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Definition

Blood vessels compose a continuous system of channels through which blood transports oxygen and nutrients to and waste materials from all body tissues.

Description

Structure

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

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.

Function

Blood pressure

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.

Capillary dynamics

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 semi-permeable 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 2001 "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 40.6% of all U.S. deaths in 1998; over 60 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 birth-marks 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.

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KEY TERMS

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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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Stephanie Islane Dionne