blood vessels Flashcards
Bblood vessels
- examples
- when aren’t arteries carrying oxygenated blood?
- delivery system of dynamic structures that begins and ends at the heart
- arteries, capillaries, veins
- pulmonary circulation and umbilical vessels of fetus
structures of blood vessel walls
- lumen
- tunica intima
- tunica media
- tunica externa
capilaries = endothelium with sparse basal lamina
Tunica intima
- endothelium lines lumen of all vessels
- continuous with endocardium
- slick surface reduces friction
- subendothelial layer in cessels larger than 1 mm = CT basement membrane
- internal elastic lamina in arteries
Tunica media
smooth muscle and sheets of elastin
-sympathetic vasomotor nerve fibers control vasoconstriction and vasodilation of vessels (affects blood flow and pressure)
Tunica externa (tunica adventitia)
- collagen fibers protect and reinforce; anchor to surrounding structures
- contain nerve fibers and lymphatic vessels
- vaso vasorum of larger vessels nourishes external layer
Elastic arteries
- “conducting arteries”
- large, thick-walled arteries with elastin in all three tunics
- aorta and its major branches
- large lumen offers low resistance
- inactive in vasoconstriction
- acts as pressure reservoirs - expand and recoil as blood ejected from heart
- smooth pressure downstream
- diameter = 1.5 cm; thickness = 1 mm
muscular arteries
- “distributing arteries”
- distal to elastic arteries
- deliver blood to body organs
- thick tunica media with more smooth muscle
- active in vasoconstriction
- diameter = 6 mm; thickness = 1 mm
arterioles
- -“resistance vessels”
- smallest arteries
- lead to capillary beds
- control flow into capillary bed via vasodilation and vasoconstriction
- diameter = 37 um; thickness = 6 um
capillaries
-walls and size
- microscopic blood vessels
- walls of thin tunica intima
- in the smallest, one cell forms entire circumference
- diameter allows only one rbc to pass at a time
capillaries
-where?
function?
in all tissues except cartilage, epithelia, cornea, and lens of eye
- provide direct access to almost every cell
- func: exchange of gases, nutrients, wastes, hormones, etc… bt blood and interstitial fluid
Three types of capillaries
- continuous= least permeable; most common; tight junctions; small intercellular cleft; skin and muscle
- fenestrated = large fenestration; tight junctions; intercellular cleft; kidney and small intestine
- sinusoid = few tight junctions; pores and intercellular cleft; big lumen; macrophages in lining; liver and bone marrow and spleen and adrenal medulla
blood flow in sinusoid capillaries
- slugginsh –> allows modification
- big molecules and blood cells pass bt blood and surrounding tissues
Microcirculation in capillary beds
- interwoven networks of capillaries between arterioles and venules
- teminal arteriole –> metarteriole –> throughfare channel –> postcapillary venule
two types of vessels in capillary beds
- vascular shunt (metarteriole to thoroughfare channel) - directly connects terminal arteirole and postcapillary venule
- true capillaries = 10-100 exchange vessels per capillary bed –> branch off metarteriole or terminal arteriole and return to thoroughfare channel
precapillary sphincters
- regulate blood flow into true capillaries
- blood may go into true capillaries or to shunt
- regulated by local chemical conditions and vasomotor nerves
venules
- formed when capillary beds unite
- smallest postcapillary venules = endothelium and a few pericytes - very porous, allowing fluids and wbcs into tissues
- larger venules have 1-2 layers of smooth muscle cells
veins
- anatomy
- pressure
- formed when venules converge
- have thinner walls, bigger lumens than arteries
- BP is lower than arteries
- thin tunica media; thick tunica externa of collagen fibers and elastic networks
- “capacitance vessels” (blood reservoirs) w/ up to 65% of blood supply
% of blood everywhere
capillaries = 5% heart = 8% pulmonary blood vessels = 12% systemic arteries and arterioles = 15% systemic veins and venules = 60%
vein adaptations to ensure return of blood to heart despite low pressure
- big diameter lumens offer little resistance
- venous valves prevent backflow of blood (most abundant in veins of limbs)
- venous sinuses = flattened veins with extremely thin walls (coronary sinus of the heart and dural sinuses of the brain)
dimmensions of capillary, venule and vein
capillary: D = 9 um; T = 0.5 um
venule: D = 20 um; T = 1 um
vein: D = 5 mm; T = 0.5 mm
vascular anastomoses
- interconnections of blood vessels
- Arterial anastomoses provide alternate pathways (collateral channels) to a given body region (joinds, abdominatl orgains, brain, and heart –> NOT retina, kidney or spleen)
- Venous anastomoses are common
Blood flow
- volume of blood flowing through a vessel, organ, or entire circulation in given period
- measured in ml/min
- CO for entire system
- constant at rest
- varies widely through individual organs based on needs
blood flow vs blood pressure vs resistance
- blood flow (F) is directly proportional to blood pressure gradient (delta P)
- Blood flow inversely proportional to peripheral resistance (R)
- F = (delta P)/(R)
- R is more important in influencing local blood flow because its easily changed by altering blood vessel diameter
Resistance
- definition
- sources
- opposition to flow
- amount of friction blood encounters with vessel walls, generally in peripheral (systemic) circulation
- 3 sources:
1. blood viscosity
2. total blood vessel length (every 2.2 lb of adipose tissue contains 400 miles of blood)
3. blood vessel diameter
factors affecting resistance that remain relatively constant
blood viscosity
- “stickiness” of blood due to formed elements and plasma proteins
- increased viscosity = increased resistance
blood vessel length
-longer vessel = greater resistance encountered
how does blood vessel diameter impact resistance (use math)?
resistance varies inversely with the fourth power of vessel radius
-e.g. if radius doubles, resistance is 1/16 what it used to be
Atherosclerosis
- increase in plaque in blood vessels
- increases resistance
- disrupts laminar flow and causes turbulent flow
- irregular fluid motion –> increased resistance
Blood pressure
- force per unit area exerted on wall of blood vessel by blood
- pressure results when flow is opposed by resistance
- mm Hg
- measured as systemic arterial BP in large arteries near heart
- Pressure gradient provides driving force that keeps blood moving from higher to lower pressure areas
systemic blood pressure
- highest in aorta
- declines throughout pathway
- 0 mm Hg in right atrium
- steepest drop occurs in arterioles
Arterial blood pressure
-what it reflects
reflects 2 factors of arteries close to heart:
- elasticity (compliance or distensibility)
- volume of blood forced into them at any time
-BP near heart is “pulsatile”
Arterial BP
- systolic pressure
- diastolic pressure
- pulse pressure
Systolic = pressure exerted in aorta during ventricular contraction (about 120 mm Hg in normal adult)
Diastolic = lowest level of aortic pressure
Pulse pressure = dif bt systolic and diastolic pressure
-“pulse” = throbbing of arteries
Mean arterial pressure
- pressure that propels blood into tissues
- MAP = diastolic pressure + 1/3 pulse pressure
- pulse pressure and MAP both decline with increasing distance from heart
- BP: 120/80 and MAP = 93 mm Hg near heart
capillary blood pressure
- ranges from 17 to 35 mm Hg
- low capillary pressure is desirable
- high BP would rupture the thin walls
- most are very permeable, so low pressure forcesfiltrate into interstitial spaces
venous BP
- changes little during cardiac cycle
- smallest pressure gradient: about 15 mm Hg
- low pressure due to cumulative effects of peripheral resistance (energy of blood pressure lost as heat during each circuit)
Factors aiding venous return
- muscular pump: contraction of skeletal muscles “milks” blood toward heart; valves prevent backflow
- Respiratory pump: pressure changes during breathing move blood toward heart by squeezing abdominal veins as thoracic veins expand
- Venoconstriction: under sympathetic control pushes blood toward heart
Maintaining BP
- requirements
- factors influencing BP
Requirements
- cooperation of heart, blood vessels, and kidneys
- supervision by brain
Main factors influencing BP
- Cardiac output (CO)
- Peripheral resistance (PR)
- Blood volume
- change in one variable quickly compensated for by change in another
