The circulatory system consists of the heart, blood vessels, and blood. The heart pumps blood through arteries, capillaries, and veins to deliver oxygen and nutrients to tissues throughout the body. It has three layers - the inner endocardium, middle muscular myocardium, and outer epicardium. The heart's conducting system coordinates heartbeats and includes the sinoatrial and atrioventricular nodes and bundle of His. Blood vessels have three layers - the inner endothelium-lined tunica intima, middle smooth muscle-containing tunica media, and outer connective tissue tunica externa. Capillaries are the sites of nutrient exchange and have only an endothelium.

1. The Circulatory System
Rosemarie C. Quilala MD

2. The Circulatory System
-pumps and directs blood cells and substances carried in blood to all
tissues of the body.
- It includes both the blood and lymphatic vascular system.

3. Lecture outline:
• Heart
• 3 major layers
• Innate cardiac conducting system
• Purkinje fibers
• Cardiac skeleton

4. Lecture outline:
• Heart
• Vasculature
- 3 major tunics
- Endothelial cells
- Arteries (small, medium, large)
-Microvasculature (arteriole, capillaries,venules)
-Metarterioles
-Capillaries

5. Blood
Vascular
System
or
Cardiovascular
System

6. The CVS is composed of:
Heart- an organ whose function is to pump the blood.
Arteries- a series of efferent vessels that become smaller as they branch into
the various organs.
- carry the blood to the tissues.
Capillaries-the smallest blood vessels
- site of oxygen and carbon dioxide, nutrient and waste product
exchange between blood and tissues.
Veins- which result from the convergence of capillaries into a system of
larger channels that continue enlarging as they approach the heart,
toward which they convey the blood to be pumped again.

7. Endothelium
-is a single layer of a squamous epithelium that lined the internal
surface of all components of the blood.
Function:
1. Acts as a selectively permeable,antithrombogenic (inhibitory to clot
formation) barrier
2. Determines when and where white blood cells leave the circulation
for the interstitial space of tissues
3. Secretes a variety of paracrine factors for vessel dilation,
constriction and growth of adjacent cells.

8. Heart
• The heart is a muscular organ
that contracts rhythmically,
pumping the blood through the
circulatory system
• The right and left ventricles
pump blood to the lungs and the
rest of the body respectively
• The right and left atria receive
blood from the body and the
pulmonary veins respectively.

9. The Heart’s 3 major layers:
1. The inner endocardium of the endothelium and subendothelial
connective tissue.
2. The myocardium of cardiac muscle.
3. The epicardium, connective tissue with many adipocytes and
covered mesothelium.

10. The endocardium consists of:
Endothelium (En) -a thin layer of
connective tissue with smooth
muscle cells.
Subendothelial layer (Sen)- a layer of
variable thickness lacking smooth
muscle

11. Subendocardial layer
consists of:
• Small nerves
• Conducting (Purkinje) fibers (P) -
These fibers are cardiac muscle
cells joined by intercalated disks
but specialized for impulse
conduction rather than
contraction.
• Purkinje fibers are usually larger
than contractile cardiac muscle
fibers with large amounts of lightly
stained glycogen filling most of the
cytoplasm and displacing sparse
myofibrils to the periphery.

12. The cardiac conducting system
• This system consists of two nodes located in the right
atrium—the sinoatrial (SA) node (pacemaker) and the
atrioventricular (AV) node—and the atrioventricular
bundle (of His).
• The SA node is a small mass of modified cardiac muscle
cells that are fusiform, smaller and with fewer myofibrils
than neighboring muscle cells.
• The cells of the AV node are similar to those of the SA
node but their cytoplasmic projections branch in various
directions, forming a network.
• Distally fibers of the AV bundle become larger than
ordinary cardiac muscle fibers and acquire a distinctive
appearance. These conducting myofibers or Purkinje
fibers have one or two central nuclei and their cytoplasm
is rich in mitochondria and glycogen. Myofibrils are
sparse and restricted to the periphery of the cytoplasm

13. The Myocardium
• The thickest layer
• Consist mainly of cardiac muscle
with its fibers arranged spirally
around each heart chamber

14. The Epicardium
• Simple squamous mesothelium supported by a
layer of connective tissue containing blood vessels
and nerves.
• The epicardium, is the site of the coronary vessels
and contains considerable adipose tissue. This
section of atrium shows part of themyocardium
(M) and epicardium (Ep). The epicardium consists
of loose connective tissue (CT) containing both
autonomic nerves (N) and fat (F).
• The epicardium is the visceral layer of the
pericardium and is covered by the simple
squamous-to-cuboidal epithelium (arrows) that
also lines the pericardial space. These mesothelial
cells secrete a lubricate fluid that prevents friction
as the beating heart contacts the parietal
pericardium on the other side of the pericardial
cavity.

15. Cardiac skeleton
• Masses of dense irregular
connective tissue.
• Surrounds the bases of all heart
valves, separates the atria from
the ventricles, and provides
insertions for cardiac muscles.
• These fibrous tissue includes the
chordae tendinae, cords that
extend from the cusps and the
chordae tendinae to which they
attached.

16. Cardiac skeleton
Function:
• Anchors and supports the heart
valves
• Provides firm points of insertion
for cardiac muscle
• Helps coordinate the heartbeat
by acting a s electrical insulation
between atria and ventricles

17. Lecture outline:
• Heart
• Vasculature
- Endothelial cells
-3 major tunics
- Arteries (small, medium, large)
-Microvasculature (arteriole, capillaries,venules)
-Metarterioles
-Capillaries

18. Tissues of the vascular wall
• Walls of all blood vessels except capillaries contain smooth muscle
and connective tissue in addition to the endothelial lining.
• The endothelium is a specialized epithelium that acts as a
semipermeable barrier between two internal compartments: the
blood plasma and the interstitial tissue fluid.

19. Functions of Endothelial cells
1. Nonthrombogenic surface
2. Regulate local vascular tone and blood flow
3. Inflammation and local immune responses
4. Secrete growth factors

20. Walls of arteries, veins, capillaries
• tunica intima, tunica media, and
tunica externa (or adventitia)
• An artery has a thicker tunica media
and relatively narrow lumen.
• A vein has a larger lumen and its
tunica externa is the thickest layer.
The tunica intima of veins is often
folded to form valves.
• Capillaries have only an endothelium,
with no subendothelial layer or other
tunics.

21. • Simple squamous endothelial cells (arrows)
line the tunica intima (I) which has
subendothelial loose connective tissue and is
separated from the tunica media by the
internal elastic lamina (IEL), a prominent sheet
of elastin.
• The media (M) contains elastic lamellae and
fibers (EF) and multiple layers of smooth
muscle not seen well here. The tunica media is
much thicker in large arteries than veins, with
relatively more elastin.
• Elastic fibers are also present in the outer
tunica adventitia (A), which is relatively thicker
in large veins.
• Vasa vasorum (V) are seen in the adventitia of
the aorta. The connective tissue of the
adventitia always merges with the less dense
connective tissue around it.
Aorta Vena Cava

23. Microvasculature
Arterioles (A), capillaries (C), and venules
(V) comprise the microvasculature where,
in almost every organ, molecular exchange
takes place between blood and the
interstitial fluid of the surrounding tissues.
Lacking media and adventitia tunics and
with diameters of only 4-10 μm, capillaries
(C) in paraffin sections can be recognized by
nuclei adjacent to small lumens or by highly
eosinophilic red blood cells in the lumen.

24. Microvascular bed
structure and perfusion
• In metarterioles, the smooth muscle cells are
dispersed as bands that act as precapillary
sphincters.
• The thoroughfare channel, lacks smooth
muscle cells and merges with the
postcapillary venule.
• The precapillary sphincters regulate blood
flow into the true capillaries.
• At any given moment, most sphincters are at
least partially closed and blood enters the
capillary bed in a pulsatile manner for
maximally efficient exchange of nutrients,
wastes, O2, and CO2 across the endothelium

25. Capillaries
Capillaries consist only of an
endothelium rolled as a tube, across
which molecular exchange occurs
between blood and tissue fluid. (a)
Capillaries are normally associated with
perivascular contractile cells called
pericytes (P) that have a variety of
functions. The more flattened nuclei
belong to endothelial cells.

26. Types of capillaries
(a) Continuous capillaries- have tight,
occluding junctions sealing the intercellular
clefts between all the endothelial cells to
produce minimal fluid leakage.
(b) Fenestrated capillaries- also have tight
junctions, but perforations (fenestrations)
through the endothelial cells allow greater
exchange across the endothelium.
c) Sinusoids, or discontinuous capillaries,
usually have a wider diameter than the other
types and have discontinuities between the
endothelial cells, large fenestrations through
the cells, and a partial, discontinuous
basement membrane.

27. Venules
• the last segment of the microvasculature where capillary beds
generally drain.
• Postcapillary venules- are the sites at which white blood cells enter
damaged or infected tissue

28. Venules
(a)Compared to arterioles (A), postcapillary venules (V) have
large lumens and an intima of simple endothelial cells, with
occasional pericytes (P).
(b)Larger collecting venules (V) have much greater diameters
than arterioles (A), but the wall is still very thin, consisting of an
endothelium with more numerous pericytes or smooth muscle
cells.
(c) The muscular venule cut lengthwise here has a better
defined tunica media, with as many as three layers of smooth
muscle (M) in some areas, a very thin intima (I) of endothelial
cells (E), and a more distinct adventitia (Ad). Part of an arteriole
(A) shows a thicker wall than the venule.
Postcapillary venules are important as the site in the vasculature
where these cells leave the circulation to become functional in
the interstitial space of surrounding tissues when such tissues
are inflamed or infected.
(d) Postcapillary venule (V) from an infected small intestine
shows several leukocytes adhering to and migrating across the
intima.

29. Arteriovenous shunts
In skin blood flow can be varied according to external
conditions by arteriovenous (AV) shunts, or anastomoses,
commonly coiled, which directly connect the arterial
and venous systems and temporarily bypass capillaries.
In venous portal systems one capillary bed drains into
a vein that then branches again into another capillary bed.
This arrangement allows molecules entering the blood in
the first set of capillaries to be delivered quickly and at high
concentrations to surrounding tissues at the second capillary
bed, which is important in the anterior pituitary gland
and liver.

30. Veins
(a) Small vein (V) shows a relatively large lumen
compared to the small muscular artery (A) with
its thick media (M) and adventitia (Ad). The wall
of a small vein is very thin, containing only two
or three layers of smooth muscle.
(b) Convergence between two small veins shows
valves (arrow). Valves are thin folds of intima
projecting well into the lumen, which act to
prevent backflow of blood.
(c) Medium vein (MV) shows a thicker wall but
still less prominent than that of the
accompanying muscular artery (MA). Both the
media and adventitia are better developed, but
the wall is often folded around the relatively
large lumen.
(d) Medium vein contains blood and shows
valve folds (arrows).

31. Wall of Large vein with valve
Large veins have a muscular
media layer (M) that is very thin
compared to the surrounding
adventitia (A) of dense irregular
connective tissue.
The wall is often folded with the
intima (I) projecting into the
lumen as a valve (V) composed of
the subendothelial connective
tissue with endothelium on both
sides.