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Blood Flow and Pressure Exchange

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.

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Blood Flow and Pressure Exchange

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  1. Blood Flow and Pressure Exchange

  2. 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

  3. 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.

  4. 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 Components of the Vascular System - Blood Vessels

  5. Comparison of Veins and Arteries Arteries: Veins:

  6. Histological Structure of Blood Vessels

  7. 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 Arteries

  8. 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 Atrialnatrietic peptide ADH Alcohol Histamine—other vasodilators Venous System

  9. 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. Hypertension

  10. 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. Atherosclerosis

  11. 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.

  12. 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.

  13. 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.

  14. 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.

  15. 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.

  16. 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.

  17. 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.

  18. 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 VenaeCavae8

  19. Normal Blood Pressures in Vasculature

  20. Ohm’s Law Applied to Blood Flow

  21. 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.

  22. 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.

  23. 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.

  24. Vessel Diameter and Blood Flow – Changes in Resistance

  25. Laminar Flow

  26. Poiseuille’s Law

  27. Turbulant Flow

  28. Adding Resistance in Series and Parallel

  29. Effect of Viscosity on Resistance and Blood Flow

  30. Summary of Blood Flow Physics

  31. 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

  32. Calculate Distensibility • Fractional increase in volume per rise in pressure: Vascular = Increase in Volume DistensibilityIncr in P xorigVol If  1mm  Hg  increases  a  vessel  from  10mm  to   11mm,  the  distensibility  would  be  0.1  per  mm   Hg  or  10%  per  mm  Hg.

  33. 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.

  34. 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 =distensibilityxvol

  35. Arterial and Venous Volume-Pressure Curves

  36. Damping of Pulse Pressure in Arterial System

  37. Athersclerosis – Arteries become less Compliant

  38. 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.

  39. 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.

  40. The Distribution of Blood

  41. 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.

  42. Capillaries • Exchange  nutrients  and  waste  with  tissues. • ~ 10  billion  capillaries  with  500 – 700 m2 total   surface  area  in  whole  body.

  43. 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).

  44. Capillary Structure

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