Venous Circulation

VENOUS CIRCULATION

• Are low resistance conduits & capacitance vessels.

Structure of veins:

• Thin wall, less smooth muscles & poor elastic tissue content.
• Hence, highly distensible & collapsible.

Functions:

2 major function:

• Return blood to heart from tissues.
  • Referred as venous return.
  • Thus acting as conduits.
• Holding more than half (about 40%) of total blood volume at any given moment.

Types of venous pressure:

• Central venous pressure
• Peripheral venous pressure

1. Central venous pressure (CVP):

• Is pressure in right atrium –
• Because all systemic veins open into right atrium.
• Normal right atrial pressure (CVP) – 2 mm/Hg.

2. Peripheral venous pressure:

• Pressure in various part of venous system :-

In peripheral venules:

• About 10 mm Hg.
• Also, 4-7 mm Hg greater than right atrial pressure (CVp).

Great vein near heart:

• About 5 mm Hg.

VENOUS RETURN

• Amount of blood flowing back to heart per minute via veins. (More specifically to right atrium). Equal to cardiac output (about 5 lit/min).

Calculating venous return:

• Blood flow is from high to low pressure, hence, venous return (VR) is calculated as “Difference between mean systemic filling pressure (MSFP)&  PRA – Pressure in right atrium”.
• VR = (MSFP – PRA) / RVR.

Determinants of venous return:

Principal determinants:

1. Right atrial pressure:

• Exerts backward force on veins impeding blood flow from veins into right atrium.

2. Mean systemic filling pressure (MSFP):

• Degree of filling of systemic circulation.
• Is the pressure inside ‘systemic’ circulation when blood flow stopped completely (zero blood flow).
• MSFP represents pressure generated by elastic recoil in systemic circulation during no-flow state.

3. Mean circulatory filling pressure (MCFP):

• Equilibrium pressure reached by entire CVS, when cardiac output is stopped completely.
• Normal value of MCFP & MSFP – 7 mm/Hg.

MSFP is nearly equal to MCFP – 

• As pulmonary circulation has <1/8th capacitance as systemic circulation & only about 1/10th blood volume.
• At a blood volume of about 4000 ml, MCFP is Zero.

4. Sympathetic stimulation:

• Causes vasoconstriction & decrease in compliance (capacitance) → Increased filling pressure.

5. Resistance to venous return (RVR):

• Resistance between peripheral vessels & right atrium.

Factors affecting venous return:

1. Pumping action of heart (ventricular contraction):

• Most fundamental factor responsible for venous flow.
• Referred as ” Vis a tergo” (Force from behind).

2. Suction:

• During rapid ejection phase of cardiac cycle, intraatrial pressure becomes negative & creates suction force for blood in SVC & IVC.
• Referred as “Vis a fronte” (force from front).

3. Thoracic pump.

4. Positive intra-abdominal pressure.

5. Gravity.

6. Muscle pump:

• Flow in limb veins is assisted by frequent skeletal muscle contraction in legs (calf muscles).
• Efficiency of muscle pump is aided by tight sleeve of deep fascia.
• Deep fascia prevents outward bulge of contracting muscles → Aiding in more effective venous compression.

7. Role of venous valves

8. Blood volume.

9. Sympathetic discharge:

• Increased sympathetic discharge causes venoconstriction & decreases venous capacitance → in
• Increases MSFP & venous return.

10. Right atrial pressure.