Blood pumped by the heart flows through a series of vessels known as arteries, arterioles, capillaries, venules, and veins before returning to the heart. Arteries transport blood away from the heart and branch into smaller vessels, forming arterioles. Arterioles distribute blood to capillary beds, the sites of exchange with the body tissues. Capillaries lead back to small vessels known as venules that flow into the larger veins and eventually back to the heart.
The arterial system is a relatively high-pressure system, so arteries have thick walls that appear round in cross section. The venous system is a lower-pressure system, containing veins that have larger lumens and thinner walls. They often appear flattened. Arteries, arterioles, venules, and veins are composed of three tunics known as the tunica intima, tunica media, and tunica externa. Capillaries have only a tunica intima layer. The tunica intima is a thin layer composed of a simple squamous epithelium known as endothelium and a small amount of connective tissue. The tunica media is a thicker area composed of variable amounts of smooth muscle and connective tissue. It is the thickest layer in all but the largest arteries. The tunica externa is primarily a layer of connective tissue, although in veins, it also contains some smooth muscle. Blood flow through vessels can be dramatically influenced by vasoconstriction and vasodilation in their walls.
arteriole
(also, resistance vessel) very small artery that leads to a capillary
arteriovenous anastomosis
short vessel connecting an arteriole directly to a venule and bypassing the capillary beds
artery
blood vessel that conducts blood away from the heart; may be a conducting or distributing vessel
capacitance
ability of a vein to distend and store blood
capacitance vessels
veins
capillary
smallest of blood vessels where physical exchange occurs between the blood and tissue cells surrounded by interstitial fluid
capillary bed
network of 10–100 capillaries connecting arterioles to venules
continuous capillary
most common type of capillary, found in virtually all tissues except epithelia and cartilage; contains very small gaps in the endothelial lining that permit exchange
elastic artery
(also, conducting artery) artery with abundant elastic fibers located closer to the heart, which maintains the pressure gradient and conducts blood to smaller branches
external elastic membrane
membrane composed of elastic fibers that separates the tunica media from the tunica externa; seen in larger arteries
fenestrated capillary
type of capillary with pores or fenestrations in the endothelium that allow for rapid passage of certain small materials
internal elastic membrane
membrane composed of elastic fibers that separates the tunica intima from the tunica media; seen in larger arteries
lumen
interior of a tubular structure such as a blood vessel or a portion of the alimentary canal through which blood, chyme, or other substances travel
metarteriole
short vessel arising from a terminal arteriole that branches to supply a capillary bed
microcirculation
blood flow through the capillaries
muscular artery
(also, distributing artery) artery with abundant smooth muscle in the tunica media that branches to distribute blood to the arteriole network
nervi vasorum
small nerve fibers found in arteries and veins that trigger contraction of the smooth muscle in their walls
perfusion
distribution of blood into the capillaries so the tissues can be supplied
precapillary sphincters
circular rings of smooth muscle that surround the entrance to a capillary and regulate blood flow into that capillary
sinusoid capillary
rarest type of capillary, which has extremely large intercellular gaps in the basement membrane in addition to clefts and fenestrations; found in areas such as the bone marrow and liver where passage of large molecules occurs
thoroughfare channel
continuation of the metarteriole that enables blood to bypass a capillary bed and flow directly into a venule, creating a vascular shunt
tunica externa
(also, tunica adventitia) outermost layer or tunic of a vessel (except capillaries)
tunica intima
(also, tunica interna) innermost lining or tunic of a vessel
tunica media
middle layer or tunic of a vessel (except capillaries)
vasa vasorum
small blood vessels located within the walls or tunics of larger vessels that supply nourishment to and remove wastes from the cells of the vessels
vascular shunt
continuation of the metarteriole and thoroughfare channel that allows blood to bypass the capillary beds to flow directly from the arterial to the venous circulation
vasoconstriction
constriction of the smooth muscle of a blood vessel, resulting in a decreased vascular diameter
vasodilation
relaxation of the smooth muscle in the wall of a blood vessel, resulting in an increased vascular diameter
vasomotion
irregular, pulsating flow of blood through capillaries and related structures
vein
blood vessel that conducts blood toward the heart
venous reserve
volume of blood contained within systemic veins in the integument, bone marrow, and liver that can be returned to the heart for circulation, if needed
venule
small vessel leading from the capillaries to veins
1. The endothelium is found in the ________.
A) tunica intima
B) tunica media
C) tunica externa
D) lumen
A
2. Nervi vasorum control ________.
A) vasoconstriction
B) vasodilation
C) capillary permeability
D) both vasoconstriction and vasodilation
D
3. Closer to the heart, arteries would be expected to have a higher percentage of ________.
A) endothelium
B) smooth muscle fibers
C) elastic fibers
D) collagenous fibers
C
4. Which of the following best describes veins?
A) thick walled, small lumens, low pressure, lack valves
B) thin walled, large lumens, low pressure, have valves
C) thin walled, small lumens, high pressure, have valves
D) thick walled, large lumens, high pressure, lack valves
B
5. An especially leaky type of capillary found in the liver and certain other tissues is called a ________.
A) capillary bed
B) fenestrated capillary
C) sinusoid capillary
D) metarteriole
C
1. Arterioles are often referred to as resistance vessels. Why?
Arterioles receive blood from arteries, which are vessels with a much larger lumen. As their own lumen averages just 30 micrometers or less, arterioles are critical in slowing down—or resisting—blood flow. The arterioles can also constrict or dilate, which varies their resistance, to help distribute blood flow to the tissues.
2. Cocaine use causes vasoconstriction. Is this likely to increase or decrease blood pressure, and why?
Vasoconstriction causes the lumens of blood vessels to narrow. This increases the pressure of the blood flowing within the vessel.
3. A blood vessel with a few smooth muscle fibers and connective tissue, and only a very thin tunica externa conducts blood toward the heart. What type of vessel is this?
This is a venule.