HHHoldorf 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 ID: 706817
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Vascular TechnologyLecture 23: Venous HemodynamicsHHHoldorfSlide2
Venous physiology and HemodynamicsVenous ResistancePeripheral 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 shapeSlide3Slide4
Pressure/volume RelationshipsWhen 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 veinSlide5
Low transmural pressure: Low volume of blood results in dumbbell shapeHigh transmural pressure: high volume results in circular shapeSlide6Slide7
Small pressure changes required to expand or distend vein from normal dumbbell shape to a circular oneOnce completely distended, greater pressure ranges required to accommodate further increases in volumeSlide8
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 heartSlide9
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)Slide10Slide11
StandingHP gradually increases from level to level down the body, reaching approximately 100 mmHg at the ankle.Slide12Slide13
Body part above heart:Negative HP.Measured pressure less than circulatory P.Slide14Slide15
Factors Affecting Venous FlowVenous/Skeletal muscle pump /’venous heart’Muscle contraction squeezes vein propelling blood toward the heartSlide16
Effective calf muscle pumpBlood moves from superficial system (S) to deep system (D)Competent valves prevent reflux
Venous volume and pressure decreases
Venous return to heart increasesSlide17Slide18
Ineffective calf muscle pumpIncompetent valves cause refluxVenous volume and pressure increasesResults in venous pooling and ambulatory venous hypertensionSlide19
RespirationInspirationDecrease in intra-thoracic pressureIncreases blood flow from upper extremities
Increase in intra-abdominal pressure
Decreases blood flow from lower extremitiesSlide20Slide21
ExhalationIncrease in intra-thoracic pressureDecreases blood flow from upper extremitiesDecrease in intra-abdominal pressureIncreases blood flow from lower extremitiesSlide22
Valsalva ManeuverPatient takes in deep breath and holds it, then bears down as if having a bowel movementIntra-thoracic and intra-abdominal pressure increases significantlyAll venous return is halted
This maneuver equates with proximal compression while performing Doppler assessment of the lower extremitiesSlide23Slide24
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 mmHgSlide25
HomeworkTextbookChapter 25 Venous HemodynamicsPages 277 – 280
SDMS assignmentsSlide26