PHS 3342 Lecture Notes - Lecture 7: Tunica Media, Blood Vessel, Metarteriole
28 Apr 2018
School
Department
Course
Professor
Jan 30, 2018
Vascular Physiology
Types of Blood Vessels
Structure of vessels (in general): three layers
-Tunica interna:
•Single layer of endothelium on a basement membrane (inner layer)
•Internal elastic lamina (arteries only)
-Tunica media:
•Smooth muscle: regulates vessel diameter
•External elastic lamina (arteries only)
-Tunica externa: outermost layer of CT
Arteries: deliver blood away from the heart
-Smaller inner diameter than veins - contains extra layers of elastic fibres/more muscular walls
-Elastic arteries: aorta and main arteries branching off the aorta
•Large diameter
•Elastic fibres in tunica media
•Thin walls
•Show good stretch/recoil
-Muscular arteries: “distributing arteries”
•Medium diameter
•Muscular walls
•Vasoconstriction/vasodilation to distribute blood as needed, depending upon how active that tissue is at that
moment in time
Aterioles: deliver blood to capillaries
-Small diameter
-Not elastic - blood pressure has dropped significantly, no longer accommodating large volumes of blood and recoil
-Vasoconstriction/vasodilation to regulate capillary bed perfusion
Capillaries: microcirculation metarteriole to capillary bed, nutrient exchange
-Composed of a single layer of endothelial cells with a basement membrane wrapped around
-Vasomotion
1
Jan 30, 2018
Venules: collect blood from capillaries
-Small diameter
-Collect from capillaries
-Smallest are very porous (WBC entry)
Veins: deliver blood to the heart
-Large lumen - larger inner diameter than arteries but same outer diameter
•Increasing diameter helps encourage blood to enter veins - not needed in arteries because of the greater BP
-Not elastic, but are distensible
-Contains vales - prevents back flow of blood (especially important for counteracting gravity)
Blood vessel compliance: ability of a blood vessel wall to expand and contract passively with changes in pressure
-Important function of large arteries and veins
-Allows the aorta to almost act as a secondary pump
Definitions
Blood flow: volume of blood moving past a certain point during a given period of time
-Flow through systems in parallel can be differentially regulated (ex. GI tract vs. Brain)
-Flow occurs in response to pressure gradient - determined by pressure differentials
•Blood flows from areas of higher pressure to lower pressure
•Rate of flow is proportional to the pressure gradient - F ∝ ΔP
•Flow is determined by relative pressures, not absolute pressures
-Inversely related to total cross-sectional area
•CSA is smallest in aorta (flow is fastest), greatest in capillaries (flow is slowest - one RBC at a time)
Blood pressure: force that causes blood to flow through vessels (mmHg)
-NB: the greatest drop in blood pressure occurs from arteries to arterioles
•Great increase in resistance due to smaller diameter of the vessel - more blood will being rubbing against the
walls
Resistance: frictional forces that impede flow
-Inversely related to flow
-Determined by length/diameter of vessels, blood viscosity, etc.
-R = 8Ln/r4
•L = length of tube (length of circulatory pathway)
2
Jan 30, 2018
-Doesn’t tend to have much of an effect because this tends to remain constant day to day - may change over
time though (due to growing, increased fat deposition, etc.)
•n = fluid viscosity
-Usually doesn’t have much of an effect because the body tends to be good at regulating viscosity and
keeping it constant
•r = inner radius of the tube
-Radius has a HUGE effect on resistance - vasoconstriction, vasodilation
-Altering radius is used all the time to affect where blood is flowing - will take the path of least resistance
Pulse Pressure
Pulse pressure = systolic BP - diastolic BP
Indicates vigour of contraction of ventricle
Provides information on elasticity of aorta and major arteries
-Will stretch to accommodate the increase in pressure and recoil to go back
down
-Diastolic value is most important when looking at cardiovascular health - not
getting good recoil, vessels are losing elasticity, plaque may be building up —> atherosclerosis
Magnitude of pules pressure depends on:
-Stroke volume: increases systolic pressure = increased pulse pressure
-Arterial compliance: ability to recoil and give lower diastolic BP
Mean Arterial Pressure
MAP = ⅓ pulse pressure + diastolic pressure
Single value to give meaning of pulsatile pressure and give some indication of flow
-Mean pressure on the graph above
NB: does not represent the mean of systolic and diastolic because diastole is longer than systole
Ex. For BP of 120/80
-MAP = 80 + 40/3 = 93 mmHg
-F = arterial pressure/R = MAP/R
Arterioles: Major Source of Resistance to Blood Flow
Blood vessel diameter is the major determinant of resistance: R ∝ 1/r4
3
