The circulatory system consists of the cardiovascular system and lymphatic system. The cardiovascular system includes the heart and blood vessels that circulate blood throughout the body. It transports oxygen, nutrients, hormones, and immune cells and removes carbon dioxide and wastes. The circulatory system is divided into the systemic and pulmonary circuits. Arteries branch into smaller arterioles and capillaries, where gas and nutrient exchange occurs, before venules and veins return blood to the heart. The lymphatic system drains excess fluid from tissues and transports it back to the bloodstream.

1. Introduction to Circulatory
System
1

2. Outline
• Overview
• Functional components of the circulatory system
• Cardiovascular system
– Functions
– Vessels of the blood circulatory system
– Blood Circulation
– Anastomoses, collateral circulation, and terminal (end)
arteries, vasa vasorum
– Cross sectional areas of blood vessels
– Blood volumes
• Lymphatic System
– Introduction
– Lymph
– Lymphatic vessels
– Lymphoid organs
2

3. Overview
• Beyond a distance of a few cell diameters, diffusion is
not sufficiently rapid to meet the metabolic
requirements of cells
• In large multicellular organisms, therefore, some
mechanism other than diffusion is needed to transport
molecules rapidly over the long distances between
internal cells and the body’s surface and between the
various specialized tissues and organs
• In the animals this is achieved by the circulatory
system, which comprises
– The cardiovascular system
– The lymphatic system
3

4. Functional components of the circulatory
system
• The circulatory system has two functional
components:
– Blood vascular system
– Lymph vascular system
• Blood circulatory system
– Comprises a circuit of vessels through which
blood flow is initiated by continuous action of a
central muscular pump, the heart
– The arterial system provides a distribution
network to the tiny peripheral microcirculation,
the capillaries, which are the main sites of
interchange of gas and metabolite molecules
between the tissues and the blood
– The venous system carries blood from the
capillary system back to the heart
4

5. • Lymph vascular system
– A network of drainage vessels for returning
excess extravascular fluid, the lymph, to the
blood circulatory system, and for transporting
lymph to the lymph nodes for immunological
screening
– Begins with the lymphatic capillaries, which are
closed-ended tubules that merge to form vessels
of steadily increasing size
– These vessels terminate in the blood vascular
system emptying into the large veins near the
heart
– The lymphatic system has no central pump but in
all but the tiniest lymphatics there is an intrinsic
pumping system effected by contractile smooth
muscle fibers in the lymph vessel wall, combined
with a valve system preventing back flow 5

6. The Cardiovascular System
6

7. Components of the Cardiovascular
System
• Blood: the carrier of substances
• Blood vessels, or vascular system: set
of interconnected tubes through which
the blood flows
• Heart: a pump that produces this flow
7

8. Functions of the Cardiovascular System
• Mediates the continuous movement of all body
fluids
– Transport system that supplies O₂ (from the lungs) and nutrients
(absorbed from the gastrointestinal tract) to the tissues, returns
CO₂ to the lungs and other waste products of metabolism to the
kidneys
• Distribute hormones and cells of the immune
system
• Involved in temperature regulation; blood
participates in heat distribution
• Blood is also involved in maintenance of acid-base
and osmotic balance
• Blood cells are important in defense against
infection 8

9. Vessels of the blood circulatory system
• The heart is the principal organ of the blood circulatory
system, pumping blood throughout the body and
providing the force by which nutrients leave the
capillaries and enter tissues
• Large elastic arteries leave the heart and branch to
form muscular arteries
• These arteries branch further and enter organs, where
they branch much further to form arterioles
• Arterioles branch into the smallest vessels, the
capillaries, the site of exchange between blood and
surrounding tissue
• Capillaries then merge to form venules, which merge
further into small and then medium-sized veins
• These veins leave organs, form larger veins which
eventually bring blood back to the heart
9

10. 10

11. 11
Blood Circulation
• Divided into:
– Systemic circulation
– Pulmonary circulation
• The blood is pumped through a closed system of
blood vessels by the heart, which in mammals is
really two pumps in series with each other
• Systemic circulation
– The flow of blood through vessels from left
ventricle to the tissue of the body and back to
the right atrium
 From the left ventricle, blood is pumped
through the arteries and arterioles to the
capillaries, where it equilibrates with the
interstitial fluid
 The capillaries drain through venules into the
veins and back to the right atrium

12. • Pulmonary circulation
– The flow of blood through vessels that
carry blood from the right ventricle to
the lungs, and back to the left atrium
 From the right atrium, blood flows to
the right ventricle, which pumps it
through the vessels of the lungs and
back to the left atrium then to the left
ventricle
 In the pulmonary capillaries, the blood
equilibrates with the O₂ and CO₂ in the
alveolar air
12

13. 13

14. • The circulatory system consist of
– Macrovasculature
• vessels that are more than 0.1 mm in
diameter
• large arterioles, muscular and elastic
arteries, and muscular veins
– Microvasculature
• small arterioles, capillaries, and
postcapillary venules
• visible only with a microscope
• site of interchanges between blood and
the surrounding tissues
14

15. Microcirculation
• Types
1. Usual
– Arteriole – capillary – venule
2. Arteriovenous anastomosis
– Arteriole – venule
3. Portal system
– Arteriole - capillary – portal vein/venule –
capillary - venule
15

16. Blood vessels
• The blood vessels of the body form a closed
delivery system that begins and ends at the
heart
• It is a dynamic system of structures that
pulse, constrict and relax and even proliferate
to meet changing body needs
• The pattern of distribution starts with
arteries then to arterioles to capillaries to
venules to veins
• The blood vessels in the adult human body
carry blood in a distribution network that is
approximately 60,000 miles in length
• Only capillaries come into intimate contact with
tissue cells and serve cellular needs
16

17. Types of blood vessel
• The major types of blood vessels are
– Arteries: large distributing vessels that
carry blood away from heart
– Arterioles: bring blood to the capillaries
– Capillaries: tiny vessels that distribute blood
to the cells; branch to form networks in the
tissue
– Venules: collect blood from several capillaries
– Veins: large collecting vessels that bring
blood back to the heart
17

18. Tissues of the Vascular Wall
• Walls of larger blood vessels contain
three basic structural components:
– Endothelium: a simple squamous epithelium
– Smooth muscle
– Connective tissue: elastic and collagen
fibers
• The amount and arrangement of these
tissues in vessels are influenced by
– mechanical factors: primarily blood
pressure
– metabolic factors: local needs of tissues
18

19. Structural Plan of Blood Vessels
• All blood vessels other than capillaries have
many structural features in common and
present a similar plan of construction
• The distinction between different types of
vessels often is not clear-cut because the
transition from one type to another is gradual
• Blood vessels’ wall is composed of 3 layers, or
tunics
– Tunica intima
– Tunica media
– Tunica externa
19

20. 20

21. • Tunica intima
– Has one layer of endothelial cells supported
by a thin subendothelial layer of loose
connective tissue with occasional smooth
muscle cells
– In arteries, the intima is separated from the
media by an internal elastic lamina, the most
external component of the intima
• composed of elastin, has holes (fenestrae) that
allow the diffusion of substances to nourish cells
deep in the vessel wall
21

22. • Tunica media
– The middle layer
– Consists chiefly of concentric layers of
helically arranged smooth muscle cells
– Interposed among the smooth muscle cells
are variable amounts of elastic fibers and
lamellae, reticular fibers of collagen type
III, proteoglycans, and glycoproteins
– In arteries, the media has a thinner external
elastic lamina, which separates it from the
tunica adventitia
22

23. • Tunica adventitia or tunica externa
– Consists principally of type I collagen and
elastic fibers
– Gradually continuous with the stromal
connective tissue of the organ through which
the blood vessel runs
23

24. Aorta (a) and Vena cava (b): Simple squamous endothelial cells
(arrows); tunica intima (I); internal elastic lamina (IEL); media (M);
elastic fibers (EF); tunica adventitia (A); Vasa vasorum (V) 24

25. Arteries
• Arteries are vessels that carry blood away from the
heart
• Transport blood under high pressure to the tissues
– Need strong muscular walls
• Rapid blood flow
• All arteries carry oxygen rich blood with the
exception of pulmonary arteries
• 2 major types
– Elastic Arteries
• Help to stabilize the blood flow
• Include aorta and its large branches
– Muscular Arteries
• Control blood flow to organs by contracting or
relaxing the smooth muscle cells of the tunica
media 25

26. Arterioles
• Muscular arteries branch repeatedly into smaller and
smaller arteries, until reaching a size with only two or
three medial layers of muscle
• The smallest arteries branch as arterioles, and
indicate the beginning of an organ's microvasculature
where exchanges between blood and tissue fluid occur
• Arterioles are generally less than 0.5 mm in diameter
• Larger arterioles exhibit all three tunics, but their
tunica media is chiefly a few scattered muscle fibers
• The smaller arterioles that lead into capillary beds,
are little more than a single layer of smooth muscle
cells spiraling around the endothelial lining
26

27. Arterioles
• The diameter of each arteriole is
regulated in two ways:
– Local factors in the tissues
• signal the smooth musculature to
contract or relax, thus regulating the
amount of blood sent downstream to each
capillary bed
– Sympathetic nervous system
• adjusts the diameter of arterioles
throughout the body to regulate systemic
blood pressure
27

28. Capillaries
• Capillaries are the body’s most important blood vessels
because they renew and refresh the interstitial fluid
with which all cells in the body are in contract
• Capillaries permit different levels of metabolic
exchange between blood and surrounding tissues
• Capillaries deliver to interstitial fluid the oxygen and nutrients
that cells need while removing carbon dioxide and other wastes
that cells deposit in the fluid
• They are composed of a single layer of endothelial cells
rolled up in the form of a tube
• The average diameter of capillaries varies from 5 to 10
µm and their individual length is usually not more than
50 µm
• Walls are very thin and contain lots of tiny capillary
pores that are permeable to water and other small
molecules
• Form an interconnected network
28

29. • Given their location and the thinness of their walls
capillaries are ideally suited for their role of providing
access to nearly every cell
• Because of their thin walls and slow blood flow,
capillaries are a favorable place for the exchange of
water, solutes, and macromolecules between blood and
tissues
• The capillaries are often referred to as exchange
vessels, since it is at these sites that O2, CO2,
substrates, and metabolites are transferred from blood
to the tissues and from the tissues to blood
• Some capillaries also perform site-specific functions
– Lungs: gas exchanges
– Endocrine glands: pick up hormones
– Small intestine: take up nutrients
– Kidneys: removal of nitrogenous wastes
29

30. Capillary Beds
• A network of the body’s smallest vessels that run
throughout almost all tissues, especially the loose
connective tissue
• The flow of blood in these vessels is called
microcirculation
• In most body regions, a capillary bed consists of two
types of vessels:
• a vascular shunt (metarteriole)
• true capillaries
• The terminal arteriole leads into a metarteriole
which is directly continuous with the thoroughfare
channel
• The thoroughfare channel joins the post-capillary
venule that drains the capillary bed
30

31. • A cuff of smooth muscle fibers, called a pre-
capillary sphincter surrounds the root of each
true capillaries and acts as a valve to regulate
the flow of blood into the capillary
– When the precapillary sphincters are relaxed,
blood flows through the true capillaries and
takes part in exchange with tissue cells
– When the precapillary sphincters are
contracted, blood flows through the shunts
and bypasses the tissue cells
31

32. • The relative amount of blood entering a
capillary bed is regulated by vasomotor
nerve fibers and local chemical
conditions
• A capillary bed may be flooded with
blood or almost completely bypassed,
depending on conditions in the body or in
that specific organ
32

33. 33

34. 34

35. Types of capillaries
35
• Capillaries have structural variations which
permit different levels of metabolic exchange
between blood and surrounding tissue
• They can be grouped into three types,
depending on the continuity of the endothelial
cells and the external lamina
– Continuous capillary
– Fenestrated capillary
– Sinusoid or discontinuous capillary

36. Venules
• Collect blood from capillaries
• Ranging from 15 to 100 m in diameter
• Formed when capillaries unite
• Gradually join to form larger veins
• The smallest venules, the post capillary venules,
consist of endothelium on which lie pericytes
• Very thin walled
• Adventitia is relatively thicker
• Media contain pericytes and few smooth muscles
• Smaller venules serve for interchange of
metabolites
36

37. Veins
• Veins are the blood vessels that conduct blood
from the capillaries back to the heart
• Because blood pressure declines substantially
while passing through the high-resistance
arterioles and capillary beds, blood pressure in
the venous part of the circulation is much lower
than in the arterial part
• Because they need not withstand as much
pressure, the walls of veins are thinner than
those of comparable arteries
• The venous vessels increase in diameter, and
their walls gradually thicken as they progress
from venules to the larger and larger veins
leading to the heart
37

38. Veins
• Major reservoir of blood
• Walls are muscular enough to contract or
expand so veins can act as a controllable
reservoir for extra blood
• With their large lumens and thin walls,
veins can accommodate a fairly large
blood volume
• Up to 65% of the body’s total blood
supply is found in the veins at any one
time although the veins are normally only
partially filled with blood 38

39. Veins: valves
• Formed from folds of
the tunica intima
• Prevent backflow of
blood
• Most abundant in the
veins of the limbs,
where the upward flow
of blood is opposed by
gravity
39

40. Veins: valves
• A few valves occur in the veins of the
head and neck, but none are located in
veins of the thoracic and abdominal
cavities
• A functional mechanism that aids the
return of venous blood to the heart is
the normal movement of our body and
limbs
40

41. Veins: Musculovenous pump
• Contracting muscles
press against the
thin-walled veins
forcing valves
proximal to the
contraction to open
and propelling the
blood toward the
heart
41

42. Musculovenous pump
42

43. Anastomoses
• Anastomoses are communications between multiple
branches of an artery or arteries/vein or veins
• Most organ receive blood from more than one arterial
branch and arteries supplying the same area often merge
at their distal end, forming arterial anastomoses
• Arterial anastomoses provide alternative routes for blood
flow if the usual pathway is obstructed by compression,
the position of a joint, pathology, or surgical ligation
• Arterial anastomoses are abundant around joints, where
active movement may hinder blood flow through one
channel
• Anastomoses are also prevalent in the abdominal organs,
brain, and heart
• Veins anastomose much more freely than arteries and
because of abundant collateral circulation occlusion of a
vein rarely blocks blood flow leading to tissue death 43

44. Collateral circulation
• If a main channel is occluded,
the smaller alternate channels
can usually increase in size,
providing a collateral
circulation that ensures the
blood supply to structures
distal to the blockage
• However, collateral pathways
require time to develop; they
are usually insufficient to
compensate for sudden
occlusion or ligation
44

45. End arteries
• Arteries that do not anastomose with adjacent arteries
are true terminal (end) arteries
• Arteries that do not anastomose, or which have a poorly
developed collateral circulation are vulnerable if their
blood flow is interrupted
• Occlusion of a terminal artery disrupts the blood supply
to the structure or segment of an organ it supplies
• For example, occlusion of the terminal arteries of the
retina will result in blindness
• Although not true terminal arteries, functional terminal
arteries (arteries with ineffectual anastomoses) supply
segments of the brain, liver, kidney, spleen, and
intestines
45

46. Vasa Vasorum
• The wall of the blood vessels contain living
cells and therefore require a blood supply of
their own
• For this reason the larger arteries and veins
have tiny arteries, capillaries and veins in
their tunica externa
• These tiny vessels are called vasa vasorum
• Nourish the outer half of the wall of a large
vessel with the inner half being nourished by
the blood in the lumen
46

47. Cross sectional areas of blood vessels
• If the vessels of each type were put side by
side their total cross sectional area would be:
47
Vessel Cross sectional area (m2)
Aorta 2.5
Small arteries 20
Arterioles 40
Capillaries 2500
Venules 250
Small veins 80
Vena cavae 8

48. Blood volumes
• Pulmonary circulation 9%
• Heart 7%
• Arteries 13%
• Arterioles and capillaries 7%
• Veins, venules and venous sinuses 64%
48

49. The Lymphatic System
49

50. Introduction
• The lymphatic system supports the
function of the cardiovascular and
immune systems of the body
• The lymphatic system consists of two
semi-independent parts
–A network of lymphatic vessels
–Lymphoid organs scattered
throughout the body
50

51. Introduction
• The lymphatic vessels transport fluids that
have escaped from the cardiovascular system
• The main components of the immune system
(lymphocytes, lymphoid tissue, and lymphoid
organs) fight infections and confer immunity
to disease
• An elaborate system of lymphatic vessels runs
throughout the body
• These vessels collect a fluid called lymph from
the loose connective tissue around blood
capillaries and carry this fluid to the great
veins at the root of the neck
51

52. Introduction
• Because lymph
flows only toward
the heart, the
lymphatic vessels
form a one-way
system rather
than a full circuit
52

53. Lymphatic vessels
• There are several
orders of vessels
– Lymph capillaries
– Lymph collecting
vessels
– Lymph nodes
– Lymph trunks
– Lymph ducts
53

54. Lymph
• Blood capillaries are
surrounded by a
loose connective
tissue that contains
tissue fluid or
interstitial fluid
• The fluid arises
from blood filtered
through the
capillary walls
54

55. • Tissue fluid consists of small molecules of blood
plasma, water, various ions, nutrient molecules,
and respiratory gases
• Tissue fluid is continuously leaving and re-
entering the blood capillaries
• Slightly more fluid arises from the arteriole end
than re-enters the venule end
– This amounts to about 3 liters a day
• The lymphatic vessels collect this excess fluid
and return it to the bloodstream
• Any blockage of the lymphatic vessels causes
the affected body region to swell with excess
tissue fluid resulting in edema
55

56. • Blood proteins leak slowly from blood
capillaries into the surrounding tissue
fluid
• Lymph vessels return leaked proteins to
the bloodstream
• This is important because proteins in
blood generate osmotic forces that are
essential for keeping water in the
bloodstream
56

57. Lymph Capillaries
• Lymph capillaries are permeable vessels
that receive the tissue fluid
• They are located near blood vessels in
the loose connective tissue
• Like blood capillaries their walls
consists of a single layer of endothelial
cells
• Lymph capillaries are very permeable
• They have few intercellular junctions
57

58. • In lymph capillaries the adjacent edges of cells
overlap to form easily opened minivalves
• These valves open to allow tissue fluid to enter
the lymphatic capillary
• Collagen filaments anchor portions of the cell
membrane to the surrounding connective tissue
• Any fluid pressure in the volume of the tissue
fluid separates the mini valves
• This opens gaps in the wall of the capillary
allowing fluid to enter
• Once in the lymphatic capillary, it cannot leak
out, because backflow pressure forces the mini
valve flaps together
58

59. 59

60. • Once interstitial fluid enters the lymphatic
capillaries it is called lymph
• Although the high permeability of lymph capillaries
allows the uptake of large quantities of tissue fluid
and protein molecules it also allows bacteria,
viruses, or cancer cells in the loose connective
tissue to enter these capillaries with ease
• These pathogenic agents can then travel
throughout the body via the lymphatic vessels
• However, most pathogenic agents are destroyed in
the lymph nodes by various antibodies before
reaching the general circulation
• Cancer cells can be the most problematic and can
actually use lymph nodes as a site to metastasize
further 60

61. Distribution of Lymphatic Vessels
• Lymph capillaries are widespread, occurring
almost everywhere blood capillaries occur
• Lymph capillaries are absent from bone and
teeth, bone marrow, and the entire central
nervous system
• In the CNS excess fluid drains into the
cerebrospinal fluid and then returns it to the
blood at the superior sagittal sinus
61

62. Lymph Collecting Vessels
• From the lymph capillaries, lymph enters lymph
collecting vessels which accompany blood
vessels
• In general, the superficial lymphatic collecting
vessels in the skin travel with superficial veins
• Deep lymphatic collecting vessels of the trunk
and digestive viscera travel with the deep
arteries
• The vessels have the same tunics as blood
vessels but their walls are always thinner
• The thinness reflects the fact that lymph flows
under very low pressure and have no pump to
move the lymph along 62

63. • To direct the flow of
lymph, lymphatic
collecting vessels
contain more valves
than do veins
• At the base of each
valve, the vessel
bulges, forming a
pocket in which
lymph collects and
forces the valve shut
• Because of these
bulges, each
collecting vessel
resembles a string of
beads 63

64. Lymph flow
• Lymph is propelled through lymph vessels by a
series of weaker mechanisms
• The action of contracting skeletal muscle and
the pulsation of nearby arteries push on lymph
vessel, squeezing lymph through them
• The muscular tunica media of the lymph vessels
also contracts to help propel the lymph
• Additionally, the normal movements of the limbs
and trunk keep the lymph flowing
• Despite these mechanisms, the transport of
lymph is slow
• People who are inactive or who stand for long
times often develop edema
64

65. Lymph Nodes
• Bean shaped organs
situated along
lymphatic
collecting vessels
• Lymph nodes
cleanse the lymph
of pathogens
• There are about
500 lymph nodes in
the body
65

66. • Large clusters of superficial lymph nodes are
found in the neck, axillary, and inguinal regions
• Cervical lymph nodes
– along the jugular veins
– receive lymph from the head and neck
• Axillary lymph nodes
– in the armpit
– filter lymph from the upper limbs
• Inguinal lymph nodes
– in the superior thigh
– filter lymph from the lower limb
66

67. • Tracheobronchial lymph nodes
–in mediastinum
–receive lymph from the thoracic
viscera
• Aortic lymph nodes
–along the abdominal aorta
–filter lymph from the posterior
abdominal wall
• Iliac lymph nodes
–along the iliac arteries
–filter lymph from the pelvic organs and
the lower limbs
67

68. Lymph Trunks
• After leaving the lymph nodes, the largest lymphatic
collecting vessels converge to form lymph trunks
• These trunks drain large areas of the body
Lumbar and Intestinal Trunks
• The paired lumbar trunks lie along either side of the
aorta in the inferior abdomen
– They receive all lymph draining from the lower
limbs, pelvic organs and some of the anterior
abdominal wall
• The unpaired intestinal trunk lies near midline on the
posterior abdominal wall
– It receives fatty lymph (chyme) from the stomach,
intestines, and other digestive organs
68

69. Bronchomediastinal Trunks
• paired trunks ascend near the sides of the trachea
• collect lymph from the thoracic viscera and thoracic
wall
Subclavian Trunks
• paired trunks located near the subclavian vein
• receive lymph from the upper limbs
• also drain the inferior neck and the superior thoracic
wall
Jugular Trunks
• located in the neck at the base of each internal jugular
vein
• drain lymph from the head and neck
69

70. 70

71. Lymph Ducts
• The lymph trunks drain into the largest
vessels - the lymph ducts
–Thoracic duct
–Right lymphatic duct
71

72. Thoracic Duct
• Begins from a sac called cisterna chyli, located
at the union of the lumbar and intestinal trucks
which lies on the bodies of vertebrae L1 & L2
• Ascends along the vertebral bodies
• In the superior thorax, it turns to left and
empties into the venous circulation at the
junction of the left internal jugular and left
subclavian veins
• It drains all parts of body below diaphragm and
left thorax, left upper limb, left part of head
and neck
72

73. Right lymphatic Duct
• Formed by the union of the right
jugular, subclavian, and
bronchomediastinal trunks
• Empties into the venous circulation at
the junction of the right internal
jugular and subclavian veins
• Drains the right thorax, right upper
limb, right part of head and neck
73

74. 74

75. Lymphoid Tissue
• Lymphoid tissue is an important component of
the immune system because it
– Houses and provides a proliferation site for
lymphocytes
– Furnishes an ideal surveillance vantage point for both
lymphocytes and macrophages
• This tissue is the main battleground in the fight
against infection
• Lymphoid tissue is where most lymphocytes
become activated and most effector and
memory lymphocytes are generated
75

76. Lymphoid Tissue
• This tissue has two general locations
– Mucous membranes
– Lymphoid organs
• Mucous membranes
– Found within the digestive, respiratory,
urinary and reproductive tracts where it is
called mucosa-associated lymphoid tissue
(MALT)
• Lymphoid Organs
– Lymph nodes, spleen, thymus, tonsils,
aggregated lymphoid nodules
76

77. Lymphoid Organs
• Lymphoid organs are
– Lymph nodes
– Spleen
– Thymus
– Tonsils
– Aggregated
lymphoid nodules in
the small intestine
and appendix
77