Prof. Dr. HUDA AL_KHATEEB
The Circulatory System
Aim :study the histology of the circulatory system
1. Study the layers and the sub-layers of the heart
2. Study the impulse conducting system structure
3. Study the types of blood vessels
4. Study the classification of the arteries
5. Study the sorting of veins
6. Study the light and electron microscopical
features of the capillaries
7. Classification of capillaries according to their
8. Throw light on the lymphatic vessel
The circulatory system is categorized as:
1. blood vascular system:
2. Lymph vascular system:
1. lymph vessels
Circulatory system can be divided into:
1. Macro-vasculature (includes vessels with more than 0.1mm in
diameter). These vessels are seen grossly.
2. Micro-vasculature (includes arterioles, capillaries and post-
capillary venules). These vessels are seen by microscope.
Blood Vascular System
-it is a muscular, highly specialized portion of the vascular system.
-it consists of 4 chambers:
The heart wall consists of 3 layers
1. Endocardium (inner layer).
2. Myocardium (middle layer).
3. Epicardium (outer layer).
The fibrous central region of the heart is called fibrous skeleton, which
serves as base of the valves and site of origin and insertion of cardiac
muscle cells. Histologically, fibrous skeleton is composed of dense
irregular connective tissue, with separated nodules of fibrocartilage.
-it lines all internal surfaces of the heart.
-it is thicker in atria than ventricles (Figure 1 &2)
-it has three layers:
1. The endothelium- it is the inner most layer. It is continues with that of
blood vessels entering and leaving the heart. It is composed of simple
2. Subendothelial layer- consists of narrow zone of loose connective t.
that is mainly composed of fine collagen, elastic and smooth muscle
3. subendocardial layer- which is composed of connective t. that
+branches of impulse-conducting system of the heart (Purkinje
1. composed of cardiac m. fibers that run in different directions
(complex-spiral) and usually inserted into the fibrous skeleton of the
2. form the main mass of the heart wall.
3. It is the thickest layer in the heart wall. Its thickness varies in
different parts of the heart being thinnest in the atria, thickest in the
Epicardium (it is the visceral pericardium)
-covered externally by a single layer of simple squamous epithelium
(mesothelium), that is supported by very thin layer of connective t.
containing elastic fibers (subepicardial layer).
-subepicardial layer-composed of loose connective t. containing:
3. adipose t.
(1) atrio-ventricular valves (A-V valves):(tricuspid & mitral)
-they are composed of core of dense fibrous connective t. (central core)
that is lined on both sides by endothelium. The bases of the valves are
attached to the fibrous skeleton.
(2)Semi lunar valves (aortic and pulmonary valves): are similar in
structure to the A-V valves, but they have thinner central core.
1. Sino-atrial node (SA node)
2. Atrio ventricular node (AV node)
3. Atrio-ventricular bundle (AV bundle);includes;
A. Bundle of His
B. Right and left bundle branches
C. Purkinji fibers
All cells of impulse-conducting system are modified cardiac muscle cells
(fuseform cells and smaller than atrial cardiac muscle fibers), except
Purkinji fibers, which are larger.
-they are modified cardiac m. fibers. They conduct impulses faster than
the ordinary heart m. fibers.
-After traveling in subendocardium, they penetrate myocardium of
ventricles. This arrangement is important because it allows the stimulus
to get into the outermost layers of ventricular musculature.
Histological features of Purkinji fibers:
I. Light microscopical (L.M.) features:
Purkinje fibers resemble ordinary cardiac m. in that:
1. they have central nuclei.
2. they have cross striation.
However they differ from them in that:
1. they are generally larger and paler.
2. they have more sarcoplasm.
3. their nuclei are surrounded by clear perinuclear area.
II. Electron microscopical (E.M.) features:
Ultrastructurally Purkinje fibers have the following features:
1. they contain large amount of glycogen and mitochondria.
2. they contain less amount of myofibrils which tend to lie peripherally
(this explain the presence of clear perinuclear area).
3. sarcoplasmic reticulum is not well developed as in cardiac m.s
INNERVATION OF THE HEART
Heart is innervated by:
1. Parasympathetic nerve (vagus) - ends near SA node. Its
stimulation leads to reduction of the heart rate.
2. Sympathetic nerve - ends near SA and AV nodes. Its
stimulation leads to increase of heart rate.
3. Free nerve ending - ends between cardiac muscle fibers. It is
related to pain sensation.
-Partial obstruction of coronary artery or any of its branches leads to
reduction of O2 supply to myocardium that leads to temporary
pain(conducted by free nerve ending). This case is called Angina
-Complete obstruction of coronary artery or any of its branches (by a
thrombus ) leads to sever pain (conducted by free nerve ending). This
case is called myocardial infarction.
Prof. Dr. Huda Al-Khateeb
General structure of blood vessels (BV):
Blood vessels (BV) have three basic types of tissues. These are:
2. Smooth muscle cells
3. Connective tissue
The amount and the arrangement of these three types of tissues,
within BV wall, are influenced by:
1. Mechanical factor – represented primarily by blood pressure.
2. Metabolic factor – reflects the local needs of tissue.
It is a special type of simple squamous epithelium. It serves many functions,
1. It forms a semipermeable barrier between plasma and interstitial
2. It converts angiotensin I to angiotensin II. The latter increase blood
3. It converts bredykinin, serotonin, prostaglandins and noradrenaline
------------etc. to biologically inert compounds.
4. It enhances lipolysis leading to formation of triglycerides and
5. It produces endothelin, which is a vasoconstrictive factor.
6. It produces nitric oxide, which is a relaxing agent.
7. It produces vascular endothelial growth factors (VEGFs). In embryo,
VEGFs is responsible for the formation of vascular system, while in
adults, they regulate capillary growth in normal and pathological
8. It has anti-thrombogenic action (preventing blood coagulation).
Damage to endothelium will uncover connective tissue and induce
platelates aggregation, thus cause thrombus and emboli formation.
VASCULAR SMOOTH MUSCLE CELLS
Found in all vessels, except capillaries and venules.
They are arranged in helical layers in tunica media.
Each muscle is enclosed by basal lamina and connective tissue (both
are secreted by the smooth muscle).
VASCULAR CONNECTIVE TISSUE
Variable amount of connective tissue present in different BV.
Collagen fibers type I found in tunica adventitia, type III in tunica
media and type IV in the basement membrane.
Elastic fibers are responsible for shrinkage of the expanded vascular
wall. They are predominant in large arteries.
Ground substance is composed of hetrogenous gel in extracellular
spaces of the vessel wall. It affects the diffusion and permeability
across the vessel wall.
Aging causes conformational changes in collagen and elastin with
deposition of lipoproteins and calcium, in addition to atherosclerosis.
LAYERS OF WALL OF BV
Generally, each BV is composed of 3 layers; these are from inside to
1-Tunica Intima (TI) – subdivided into:
A- Endothelium- (simple sq. epith.)
B- Subendothelium- loose con.t.
C- Int. elastic lamina – composed of elastin that has gaps (for
diffusion of substances to nourish cells deep in the vessel wall). It is
found in arteries only.
2-tunica media (TM) – composed of:
A- Circular smooth m. fibers
B- Elastic fibers.
C- Reticular fibers (collagen fibers type III)
E- Ground substance (proteoglycan and glycoprotein)
F- in arteries, TM has external elastic lamina that separate it from
3-tunica adventitia (TA) – consist of connective tissue (collagen type I
and elastic fibers.
The structure & relative thickness of each layer vary according to the
type & size of the vessel.
Classification of a.s. :
1-large-size a.-(elastic a.)
2-medium-size a. and small- size a. -(muscular a.)
In arteries, TM is usually the thickest layer, while in veins, TA is
commonly the thickest layer.
large-size a. (elastic a.) (conducting a.)
includes aorta & its largest branches.
TI – relatively well developed
-the int. elastic lamina is present but difficult to be diagnosed because It
is usually mixed with the elastic lamina of TM
TM – it is the thickest layer in this type of a.s
-characterized by a distinct elastic laminae (40-70 in no., usually increase
with age) arranged in a concentrical pattern.
-interspaces between the elastic membranes are occupied by fibroblasts ,
amorphous ground substance, fine elastic net work and smooth m. cells.
TA - Loose connective tissue
Medium- & small- size a. (Muscular a.)
Characterized by thick wall & narrow lumen, when compared with
TI - prominent thick int. elastic lamina
-consists of about 40 layers of circularly arranged smooth muscle fibers.
-between smooth muscle fibers, there are small amount of con.t. that
contains elastic & reticular fibers & few fibro blasts.
-ext. elastic lamina is Prominent & composed of many layers
TA - loose connective tissue
generally less than 0.5 mm in diameter
have relatively thick wall & narrow lumen (when compared with
*Int. elastic membrane present in big arterioles only and disappear in
*has no subendothelial con.t.
*composed of (1-5) layers of circularly arranged smooth m. fibers among
which scattered elastic fibers
*the no. of smooth m. layers ↓ as diameter ↓. it becomes single layer at
about 20μm diameter arteriole.
*it has no external elastic lamina.
TA - very thin
Arterial capillary (pre-capillary) (met-arteriole)
TI - composed of endothelium only
TM - circularly oriented scattered smooth m. fibers that have branching
It is accumulation of cholesterol in the T.I. of arteries. Grossly, the
artery contains fatty streaks and plaques on its internal surface. If these
fatty thickenings become great, they occlude the vessel. Coronary
arteries are most arteries predisposed to atherosclerosis, which
sometimes leads to infarction (necrosis and tissue death) and other times
it is overcome by arterial anastomosis.
It is a pathological dilatation of an artery, which is due to embryonic
defect, disease or lesion in its TM (that leads to its weakness in TM and
dilatation in the artery). Rapture of aneurysm brings sever consequence
and may cause death.
Elastic artery Muscular artery
1. diameter of lumen Wider Narrower
2. thickness of the wall Thinner Thicker
3. TM is mainly composed 40-70 elastic lamina 40 layers of smooth m.
4. Internal and external Can't be easily diagnosed Very prominent and
elastic membranes diagnosed easily
5. TI is Thicker Thinner
6. function Stabilize arterial blood Control the amount of
pressure (during systole, blood flow to organs by
elastic lamina stretch and contracting or relaxing its
reduce the increased blood smooth m.
pressure. During diastole,
the elastic rebound increase
the reduced blood pressure)
7. example Aorta and its large Coronary artery
Prof. Dr. Huda Al-khateeb
- total length of human cap.s = 9600 km, which = 60000 mile
-connects venous to arterial sides
-(7-9) μm in diameter = diameter of a single RBC
-form a network that varies in size & shape
kidneys } have large mesh of cap. network
mucous membranes }
skeletal muscles }
gray matter of brain }
nerve } have spares cap. network
smooth muscles }
serous membranes }
Structure of cap.:
+transverse section of cap. contains (1-2) endothelial cells, whose nuclei bulge
into the cap. Lumen.
-cytoplasm is thick opposite nucleus & thin elsewhere
-organelles usu. lie in the perinuclear area
+small Golgi complex
+filaments → may be related to the contractility of endothelium
2-Basal lamina- which is a product of endothelial cells.
3-thin sheath of delicate collagen & elastic fibers.
4-pericytes- slender, elongated, highly branched cells, whose cytoplasm
contain filaments (myosin, actin and tropomyosin), which are responsible for
its contractile function. They are located between endothelial cells and their
* After tissue injury, pericytes proliferate and differentiate to form new blood
vessels and connective tissue cells, thus participating in tissue repair process.
Classification of cap.s
Cap. Are classified according to their ultrastructure into:
3-sinusoidal (or discontinuous) cap.
continuous (type I) cap.
*found in: muscle tissue
*the endothelial cytoplasm contains no. of small vesicles (pinocytotic vesicles)
of (50-70) nm in diameter.
Functionally, it appears that they are involved in the transport of fluid &
macromolecules across the cap. wall.
*few or no pinocytotic vesicles are encountered in continuous cap.s of nervous
system. this feature accounts for the existence of B. Brain Barrier.
fenestrated (type ІІ) cap.s
*found in: intestinal mucosa
*peripheral cytoplasm of endothelium is perforated at intervals by “pores”
ranging (30-50) nm. the pores are closed by thin diaphragm, except in cap. of
renal glomeruli.(pores have no diaphragm)
Sinusoidal cap.s (sinusoids):
*the cap. lumen is greater than other types of cap.s (30-40) μm In diameter
*the wall of sinusoid is composed of discontinuous layer of endothelial cells
* cytoplasm of endothelial cells shows multiple fenestration without
*macrophages are closely associated with the endothelial cells. both within
& around the sinusoidal wall.
*basal lamina is incomplete.
*sinusoids are found in liver and haemopoietic organs (ex. Bone marrow &
Venus cap.s (post-cap.s)
-diameter up to 30μm.
-wall consists of:
2-thin con.t. coat contains pericytes (slender, elongated, highly branched cells
found in cap.s & postcap.s surrounding endothelium). they are greater in no.
in post cap. than that in cap.s.
classification of v.s
2- small to medium size v.s
3- large size v.s
*the smallest venule (40μm in diameter) has
-T.I. that possess endothelium with basal lamina
-T.A.- outer thin sheath of collagenous fibers
*in venules of 50μm diameter smooth m. fibers appear between
endothelium & con.t. (i.e. T.M. appears).
*in 200μm diameter venule- the circular muscle fibers form a
continuous layer (T.M.)(1-3) cells thick.
-T.A. is thicker & consists of longitudinally oriented collagenous fibers
, scattered elastic fibers & fibroblasts .
Small & medium size v.s
-diameter ranges (1-9)mm
-T.M. has no elastic membrane
*best developed in v.s of lower limb.
-thick longitudinal collagenous bundles and frequently few smooth
m. fibers which are arranged longitudinally along the vessels .
-includes the sup. & inf. Vena cava & their main tributaries.
-same as medium sized v.
-it may be thicker.
-poorly developed or absent
-thickest composed of longitudinal coarse collagen fibers & smooth m.s
- found in medium size v.s especially of lower limb are provided with valves
that prevent the flow of b. away from the heart.
-valves are folding of the T.I.
BV of BV (vasa vasorum)
-a.s & v.s of diameter over 1 mm are supplied by small , nutrient BV that is
called vasa vasorum .
-vasa vasorum usually inters T.A. & terminates in a dense cap. network which
penetrate as far as the deepest layer of the T.M.
-generally no cap.s are found in T.I. However , in some large v.s, cap.s penetrate
as far as T.I. (probably because of low venous pressure & O2 tension) .
Lymph vascular system
Consists of :
their structure is usu. Similar to the structure of the corresponding size v.s
2-organs → lymph nodes