Chapter 21 BLOOD VESSELS & HEMODYNAMICS THERE ARE 5 MAIN TYPES OF BLOOD VESSELS Arteries Arterioles Capillaries Venules Veins
Arteries carry blood away from the heart.
Large elastic arteries leave the heart and divide into muscular medium sized arteries that branch out into the body.
Muscular arteries then divide into small arteries that divide into smaller arterioles. Arterioles in some tissue then divide into capillaries.
Gas and nutrient exchange occurs in the thin vessel walls of capillaries, which eventually collapse into venules.
These small veins collapse into larger vessels called veins.
Veins and venules carry blood back to the heart.
Because venous blood is low in oxygen, large veins require vasa vasorum, which is the vasculature in their walls that supplies them with oxygen rich blood.
Arteries have three layers called tunics.
Closest to the vessel lumen is the tunica interna, or intima, consists of endothelium, a basement membrane, and the internal elastic lamina. The simple squamous cells of the endothelium are the only tissue in direct contact with the blood.
In the middle is the tunica media which consists of smooth muscle cells that encircle the vessel lumen and elastic fibers.
The outer coat is called the tunica externa and in muscular arteries it is separated from the tunica media by the elastic lamina.
The tunica externa consists of elastic and collagen fibers. Sympathetic neurons typically innervate vascular smooth muscle, with sympathetic stimulation usually causing smooth muscle contraction that decreases the diameter of the artery’s lumen, a process called vasoconstriction.
When sympathetic tone is low (low rate of stimulatory action potentials), or when there are high serum levels of K+ , H+ , lactic acid, or nitric oxide around, smooth muscle relaxes and will mediate an increase the luminal diameter of arteries in a process called vasodilation. These are the same smooth muscles that mediate vascular spasm when vessels are damaged.
Elastic arteries Elastic arteries are the largest diameter arteries.
Examples include the aorta, brachiochephalic, common carotid, subclavian, vertebral, pulmonary, and common iliac arteries.
Elastic arteries stretch during ventricular systole and help push the blood along when the ventricles are relaxing (diastole) because their elastic fibers contract as blood pressure falls.
This storage of mechanical energy in elastic fibers results reservoir for the circulating blood that serves to propel blood on as the elastic fibers release their energy during elastic fiber recovery (contraction).

Muscular arteries are medium sized, and they have more smooth muscle in their tunica media with fewer elastic fibers.
Vasoconstriction and vasodilation mediated by the muscle component serves to regulate the flow of blood.
Arteriole are small, nearly microscopic arteries that deliver blood to capillaries.
At portions near to where they branch from arteries, arterioles have a muscular tunica media with a bit of elasin and a tunica externa with with a bit of elasin and a tunica externa with lots of collagen and elastic fibers. At segments near capillaries, arterioles are not much more than a ring of endothelium and a bit of smooth muscle cells .
Arterioles regulate blood flow into capillaries by controlling resistance, which refers to the opposition to blood flow.
When vessel diameter is small, resistance to flow caused by endothelial wall-blood cell associated friction is greater. By contracting or relaxing smooth muscle cells and causing vasoconstriction or vasodilation, arterioles increase or decrease resistance of blood flow into capillaries. These lumen diameter changes can significantly change blood pressure (contracted increases blood pressure).
Capillaries are microscopic arteries that connect venules and arterioles.
The flow of blood from arterioles though capillaries into venules is called