Ch 15 Circulation.pdf

Chapter 15 : Vascular Distensibility and Functions of the Arterial and Venous Systems Vascular Distensibility (D): Ability to dilate in response to pressure (stretch). D = increase in volume increase in pressure x original volume Veins - most distensible (8x’s more) important “blood reservoir” fu...

Chapter 15 : Vascular Distensibility and Functions of the Arterial and Venous Systems Vascular Distensibility (D): Ability to dilate in response to pressure (stretch). D = increase in volume increase in pressure x original volume Veins - most distensible (8x’s more) important “blood reservoir” function Arteries – are less distensible than veins, arteries have stronger walls than veins; thus, the arterial wall can accommodate the pulsatile output of heart leading to smooth, continuous blood flow through smaller arteries. What accounts for difference in distensibility of arteries vs. veins? Slide 18 Anatomy What accounts for difference in distensibility of arteries vs. veins? Distensibility (D): Ability to dilate in response to pressure (to stretch). Compliance (C): Relationship between volume and pressure. Is the ability of a vessel to stretch and hold volume. C = Total Distensibility (D) x Volume Vascular C = increase in volume increase in pressure Compliance of a vein is 24 times that of its corresponding artery as it is 8 times as distensible. Veins ~3 times greater surface area It is more important to know the total quantity of blood that can be stored in any given portion of the circulation that to know the distensibility of the individual vessel. ↑ D with ↓volume = ↓ C ↓ D with ↑ volume = ↑ C Volume most important !! Slide 19 Effect of the Sympathetic Stimulation or Inhibition on Volume – Pressure Relationship Arterial Low compliance. Small ∆Volume leads to big ∆Pressure. Venous High compliance. Big ∆Volume leads to small ∆Pressure. Figure 15-1 Slide 20 Volume Pressure Curve Vascular compliance Although the veins are most compliant, arteries also have some compliance which influences cardiovascular function. Compliance: Is the ability of a vessel to stretch and hold volume. Stress Relaxation Vessel exposed to increase in volume at first has a large increase in pressure, but progressive delayed stretching of the vessel allows pressure to return to normal over time. Delayed Compliance Vascular compliance is one mechanism that is used to maintain blood pressure constant during changes in blood volume, ie. extra blood (transfusion) or hemorrhage. Vascular compliance: ability of vessel wall to expand and contract passively with changes in pressure (or volume). Slide 21 Example of vascular compliance in action. ↑ Volume→ ↑ Pressure Remove small volume Inject small volume Pressure will decrease with time with constant volume. Vascular compliance: ability of vessel wall to expand and Figure 15-2 Delayed compliance is due to a slow “adaptation” of length of fibers maintaining vascular tone. This figure demonstrates the effect on the intravascular pressure of injecting a small volume of blood into a venous segment and minutes later removing the excess blood. Slide 22 contract passively with changes in pressure (or volume). Arterial distensibility helps maintain continuous blood flow. Pressure trace from aorta during cardiac cycle. Systolic Distensibility: ability to respond to pressure. Pulse Pressure (PP) = Difference between Systolic – Diastolic (~40 mmHg) Diastolic Figure 15-3 What happens during ejection? Blood rushes out of left ventricle into aorta. The increase in blood volume/pressure stretches the aorta. This helps prevent systolic pressure from going too high. At the end of ejection, the drop in pressure leads to relaxation/recoiling of aorta wall and allows run-off of blood and prevents diastolic pressure from dropping too low. Arterial distensibility influences pulse pressure and allows continuous blood flow. Pulse pressure would be very high if vessels were not distensible--flow during systole, but not during diastole. ARTERIAL DISTENSIBILITY INFLUENCES SHAPE OF AORTA PRESSURE TRACE Slide 23 Factors affecting Pulse Pressure (PP) Stiff Vessel ¤C C = Compliance Factors that affect PP: 1. Compliance (¤ C ¢ £ PP) ¤ SV 2. Stroke volume (£ SV ¢ £ PP) SV = blood pumped from ventricle Pulse Pressure PP determined by ratio of Stroke Volume (SV) to C of arterial tree. Systolic Abnormal PP due to: Arteriosclerosis = stiff vessels (¯C) Diastolic Difference = PP Figure 15-4 Slide 24 Aortic stenosis = ¯ diameter of the aortic valve opening leading to diminished flow out (¯Stroke Volume) Patent ductus arteriosus = backward flow into ductus (hole) Aortic regurgitation = incomplete closing of (backward flow) absence or aortic valve Transmission of pressure wave in vascular tree. Ejection of blood from heart into proximal portion of aorta becomes extended a distended vessel a rising pressure a extension of distention along aorta. Progressively less The more compliant a vessel, the slower the transmission of the pressure wave. Figure 15-6 Dampening = progressive diminution of pulsations Dampening of pulse pressure wave due to: 1. Resistance of blood movement in vessels (must distend vessels as moves forward). 2. Compliance of vessels (more compliant, the greater the flow is needed to increase the pressure. Degree of dampening = C x R Slide 25 How to Measure Blood Pressure: Dr. Nikolai Korotkov, 1905 systolic diastolic Auscultatory or “cuff” method. Indirect Place stethoscope over antecubital artery. Raise pressure in cuff. High cuff pressure occludes blood flow. Pressure is slowly released from cuff. Flow resumes once cuff P reaches systolic pressure. Heard as “Korotkoff sounds”-blood jetting thru artery. When cuff pressure = diastolic pressure, no more sounds are heard. Figure 15-7 Slide 26 How to Measure Blood Pressure: Cannula needle Direct / Invasive method Most precise. Measures blood pressure through direct measure via a cannulae needle placed in an arterial line. Cannulae is connected to a sterile, fluidfilled system, which connected to an electronic pressure transducer. Requires direct supervision Slide 27 Systolic and diastolic pressure progressively increase with age. Slight rise in systolic-usually due to atherosclerosis. PP also increases. (¤ C ¢ £ PP) Mean Arterial Pressure: Average blood pressure over time MAP = DBP + 1/3 (SBP-DBP) MAP nearer DBP Why 1/3? Blood pressure: Figure 15-8 Systolic pressure/Diastolic pressure Normal BP Hypertensive Slide 28 120/80 135-140/90 Systolic and diastolic pressure progressively increase with age. Slight rise in systolic-usually due to atherosclerosis. PP also increases. (¤ C ¢ £ PP) Mean Arterial Pressure: Average blood pressure over time MAP nearer DBP MAP = DBP + 1/3 (SBP-DBP) Why 1/3? Blood pressure: Sprint Trial, 11/2017 Figure 15-8 Slide 29 Function of the Veins, Venous Pressure Veins play important role in the circulation: -pathway for blood to return to heart -blood reservoir due to high compliance -can also propel blood towards heart – “venous pump” -contribute to cardiac output Large veins have so little resistance to blood flow when distended, resistance is almost zero. But as they enter the thorax they can be compressed by surrounding tissue. When venous pressure is very low, high pressures can lead to collapse -abdominal pressure and atmospheric pressure. Example: Neck. When intra-abdominal pressure rises, pressure in the veins of the leg must increase to allow blood to flow to the heart. Slide 30 Figure 15-9 Function of the Veins, Venous Pressure Gravitational effect on venous pressure. aka - hydrostatic pressure directly proportional to height Adult standing still: Right Atrial Pressure (RAP) = 0 Venous pressure at the feet = 90 mm Hg Figure 15-10 Slide 31 Function of Veins, Right Atrial Pressure Blood flows from all systemic veins to right atrium. Right atrial pressure (RAP) termed “central venous pressure”. Normal RAP = 0 mm Hg Control of RAP 1) Tendency for blood to flow from veins into right atrium (venous return) (↑VR → ↑BV) 2) Ability of heart to pump blood out (cardiac function) of the right atrium and ventricle into lungs. RAP is a balance between what blood comes back to the heart (venous return) and what the heart can pump out (cardiac function). Weak heart Rapid flow into heart Heart failure, transfusion Venous return > cardiac function £ RAP Hypereffictive heart Venous return < cardiac function ¤RAP Slide 32 Function of Veins and Venous Return 1. Venous Pump Contraction of skeletal muscles compresses veins and helps propel blood to heart. Only effect with muscle contraction. Figure 15-11 Slide 33 2. Venous valves Helps maintain VP prevents backflow Incompetence - varicose veins. Valve failure leads to increases in venous pressure which destroys the function of the valves. Function of Veins, Blood Reservoir Function High compliance of veins make them a blood reservoir. Specific blood reservoirs: Abdominal Veins Spleen Liver Venous plexus beneath skin. Heart Spleen Can release up to 100 mls with SNS = blood volume reservoir 1. venous sinuses-whole blood 2. venous pulp-special store of RBC cells Figure 15-13 Slide 34 Fragile RBCs are destroyed in pulp--destroys old cells. Distensibility vs. Compliance • Distensibility is the ability of a vessel to stretch (distend) • Compliance is the ability of a vessel to stretch and hold volume Slide 35 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 36 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)?