lecture 18 Flashcards
(19 cards)
venosu circulation
a conduit to return blood to the right side of the heart from the periphery
venous side —> 64% veins
arterial side —> 13% of blood is arteries
- atrium —> ventricle —> aorta
“reservoir” for blood volume
venour return and cardiac output
a large amount of blood can be returned to the heart when needed
during exercise, an increase in VR —> increase RV and LV filling
same delivery of blood out of the heart as back into the heart (h to be equal)
positively linear line
how does venous return increase?
from hemodynamic perspective
total volume of blood returned to the right atrium each minute (L of blood per minute)
flow = pressure / resistance
VR = P / TVR (ohm’s law)
P = Pvenous - Pra
- TVR is resistance in the large veins and vena cava
- Pvenous is peripheral venous pressure (i.e. pressure inthe peripheral veins draining tissue)
- Pra is central venous pressure
driving pressure (P) in venous circulation
blood flow from LV to muscle (high pressure system)
- Q = (P arterial - P capillary) / TPR
blood flow from muscle to RA (low pressure system)
- VR = (P ven - P ra) / TVR
low resistance in the venous circulation
compliance = volume / pressure
how does increase venous return = increase cardiac output?
total volume of blood pumped by the ventricle each minute (L of blood per minute)
Q = HR x SV
SV is made up of preload, contractility, and afterload
preload — volume of blood received by the heart during diastole (EDV)
- increase VR —> increase preload
contractility
afterload — resistance in circulation for the heart to pump though
end-diastolic filling
during exercise: increase venous return —> increase EDV (increase preload) and cardiac ouput
effect of posture on venous pressure
- when standing rom supine, gravitational forces “pull” venous blood to the lower limbs
- due to the high compliance of veins, ~500 mL of blood can redistribute to peripheral veins
- slowed transit time of blood thru venous circulation drops SV by ~40%
- ever felt light headed when standing?
why don’t we get venous pooling in our feet?
- 90mL mercury to bring blood up
how do we increase venous return?
an increase venous return is necessary to increase cardiac output
VR = (P ven - P ra) / TVR
red boxes are positive influences
- venous valves (mechnically prevent backflow of blood)
- respiratory pump (decrease pressure in chest veins —> increase pressure gradient)
- skeletal muscle pump (increase venous pressure —> increase pressure gradient)
all add to
increase venous return —> increase end-diastolic volume —> increase stroke volume —> increase cardiac output
venous blood flows in one direction
- thin, mebranous flap-like valves spaced at short intervals within veins permit one-way blood flow back to heart
- pressure of blood in the large veins in very low (i.e. dificult to return blood to the heart)
- vessels within muscle are well tethered to the surrounding tissue = transmission of forces
- in the upright posture, gravity opposes the upward flow of blood from the lower body
- active mechanisms are required to help increase venous return
valves open
valves closed
skeletal muscle pump
“the second heart”
- during exercise, if there were no muscle pump, high flow of blood through muscle would lead to pooling in compliant vasculature
- muscle pump prevents this pooling by maintaining a low volume of blood within muscle veins and displacing it back to heart
- also increase driving pssure for blood flow through the muscle
- tethering causes negative venous pressure which “sucks” blood through muscle above what could be provided by LV force alone
P= 80mmHg passive unright rest
muscle contraction
P = 200mmHg immediate post contraction
respiratory muscle pump
- inspiration increase intrathoracic pressure and increase pressure gradients between the RA and system venous circuit (outside thoracic cavity)
- descent of diaphragm during inspiration increase intra-abdominal pressure and increase pressure gradient between thorax and abdomen, causing translocation of blood centrally
- opposite occurs during expiration
- oscillations, squeeze blood for the lower veins to the chest increase VR
respiratory muscle pump
- PAV eliminates megative swing in ITP
- therefore, no fall in P ra with inspiration - same ventilation achieved with lower ITP swing
PAV = proportional assist ventilator breathing
SPON = spontaneous breathing
Wb = work of breathing
ITP = intrathoracic pressure
esophageal pressure ~ ITP
driving pressures
blood flow from LV to muscle
- Q lv = (P arterial - P muscle / TPR
driven by force provided by LV
blood flow from muscle to RV
- VR = (P ven - P ra) / TVR
Pven = muscle pump
Pra = respiratory pump
what about the right heart?
venous return = RV output = LV output
the right and left heart both circulate the same volume blood at the same frequency:
- rest: Q = 5 L/min
- severe exercise: Q = 20+ L/min
“closed loop”
Pulmonary —> LV (CO) —> systemic —> RV (VR) —> back to pulmonary
- each depends on the other
- factors that affect one will affect the other
pulmonary vasculature
- low resistance circulation; pressure generated by RV is much less than left
LV - oxygenated blood
RV - deoxygenated blood
graphs
systemic arterial (120/80 = RHR) vs pulmonary arterial
systemic peripheral vs pulmonary (40/10 = RHR) vascular resistance
vasodilation occurs
ventricular dimensions
effects of exercise training
12 previously sedentary participants (7 male and 5 female; 29 +- 6 years)
LV mass, g = 168 +- 38
RV mass, g = 63+- 9
- RV doesn’t have to produce as much force as less cardiac muscle than LV
aerobc fitness and pulmonary vasculature
greater aerobic fitness = less resistnace in the pulmonary vasculature
- beneficial to absurb the higher RV stroke volume in endurance trained individuals and optimize gas exchange
