Physiology Lecture 2 -- Cardiac Output and Venous Return Flashcards
Average cardiac output
5 L/min
Average pumping capacity
10 - 13 L/min
Effect of athleticism on cardiac output and pumping capacity
Multiply by 2
How does periphery impact CO
Dilation causes increased bloodflow = higher CO
Effect of AV fistula on cardiac output and why
Causes compensatory response in peripheral veins to decrease resistance to venous return, causing increase in CO
Cardiac output equation
CO = HR x SV
Cardiac output curve
RAP vs. CO
Normal intrapleural pressure
-4 mm Hg
Effect of intraplerual pressure on cardiac output
Increase = shift curve to right
Decrease = shift curve to left
Where does the venous return curve plateau
Great veins (SVC, IVC)
Why does the venous return curve plateau?
Pressure in great veins becomes subatmospheric = collapse so that venous return cannot increase any further
Venous return equation
Pv = venous pressure = mean systemic pressure
PRA = right atrial pressure
Rv = venous resistance
What variable is most significant in determining venous return? Explain why
In steady state, PRA is usually 0
Mean systemic pressure is usually kept constant
Therefore resistance is the biggest factor
Effect of right atrial pressure on venous return
Increased right atrial pressure = decreased venous return
Define right atrial pressure
Pressure that the periphery must overcome to bring blood back to the heart (must have P gradient)
Define mean systemic filling pressure
If heart is in cardiac arrest (i.e. no pumping = venous return is 0), P would be equal in ANY vessel
Normal mean systemic filling pressure
7 mm Hg
What influences mean systemic filling pressure?
Blood volume in heart and veins
Venous return curve
Equation for PMS
mean systemic filling pressure = central volume / peripheral compliance
How can one induce an increase in PMS
Give physiological saline to increase volume
Activate sympathetic nervous system
Effect of sympathetic nervous system on PMS
Sympathetic nervous system decreases venous compliance to move blood from veins to the heart = increase PMS
Normal percentage of blood in veins
80%
Effect of PMS on VR
Increase PMS = shift VR curve to the right
Decrease = shift to left
General roles of arteries and veins in terms of physical characteristics
Arteries = resistance (high resistance)
Veins = storage (high compliance)
When talking about resistance and VR, what kind of resistance is it?
Resistance to venous return = TPR (total peripheral resistance)
Effect of TPR on VR
Change slope of VR curve
Decrease R (i.e. by dilation) = shift curve right and up
Increase R (i.e. by constriction) = shift curve left and down
When can veins have a significant role in TPR?
Normally they don’t
Increased resistance in veins can occur if a great mass compresses a great vein (i.e. large tumor or a really large multi-child pregnancy)
How can one determine cardiac output at a given point using CO and VR curves?
Intersect
Effect of transfusion on CO VR curves
Increase volume = increase PMS = increase CO
Why does the slope of the VR curve change in the event of a transfusion?
Fluid given is saline, so blood becomes dilated, thus decreasing resistance
If given blood, R would not change, so no slop change
Effect of exercise on CO VR curve
Sympathetic NS activates so PMS increases
Muscles pump veins = blood pushed to heart = increase central V = increase PMS
Arterioles vasodilate = decrease resistance to venous return (net effect despite sympathetic activation)
Explain the sequence of events A to B in this diagram involving heart failure
A –> B = Super hypoeffective pump decreases CO due to myocardial infarct
Explain the events B –> C in this diagram involving heart failure
B –> C = ANS activates to attempt restoration of BP so shift curve to the right (decreased peripheral C)
Explain events C –> D in this diagram involving heart failure
Kidneys retain fluid, so VR shift due to increased V
Explain the net effect of events A –> D in this diagram involving heart failure
D reaches the same CO as A, however PRA is increased, so more susceptible to pulmonary congestion since fillling pressure in increased
Define decompensated heart failure
Non-viable heart (i.e. EF of 5 - 10%) that cannot surpass the critical point, so essentially drowning due to too much fluid retention
NYHA clasification for decompensated heart failure
NYHA class 4
Critical CO for normal fluid balance
5 L/min
