1. Physics of Blood flow in the circulation
Hemodynamics
Physics of Blood flow in the circulation

2. Circulatory System Heart:
Has 2 collecting chambers - (Left, Right Atria)
Has 2 Pumping chambers - (Left, Right Ventricles)

4. Circulation Schematic
Left Side of Heart
Pulmonary Vein
Aorta A V
Aortic Valve
Mitral Valve
Tissues
Lungs
Tricuspid Valve
Pulmonary Valve V A
Pulmonary Artery
Right Side of Heart
Sup. & Inf. Vena Cava

5. Heart Valves
Atrioventricular (A-V) valves - separate Atria from Ventricles
Bicuspid (Mitral) - Left Side
Tricuspid - Right Side
Semi-Lunar Valves - separate ventricles from Arteries

6. Opening, Closing of Valves - Depends on Pressure
differences between blood
in adjacent areas

7. Heart Sounds ‘Lubb’ (1st sound) - Closure of A-V valves
‘Dupp’ (2nd sound) - Closure of S-L valves
Caused by Turbulence on closing.
Anything extra ’Murmur’ (swishing of blood)
Could be due to:
Stenosis of Valves (calcification)
Valves not closing properly
(Incompetence, Insufficiency)
Increases Pressure on heart

8. Blood Vessels Arteries Capillaries Veins Systemic Pathway:
Left Ventricle Aorta Arteries Arterioles
of Heart
Venules Veins Right Atrium

9. Lumen of Coronary Artery

10. Blood Vessel Walls - Layers
Arteries:
Tunica Externa (Fibrous Connective Tissue)
Tunica Media (Elastic Connective Tissue
and/or Smooth Muscle)
Tunica Intima (Endothelium)
Capillaries:
Endothelium
Veins - same as Arteries except:
Less smooth muscle in Tunica Media
Have valves to control blood flow direction

11. Blood Composition: Approx 45% by Vol. Solid Components
Red Blood Cells (12m x 2 m)
White Cells
Platelets
Approx 55% Liquid (plasma)
91.5% of which is water
7% plasma proteins
1.5% other solutes

12. Blood Functions Transportation of blood gases, nutrients, wastes
Homeostasis (regulation)
of Ph, Body Temp, water content
Protection

13. As a Result …….
Blood behaves as a simple Newtonian Fluid when flowing in blood vessels
i.e. Viscous stresses  Viscosity, strain rate y
u(y)
No slip
at wall

14. Viscosity of Blood = 3 3.5 times of water
Blood acts as a non-newtonian fluid in smaller vessels (including capillaries)

15. Cardiac Output Flow of blood is usually measured in l/min
Total amount of blood flowing through the circulation = Cardiac Output (CO)
Cardiac Ouput = Stroke Vol. x Heart Rate
= 5 l/min
Influenced by Blood Pressure & Resistance
Force of blood
against vessel wall
Blood viscosity
Vessel Length
Vessel Elasticity
Vasconstriction / Vasodilation
 with water retention
 with dehydration, hemorrage

16. Overall Greater Pressure  Greater Blood Differences Flow
Greater Resistance  Lesser Blood Flow

17. Elastic arterial walls expand and recoil
Blood Pressure
Driving force for blood flow is pressure created by ventricular contraction
Elastic arterial walls expand and recoil
continuous blood flow

18. Blood pressure is highest in the arteries (Aorta
Blood pressure is highest in the arteries (Aorta!) and falls continuously . . .
Systolic pressure in Aorta: 120 mm Hg
Diastolic pressure in Aorta: 80 mm Hg
Diastolic pressure in ventricle: ?? mm Hg

19. Ventricular pressure difficult to measure
arterial blood pressure assumed to indicate driving pressure for blood flow
Arterial pressure is pulsatile
useful to have single value for driving pressure: Mean Arterial Pressure
MAP = diastolic P + 1/3 pulse pressure

20. Pulse Pressure = systolic pressure - ??
= measure of amplitude of blood pressure wave

21. MAP influenced by Cardiac output Peripheral resistance
MAP CO x Rarterioles
Blood volume
fairly constant due to homeostatic mechanisms (kidneys!!)

22. BP too low: Driving force for blood flow unable to overcome gravity
O2 supply to brain 
Symptoms?

23. BP too high: Weakening of arterial walls - Aneurysm
Risk of rupture & hemorrhage
Cerebral hemorrhage: ?
Rupture of major artery:

24. BP estimated by Sphygmomanometry
Auscultation of brachial artery with stethoscope
Laminar flow vs. turbulent flow

25. Principles of Sphygmomanometry
Cuff inflated until brachial artery compressed and blood flow stopped what kind of sound?

26. Slowly release pressure in cuff:
turbulent flow

27. Pressure at which . . . . . . sound (= blood flow) first heard:
. . . sound disappeared:

28. Pressure can be stated in terms of column of fluid.
Pressure Units
mm Hg cm H2O PSI ATM

29. Pressure = Height x Density or P = gh
If Right Atrial pressure = 1 cm H2O in an open column of blood
 Pressure in feet = 140 cm H2O
 Rupture
 Venous Valves
Density of blood
= that of water
Incompetent venous valves
 Varicosities
Actual Pressure in foot
= 4-5 cm H2O

30. Dynamic Fluid Mechanics
Blood flowing through circulation follows physical principles of pressure, flow
Under conditions of constant flow, velocity must increase to allow the same flow through a smaller space
i.e. the continuity equation

31. (This is the usual pattern of flow in the vascular system.)
Types of Blood Flow
Laminar: When velocity of blood flow is below a critical speed, the flow is orderly and streamlined
(This is the usual pattern of flow in the vascular system.)
Turbulent: disorderly flow with eddies & vortices

32. R = Velocity * Diameter * Density / Viscosity
Turbulent flow is noisy
In the vascular system high flow velocities cause turbulent flow and produce sound
The murmur heard when blood flows through a narrowed heart valve is due to turbulent flow
To determine whether flow is laminar or turbulent, calculate the Reynolds Number (R) (dimensionless no.)
R = Velocity * Diameter * Density / Viscosity
Values below 2000 define laminar flow

33. Energy = [V2/2g] + [P/r] + H = Constant
Bernoulli Equation
When blood flows through an artery, the total energy of the fluid at any point is assumed to be constant
i.e. The sum of energy stored in pressure, energy provided by flow and potential energy due to the height of the blood above a reference point is constant
Energy = [V2/2g] + [P/r] + H = Constant
In an arterial stenosis, the increase in velocity causes a fall in pressure in the stenosis

34. Pressure = Flow x Resistance
Poiseulle equation:
Pressure drop across
a length of vessel
Radius
Flow
Length of Vessel

35. This is most effective in the arterioles
For a given pressure, there is greater flow when the blood vessel is short and wide and the blood is thin
Since BP is constant most of the time, flow is controlled by small changes in r--the vessel radius
This is most effective in the arterioles
Resistance is measured in Wood units
Pulmonary = (PA pressure - LA pressure) / CO
resistance
e.g. R = (14 mm Hg - 7 mm Hg) / 5 l/min
= 1.4 Wood units
Pulmonary Artery

36. Pressures in the circulation
Pressures in the arteries, veins and heart chambers are the result of the pumping action of the heart
The right and left ventricles have similar waveforms but different pressures
The right and left atria also have similar waveforms with pressures that are similar but not identical

37. 5. Then the aortic valve closes and LV pressure falls to LA pressure
3. As blood enters the aorta, the aortic pressure begins to rise to form the systolic pulse
4. As the LV pressure falls in late systole the aortic pressure falls until the LV pressure is below the aortic diastolic press.
2. Pressure rises until the LV pressure exceeds the aortic pressure
5. Then the aortic valve closes and LV pressure falls to LA pressure
 The blood begins to move from the ventricle to the aorta
1. The LV pressure begins to rise after the QRS wave of the ECG

38. The first wave of atrial pressure (the A wave) is due to atrial contraction
The second wave of atrial pressure (the V wave) is due to ventricular contraction

39. Normal Pressures RV and pulmonary systolic pressure are 12-15 mm Hg
Pulmonary diastolic pressure is 6-10 mm Hg
LA pressure is difficult to measure because access to the LA is not direct

40. AS produces a pressure gradient between the aorta and LV
The severity of AS is determined by the pressure drop across the aortic valve or by the aortic valve area
The high velocity of blood flow through the narrowed valve causes turbulence and a characteristic murmur AS can be diagnosed with a stethoscope
i.e. For blood to move rapidly through a narrowed aortic valve orifice, the pressure must be higher in the ventricle

41. AV Area The AV area can be calculated from pressure and cardiac output
The calculate the AV area, use the Gorlin equation
In the formula, flow is calculated for the time of systole since systolic time is the only time in the heart cycle when blood flows across the AV
Flow
Mean Gradient between LV and Aorta
AV Area

42. Pressure Measurement
Accurate pressure measurements are essential to understanding the status of the circulation
In 1733 Steven Hales connected a long glass tube directly to the left femoral artery of a horse and measured the height of a column of blood (8 feet, 3 inches) to determine mean BP
Direct pressure measurements are made frequently in the cardiac catheterization laboratory, the ICU and the OR

43. A tube is inserted into an artery and connected to an electrical strain gauge that converts pressure into force that is sensed electrically
The output of the transducer is an electrical signal that is amplified and recorded on a strip chart
For correct pressure measurements the cannula and transducer must be free of air, the cannula should be stiff and short

44. Cardiac Output (CO)Measurement
The measurement of blood flow through the circulation is usually done clinically using either the Fick method
The Fick method states that the cardiac output is equal to the oxygen consumption divided by the arterial-venous oxygen difference
CO = Oxygen consumption / A-V O2

45. The measurement is done by determining the oxygen consumption using respiratory gas measurements and the O2 content of arterial and mixed venous blood
The mixed venous blood sample is obtained from a PA with a catheter
The arterial sample can be drawn from any artery