Histology of the cardiovascular system

1. THE HISTOLOGY OF THE
CARDIOVASCULAR SYSTEM
DR. A.E.OMONISI
EKSUCOM, ADO-EKITI

2. OBJECTIVES OF THIS LECTURE
At the end of this lecture , students should be able to:
1. Identify and describe the various components of the
cardiovascular system.
2. Identify the 3 layers found in blood vessels.
3. Distinguish between arteries ,veins and capillaries.
4. Identify and differentiate b/w different types of veins.
5. Identify and differentiate b/w different types of arteries.
6. Identify and differentiate b/w different types of capillaries
7. Identify the layers of the atrial and ventricular walls and
understand how differences in the thickness of these layers
contribute to heart function.

3. Introduction
• Cardiovascular system is a closed system that
circulates blood.
• There are two groups of blood vessels:
1. One supplies the lungs ( the pulmonary circuit).
2. The other supplies the rest of the body ( the
systemic circuit).
Blood is pumped from the heart into both the
pulmonary and systemic ( aortic) trunks
simultaneously.

5. The Pulmonary Circuit: (b) Coronary
Angiogram

6. Introduction
• The cardiovascular system is concerned with
the transport of blood and lymph through the
body. It may be divided into four major
components:
• Heart,
• Macrocirculation,
• Microcirculation,
• Lymph vascular system.

7. General Structure of Blood Vessels
• A common structural pattern that can be seen
in all blood vessels with the exception of
capillaries, i.e. the division of the walls of the
blood vessels into three layers or tunics

9. Histological Organization of Blood
Vessels
• Blood vessel walls
1. Tunica intima
* Inner layer
* Endothelium and CT
2. Tunica media
* Middle layer
*Smooth muscle and CT
3. Adventitia
* Outer layer
* Mostly CT

10. Tunica intima
Innermost and consists of:-
• a)Endothelium. Simple squamous epithelium
• b) Subendothelium A thin layer of loose CT
(connective tissue)
• c) Internal elastic lamina

11. Tunica media
• Middle layer
• a) Concentric layers of smooth muscle cells
• b) External elastic lamina

12. Tunica Adventitia
• Outermost layer
• Composed of supporting tissue

13. Histological Comparison of Typical
Arteries and Veins

14. Structure of vessels - General relationship between tunics and arterial
and venous vessels

15. • Compared to veins, arteries
– Have thicker walls
– Have more smooth muscle and elastic fibers
– Are more resilient
Differences between arteries and veins

16. • Blood flows through the blood vessels from the
heart and back to the heart in the following
order:
– Elastic Arteries e.g. Aorta, pulmonary artery
– Muscular Arteries
– Arterioles
– Capillaries – the only vessels that allow exchange
– Venules
– Medium Veins
– Large Veins e.g. vena cava, pulmonary vein
Blood Flow Through the Blood Vessels

17. Diagram of general pattern of cardiovascular circulation

18. • As blood flows from the aorta toward the
capillaries and from capillaries toward the vena
cava:
– Pressure decreases
– Flow decreases
– Resistance increases
Blood Flow Through the Blood Vessels

19. Histological Organization of Blood
Vessels

20. Variations of Vessel Wall Structure
• Arteries.
• All arterial vessels originate with either the
pulmonary trunk (from the right ventricle) or the
aorta (from the left ventricle).
• Specialisations of the walls of arteries relate
mainly to two factors: the pressure pulses
generated during contractions of the heart
(systole) and the regulation of blood supply to the
target tissues of the arteries.
• The tunica media is the main site of histological
specialisation in the walls of arteries.

21. Muscular arteries
• The tunica intima is thinner than in elastic
arteries. Sub- endothelial connective tissue other
than the internal elastic lamina is often difficult to
discern.
• The internal elastic lamina forms a well defined
layer.
• The tunica media is dominated by numerous
concentric layers of smooth muscle cells. Fine
elastic fibres and and a few collagen fibres are
also present.

22. Muscular arteries
• The external elastic lamina can be clearly
distinguished although it may be incomplete
in places.
• The thickness and appearance of the tunica
adventitia is variable.

23. Arterioles
• Arterioles are arterial vessels with a diameter
below 0.1 - 0.5 mm .
• Endothelial cells are smaller than in larger
arteries, and the nucleus and surrounding
cytoplasm may 'bulge' slightly into the lumen of
the arteriole.
• The endothelium still rests on a internal elastic
lamina, which may be incomplete and which is
not always well-defined in histological sections

24. Arterioles
• The tunica media consists of 1-3 concentric layers of
smooth muscle cells.
• It is difficult to identify an external elastic lamina or to
distinguish the tunica adventitia from the connective
tissue surrounding the vessel.
• The smooth muscle of arterioles and, to some extent,
the smooth muscle of small muscular arteries regulate
the blood flow to their target tissues.
• Arterioles receive both sympathetic and
parasympathetic innervation

25. Capillaries
• The sum of the diameters of all capillaries is
significantly larger than that of the aorta (by
about three orders of magnitude), which
results in decreases in blood pressure and
flow rate.
• Also, capillaries are very small vessels. Their
diameter ranges from 4-15 µm.
• The wall of a segment of capillary may be
formed by a single endothelial cell

26. Capillaries
• This results in a very large surface to volume
ratio. The low rate of blood flow and large surface
area facilitate the functions of capillaries in:
• providing nutrients and oxygen to the
surrounding tissue, in
• the absorption of nutrients, waste products and
carbon dioxide, and in
• the excretion of waste products from the body.

27. Veins
• The walls of veins are thinner than the walls of
arteries, while their diameter is larger. In
contrast to arteries, the layering in the wall of
veins is not very distinct.
• The tunica intima is very thin.
• Only the largest veins contain an appreciable
amount of subendothelial connective tissue.
• Internal and external elastic laminae are
absent or very thin.

28. Veins
• The tunica media appears thinner than the tunica
adventitia, and the two layers tend to blend into
each other.
• The appearance of the wall of veins also depends
on their location.
• The walls of veins in the lower parts of the body
are typically thicker than those of the upper parts
of the body, and the walls of veins which are
embedded in tissues that may provide some
structural support are thinner than the walls of
unsupported veins.

29. Venules.
• They are larger than capillaries. Small venules
are surrounded by pericytes. A few smooth
muscle cells may surround larger venules. The
venules merge to form;
1.Small to medium-sized veins.
2.The largest veins of the abdomen and thorax

30. Small to medium-sized veins
• Small to medium – sized veins which contain
bands of smooth muscle in the tunica media.
The tunica adventitia is well developed. In
some veins (e.g. the veins of the pampiniform
plexus in the spermatic cord)the tunica
adventitia contains longitudinally oriented
bundles of smooth muscle.

31. Small to medium-sized veins
• Aside from most veins in the head and neck,
small to medium-sized veins are also
characterised by the presence of valves.
• The valves are formed by loose, pocket-
shaped folds of the tunica intima, which
extend into the lumen of the vein.
• The opening of the pocket will point into the
direction of blood flow towards the heart. One
to three (usually two) pockets form the valve.

32. The largest veins of the abdomen and
thorax
• The largest veins of the abdomen and thorax do
contain some sub-endothelial connective tissue
in the tunica intima, but both it and the tunica
media are still comparatively thin. Collagen and
elastic fibres are present in the tunica media. The
tunica adventitia is very wide, and it usually
contains bundles of longitudinal smooth muscle.
• The transition from the tunica adventitia to the
surrounding connective tissue is gradual. Valves
are absent

33. The largest veins of the abdomen and
thorax
• Vasa vasorum are more frequent in the walls
of large veins than in that of the
corresponding arteries - probably because of
the lower oxygen tension in the blood
contained within them.

36. The Organization of a Capillary Bed

37. Additional Specialisations of Vessels
• Small arteries and veins often form anastomosing
networks, which provides routes for alternative
blood supply and drainage if one of the vessels
should become occluded because of pathological
or normal physiological circumstances.
• Some arteries are however the only supply of
blood to their target tissues. These arteries are
call end arteries.
• Tissues which are supplied by end arteries die if
the arteries become occluded.

38. Additional Specialisations of Vessels
• Arteries and veins may also
form arteriovenous shunts, which can shunt
the blood flow that otherwise would enter the
capillary network between the vessels.

39. Lymphatic Vessels
• Parts of the blood plasma will exude from the
blood vessels into the surrounding tissues
because of transport across the endothelium
or because of blood pressure and the
fenestration of some capillaries .
• The fluid entering tissues from capillaries adds
to the interstitial fluid normally found in the
tissue.

40. Lymphatic Vessels
• The surplus of liquid needs to be returned to
the circulation.
• Lymph vessels are dedicated to
this unidirectional flow of liquid, the lymph.

41. Lymph capillaries
• are somewhat larger than blood capillaries
and very irregularly shaped. They begin as
blind-ending tubes in connective tissue. The
basal lamina is almost completely absent and
the endothelial cells do not form tight
junctions, which facilitates the entry of liquids
into the lymph capillary.

42. Lymph collecting vessels
• Are larger and form valves but otherwise
appear similar to lymph capillaries.
• The lymph is moved by the compression of
the lymph vessels by surrounding tissues.
• The direction of lymph flow is determined by
the valves.

43. Lymph ducts
Lymph ducts which contain one or two layers of
smooth muscle cells in their wall .
They also form valves. The walls of the lymph
ducts are less flexible in the region of the
attachment of the valves to the wall of the duct,
which may give a beaded appearance to the
lymph ducts.

44. THE HISTORY OF THE HEART

45. Chambers of the heart; valves

46. Serous membrane
Continuous with
blood vessels

47. NOTE:
- The heart serves as a mechanical pump to supply
the entire body with blood, both providing nutrients
and removing waste products.
- The great vessels exit the base of the heart.
- Blood flow: body→vena cava→right atrium→right
ventricle→lungs→left atrium→left ventricle→body
- The heart consists of 3 layers – the endocardium,
myocardium, and epicardium. The epicardium
(bottom left) consists of arteries, veins, nerves,
connective tissue, and variable amounts of fat.
- The myocardium contains branching, striated
muscle cells with centrally located nuclei. They
are connected by intercalated disks (arrowheads).

48. Heart Wall
Epicardium
Myocardium
Endocardium

49. Endocardium
• Innermost layer
• Composed of:
– Simple squamous
epithelium (endothelium)
– Connective Tissue
– Subendocardium: in
contact with cardiac
muscle and contains small
vessels, nerves, and
Purkinje Fibers.

50. Purkinje Fibers
• Impulse conducting fibers
• Large modified muscle
cells
– Cluster in groups together
– 1-2 nuclei and stain pale
due to fewer myofibrils
• Terminal branches of the
AV bundle branches
located in the
subendocardial connective
tissue

51. Myocardium
• Thickest layer of the heart
• Thickest in left ventricle because must
pump hard to overcome high pressure
of systemic circulation
• Right atrium the thinnest because of
low resistance to back flow
• Consist of cardiac muscle cells =
myocytes
– Different from smooth or skeletal
muscle cells due to placement of
nuclei, cross striations, and intercalated
disks
• Intercalated disks
– Junctional complexes that contain
fascia adherens, desmosomes, and
gap junction to provide connection
and communication.
– Bind myocytes and allow ion
exchange to facilitate electrical
impulses to pass

52. Cardiac Myocytes
Central nuclei
Fibers with Cross Striations
Branching
myocytes

53. Cardiac Muscle
Intercalated Disc

54. Smooth Muscle
Long, slender central nuclei,
lying within narrow, fusiform cells.
No cross striations
Skeletal muscle
Fibers with cross-striations and
peripheral nuclei.

55. Epicardium
• Outermost layer of the heart
• Composed of connective tissue with nerves,
vessels, adipocytes and an outer layer of
mesothelium
• Mesothelium secretes pericardial fluid
• Covers and protects the heart

56. Cardiac Valves
• 4 valves
– 2 AV (mitral and tricuspid) in the
chambers
– 2 semilunar (aortic /pulmonary)
• Composed of connective tissue
layers covered by endothelium on
each side; 3 layers
– Spongiosa: loose collagen
– Fibrosa: dense core of connective
tissue
– Ventricularis: dense connective
tissue with many elastic and
collagen fibers

57. Anatomy-Histology Clinical Correlate
By: Michael Lu, Class of ‘07

58. Clinical Correlates of the Histology of
the Cardiovascular System
1. Vasculitis
2. Gangrene resulting from ligation of end arteries
3. Atherosclerosis/ arteriolosclerosis
4. Coronary artery disease
5. Cardiomyopathy
6. Hypertensive heart disease
7. Valvular heart disease
8. Congenital heart disease
9. Heart failure

59. THANK YOU