Circulatory System
The circulatory system consists of the heart and blood vessels.
The heart is a specialized blood vessel that acts as a pump to circulate the blood.
Knowledge of the structure and function of blood vessels is important in understanding
vascular diseases, the leading cause of death all over the world.
Blood vessels are divided into three groups:
Arteries
Veins
Capillaries.
Arteries and veins are further divided, according to size, into large, medium, and small
blood vessels.
Arteries Veins Capillaries
 The vascular system is subjected to varying degrees of hydrostatic pressure.
 The structure of vessels varies in an adaptive fashion.

Blood vessels are thickest and their walls more complex in the immediate
vicinity of the heart, where hydrostatic pressure is greatest.
 As blood vessels decrease in size their wall becomes thinner and less complex.
Cross Section of a Blood Vessel
Microscopic Structure of Blood Vessels
Wall of blood vessels are composed of three layers:
a) Tunica intima (innermost layer)
b) Tunica media (middle layer)
c) Tunica adventitia (outermost layer)
.
Microscopic Structure of Blood Vessels
The tunica intima consists of:
1-A layer of endothelial cells lining the lumen of the vessel .
2- A sub-endothelial layer, made up of mostly loose connective tissue.
Often, the internal elastic lamina separates the tunica intima from the tunica media
The tunica media is composed chiefly of circumferentially arranged smooth muscle cells.
The external elastic lamina often separates the tunica media from the tunica adventitia.
Finally, the tunica adventitia is primarily composed of loose connective tissue made up of
fibroblasts and associated collagen fibers.
Wall of Blood Vessel
1-Tunica intima
• Endothelial cell lining
• Sub-endothelial layer
• Internal elastic lamina
2- Tunica media
• Smooth muscle cells, collagen fibers, and ground substance
• Fenestrated elastic lamellae (esp. in elastic arteries)
• External elastic lamina
3- Tunica adventitia
• Mostly collagenous fibers
• Elastic fibers (not lamellae)
• Fibroblasts and macrophages
• Vasa vasorum
Classification of Arteries
1-Arterioles and small arteries
2- Medium sized arteries
3- Large arteries
1-Arterioles
All the three tunica are distinguished.
Intima. Sub-endothelial connective tissue is absent.
Media consists of few layers of smooth muscle fibers.
Adventitia consists of thin layers of longitudinally. arranged collagen and elastic fibers.
These vessels chiefly control the blood pressure.
2-Medium Sized Artery (typical structure)
(muscular arteries/ distribution arteries)
Tunica intima:
All the three layers , endothelium, sub-endothelial connective tissue and internal elastic
lamina are present
Internal elastic lamina is very prominent.
Tunica media:
Very thick, many layers of smooth muscle.
Adventia:
Consists of collagen and elastic fibers.
Example of medium sized arteries is radial, femoral etc.
3- Large Arteries
(Elastic arteries/ conducting arteries)
They are thin walled, having large diameter.
Large amount of elastic tissue is present in the wall.
Intima
Endothelium
Sub- endothelial tissue containing collagen and elastic fibers.
There is no internal elastic lamina.
Media
There are circularly arranged membranes made up of elastic fibers, 40-70 in number.
Smooth muscles and ground substance in moderate amount
No external elastic lamina
Adventia
Adventia of elastic arteries is made up of collagen fibers.
Example of elastic arteries are Aorta, pulmonary arteries, common carotid , subclavian
and brachiocephalic trunk
Capillaries
These are the vessels which connect arteries and veins.
Diameter is 7-9 microns.
They are found in the tissues in the form of network.
Wall consists of endothelial cells and basal lamina.
Cells are arranged longitudinally
Capillaries
In transverse section only two or three cells are visualized which are joined by occluding
junctions.
Certain elongated cells with cytoplasmic processes are present between the endothelial
cells and basal lamina known as pericytes.
Classification of Capillaries
I.
Continuous
II.
Fenestrated
III.
Sinusoidal
I.
Continuous Capillaries
Continuous capillaries are found in muscle, nervous tissue, lungs, skin and in connective
tissue.
Their wall is continuous without any pore.
Many pinocytotic vesicles are found in the cytoplasm for the transport of fluid across the
capillary wall.
II.
Fenestrated Capillaries
They are found in kidneys, endocrine glands, intestinal mucosa and choroid plexus of
brain.
They are characterized by the presence of pores in the wall
The pores are 60 to 90 nm size, and covered by a diaphragm.
Rapid exchange of substances takes place through their walls
III.
Sinusoidal Capillaries
They are found in liver, spleen and bone marrow.
Their diameter is much greater than ordinary capillaries (30-40um).
Walls are not regular.
There are large pores in their walls, without any diaphragm
Phagocytic cells are frequently found in the walls of the sinusoids.
They allow very rapid exchange of substances between the blood and tissue.
VEINS
Basic structure of veins is similar to arteries.
Lumen of the veins is larger as compared to arteries of same size.
Classification of veins
1- Venule
2- Medium sized vein
3-Large veins
1-Venules- 0.2-1mm in diameter
Intima –
endothelium only
Media –
few layers of smooth muscle
Adventia - thick
Medium Size Vein
1-9 mm in diameter.
Intima- Endothelium and a thin layer of sub-endothelium.
Media- Thin layer of smooth muscle.
Adventia- Well developed, consists of collagen fibers.
Examples are axillary vein, and femoral vein etc.
VEIN (T.S)
Large Veins
Intima of large veins is endothelium and sub-endothelium
Internal elastic lamina is either poorly developed or absent.
Media thin or poorly developed, contains collagen fibers only.
Adventia is thickest in these veins. It consists of three layers:
a) inner layer of coarse collagen fibers-run spirally
b) middle layer of smooth muscle cells - run longitudinally
c) outermost layer of coarse collagen and elastic fibers
Histology of Heart
The wall of the heart consists of same three layers,
tunica intima, tunica media and tunica adventitia.
The tunica intima in the heart is known as endocardium.
The endocardium lies on the luminal side of the myocardium.
Its inner surface is covered with endothelial cells – the squamous epithelium lining the
inside of the heart and blood vessels.
Beneath the endothelium is a layer of fairly loose, well-vascularized connective tissue.
This becomes a bit denser closer to the myocardium.
The thickness of the endocardium varies inversely with the thickness of the myocardium.
The layer of C.T closest to the myocardium is called the sub-endocardial layer.
It contains veins and nerves, as well as the Purkinje fibers when present.
This layer accommodates movements of myocardium without damage to endothelium
The tunica media is known as Myocardium.
It is thickest in the ventricular walls.
The myocardium is made up of cardiac muscle.
The extension of cardiac muscle is the papillary muscle.
These papillary muscles are connected to the valves of the heart via chorda tendinae.
Myocardium (medium magnification)
Myocardium (high magnification)
Myocardium
It is involuntary and striated.
Cells are branched and arranged in the form of chains.
These branches are joined by special cellular junctions called intercalated disc.
There is rich capillary network in around muscle fiber
Intercalated disc
The intercalated discs are made up of:
1. Fasciae adherens.
2. Maculae adherens.
3. Gap junctions
Myocardium
Nucleus is single and central.
Sarcoplasm is abundant contain myofibril as well as cell organelles.
Mitochondria are abundant, there is smooth and rough endoplasmic reticulumand golgi
appatus.
Glycogen granules, fat droplets and lipofuscin granules are also present.
Epicardium
The tunica adventitia is known as epicardium.
The epicardium is the delicate, inner visceral layer of the pericardium.
The outer part of the epicardium is lined with mesothelium: which is a simple squamous
epithelium.
Large blood vessels and nerves are found in the epicardium, and adipose tissue can be
abundant.
Epicardium
Purkinji Fibers
These are modified cardiac muscle fibers.
They mediate the coordinated contraction of cardiac muscle in ach cycle.
The wave of excitation starts from sino-atrial node then spreads throughout the atria.
This wave then reaches the atrio-ventricular node.
From this AV node the wave reaches the ventricle through bundle of His.
The bundle of His divides into branches called purkinji fibers.
The purkinji fibers lie in sub-endocardium.
Microscopically these fibers are larger than cardiac muscle cell.
Cytoplasm contains few myofibrils which are arranged in irregular manner.
The cytoplasm is rich in mitochondria and glycogen.
There are no T-tubules.
No intercalated discs.
Cells are joined by gap junctions or desmosomes.
The cells of SA and AV nodes are supplied by large number of autonomic nerve fibers.
Myocardium and Purkinji Fibers
Heart valves
The heart valve consists of a layer of collagenous tissue covered on both sides with
endothelium.
The endothelium is continuous with that of the chambers of heart or great vessels.
Central fibrous sheath is known as lamina fibrosa.
The lamina fibrosa merges with the fibro-elastic supporting layer the sub-endothelium.
Heart Valve (L.S)