Blood Vessel Terms


Aneurysm: A saclike enlargement of a blood vessel or heart chamber caused by a weakening of its wall.


Arteries: Vessels that carry blood away from the heart; they are oxygenated, except for arteries in the pulmonary circulation and umbilical cord of a fetus.


Arterioles: The smallest of arteries that regulate blood flow into capillary beds.


Atherosclerosis: Changes in the walls of large arteries consisting of lipid deposits on the artery walls; the early stage of arteriosclerosis.


Baroreceptor reflexes: Feedback mechanism to maintain blood pressure homeostasis, baroreceptors in the aortic arch and carotid sinus detect stretch caused by increased BP; impulses are sent to the CV center, which responds to high BP by reflexively causing vasodilation, and low BP by reflexively causing vasoconstriction.


Blood flow: The volume of blood flowing through a vessel per unit time, in L/min; fluid flow (F) is equal to the change in pressure (ΔP) divided by resistance (R), according to the formula: F = ΔP/R. Total blood flow through the entire cardiovascular system is equivalent to cardiac output (CO), and is equal to blood pressure (BP) divided by peripheral resistance (PR), according to the formula: CO = BP/PR


Blood pressure (BP): The force per unit area exerted by blood against a vessel wall, in units of mm Hg (torr). Systolic blood pressure (SBP) is the force exerted by blood on arterial walls during systole (ventricular contraction); SBP is the highest blood pressure measured in the large arteries, normally about 120 in a young adult. Diastolic blood pressure (DBP) is the force exerted by blood on arterial walls during diastole (ventricular relaxation); DBP is the lowest blood pressure measured in the large arteries, normally about 80 in a young adult. Hypertension is high blood pressure with SBP > 140 and DBP > 90. Hypotension is abnormally low blood pressure with SBP < 90 or DBP < 60.


Blood vessel walls: The tissue that forms the blood vessels, which consist of three layers: the tunica intima is the inner wall that contains an endothelial lining, a subendothelial connective tissue basement membrane, and an internal elastic lamina in vessels larger than 1 mm; the tunica media is the middle wall that contains smooth muscle, an external elastic lamina, and sympathetic vasomotor nerve fibers that control the vascular tone and produce vasoconstriction or vasodilation; the tunica externa is the outer wall that contains collagen and elastic fibers, which protects, reinforces and anchors the vessel to the surrounding structures; larger vessels contain a vasa vasorum (vessels of the vessels) to nourish the tissue in the outer and middle walls.


Capillaries: Microscopic blood vessels, with only a thin tunica intima and pericytes that help stabilize their walls and control permeability; capillaries function in the exchange of substances between the blood and interstitial fluid. There are three structural types: continuous, fenestrated, and sinusoidal capillaries.


Capillary beds: Interwoven networks of capillaries that form the microcirculation between arterioles and venules, and consist of two types of vessels. The true capillaries consist of 10 to 100 exchange vessels; the vascular shunt (metarteriole-thoroughfare channel) directly connects the terminal arteriole to a postcapillary venule, and has a precapillary sphincter which is a cuff of smooth muscle that surrounds each capillary at the metarteriole and acts as a valve to regulate blood flow into the true capillaries.


Capillary Net Filtration Pressure (NFP): All forces acting on a capillary bed, and equal to the difference of net hydrostatic pressure (HP) minus net osmotic pressure (OP), according to the formula: NFP = HPnet - OPnet .


Cardiovascular centers: Nuclei of the reticular formation in the medulla oblongata that include the cardiac centers and the vasomotor centers. Also called CV center.


Cardiac centers: Nuclei of the reticular formation in the medulla oblongata that regulate the heart; the cardioacceleratory center innervates the SA and AV nodes, myocardium and coronary arteries through the sympathetic division of the ANS; the cardioinhibitory center sends inhibitory nerve impulses to the SA and AV nodes through the parasympathetic division of the ANS.


Continuous capillaries: The most common capillaries, found in the skin, muscles, and brain; they contain tight junctions that connect endothelial cells, and intercellular clefts that allow passage of fluids and small solutes; the capillaries of the brain have complete tight junctions that form the blood-brain barrier.


Elastic arteries: Include the aorta and its major branches, which contain large lumens for low resistance, and large thick-walled arteries with elastin; they act as pressure reservoirs to expand, recoil and smooth out pressure fluctuations as blood is ejected from the heart. Also called conducting arteries.


Fenestrated capillaries: Function in absorption in the small intestines, and filtration in the kidneys; they are very permeable to fluids and solutes because they contain fenestrations (pores) and intercellular clefts.


Mean arterial pressure (MAP): The pressure that propels the blood to the tissues, measured as the average pressure in the large arteries near the heart, according to the formula: MAP = DBP + PP/3, where PP is the pulse pressure, measured as the difference of SBP minus DBP, according to the formula: PP = SBP DBP.


Muscular arteries: Deliver blood to the body organs, and have thick tunica media with more smooth muscle, making them more active in vasoconstriction. Also called distributing arteries.


Peripheral resistance (PR): The opposition to flow (resistance) encountered in the peripheral systemic circulation, which is dependent on the amount of friction between blood and the vessel wall. PR is equal to the blood viscosity (μ) times the total blood vessel length (L), divided by the blood vessel radius raised to the fourth power (r4 ), according to the formula: PR = μ L / r4 .


Sinusoidal capillaries: Found in the liver, spleen, and bone marrow; they are leaky capillaries with few tight junctions, many fenestrations, large intercellular clefts, and large lumens; they allow large molecules and blood cells to pass between the blood and surrounding tissues.


Varicose veins: Veins with permanent dilation and many twists, most commonly seen in the legs.


Vascular anastomoses: Connecting channels that allow blood to be supplied to arteries, or drained from veins, even if one channel is blocked; many are found in the brain and heart.


Vasomotor centers: Nuclei of the reticular formation in the medulla oblongata that control changes in the diameter of blood vessels (vascular tone) through the sympathetic division of the ANS; the vasopressor center produces vasoconstriction and an increase in blood pressure; the vasodepressor center produces vasodilation and a decrease in blood pressure.


Veins: Vessels that carry blood toward the heart; they are formed where venules converge, and have thin walls, large lumens, and valves to prevent backflow of blood; they are capacitance vessels that act as blood reservoirs and contain 60% of the blood supply.


Venous sinuses: Flattened veins with thin walls; found in heart (coronary sinus) and brain (dural sinuses).


Venules: Vessels that are formed where capillaries converge, and are very porous for reabsorption of fluids into the vessel, and for escape of white blood cells into the tissues.