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)
what is respiratory muscle pump?
- a pump to bring blood back to the heart
- inspiration decreases intrathoracic pressure and increases pressure gradients between RA and outside the thoracic cavity
- during inspiration, the diaphragm increases abdominal pressure and the pressure gradient between the thorax and abdomen
- opposite for expiration
what are oscillations?
- help expel blood back to the heart
- increase VR
what are the driving pressures?
- LV to Muscle –> QLV = (Parterial - Pmuscle) / TPR (driven by force provided by LV)
- Muscle to RV –> VR = (Pven - PRA) /TPR (where Pven = muscle pump and PRA = respiratory pump)
How does Q change with exercise?
Q (rest) = 5L/min
Q (severe exercise) = 20+ L/min
Describe the path of CO2 during pulmonary circulation.
- starts in muscle cells
- travels to the heart (through veins)
- enters the vena cava
- travels to the lungs (through pulmonary arteries)
- leaves the body through the lungs
describe the path of O2 during pulmonary circulation.
- starts in the lungs
- travels to the heart through the pulmonary veins
- enters the aorta
- travels to the muscle (through systemic arteries)
- provides muscles with oxygen
why is pulmonary vasculature low pressure?
- low resistance and low pressure
- different structures of the muscle
- this is because you NEVER want to cut the oxygen flow to your lungs (you will die)
how do you calculate QLV?
- QLV = (P arterial - P muscle) / TPR
- systemic arterial pressure - systemic muscle pressure
- will be the same as QRV
how do you calculate QRV?
- QRV = (P PA - P PV) / PVR
- pulmonary arterial pressure - pulmonary venous pressure
- will be the same as QLV
How does exercise affect ventricular dimensions?
- exercise increases the muscle mass of the left and right ventricles (this supplies the heart with a higher stroke volume)
- exercise increases the heart’s capabilities of doing its required job
what is the relationship between aerobic fitness and pulmonary vasculature?
- greater aerobic fitness = less resistance in the pulmonary vasculature
- lower resistance = higher VO2 max
