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‫االسراء اية ‪58‬‬
‫‪dr abdelaziz Hussein, Mansoura Faculty‬‬
‫‪of Medicine‬‬
By
Dr. Abdel Aziz M. Hussein
Lecturer of Medical Physiology
Member of American Society of Physiology
dr abdelaziz Hussein, Mansoura Faculty
of Medicine
dr abdelaziz Hussein, Mansoura Faculty
of Medicine
dr abdelaziz Hussein, Mansoura Faculty
of Medicine
• They are extremely thin vessels to allow
exchange of nutrients and cellular excreta ( )
the tissues and the circulating blood occur
No
• About 10 billion capillaries
Surface area
• Total surface area is about 500 to 700 m2.
dr abdelaziz Hussein, Mansoura Faculty
of Medicine
dr abdelaziz Hussein, Mansoura Faculty
of Medicine
dr abdelaziz Hussein, Mansoura Faculty
of Medicine
Thickness 0.5 um
Endothelial
cell
Diameter 4-9
um
Basement membrane
dr abdelaziz Hussein, Mansoura Faculty
of Medicine
dr abdelaziz Hussein, Mansoura Faculty
of Medicine
•Blood in capillary is not continuous, but it is
intermittent
Cause:
• It is due to intermittent contraction of the
metarterioles and precapillary sphincters (
Vasomotion)
• There is a cycle of constriction and relaxation for 5 to
10 times per min, so only about 10-20% of the
capillaries are opened at a time.
dr abdelaziz Hussein, Mansoura Faculty
of Medicine
Regulation of capillary Vasomotion:
• Vasomotion is controlled by the O2 concentration in
the tissues.
• When the O2 concentration is very low→ the
intermittent periods of blood flow occur more often
and the duration of each period of flow lasts for a long
time, thereby allowing the blood to carry ↑ed
quantities of O2 to the tissues.
dr abdelaziz Hussein, Mansoura Faculty
of Medicine
Regulation of capillary Vasomotion:
Low O2 tension
dr abdelaziz Hussein, Mansoura Faculty
of Medicine
Regulation of capillary Vasomotion:
High O2 tension
dr abdelaziz Hussein, Mansoura Faculty
of Medicine
• It is about 0.5-1 mm/second→ slow blood flow→
suitable for exchange of materials between the
plasma and interstitial fluid.
dr abdelaziz Hussein, Mansoura Faculty
of Medicine
Value:
• At the arterial end, it is about 30-35 mmHg.
• At the venous end, it is about 10-15 mmHg.
• The mean capillary blood pressure is 25 mmHg.
Arterial End
30-35 mmHg
Mean Cap. P.
25 mmHg
dr abdelaziz Hussein, Mansoura Faculty
of Medicine
Venous End
10-15 mmHg
Extracapillary Passive
Factors
1. Arteriolar diameter
Active Capillary
Contraction
1. Chemical factors
2. Venous Pressure
2. Nervous factors
3.Gravity
3. Physical factors
dr abdelaziz Hussein, Mansoura Faculty
of Medicine
a. Diameter of arterioles:
•Arteriolar VD →↑es the capillary blood flow and the capillary pressure.
•VC of the arterioles has the reverse effect.
b. Venous pressure:
-↑ed venous pressure → ↑capillary BP and ↓capillary blood flow.
-↑ed venous pressure occurs in;
Congestive heart failure.
Local obstruction of the vein by thrombosis or tumour.
c. Gravity:
It ↑es the capillary blood pressure in parts below the level of the heart by
antagonizing the VR.
It ↓es the capillary BP in parts above the level of the heart.
dr abdelaziz Hussein, Mansoura Faculty
of Medicine
Arteriolar
dilatation
Arteriolar
Constriction
dr abdelaziz Hussein, Mansoura Faculty
of Medicine
↑ capillary
B.P.
↓ capillary
B.P.
↑ capillary
B.P.
↓ capillary
B.P.
dr abdelaziz Hussein, Mansoura Faculty
of Medicine
↑ venous
B.P.
↓ Venous
B.P.
dr abdelaziz Hussein, Mansoura Faculty
of Medicine
dr abdelaziz Hussein, Mansoura Faculty
of Medicine
Swelling of
endothelial cells
Contraction of
metarterioles
Contraction of
precapillary sphincter
dr abdelaziz Hussein, Mansoura Faculty
of Medicine
1. Vasoconstrictor substances
a. Adrenaline:
• VC of cutaneous and splanchnic
capillaries.
b. Noradrenaline →VC of all the
capillaries except the coronaries.
c. ADH → generalized VC of the
capillaries
d. Angiotensin II → generalized VC
of the capillaries.
dr abdelaziz Hussein, Mansoura Faculty
of Medicine
2. Vasodilator substances
a. Adrenaline:
• VD of Sk. Ms and coronary
capillaries.
b. Acetyl choline →VD of all the
capillaries including coronaries
c. Histamine → generalized VD
of the capillaries
d. Gases → CO2 excess, ↑ed H
and O2 lack produce capillary
dilatation.
dr abdelaziz Hussein, Mansoura Faculty
of Medicine
• True capillaries receive no nerve supply, so it is not affected
by nervous factors.
• Capillary dilatation or constriction is dependent on the tone
of the precapillary sphincter which is influenced by the local
regulatory mechanisms.
dr abdelaziz Hussein, Mansoura Faculty
of Medicine
• Warming causes capillary VD because heat relaxes the smooth
ms fibers of the precapillary sphincter.
• Cooling of the skin for a short time produces VC, while prolonged
cooling causes VD due to accumulation of metabolites.
Skin colour and temperature
• The colour of the skin depends on the
state of capillaries
• The temperature of the skin depends
on the state of the arterioles
dr abdelaziz Hussein, Mansoura Faculty
of Medicine
◊Def,
•It is the liability of the capillary wall to rupture on raising the
capillary pressure.
• The capillaries do not rupture easily under normal conditions because
of their very small diameter→ according to the law of Laplace.
Law of Laplace
• This law states that the tension
developed in the wall of a hollow organ
(T) is equal to the distending pressure
(P) x the radius (r) of the organ
T=Pxr
dr abdelaziz Hussein, Mansoura Faculty
of Medicine
Law of Laplace
• As the pressure in the aorta is 4 times the pressure in the capillary
and its radius is 4000 times as much as the radius of the capillary,
so the tension in the aortic wall is 16000 times the tension in the
capillary wall.
• This very low tension in the capillary wall (1/16000 that in the aortic
wall) explains the little liability of the capillary wall to rupture.
◊Causes of ↑ed capillary fragility:
a.Defects in their wall (e.g. vitamin C deficiency, certain toxins,
allergic conditions, old age).
b.Defect in blood (e.g. thrombocytopenic purpura).
dr abdelaziz Hussein, Mansoura Faculty
of Medicine
dr abdelaziz Hussein, Mansoura Faculty
of Medicine
THANKS
dr abdelaziz Hussein, Mansoura Faculty
of Medicine