Prof. Dr. Huda Al-khateeb
Lec. 2&3
The Circulatory System
Blood vessels (BV) have three basic types of tissues. These are:
Endothelium.
Smooth muscle cells
Connective tissue
The amount and the arrangement of these three types of tissues,
within BV wall, are influenced by:
Mechanical factor – represented primarily by blood pressure.
Metabolic factor – reflects the local needs of tissue.
General structure of blood
vessels (BV):
It forms a semipermeable
barrier
between plasma and interstitial fluid.
It
converts
angiotensin I to angiotensin II. The latter increase blood pressure.
It
converts
bredykinin, serotonin, prostaglandins and noradrenaline to biologically inert
compounds.
It enhances
lipolysis
leading to formation of triglycerides and cholesterol.
It produces
endothelin
, which is a vasoconstrictive factor.
It produces
nitric oxide
, which is a relaxing agent
It produces vascular endothelial growth factors (
VEGFs
). In embryo, VEGFs is responsible
for the formation of vascular system, in adults, they regulate capillary growth in normal and
pathological conditions.
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 ENDOTHELIUM
)simple squamous epithelium (functions, are:
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 SMOOTH MUSCLE
CELLS
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. 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
VASCULAR CONNECTIVE
TISSUE
subdivided into:
–
(TI)
Intima
Tunica
-
1
A- Endothelium- (simple sq. epith.)
B- Subendothelium- loose con.t.
2-tunica media (TM) – composed of:
A- Circular smooth m. fibers
B- Elastic fibers.
C- Reticular fibers (collagen fibers type III)
D- Fibroblast
E- Ground substance (proteoglycan and glycoprotein)
F- in arteries, TM has external elastic lamina that separate it from TA
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.
LAYERS OF WALL OF BV
Generally, each BV is composed of 3 layers; these are from inside to outside:
1-large-size a.-(elastic a.)
2-medium-size a. and small- size a. -(muscular a.)
3-arteriol
In arteries, TM is usually the thickest layer, while in
veins, TA is commonly the thickest layer.
Classification of a.s.
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 very thick and 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
large-size a. (elastic a.)
(conducting a.)
Characterized by thick wall & narrow lumen, when compared
with elastic a.
TI
- prominent thick int. elastic lamina
TM
-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
Medium- & small- size a.
(Muscular a.) (Distributing a.)
generally less than 0.5 mm in diameter
have relatively thick wall & narrow lumen (when compared with venule).
TI
*Int. elastic membrane present in big arterioles only and disappear in
smaller arterioles.
*has no subendothelial con.t.
TM
*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
Arterioles
TI
- composed of endothelium only
TM
- circularly oriented scattered smooth m. fibers
that have branching processes.
Arterial capillary (pre-
capillary) (met-arteriole)
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 will 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 overcomed by arterial anastomosis
ATHEROSCLEROSIS:
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.
ANEURYSM:
- 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
ex: lung , liver , kidneys , glands , mucous membranes , skeletal
muscles, gray matter of brain, have
large mesh of cap. network
while , tendon, nerve, smooth muscles, serous membranes have
spares cap. network
Capillaries (cap.):
L.M.+transverse section of cap. contains (1-2) endothelial cells, whose
nuclei bulge into the cap. Lumen.
E.M.
1-endothelium-
cytoplasm is thick opposite nucleus & thin elsewhere
-organelles usu. lie in the perinuclear area ,these are:+small Golgi
complex+few mitochondria +free ribosomes +RER+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-
Structure of cap.:
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 basal lamina.
*
After tissue injury
, pericytes proliferate and
differentiate to form new blood vessels and
connective tissue cells, thus participating in tissue
repair process.
Pericytes
Cap. Are classified according to their ultrastructure into:
1-continuous cap.
2-fenestrated cap.
3-sinusoidal (or discontinuous) cap.
Classification of cap.s
*
found in: muscle tissue
lung, CNS, Skin, Connective tissue, Exocrine gland
*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.
continuous (type I) cap.
*found in: intestinal mucosa
endocrine glands, renal Glomerulus, pancreas
*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)
fenestrated (type ІІ) cap.s
*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
diaphragms.
*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
& spleen).
Sinusoidal cap.s (sinusoids):
-diameter up to 30μm.
-wall consists of:
1-endothelium
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.
Venus cap.s (post-cap.s)
venules
small to medium size v.s
large size v.s
classification of 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 .
Venules
-diameter ranges (1-9)mm
-T.M. has no elastic membrane
T.M.-thin
*best developed in v.s of lower limb.
T.A. -thickest layer
-thick longitudinal collagenous bundles and
frequently few smooth m. fibers which are arranged
longitudinally along the vessels .
Small & medium size v.s
-includes the sup. & inf. Vena cava & their main tributaries.
T.I.
-same as medium sized v.
-it may be thicker.
T.M.
-poorly developed or absent
T.A.
-thickest composed of longitudinal coarse collagen fibers &
smooth m.s
Large v.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.
venous valves
-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 & O
2
tension) .
BV of BV (vasa vasorum)
Consists of :
1-lymphatic vessels
their structure is usu. Similar to the
structure of the corresponding size v.s
2-organs → lymph nodes
Lymph vascular system
Thank you
