Blood Vessels
Blood is carried in a closed system of vessels that begins and ends at the heart
The three major types of vessels are arteries, capillaries, and veins
Arteries carry blood away from the heart, veins carry blood toward the heart
Capillaries contact tissue cells and directly serve cellular needs
Generalized Structure of Blood Vessels
Arteries and veins are composed of three tunics- tunica interna, tunica media, and tunica externa
Lumen- central blood- containing space surrounded by tunics
Capillaries are composed of endothelium with sparse basal lamina
Tunics
Tunica interna (tunica intima)
Endothelial layer that lines the lumen of all vessels
In vessels larger than 1 mm, a subendothelial connective tissue basement membrane is present
Tunica media
Smooth muscle and elastic fiber layer, regulated by sympathetic nervous system
Controls vasoconstriction/vasodilation of vessels
Tunica externa (tunica adventitia)
Collagen fibers that protect and reinforce vessels
Larger vessels contain vasa vasorum
Elastic (conducting) Arteries
Thick-walled arteries near the heart; the aorta and its major branches
Large lumen allow low-resistance conduction of blood
Contain elastin in all three tunics
Withstrand and smooth out large blood pressure flutuations
Allow blood to flow fairly
Muscular (Distributing) Arteries and Arterioles
Muscular arteries- distal to elastic arteries; deliver blood to body organs
Have thick tunica media with more smooth muscle and less elastic tissue
Active in vasoconstriction
Arterioles- smalest srteries; lead to cappillay beds
Control flow into capillary beds via vasodilation and constriction
Capillaries
Capillaries are the smallest blood vessels
Walls consisting of a thin tunica interna, one cell thick
Allow only a single RBC to pass at a time
Pericytes on the outer surface stabilize their walls
There are three structural types of capillaries: continuous, fenestrated, and sinusoids
Venous System: Venules
Are formed when capillary beds unite
Allow fluids and WbCs to pass from the bloodstream to tissues
Postcapillary venules- smallest venuels, composed of endothelium and a few pericytes
Large venules have one or two layers of smooth muscle (tunica media)
Venous System: Veins
Veins are:
Formed when venules convege
Composed of three tunics, with a thin tunica media and a thick tunica externa consisting of a collagen fibers and elastic networks
Capacitance vessels (blood reservoirs) that contain
Veins have much lower blood pressure and thinner walls than arteries
To return blod to the heart, veins have special adaptations
Large-diameter lumens, which offer little resistance to flow
Valves (resembling semilunar heart valves), which are
Anastomoses
Blood will flow where it can
Blood Flow
Actual volume of blood flowing through a vessel, an organ, or the entire circulation in a givn period:
Is measured in ml per min
Is equivalent to cardiac output (CO), considering the entire vascular system
Is
Blood Pressure
Force per unit area exerted on the wall of a blood vessel by its contained blood
Expressed in millimeters of mercury (mmHg)
Measured in reference to systemic arterial BP in large arteries near the heart
The differences in BP within the vasclar system provide the driving forse that keeps blood moving from higher to lower pressure areas
Resistance
Resistance- opposition to flow
Measure of the amount of friction blood encounters as it passes through vessels
Generally encountered in the systemic circulation
Referred to as peripheral resistance (PR)
The three important sources of the resistance are blood viscosity, total blood vessel length, and blood vessel diameter
Resistance Factors: Viscosity and Vessel Lenght
Resistance factors that remain relatively constant are:
Blood vi
Blood Vessel Diameter
Changes in vessel diameter are frequent and significantly
Small-diameter arterioles are the major determinants of peripheral resistance
Fatty plaques from atherosclerosis:
Cause turbulent blood flow
Dramatically increase resistance due to turbulance
Blood Flow, Blood Pressure, and Resistance
blood flow (F) is directly proportional to the difference in blood pressure between two points in the circulation
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