Venous Hemodynamics Flashcards
how much blood resides in the extrapulmonary venous system
2/3
what is the primary function of the venous system
return blood to the heart
what is required for an effective venous return
central pump, pressure gradient, peripheral venous pump, competent venous valves
what are the two types of pressures in the venous system
dynamic pressure and hydrostatic pressure
what is dynamic pressure
pressure from cardiac contraction
drives blood through the system
when supine 8mm/Hg in the leg and 0 mm/Hg at the right atrium
due to low venous pressures, respiratory motions influences venous return
what is hydrostatic pressure
the weight of the column of blood from the heart to any given spot on the body
pressure at the ankle of a normal height person standing is approximately 100mm/Hg
what is the hydrostatic pressure of the body parts above the aright atrium
-50 mm/Hg
but veins collapse at 0 mm/Hg
0mm/Hg is the minimum measurable
describes what happens during inspiration
lowers the diaphragm, decreases intra-thoracic pressure and increases intra-abdominal pressure
decreases venous return from the lower extremities
increases venous return from the upper extremities
describes what happens during expiration
raises the diaphragm, increases intrathoracic pressure and decreases intraabdominal pressure
increases venous return from the lower extremities
decreases venous return from the upper extremities
describe what happens during Valsalva maneuver
increases intrathoracic and intraabdominal pressure - all venous flow stops
used as a proximal augmentation maneuver for eval of valve function
what is the function of peripheral pumps
to overcome the hydrostatic pressure, peripheral muscular pumps propel blood upward. most important one is the calf muscle pump and is activated when walking
without it pressure is inadequate to return blood from the dependent lib to the heart and venous hypertension starts
describe what happens during calf muscle contraction
muscles contract squeezing deep veins
blood flows toward the heart in both superficial and deep systems
perforator valves are closed
valves prevent venous backflow
describes what happens during calf muscle relaxation
deep veins decompress and expand
blood is drawn from superficial to deep veins through the perforators
blood is sucked from capillaries and venules through the calf
upstream valves are closed, reducing local venous pressure
describe what happens when calf muscle pumps become ineffective or have outflow obstruction
blood flows outward from deep to superficial veins through h incompetent perforators
blood flows distally towards foot
inefficient circular flow of blood
results in venous pooling and ambulatory venous hypertension
valves are susceptible to
disease, damage - scarring, incompetence,
describe venous properties
they have the property of high compliance and high capacitance - this allows the venous system to serve as a reservoir for blood volume, primarily in the lower extremities
describe compliance of venous properties
ability to accommodate a large change in VOLUME with a small change in PRESSURE
describe capacitance of venous properties
ability to accommodate a large change in VOLUME in a short period of TIME
what is transmural pressure
relative pressure difference from within the vessel (intravascular pressure) to the outside (tissue pressure)
what happens when transmural pressure is high
high transmural pressure = low tissue pressure (distended vein - lower resistance)
what happens when transmural pressure is low
low transmural pressure = high tissue pressure (collapsed vein - high resistance)
describe venous resistance
veins offer resistance to flow similar to the arteries but can vary significantly
volume of blood carried by the arteries equal the volume carried by the veins because cardiac output equals the venous return
resistance in veins varies due to elasticity of the vessel
