Microcirculation - 35

Question 1

The Microcirculation

Answer 1

It is a term comprises the smallest arterioles, and the exchange
vessels, including the capillaries and the postcapillary venules.

Question 2

exchange vessels

Answer 2

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.

Question 3

Capillary Blood Flow

Answer 3

• 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.

Question 4

Vasomotion

Answer 4

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

Question 5

Regulation of capillary Vasomotion

Answer 5

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.

Question 6

precapillary sphincter

Answer 6

Relax when low O2 tention.. reverse is right

Question 7

Capillary Blood Pressure

Answer 7

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.

Question 8

Factors affecting Capillary B.P. and B. Flow:

Answer 8

1- Local regulatory mechanisms
2-Active Capillary Contraction
3-Extracapillary Passive Factors:

Question 9

2-Active Capillary Contraction

Answer 9

A.Chemical control.
B.Nervous control.
C.Physical control.
D.Mechanical control.

Question 10

A-Chemical Factors

Answer 10

1. Vasoconstrictor substances:

2. Vasodilator substances

Question 11

1. Vasoconstrictor substances:

Answer 11

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.

Question 12

2. Vasodilator substances

Answer 12

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

Question 13

Nervous factors

Answer 13

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

Question 14

C-Physical Factors

Answer 14

• 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

Question 15

D. Mechanical Factors
(Skin Reaction To Mechanical Stimulation)
( Triple Response Of Lewis)

Answer 15

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.

Question 16

3-Extracapillary Passive Factors:

Answer 16

A.Diameter of arterioles.
B.Venous pressure.
C.Gravity.

Question 17

A. Diameter Of Arterioles

Answer 17

• Arteriolar VD →↑es the capillary blood flow and the capillary pressure.
• Arteriolar VC has the reverse effect.

Question 18

B. Venous pressure

Answer 18

• ↑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.

Question 19

C. Gravity

Answer 19

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.

Question 20

Capillary Fragility

Answer 20

• 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

Question 21

• Causes of ↑ed capillary fragility:

Answer 21

i. Defects in their wall (e.g. vitamin C deficiency, certain toxins,
allergic conditions, old age).
ii. Defect in blood (e.g. thrombocytopenic purpura).

Question 22

Venous Pressure

Answer 22

It is the pressure inside veins.

Question 23

Central venous pressure (CVP)

Answer 23

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.

Question 24

Causes Of Increase Or Decrease CVP

Answer 24

see

Question 25

Factors affecting venous pressure

Answer 25

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

Question 26

1) Gravity

Answer 26

- In recumbent position :gravity has no effect on circulation. - On standing: CVP is ↓ed →↓ VR and the cardiac filling.

Question 27

Prolonged standing leads to :(harmful effect of gravity):

Answer 27

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.

Question 28

Acceleration force

Answer 28

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.

Question 29

a) Blacking out:

Answer 29

``` 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) ```

Question 30

b) Reddening out:

Answer 30

``` - 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. ```

Question 31

2) Rate of blood inflow into veins:

Answer 31

-VD of arterioles without capillary dilatation →↑inflow into veins →↑es VP. -If the capillaries are dilated→↓ the blood inflow to veins→ ↓ VP

Question 32

3) Rate of blood outflow from veins:

Answer 32

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).

Question 33

4) Venomotor tone

Answer 33

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.

Question 34

5) Blood volume

Answer 34

- VP is↑ed in hypervolemia (blood transfusion). | - VP is↓ed in hypovolemia (hemorrhage).

Question 35

6) Muscular exercise

Answer 35

Contracting skeletal ms press on veins, ↑ing the VP. | - This is due to arteriolar VD of active ms , ↑ing the VP.

Question 36

7) Respiratory movement

Answer 36

- 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.

Question 37

- Forced inspiration with closed glottis (Muller's experiment)

Answer 37

The -ve intrathoracic pressure is ↑ed to -30 mmHg → marked ↑ in the VR and ↓VP.

Question 38

- Forced expiration with closed glottis (Valsalva's experiment).

Answer 38

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.