This is not true for the veins as the pressure in the veins is less as there is no pumping of blood into the veins, unlike the arteries. The structure of the arterial walls is explained in detail below. This explanation also provides the answer to the question - Why do arteries have thick walls? Arteries, as it is very well known, carry oxygenated blood from the heart to the different parts of the body.
The arteries that actually carry the blood from the heart to different organs is known as the systemic artery. In exception, there are two arteries: the pulmonary artery and umbilical artery which carry the blood to the organs that oxygenate the blood. The walls of the arteries can be divided into three layers. Those are from outside to inside: i Tunica externa, ii Tunica media and, iii Tunica intima.
The first layer Tunica externa or also known as tunica adventitia is composed of collagen fibres and elastic tissue.
This layer does not have a well-defined boundary as we go towards the lumen of the artery that carries the blood. Normally, it is considered when it reaches or touches the connective tissue layer or the Tunica media. Symptoms of atherosclerosis develop gradually, vary depending on the affected artery and may resemble symptoms of other cardiac disease. For example, if a major artery is blocked, symptoms can be severe, such as those associated with a heart attack, stroke, aneurysm or blood clot.
If blockages in your arteries develop slowly, you can develop symptoms from a lack of sufficient blood flow to your heart. You may have chest pain, shortness of breath and become easily fatigued with physical activity. Over time, this impairment of blood flow can damage your heart, reduce its ability to pump effectively and can cause congestive heart failure. As plaque gradually builds up in your arteries, blood flow is decreased and the oxygen supply is reduced to vital organs and extremities.
A decrease in oxygenated blood to your heart may result in a heart attack. A decrease in oxygenated blood to your brain may result in a stroke. If the oxygenated blood supply to your arms and legs is reduced, limb loss can occur. Reduced blood flow or the availability of oxygen to any of your organs can impair their function, leading to kidney problems, muscle pain and weakness in the legs and impaired memory.
The Ohio State University Wexner Medical Center has expertise in complex and high-risk coronary catheter and surgical treatment options, including stents and coronary artery bypass graft CABG. When stents and coronary artery bypass crafts are not reasonable options, advanced treatment is available, including laser therapies transmyocardial revascularization , long-term artificial heart pumps ventricular assist devices and heart transplantation.
If your physician suspects you have atherosclerosis, you will have a complete examination and diagnostic tests that may include:. Treatment may include modification of risk factors, such as quitting smoking; exercising; improving dietary habits; losing weight; and reducing blood sugar levels, blood pressure and cholesterol levels. Tight control of blood sugar levels in individuals who have diabetes has proved to be an important factor. Antiplatelet, antihyperlipidemic statins and antihypertensive medications may be prescribed to treat atherosclerosis.
It is caused by a buildup of plaque in the inner lining of an artery. Plaque is made up of deposits of fatty substances, cholesterol, cellular waste products, calcium, and fibrin. As it builds up in the arteries, the artery walls become thickened and stiff. Atherosclerosis is a slow, progressive disease that may start as early as childhood. However, it can progress rapidly. It's not clear exactly how atherosclerosis starts or what causes it.
However, a gradual buildup of plaque or thickening due to inflammation occurs on the inside of the walls of the artery. This reduces blood flow and oxygen supply to the vital body organs and extremities. Signs and symptoms of atherosclerosis may develop gradually, and may be few, as the plaque gradually builds up in the artery. Symptoms may also vary depending on the affected artery.
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.
The structural features of arteries and veins are dictated by the pressure of blood passing through them. Arteries have thick muscular walls to resist bursting under the high pressure of blood passing through them. Large arteries near the heart also have a lot of elastic fibers, so they may expand and contract in response to the pulse pressure.
Arteries have moderately sized lumens and no valves, as blood flow is easily maintained by the high pressure in the vessels. Veins carry blood that is low in pressure. They do not need to guard against rupture due to high blood pressure. To the contrary, veins must have wide lumens to encourage blood flow, and thin walls to maximize the effect of the skeletomuscle and thoracic pumps. Veins also posses valves to prevent back flow, as the low pressure of blood increases the likelihood that blood may flow backwards.
Blood pumped by the heart flows through a series of vessels known as arteries, arterioles, capillaries, venules, and veins before returning to the heart.
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