1. Overview of Circulation; Pressure, Flow, Resistance
Prof. dr. Zoran Valić
Department of Physiology
University of Split School of Medicine

2. to transport nutrients to the body tissues
to transport waste products away
to transport hormones
to maintain an appropriate environment
rate of blood flow through many tissues is controlled mainly in response to tissue need for nutrients

4. systemic circulation (greater circulation or peripheral circulation)
pulmonary circulation

5. arteries (high pressure, strong vascular walls, blood flows at a high velocity)
arterioles (control conduits, strong muscular walls)
capillaries (exchange, thin walls, pores)
venules (collect blood)
veins (transport of blood from the venules back to the heart, major reservoir of extra blood, low pressure, thin walls – muscle)

7. veins on average 4x bigger

8. CO is equal in all vascular segments
velocity averages 33 cm/sec in the aorta
velocity averages 0.3 mm/sec in the capillaries (blood remains in the capillaries for only 1 to 3 seconds)

10. mean pressure in the aorta is high, averaging about 100 mm Hg (systolic pressure level of 120 mm Hg and a diastolic pressure level of 80 mm Hg)
pressure in the systemic capillaries varies between 35 mm Hg at arteriolar ends and 10 mm Hg at venous ends (average "functional" pressure is about 17 mm Hg)
mean pulmonary arterial pressure is 16 mm Hg (systolic is 25 and diastolic is 8 mm Hg)

11. Principles of Circulatory Function
The rate of blood flow to each tissue of the body is almost always precisely controlled in relation to the tissue need ( up to 30 x)
CO is controlled mainly by the sum of all the local tissue flows
Arterial pressure regulation is generally independent of either local blood flow control or CO control

12. Interrelationships of Pressure, Flow, and Resistance
F = ΔP / R (mL/min)
(ΔP = F x R,
R = ΔP / F)
CO  5000 mL/min

14. electromagnetic flowmeter
records changes in flow in less than 1/100 of a second, allowing accurate recording of pulsatile changes in flow, as well as steady flow

16. ultrasonic Doppler flowmeter
minute piezoelectric crystal
sound is reflected by the red blood cells
records rapid, pulsatile changes in flow, as well as steady flow

18. laminar or streamline flow, and turbulent or disorderly flow (whorls in the blood, called eddy currents)

19. parabolic profile for velocity of blood flow

20. Re = v x d x ρ / η
Re 
Re  2000 (turbulence will usually occur even in a straight, smooth vessel)

21. occasionally, pressure is measured in centimeters of water (cm H2O)
occasionally, pressure is measured in centimeters of water (cm H2O) (1 mmHg = 1,36 cm H2O)
1 mmHg = 133,322 Pa
1 kPa = mmHg (0°C)

24. resistance cannot be measured by any direct means (in lungs 1/7 resistance)
conductance = 1 / resistance
slight changes in the diameter of a vessel cause tremendous changes in conductance
Poiseuille's law:
F = ΔPΠr4 / 8ηl

26. 2/3 of the total systemic resistance to blood flow is arteriolar resistance in the small arterioles (4-25 μm)
strong vascular walls allow the internal diameters to change tremendously, often as much as fourfold (flow 256x)

27. parallel arrangement : 1/ RTOTAL = 1/ R1 + 1/ R2 + 1/ R3 + ...
series arrangement :
RTOTAL = R1 + R2 +R
parallel arrangement :
1/ RTOTAL = 1/ R1 + 1/ R2 + 1/ R
adding more parallel blood vessels to a circuit reduces the total vascular resistance

29. blood is highly viscous due to presence of RBC (3 x as great as the viscosity of water)
proportion of the blood that is red blood cells is called the hematocrit (42 in men, 38 in women)

32. in isolated blood vessels or in tissues that do not exhibit autoregulation increased arterial pressure not only increases the force that pushes blood through the vessels but it also distends the elastic vessels, actually decreasing vascular resistance
role of sympathetic nervous system

35. Vascular Distensibility

36. all blood vessels are distensible
distensible nature of the arteries allows them to accommodate the pulsatile output of the heart and to average out the pressure pulsations
provides smooth, continuous flow of blood

37. most distensible are the veins
slight increases in venous pressure cause the veins to store 0.5 to 1.0 liter of extra blood
reservoir function

38. distensibility = ΔV / ΔP x original V
vascular distensibility is expressed as the fractional increase in volume for each millimeter of mercury rise in pressure
distensibility = ΔV / ΔP x original V
veins are about eight times more distensible than the arteries
pulmonary vein distensibilities are similar to those of the systemic circulation, pulmonary arteries are six times more distensibile than systemic arteries

39. compliance or capacitance of vascular bed – total quantity of blood that can be stored in a given portion of the circulation for each mm Hg pressure rise
compliance = ΔV / ΔP
compliance = distensibility x original V
veins are on average 24 x more compliant then arteries (8x distensible + 3x greater V)

40. relation of pressure to volume in a vessel is expressed by volume-pressure curve
effect of sympathetic stimulation and inhibition
highly important during hemorrhage (loss of 25 percent of the total blood volume)

42. delayed compliance (stress-relaxation)
after too large a transfusion or serious hemorrhage

44. Arterial Pressure Pulsations
were it not for distensibility of the arterial system blood flow would occur only during cardiac systole
tissue blood flow is mainly continuous with very little pulsation

46. pulse pressure equals about 40 mm Hg (120 – 80 mm Hg)
depends on: 1) the stroke volume output
2) the compliance (total distensibility) of the arterial tree (arteriosclerosis)
3) character of ejection from the heart

48. Transmission of Pressure Pulses
in the normal aorta 3 to 5 m/sec; in the large arterial branches 7 to 10 m/sec; and in the small arteries 15 to 35 m/sec
the greater the compliance of each vascular segment, the slower the velocity
pressure pulse is simply a moving wave of pressure that involves little forward total movement of blood volume

51. Damping of the Pressure Pulses
resistance to blood movement in the vessels
compliance of the vessels

54. mean arterial pressure is determined about 60 percent by the diastolic pressure and 40 percent by the systolic pressure

55. Veins and Their Functions
in the past – nothing more than passageways for flow of blood to the heart
making blood available when it is required by the remainder of the circulation
pressure in the right atrium is called the central venous pressure

56. right atrial pressure is regulated by a balance between:
the ability of the heart to pump blood out of the right atrium and ventricle into the lungs and
the tendency for blood to flow from the peripheral veins into the right atrium

57. factors that can increase venous return: 1) increased blood volume
2) increased large vessel tone throughout the body with resultant increased peripheral venous pressures
3) dilatation of the arterioles, which decreases the peripheral resistance and allows rapid flow of blood from the arteries into the veins

58. normal right atrial pressure is about 0 mm Hg
increase to 20 to 30 mm Hg under very abnormal conditions (serious heart failure, after massive transfusion)
-3 to -5 mm Hg (when the heart pumps with exceptional vigor, after severe hemorrhage)

61. “venous pump” or “muscle pump”
instead +90 mm Hg less than +20 mm Hg
10 to 20 percent of the blood volume can be lost from the circulatory system within the 15 to 30 minutes of standing absolutely still

63. varicose veins (pregnancy or work)
pressure in the veins and capillaries of the legs increases greatly – edema
prevents diffusion of nutritional materials – pain, gangrene and ulcers

65. Blood Reservoir Function of the Veins
more than 60 percent of all the blood in the circulatory system is usually in the veins + great compliance
hemorrhage – activation of sympathetic nervous system

66. Specific Blood Reservoirs
spleen (as much as 100 mL of blood)
liver (several hundred mL)
large abdominal veins (as much as 300 mL)
venous plexus beneath the skin (several hundred mL)
heart ( mL) and lungs ( mL)