELS

BLOOD VESSELS

Wall of an artery consists of three (3) distinct layers of tunics
Tunica intima
- Composed of simple, squamous epithelium called endothelium.
- Rests on a connective tissue membrane that is rich in elastic and collagenous fibers.
Tunica media
- Makes up the bulk of the arterial wall.
- Includes smooth muscle fibers, which encircle the tube, and a thick layer of elastic connective tissue.
Tunica adventitia
- Is relatively thin.
- Consists chiefly of connective tissue with irregularly arranged elastic and collagenous fibers.
- This layer attaches the artery to the surrounding tissues.
- Also contains minute vessels (vasa vasorum--vessels of vessels) that give rise to capillaries and provide blood to the more external cells of the artery wall.
Smooth muscles in the walls of arteries and arterioles are innervated by the sympathetic branches of the autonomic nervous system. 17986

Impulses on these vasomotor fibers cause the smooth muscles to contract causing vasoconstriction.
If these impulses are inhibited, the muscle fibers relax and the diameter of the vessel increases--vasodilation.

Flow of blood through the capillaries is regulated by vessels with smooth muscles in their walls.
- Metarteriole--is a vessel that emerges from an arteriole, passes through the capillary network and empties into a venule.
  - Proximal portions of the metarterioles are surrounded by scattered smooth muscle cells whose contraction and relaxation help regulate the amount and force of the blood.
  - Distal portion of a metarteriole has no smooth muscle fibers and is called a thoroughfare channel.
    - Serves as a low resistance channel that increases blood flow.
True Capillaries
- Emerge from arterioles or metarterioles and are not on the direct flow route from arteriole to venule.
- At their site of origin, there is a ring of smooth muscle fibers called a precapillary sphincter that controls the flow of blood entering a true capillary.
Continuous Capillaries 17991

Are named because the cytoplasm of the endothelial cells is continuous when viewed in cross-section through a microscope.
- Cytoplasm appears as an uninterrupted ring, except for the endothelial junction.

Differ from continuous capillaries in that their endothelial cells have numerous pores or fenestrations where the cytoplasm is very thin or absent.
Found in kidneys, villi of the small intestine, choroid plexi of the ventricles of the brain, and endocrine glands.

Are wider than capillaries and more torturous
- Contain spaces between endothelial cells instead of having the usual endothelial lining.
Basal lamina is incomplete or missing.
- In addition, sinusoids contain specialized lining cells that are adapted to the function of the tissue.
- In the liver, sinusoids contain phagocytic cells called stellate reticuloendothelial (Kupffer) cells.
- Other regions containing sinusoids include the spleen, parathyroid glands, adrenal cortex, and bone marrow.

Venules are the microscopic vessels that continue from the capillaries and merge to form veins.
Veins which carry blood back to the heart, follow pathways roughly parallel to those of the arteries.
Walls of veins are similar to those of arteries, in that they are composed of three distinct layers.
- Middle layer is poorly developed.
- As a result, veins have thinner walls that contain less smooth muscle and less elastic tissue than arteries.
Many veins, particularly those in the arms and legs, have flaps or valves which project inward from the lining.
- Valves are usually composed of two leaflets that close if the blood begins to back up in the veins.
  - Valves are open as long as the blood flow is toward the heart and closed if it is in the opposite direction.
Veins also function as blood reservoirs that can be drawn upon in time of need.
- If a hemorrhage accompanied by drop in blood pressure occurs, the muscular walls of the veins are stimulated reflexively by the sympathetic nervous system.
  - Veins constrict and help to raise the blood pressure.
  - This mechanism ensures a nearly normal blood flow even if as much as 25% of the blood volume is lost.