1. Vascular Technology Lecture 23: Venous Hemodynamics HHHoldorf

2. Venous physiology and Hemodynamics
Venous Resistance
Peripheral venous and arterial resistances are similar
Both arteries and veins carry same amount of blood
This Paradox is explained by the “collapsible” nature of the venous wall.
Flattened shape offers more flow resistance than circular shape

4. Pressure/volume Relationships
When distended, cross-sectional area of the vein is about 3-4 times that of the corresponding artery. The extra-pulmonary veins carry about 2/3’s of the blood in the body
Shape of veins determined by transmural pressure (distension pressure), e.g., the pressure within the veins versus pressure outside the vein

5. Low transmural pressure: Low volume of blood results in dumbbell shape
High transmural pressure: high volume results in circular shape

7. Small pressure changes required to expand or distend vein from normal dumbbell shape to a circular one
Once completely distended, greater pressure ranges required to accommodate further increases in volume

8. Hydrostatic Pressure (HP)
Equivalent to the weight of a column of blood pressing against the vessels of the body; uses the heart as a reference point (HP is zero at the heart level).
HP = pgh
P = specific gravity of blood
g = acceleration due to gravity
h = distance from the heart

9. HP is added to the existing circulatory pressure and is related to position:
Supine: HP exerted on veins and arteries negligible, assumed to be zero (Pressure (P) measured at all levels = actual circulatory P)

11. Standing
HP gradually increases from level to level down the body, reaching approximately 100 mmHg at the ankle.

13. Body part above heart: Negative HP.
Measured pressure less than circulatory P.

15. Factors Affecting Venous Flow
Venous/Skeletal muscle pump /’venous heart’
Muscle contraction squeezes vein propelling blood toward the heart

16. Effective calf muscle pump
Blood moves from superficial system (S) to deep system (D)
Competent valves prevent reflux
Venous volume and pressure decreases
Venous return to heart increases

18. Ineffective calf muscle pump
Incompetent valves cause reflux
Venous volume and pressure increases
Results in venous pooling and ambulatory venous hypertension

19. Respiration Inspiration Decrease in intra-thoracic pressure
Increases blood flow from upper extremities
Increase in intra-abdominal pressure
Decreases blood flow from lower extremities

21. Exhalation Increase in intra-thoracic pressure
Decreases blood flow from upper extremities
Decrease in intra-abdominal pressure
Increases blood flow from lower extremities

22. Valsalva Maneuver
Patient takes in deep breath and holds it, then bears down as if having a bowel movement
Intra-thoracic and intra-abdominal pressure increases significantly
All venous return is halted
This maneuver equates with proximal compression while performing Doppler assessment of the lower extremities

24. Additional Notes:
When distended, cross-sectional area of the vein is about 3-4 times that of he corresponding artery.
Excluding pulmonary veins, extra-pulmonary veins carry about 2/3’s of the blood in the body- because they can STRETCH.
Hydrostatic pressure (HP)
Standing
Heart 0 mm Hg
Ankles 100+ mm Hg
Arm raised -50 mmHg

25. Homework Textbook SDMS assignments Chapter 25 Venous Hemodynamics
Pages 277 – 280
SDMS assignments