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. The precise diameter of the lumen of an arteriole at any given moment is determined by neural and chemical controls, and vasoconstriction and vasodilation in the arterioles are the primary mechanisms for distribution of blood flow.
A capillary is a microscopic channel that supplies blood to the tissues themselves, a process called perfusion. Exchange of gases and other substances occurs in the capillaries between the blood and the surrounding cells and their tissue fluid interstitial fluid.
The diameter of a capillary lumen ranges from 5—10 micrometers; the smallest are just barely wide enough for an erythrocyte to squeeze through.
Flow through capillaries is often described as microcirculation. The wall of a capillary consists of the endothelial layer surrounded by a basement membrane with occasional smooth muscle fibers.
There is some variation in wall structure: In a large capillary, several endothelial cells bordering each other may line the lumen; in a small capillary, there may be only a single cell layer that wraps around to contact itself.
For capillaries to function, their walls must be leaky, allowing substances to pass through. The most common type of capillary, the continuous capillary , is found in almost all vascularized tissues.
Continuous capillaries are characterized by a complete endothelial lining with tight junctions between endothelial cells. Although a tight junction is usually impermeable and only allows for the passage of water and ions, they are often incomplete in capillaries, leaving intercellular clefts that allow for exchange of water and other very small molecules between the blood plasma and the interstitial fluid.
Substances that can pass between cells include metabolic products, such as glucose, water, and small hydrophobic molecules like gases and hormones, as well as various leukocytes. Continuous capillaries not associated with the brain are rich in transport vesicles, contributing to either endocytosis or exocytosis.
Those in the brain are part of the blood-brain barrier. Here, there are tight junctions and no intercellular clefts, plus a thick basement membrane and astrocyte extensions called end feet; these structures combine to prevent the movement of nearly all substances.
Figure 4. The three major types of capillaries: continuous, fenestrated, and sinusoid. A fenestrated capillary is one that has pores or fenestrations in addition to tight junctions in the endothelial lining. These make the capillary permeable to larger molecules. The number of fenestrations and their degree of permeability vary, however, according to their location.
Fenestrated capillaries are common in the small intestine, which is the primary site of nutrient absorption, as well as in the kidneys, which filter the blood. They are also found in the choroid plexus of the brain and many endocrine structures, including the hypothalamus, pituitary, pineal, and thyroid glands. A sinusoid capillary or sinusoid is the least common type of capillary. Sinusoid capillaries are flattened, and they have extensive intercellular gaps and incomplete basement membranes, in addition to intercellular clefts and fenestrations.
This gives them an appearance not unlike Swiss cheese. These very large openings allow for the passage of the largest molecules, including plasma proteins and even cells.
Blood flow through sinusoids is very slow, allowing more time for exchange of gases, nutrients, and wastes. Sinusoids are found in the liver and spleen, bone marrow, lymph nodes where they carry lymph, not blood , and many endocrine glands including the pituitary and adrenal glands. Without these specialized capillaries, these organs would not be able to provide their myriad of functions.
For example, when bone marrow forms new blood cells, the cells must enter the blood supply and can only do so through the large openings of a sinusoid capillary; they cannot pass through the small openings of continuous or fenestrated capillaries. The liver also requires extensive specialized sinusoid capillaries in order to process the materials brought to it by the hepatic portal vein from both the digestive tract and spleen, and to release plasma proteins into circulation.
A metarteriole is a type of vessel that has structural characteristics of both an arteriole and a capillary. Slightly larger than the typical capillary, the smooth muscle of the tunica media of the metarteriole is not continuous but forms rings of smooth muscle sphincters prior to the entrance to the capillaries.
Each metarteriole arises from a terminal arteriole and branches to supply blood to a capillary bed that may consist of 10— capillaries. The precapillary sphincters , circular smooth muscle cells that surround the capillary at its origin with the metarteriole, tightly regulate the flow of blood from a metarteriole to the capillaries it supplies.
Their function is critical: If all of the capillary beds in the body were to open simultaneously, they would collectively hold every drop of blood in the body and there would be none in the arteries, arterioles, venules, veins, or the heart itself.
Normally, the precapillary sphincters are closed. When the surrounding tissues need oxygen and have excess waste products, the precapillary sphincters open, allowing blood to flow through and exchange to occur before closing once more see Figure 5.
If all of the precapillary sphincters in a capillary bed are closed, blood will flow from the metarteriole directly into a thoroughfare channel and then into the venous circulation, bypassing the capillary bed entirely. This creates what is known as a vascular shunt. In addition, an arteriovenous anastomosis may bypass the capillary bed and lead directly to the venous system.
Although you might expect blood flow through a capillary bed to be smooth, in reality, it moves with an irregular, pulsating flow. This pattern is called vasomotion and is regulated by chemical signals that are triggered in response to changes in internal conditions, such as oxygen, carbon dioxide, hydrogen ion, and lactic acid levels.
For example, during strenuous exercise when oxygen levels decrease and carbon dioxide, hydrogen ion, and lactic acid levels all increase, the capillary beds in skeletal muscle are open, as they would be in the digestive system when nutrients are present in the digestive tract. During sleep or rest periods, vessels in both areas are largely closed; they open only occasionally to allow oxygen and nutrient supplies to travel to the tissues to maintain basic life processes.
Figure 5. In a capillary bed, arterioles give rise to metarterioles. Precapillary sphincters located at the junction of a metarteriole with a capillary regulate blood flow. A thoroughfare channel connects the metarteriole to a venule. An arteriovenous anastomosis, which directly connects the arteriole with the venule, is shown at the bottom. Capillaries are surrounded by a thin basal lamina of connective tissue. Capillaries form a network through body tissues that connects arterioles and venules and facilitates the exchange of water, oxygen, carbon dioxide, and many other nutrients and waste substances between blood and surrounding tissues.
The thin wall of the capillary and close association with its resident tissue allow for gas and lipophilic molecules to pass through without the need for special transport mechanisms.
This allows bidirectional diffusion depending on osmotic gradients. During embryological development, new capillaries are formed by vasculogenesis, the process of blood vessel formation occurring by de novo production of endothelial cells and their formation into vascular tubes.
The term angiogenesis denotes the formation of new capillaries from pre-existing blood vessels. Capillaries do not function independently. The capillary bed is an interwoven network of capillaries that supplies an organ. The more metabolically active the cells, the more capillaries required to supply nutrients and carry away waste products.
A capillary bed can consist of two types of vessels: true capillaries, which branch mainly from arterioles and provide exchange between cells and the circulation, and vascular shunts, short vessels that directly connect arterioles and venules at opposite ends of the bed, allowing for bypass.
Capillary beds may control blood flow via autoregulation. This allows an organ to maintain constant flow despite a change in central blood pressure. This is achieved by myogenic response and by tubuloglomerular feedback in the kidney.
When blood pressure increases, the arterioles that lead to the capillary bed are stretched and subsequently constrict to counteract the increased tendency for high pressure to increase blood flow. In the lungs, special mechanisms have been adapted to meet the needs of increased necessity of blood flow during exercise. When heart rate increases and more blood must flow through the lungs, capillaries are recruited and are distended to make room for increased blood flow while resistance decreases.
Privacy Policy. Skip to main content. Cardiovascular System: Blood Vessels. Search for:. Artery Function Arteries are high-pressure blood vessels that carry oxygenated blood away from the heart to all other tissues and organs. Learning Objectives Distinguish the function of the arterial system from that of venous system.
Key Takeaways Key Points Arteries are blood vessels that carry blood away from the heart. This blood is normally oxygenated, with the exception of blood in the pulmonary artery. Arteries typically have a thicker tunica media than veins, containing more smooth muscle cells and elastic tissue. This allows for modulation of vessel caliber and thus control of blood pressure. The arterial system is the higher-pressure portion of the circulatory system, with pressure varying between the peak pressure during heart contraction systolic pressure and the minimum diastolic pressure between contractions when the heart expands and refills.
The increase in arterial pressure during systole, or ventricular contraction, results in the pulse pressure, an indicator of cardiac function. Key Terms systolic pressure : The peak arterial pressure during heart contraction. Elastic Arteries An elastic or conducting artery has a large number of collagen and elastin filaments in the tunica media.
Learning Objectives Distinguish the elastic artery from the muscular artery. Key Takeaways Key Points Elastic arteries include the largest arteries in the body, those closest to the heart. They give rise to medium-sized vessels known as muscular, or distributing, arteries.
Elastic arteries differ from muscular arteries both in size and in the relative amount of elastic tissue contained within the tunica media. Arterial elasticity gives rise to the Windkessel effect, which helps to maintain a relatively constant pressure in the arteries despite the pulsating nature of blood flow.
Key Terms elastic arteries : An artery with a large number of collagen and elastin filaments, giving it the ability to stretch in response to each pulse. The Aorta Due to position as the first part of the systemic circulatory system closest to the heart and the resultant high pressures it will experience, the aorta is perhaps the most elastic artery, featuring an incredibly thick tunica media rich in elastic filaments. The heart muscle is supplied by the coronary arteries.
The left coronary artery and the right coronary artery branch off of the aorta and the left coronary artery further divides into the circumflex artery and the left anterior descending artery. These four arteries are the ones that may be replaced in coronary artery bypass graft CABG surgery. A quadruple bypass replaces all four arteries. Hardening of the arteries is the common term for atherosclerosis and peripheral arterial disease PAD. This occurs when plaque forms from fat, cholesterol, calcium, protein, and inflammatory cells, narrowing or blocking the arteries.
When this happens in the arteries of the heart, it is coronary artery disease CAD. Risk factors for PAD include smoking, diabetes, high blood pressure, and high cholesterol. PAD can lead to heart attack, stroke, transient ischemic attack, renal artery disease, and amputation. Looking to start a diet to better manage your high blood pressure?
Our nutrition guide can help. Pugsley MK, Tabrizchi R. The vascular system. An overview of structure and function.
J Pharmacol Toxicol Methods. Shammas NW. Epidemiology, classification, and modifiable risk factors of peripheral arterial disease. Vasc Health Risk Manag. Your Privacy Rights. To change or withdraw your consent choices for VerywellHealth. At any time, you can update your settings through the "EU Privacy" link at the bottom of any page.
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