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About 10 percent of the total blood volume is in the systemic arterial system at any given time. The wall of an artery consists of three layers. The innermost layer, the tunica intima also called tunica interna , is simple squamous epithelium surrounded by a connective tissue basement membrane with elastic fibers.
The middle layer, the tunica media , is primarily smooth muscle and is usually the thickest layer. It not only provides support for the vessel but also changes vessel diameter to regulate blood flow and blood pressure.
The outermost layer, which attaches the vessel to the surrounding tissue, is the tunica externa or tunica adventitia. This layer is connective tissue with varying amounts of elastic and collagenous fibers. The connective tissue in this layer is quite dense where it is adjacent to the tunic media, but it changes to loose connective tissue near the periphery of the vessel. Capillaries, the smallest and most numerous of the blood vessels, form the connection between the vessels that carry blood away from the heart arteries and the vessels that return blood to the heart veins.
The primary function of capillaries is the exchange of materials between the blood and tissue cells. Capillary distribution varies with the metabolic activity of body tissues. Tissues such as skeletal muscle , liver , and kidney have extensive capillary networks because they are metabolically active and require an abundant supply of oxygen and nutrients.
Other tissues, such as connective tissue, have a less abundant supply of capillaries. They connect these two things and can be considered an exchange stops. Capillaries remove nutrients and oxygen from the blood found in the arteries and distribute them to the tissues throughout the body.
They are also there to place cellular waste into the veins, including carbon dioxide. Capillaries are essential to the circulatory system.
In fact, if you press a pink area of your skin, blood is pressed out of the capillaries, which causes blanching. Capillaries, therefore, are involved in hundreds of actions such as these. Capillary action is the spontaneous flow of a liquid of any type through either a porous material or even a narrow tube.
The action is not dependent upon gravity; in fact, the movement can occur in spite of and in opposition to gravity. Examples of capillary action include:. Capillary action needs two things to occur: adhesive forces, which are found between the tube material and the liquid; and cohesive forces, which are found in the liquid itself. Labeled as types of intermolecular forces, adhesion and cohesion pull the liquid into the tube; however, for wicking to occur as in the example with the paintbrush , the tube must be very small in diameter.
Artery: A blood vessel which carries blood from the heart to the rest of the body. If an artery is diseased, the organ that is supposed to be receiving the blood can become damaged because there are no nutrients or oxygen supplied to it.
Brachial Artery: This artery is found in the arm and is usually used in dialysis to create AV fistulas. Calcified Vessels: Calcified vessels occur when arteries harden due to deposits of calcium in the wall.
They directly affect the ability to make certain pressure measurements in the legs, and they are very often seen in diabetic patients. Carotid Artery: This is a major artery whose function is to provide blood from the heart to the brain.
Located in the front of the neck, it can become diseased if enough plaque develops in the neck area. This action causes the carotid artery to separate from the artery, called an embolism, and travel upwards to the brain, which can cause a stroke in some instances.
Celiac Artery: This is the artery that supplies blood to various internal organs, particularly the stomach. Deep Vein Thrombosis DVT : This occurs with there is clotting found inside of a vein, which is caused by either immobility or even an injury.
Usually found in the leg, DVT can also occur due to abnormal clotting factors. Diffusion: If molecules spread into an available space, this is called diffusion, and this tendency occurs because of the heat which is found in all molecules that have temperatures above absolute zero.
Endothelial Cells: These are cells that form the inner lining of two structures — the lymphatic system and the cardiovascular system. The inner layers of numerous organs also contain endothelial cells, including the heart, skin, brain, and lungs. They are responsible for creating new blood vessels. Endovascular: This is a treatment that involves either reconstructing or repairing an artery and usually includes techniques that are minimally invasive. False Aneurysm: This refers to when the layers of the artery are injured, which causes blood to leak outside of the artery and which is then held onto by the tissues that surround it.
Femoral Artery: The femoral artery is very large and located in the leg, extending from the hip to the knee. When physicians are performing bypass grafts, they usually start around the femoral artery. Fibromuscular Dysplasia FMD : This term refers to the narrowing of a particular artery and is usually caused by a spasm in the muscle layers of the artery wall, which is exacerbated by the fact that the layers are so thin.
Hypertension: Also known as high blood pressure, this occurs when the pressure within the arteries remains above a certain level, a level that is recommended to be healthy. Iliac Artery: The iliac artery is found deep down inside of the abdomen and carries the blood to the legs and pelvic area from the aorta. Jugular Vein: The jugular vein is located in the neck and is very large; it is also used to place the catheters used in dialysis.
Leg Bypass: In a leg bypass, either a plastic tube or an actual vein gets sewn in place so that blood flow can be redirected around another blood vessel in the leg. Leg Ulcers: Leg ulcers are essentially the breakdown of an area of skin, caused when there is an impairment of blood flow in open areas.
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. Vascular surgery is a specialty in which the physician deals primarily with diseases of the vascular portion of the cardiovascular system. This includes repair and replacement of diseased or damaged vessels, removal of plaque from vessels, minimally invasive procedures including the insertion of venous catheters, and traditional surgery. Following completion of medical school, the physician generally completes a 5-year surgical residency followed by an additional 1 to 2 years of vascular specialty training.
Vascular technicians are specialists in imaging technologies that provide information on the health of the vascular system. They may also assist physicians in treating disorders involving the arteries and veins. This profession often overlaps with cardiovascular technology, which would also include treatments involving the heart.
Although recognized by the American Medical Association, there are currently no licensing requirements for vascular technicians, and licensing is voluntary. The United States Bureau of Labor projects this profession to grow by 29 percent from to 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. Capillaries lead back to small vessels known as venules that flow into the larger veins and eventually back to the heart.
The arterial system is a relatively high-pressure system, so arteries have thick walls that appear round in cross section. The venous system is a lower-pressure system, containing veins that have larger lumens and thinner walls. They often appear flattened. Arteries, arterioles, venules, and veins are composed of three tunics known as the tunica intima, tunica media, and tunica externa.
Capillaries have only a tunica intima layer. The tunica intima is a thin layer composed of a simple squamous epithelium known as endothelium and a small amount of connective tissue. The tunica media is a thicker area composed of variable amounts of smooth muscle and connective tissue. It is the thickest layer in all but the largest arteries. The tunica externa is primarily a layer of connective tissue, although in veins, it also contains some smooth muscle.
Blood flow through vessels can be dramatically influenced by vasoconstriction and vasodilation in their walls. Answer the question s below to see how well you understand the topics covered in the previous section. Skip to main content.
Search for:. Structure and Function of Blood Vessels Learning Objectives By the end of this section, you will be able to: Compare and contrast the three tunics that make up the walls of most blood vessels Distinguish between elastic arteries, muscular arteries, and arterioles on the basis of structure, location, and function Describe the basic structure of a capillary bed, from the supplying metarteriole to the venule into which it drains Explain the structure and function of venous valves in the large veins of the extremities.
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. Careers in Action: Vascular Surgeons and Technicians Vascular surgery is a specialty in which the physician deals primarily with diseases of the vascular portion of the cardiovascular system.
Visit this site to learn more about vascular surgery. Visit this site to learn more about vascular technicians. Critical Thinking Questions Arterioles are often referred to as resistance vessels. Cocaine use causes vasoconstriction. Is this likely to increase or decrease blood pressure, and why?
A blood vessel with a few smooth muscle fibers and connective tissue, and only a very thin tunica externa conducts blood toward the heart. What type of vessel is this? Show Answers Arterioles receive blood from arteries, which are vessels with a much larger lumen. As their own lumen averages just 30 micrometers or less, arterioles are critical in slowing down—or resisting—blood flow.
The arterioles can also constrict or dilate, which varies their resistance, to help distribute blood flow to the tissues. Vasoconstriction causes the lumens of blood vessels to narrow. This increases the pressure of the blood flowing within the vessel. This is a venule. Licenses and Attributions. CC licensed content, Shared previously. Endothelium usually appears wavy due to constriction of smooth muscle; Internal elastic membrane present in larger vessels.
Normally the thickest layer in arteries; Smooth muscle cells and elastic fibers predominate the proportions of these vary with distance from the heart ; External elastic membrane present in larger vessels. Normally thinner than the tunica externa; Smooth muscle cells and collagenous fibers predominate; Nervi vasorum and vasa vasorum present; External elastic membrane absent.
Normally thinner than the tunica media in all but the largest arteries; Collagenous and elastic fibers; Nervi vasorum and vasa vasorum present.
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