Why does an artery have thick walls

This site complies with the HONcode standard for trustworthy health information: verify here. Common Health Topics. Arteries and arterioles. Venules and veins. Biology of the Heart and Blood Vessels.

Test your knowledge. An aortic dissection occurs when the inner layer lining of the aortic wall tears and separates from the middle layer of the wall.

Aortic dissection is frequently fatal. Among which of the following groups is aortic dissection most common? More Content. Click here for the Professional Version. Blood Vessels: Circulating the Blood Blood travels from the heart in arteries, which branch into smaller and smaller vessels, eventually becoming arterioles. 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. A venule is an extremely small vein, generally 8— micrometers in diameter. Postcapillary venules join multiple capillaries exiting from a capillary bed.

Multiple venules join to form veins. The walls of venules consist of endothelium, a thin middle layer with a few muscle cells and elastic fibers, plus an outer layer of connective tissue fibers that constitute a very thin tunica externa. Venules as well as capillaries are the primary sites of emigration or diapedesis, in which the white blood cells adhere to the endothelial lining of the vessels and then squeeze through adjacent cells to enter the tissue fluid.

A vein is a blood vessel that conducts blood toward the heart. Compared to arteries, veins are thin-walled vessels with large and irregular lumens see Figure 6. Figure 6. Many veins have valves to prevent back flow of blood, whereas venules do not. In terms of scale, the diameter of a venule is measured in micrometers compared to millimeters for veins.

Because they are low-pressure vessels, larger veins are commonly equipped with valves that promote the unidirectional flow of blood toward the heart and prevent backflow toward the capillaries caused by the inherent low blood pressure in veins as well as the pull of gravity. Table 2 compares the features of arteries and veins. Higher in pulmonary veins Valves Not present Present most commonly in limbs and in veins inferior to the heart Disorders of the 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. Figure 7. Varicose veins are commonly found in the lower limbs. Edema may be accompanied by varicose veins, especially in the superficial veins of the legs.

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. In addition to their primary function of returning blood to the heart, veins may be considered blood reservoirs, since systemic veins contain approximately 64 percent of the blood volume at any given time. 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. When blood flow needs to be redistributed to other portions of the body, the vasomotor center located in the medulla oblongata sends sympathetic stimulation to the smooth muscles in the walls of the veins, causing constriction—or in this case, venoconstriction.

This increases pressure on the blood within the veins, speeding its return to the heart. As you will note in the image above, approximately 21 percent of the venous blood is located in venous networks within the liver, bone marrow, and integument. This volume of blood is referred to as venous reserve. What is heart disease? What is the primary cause of heart disease and what symptoms does it How does blood flow back to the heart from the lower extremities, since it is fighting against gravity?

In a heart transplant, if the vagus nerve is not re-transplanted, how is the heart rate kept in check? Is heart disease hereditary? Apart from the main arteries, the arterioles which carry oxygenated blood to extreme ends of the body also have thick elastic and muscular media. They have a major influence on the local blood pressure and overall blood pressure.

The combination of the cardiac output of blood and systemic vascular resistance which refers to collective resistance of blood arterioles are the major deciding factors of arterial blood pressure at any given moment. Veins, on the other hand, have a smaller Tunica media, comprising less elastic and muscular layer than arteries because veins do not work in a contractile manner like the arteries and are not subject to high systolic pressure.

Hence, to the question - Why are arteries thicker than veins? The answer is - Veins are not subject to high blood pressure, unlike the arteries. Similarly, for the question why arteries are thick-walled, it is because the thick elastic and muscular walls of the arteries not only help them sustain the cardiac output pressure but also maintain blood pressure throughout the circulatory system of the body.

The collapse or change in the blood pressure can be catastrophic for the body and hence fatal to the living organism. It is highly important to maintain a constant and regulated blood pressure that works towards making the blood reach different parts of the body. The arteries work in a contractual manner that allows them to make the blood reach the extreme portions.

In that process, the pressure difference between the arterioles and the arteries has a huge role to play.