Lecture 18 - Venous Return and the Pulmonary Circulation Flashcards
what is venous circulation?
- a conduit to return blood to the right side of the heart from the periphery
- a “reservoir” for blood volume
- carries deoxygenated blood back to the right side of the heart
what is venous return (VR)?
- volume of blood that returns from the veins to the atria each minute
how are venous return and cardiac output connected?
- creates a closed loop
- the amount of blood leaving the right side = amount of blood leaving the right side = amount of blood going from the right to left side of the heart
what is the equation for venous return with relation to cardiac output?
venous return = RV output = LV output
what happens if venous return, RV output and LV output are not equal?
- edema (backed up blood)
- could be edema of the body or in the lungs
what is the relationship between venous return and cardiac output during exercise?
- increase in VR –> increase in RV and LV filling
what is venous return? it’s equation?
- total volume of blood returned to the right atrium each minute
- VR = deltaP / TVR (TVR = resistance in the large veins and vena cava)
- delta P = Pvenous - PRA (peripheral venous pressure - central venous pressure)
- PRA = pressure in the right atrium
What is the driving pressure (deltaP) in venous circulation?
- left ventricle has high pressures because there is more resistance –> more pressure is required to send blood away from the heart and into the body
- in venules/veins, pressure is much lower so pressure differential is much lower –> resistance therefore is lower because the systems have to be equal
what is the structural difference between veins and arteries?
- veins have thinner walls
- veins lack smooth muscle (cannot contract)
- veins lack sympathetic “tone” (no CNS)
- veins are essentially just rubber tubes to transport blood
- veins are more compliant (arteries will resist expansion because of smooth muscle) –> compliance = volume/pressure
- this is why veins are so much easier to cut off circulation, and arteries need a cuff to measure BP
why does increased venous return = increased cardiac output?
- Q = HR x SV
- SV = preload, contractility and afterload
- increased preload = increased VR (where preload = volume of blood received by the heart during diastole)
how are venous return and EDV related during exercise?
- increase in venous return = increase in end-diastolic volume (increased preload)
what are the effects of posture on venous pressure?
- gravity “pulls” venous blood to the lower limbs
- gravity prevents the flow of blood to the heart (pools in the legs)
- pressure would be highest at furthest distance from the heart
- light-headedness comes from a lack of blood in the brain when standing up
how do we increase venous return during exercise?
- an increase in VR is necessary to increase cardiac output
- during exercise; venous valves, respiratory pump and skeletal muscle pump affect venous return
how does venous blood flow?
- in one direction
- has thin membranous valves to prevent backflow
- low pressure in the veins
- vessels are well tethered to surrounding tissue
- valves are closed to prevent gravity from causing backflow
- active mechanisms are required to help increase venous return
what is skeletal muscle pump?
- the “second heart” (aka the main mechanism to increase venous return)
- pump prevents pooling in muscle vasculature
- maintains a low volume of blood in muscle veins, displaces it back to the heart
- increased driving pressure for blood flow through the muscle
- tethering = negative pressure, sucks blood through muscle above (like a vacuum, caused by forceful muscle relaxation)
- contracted muscles squish the veins, (helps with the venous flow), relaxed muscles open the veins (helps with the arterial flow)
