block 3 chap 14.pptx

Chapter 14: Introduction to the circulation and the mechanics of vessels: pressure, flow and resistance Function: Components: To service the Systemic or peripheral needs of the body tissues. circulation Pulmonary circulation Blood flow route: Pulmonary circulation  Right heart Left heart  ...

Chapter 14: Introduction to the circulation and the mechanics of vessels: pressure, flow and resistance Function: Components: To service the Systemic or peripheral needs of the body tissues. circulation Pulmonary circulation Blood flow route: Pulmonary circulation  Right heart Left heart   Vena cava Aorta   Large veins large arteries   small veins small arteries   arterioles  capillaries  venules Figure 14-1 Slide 1 Pulmonary circulation: Dr. William Harvey, 1628 Right heart  pulmonary artery  pulmonary capillaries pulmonary veins  left heart Characteristics of the vascular tree components Vessel Characteristics Cross-sectional Area (cm2) Aorta Arteries Arterioles 40 2.5 high pressure thick muscular wall 20 thinner, but still muscular wall “blood flow control conduits“ Capillaries nutrient exchange very thin wall Venules low pressure thin muscle wall collect blood from capillaries Veins low pressure 80 conduit for blood transport back to heart major reservoir for extra blood Slide 2 2500 250 Systolic Pulsatile Diastolic Pulsatile Low pressureAll needed for exposure to O2 and gas exchange Ave ~17 mmHg Prevent leakage of plasma But allow nutrient diffusion ~0 mmHg Figure 14-2 Volume 16% Arterial System High pressure Low volume Low compliance (∆V/∆P) Slide 3 4% 64% Venous System Low pressure High volume High compliance 4% Where is most of the blood volume? Where is the site of most vascular resistance? Where is the pressure lowest? Why? Why is the venous system high compliance (capacitance)? Basic Theory of Circulatory Function Goal of the circulatory system: deliver nutrients and remove wastes. At the same time, blood flow to each organ/tissue is precisely controlled and arterial pressure is controlled independently. 1. Rate of blood flow to each tissue is almost always precisely controlled in relation to tissue need. (BF = volume of blood that flows past a certain point during a specified amount of time) 2. Cardiac output is controlled mainly by the sum of all the local tissue flows. (CO = volume of blood pumped by the heart) 3. Generally, arterial pressure is controlled independently of either local blood flow control or cardiac output. Slide 4 (BP = force that drives the blood throughout the body) Pressure, Flow and Resistance #1 Figure 14-3 ∆P = P1 - P2 Ohms Law ∆P = F x R or F = ∆P/R #2 Blood Flow through a vessel is determined by: # 1. Pressure difference (∆P) between 2 ends F = blood flow ∆P = pressure difference R = resistance # 2. Impediment of flow through vessel (Resistance) Friction •Pressure, flow, and resistance are related. •Pressure drops along a resistance vessel. Slide 5 Pressure, Flow and Resistance #1 Figure 14-3 ∆P = P1 - P2 Ohms Law ∆P = F x R or F = ∆P/R #2 Blood Flow through a vessel is determined by: # 1. Pressure difference (∆P) between 2 ends F = blood flow ∆P = pressure difference R = resistance # 2. Impediment of flow through vessel (Resistance) •Pressure, flow, and resistance are related. •Pressure drops along a resistance vessel. Figure 14-2 Slide 6 Blood Flow: Quantity of blood that passes a certain point at a given period of time. (ml/min) Laminar Flow – when blood flows at a steady rate: flows in streamline. • Each layer of blood remains the same distance from wall. • Type of flow = laminar or streamline. Turbulent Flow – blood flowing in all directions, continually mixing. No flow Laminar Turbulent Figure 14-6 Slide 7 “Parabolic”- molecules in contact with wall barely move, next layers slip over preceding layers. Thus, center layer flows most rapidly. Can occur with obstruction, turn, rough surface. Flow no longer linear or streamline. Develop eddy currents which will increase resistance to flow. Resistance to Blood Flow F = ∆P/R Resistance: Impediment to blood flow in a vessel. Cannot be measured directly. R = ∆P/F Resistance of the entire circulatory system is called total peripheral resistance (TPR). Conductance: Measure of pressure difference. blood conductance = 1 / resistance Slide 8 flow through a vessel for a given Blood Flow Reynolds’ number (Re): measure of the tendency for turbulence to occur. Low Re = laminar flow Re = (flow velocity)(diameter)(density) / viscosity Re = v d p / n Density: amount of mass that can be contained in a volume Viscosity: a liquid’s resistance to flow Re decreases as blood viscosity increases. Anemia or ↓ blood viscosity can contribute to turbulence. Slide 9 Resistance to Blood Flow BF: Resistance most sensitive to changes in radius/diameter. 4th power Law: Arterioles control blood flow Different Blood Flows Arteriole Slight change in diameter can cause tremendous changes in ability to conduct blood. • A 4 fold increase in diameter leads to a 256fold decrease in resistance and a 256 increase in flow (44 = 256). (2 fold = 16 fold) • A 4 fold increase in vessel length leads to a 4-fold increase in resistance and a 4-fold decrease in flow. Figure 14-8 (2 fold = 2 fold) Poiseuille Law F = p ∆P r4 / 8 h L F = blood flow L = length ∆P = pressure difference h = viscosity r = radius Slide 10 Organization of Resistance Beds Determines Total Resistance (Rtotal ) Total peripheral resistance (Rtotal): Series Equals sum of resistance of entire peripheral circulation. Parallel Figure 14-9 Summation of Resistances: Series: R1 + R2 = Rtotal Parallel: 1/R1 + 1/R2 + 1/R3 + 1/R4 = 1/Rtotal Independent regulation of blood flow. Total R is less than the vessel with the lowest resistance. •Resistance beds in parallel offer less resistance vs. beds in series. •Loss of a resistance bed for a parallel circuit leads to an increase in total resistance. Slide 11 Parallel provides another pathway for conductance. Hematocrit and viscosity Hematocrit •percentage of cells in blood, typically RBCs •Anemia - too few •Polycythemia - too many Figure 14-10 Normal 38 (F) - 42 (M) Slide 12 Anemia Polycythemia Hematocrit and viscosity Poiseuille Law F = p ∆P r4 / 8 h L Hematocrit is an important determinant of viscosity.  Hematocrit   viscosity 4 fold viscosity of water Figure 14-11 Viscosity  less flow in vessel Normal 38 (F) - 42 (M) Slide 13 if all other factors are constant. Viscosity: force to make a fluid move Anemia and Dental Care Anemia can increase the risk of infection with a dental procedure. Antibiotic prophylaxis may be indicated before any invasive dental procedures to prevent possible infection risk. Because anemia reduces the number of red blood cells present, one common symptom is paleness in the gums, which some have come to consider anemia gums. Instead of a normal, healthy pink, they begin to take on a faded or even whiter shade of their normal color. Clinical Correlation Methods for Measuring Blood Flow Electromagnetic flowmeter- electrical voltage proportional to rate of flow of vessel placed in a magnetic field. Utilizes the generation of electromotive force in blood moving through a magnetic field. Ultrasonic doppler flowmeter – transmission of ultrasound frequency with the sound wave reflected off a moving blood cell proportional to flow. Slide 14 Systolic Pulsatile Diastolic Pulsatile Low pressureAll needed for exposure to O2 and gas exchange Ave ~17 mmHg Prevent leakage of plasma But allow nutrient diffusion ~0 mmHg Figure 14-2 Volume 16% Arterial System High pressure Low volume Low compliance (∆V/∆P) Slide 15 4% 64% Venous System Low pressure High volume High compliance 4% Where is most of the blood volume? Where is the site of most vascular resistance? Where is the pressure lowest? Why? Why is the venous system high compliance (capacitance)? Arterial pressure, vascular resistance and blood flow. 7 Ohms Law Blood flow (ml/min) 6 5 ∆P = F x R or F = ∆P/R 4 3 2 1 Distend 0 20 40 60 80 100 120 140 160 200 Arterial pressure (mm Hg) Slide 16 F = blood flow ∆P = pressure difference R = resistance Effect of pressure on vascular resistance and blood flow. The relationship between pressure and flow is not linear as one might expect based on the P = F x R equation. Dilates vessels and  blood flow. Vessels are not rigid. Constricts vessels and  blood flow. Figure 14-13 Slide 17 Increases in P increases the driving force for flow, but also distends vessels at the same time which decreases Resistance (R). Changes in flow can be caused by changes in sympathetic nerve stimulation. At a given P, increased stimulation decreases flow due to vasoconstriction.