This document provides an overview of the circulatory system and its components. It discusses: - The two circuits of circulation - systemic and pulmonary. - The main components and their functions, including arteries, veins, capillaries and their types. - Blood flow pathways and pressure differences at different points in the circulatory system. - Factors that influence blood flow such as vessel size, elasticity and resistance according to principles like Poiseuille's law and Laplace's law.

1. GENERAL PRINCIPLES OF CIRCULATION
Dr Raghuveer Choudhary
Associate Professor
Dept. of Physiology
Dr. S.N.Medical College, Jodhpur
12/3/2022
1

2. 12/3/2022
2
PARTS OF THE CIRCULATORY SYSTEM
The circulatory system forms two circuits in
series with each other:-
- Systemic circulation (greater circulation)
- Pulmonary circulation (lesser circulation)

3. 12/3/2022
3

4. 12/3/2022
4

5. General Pathway of Blood Flow
heart -> arteries -> arterioles -> capillaries -
> venules -> veins -> heart
12/3/2022
5

6. Arteries are strong, elastic
vessels adapted for carrying
blood away from the heart
under high pressure.
Three distinct layers:
Endothelium – Inner most
layer. Rich in elastic and
collagenous fibers. Called the
tunica interna.
Middle layer – Tunica media.
Smooth muscle fibers, thick
layer of elastic connective
tissue.
Outer layer – Tunica externa.
Attaches the artery to tissues.
Contains vasa vasorum that gives
Rise to capillaries
12/3/2022
6

7. TYPES OF ARTERIES
ELASTIC ARTERIES
(LARGE-SIZED)
MUSCULAR ARTERIES
(MEDIUM-SIZED)
RESISTANCE ARTERIES
(SMALL-SIZED)
12/3/2022
7

8. 12/3/2022
8

9. 21-9
ELASTIC ARTERIES
 Largest-diameter arteries have lot of elastic
fibers in tunica media
 Help propel blood onward despite ventricular
relaxation (stretch and recoil)
12/3/2022

10. Windkessel Effect
12/3/2022
10

11. 12/3/2022
11

12. In old age elasticity of these vessels is lost due to degenerative &
atherosclerotic changes. So SBP rises,DBP falls, Pulse Pressure Rises
resulting in defective perfusion in periphery
12/3/2022
12

13. 21-13
MUSCULAR ARTERIES
 Medium-sized arteries with more muscle
than elastic fibers in tunica media
 Capable of greater vasoconstriction and
vasodilation to adjust rate of flow
 walls are relatively thick
12/3/2022

14. 12/3/2022
14

15. 12/3/2022
15

16. 12/3/2022
16

17. 12/3/2022
17

18. 12/3/2022
18

19. 21-19
TYPES OF CAPILLARIES
 Continuous capillaries
 gaps between neighboring cells
 muscle and lungs
 Fenestrated capillaries
 plasma membranes have many holes
 kidneys, small intestine & endocrine
glands
 Sinusoids
 very large fenestrations
 incomplete basement membrane
 liver, bone marrow, & spleen
12/3/2022

20. Hydraulic conductivity of capillaries in
various parts of the body
(Ganong, Medical Physiology 2001)
Organ Conductivity Type of Endothelium
Brain (except CVO) 3
Skin 100
Skeletal muscle 250 Continuous
Lungs 340
Heart 860
GIT (intestinal mucosae) 13,000
Fenestrated
Kidney (glomerulus)
Liver
Bone marrow
Endocrine glands Sinusoidal
Lymphoid tissue
(Marieb, Human Anatomy and Physiology)
12/3/2022
20

21. 12/3/2022
21

22. FLUID MOVEMENT IN THE CAPILLARY
 Arteriole side: fluid moves
toward the tissues
 Venous side: fluid reenters the
capillary
 Overall: for every 1 liter of fluid
entering the tissues, only 0.85 l
reenter the capillary
 The remaining 0.15 l is
reabsorbed as lymph by
lymphatic capillaries and
eventually returned back to
blood circulation
 When this system fails: Edema
12/3/2022
22

23. CAUSES OF EDEMA
 Increased hydrostatic
blood pressure
- heart failure (left or right),
- excess fluid in the blood
 Decreased blood osmotic
pressure
 Liver, kidney diseases,
malnutrition (kwashiorkor),
burn injuries
 Increased interstitial
hydrostatic pressure
(lymphatic capillary
blockage)
- breast cancer surgery,
elephantiasis
 Leaking capillary wall
- histamine release during
allergic reaction
12/3/2022
23

24. 12/3/2022
24

25. VEINS
 Transport blood
under low pressure.
 8x more distensible
than arteries
 transport blood
towards the heart
 carry deoxygenated
blood.
Great veins
 no valves, thin and easily
distended
Venules
 no valves, walls slightly
thicker than capillaries
12/3/2022
25

26. 12/3/2022
26

27. 12/3/2022
27

28. 12/3/2022
28

29.  Windkessel vessels: large arteries
 Resistance vessels: small arteries
 Exchange vessels: formed by a
single layer of endothelial cells
 Capacitance vessels: veins
 Shunt vessels: Meta-arterioles
12/3/2022
29

30. 12/3/2022
30

31. 12/3/2022
31

32. SYSTEMIC CIRCULATION
Left Atrium 7-8/0
 Left Ventricle 120/0
 Aorta&large arteries 120/80
Arterioles 60
 Capillaries 25
 Venules&large veins 10
Vena cava (SVC&IVC) 2

12/3/2022
32

33. PULMONARY CIRCULATION
 Structure Pressure in mmHg
 Right Atrium 4-6/0
 Right Ventricle 25/0
 Pulmonary arteries 25/8
 Arterioles 10
 Capillaries 6-8
 Venules & larger branches 5
 Pulmonary veins 2

12/3/2022
33

34. 12/3/2022
34

35. Vessel % of blood
volume
Systemic 84 %
Arteries 13 %
Arteriole 1-2 %
Capillary 5 %
Veins 64 %
(54 %)
Pulmonary/Heart 16 %
Lungs 9 %
Heart 7 %
BLOOD DISTRIBUTION
12/3/2022
35

36. Pressure
12/3/2022
36

37. 12/3/2022
37

38. 12/3/2022
38

39. 12/3/2022
39

40. 12/3/2022
40

41. 12/3/2022
41

42. 12/3/2022
42

43. BLOOD FLOW, VELOCITY,
AND PRESSURE
12/3/2022
43

44. 12/3/2022
44

45. POISEUILLE-HAGEN EQATION
12/3/2022
45

46. BLOOD FLOW
Hagen-Poisseuille LawTM
12/3/2022
46

47. 12/3/2022
47

48. 12/3/2022
48

49. 12/3/2022
49

50. Poiseuille's Law
The biggest surprise in the application of Poiseuille's law to fluid flow is the dramatic effect of changing the radius.
A decrease in radius has an equally dramatic effect, as shown in blood flow examples.
12/3/2022
50

51. 12/3/2022
51

52. Blood Flow Examples
Suppose you have an emergency requirement for a five-fold increase in blood volume flowrate
(like being chased by a big dog)? How does your body supply it?
According to Poiseuille's law, a five-fold increase in blood pressure would be required if
the increase were supplied by blood pressure alone!
But the body has a much more potent method for increasing volume flowrate in the
vasodilation of the small vessels called arterioles.
Since the smaller vessels provide most of the resistance to flow, the arterioles in their
position just prior to the capillaries can provide a major controlling influence on the
volume flowrate. This system of small vessels can constrict flow to one part of the body
while enhancing the flow to another to meet changing demands for oxygen and nutrient
12/3/2022
52

53. 12/3/2022
53

54. Blood Flow Examples 12/3/2022
54

55. 12/3/2022
55

56. 12/3/2022
56

57. LAMINAR
FLOW
12/3/2022
57

58. TURBULENT
FLOW
12/3/2022
58

59. BLOOD FLOW PATTERNS
 But how do we know
which way a fluid will flow?
 We use an Engineering Trick:
 Dimensionless Numbers
12/3/2022
59

60. REYNOLD’S NUMBER
 Invented by an Engineer:
 Predicts Laminar flow versus Turbulent flow
 Low Number means Laminar Flow
 High Number means Turbulent Flow
12/3/2022
60

61. REYNOLD’S NUMBER
 Reynold’s Number is:
 The ratio of Inertial forces to Viscous forces
 Reynold’s Number = vpL/u
12/3/2022
61

62. REYNOLD’S NUMBER
 Reynold’s Number = vpL/u
 p is the weight-density of the fluid
 u is the dynamic viscosity of the fluid
 v is the velocity of the fluid flow
 L the Characteristic Length
12/3/2022
62

63. 12/3/2022
63

64. INTRINSIC REGULATION OF BLOOD FLOW
(AUTOREGULATION)
 Maintains fairly constant blood flow despite BP
variation
 Myogenic control mechanisms occur in some
tissues because vascular smooth muscle
contracts when stretched & relaxes when not
stretched
 E.g. decreased arterial pressure causes cerebral
vessels to dilate & vice versa
14-39
12/3/2022
64

65. INTRINSIC REGULATION OF BLOOD FLOW
(AUTOREGULATION) CONTINUED
 Metabolic control mechanism matches
blood flow to local tissue needs
 Low O2 or pH or high CO2, adenosine, or
K+ from high metabolism cause
vasodilation which increases blood flow (=
active hyperemia)
14-40
12/3/2022
65

66. 12/3/2022
66

67. 12/3/2022
67

68. 12/3/2022
68

69. Wall Tension
Pascal's principle requires that the pressure is everywhere the same inside the balloon at equilibrium. But examination
immediately reveals that there are great differences in wall tension on different parts of the balloon. The variation is
described by Laplace's Law.
12/3/2022
69

70. LaPlace's Law
The larger the vessel radius, the larger the wall tension required to withstand a given
internal fluid pressure.
For a given vessel radius and internal pressure, a spherical vessel will have half the
wall tension of a cylindrical vessel.
Why does the wall tension increase with radius?
12/3/2022
70

71. Why does wall tension increase with radius?
If the upward part of the fluid pressure remains the same,
then the downward component of the wall tension must
remain the same. But if the curvature is less, then the total
tension must be greater in order to get that same downward
component of tension.
12/3/2022
71

72. Tension in Arterial Walls
The tension in the walls of arteries and veins in the human body is a classic
example of LaPlace's law. This geometrical law applied to a tube or pipe
says that for a given internal fluid pressure, the wall tension will be
proportional to the radius of the vessel.
The implication of this law for the large arteries, which have comparable
blood pressures, is that the larger arteries must have stronger walls since an
artery of twice the radius must be able to withstand twice the wall tension.
Arteries are reinforced by fibrous bands to strengthen them against the risks
of an aneurysm. The tiny capillaries rely on their small size.
12/3/2022
72

73. Capillary Walls
The walls of the capillaries of the human circulatory system
are so thin as to appear transparent under a microscope,
yet they withstand a pressure up to about half of the full
blood pressure. LaPlace's law gives insight into how they
are able to withstand such pressures: their small size
implies that the wall tension for a given internal pressure is
much smaller than that of the larger arteries.
Given a peak blood pressure of about 120 mmHg at the
left ventricle, the pressure at the beginning of the capillary
system may be on the order of 50 mmHg. The large radii
of the large arteries imply that for pressures in that range
they must have strong walls to withstand the large
resulting wall tension. The larger arteries provide much
less resistance to flow than the smaller vessels according
to Poiseuille's law, and thus the drop in pressure across
them is only about half the total drop. The capillaries offer
large resistances to flow,but don’t required much strength
12/3/2022
73

74. 12/3/2022
74

75. 12/3/2022
75