Microcirculation - 35 Flashcards
The Microcirculation
It is a term comprises the smallest arterioles, and the exchange
vessels, including the capillaries and the postcapillary venules.
exchange vessels
The transfer of gases, water, nutrients, waste materials and
other substances between the blood and body tissues carried out
by the exchange vessels is the ultimate function of the
cardiovascular system.
Capillary Blood Flow
• The flow of blood through capillaries is not continuous, but it
is intermittent. It is due to a phenomena called Vasomotion .
Its velocity is about 0.5-1 mm/second→ slow blood flow→
suitable for exchange of materials between the plasma and
interstitial fluid.
Vasomotion
It is the intermittent contraction of the metarterioles and
precapillary sphincters.
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
Regulation of capillary Vasomotion
It 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.
precapillary sphincter
Relax when low O2 tention.. reverse is right
Capillary Blood Pressure
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.
Factors affecting Capillary B.P. and B. Flow:
1- Local regulatory mechanisms
2-Active Capillary Contraction
3-Extracapillary Passive Factors:
2-Active Capillary Contraction
A.Chemical control.
B.Nervous control.
C.Physical control.
D.Mechanical control.
A-Chemical Factors
- Vasoconstrictor substances:
2. Vasodilator substances
- 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.
- 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
Nervous factors
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
C-Physical Factors
• 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
D. Mechanical Factors
(Skin Reaction To Mechanical Stimulation)
( Triple Response Of Lewis)
It is a cutaneous response that occurs from firm stroking of the skin ,
which produces an initial red line, followed by a flare around that
line, and then finally a wheal.
• It is due to the release of histamine .
1. Red spot: due to capillary dilatation.
2. Flare: redness in the surrounding area due to arteriolar dilatation
mediated by axon reflex.
3. Wheal: due to exudation of fluid from capillaries and venules.
3-Extracapillary Passive Factors:
A.Diameter of arterioles.
B.Venous pressure.
C.Gravity.
A. Diameter Of Arterioles
- Arteriolar VD →↑es the capillary blood flow and the capillary pressure.
- Arteriolar VC has the reverse effect.
B. Venous pressure
• ↑ed venous pressure → ↑capillary BP and ↓capillary
blood flow.
• ↑ed venous pressure occurs in Congestive heart failure
and local obstruction of the vein by thrombosis or
tumor.
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.
Capillary Fragility
• 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
• Causes of ↑ed capillary fragility:
i. Defects in their wall (e.g. vitamin C deficiency, certain toxins,
allergic conditions, old age).
ii. Defect in blood (e.g. thrombocytopenic purpura).
Venous Pressure
It is the pressure inside veins.
Central venous pressure (CVP)
It is VP in big veins at right atrium (intrathoracic portions of vena
cavae).
CVP averages 4-6 mmHg in recumbancy and 2 mmHg on standing.
Causes Of Increase Or Decrease CVP
see
Factors affecting venous pressure
1) Gravity
2) Rate of blood inflow into veins:
3) Rate of blood outflow from veins:
4) Venomotor tone
5) Blood volume
6) Muscular exercise
7) Respiratory movement
1) Gravity
- In recumbent position :gravity has no effect on circulation.
- On standing: CVP is ↓ed →↓ VR and the cardiac filling.
Prolonged standing leads to :(harmful effect of gravity):
a) Filtration of fluid from the blood to the tissues → edema in lower limbs.
b) Distended veins accommodate a large portion of blood volume →↓ the VR,
COP and ABP (postural hypotension) &cerebral ischemia and fainting.
c) Varicose veins occur when the valves are damaged and the veins are twisted,
enlarged . It is common in the veins of legs and scrotum.
Acceleration force
Constant speed has no effect on the circulation.
Anteroposterior acceleration has no effect on the circulation.
Acceleration in the direction of longitudinal axis of the body → blacking out or
reddening out.
a) Blacking out:
When the head is upward, the blood is pooled in the lower limbs ↓ VR&COP → cerebral ischemia →↓ blood in retinal vessels → temporary blindness (blackout) and loss of consciousness (fainting) occurs . It occurs in: Jet plane pilots (during ascent)
b) Reddening out:
- When the head is downward, the blood is shifted from the lower limbs to the head → marked congestion of the eyes occurs and the fields of vision become suddenly bright red with temporary loss of vision (reddening out). -It occurs in: Pilots during descent.
2) Rate of blood inflow into veins:
-VD of arterioles without capillary dilatation →↑inflow into veins
→↑es VP.
-If the capillaries are dilated→↓ the blood inflow to veins→ ↓ VP
3) Rate of blood outflow from veins:
If the outflow is less than the inflow, the VR is ↓ed and consequently
the VP is↑ed e.g.:
1. Rt side heart failure or local venous obstruction.
2. ↑ed +ve intrathoracic pressure (straining).
4) Venomotor tone
Tonic contraction in veins, maintains normal VP.
Venous tone is ↑ed reflexly in haemorrhage to maintain VP.
↑ venous tone by symp. stimulation or noradrenaline→↑ VP.
↓ venous tone by α- adrenergic blockers→↓ VP.
5) Blood volume
- VP is↑ed in hypervolemia (blood transfusion).
- VP is↓ed in hypovolemia (hemorrhage).
6) Muscular exercise
Contracting skeletal ms press on veins, ↑ing the VP.
- This is due to arteriolar VD of active ms , ↑ing the VP.
7) Respiratory movement
- Inspiration:
The diaphragm descends, the VP in abdominal veins is ↑ed and the VP
in intrathoracic veins is ↓ed →↑ venous return → ↓VP to the normal
level. - Expiration:
The opposite effect occurs.
- Forced inspiration with closed glottis (Muller’s experiment)
The -ve intrathoracic pressure is ↑ed to -30 mmHg → marked ↑ in the
VR and ↓VP.
- Forced expiration with closed glottis (Valsalva’s experiment).
The VP in intrathoracic veins is ↑ed (+40 to +50 mmHg) →stops the
VR in the extrathoracic veins with bulging of the neck veins and VP is
much ↑ed.
