Blood vessels, except the smallest ones, are made of three layers: the tunica interna, tunica media and tunica externa or adventitia. The tunica interna innermost layer is a single layer of squamous flat epithelial cells called the endothelium; this smooth lining in direct contact with the blood offers little resistance to blood flow Marieb and Hoehn, The endothelial cells can easily be damaged by hypertension, toxins such as cigarette smoke, or hyperglycaemia; this damage can result in atherosclerosis.
These delicate cells rest on a thin layer of connective tissue made of elastin and collagen elastic and structural support fibres that anchors the tunica interna to the tunica media. The endothelium regulates blood flow and prevents clotting; it produces chemicals such as nitric oxide that help regulate blood flow by relaxing the smooth muscle within blood vessels.
The tunica media middle layer takes up most of the arterial vessel wall and is composed of smooth muscle fibres and elastin. This is where an activated sympathetic nervous system can stimulate the smooth muscle fibres to contract, provoking blood vessel narrowing vasoconstriction and decreasing blood flow Marieb and Hoehn, When the sympathetic nerves are inhibited, the muscle fibres of the tunica media relax, the blood vessels increase in diameter vasodilation and blood flow increases.
The tunica externa outer layer consists mainly of connective tissue fibres that protect the blood vessels and attach them to any surrounding tissues. In larger blood vessels, additional small vessels — vasa vasorum — supply blood and nutrients to the tunica externa and tunica media. Arteries supply the body with oxygenated blood — with the exception of the pulmonary arteries from the heart; these carry deoxygenated blood to the lungs, and the umbilical artery, which carries deoxygenated blood from the foetus to the placenta.
Blood travels from the arteries to the arterioles and on to the capillaries, where gaseous exchange takes place. The largest artery is the aorta, which extends from the left ventricle down the left side of the body. It divides into four major regions, the ascending aorta, aortic arch, thoracic aorta and abdominal aorta. Table 2 lists the major branches off the aorta. Arteries can be divided into elastic arteries, muscular arteries and arterioles.
The elastic arteries are the largest They have a large lumen with low resistance to blood flow, and can expand and recoil to accommodate changes in blood volume. Muscular arteries regulate local blood flow and deliver blood to individual organs.
They measure 0. The arterioles are the smallest arteries 0. In certain areas, they have all three vascular layers tunica intima, media and externa. When they are close to the capillaries they comprise a single smooth muscle layer overlying endothelial cells. Blood flow into the capillaries is determined by the diameter of the arterioles and can be increased through vasodilation.
The veins are thin, elastic vessels that act as a reservoir of blood. They do not need large amounts of elastin and smooth muscle, since they transport low-pressure blood back to the heart. They have a large lumen, as well as valves that ensure a one-way flow of blood to the heart. The venules join to form veins, in which the tunica externa, consisting of thick collagenous bundles, is the largest layer. The largest veins — the superior and inferior venae cavae — have a large tunica externa further thickened by smooth muscle bands Marieb and Hoehn, The venous system is an irregular network that tends to follow the course of the arteries.
The capillaries can be compared to the smallest branches of a tree and connect arterioles to venules. The arteries divide into arterioles, which in turn divide into capillaries. These feed blood back into the venules, which connect to larger veins and ultimately to the superior or inferior vena cava. There are three main types of capillaries: continuous, fenestrated and sinusoidal. Table 3 lists their features and gives examples of where they are found in the body.
Capillaries act as a semipermeable membrane allowing the diffusion of gases and transfer of nutrients and waste products. The single layer of flattened endothelial cells of the capillaries facilitate the exchange of substances between capillaries and tissues. Gases, such as O2 and CO2, metabolic waste products, lactate, glucose and other nutrients are transferred across the walls of the capillaries through small slits in the endothelial cells known as pores or fenestrations.
To prevent capillaries from losing vital substances such as plasma proteins, the slits in the endothelial cells are smaller than these proteins. How do gas exchange and nutrient transfer happen between capillaries and tissues? According to the Starling principle named after physiologist Ernest Starling who described it in , fluid movement through the capillary walls is governed by hydrostatic pressure and oncotic pressure.
Like any fluid pushed through a confined space, blood in a capillary exerts pressure on the wall of the vessel because of the pressure exerted upstream by the blood coming from the arteriole. The blood pressure BP generates hydrostatic pressure, which expels fluid from the pores of the capillary into the interstitial compartment. The systemic circulation includes the left heart pump which supplies blood to the systemic organs.
The right and left heart pumps function in a series arrangement, thus both will circulate an identical volume of blood in a given minute cardiac output: liters per minute. All arteries of the systemic circulation branch from the aorta this is the largest artery of the body, with a diameter of cm , and divide into progressively smaller vessels.
The aorta's four principal divisions are the ascending aorta begins at the aortic valve where, close by, the two coronary artery branches have their origin , arch of the aorta, thoracic aorta, and abdominal aorta. The smallest of the arteries eventually branch into arterioles.
Next blood exits the capillaries and begins its return to the heart via the venules. Microcirculation is a term coined to collectively describe the flow of blood through arterioles, capillaries and the venules Fig. Figure 1. The major paths of blood flow through pulmonary and systemic circulatory systems. Capillaries, which are the smallest and most numerous blood vessels in the human body ranging from 5 to 10 micrometers in diameter and numbering around 10 billion are also the thinnest walled vessels; an inner diameter of 5 um is just wide enough for an erythrocyte to squeeze through.
Further, it is estimated that there are 25, miles of capillaries in an adult, each with an individual length of about 1 mm. Most capillaries are little more than a single cell layer thick, consisting of a layer of endothelial cells and a basement membrane.
As mentioned above, small molecules e. Nevertheless, the relative permeability of capillaries varies from region to region with regard to the physical properties of these formed walls. Based on such differences, capillaries are commonly grouped into two major classes: continuous and fenestrated capillaries. Figure 2. The microcirculation including arterioles, capillaries and venules. The capillaries lie between, or connect, the arterioles and venules. Capillaries form extensive branching networks that dramatically increase the surface areas available for the rapid exchange of molecules.
A metarteriole is a vessel that emerges from an arteriole and supplies a group of 10 to capillaries. 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.
Some bands of elastic fibers are found here as well. The tunica externa in veins also contains groups of smooth muscle fibers. This is normally the thickest tunic in veins and may be thicker than the tunica media in some larger arteries.
The outer layers of the tunica externa are not distinct but rather blend with the surrounding connective tissue outside the vessel, helping to hold the vessel in relative position.
If you are able to palpate some of the superficial veins on your upper limbs and try to move them, you will find that the tunica externa prevents this. If the tunica externa did not hold the vessel in place, any movement would likely result in disruption of blood flow. An artery is a blood vessel that conducts blood away from the heart.
All arteries have relatively thick walls that can withstand the high pressure of blood ejected from the heart. However, those close to the heart have the thickest walls, containing a high percentage of elastic fibers in all three of their tunics.
This type of artery is known as an elastic artery see Figure 3. Vessels larger than 10 mm in diameter are typically elastic. Their abundant elastic fibers allow them to expand, as blood pumped from the ventricles passes through them, and then to recoil after the surge has passed. If artery walls were rigid and unable to expand and recoil, their resistance to blood flow would greatly increase and blood pressure would rise to even higher levels, which would in turn require the heart to pump harder to increase the volume of blood expelled by each pump the stroke volume and maintain adequate pressure and flow.
Artery walls would have to become even thicker in response to this increased pressure. The elastic recoil of the vascular wall helps to maintain the pressure gradient that drives the blood through the arterial system. An elastic artery is also known as a conducting artery, because the large diameter of the lumen enables it to accept a large volume of blood from the heart and conduct it to smaller branches.
Figure 3. Comparison of the walls of an elastic artery, a muscular artery, and an arteriole is shown. In terms of scale, the diameter of an arteriole is measured in micrometers compared to millimeters for elastic and muscular arteries.
The artery at this point is described as a muscular artery. The diameter of muscular arteries typically ranges from 0. Their thick tunica media allows muscular arteries to play a leading role in vasoconstriction. In contrast, their decreased quantity of elastic fibers limits their ability to expand. Fortunately, because the blood pressure has eased by the time it reaches these more distant vessels, elasticity has become less important.
Rather, there is a gradual transition as the vascular tree repeatedly branches. In turn, muscular arteries branch to distribute blood to the vast network of arterioles. For this reason, a muscular artery is also known as a distributing artery. An arteriole is a very small artery that leads to a capillary. Arterioles have the same three tunics as the larger vessels, but the thickness of each is greatly diminished.
The critical endothelial lining of the tunica intima is intact. The tunica media is restricted to one or two smooth muscle cell layers in thickness. The tunica externa remains but is very thin see Figure 3. With a lumen averaging 30 micrometers or less in diameter, arterioles are critical in slowing down—or resisting—blood flow and, thus, causing a substantial drop in blood pressure.
Because of this, you may see them referred to as resistance vessels. The muscle fibers in arterioles are normally slightly contracted, causing arterioles to maintain a consistent muscle tone—in this case referred to as vascular tone—in a similar manner to the muscular tone of skeletal muscle. In reality, all blood vessels exhibit vascular tone due to the partial contraction of smooth muscle.
The importance of the arterioles is that they will be the primary site of both resistance and regulation of blood pressure.
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