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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