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

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


3 Learning Objectives Describe the organization of the circulatory system and its function 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 Functions of circulation Supply the tissues with nutrients Removal of waste product of tissue metabolism Control blood flow to the skin and limbs to regulate heat loss Aids in body’s defence mechanisms by delivering antibodies,platelets and leucocytes to the affected area of the body.

7 General features of the cardiovascular system




11 120


13 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



16 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

17 Types of blood vessels 1.Windkessel vessels/Distensible vessels- aorta,pulmonary artery and their large branches. 2.Resistance vessels– arterioles,metarterioles and pre capillary sphincter. 3.Exchange vessels—capillaries 4.Capacitance vessels- venules and veins 5.Shunt vessels or Thoroughfare vessels/A-V shunts


19 WINDKESSEL (DISTENSIBLE) VESSELS (large arteries) Highly elastic Windkessel means elastic reservoir During systole they distend withstanding the high systolic pressure Once systole is over they recoil contributing to the diastolic pressure



22 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.


24 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.


26 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.

27 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


29 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.

30 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.

31 Because of the high compliance of veins, large increases of pressure occur mainly with substantial increases of volume, as in congestive heart failure, or with massive sympathetic activity that reduces compliance. Similarly, substantial decreases of central venous pressure occur with large loss of volume. An exception is the effect of posture, which can lower central venous pressure, even though blood volume has not changed. This is because gravity causes blood to pool in the dependent veins. The actual venous return to the heart is determined by the venous pressure gradient.


33 Cross-Sectional Area

34 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., cm 3 /sec).

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

36 Total cross sectional area



39 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.

40 Velocity of circulation


42 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.


44 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

45 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

46 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

47 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 a vessel - Turbulent flow In laminar flow, the velocity of flow is greater in the center than the outer edges.


49 Axial streaming and flow velocity

50 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.


52 CRITICAL VELOCITY The maximal velocity at which the flow becomes turbulent. Expressed in REYNOLDS NUMBER. R= PDV /  P= Density of blood (1), D = diameter of vessel, V= Velocity of blood flow (cm/sec),  = viscosity in poises When number is 2000 – TURBULENCE occurs. Velocity, Cross section eg; Stenosis Velocity, Viscosity eg; Anaemia

53 The following promote the development of turbulent flow (i.e., increase Reynolds’ number): Increasing tube diameter Increasing velocity Decreasing blood viscosity, e.g., anemia Vessel branching Narrow orifice (severe stenosis)—due to very high velocity of flow The vessel in the systemic circuit that is closest to the development of turbulent flow is the aorta. It is a large-diameter vessel with high velocity. This is where turbulence should appear first in anemia

54 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.

55 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.


57 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.

58 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


60 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


62 IN THE CLINIC- Dilated heart If the heart becomes greatly distended with blood during diastole, as may occur with cardiac failure, it functions less efficiently. More energy is required (greater wall tension) for the distended heart to eject a given volume of blood per beat than is required for a normal undilated heart. The less efficient pumping of a distended heart is an example of Laplace's law, which states that the tension in the wall of a vessel or chamber (in this case the ventricles) equals transmural pressure (pressure across the wall, or distending pressure) times the radius of the vessel or chamber.

63 Estimation of blood flow through various parts of the body Use of flow meters – (Direct method) In animals Electromagnetic flow meter Plethysmography Ficks principle – Also used to measure Cardiac output,renal/coronary / cerebral blood flow can be estimated Indicator dilution technique PAH clearance By doppler study

64 Plethysmography

65 Poiseuille’s Law Describes the Relationship Between Pressure and Flow

66 In electrical theory, Ohm's law states that the resistance, R, equals the ratio of voltage drop, E, to current flow, I.


68 Poiseuille Equation





73 Viscosity Viscosity is a property of a fluid that is a measure of the fluid’s internal resistance to flow. Viscosity is the frictional resistance in between the laminae of the flowing fluid. Frictional resistance is due to red cells and plasma proteins. The greater the viscosity, the greater the resistance. The prime determinant of blood viscosity is the hematocrit.





78 Q A 53-year-old woman is found, by arteriography, to have 50% narrowing of her left renal artery. What is the expected change in blood flow through the stenotic artery? (A)Decrease to ½ (B) Decrease to ¼ (C) Decrease to 1/8 (D) Decrease to 1/16 (E) No change

79 Critical Closing Pressure


81 Hemodynamics - Summary

82 A 56 yr old female is admitted to the hospital for a hysterectomy. After surgery, she is transferred to the intensive care unit. Her mean systemic blood pressure is 100mmHg and her resting cardiac output is 4 L/min. Which of the following is total peripheral resistance in this patient? a.0.025(ml/min)/mmHg b mmHg/(mL/min) c.40( ml/min)/mmHg d.40 (mmHg/(mL/min) e.4000 (ml/min)/mmHg f.4000 (mmHg/(mL/min)

83 Series Versus Parallel Circuits Series


85 Parallel



88 3.6ml/min 45ml/min 90ml/min 135ml/min 160ml/min

89 0.0625mm hg/l/min 0.05 mm hg/l/min 0.04 mm hg/l/min 0.03 mm hg/l/min

90 A MD 2 student is performing experiments on blood flow in various vessels. She came to the conclusion that the velocity of blood flow is slowest in the capillaries. The most likely reason for this is: A.Capillaries have the smallest cross-sectional area B.Capillaries have the largest cross-sectional area C.Decreased in blood viscosity in the capillaries D.Single stream of blood flow E.Decreased in turbulence

91 The circuit below has an inflow pressure of 120 mmHg and an outflow pressure of 40 mmHg. Resistance is each of the vessel shown is 2mmHg/ml/min( R1=R2=R3=R4=2mmHg/ml/min).What is the total peripheral resistance of the circuit shown in the picture below? a.8 mmHg/ml/min b.4 mmHg/ml/min c.2 mmHg/ml/min d.1 mmHg/ml/min e.0.5 mmHg/ml/min

92 In order to maintain constant flow through a tube with varying diameters, which of the following would be true( where A1 and A2 represent cross sectional areas, and V1 and V2 represents the corresponding flow velocities)? a.V1=V2 b.V1=A1× V2 c.A2= A1 ×V1/V2 d.V1=A1 ×A2/V2 e.V1 ×A2=V2 ×A1

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