In contrast, the pulmonary circuit begins in the right atrium and ends in the pulmonary veins that drain into the left atrium. Pulmonary arteries carry blood low in oxygen exclusively to the lungs for gas exchange.
Pulmonary veins then return freshly oxygenated blood from the lungs to the heart to be pumped back out into systemic circulation. Although arteries and veins differ structurally and functionally, they share certain features. Different types of blood vessels vary slightly in their structures, but they share the same general features.
Each type of vessel has a lumen —a hollow passageway through which blood flows. Arteries have smaller lumens than veins, a characteristic that helps to maintain the pressure of blood moving through the system. Together, their thicker walls and smaller diameters give arterial lumens a more rounded appearance in cross section than the lumens of veins. By the time blood has passed through capillaries and entered venules, the pressure initially exerted upon it by heart contractions has diminished.
In other words, in comparison to arteries, venules and veins withstand a much lower pressure from the blood that flows through them. Their walls are considerably thinner and their lumens are correspondingly larger in diameter, allowing more blood to flow with less vessel resistance.
In addition, many veins of the body, particularly those of the limbs, contain valves that assist the unidirectional flow of blood toward the heart. This is critical because blood flow becomes sluggish in the extremities, as a result of the lower pressure and the effects of gravity. The walls of arteries and veins are largely composed of living cells and their products including collagen and elastic fibers ; the cells require nourishment and produce waste.
Further, the walls of the larger vessels are too thick for nutrients to diffuse through to all of the cells. The lower pressure within veins allows the vasa vasorum to be located closer to the lumen.
The restriction of the vasa vasorum to the outer layers of arteries is thought to be one reason that arterial diseases are more common than venous diseases, since their location makes it more difficult to nourish the cells of the arteries and remove waste products. There are also minute nerves within the walls of both types of vessels that control the contraction and dilation of smooth muscle. These minute nerves are known as the nervi vasorum.
Both arteries and veins have the same three distinct tissue layers, called tunics from the Latin term tunica, for the garments first worn by ancient Romans. Normally the thickest layer in arteries Smooth muscle cells and elastic fibers predominate the proportions of these vary with distance from the heart Nervi vasorum and vasa vasorum present External elastic membrane present in larger vessels.
The tunica intima also called the tunica interna is composed of epithelial and connective tissue layers. Lining the tunica intima is the specialized simple squamous epithelium called the endothelium, which is continuous throughout the entire vascular system, including the lining of the chambers of the heart. Damage to this endothelial lining and exposure of blood to the collagen fibers beneath is one of the primary causes of clot formation.
Until recently, the endothelium was viewed simply as the boundary between the blood in the lumen and the walls of the vessels.
Recent studies, however, have shown that it is physiologically critical to such activities as helping to regulate capillary exchange and altering blood flow. The endothelium releases local chemicals called endothelins that can constrict the smooth muscle within the walls of the vessel to increase blood pressure. Uncompensated overproduction of endothelins may contribute to hypertension high blood pressure and cardiovascular disease. Next to the endothelium is the basement membrane, or basal lamina, that effectively binds the endothelium to the connective tissue.
The basement membrane provides strength while maintaining flexibility, and it is permeable, allowing materials to pass through it. The thin outer layer of the tunica intima contains a small amount of areolar connective tissue that consists primarily of elastic fibers to provide the vessel with additional flexibility; it also contains some collagen fibers to provide additional strength.
In larger arteries, there is also a thick, distinct layer of elastic connective tissue known as the internal elastic membrane also called the internal elastic lamina at the boundary with the tunica media. Like the other components of the tunica intima, the internal elastic membrane provides structure while allowing the vessel to stretch. It is permeated with small openings that allow exchange of materials between the tunics.
The internal elastic membrane is not apparent in veins. In addition, many veins, particularly in the lower limbs, contain one-way valves formed by sections of thickened endothelium that are reinforced with connective tissue, extending into the lumen.
Under the microscope, the lumen and the entire tunica intima of a vein will appear smooth, whereas those of an artery will normally appear wavy because of the partial constriction of the smooth muscle in the tunica media, the next layer of blood vessel walls. It is generally the thickest layer in arteries, and it is much thicker in arteries than it is in veins. The tunica media consists of layers of smooth muscle supported by connective tissue that is primarily made up of elastic fibers, most of which are arranged in circular sheets.
Toward the outer portion of the tunic, there are also layers of longitudinal smooth muscle. Contraction and relaxation of the circular muscles decrease and increase the diameter of the vessel lumen, respectively. Specifically in arteries, vasoconstriction decreases blood flow as the smooth muscle in the walls of the tunica media contracts, making the lumen narrower and increasing blood pressure. Similarly, vasodilation increases blood flow as the smooth muscle relaxes, allowing the lumen to widen and blood pressure to drop.
Neural and chemical mechanisms reduce or increase blood flow in response to changing body conditions, from exercise to hydration. The smooth muscle layers of the tunica media are supported by a framework of collagen fibers that also binds the tunica media to the inner and outer tunics.
Along with the collagen fibers are large numbers of elastic fibers that appear as wavy lines in prepared slides. Separating the tunica media from the outer tunica externa in larger arteries is the external elastic membrane also called the external elastic lamina , which also appears wavy in slides. This structure is not usually seen in smaller arteries, nor is it seen in veins. The outer tunic, the tunica externa also called the tunica adventitia , is a substantial sheath of dense irregular connective tissue composed primarily of collagen fibers.
Some bands of elastic fibers are found here as well. The tunica externa in veins also contains groups of smooth muscle fibers. This is normally the thickest tunic in veins and may be thicker than the tunica media in some larger arteries. The outer layers of the tunica externa are not distinct but rather blend with the surrounding connective tissue outside the vessel, helping to hold the vessel in relative position.
If you are able to palpate some of the superficial veins on your upper limbs and try to move them, you will find that the tunica externa prevents this. If the tunica externa did not hold the vessel in place, any movement would likely result in disruption of blood flow.
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. 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 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 gives it a low resistance and enables it to accept a large volume of blood from the heart which is conducted to smaller branches within regions of the body.
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 which controls blood flow to individual organs.
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 that deliver blood to capillaries within specific organs and tissues. 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. This can cause an aortic aneurysm. This is a bulging, weakened area in the wall of a blood vessel due to an abnormal widening or ballooning.
It can also cause coarctation of the aorta. This is narrowing of the aorta, the largest artery in the body. It can also cause Takayasu arteritis. This is a rare inflammatory disease that affects the aorta and its branches.
Thoracic vascular disease. It can cause thoracic aortic aneurysm. This is a bulging, weakened area in the wall of a blood vessel. It causes an abnormal widening or ballooning in the chest thoracic part of the aorta. Abdominal vascular disease. It can cause an abdominal aortic aneurysm. It causes an abnormal widening or ballooning in the belly abdominal part of the aorta.
Peripheral venous disease. This can cause deep vein thrombosis DVT. DVT is a blood clot in a deep vein in the muscles of the leg. This disease can also cause varicose veins. Lymphatic vascular diseases. These can cause lymphedema. This is swelling caused by problems of the normal draining of the lymph nodes.
Vascular diseases of the lungs. These can cause granulomatosis with polyangiitis. This is an uncommon disease in which the blood vessels are inflamed. It mainly affects the respiratory tract and the kidneys. Other diseases include angiitis inflammation of blood vessels and hypertensive pulmonary vascular disease.
This is high blood pressure in the lungs' blood flow. Renal kidney vascular diseases. These can cause renal artery stenosis blockage of a renal artery. They can also cause fibromuscular dysplasia. Venule : Venules form when capillaries come together and converging venules form a vein.
Venule walls have three layers: an inner endothelium composed of squamous endothelial cells that act as a membrane, a middle layer of muscle and elastic tissue, and an outer layer of fibrous connective tissue.
The middle layer is poorly developed so that venules have thinner walls than arterioles. Venules are extremely porous so that fluid and blood cells can move easily from the bloodstream through their walls. In contrast to regular venules, high-endothelial venules HEV are specialized post-capillary venous swellings. They are characterized by plump endothelial cells as opposed to the usual thinner endothelial cells found in regular venules.
HEVs enable lymphocytes white blood cells circulating in the blood to directly enter a lymph node by crossing through the HEV. Veins are blood vessels that carry blood from tissues and organs back to the heart; they have thin walls and one-way valves. Veins are blood vessels that carry blood towards the heart. Most carry deoxygenated blood from the tissues back to the heart, but the pulmonary and umbilical veins both carry oxygenated blood to the heart.
The difference between veins and arteries is the direction of blood flow out of the heart through arteries, back to the heart through veins , not their oxygen content. Veins differ from arteries in structure and function. For example, arteries are more muscular than veins, veins are often closer to the skin, and veins contain valves to help keep blood flowing toward the heart, while arteries do not have valves and carry blood away from the heart.
The heart pumps these products to the organs, while the vessels transport them to and from the organs. Arteries perfuse the organs and veins drain the organs of waste products. The lymphatic system helps in draining excess tissue fluid to the bloodstream.
Two circulatory loops are most important to survival: the pulmonary circulation and the systemic circulation. The pulmonary circulation pumps blood from the right ventricle to the pulmonary artery.
Blood exchanges carbon dioxide for oxygen while passing through the lung and the newly oxygenated blood drains into the left atrium from the pulmonary veins. The other circulatory loop is the systemic circulation, which pumps blood from the left ventricle to the aorta to the rest of the body.
It transports nutrients to the intestines and hormones to endocrine glands. Waste excretion then occurs via the kidneys, intestines, lungs, and skin. Blood returns to the right atrium from the superior and inferior vena cava. There are four main types of vessels in the body and have a specific role in blood flow.
Arteries are thick-walled vessels that perfuse organs. Capillaries have the largest total cross-sectional and surface area. Veins are thin-walled blood vessels that return deoxygenated blood to the heart. Sinusoids are substitutes for capillaries in some organs such as the spleen, liver, red bone marrow, etc.
Each vessel plays a specific role in blood flow and is regulated by several different factors that alter flow physiology. Arterioles are branches of arteries that are innervated by autonomic nerve fibers. There are alphaadrenergic receptors on arterioles of the skin, splanchnic and renal circulations, and betaadrenergic receptors on skeletal muscle.
Alpha1-adrenergic receptor activation results in vasoconstriction, and beta2-adrenergic receptor activation results in vasodilation.
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