Blood vessels compose a continuous system of channels through which blood transports oxygen and nutrients to and waste materials from all body tissues.
All blood vessels (except capillaries) share a similar three-layered structure. The innermost layer, called the tunica intima, is composed of a monolayer of endothelial cells called the endothelium. The tunica intima helps to restrict the entry of substances into the vascular wall, control blood vessel diameter, and regulate coagulation. The hollow center of a blood vessel is called the lumen and is the space through which blood flows.
The middle layer is called the tunica media and is separated from the tunica intima by a sheath of high-flexible material called the internal elastic lamina. The tunica media is composed of a circular arrangement of smooth muscle cells, collagen, and elastic fibers; it composes the bulk of the wall of most arteries but in veins is thinner and contains fewer smooth muscle cells. Smooth muscle contains contractile elements that are responsible for contraction (vasoconstriction) and relaxation (vasodilation). The tunica media, therefore, imparts strength, elasticity, and contractile abilities to the vessel wall.
Surrounding the tunica media is the tunica adventitia (the two layers are separated by the external elastic lamina). This outermost layer contains a matrix of collagen and elastic fibers that support fibroblasts (cells that secrete the fibrous proteins collagen and elastin), nerves, and vasa vasorum (small blood vessels that supply the walls of large arteries and veins with oxygen and nutrients).
Arteries and arterioles
Arteries are blood vessels that carry blood away from the heart. Arterial blood is oxygen-rich, with the exception of blood carried by the pulmonary artery from the heart to the lungs to be oxygenated. The aorta is the largest artery in the human body and originates at the left ventricle of the heart. This vessel and its major branches (the common carotid, common iliac, subclavian, and brachiocephalic arteries) are called elastic arteries because they expand and recoil in response to the pulsing flow of blood and to changing blood volume.
The elastic arteries branch to become muscular arteries, vessels with thick walls that transport blood to specific organs. Muscular arteries give rise to resistance vessels; these include small arteries and arterioles. As arteries become smaller, their walls become thinner and are composed of less collagen and elastin. The walls of small arteries have multiple layers of smooth muscle cells, while arterioles have only one or two. Resistance vessels are thus less stretchy but more active in regulating the flow of blood into capillary beds.
Anastomoses are formed where arteries and arterioles merge to provide alternative channels for blood delivery. They provide collateral circulation in the event that an artery becomes occluded (blocked).
Exchange vessels include capillaries and postcapillary venules. The walls of capillaries are composed of only a tunica intima (a thin layer of endothelial cells). The average diameter of the lumen is just large enough to allow erythrocytes (red blood cells) to pass through in single file. Exchange vessels are the site where gases, nutrients, and wastes are exchanged between blood and surrounding tissues.
There are three major type of capillaries: continuous, fenestrated, and discontinuous. Continuous capillaries are the most abundant type in the human body and are found in skin, muscle, lungs, and the central nervous system. They have low permeability and therefore allow only limited passage of substances across the capillary wall. Fenestrated capillaries are much more permeable than continuous capillaries; their walls contain circular pores or fenestrae closed by a thin diaphragm. Discontinuous capillaries, also called sinusoids, have gaps between endothelial cells that are large enough to allow even erythrocytes to pass through the capillary wall. They are found in the liver, spleen, and bone marrow, as well as some endocrine glands.
The capillary bed is a network of capillaries that connect arterioles with venules; there are typically 10 to 100 capillaries per bed. Arterioles give rise to either capillaries or metarterioles, vessels that are wider than true capillaries and directly connect arterioles to venules. True capillaries branch off arterioles or metarterioles and are encircled at their origin by the precapillary sphincter, permitting the regulation of blood flow into the capillary. Arteriovenous (A-V) shunts are anastomoses that bypass the capillary bed completely; they are frequently seen in tissues that require increased blood flow.
Veins and venules
Veins are blood vessels that carry blood from the capillary beds to the heart. Capillaries give rise to venules (small veins that have walls composed of a thin layer of endothelial cells), which in turn converge to form veins. Blood from the head, neck, and arms is carried to the superior vena cava, while the inferior vena cava receives blood from the trunk and legs; these large veins empty into the right atrium of the heart. The veins carry blood that is oxygen-poor, with the exception of the pulmonary vein, which carries oxygenated blood from the lungs to the heart.
The walls of veins are thinner and the lumens larger than those of arteries. They can accommodate a large blood volume and may act as blood reservoirs, containing up to 70% of the body's total blood volume. Veins and venules are therefore called capacitance vessels. Most veins have a system of valves, paired folds of the tunica intima that prevent the backflow of blood.
Blood pressure is defined as the force per unit area that flowing blood exerts on the wall of a vessel; it can be represented by the equation Blood pressure = flow × resistance. Blood pressure is typically expressed in mm Hg (read as "millimeters of mercury"). It is usually recorded as two numbers: systolic pressure over diastolic pressure. Systole is the period of the cardiac cycle in which the aortic valve opens and blood flows into the aorta; systolic pressure is the maximal pressure during systole. Likewise, diastole is the period in which the left ventricle relaxes so it can refill with blood; diastolic pressure is therefore measured during diastole. It is generally assumed that a healthy young adult should have a blood pressure of 120/80 mm Hg (i.e. systolic pressure of 120 mm Hg and diastolic pressure of 80 mm Hg).
Blood pressure is proportional to blood flow (the amount of blood flowing through a vessel per unit time) and vascular resistance. Pressures vary throughout the cardiovascular system depending on the type and size of blood vessel. The highest systemic blood pressure is found in the aorta and diminishes progressively along the arterial system; it reaches its lowest point in the veins.
There are a number of factors that influence blood pressure. An individual's physical characteristics (i.e. sex, age, weight, race, or socioeconomic status) may positively or negatively affect blood pressure. Activities such as eating, drinking, sleeping, or smoking cause changes in pressure, as do mental activities or emotions such as anxiety or apprehension. Various disorders such as atherosclerosis, anemia, and diabetes mellitus have adverse affects on blood pressure.
The capillary bed is the site at which gases, nutrients, and wastes are exchanged between the blood and surrounding tissues. It is surrounded by interstitial fluid, or lymph, which is produced by the lymphatic system. Substances are moved between blood and interstitial fluid across the capillary wall by means of diffusion (movement from a high to a low concentration). Oxygen and nutrients move from the blood to interstitial fluid, while carbon dioxide and wastes move in the opposite direction. Gases such as oxygen or carbon dioxide and lipid-soluble nutrients diffuse across the cell membranes of endothelial cells. Small openings in the capillary wall called slit pores or clefts exist where endothelial cells border each other; small water-soluble nutrients or wastes may diffuse through these clefts.
There are two types of pressure that are involved in capillary dynamics. Hydrostatic pressure is the force per unit area exerted by a fluid (blood) against a vessel wall. Colloid osmotic pressure is the pressure required to prevent osmosis of fluid across a semipermeable membrane. Transcapillary filtration is determined not only by these pressures inside the blood vessels, but also by the same pressures outside the blood vessels. Osmotic pressure is an indirect measurement of the relative concentrations of water and solute in a solution; the higher the osmotic pressure of the solution, the lower the water concentration and therefore the higher the solute concentration of the solution. In a capillary, osmotic forces are exerted primarily by proteins, which are relatively impermeable to the capillary wall.
Role in human health
The 2006 "Heart and Stroke Statistical Update," published by the American Heart Association, states that cardiovascular diseases (CVD) have been the leading cause of death in the United States every year since 1900, with the exception of 1918. CVD accounted for 37.3% of all U.S. deaths in 2003; over 71 million Americans are estimated to suffer from one or more CVD.
There are numerous factors that increase the risk of cardiovascular disease. These include:
- Major risk factors: tobacco smoke, race, genes, diabetes mellitus, high cholesterol levels, hypertension, physical inactivity, and obesity.
- Contributing risk factors: stress, high triglycerides, alcohol, oral contraceptives, pregnancy, menopause, and Syndrome X (a cluster of risk factors that include obesity, glucose intolerance, hypertension, and high cholesterol).
Blood vessels and blood flow can respond to a variety of local control factors, including neural (such as shock ) or hormonal impulses (such as anger or fear). Blood vessels themselves can also grow (a process called angiogenesis) or remodel themselves in response to diseases such as ischema and hypertension.
Common diseases and disorders
- Atherosclerosis: According to the American Heart Association, atherosclerosis accounts for nearly 75% of all cardiovascular-related deaths. It involves the accumulation of lipids and other substances on the inner lining of an artery; the area of buildup is called a plaque. As a result, the arterial wall thickens and hardens, losing elasticity. Thrombi (blood clots) form when plaques rupture; if these occlude the artery, a heart attack or stroke may result.
- Stroke: A stroke occurs when the brain has been deprived of oxygen due to interrupted blood flow, often caused by a blood clot or burst blood vessel. Depending on the area of the brain that is damaged, neurological damage may be reversible or irreversible and may include coma, paralysis, visual or speech problems, seizures, or impaired memory.
- Varicose veins: Permanent changes in the diameter and/or length of veins may result from damage to or failure of the venous valves. Gravity, obesity, pregnancy, and increasing age may also play a role in the development of varicose veins.
- Hemangiomas: These are usually benign vascular anomalies that may result in small, harmless birthmarks or sacs of vascular tissues of varying sizes that may protrude from the skin. Hemangiomas are often only cosmetic blemishes but may, depending on their location, cause obstruction of the airway, block vision, or obstruct a vital organ.
- Aneurysm: An aneurysm results from the dilation of the wall of a blood vessel due to the weakening of the wall by disease, high blood pressure, or congenital defects. An abdominal aortic aneurysm is the most common type. A ruptured aneurysm is a serious medical emergency and requires surgical intervention.
Anastomoses— Connections formed where arteries and arterioles merge to provide alternative channels for blood delivery.
Arteries— Blood vessels that carry blood away from the heart.
Diastolic pressure— Diastole is the period in which the left ventricle relaxes so it can refill with blood; diastolic pressure is therefore measured during diastole.
Hydrostatic pressure— Force per unit area exerted by a fluid (blood) against a vessel wall.
Lumen— The hollow center of a blood vessel.
Osmotic pressure— The pressure required to prevent osmosis of fluid across a semi-permeable membrane. It is an indirect measurement of the water and solute concentrations of the solution.
Systolic pressure— Systole is the period of the cardiac cycle in which the aortic valve opens and blood flows into the aorta; systolic pressure is the maximal pressure during systole.
Tunica intima, media, adventitia— The three layers that compose the walls of large arteries and veins.
Vasa vasorum— Small blood vessels that supply the walls of large arteries and veins with oxygen and nutrients.
Veins— Blood vessels that carry blood from the capillary beds to the heart.
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The thickness of blood vessel walls also varies enormously, being largest in the large arteries, much less in veins of comparable diameter, and only a single cell thick in the capillaries. Despite the range of sizes the components of the blood vessel walls have a common pattern. All vessels are lined with a single layer of flattened cells called the endothelium. Except for capillaries, all vessels also contain elastic fibres, stiff collagen fibres (similar structure to muscle tendons), and smooth muscle fibres which can constrict or dilate in response to chemical and nervous stimuli. The relative proportions of these components vary in different blood vessels in accordance with their functions.
Recently, the endothelium has been recognized to be of importance in the regulation of the state of constriction or dilatation of the vessels themselves. Of particular note in this respect is ‘endothelial derived relaxation factor’, later shown to be nitric oxide: when this is released, notably in response to the shearing force of the blood on the vessel, it causes dilatation of the vessel.
Large arteriesThe aorta and its main branches are called elas-tic arteries. Although they also possess fibrous collagen tissue and smooth muscle, about half of their structure is composed of elastic fibres. These give large arteries a characteristic pale yellow colour. Their wide bore means that they offer little resistance to blood flow, so there is little pressure drop throughout the system of major arteries. The physiological significance of the elastic fibres is that they allow the vessels to expand when blood is ejected intermittently into them from the heart and to constrict again as blood flows out of them into the smaller vessels. The combination of a distensible large vessel and a downstream resistance (arterioles) transforms an intermittent cardiac ejection into a continuous capillary flow.
Small arteries and arteriolesThese are the resistance vessels of the circulation and are responsible for determining blood pressure. Arterioles are the vessels at the end of the arterial tree and have a diameter of 20 to 30 μm. Their particular significance is that they have very thick walls in relation to their diameters. Furthermore, the main constituent in their walls is smooth muscle, and the degree of contraction of this muscle regulates the diameter of the vessels and consequently the amount of blood flowing through them. Arterioles are responsible for the largest pressure drop in the circulation. Blood pressure in arteries typically varies from 120 to 80 mm Hg, depending on the phase of the cardiac cycle. In capillaries, the pulsatility is lost and pressure is only about 30 mm Hg.
The muscle in the walls of arterioles possesses an inherent tone. That means that they are normally partly contracted, reducing the size of the lumen to less than the widest possible. The degree of contraction is modified by factors external to the vessels. In particular, the chemical products that are formed as tissues use up energy — the ‘metabolites’ — reach the muscle fibres in the walls of the arterioles and cause them to relax and dilate. This local vasodilatation has the effect of matching local blood flow to tissue energy requirement.
Arterioles can also be regulated by nerves and hormones. These effects tend to be widespread and are concerned mainly with the regulation of arterial blood pressure. Sympathetic nerves have an important role in the control of arterioles. As the frequency of sympathetic nerve impulses increases, more of the transmitter, noradrenaline, is released at the nerve endings, and this causes arterioles to constrict. The adrenal glands also release noradrenaline into the blood but their secretion is mainly of adrenaline. Adrenaline also constricts blood vessels — except those in skeletal muscle, where it dilates them. This diverts blood to the muscle and prepares the body for emergencies as part of the ‘fight or flight’ response.
CapillariesThese are the ‘exchange vessels’, allowing passage of substances between blood and the fluids outside them which surround the body cells. They consist of a single layer of endothelial cells, with microscopic spaces between adjacent cells which allow the solutes of the blood, including salts, glucose, and dissolved oxygen, to pass into the tissues, and products of tissue metabolism, including carbon dioxide, to pass into the blood. The number of capillaries is so vast that even though they are microscopic their overall resistance to blood flow is low and blood passes through them slowly. The high density of capillaries means the distance for diffusion by the nutrients and gases is small. The more active tissues tend to have a denser supply of capillaries.
Capillaries are formed as a complex system of branching blood vessels between arterioles and venules (microscopic veins). Those near the arteries are at a higher pressure than those near veins. The gaps between endothelial cells are small enough to be almost impermeable to the protein molecules present in the blood, causing the capillary bed to function as a semipermeable membrane. These molecules exert an osmotic force which tends to draw fluid from the tissue spaces into the capillary. This is opposed by the hydrostatic pressure forcing fluid out. A dynamic equilibrium is established, such that at the higher pressure capillaries fluid leaves the circulation, and at the lower pressure ones it is drawn back in. An additional system of vessels, the lymphatics, are fine tubes which provide an alternative route for tissue fluid, via the lymph nodes and back to the circulation.
Disturbance of the balance of the fluid exchange at capillaries can lead to oedema, which is swelling caused by excess tissue fluid. Major causes of this are: a generalized increase in tissue fluid as in heart failure; obstruction to flow through veins or lymphatic vessels such as by cancer growths; and deficiency of blood protein, as in liver or kidney disease or malnutrition, which reduces the osmotic reabsorption force.
VeinsBlood returns from the tissues to the heart along veins. Larger veins possess valves which ensure that blood travels in the correct direction and prevents the development of undue back pressure. Sometimes the valves may cease to function, causing veins to distend abnormally and permanently. This is the cause of varicose veins.
Veins have another important role in addition to being conduits. Approximately 70% of the entire blood volume is contained within the veins, and these are very distensible. This means that they can readily accommodate quite large changes in their volume, either as a result of a change in the total quantity of blood in the circulation (haemorrhage or transfusion), or because of changes in blood distribution (leg veins distend on standing up, for example). The reason that veins can change their volume with little change in pressure is partly because they collapse when empty, which applies to veins above heart level. When filled, the elastic tissue in their walls is readily distensible, although expansion is eventually limited by the relatively indistensible fibrous tissue (collagen).
There is another, active, way in which the volume of blood in veins can be controlled: some veins have the ability to constrict in response to nerve stimulation. Sympathetic nerves supply smooth muscle in the vein walls, and an increase in sympathetic activity, resulting for example from a decreased stimulus to baroreceptors (falling blood pressure), causes venous volume to decrease. The effect of this is to increase filling of the heart and to enhance its output.
Pulmonary vesselsAlthough the total flow of blood per minute through the lungs is the same as that through the systemic circulation, the pressures are very much lower. Pressure in the pulmonary artery is typically 25/12 mm Hg (systolic/diastolic) compared with 120/80 mm Hg in the aorta and its main branches. The pressure in the lung vessels is lower because they are shorter, wider, have less muscle in their walls, and are very numerous. In particular, there are no muscular resistance vessels like those in the systemic circulation. The pulmonary vessels form a vast low-resistance capillary network which encircles the microscopic air-sacs (alveoli). Gas exchange — of oxygen for carbon dioxide — takes place between blood in the pulmonary capillaries and air in the alveoli.
See cardiovascular systemSee also blood circulation; blood pressure; body fluids; lymphatic system; microscopy.
The cardiovascular system includes the heart (cardio) and blood vessels (vascular). The heart pumps blood throughout the body. Sixty thousand miles of blood vessels transport the blood, enough to encircle Earth more than twice. Arteries carry blood away from the heart; capillaries reach all of the body's seventy trillion cells; and veins carry blood back to the heart. Because blood vessels form a circular route, this system is also called the circulatory system.
The cardiovascular system has two main parts. In the pulmonary circuit, blood is pumped from the right ventricle of the heart through the pulmonary arteries, which lead to the lungs. Here the blood gives up carbon dioxide and picks up oxygen. The oxygen-rich blood returns to the left atrium of the heart through pulmonary veins. From the left atrium, blood passes to the left ventricle of the heart, which pushes the blood through the systemic circuit beginning with the aorta, which branches to all body parts. After delivering oxygen and picking up carbon dioxide, blood returns to the right atrium of the heart and then to the right ventricle. The journey begins anew.
Thick walls enable arteries to withstand the pressure created by the pumping of the heart (blood pressure). The pulmonary arteries and the aorta are the largest arteries (the aorta is as wide as a thumb!). Some arteries are named for the organ that they supply, such as the hepatic artery (liver) and the coronary arteries (heart). Others have special names, such as the carotid arteries that supply the head and brain. Arteries branch many times into smaller arteries and eventually into minute branches called arterioles.
Arteries consist of an inner lining, one cell thick, called endothelium, a middle layer of smooth muscle and elastic tissue, and an outer layer that is mostly loose connective tissue , which holds the multilayered tube together. The muscle layer in arteries and arterioles is thick and the overall structure quite elastic, enabling these vessels to withstand greater blood pressure than can veins.
Veins and arteries are so similar that portions of veins are used to replace damaged arteries in coronary artery bypass surgery. Veins have the same three layers as arteries and are elastic, but they have a less-muscular middle layer, making their walls thinner. Also, unlike arteries, some veins have valves (tissue flaps) that permit blood to flow in only one direction, back to the heart. Valves help maintain blood flow in places such as the legs where the blood pressure has to push blood uphill, against the force of gravity. Despite the valves, accumulation of blood in leg veins can stretch the thin walls, resulting in varicose veins.
Veins are named in much the same way as arteries. Pulmonary veins return blood from the lungs to the heart, and a hepatic vein returns blood from the liver. Some veins have special names. The jugular veins return blood from the head, and the great saphenous veins return blood from the legs; these are used as grafts in coronary artery bypass surgery. The median cubital vein, which extends from side to side in the bend of the elbow, is a common site for drawing blood. The smallest veins arise from minute venules, and then merge to form larger and larger veins.
Capillaries are the shortest, narrowest, and thinnest blood vessels. They connect arterioles to venules to complete the circuit. Capillaries consist only of endothelium with some connective tissue binding the cells. Red blood cells squeeze through capillaries single file. Unlike arteries and veins, capillaries do not have specific names, but are named collectively for the region that they supply. Capillaries in the lungs, for example, are called pulmonary capillaries, and those in the stomach are the gastric capillaries.
The body will always have one heart, but the number of blood vessels may change. Because blood vessels bring oxygen-rich blood to cells, areas that have increased oxygen demands actually develop more blood vessels, primarily capillaries. New blood vessel growth is called angiogenesis. For example, new capillaries permeate the muscles of a conditioned athlete. Cancerous tumors also grow new capillary networks. One approach to fight cancer is to starve it with drugs that block angiogenesis.
see also Blood; Blood Clotting; Cardiovascular Diseases; Circulatory Systems; Heart and Circulation
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