Venous Circulation

Veins REF: Ganong’s 22^nd ed p.595

Veins are also called capacitance vessels because they contain the maximum amount of blood in circulation

Factors affecting venous return from lower limbs:

a. Muscle contraction- Venous return from lower limb depends largely on muscular activity, especially contraction of the calf muscles, known as the ‘calf pump’.

b. Deep fascia 

c. Valves 

d. Negative intrathoracic pressure made more negative during inspiration.

e.Arterial pulsation of the arteries accompanying veins.

A i.e. Arterial blood pressure

– In standing position, venous return from the lower limb depends largely on muscular activity, especially contraction of calf and foot muscles, known as muscle pump, whose efficiency is aided by the tight sleeve of deep fascia and valves in perforating veins

– In standing position, venous return from leg depends largely on the muscular activity of the triceps surae. The efficiency of calf pump is improved by the deep fascia that invests the muscle like an elastic stocking. The blood is prevented from flowing back in superficial veins by presence of valves in perforating veins. If valves become incompetent the blood is forced into superficial veins during calf muscle contraction and by hydrostatic pressure (of blood column) when straining of standing and these veins become high pressure leaks during muscle contraction leading to varicosity .

– Within the closed fascial compartment, thin walled, valved deep veins are subjected to intermittent pressure. The pulsation of adjacent arteries and contraction of large muscles within the compartment compress these deep veins and force the blood up in limb.  Presence of valves in perforating veins prevent backward flowing of blood from high pressure deep veins to low pressure superficial veins. More over as the muscles in closed deep fascial compartment relax, venous blood is sucked from superficial into deep veins. 

– So by this time it must be very clear that venous return in lower limb is dependent on muscle pump, deep fascia, valves in perforators, and pulsation of adjacent arteriesQ

– Blood flow in blood vessels including veins is primarily because of pumping action of heart. However, venous flow is aided by heart beat (in great veins draining into atrium), the increase in negative intrathoracic pressure (especially in great veins, intrathoracic & intraabdominal veins), and muscle pump (lower limb veins).

C i.e. Sleeves of deep facia 

• Gravitational increase in arterial pressure during (quite) standing does not facilitate (enhance/improve) venous return to heart because as a result of same increment in venous pressure the pressure gradient driving the circulation is not altered. 
• The amount of gravitational effect is 0.77 mm Hg/cm of vertical distance below (+) or above (-) the heart level at the density of normal blood, both for arterial & venous pressure. So the pressure in any vessel (artery or vein) below heart level is increased or above heart level is decreased by 0.77 mm Hg per centimeter d/t effect of gravity.

A i.e. Rise in central venous pressure

On assumption of erect posture, the force of gravity opposes the return of blood   incentral venous pressure)Q

B i.e. Venous return to heart increases immediately

In standing posture, 300- 500m1 of blood pools in venous capacitance vessels of lower extremities, which is immediately returned towards heartQ on assuming lying down posture.

Cardiovascular system

D i.e. Arterial pressure taken at the point when heart stops beating

Measures of Venous Return & Degree of filling of veins

Mean Circulatory Filling Pressure (MCFP)                                                Mean systemic filling pressure (MSFP)

1                                                                                                                                                                                                                                                                                                                                                 1

It is the pressure that would exist every where in the                 It is slightly different from the MCFP. It is the pressure measured

circulation if the heart pumping is stoppedQ (by shocking           every where in the systemic circulation after blood flow has been

with electricity to cause VF or in any other way), and the        stopped by clamping the large blood vessels at the heart, so that

pressure is allowed to equilibrate through out the system.     the pressure in the systemic circulation can be measured

The normal value is 7 mm Hg at (5 lit blood volume). An       indepently from those in pulmonary circulation.

increase in Blood volume increases the MCFP, while              It is almost impossible to measure in living animals. Although it is

hypovolaemia (as in dehydration & haemorrhage) decreases        it important pressure for determining venous returnQ

At 4 litre blood volume, the MCFP is close to ‘0’ and this              It is almost always nearly equal to MCFPQ because the

volume is k/a unstressed volume of circulation. The MCFP          pulmonary circulation has only 1/10 as much blood & < 1/8 as

increases almost linearly with increase in volumes                      much capacitance as the systemic circulation.

Strong sympathetic stimulation constricts all systemic blood       The normal MSFP is 7 mmHg venous return curves demonstrate

vessels, larger pulmonary vessels & even chambers of heart,      that greater the MSFP (eg 14), the greater the tightness with which

therefore, decreasing the capacity of system. So that at each      circulatory system is filled & the more the venous return curve

level of blood volume MCFP is increased.                                  At normal blood shifts upward & to the right. And vice versa. In other words, the

volume maximal sympathetic stimulation & inhibition cause          greater the system is filled the easier it is for blood to flow into

MCFP to rise / decrease to 17 mm Hg & 4 mm Hg respectively.      the heart. The less the filling, the more difficult it is for blood to flow into heart.

C i.e. VR = (MSFP-RAP)/RVR

A i.e. Large veins

The velocity of blood is inversely proportional to the total cross – sectional area at that point.

Ans. is ‘a’ i.e., Tissue perfusion 

Ans. is ‘c’ i.e., Pressure in right atrium

Central venous pressure (CVP) refers to pressure in right atrium because all the systemic veins open into right atrium.

Normal right atrial pressure (CVP) is about 2 mm Hg.