The pulmonary circuit moves blood from the right side of the heart to the lungs and back to the heart. The systemic circuit moves blood from the left side of the heart to the head and body and returns it to the right side of the heart to repeat the cycle. The arrows indicate the direction of blood flow, and the colors show the relative levels of oxygen concentration.
Different types of blood vessels vary slightly in their structures, but they share the same general features. Arteries and arterioles have thicker walls than veins and venules because they are closer to the heart and receive blood that is surging at a far greater pressure Figure 2. Each type of vessel has a lumen —a hollow passageway through which blood flows. Arteries have smaller lumens than veins, a characteristic that helps to maintain the pressure of blood moving through the system.
Together, their thicker walls and smaller diameters give arterial lumens a more rounded appearance in cross section than the lumens of veins. Figure 2. By the time blood has passed through capillaries and entered venules, the pressure initially exerted upon it by heart contractions has diminished.
In other words, in comparison to arteries, venules and veins withstand a much lower pressure from the blood that flows through them. Their walls are considerably thinner and their lumens are correspondingly larger in diameter, allowing more blood to flow with less vessel resistance.
In addition, many veins of the body, particularly those of the limbs, contain valves that assist the unidirectional flow of blood toward the heart. This is critical because blood flow becomes sluggish in the extremities, as a result of the lower pressure and the effects of gravity. The walls of arteries and veins are largely composed of living cells and their products including collagenous and elastic fibers ; the cells require nourishment and produce waste.
Further, the walls of the larger vessels are too thick for nutrients to diffuse through to all of the cells. Since the pressure within arteries is relatively high, the vasa vasorum must function in the outer layers of the vessel or the pressure exerted by the blood passing through the vessel would collapse it, preventing any exchange from occurring. The lower pressure within veins allows the vasa vasorum to be located closer to the lumen.
The restriction of the vasa vasorum to the outer layers of arteries is thought to be one reason that arterial diseases are more common than venous diseases, since its location makes it more difficult to nourish the cells of the arteries and remove waste products.
There are also minute nerves within the walls of both types of vessels that control the contraction and dilation of smooth muscle. These minute nerves are known as the nervi vasorum. Both arteries and veins have the same three distinct tissue layers, called tunics from the Latin term tunica , for the garments first worn by ancient Romans; the term tunic is also used for some modern garments.
From the most interior layer to the outer, these tunics are the tunica intima, the tunica media, and the tunica externa. Table 1 compares and contrasts the tunics of the arteries and veins. The tunica intima also called the tunica interna is composed of epithelial and connective tissue layers. Lining the tunica intima is the specialized simple squamous epithelium called the endothelium, which is continuous throughout the entire vascular system, including the lining of the chambers of the heart.
Damage to this endothelial lining and exposure of blood to the collagenous fibers beneath is one of the primary causes of clot formation.
Until recently, the endothelium was viewed simply as the boundary between the blood in the lumen and the walls of the vessels. Recent studies, however, have shown that it is physiologically critical to such activities as helping to regulate capillary exchange and altering blood flow.
The endothelium releases local chemicals called endothelins that can constrict the smooth muscle within the walls of the vessel to increase blood pressure. Uncompensated overproduction of endothelins may contribute to hypertension high blood pressure and cardiovascular disease. Next to the endothelium is the basement membrane, or basal lamina, that effectively binds the endothelium to the connective tissue. The basement membrane provides strength while maintaining flexibility, and it is permeable, allowing materials to pass through it.
The thin outer layer of the tunica intima contains a small amount of areolar connective tissue that consists primarily of elastic fibers to provide the vessel with additional flexibility; it also contains some collagenous fibers to provide additional strength.
In larger arteries, there is also a thick, distinct layer of elastic fibers known as the internal elastic membrane also called the internal elastic lamina at the boundary with the tunica media. Like the other components of the tunica intima, the internal elastic membrane provides structure while allowing the vessel to stretch.
It is permeated with small openings that allow exchange of materials between the tunics. The internal elastic membrane is not apparent in veins. In addition, many veins, particularly in the lower limbs, contain valves formed by sections of thickened endothelium that are reinforced with connective tissue, extending into the lumen. Under the microscope, the lumen and the entire tunica intima of a vein will appear smooth, whereas those of an artery will normally appear wavy because of the partial constriction of the smooth muscle in the tunica media, the next layer of blood vessel walls.
The tunica media is the substantial middle layer of the vessel wall see Figure 2. It is generally the thickest layer in arteries, and it is much thicker in arteries than it is in veins. The tunica media consists of layers of smooth muscle supported by connective tissue that is primarily made up of elastic fibers, most of which are arranged in circular sheets. Toward the outer portion of the tunic, there are also layers of longitudinal muscle. Contraction and relaxation of the circular muscles decrease and increase the diameter of the vessel lumen, respectively.
Specifically in arteries, vasoconstriction decreases blood flow as the smooth muscle in the walls of the tunica media contracts, making the lumen narrower and increasing blood pressure.
Similarly, vasodilation increases blood flow as the smooth muscle relaxes, allowing the lumen to widen and blood pressure to drop. These are generally all sympathetic fibers, although some trigger vasodilation and others induce vasoconstriction, depending upon the nature of the neurotransmitter and receptors located on the target cell. Parasympathetic stimulation does trigger vasodilation as well as erection during sexual arousal in the external genitalia of both sexes.
Nervous control over vessels tends to be more generalized than the specific targeting of individual blood vessels. Local controls, discussed later, account for this phenomenon.
Seek additional content for more information on these dynamic aspects of the autonomic nervous system. Hormones and local chemicals also control blood vessels.
Together, these neural and chemical mechanisms reduce or increase blood flow in response to changing body conditions, from exercise to hydration. Regulation of both blood flow and blood pressure is discussed in detail later in this chapter.
The smooth muscle layers of the tunica media are supported by a framework of collagenous fibers that also binds the tunica media to the inner and outer tunics. Along with the collagenous fibers are large numbers of elastic fibers that appear as wavy lines in prepared slides. Separating the tunica media from the outer tunica externa in larger arteries is the external elastic membrane also called the external elastic lamina , which also appears wavy in slides.
This structure is not usually seen in smaller arteries, nor is it seen in veins. The outer tunic, the tunica externa also called the tunica adventitia , is a substantial sheath of connective tissue composed primarily of collagenous fibers. Arteries have to carry oxygenated blood away from the heart to the tissues at high pressure. Small lumen relative to the large, muscular vessel ensure this pressure is maintained as the blood is transported around the body.
Veins carry unoxygenated blood towards the heart, away from tissues at low pressure so the lumen is large. Blood moves more slower and often against gravity so valves and a larger lumen ensure it is still transported efficiently. But unlike the arteries, the venous pressure is low. Veins are thin-walled and are less elastic. This feature permits the veins to hold a very high percentage of the blood in circulation. The venous system can accommodate a large volume of blood at relatively low pressures, a feature termed high capacitance.
At any point in time, nearly three-fourths of the circulating blood volume is contained in the venous system. One can also find one-way valves inside veins that allow for blood flow, toward the heart, in a forward direction. Muscle contractions aid the blood flow in the leg veins. The forward blood flow from the lower extremities to the heart is also influenced by respiratory changes that affect pressure gradients in the abdomen and chest cavity. This pressure differential is highest during deep inspiration, but a small pressure differential is observable during the entire respiratory cycle.
Excerpt The peripheral vascular system PVS includes all the blood vessels that exist outside the heart. The peripheral vascular system is classified as follows: The aorta and its branches: The arterioles The capillaries The venules and veins returning blood to the heart The function and structure of each segment of the peripheral vascular system vary depending on the organ it supplies.
Aside from capillaries, blood vessels are all made of three layers: The adventitia or outer layer which provides structural support and shape to the vessel The tunica media or a middle layer composed of elastic and muscular tissue which regulates the internal diameter of the vessel The tunic intima or an inner layer consisting of an endothelial lining which provides a frictionless pathway for the movement of blood Within each layer, the amount of muscle and collagen fibrils varies, depending on the size and location of the vessel.
Arteries Arteries play a major role in nourishing organs with blood and nutrients. Simultaneously, valves inferior to the contracting muscles close; thus, blood should not seep back downward toward the feet. Military recruits are trained to flex their legs slightly while standing at attention for prolonged periods.
Failure to do so may allow blood to pool in the lower limbs rather than returning to the heart. Consequently, the brain will not receive enough oxygenated blood, and the individual may lose consciousness. The respiratory pump aids blood flow through the veins of the thorax and abdomen. During inhalation, the volume of the thorax increases, largely through the contraction of the diaphragm, which moves downward and compresses the abdominal cavity.
The elevation of the chest caused by the contraction of the external intercostal muscles also contributes to the increased volume of the thorax.
The volume increase causes air pressure within the thorax to decrease, allowing us to inhale. Additionally, as air pressure within the thorax drops, blood pressure in the thoracic veins also decreases, falling below the pressure in the abdominal veins. This causes blood to flow along its pressure gradient from veins outside the thorax, where pressure is higher, into the thoracic region, where pressure is now lower.
This in turn promotes the return of blood from the thoracic veins to the atria. During exhalation, when air pressure increases within the thoracic cavity, pressure in the thoracic veins increases, speeding blood flow into the heart while valves in the veins prevent blood from flowing backward from the thoracic and abdominal veins. Cardiovascular System: Edema and Varicose Veins.
Despite the presence of valves and the contributions of other anatomical and physiological adaptations we will cover shortly, over the course of a day, some blood will inevitably pool, especially in the lower limbs, due to the pull of gravity. Any blood that accumulates in a vein will increase the pressure within it, which can then be reflected back into the smaller veins, venules, and eventually even the capillaries.
Increased pressure will promote the flow of fluids out of the capillaries and into the interstitial fluid. The presence of excess tissue fluid around the cells leads to a condition called edema. Most people experience a daily accumulation of tissue fluid, especially if they spend much of their work life on their feet like most health professionals. However, clinical edema goes beyond normal swelling and requires medical treatment.
Edema has many potential causes, including hypertension and heart failure, severe protein deficiency, renal failure, and many others. In order to treat edema, which is a sign rather than a discrete disorder, the underlying cause must be diagnosed and alleviated.
This disorder arises when defective valves allow blood to accumulate within the veins, causing them to distend, twist, and become visible on the surface of the integument. Varicose veins may occur in both sexes, but are more common in women and are often related to pregnancy. More than simple cosmetic blemishes, varicose veins are often painful and sometimes itchy or throbbing. Without treatment, they tend to grow worse over time. The use of support hose, as well as elevating the feet and legs whenever possible, may be helpful in alleviating this condition.
Laser surgery and interventional radiologic procedures can reduce the size and severity of varicose veins. Severe cases may require conventional surgery to remove the damaged vessels. As there are typically redundant circulation patterns, that is, anastomoses, for the smaller and more superficial veins, removal does not typically impair the circulation.
There is evidence that patients with varicose veins suffer a greater risk of developing a thrombus or clot. Their ability to hold this much blood is due to their high capacitance , that is, their capacity to distend expand readily to store a high volume of blood, even at a low pressure. The large lumens and relatively thin walls of veins make them far more distensible than arteries; thus, they are said to be capacitance vessels.
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. Arteries, arterioles, venules, and veins are composed of three tunics known as the tunica intima, tunica media, and tunica externa.
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 increased or decreased by vasoconstriction and vasodilation. 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. Capillaries have only a tunica intima layer. A: 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. 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?
Outside of work, she engages in no physical activity. She confesses that, because of her weight, she finds even walking uncomfortable. People who stand upright all day and are inactive overall have very little skeletal muscle activity in the legs. Pooling of blood in the legs and feet is common. Venous return to the heart is reduced, a condition that in turn reduces cardiac output and therefore oxygenation of tissues throughout the body.
By the end of this section, you will be able to: Compare and contrast the three tunics that make up the walls of most blood vessels Distinguish between elastic arteries, muscular arteries, and arterioles on the basis of structure, location, and function Compare and contrast the three types of capillaries on the basis of structure, location, and function Describe the basic structure of a capillary bed, from the supplying metarteriole to the venule into which it drains Compare and contrast veins, venules, and venous sinuses on the basis of structure, location, and function Discuss several factors affecting blood flow in the venous system.
Structural Characteristics of Vessels Different types of blood vessels vary slightly in their structures, but they share the same general features. Note the relative differences in wall thickness and compare the round lumen of the artery to the flattened lumen of the vein. Tunica Externa The outer tunic, the tunica externa also called the tunica adventitia , is a substantial sheath of dense irregular connective tissue composed primarily of collagen fibers.
Arteries An artery is a blood vessel that conducts blood away from the heart. A comparison of the walls of an elastic artery, a muscular artery, and an arteriole is shown. The scale of each vessel's wall has been adjusted for comparison in this illustration. The tunica media is thickest in muscular arteries in proportion to the diameter of the lumen. Tunica externa is very thin and incomplete in arterioles. Arterioles An arteriole is a very small artery that leads to a capillary.
Cardiovascular System: Arteriosclerosis Compliance allows an artery to expand when blood is pumped through it from the heart, and then to recoil after the surge has passed. Capillaries A capillary is a microscopic channel that supplies blood to the tissues, through a process called perfusion.
Continuous Capillaries The most common type of capillary, the continuous capillary , is found in almost all vascularized tissues. The three major types of capillaries: continuous, fenestrated, and sinusoid. Fenestrated Capillaries A fenestrated capillary is one that has pores or fenestrations in addition to tight junctions in the endothelial lining. Sinusoid Capillaries A sinusoid capillary or sinusoid is the least common type of capillary and the most permeable.
Metarterioles and Capillary Beds A metarteriole is a type of vessel that has structural characteristics of both an arteriole and a capillary.
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