Circuitry of cardiovascular system and structure-function relationship

1. Circuitry of cardiovascular system and structure-function relationship Dr. Shafali

3. Learning Objectives • Describe the organization of the circulatory system • Explain how the systemic and pulmonary circulations are linked physically and physiologically • Understand the relationship between flow, velocity, and cross-sectional area

4. Learning Objectives • Understand the relationship between pressure, flow, and resistance in the vasculature . • Define resistance and conductance. Understand the effects of adding resistance in series vs. in parallel on total resistance and flow.

6. General features of the cardiovascular system

12. Cardiac output and heart rate of the two circuits are equal, so stroke volumes are the same. • Despite this, all pressures are higher in the systemic (peripheral) circuit. This shows that the vessels of the circuits are very different. The systemic circuit has much higher resistance and much lower compliance than the pulmonary circuit. • The lower pressures mean that the work of the right ventricle is much lower. • In addition, the lower capillary pressure protects against the development of pulmonary edema

14. Local arteriolar dilation decreases arteriolar resistance, which increases flow and pressure downstream (more pressure and more flow get downstream). • Local arteriolar constriction increases arteriolar resistance, and flow and pressure decrease downstream

15. COMPLIANCE OF BLOOD VESSELS The compliance or capacitance of a blood vessel describes the volume of blood the vessel can hold at a given pressure. Compliance is related to distensibility and is given by the following equation:where C ,Compliance (mL/mm Hg) ,V Volume (mL), P Pressure (mm Hg) The equation for compliance states that the higher the compliance of a vessel, the more volume it can hold at a given pressure.

17. Compliance is essentially how easily a vessel is stretched. • If a vessel is easily stretched, it is considered very compliant. The opposite is noncompliant or stiff. • Elasticity is the inverse of compliance. A vessel that has high elasticity (a large tendency to rebound from a stretch) has low compliance.

18. CHARACTERISTICS OF SYSTEMIC VEINS • Systemic veins are about 20 times more compliant than systemic arteries. • Veins also contain about 70% of the systemic blood volume and thus represent the major blood reservoir. • In the venous system, then, a small change in pressure causes a large change in venous volume

20. Example-Hemorrhage • Cause venous pressure to decreases. • Because veins are very compliant vessels, this loss of distending pressure causes a significant passive constriction of the veins and a decrease in blood stored in those veins. • The blood removed from the veins will now contribute to the circulating blood volume (cardiac output), a compensation for the consequences of hemorrhage.

21. Volume loading (infusion of fluid) • Increases venous pressure. The increased pressure distends the veins; this is a passive dilation. • The volume of fluid stored in the veins increases, which means that some of the infused volume will not contribute to cardiac output. • The large volume and compliant nature of the veins act to buffer changes in venous return and cardiac output.

23. Blood Volume • The largest blood volume in the cardiovascular system is in the systemic veins. • The second largest blood volume is in the pulmonary system. • Both represent major blood reservoirs. • The systemic veins and the pulmonary vessels have very high compliance compared to the systemic arteries; this is primarily responsible for the distribution of blood volume.

24. Cross-Sectional Area

25. Velocity of the Bloodstream • Velocity, as relates to fluid movement, is the distance that a particle of fluid travels with respect to time, and it is expressed in units of distance per unit time (e.g., cm/sec). • Flow, is the rate of displacement of a volume of fluid, and it is expressed in units of volume per unit time (e.g., cm3/sec).

26. In a rigid tube, velocity (v) and flow (Q) are related to one another by the cross-sectional area (A) of the tube

30. Factors influencing velocity • Cross sectional area of segment • Phase – Systolic phase ↑ velocity Diastolic phase ↓ velocity Viscosity - ↑viscosity ↓ velocity ↓ viscosity ↑ velocity Applied physiology Velocity decreases in heart failure.

31. Velocity of circulation

32. Bernoulli’s principle A B The lateral (distending) pressure drops at the level of the constriction in the schematic above. Bernoulli’s principle: Velocity of flow increases at the level of the constriction. Also there is a pressure drop .

34. In most arterial locations, the dynamic component will be a negligible fraction of the total pressure. • However, at sites of an arterial constriction or obstruction, the high flow velocity is associated with a large kinetic energy, and therefore the dynamic pressure component may increase significantly. • Hence, the pressure would be reduced and perfusion of distal segments will be correspondingly decreased.

36. Q. The greatest pressure decrease in the circulation occurs across the arterioles because (A) they have the greatest surface area (B) they have the greatest cross-sectional area (C) the velocity of blood flow through them is the highest (D) the velocity of blood flow through them is the lowest (E) they have the greatest resistance

37. Q. A 25 year old graduate student while going for her lectures on her power bike skids off the road and sustains a fracture to her right leg. The fractured leg is bleeding profusely. At the ER, her blood pressure is determined to be low. Homeostatic mechanisms in stabilizing the blood pressure will include increases in total peripheral resistance. The site of highest resistance in the vasculature is in the; A. Arterioles B. Venules C. Capillaries D. Large arteries E. Veins

38. Q. 12 A healthy 32-year-old woman participates in a clinical study. Her blood volume is 5,200mL. Images are obtained to determine the volume of blood in various vessels in various body positions at rest and during exercise. While lying supine, which of the following vascular structures will most likely contain the largest portion of the total blood volume in this woman? A. The left ventricle B. The right ventricle C. The pulmonary vasculature D. Veins and venules E. Vena cavae F. Capillaries G. Arterioles

39. BLOOD FLOW • Quantity of blood that passes a given point of circulation in a given period of time. Units= ml/min • Normal blood flow is – streamline or laminar(Silent) • Random flow in avessel - Turbulent flow • In laminar flow , the velocity of flow is greater in the center than the outer edges .

40. DEMONSTRATION OF LAMINAR & TURBULENT BLOOD FLOW

41. Axial streaming and flow velocity

42. Laminar flow is flow in layers. • Laminar flow occurs throughout the normal cardiovascular system, excluding flow in the heart. • The layer with the highest velocity is in the center of the tube. • Turbulent flow is non layered flow. • It creates murmurs. These are heard as bruits in vessels with severe stenosis. • It produces more resistance than laminar flow.

44. CRITICAL VELOCITY • The maximal velocity at which the flow becomes turbulent . • Expressed in REYNOLDS NUMBER . • R= PDV /  • P= Density of blood , D = diameter of vessel , V= Velocity of blood flow ,  = viscosity in poises • When number is 2000 – TURBULENCE occurs . Velocity , Cross section eg; Stenosis Velocity , Viscosity eg; Anaemia

45. IN THE CLINIC – turbulent flow • Usually accompanied by audible vibrations, detected with a stethoscope . • When the turbulence occurs in the heart, the resultant sound is termed a murmur; when it occurs in a vessel, the sound is termed a bruit. • E.g- In severe anemia, (1) the reduced viscosity of blood and (2) the high flow velocities associated with the high cardiac output . • Blood clots, or thrombi, are more likely to develop in turbulent than in laminar flow.

46. Shear Stress on the Vessel Wall Flowing blood creates a force on the endothelium that is parallel to the long axis of the vessel. This shear stress (γ) is proportionate to viscosity (ɳ) times the shear rate (dy/dr), which is the rate at which the axial velocity increases from the vessel wall toward the lumen.

47. IN THE CLINIC -Dissecting aneurysm • In certain types of arterial disease, particularly hypertension, the subendothelial layers of vessels tend to degenerate locally, and small regions of the endothelium may lose their normal support. • The viscous drag on the arterial wall may cause a tear between a normally supported and an unsupported region of the endothelial lining. • Blood may then flow from the vessel lumen through the rift in the lining and dissect between the various layers of the artery. Such a lesion is called a dissecting aneurysm. It occurs most often in the proximal portions of the aorta and is extremely serious.

48. WALL TENSION • La Place law: States that tension in the wall of a cylinder (T) is equal to the product of the transmural pressure (P) and the radius (r) divided by the wall thickness (w): T= P r/w

49. Because of their narrow lumens (i.e., small radius), the thin-walled capillaries can withstand high internal pressures without bursting. • This property can be explained in terms of the law of Laplace