- Contrast the structure and function of arteries, arterioles, capillaries, venules, and veins
- Outline the vessels through which the blood moves in its passage from the heart to the capillaries and back
- Distinguish between pressure reservoirs and blood reservoirs
The five main types of blood vessels are:
Carry blood away from the heart to other organs.
Large, elastic arteries leave the heart and divide into medium-sized, muscular arteries that branch out into the various regions of the body
Medium-sized arteries then divide into small arteries, which in turn divide into still smaller arteries called arterioles (ar-TE¯R-e¯-o¯ ls)
As the arterioles enter a tissue, they branch into numerous tiny vessels called capillaries (hairlike). The thin walls of capillaries allow the exchange of substances between the blood and body tissues
Groups of capillaries within a tissue reunite to form small veins called venules (VEN-u¯ ls)
These in turn merge to form progressively larger blood vessels called veins.
Veins are the blood vessels that convey blood from the tissues back to the heart
Angiogenesis and Disease
Angiogenesis refers to the growth of new blood vessels.
It is an important process in embryonic and fetal development, and in postnatal life serves important functions such as wound healing, formation of a new uterine lining after menstruation, formation of the corpus luteum after ovulation, and development of blood vessels around obstructed arteries in the coronary circulation.
Several proteins (peptides) are known to promote and inhibit angiogenesis.
Clinically angiogenesis is important because cells of a malignant tumor secrete proteins called tumor angiogenesis factors (TAFs) that stimulate blood vessel growth to provide nourishment for the tumor cells.
Scientists are seeking chemicals that would inhibit angiogenesis and thus stop the growth of tumors. In diabetic retinopathy, angiogenesis may be important in the development of blood vessels that actually cause blindness, so finding inhibitors of angiogenesis may also prevent the blindness associated with diabetes.
Basic Structure of a Blood Vessel
The wall of a blood vessel consists of three layers, or tunics, of different tissues:
- an epithelial inner lining
- a middle layer consisting of smooth muscle and elastic connective tissue
- a connective tissue outer covering.
The three structural layers of a generalized blood vessel from innermost to outermost are the:
- Tunica interna (intima)
- Tunica media
- Tunica externa (adventitia)
Modifications of this basic design account for the five types of blood vessels and the structural and functional differences among the various vessel types. Structural variations correlate to the differences in function that occur throughout the cardiovascular system.
Tunica Interna (Intima)
The tunica interna (intima) forms the inner lining of a blood vessel and is in direct contact with the blood as it flows through the lumen, or interior opening, of the vessel.
Although this layer has multiple parts, these tissue components contribute minimally to the thickness of the vessel wall.
Its innermost layer is called:
which is continuous with the endocardial lining of the heart. The endothelium is a thin layer of flattened cells that lines the inner surface of the entire cardiovascular system (heart and blood vessels).
Until recently, endothelial cells were regarded as little more than a passive barrier between the blood and the remainder of the vessel wall.
It is now known that endothelial cells are active participants in a variety of vessel-related activities, including:
- physical influences on blood flow
- secretion of locally acting chemical mediators that influence the contractile state of the vessel’s overlying smooth muscle
- and assistance with capillary permeability.
In addition, their smooth luminal surface facilitates efficient blood flow by reducing surface friction.
The second component of the tunica interna is a:
deep to the endothelium. It provides a physical support base for the epithelial layer. Its framework of collagen fibers affords the basement membrane significant tensile strength, yet its properties also provide resilience for stretching and recoil.
The basement membrane anchors the endothelium to the underlying connective tissue while also regulating molecular movement. It appears to play an important role in guiding cell movements during tissue repair of blood vessel walls.
The outermost part of the tunica interna, which forms the boundary between the tunica interna and tunica media is the:
Internal elastic lamina (lamina thin plate)
The internal elastic lamina is a thin sheet of elastic fibers with a variable number of window-like openings that give it the look of Swiss cheese.
These openings facilitate diffusion of materials through the tunica interna to the thicker tunica media
The tunica media (media middle) is a muscular and connective tissue layer that displays the greatest variation among the different vessel types.
In most vessels, it is a relatively thick layer comprising mainly smooth muscle cells and substantial amounts of elastic fibers.
The primary role of the smooth muscle cells, which extend circularly around the lumen like a ring encircles your finger, is to regulate the diameter of the lumen.
An increase in sympathetic stimulation typically stimulates the smooth muscle to contract, squeezing the vessel wall and narrowing the lumen. Such a decrease in the diameter of the lumen of a blood vessel is called vasoconstriction.
In contrast, when sympathetic stimulation decreases, or in the presence of certain chemicals (such as nitric oxide, H, and lactic acid) or in response to blood pressure, smooth muscle fibers relax. The resulting increase in lumen diameter is called vasodilation.
The rate of blood flow through different parts of the body is regulated by the extent of smooth muscle contraction in the walls of particular vessels. Furthermore, the extent of smooth muscle contraction in particular vessel types is crucial in the regulation of blood pressure.
In addition to regulating blood flow and blood pressure, smooth muscle contracts when an artery or arteriole is damaged (vascular spasm) to help limit loss of blood through the injured vessel if it is small.
Smooth muscle cells also help produce the elastic fibers within the tunica media that allow the vessels to stretch and recoil under the applied pressure of the blood.
The tunica media is the most variable of the tunics.
“The structural differences in this layer account for the many variations in function among the different vessel types”
Separating the tunica media from the tunica externa is a network of elastic fibers, the external elastic lamina, which is part of the tunica media.
The outer covering of a blood vessel, the tunica externa, consists of elastic and collagen fibers.
The tunica externa contains numerous nerves and, especially in larger vessels, tiny blood vessels that supply the tissue of the vessel wall. These small vessels that supply blood to the tissues of the vessel are called vasa vasorum (VA¯ -sa va-SO¯ -rum;), or vessels to the vessels.
They are easily seen on large vessels such as the aorta. In addition to the important role of supplying the vessel wall with nerves and self-vessels, the tunica externa helps anchor the vessels to surrounding tissues.
Because arteries were found empty at death, in ancient times they were thought to contain only air.
The wall of an artery has the three layers of a typical blood vessel, but has a thick muscular-to-elastic tunica media.
Due to their plentiful elastic fibers, arteries normally have high compliance, which means that their walls stretch easily or expand without tearing in response to a small increase in pressure
Elastic arteries are the largest arteries in the body, ranging from the garden hose–sized aorta and pulmonary trunk to the finger sized branches of the aorta.
They have the largest diameter among arteries, but their vessel walls (approximately one-tenth of the vessel’s total diameter) are relatively thin compared with the overall size of the vessel.
These vessels are characterized by:
- Well-defined internal and external elastic laminae
- A thick tunica media that is dominated by elastic fibers, called the elastic lamellae (la-MEL-e¯)
Elastic arteries include the two major trunks that exit the heart (the aorta and the pulmonary trunk), along with the aorta’s major initial branches, such as the brachiocephalic, subclavian, common carotid, and common iliac arteries.
“Elastic arteries perform an important function”
They help propel blood onward while the ventricles are relaxing. As blood is ejected from the heart into elastic arteries, their walls stretch, easily accommodating the surge of blood. As they stretch, the elastic fibers momentarily store mechanical energy, functioning as a pressure reservoir.
Then, the elastic fibers recoil and convert stored (potential) energy in the vessel into kinetic energy of the blood. Thus, blood continues to move through the arteries even while the ventricles are relaxed.
Because they conduct blood from the heart to medium-sized, more muscular arteries, elastic arteries also are called conducting arteries.
Medium-sized arteries are called muscular arteries because their tunica media contains more smooth muscle and fewer elastic fibers than elastic arteries.
The large amount of smooth muscle, approximately three-quarters of the total mass, makes the walls of muscular arteries relatively thick. Thus, muscular arteries are capable of greater vasoconstriction and vasodilation to adjust the rate of blood flow.
Muscular arteries have a well-defined internal elastic lamina but a thin external elastic lamina. These two elastic laminae form the inner and outer boundaries of the muscular tunica media.
In large arteries, the thick tunica media can have as many as 40 layers of circumferentially arranged smooth muscle cells; in smaller arteries there are as few as three layers.
Muscular arteries span a range of sizes from the pencil-sized femoral and axillary arteries to string-sized arteries that enter organs, measuring as little as 0.5 mm in diameter. Compared to elastic arteries, the vessel wall of muscular arteries comprises a larger percentage (25%) of the total vessel diameter.
Because the muscular arteries continue to branch and ultimately distribute blood to each of the various organs, they are called distributing arteries. Examples include the brachial artery in the arm and radial artery in the forearm.
The tunica externa is often thicker than the tunica media in muscular arteries. This outer layer contains fibroblasts, collagen fibers, and elastic fibers all oriented longitudinally.
The loose structure of this layer permits changes in the diameter of the vessel to take place but also prevents shortening or retraction of the vessel when it is cut.
Because of the reduced amount of elastic tissue in the walls of muscular arteries, these vessels do not have the ability to recoil and help propel the blood like the elastic arteries. Instead, the thick, muscular tunica media is primarily responsible for the functions of the muscular arteries.
The ability of the muscle to contract and maintain a state of partial contraction is referred to as vascular tone. Vascular tone stiffens the vessel wall and is important in maintaining vessel pressure and efficient blood flow.
Most tissues of the body receive blood from more than one artery.
The union of the branches of two or more arteries supplying the same body region is called an anastomosis (a-nas-to¯-MO¯ -sis)
Anastomoses between arteries provide alternative routes for blood to reach a tissue or organ. If blood flow stops for a short time when normal movements compress a vessel, or if a vessel is blocked by disease, injury, or surgery, then circulation to a part of the body is not necessarily stopped.
The alternative route of blood flow to a body part through an anastomosis is known as collateral circulation.
Anastomoses may also occur between veins and between arterioles and venules.
Arteries that do not anastomose are known as end arteries. Obstruction of an end artery interrupts the blood supply to a whole segment of an organ, producing necrosis (death) of that segment.
Alternative blood routes may also be provided by nonanastomosing vessels that supply the same region of the body.
Literally meaning small arteries, arterioles are abundant microscopic vessels that regulate the flow of blood into the capillary networks of the body’s tissues).
The approximately 400 million arterioles have diameters that range in size from 15 m to 300 m. The wall thickness of arterioles is one half of the total vessel diameter.
Arterioles have a thin tunica interna with a thin, fenestrated (with small pores) internal elastic lamina that disappears at the terminal end. The tunica media consists of one to two layers of smooth muscle cells having a circular orientation in the vessel wall.
The terminal end of the arteriole, the region called the metarteriole tapers toward the capillary junction.
At the metarteriole–capillary junction, the distal most muscle cell forms the precapillary sphincter, which monitors the blood flow into the capillary; the other muscle cells in the arteriole regulate the resistance (opposition) to blood flow.
The tunica externa of the arteriole consists of areolar connective tissue containing abundant unmyelinated sympathetic nerves. This sympathetic nerve supply, along with the actions of local chemical mediators, can alter the diameter of arterioles and thus vary the rate of blood flow and resistance through these vessels.
Arterioles play a key role in regulating blood flow from arteries into capillaries by regulating resistance, the opposition to blood flow.
“Because of this they are known as resistance vessels“
In a blood vessel, resistance is due mainly to friction between blood and the inner walls of blood vessels. When the blood vessel diameter is smaller, the friction is greater, so there is more resistance.
Contraction of the smooth muscle of an arteriole causes vasoconstriction, which increases resistance even more and decreases blood flow into capillaries supplied by that arteriole. By contrast, relaxation of the smooth muscle of an arteriole causes vasodilation, which decreases resistance and increases blood flow into capillaries.
A change in arteriole diameter can also affect blood pressure: Vasoconstriction of arterioles increases blood pressure, and vasodilation of arterioles decreases blood pressure.
Capillaries (little hair), the smallest of blood vessels, have diameters of 5–10 m, and form the U-turns that connect the arterial outflow to the venous return. Since red blood cells have a diameter of 8 m, they must often fold on themselves in order to pass single file through the lumens of these vessels.
Capillaries form an extensive network, approximately 20 billion in number, of short (hundreds of micrometers in length), branched, interconnecting vessels that course among the individual cells of the body.
This network forms an enormous surface area to make contact with the body’s cells. The flow of blood from a metarteriole through capillaries and into a postcapillary venule (venule that receives blood from a capillary) is called the microcirculation of the body.
The primary function of capillaries is the exchange of substances between the blood and interstitial fluid. Because of this, these thin-walled vessels are referred to as exchange vessels.
Capillaries are found near almost every cell in the body, but their number varies with the metabolic activity of the tissue they serve.
Body tissues with high metabolic requirements, such as muscles, the brain, the liver, the kidneys, and the nervous system, use more O2 and nutrients and thus have extensive capillary networks.
Tissues with lower metabolic requirements, such as tendons and ligaments, contain fewer capillaries.
Capillaries are absent in a few tissues, such as all covering and lining epithelia, the cornea and lens of the eye, and cartilage.
The structure of capillaries is well suited to their function as exchange vessels because they lack both a tunica media and a tunica externa.
Because capillary walls are composed of only a single layer of endothelial cells and a basement membrane, a substance in the blood must pass through just one cell layer to reach the interstitial fluid and tissue cells.
Exchange of materials occurs only through the walls of capillaries and the beginning of venules; the walls of arteries, arterioles, most venules, and veins present too thick a barrier.
Capillaries form extensive branching networks that increase the surface area available for rapid exchange of materials. In most tissues, blood flows through only a small part of the capillary network when metabolic needs are low. However, when a tissue is active, such as contracting muscle, the entire capillary network fills with blood.
Throughout the body, capillaries function as part of a capillary bed, a network of 10–100 capillaries that arises from a single metarteriole.
In most parts of the body, blood can flow through a capillary network from an arteriole into a venule as follows:
In this route, blood flows from an arteriole into capillaries and then into venules (postcapillary venules).
At the junctions between the metarteriole and the capillaries are rings of smooth muscle fibers called precapillary sphincters that control the flow of blood through the capillaries.
- When the precapillary sphincters are relaxed (open), blood flows into the capillaries
- When precapillary sphincters contract (close or partially close), blood flow through the capillaries ceases or decreases
Typically, blood flows intermittently through capillaries due to alternating contraction and relaxation of the smooth muscle of metarterioles and the precapillary sphincters. This intermittent contraction and relaxation, which may occur 5 to 10 times per minute, is called vasomotion (va¯-so¯-MO¯ -shun).
In part, vasomotion is due to chemicals released by the endothelial cells; nitric oxide is one example. At any given time, blood flows through only about 25% of the capillaries.
2. Thoroughfare channel.
The proximal end of a metarteriole is surrounded by scattered smooth muscle fibers whose contraction and relaxation help regulate blood flow.
The distal end of the vessel has no smooth muscle; it resembles a capillary and is called a thoroughfare channel. Such a channel provides a direct route for blood from an arteriole to a venule, thus bypassing capillaries.
The body contains three different types of capillaries:
- Continuous capillaries
- Fenestrated capillaries
Most capillaries are continuous capillaries, in which the plasma membranes of endothelial cells form a continuous tube that is interrupted only by intercellular clefts, gaps between neighboring endothelial cells. Continuous capillaries are found in the central nervous system, lungs, skin, muscle tissue, and the skin.
Other capillaries of the body are fenestrated capillaries. The plasma membranes of the endothelial cells in these capillaries have many fenestrations (small pores) ranging from 70 to 100 nm in diameter . Fenestrated capillaries are found in the kidneys, villi of the small intestine, choroid plexuses of the ventricles in the brain, ciliary processes of the eyes, and most endocrine glands.
Sinusoids are wider and more winding than other capillaries. Their endothelial cells may have unusually large fenestrations. In addition to having an incomplete or absent basement membrane, sinusoids have very large intercellular clefts that allow proteins and in some cases even blood cells to pass from a tissue into the bloodstream.
For example, newly formed blood cells enter the bloodstream through the sinusoids of red bone marrow. In addition, sinusoids contain specialized lining cells that are adapted to the function of the tissue.
Sinusoids in the liver, for example, contain phagocytic cells that remove bacteria and other debris from the blood. The spleen, anterior pituitary, and parathyroid and adrenal glands also have sinusoids.
Usually blood passes from the heart and then in sequence through:
and then back to the heart.
In some parts of the body, however, blood passes from one capillary network into another through a vein called a portal vein. Such a circulation of blood is called a portal system.
The name of the portal system gives the name of the second capillary location. For example, there are portal systems associated with the liver(hepatic portal circulation) and the pituitary gland (hypophyseal portal system).
Unlike their thick-walled arterial counterparts, venules (little vein) and veins have thin walls that do not readily maintain their shape. Venules drain the capillary blood and begin the return flow of blood back toward the heart.
Venules that initially receive blood from capillaries are called postcapillary venules.
They are the smallest venules, measuring 10 m to 50 m in diameter, and have loosely organized intercellular junctions (the weakest endothelial contacts encountered along the entire vascular tree) and thus are very porous.
They function as significant sites of exchange of nutrients and wastes and white blood cell emigration, and for this reason form part of the microcirculatory exchange unit along with the capillaries.
As the postcapillary venules move away from capillaries, they acquire one or two layers of circularly arranged smooth muscle cells. These muscular venules (50 m to 200 m) have thicker walls across which exchanges with the interstitial fluid can no longer occur.
The thin walls of the postcapillary and muscular venules are the most distensible elements of the vascular system;
this allows them to expand and serve as excellent reservoirs for accumulating large volumes of blood.
Blood volume increases of 360% have been measured in the postcapillary and muscular venules.
While veins do show structural changes as they increase in size from small to medium to large, the structural changes are not as distinct as they are in arteries.
Veins, in general, have very thin walls relative to their total diameter (average thickness is less than one-tenth of the vessel diameter). They range in size from 0.5 mm in diameter for small veins to 3 cm in the large superior and interior venae cavae entering the heart.
Although veins are composed of essentially the same three layers as arteries, the relative thicknesses of the layers are different.
- The tunica interna of veins is thinner than that of arteries
- The tunica media of veins is much thinner than in arteries with relatively little smooth muscle and elastic fibers
- The tunica externa of veins is the thickest layer and consists of collagen and elastic fibers.
Veins lack the internal or external elastic laminae found in arteries. They are distensible enough to adapt to variations in the volume and pressure of blood passing through them, but are not designed to withstand high pressure.
The lumen of a vein is larger than that of a comparable artery, and veins often appear collapsed (flattened) when sectioned.
The pumping action of the heart is a major factor in moving venous blood back to the heart. The contraction of skeletal muscles in the lower limbs also helps boost venous return to the heart.
The average blood pressure in veins is considerably lower than in arteries. The difference in pressure can be noticed when blood flows from a cut vessel. Blood leaves a cut vein in an even, slow flow but spurts rapidly from a cut artery.
Most of the structural differences between arteries and veins reflect this pressure difference.
For example, the walls of veins are not as strong as those of arteries.
Many veins, especially those in the limbs, also contain valves, thin folds of tunica interna that form flap-like cusps.
The valve cusps project into the lumen, pointing toward the heart. The low blood pressure in veins allows blood returning to the heart to slow and even back up; the valves aid in venous return by preventing the backflow of blood.
A vascular (venous) sinus is a vein with a thin endothelial wall that has no smooth muscle to alter its diameter. In a vascular sinus, the surrounding dense connective tissue replaces the tunica media and tunica externa in providing support.
For example, dural venous sinuses, which are supported by the dura mater, convey deoxygenated blood from the brain to the heart. Another example of a vascular sinus is the coronary sinus of the heart.
While veins follow paths similar to those of their arterial counterparts, they differ from arteries in a number of ways, aside from the structures of their walls.
First, veins are more numerous than arteries for several reasons.
Some veins are paired and accompany medium- to small-sized muscular arteries. These double sets of veins escort the arteries and connect with one another via venous channels called anastomotic veins (a-nas-to¯-MOT-ik).
The anastomotic veins cross the accompanying artery to form ladder-like rungs between the paired veins. The greatest number of paired veins occurs within the limbs.
The subcutaneous layer deep to the skin is another source of veins. These veins, called superficial veins, course through the subcutaneous layer unaccompanied by parallel arteries. Along their course, the superficial veins form small connections (anastomoses) with the deep veins that travel between the skeletal muscles.
These connections allow communication between the deep and superficial flow of blood.
The amount of blood flow through superficial veins varies from location to location within the body. In the upper limb, the superficial veins are much larger than the deep veins and serve as the major pathways from the capillaries of the upper limb back to the heart.
In the lower limb, the opposite is true; the deep veins serve as the principal return pathways. In fact, one-way valves in small anastomosing vessels allow blood to pass from the superficial to the deep veins, but prevent the blood from passing in the reverse direction.
This design has important implications in the development of varicose veins.
In some individuals the superficial veins can be seen as blue coloured tubes passing under the skin. While the venous blood is a deep dark red, the veins appear blue because their thin walls and the tissues of the skin absorb the red-light wavelengths, allowing the blue light to pass through the surface to our eyes where we see them as blue.
Blood volume at rest
- Systemic veins and venules holds about 64%
- Systemic arteries and arterioles hold about 13%
- Systemic capillaries hold about 7%
- Pulmonary blood vessels hold about 9%
- Heart holds about 7%
Because systemic veins and venules contain a large percentage of the blood volume, they function as blood reservoirs from which blood can be diverted quickly if the need arises.
For example, during increased muscular activity, the cardiovascular center in the brain stem sends a larger number of sympathetic impulses to veins.
The result is venoconstriction, constriction of veins, which reduces the volume of blood in reservoirs and allows a greater blood volume to flow to skeletal muscles, where it is needed most.
A similar mechanism operates in cases of hemorrhage, when blood volume and pressure decrease; in this case, venoconstriction helps counteract the drop in blood pressure. Among the principal blood reservoirs are the veins of the abdominal organs (especially the liver and spleen) and the veins of the skin.
Leaky venous valves can cause veins to become dilated and twisted in appearance, a condition called varicose veins or varices.
The condition may occur in the veins of almost any body part, but it is most common in the esophagus, anal canal, and superficial veins of the lower limbs. Those in the lower limbs can range from cosmetic problems to serious medical conditions.
The valvular defect may be congenital or may result from mechanical stress (prolonged standing or pregnancy) or aging.
The leaking venous valves allow the backflow of blood from the deep veins to the less efficient superficial veins, where the blood pools. This creates pressure that distends the vein and allows fluid to leak into surrounding tissue. As a result, the affected vein and the tissue around it may become inflamed and painfully tender.
Veins close to the surface of the legs, especially the saphenous vein, are highly susceptible to varicosities; deeper veins are not as vulnerable because surrounding skeletal muscles prevent their walls from stretching excessively.
Varicose veins in the anal canal are referred to as hemorrhoids. Esophageal varices result from dilated veins in the walls of the lower part of the esophagus and sometimes the upper part of the stomach. Bleeding esophageal varices are life-threatening and are usually a result of chronic liver disease.
Several treatment options are available for varicose veins in the lower limbs. Elastic stockings (support hose) may be used for individuals with mild symptoms or for whom other options are not recommended.
Sclerotherapy involves injection of a solution into varicose veins that damages the tunica interna by producing a harmless superficial thrombophlebitis (inflammation involving a blood clot). Healing of the damaged part leads to scar formation that occludes the vein.
Radiofrequency endovenous occlusion) involves the application of radiofrequency energy to heat up and close off varicose veins.
Laser occlusion uses laser therapy to shut down veins.
In a surgical procedure called stripping, veins may be removed. In this procedure, a flexible wire is threaded through the vein and then pulled out to strip (remove) it from the body.