Physiology: Cardiac Output And Venous Return

Front 1

What is the effect of vasoconstriction on cardiac output? vasodilation?

Back 1

Vasoconstriction causes afterload to go up, which will always decrease cardiac output.

Note that this applies to any central venous pressure (preload).

Also note that for vasodilation, the opposite is true

Front 2

what's the effect of rising VENOUS pressure?

Back 2

Central venous pressure rise will cause a larger preload, which we know will increase cardiac output.

note that this happens only up to a plateau (seems to plateau around 4 mm HG of venous pressure, which doesn't seem like a lot)

Front 3

On a graph of (x) = central venous pressure and (Y) = cardiac output, what can be plotted of interest?

Back 3

two lines - the cardiac function and vascular function lines.

cardiac function: trends up. As you increase the x value (venous pressure), you're increasing the preload and hence the contractility of the heart, which will INCREASE cardiac output.

vascular function curve: trends downward.
Think of Y's effect on X (counterintuitive):
Cardiac output has to equal venous return to the right atrium.

If you increase the amount of blood returning to the right atrium, you're sucking it out of the veins (which are 18x more compliant than arteries and fill with blood). this lowers the venous pressure.

Front 4

what will adding fluid do to your cardiac/vascular curve?

Back 4

adding fluid will shift the vascular curve to the right (increasing the mean venous pressure), removing fluid will shift it left.

Front 5

Vascular curve: what's a good way to remember which way the curve will shift?

Back 5

if what you're doing is likely to increase venous pressure, it'll shift right (look at where it crosses the x-axis, which IS venous pressure).

So, if adding fluid OR increasing vagal constriction tone, pressure will rise and curve will shift right.

Front 6

what's the effect of vasoconstriction on the arteries on the vascular function curve?

Back 6

this is the odd one - the whole line doesn't shift, the x-intercept stays the same (think that when cardiac output is zero, changing constriction of the arteries doesn't change the venous pressure).

vasoconstricting = lower cardiac output, so the "Y" intercept goes DOWN.

Front 7

when superimposing the curves, what are the effects of arterial constriction and dilation?

Back 7

remember that arterial constriction raises afterload, which brings the cardiac function curve DOWN.

also recall that clamping down on the arteries lowers cardiac output, so the vascular function curve moves down - but pivots on the x-intercept.

the setpoint moves straight down.

dilation is the opposite.

Front 8

what can't you determine from shifting curves?

Back 8

heart rate doesn't result in a moved line.