Dr. Abdulhalim Serafi, MB ChB,MSc,PhD,FESC
Assistant Professor & Consultant Cardiologist
Faculty of Medicine
Umm Al-Qura University
LECTURE VII: CAPILLARY CIRCULATION
- Structure and functions of the blood capillaries
- Capillary blood flow and capillary tone
- Capillary blood pressure & capillary reactions to
- The triple response.
- Tissue fluid formation and drainage
- Forces controlling fluid movement across the capillary
- Edema (definition, causes and types)
• Guyton: Textbook of Medical Physiology
• Ganong: Review of Medical Physiology
THE BLOOD CAPILLARIES are thin narrow vessels which
form networks connecting the arterioles with the venules.
Exchange of gases and materials between blood and
tissue cells occurs only in the blood capillaries. Therefore,
they are called exchange vessels.
Structure of the blood capillaries:
• The blood capillary is about 10 microns in diameter and
about 500-1000 microns in length.
• The human body contains about 10 billion capillaries with
a total surface area estimated to be 500-700 square
• The wall of the capillary is formed of a single layer of
endothelial cells. These cells arranged close to each other
and they are surrounded by a thin basement membrane on
In the capillary wall and in between the endothelial cells,
there are thin pores which determine the capillary
The total thickness of the capillary wall is about 0.5-1
• The arteriolar ends of the blood capillaries are surrounded
by a small rings of smooth muscle fibers called precapillary
sphincters. These sphincters: have sympathetic enervation
and they regulate the blood flow to the capillaries. Like
arterioles, the sphincters receive vasoconstrictor impulses
from the VMC.
- Capillary blood flow and capillary tone:
• The flow of blood in the capillaries is very slow (about 0.5
mm/second) to give enough time for exchange of
materials between blood and tissues.
• The blood flow in the capillaries is intermittent, because
they show. Alternating periods of closure and opening which
occur 6-12 times/ minute. This is called alternation
phenomenon or vasomotion.
• At resting tissues, the majority of capillaries are closed
(about 90% closed and 10% open)
During increased tissue activity, the number of the open
capillaries is increased.
If all capillaries open simultaneously, this leads to
accumulation of blood in the dilated capillaries this leads
to accumulation of blood in the dilated capillaries
marked of VR marked of COP marked of ABP
shock. This occurs after intravenous injection of histamine
in experimental animals (histamine shock). In man, this
may occur in extensive tissue injury e.g. in burns due to
release of histamine like substances.
CAPILLARY BLOOD PRESSURE
- The normal capillary blood pressure is about:
25-40 mm Hg at the arterial ends of the capillaries.
10-15 mm Hg at the venular ends of the capillaries.
- It is affected by some factors e.g.
Condition of the arterioles
- Dilation of the arterioles cap. Blood flow cap. Blood
- Constriction of the arterioles cap blood flow cap
- venous pressure capillary blood pressure
CAPILLARY REACTION TO MECHANICAL STIMULI:
1. White Line is produced by striking the skin gently with a
blunt-pointed object. This occurs within 15 seconds. The
white line response is due to contraction of the
precapillary sphincters (produced directly by the
mechanical stimulus) capillary blood flow i.e. capillary
2. Red Line is produced by scratching the skin firmly by a
sharp-pointed object e.g. pin. It is the first reaction during
a triple response.
THE TRIPLE RESPONSE:
- Irritation or injury of the skin e.g. by scratching with sharp
object leads to triple response which consists of 3
1. Red line due to capillary dilatation by histamine:
The red line (local redness) appears immediately after the
It is due to dilatation of the capillaries by histamine
released from the injured skin cells.
2. Spreading flare due to arteriolar dilatation by local axon
Spreading flare occurs within 30 seconds in the form of a
diffuse irregular area of mottled redness surrounding the
It is due to arteriolar vasodilatation which is produced by
local axon reflex I.e. impulses are conducted
antidromically via a branch of the afferent neuron to the
periphery where they cause a liberation of a vasodilator
substance, most probably substance P, which produces
3. Local oedema (wheal) due to capillary dilatation by
It is due to cap dilatation which causes cap permeability
accumulation of excess tissue fluid.
Dynamics of fluid exchange through the capillary wall
(tissue fluid formation and drainage)
- The cardiovascular system is a closed system I.e. the blood
does not come into direct contact with the tissue cells. The
nutrition of the tissues is maintained through a fluid
filtered from the blood capillaries and surrounds the tissue
cells and is called tissue fluid or interstitial fluid.
Thus, the interstitial fluid acts as a medium for the passage
of O2 and nutrients from the blood to the tissue cells and
CO2 & waste products in the opposite direction.
- The interstitial fluid is filtered at the arterial end of the
capillaries and is reabsorbed back to the blood at the
venous end of the capillaries. Excess fluid is drained by the
lymph vessels as lymph.
Forces controlling fluid movement through the
There are 4 primary forces (called Starling forces)
controlling fluid movement across the capillary membrane.
1. Capillary Blood Pressure:
- This acts as filtering force that drives fluid from the
capillaries to the interstitial spaces. It is about:
30 mm Hg (25-40 mm Hg) at the arteriolar ends of the
10 mm Hg (10-15 mm Hg) at the venular ends of the
2. Interstitial Fluid Pressure:
- This is the pressure in the interstitial fluid outside the
capillary wall. This pressure is negative (subatmospheric)
in the most tissues, it is about-3 mm Hg. Thus it acts as
suction force moving fluid from the capillary into the
3. Colloidal osmotic pressure of the interstitial fluid:
- It is caused by little amount of proteins present in the
interstitial fluid (about 2 gm/dL). It is about 8 mm Hg
and it acts as a force moving fluid from the capillaries
into the interstitial spaces.
4. Colloid Osmotic Pressure of the plasma proteins:
- The colloid or oncotic OP is produced by plasma proteins
(7 gm%) mainly albumin & globulin. It acts as
reabsorbing force i.e. it moves fluid from the interstitial
spaces into the blood capillaries. It is about 28 mm Hg
(25-30 mm Hg).
The net force for fluid movement through the capillary
- The net force is the difference between
a. Out ward force (force moving fluid out) &
b. Inward force (force moving fluid into the capillaries).
At the arteriolar end of the capillary:
Outward force > Inward force filtration
The net force = (30+3+8) - (28).
= 41 - 28
= 13 mm Hg
The direction of this force will cause fluid to move from the
capillaries into the interstitial spaces i.e. filtration.
At the venular end of the capillary:
Inward force > outward force reabsorption
The net force = (28) - (10+3+8)
= 28 - 21
= 7 mm Hg
The direction of the net force will cause fluid to move from the
interstitial spaces into the capillaries i.e. reabsorption.
Edema means swelling of the tissues due to abnormal
accumulation of excess tissue fluid in the interstitial spaces.
Causes and mechanisms of edema:
1) Increase of capillary blood pressure (=increase of
- Increase of capillary BP increases filtration of fluid from
the capillaries to the interstitial spaces producing edema.
Increase of capillary BP may be due to increase of venous
a. Cardiac edema due to congestive heart failure (CHF);
in CHF (= right-sided heart failure), there is increase of
the venous pressure increase of capillary BP
increase of filtration edema.
b. Pregnancy edema during the last months of pregnancy;
this is because the large uterus presses on the iliac
veins increase of the venous pressure in the veins of
the lower limbs increase of filtration edema in the
2. Decrease of the colloidal osmotic pressure of the plasma
proteins (=decrease of reabsorption force):
- Decrease of colloid osmotic pressure of plasma proteins
usually occurs when the concentration of plasma proteins
decreases to 5gm/dL or less, examples:
a. Nutritional edema due to decrease of protein intake in
diet or decrease of absorption of food proteins from
the small intestine.
b. Renal edema due to loss of proteins in urine as in a
renal disease called nephrotic syndrome.
3. Increase of capillary permeability (= increase of
- Increase of capillary permeability occurs if there is
capillary dilatation due to release of vasodilator
substances such as histamine & kinins, e.g.
a. Allergic edema due to release of histamine.
b. Inflammatory edema due to release of kinins and
4. Obstruction of lymph vessels (=decrease of lymph
- Lymphatic obstruction decrease of lymphatic drainage
from the affected part lack of drainage of excess tissue
fluid which accumulates lymphatic edema, examples:
a. Elephantiasis which is a marked lymphatic edema of
the lower limbs due to obstruction of their lymph
vessels from inside by filarial worms (parasites).
b. Cancer edema due to obstruction of the lymph vessels
by malignant cells (tumour). Edema occurs in the part
drained by the obstructed lymph vessels.
5. Salt and water retention (=increase of plasma volume):
- Salt and water retention occurs in some conditions e.g.
a. Excessive secretion of Aldosterone and glucocorticoids
from the supra renal cortex in a disease called
b. Prolonged use of cortisone (cortisol) as a drug.
c. During pregnancy due to high level of oestrogen and
- Salt and water retention results from the effect of the
previous hormones on the kidney increase of NA+ and
H2O reabsorption from the renal tubules increase of
plasma volume increase of filtration through the
capillary wall edema.
TYPES OF EDEMA:
- According to its nature, edema may be
1. Soft “pitting” edema i.e it pits on pressure. Most types of
edema are pitting e.g. cardiac edema, nutritional edema…
2. Hard “non-pitting” edema which occurs in some
conditions e.g. in hypothyroidism (myxedema) due to
presence of excess mucoproteins and fluids in the
- According to its location or distribution, edema may be
1. Local edema e.g.
Edema of the triple response. Inflammatory edema
Edema in one limb (due to deep vein thrombosis in one
Generalized edema e.g.