What is an Arteriole?
Aug 04, · The function of the arteries is to carry oxygenated blood to organs and cells in the body. Because of this, arterial blood has a bright red color and flows away from the heart. Arterial walls have three layers. 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.
An arteriole is a small blood vessel that forms a connection between small arteries and capillaries. Arterioles are an important part of the circulatory system. These vessels are filled with oxygenated blood from the heart that has a bright red appearance, in contrast with the blue tinge of blood being carried toward the heart and lungs so that it can be reoxygenated. These blood vessels are between 10 and micrometers wide, about the width of a human hair or smaller.
They have thin muscular walls that can be contracted to restrict the flow of blood through a give arteriole or relaxed to increase the flow of blood. These vessels are supplied by small arteries, and small arteries in turn get their blood supply from the large arteries of the body. From the arteriole, blood passes into the capillaries. Pores in the walls of the capillaries allow for the exchange of nutrients and wastes that are carried away in the venules connected at what is guitar hero rated other side.
Arterioles play an important role in determining how much blood is distributed to organs and tissues. The body produces vasodilators and vasoconstrictors, compounds that dilate and contract blood vessels, to regulate the movement of blood through the circulatory system.
Blood supply can be increased in situations like injuries where there is a need for ample blood, or restricted if the body wants to divert resources elsewhere. This also what is positional vertigo caused from an impact on blood pressure. When the blood vessels are relaxed, blood pressure is low because there is less resistance.
The circulatory system is a highly efficient and well-oiled machine. In addition to supplying nutrients and carrying away wastes, the blood is also involved in immune system functions and injury response. On very short notice, signals can be sent to redirect supplies of blood, increase a blood supply to a given area, or circumvent a blockage to deliver blood to an area of tissue that is at risk of ischemia, or lack of oxygen. One problem that can develop within an arteriole is clotting, which leads to increased blood pressure in addition to restricting the supply of blood to an area fed by an obstructed vessel.
Diseases involving the circulatory system can limit circulation or damage individual vessels, leading to tissue death and other problems. The microcirculation involving the arterioles, capillaries, and venules is critically important to the health of every part of the body from the skin to the spleen.
Ever since she began contributing to the what is a beta 2 agonist several years ago, Mary has embraced the exciting challenge of being a wiseGEEK researcher and writer. Mary has a liberal arts degree from Goddard College and spends her free time reading, cooking, and exploring the great outdoors.
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Apr 14, · An artery is a high pressure blood vessel in the circulatory system that carries blood to an organ and away from the heart. Arteries deliver oxygen and . An afferent arteriole connects the renal artery to the glomerular capillary network in your kidney's nephron, starting the filtering process. It also takes action that controls blood pressure. What. Sep 25, · The main function of arteries and arterioles is to carry nutrients and oxygen to the different tissues of the body. Both arteries and arterioles are under the control of the sympathetic nervous system. Difference Between Arteries and Arterioles.
Blood is carried through the body via blood vessels. An artery is a blood vessel that carries blood away from the heart, where it branches into ever-smaller vessels. Eventually, the smallest arteries, vessels called arterioles, further branch into tiny capillaries, where nutrients and wastes are exchanged, and then combine with other vessels that exit capillaries to form venules, small blood vessels that carry blood to a vein, a larger blood vessel that returns blood to the heart.
Arteries and veins transport blood in two distinct circuits: the systemic circuit and the pulmonary circuit. The blood returned to the heart through systemic veins has less oxygen, since much of the oxygen carried by the arteries has been delivered to the cells. In contrast, in the pulmonary circuit, arteries carry blood low in oxygen exclusively to the lungs for gas exchange.
Pulmonary veins then return freshly oxygenated blood from the lungs to the heart to be pumped back out into systemic circulation. Although arteries and veins differ structurally and functionally, they share certain features.
Figure 1. 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. 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.