21-1: Arteries, which are elastic or muscular, and veins, which contain valves, have three-layered walls; capillaries have thin walls with only one layer
measuring pressure
blood pressure (BP): arterial pressure (mm Hg) capillary hydrostatic pressure (CHP): pressure within the capillary beds venous pressure: pressure in the venous system
arteries
carry blood away from the heart
veins
collect blood from capillaries and return it ti the heart compared to arteries, veins have larger diameters, thinner walls, and lower blood pressure
capillary beds (capillary plexus)
connect one arteriole and one venule pre capillary sphincter, which guards entrance to each capillary, and opens and closes causing capillary blood to flow in pulses
vasoconstriction
contraction of smooth muscle in arteriole walls increases blood pressure
thoroughfare channel
direct connection between arterioles and venules within the capillary bed
vascular resistance
due to friction between blood and the vessel wall dependent on vessel length (constant) and diameter (adjustable): resistance (R) increases as vessel diameter decreases
arteries
elasticity: allows arteries to absorb pressure waves that come with each heartbeat contractility: arteries change diameter controlled by sympathetic division of ANS vasoconstriction vasodilation
capillary structure
endothelial tube, inside thin basement membrane no tunica media or externa diameter is similar to that of a red blood cell
total capillary blood flow
equals cardiac output is determined by: pressure (p) and resistance (r) in the cardiovascular system
venous valves
folds of tunica intima prevent backflow of blood compression of veins pushes blood toward heart when walls of veins near the valves weaken, varicose veins or hemorrhoids may result
angiogenesis
formation of new blood vessels stimulated by vascular endothelial growth factor (VEGF) occurs in the embryo as organs develop occurs in response to factors released by cells that are hypoxic, or oxygen-starved which is most important in cardiac muscle, in response to a chronically constricted or occluded vessel
continuous capillaries
found in all tissues except epithelia and cartilage have complete endothelial lining permit diffusion of water, small solutes, and lipid soluble materials, which block blood cells and plasma proteins specialized continuous capillaries in CNS and thymus have very restricted permeability (e.g. blood brain barrier)
from heart to capillaries, arteries change
from elastic arteries to muscular arteries to arterioles
pressure (P)
generated by the heart to overcome resistance absolute pressure is less important than pressure gradient
large veins
have all 3 tunica layers thick tunica externa thin tunica media
sinusoids (sinusoidal capillaries)
have gaps between adjacent endothelial cells permit free exchange between water and large plasma proteins found in liver, spleen, bone marrow, endocrine organs proteins phagocytic cells monitor blood at sinusoids
fenestrated capillaries
have pores in endothelial lining permit rapid exchange of water and larger solutes found in choroid plexus, endocrine organs, kidneys, and intestinal tract
the distribution of blood
heart, arteries, & capillaries: 30-35% of blood volume venous system: 60-65%: 1/3 of venous blood is in the large venous networks of the liver, bone marrow, and skin
systemic veins constrict (vasoconstriction) in response to blood loss...
increasing amount of blood n arterial system and capillaries
elastic arteries (conducting arteries)
large vessels (e.g., pulmonary trunk and aorta) tunica media has many elastic fibers and few muscle cells elasticity evens out pulse force transport large volumes of blood away from the heart.
capillary function
location of all exchange functions of cardiovascular system materials diffuse between blood and interstitial fluid
muscular arteries (distributing arteries)
most arteries are medium-sized muscular arteries tunica media has many muscle cells
collateral
multiple arteries that contribute to one capillary bed allow circulation if one artery is blocked arterial anastomosis, which is the fusion of two collateral arteries arteriovenous anastomoses, which is a direct connection between arterioles and venues, and bypasses the capillary bed
circulatory pressure
must overcome total peripheral resistance: resistance (R) of entire cardiovascular system ∆P across the systemic circuit is about 85 mm Hg
tunica externa
outer layer of a blood vessel which connects it to surrounding tissues contains collagen fibers, elastic fibers, and smooth muscle cells vaso vasorum ("vessels of vessels") are small arteries in walls of large arteries and veins, they serve cells of tunica media and tunica externa
flow (F)
proportional to the pressure gradient (∆P), divided by resistance (R)
vasodilation
relaxation of arterial smooth muscle enlarging the lumen
blood viscosity
resistance (R) caused by molecules and suspended materials in a liquid whole blood viscosity is about 4x that of water
veins
return blood to the heart
arterioles (resistance walls)
small vessels have little or no tunica externa have thin or incomplete tunica media
capillaries
smallest blood vessels with thin walls location of exchange between blood and interstitial fluid
arterioles
smallest branches of arteries that lead to capillary beds
venules
smallest branches of veins that collect blood from capillaries
capillaries
smallest vessels with thin walls microscopic capillary networks permeate all active tissues
turbulence
swirling action that disturbs smooth flow of liquid occurs in heart chambers and great vessels atherosclerotic plaques cause abnormal turbulence
capacitance of a blood vessel
the ability to stretch relationship between blood volume and blood pressure veins (capacitance vessels) stretch more than arteries, which act as blood reservoirs
pressure gradient (∆P)
the difference in pressure from one end of a vessel to the other
tunica intima
the innermost layer of a blood vessel, composed of a single layer of squamous epithelial cells over a sheet of connective tissue its smooth, frictionless surface allows blood to flow smoothly through the vessel
tunica media
the middle and thickest layer of tissue of a blood vessel wall, composed of elastic tissue and smooth muscle cells that allow the vessel to expand or contract in response to changes in blood pressure and tissue demand
medium-sized veins
thin tunica media and few small smooth muscle cells tunica externa contains longitudinal bundles of elastic fibers
vessel walls have three layers
tunica intima, tunica media, tunica externa
total peripheral resistance is affected by
vascular resistance blood viscosity turbulence
venules
very small veins collect blood from capillaries
aneurysm
a bulge in an arterial wall caused by weak spot in elastic fibers pressure may rupture vessel
vasoconstriction and vasodilation affect...
after load on heart, peripheral blood pressure, capillary blood flow
differences between arteries and veins
arterial walls are thicker than a vein's walls arteries have higher blood pressure than veins a constricted artery has a small, round lumen and the endothelium is folded whereas a vein has a large, irregular lumen arteries are generally smaller in diameter than veins arteries are more elastic than veins arteries do not have valves, veins have valves