Blood Flow
Blood Flow and Pressure Exchange
Outline
Overview of circulation
Components of the Vascular system
Medical physics of blood flow
Vascular distensibility and compliance
Arterial damping of pressure pulses
Veins as reservoirs of blood
Capillary exchange
Learning Objectives
Know each component of the vascular system.
Understand blood flow using Ohms and Poiseuilles laws.
Know how vascular distensibility allows arteries to dampen pressure pulses and veins to act as reservoirs.
Know how hydrostatic and colloid osmotic forces determine the flow of fluid in the capillaries.
Components of the Vascular System - Blood Vessels
Closed circulatory system
Arteries
Arterioles
Capillaries
Venules
Veins
3 tunics
Lumen
Tunica interna
Endothelium
Connective tissue
Tunica mediaSmooth muscle
Elastin
Vasoconstriction
Vasodilation
Tunica externaCollagen fibers
Nerve fibers
Lymphatic vessels
Elastin fibers
Comparison of Veins and Arteries
Arteries:Veins:
Histological Structure of Blood Vessels
Arteries
Away from the heart
Thick, muscular walls
Very elastic
Arterioles
Diameter varies in response to neural stimuli and local chemical influences.
Capillaries
Consist of a single tunica interna
Gas, nutrient, and waste exchange
Brain capillaries
Blood-brain barrier
Capillary beds
Precapillary sphincter
Shunting of blood
Digestion
Venous System
Toward the heart
Venulesporousfree movement of fluids and white blood cells.
Veins
3 tunicsbut thin
Venous valves
Varicose veins
Incompetent valves
hemorrhoids
Maintenance of Blood Pressure
Neural control
Shunting and vasoconstriction.
Vasomotor center
BaroreceptorsCarotid and aorta
Chemoreceptors
Higher brain centers
HormonesCatecholoamines
Atrial natrietic peptide
ADH
Alcohol
Histamineother vasodilators
Hypertension
30% of people over 50
Damages arteries
Causes heart failure, vascular disease, renal failure, stroke, and blindness.
Enlargement falled by hypertrophy of the myocardium
Contributing factors:
Diet (sodium, saturated fat, cholesterol)
Obesity
Age
Race
Heredity
Stress
Smokingnicotine is a vasoconstrictor.
Atherosclerosis
Damage to the tunica interna
Viral
Bacterial
Hypertension
Reinjury
Inflammation
LDLsbad cholesterol
Foam cells
Fatty streak stage
Arteriosclerosis
Hypertension
Stroke
Heart attack
Coronary bypass
Angioplasty
tPAtissue plasminogen activator
Clot buster
HDLremoves cholesterol from vessel walls.
Arteries
Aortalargest artery
Ascending
Descending
Right and left coronary arteries
Common carotid arteriesbranch to form internal and external carotids
Externalsupply tissues of the head except the brain and orbits.
Internalsupply the orbits and most of the cerebrum.
Vertebral arteriesbranch to the cervical spinal cord, neck, cerebellum, pons, and inner ear.
Arteries to Know
Know the arteries on the proceeding chart plus:
Arteries of the armbrachial, radial, ulnar
Arteries of the legfemoral, popliteal, anterior tibial, posterior tibial
Be able to identify these arteries on a diagram. Also know the locations served by these arteries.
Veins
Dural sinusesveins of the brain drain into these enlarged chambers and drain to the internal jugular veins.
External jugular veinssuperficial head structures.
Vertebral veinscervical vertebrae and neck muscles.
Brachiocephalicmammary glands and first 2 or 3 intercostal spaces.
Veins to Know
Know the veins on the preceding chart plus:
The veins of the armscephalic, axillary, brachial, radial, ulnar.
The veins of the legsexternal iliac, femoral, popliteal, anterior tibial, posterior tibial, great saphenous vein, hepatic portal vein.
The great saphenous vein is a superficial vein. Connect with many of the deep veins of the legs and thighs.
Be able to identify these veins on a diagram. Also know the locations served by these arteries.
Overview of Circulatory System: Arteries + Veins and Everything in Between
Function of Circulatory System: To carry nutrients and hormones to tissues and wastes products away from tissues.
Basic Circulatory Function
Rate of blood flow to tissues changes based on need.
- e.g., during exercise, blood flow to skeletal muscle increases.- In most tissues, blood flow increases in proportion to the metabolism of that tissue.Cardiac output is mainly controlled by venous return.
Generally, arterial pressure is controlled independently of local blood flow or cardiac output control.
Parts of the Vasculature
Aorta receives blood from left ventricle.
Arteries transport under high pressure, strong vascular walls.
Arterioles control conduits, last branch of arterial system, strong muscular walls that can strongly constrict or dilate.
Capillaries exchange substances through pores.
Venules collect blood from capillaries.
Veins low pressure, transport blood back to the heart, controllable reservoir of extra blood.
Blood Volume and Vasculature Cross-Sectional Area
Cross-sectional area (cm2)Aorta 2.5Small Arteries 20Arterioles 40Capillaries 2500Venules 250Small Veins 80Venae Cavae 8
Normal Blood Pressures in Vasculature
Ohms Law Applied to Blood Flow
Blood Pressure
BP is the force exerted by the blood against the vessel wall.
- Typically measured as mm Hg.- E.g., 100 mm Hg is the force needed to push a column of Hg to a level of 100 mm.
Resistance
Resistance is the impediment to blood flow.
Not measured directly, but determined from pressure and flow measurements.
- If P = 1 mm Hg and F = 1 ml/sec, then R = 1 PRU (peripheral resistance unit).- In the adult systemic circulatory system, P = 100 mm Hg, and F = 100 ml/sec; so R = 1 PRU.- In the pulmonary system, P = 14 mm Hg and F = 100 ml/sec; so R = 0.14 PRU.
Conductance
Conductance is the opposite of resistance:
Conductance = 1/resistanceConductance may be easier to conceptualize than resistance and is sometimes easier to use in calculating the total resistance of parallel vessels.
Vessel Diameter and Blood Flow Changes in Resistance
Laminar Flow
Poiseuilles Law
Turbulant Flow
Adding Resistance in Series and Parallel
Effect of Viscosity on Resistance and Blood Flow
Summary of Blood Flow Physics
Vascular Distensibility
Vascular distensibility is the ability of the vascular system to expand with increased pressure, which
- Increases blood blow as pressure increases.- In arteries, averages out pulses.- Allows veins to act as reservoirs
Calculate Distensibility
Fractional increase in volume per rise in pressure:
Vascular = Increase in VolumeDistensibilityIncr in P x orig VolIf 1mm Hg increases a vessel from 10mm to 11mm, the distensibility would be 0.1 per mm Hg or 10% per mm Hg.
Distensibility of Arteries and Veins
Artery walls are much stronger than those of veins and thus, much less distensible.
The larger distensibility of veins allows them to act as blood reservoirs.
Vascular Compliance
The quantity of blood that can be stored in a particular portion of the vasculature for a rise in pressure:
Vascular compliance = Increase in volumeIncrease in pressureCompliance = distensibility x vol
Arterial and Venous Volume-Pressure Curves
Damping of Pulse Pressure in Arterial System
Athersclerosis Arteries become less Compliant
Veins
Can distend to hold large amounts of blood.
Contraction of skeletal muscles can constrict the veins and propel blood to the heart and increase cardiac output.
The contraction-induced constriction and the valves prevent the venous pressure from building up on the feet of standing adults.
Veins as Blood Reservoirs
> 60% of blood in thecirculatorysystemisin the veins.
When blood islost,sympatheticstimulation causes veins to constrictand make upfor the lost blood.
Conversely, veinscandistend tohold excess blood if toomuch is givenduringa transfusion.
The Distribution of Blood
Blood Volume
Distribution of H2O within the body:
Intracellular compartment:
2/3 of total body H2O within the cells.
Extracellular compartment:1/3 total body H2O.
80% interstitial fluid.
20% blood plasma.
Maintained by constant balance between H2O loss and gain.
Capillaries
Exchange nutrients and waste with tissues.
~ 10 billion capillaries with 500 700 m2 total surface area in whole body.
Capillaries are Porous
The exchange of water-soluble nutrients and waste between blood plasma and interstitial fluid occurs by diffusion through pores in the capillary walls.
Lipid-soluble substances pass directly through the capillary wall (e.g., O2 and CO2).
Capillary Structure
Capillary Exchange
Diffusion:
Filtration:
Reabsorption:
Capillary Exchange
Molecular Weight and Capillary Porosity
Colloid Osmotic Pressure
Starling force=(Pc + Pif) - (Pif + Pp)
Pc
Hydrostatic pressure in the capillary.
PifColloid osmotic pressure of the interstitial fluid.
Pif Hydrostatic pressure in the interstitial fluid.
PpColloid osmotic pressure of the blood plasma.
Volume of blood pumped/min. by each ventricle.
Pumping ability of the heart is a function of the beats/ min. and the volume of blood ejected per beat.
CO = SV x HRTotal blood volume = about 5.5 liters.
Each ventricle pumps the equivalent of the total blood volume/ min.
Cardiac Output (CO)
Recall the Frank-Starling Mechanism?
Forces Determining the Flow of Substances in the Capillary
Forces at Arterial End of Capillary
Forces at Venous End of Capillary
Mean Capillary Forces
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