Chapter 14 Overview of the Circulation Medical Physics

Chapter 14: Overview of the Circulation; Medical Physics of Pressure, Flow, and Resistance

Overall Objectives ü Physical characteristics of the circulation: distribution of blood volume total cross sectional area velocity blood pressure ü Determinants of blood flow ü Define and calculate blood flow, resistance, and pressure ü Define and calculate conductance ü Know Poiseulle’s law

The Circulatory System Services the Needs of the Tissues • Transporting nutrients to the tissues • Transporting waste products away from the tissues • Transporting hormones

Components of the Circulation .

The Circulatory System is Composed of the Systemic and Cardiopulmonary Circulation • Systemic Circulation - Serves all tissues except the lungs - Contains 84% of blood volume - Also called the peripheral circulation • Pulmonary Circulation - Serves the lungs - Lungs contain 9% of blood volume and heart 7%

The Capillaries Have the Largest Total Cross-sectional Area of the Circulation cm Aorta Small Arterioles Capillaries Venules Small Veins Venae Cavae 2. 5 20 40 250 80 8

Velocity of Blood Flow is Greatest in the Aorta Velocity of Blood Flow = Blood Flow Cross sectional area Aorta >Arterioles> Small veins >Capillaries

The Majority of Blood Volume is in the Veins .

Blood Pressure Profile in the Circulatory System 20 0 Systemic Pulmonary viens Venules Capillaries Arterioles Venae cavae Large viens Small viens Venules Capillaries 40 Small arteries Arterioles 60 Large arteries 80 Aorta Pressure (mm. Hg) 100 Pulmonary arteries 120 Pulmonary • High pressures in the arterial tree • Low pressures in the venous side of the circulation • Large pressure drop across the arteriolar-capillary junction

Basic Theory of Circulatory Function • Blood flow to tissues is controlled in relation to tissue needs. • Cardiac output is mainly controlled by local tissue flow. • Arterial pressure is controlled independent of either local blood flow control or cardiac output control.

Variations in Tissue Blood Flow ml/min/ 100 gm 50 70 25 360 95 Per cent ml/min Brain 14 700 Heart 4 200 Bronchi 2 100 Kidneys 22 1100 Liver 27 1350 Portal (21) (1050) Arterial (6) (300) Muscle (inactive state) 15 750 4 Bone 5 250 3 Skin (cool weather) 6 300 3 Thyroid gland 1 50 160 Adrenal glands 0. 5 25 300 Other tissues 3. 5 175 1. 3 Total 100. 0 5000 ---

Relationship between Pressure, Flow, and Resistance • Q= P/R • Flow (Q) through a blood vessel is determined by: • 1) The pressure difference ( P) between the two ends of the vessel • 2) Resistance (R) of the vessel

Determinants of Blood Flow FLOW = arterial - venous pressure ( P) resistance (R) 100 mm. Hg A 0 mm. Hg R =. 1 mm. Hg/ml/min B 20 mm. Hg R =. 1 mm. Hg/ml/min FLOW = 100 - 0 mm. Hg. 1 mm. Hg/ml/min FLOW = 100 - 20 mm. Hg. 1 mm. Hg/ml/min FLOW = 1000 ml/min FLOW = 800 ml/min

How Would a Decrease in Vascular Resistance Affect Blood Flow? FLOW = P RESISTANCE Conversely, FLOW = P RESISTANCE

Blood Flow • Blood flow is the quantity of blood that passes a given point in the circulation in a given period of time. • Unit of blood flow is usually expressed as milliliters (ml) or Liters (L) per minute. • Overall flow in the circulation of an adult is 5 liters/min which is the cardiac output. Blood Vessel

Characteristics of Blood Flow • Blood usually flows in streamlines with each layer of blood remaining the same distance from the wall, this type of flow is called laminar flow. – When laminar flow occurs, the velocity of blood in the center of the vessel is greater than that toward the outer edge creating a parabolic profile. Laminar flow Blood Vessel

Laminar Vs. Turbulent Blood Flow Causes of turbulent blood flow: • high velocities • sharp turns in the circulation • rough surfaces in the circulation • rapid narrowing of blood vessels Turbulent flow • Laminar flow is silent, whereas turbulent flow tend to cause murmurs. • Murmurs or bruits are important in diagnosing vessels stenosis, vessel shunts, and cardiac valvular lesions.

Effect of Wall Stress on Blood Vessels Turbulent flow increases wall stress Aortic Aneurysm Atherosclerosis

Blood Pressure • Blood pressure is the force exerted by the blood against any unit area of vessel wall. • Measured in millimeters of mercury (mm. Hg). A pressure of 100 mm. Hg means the force of blood was sufficient to push a column of mercury 100 mm high. • Low pressures are sometimes reported in units of mm of water. • 1 mm. Hg = 13. 6 mm of water

Resistance • Resistance is the impediment to blood flow in a vessel. • Resistance can be calculated by dividing the pressure difference between two points in a vessel by the vessel blood flow R = P = mm. Hg Q ml/min

Parallel and Serial Resistance Sites in the Circulation

Conductance • Conductance is a measure of the blood flow through a vessel for a given pressure difference. • Units ml/min per mm. Hg Poiseulle’s Law = Conductance = Q =_ Pr 4 8 l 1____ Resistance

Effect of Vessel Diameter on Blood Flow • Conductance is very sensitive to change in diameter of vessel. • The conductance of a vessel increases in proportion to the fourth power of the radius.

Hematocrit and Viscosity Effects on Blood Flow