TOPIC 4: Blood Vessels + Hemodynamics.
4.3.1: Compare + contrast the structure of the walls of arteries, capillaries
Arteries, arterioles, capillaries, venules, veins.
Arteries: blood away from the heart.
Veins: carry blood toward the heart.
Capillaries: directly serve cells.
Structure of Blood Vessel Walls (except thinnest vessels):
⇒ Tunica Intima: endothelium, subendothelial layer, internal elastic
membrane. Inner most.
⇒ Tunica Media: smooth muscle and elastic fibers, external elastic
membrane. Thicker than tunica externa.
⇒ Tunica Adventitia (externa): collagen fibers. Protect, anchor.
⇒ When not active, 60% of blood is in our veins.
⇒ Arteries have a smaller internal diameter.
4.3.2: Compare the 3 types of arterial vessels:
1.Elastic (Conducting) Arteries:
⇒ Thick walled, large diameter arteries near heart.
⇒ Highest proportion of elastin.
⇒ Smooth out pressure fluctuations.
⇒ Recoil helps to maintain pressure and flow of blood.
⇒ Aorta, bigger, stretch and recoil. 2.Muscular (Distributing) Arteries:
⇒ Deliver blood to specific organs.
⇒ More smooth muscle vs. elastin.
⇒ Smooth muscle in the walls.
⇒ Control where the blood is going at any moment in time.
⇒ Tunica media.
⇒ Single layer in smallest arterioles.
⇒ Arterioles determine which capillary beds flushed, minute to
⇒ The blood has travelled a lot of distance and developed
resistance, this allows blood pressure to drop, the blood
pressure is a lot lower in arteries.
4.3.3: Define microcirculation and compare the 3 types of capillaries.
Walls only a thin tunica intima.
Skin and muscles.
⇒ Skin and muscle.
⇒ Uninterrupted lining intercellular clefts allow limited
passage of fluids, small solutes.
⇒ Least leaky.
⇒ Not so much proteins or cells.
⇒ Transport goes through endothelial cells and transport
proteins are needed.
⇒ Similar but endothelial cells riddles with pores
⇒ Increased permeability to fluids and small solutes.
⇒ Small intestine, endocrine organs, kidney.
⇒ Highly modified, leaky capillaries (liver, bone marrow,
lymphoid tissues, endocrine organs).
⇒ Large, irregular lumens + usually fenestrated.
⇒ Few tight junctions. Absorption, secretion.
⇒ Large clefts: passage of proteins + RBC. Microcirculation: flow of blood from an arteriole to a venule through
a capillary bed.
♥ Vascular Shunt: metarteriole + throughfare channel.
♥ True Capillaries: actual exchange vessels.
♥ Precapillary sphincter: surrounds root of each true capillary.
4.3.4: Describe the structure and functions of venules + veins:
I. Venules: ⇒ Post capillary venules: just endothelium + a few fibroblasts.
⇒ Larger venules have spare tunica media and thin tunica
⇒ 3 Tunics, but walls are thinner and lumens are larger.
⇒ Relatively little elastin/smooth muscle in tunica media.
⇒ Tunica adventitia is heaviest layer.
⇒ Up to 65% of blood in veins at any or one time= capacitance
vessels or blood reservoirs.
⇒ Valves and larger internal diameters promote blood return.
4.3.5: Define: blood flow, blood pressure, resistance, and peripheral
A. Some Terms:
⇒ Blood Flow:
♥ What volume of blood passing through a particular area
in a specific point in time per unit time.
♥ Can be regulated independently for various tissues +
⇒ Blood Pressure:
♥ Force per unit area exerted on the wall of a blood vessel
by its contained blood. (mm Hg)
♥ Refers to systemic arterial blood pressure in the largest
arteries near the heart.
♥ Ex: average aortic bp = 90 mm Hg.
⇒ FLOW OCCURS IN RESPONSE TO A PRESSURE
⇒ F= P1 – P2 / R
⇒ Ex: 90 ▯90. (no flow).
⇒ Ex: 90 ▯30. (flow)
4.3.6: Illustrate the changes in blood pressure as blood travels through the
various vessels of the circulatory system.
⇒ Blood Flow = distance in blood pressure/ peripheral resistance. ⇒ Resistance: the major determinant of blood flow because a change in
blood vessel radius increases resistance to the 4 power.
⇒ Arterioles: can regulate their diameters.
4.3.7: Explain the factors that effect resistance; justify the importance of the
arterioles in the control of peripheral resistance.
Resistance: measures total of frictional forces that impede flow; flow
and resistance are inversely related.
Resistance varies inversely with radius.
Resistance is influences by: blood viscosity, vessel length/diameter.
⇒ Blood Viscosity: due to formed elements, plasma proteins.
⇒ Total Blood Vessel Length: more length=more resistance. (ex:
extra adipose tissue).
⇒ Blood Vessel Diameter: can be regulated; fluid not touching
walls moves faster.
Velocity of Blood Flow:
⇒ Velocity ~ 1/total cross sectional area.
2.20: Define Systolic + Diastolic Arterial Pressure, Pulse Pressure, MAP.
Pulse Pressure= systolic bp – diastolic bp.
⇒ Indicates vigor of contraction of ventricle.
⇒ Provides info on elasticity of aorta + major arteries. Mean Arterial Pressure (MAP)= diastolic pressure + 1/3 pulse
⇒ Pressure that propels blood to tissues during the cardiac cycle.
⇒ Not mean of systolic and diastolic pressures; diastole is longer
than ventricular systole.
⇒ Sample Calc:
♥ Bp= 120/80 mm Hg : MAP= 80+40/3 = 93 mm Hg.
⇒ MAP and pulse pressure decrease with distance from heart; by
the end of arterial tree, blood flow is steady and pulse pressure
4.3.9: Be aware of blood pressure in capillary beds.
Low pressure: 35 mm Hg at entry and 15 mm Hg by the end.
⇒ Capillaries are fragile; high pressure would rupture them.
⇒ Most capillaries are extremely permeable; lots of exchange at
low blood pressure.
Venous Blood Pressure:
⇒ Steady, changes very little during cardiac cycle. (gradient only
~20 mm Hg)
⇒ Factors Aiding Venous Return:
1)Respiratory Pump: As the blood is circulating, the
respiratory pump gives it a push.
2)Muscular Pump: Moving our body around, swollen feet,
4.3.10: Express blood pressure in terms of cardiac output + peripheral
Homeostatic mechanisms in place to maintain adequate flow to tissues
under a variety of circumstances.
Main factors influencing blood pressure are:
⇒ Cardiac Output.
⇒ Peripheral Resistance.
⇒ Blood Volume.
Blood pressure = cardiac output x peripheral resistance
What Determines Cardiac Output? Heart rate (neural + hormonal
factors) Stroke Volume (venous return, neural + hormonal factors).
Local Blood Flow: you want to open up blood vessels (vasodilate)
If you want to move blood faster you vasoconstrict.
When you are exercising, you vasoconstrict to move blood faster and
at the same time you vasodilate in your muscles.
What is pulse? How do you take a pulse? ⇒ Pressure wave due to alternating expansion + recoil of elastic
⇒ Push surface artery against firm tissues (usually radial pulse)
⇒ Also called pressure points because they can reduce blood flow
when pressed on.
⇒ Auscultatory Method: brachial artery using
⇒ Sounds of Korotokoff: as you start to let the pressure off, when
it goes below systolic pressure, there will be enough pressure to
go through the artery.
⇒ Cuff fully inflated: artery full collapsed ▯no flow)
⇒ Cuff pressure lowered to decrease peak systolic pressure (this is
⇒ Cuff pressure lowered to less than diastolic pressure.
⇒ Normal SP: 110140.
⇒ Normal DP= 7580.
4.3.11: Describe the shortterm neural + chemical mechanisms for the
regulation of blood pressure.
1) Alter blood distribution to respond to specific demands.
2) Maintain adequate MAP by altering blood vessel diameter.
Usually reflex arcs involving baroreceptors, vasomotor centre of
medulla, vascular smooth muscles.
1) Baroreceptors: respond to pressures.
2) Vasomotor Centre: responsible to regulate what’s going on with
blood vessel diameter.
3) Vascular Smooth Muscle: relaxation or constriction of blood
Short term neural mechanism: keeps you from passing out or blowing
Long term: platelets and stuff. AN INCREASE IN CO WILL LEAD TO AN INCREASE IN MAP.
Vasomotor Tone + Vasomotor Centre:
Cluster of sympathetic neurons in the medulla.