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. 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. Treatment usually involves surgically removing or closing the AVM lesion.
Medication can also help to manage symptoms, such as pain or headaches. Microcephaly-capillary malformation syndrome is a rare genetic condition that starts before birth. People with this condition have smaller heads and brains. They also have widened capillaries that increase the flow of blood near the skin surface, which can cause pinkish red spots on the skin. Mutations to this gene can result in cells dying during development, affecting the entire development process.
Treatment for this condition can involve stimulation — particularly through sound and touch — bracing to maintain posture, and anticonvulsant medication therapy for management of seizures. Capillaries are tiny blood vessels that play a big role in facilitating the exchange of various substances between your bloodstream and tissues.
There are several types of capillaries, each with a slightly different structure and function. Your venous system is a network of veins that carry blood back to your heart from other organs. These vessels are channels that distribute blood to the body. Learn the…. When a blood vessel bursts, a small amount of blood escapes from the vessel into the body.
This blood may show up just beneath the surface of the skin. Broken blood vessels on the face can happen to anyone at any age. Vasodilation is a natural process that happens in your body.
In some situations it can be harmful, yet in others causing vasodilation is important…. Temporal arteritis occurs when the temporal arteries, which supply blood to the head and brain, become inflamed or damaged. Everyday factors can make veins more noticeable.
But there are a few causes that are an emergency. Green veins are superficial veins located near the surface of skin. Learn more. Mesenteric artery ischemia is a condition that restricts blood flow to your intestines. Find out what causes it and how to treat it. Health Conditions Discover Plan Connect. Capillaries and Their Functions. Medically reviewed by Stacy Sampson, D. When the heart contracts, it pumps blood out through the arteries.
The blood pushes against the vessel walls and flows faster under this high pressure. When the ventricles relax, the vessel walls push back against the decreased force. Blood flow slows down under this low pressure. Download Blood Vessels Lab Manuals. See more from our free eBook library. Functions of the Blood. Anatomy of the Heart. Pulmonary and Systemic Circulation. Circulatory System Pathologies. When you select "Subscribe" you will start receiving our email newsletter.
Use the links at the bottom of any email to manage the type of emails you receive or to unsubscribe. See our privacy policy for additional details. Learn Site. Get our awesome anatomy emails!
0コメント