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 Ohm’s and

Poiseuille’s 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 media– Smooth muscle– Elastin– Vasoconstriction– Vasodilation

• Tunica externa– Collagen 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• Venules—porous—free movement of fluids and white blood

cells.• Veins• 3 tunics—but thin• Venous valves• Varicose veins• Incompetent valves• hemorrhoids

• Maintenance of Blood Pressure– Neural control

• Shunting and vasoconstriction.

– Vasomotor center– Baroreceptors

• Carotid and aorta– Chemoreceptors– Higher brain centers– Hormones

• Catecholoamines• Atrial natrietic peptide• ADH

– Alcohol– Histamine—other 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– Smoking—nicotine is a vasoconstrictor.

Atherosclerosis

• Damage to the tunica interna– Viral– Bacterial– Hypertension

• Reinjury• Inflammation• LDLs—”bad cholesterol”• Foam cells• Fatty streak stage

• Arteriosclerosis• Hypertension• Stroke• Heart attack• Coronary bypass• Angioplasty• tPA—tissue plasminogen activator• Clot buster• HDL—removes cholesterol from vessel walls.

Arteries

• Aorta—largest artery– Ascending– Descending– Right and left coronary arteries– Common carotid arteries—branch to form internal and

external carotids• External—supply tissues of the head except the brain and orbits.• Internal—supply the orbits and most of the cerebrum.

– Vertebral arteries—branch 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 arm—brachial, radial, ulnar– Arteries of the leg—femoral, popliteal, anterior

tibial, posterior tibial– Be able to identify these arteries on a diagram.

Also know the locations served by these arteries.

Veins

• Dural sinuses—veins of the brain drain into these enlarged chambers and drain to the internal jugular veins.

• External jugular veins—superficial head structures.• Vertebral veins—cervical vertebrae and neck

muscles.• Brachiocephalic—mammary glands and first 2 or 3

intercostal spaces.

Veins to Know

• Know the veins on the preceding chart plus:– The veins of the arms—cephalic, axillary, brachial, radial,

ulnar.– The veins of the legs—external 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 2.5 Small Arteries 20 Arterioles 40 Capillaries 2500 Venules 250 Small Veins 80 Venae Cavae 8

Normal Blood Pressures in Vasculature

Ohm’s 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 ΔP = 1 mm Hg and F = 1 ml/sec, then R = 1 PRU (peripheral resistance unit).- In the adult systemic circulatory system, ΔP = 100 mm Hg, and F = 100 ml/sec; so R = 1 PRU.- In the pulmonary system, ΔP = 14 mm Hg and F = 100 ml/sec; so R = 0.14 PRU.

Conductance

• Conductance is the opposite of resistance:Conductance = 1/resistance

• Conductance 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

Poiseuille’s 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 VolumeDistensibility Incr 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 volume

Increase in pressure• Compliance = 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 the circulatory system is in  the veins.

• When blood is lost, sympathetic stimulation   causes veins to constrict and make up for  the   lost blood.

• Conversely, veins can distend to hold excess blood if too much is given during a   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.

• Pif

– Colloid osmotic pressure of the interstitial fluid.

• Pif – Hydrostatic pressure in the interstitial fluid.

• Pp

– Colloid 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 HR– Total 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