Physiology Lecture 2 -- Cardiac Output and Venous Return

Question 1

Average cardiac output

Answer 1

5 L/min

Question 2

Average pumping capacity

Answer 2

10 - 13 L/min

Question 3

Effect of athleticism on cardiac output and pumping capacity

Answer 3

Multiply by 2

Question 4

How does periphery impact CO

Answer 4

Dilation causes increased bloodflow = higher CO

Question 5

Effect of AV fistula on cardiac output and why

Answer 5

Causes compensatory response in peripheral veins to decrease resistance to venous return, causing increase in CO

Question 6

Cardiac output equation

Answer 6

CO = HR x SV

Question 7

Cardiac output curve

Answer 7

RAP vs. CO

Question 8

Normal intrapleural pressure

Answer 8

-4 mm Hg

Question 9

Effect of intraplerual pressure on cardiac output

Answer 9

Increase = shift curve to right

Decrease = shift curve to left

Question 10

Where does the venous return curve plateau

Answer 10

Great veins (SVC, IVC)

Question 11

Why does the venous return curve plateau?

Answer 11

Pressure in great veins becomes subatmospheric = collapse so that venous return cannot increase any further

Question 12

Venous return equation

Answer 12

Pv = venous pressure = mean systemic pressure

P_RA = right atrial pressure

Rv = venous resistance

Question 13

What variable is most significant in determining venous return? Explain why

Answer 13

In steady state, PRA is usually 0

Mean systemic pressure is usually kept constant

Therefore resistance is the biggest factor

Question 14

Effect of right atrial pressure on venous return

Answer 14

Increased right atrial pressure = decreased venous return

Question 15

Define right atrial pressure

Answer 15

Pressure that the periphery must overcome to bring blood back to the heart (must have P gradient)

Question 16

Define mean systemic filling pressure

Answer 16

If heart is in cardiac arrest (i.e. no pumping = venous return is 0), P would be equal in ANY vessel

Question 17

Normal mean systemic filling pressure

Answer 17

7 mm Hg

Question 18

What influences mean systemic filling pressure?

Answer 18

Blood volume in heart and veins

Question 19

Venous return curve

Answer 19

Question 20

Equation for P_MS

Answer 20

mean systemic filling pressure = central volume / peripheral compliance

Question 21

How can one induce an increase in P_MS

Answer 21

Give physiological saline to increase volume

Activate sympathetic nervous system

Question 22

Effect of sympathetic nervous system on P_MS

Answer 22

Sympathetic nervous system decreases venous compliance to move blood from veins to the heart = increase P_MS

Question 23

Normal percentage of blood in veins

Answer 23

80%

Question 24

Effect of P_MS on VR

Answer 24

Increase PMS = shift VR curve to the right

Decrease = shift to left

Question 25

General roles of arteries and veins in terms of physical characteristics

Answer 25

Arteries = resistance (high resistance)

Veins = storage (high compliance)

Question 26

When talking about resistance and VR, what kind of resistance is it?

Answer 26

Resistance to venous return = TPR (total peripheral resistance)

Question 27

Effect of TPR on VR

Answer 27

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

Question 28

When can veins have a significant role in TPR?

Answer 28

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)

Question 29

How can one determine cardiac output at a given point using CO and VR curves?

Answer 29

Intersect

Question 30

Effect of transfusion on CO VR curves

Answer 30

Increase volume = increase PMS = increase CO

Question 31

Why does the slope of the VR curve change in the event of a transfusion?

Answer 31

Fluid given is saline, so blood becomes dilated, thus decreasing resistance

If given blood, R would not change, so no slop change

Question 32

Effect of exercise on CO VR curve

Answer 32

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)

Question 33

Explain the sequence of events A to B in this diagram involving heart failure

Answer 33

A –> B = Super hypoeffective pump decreases CO due to myocardial infarct

Question 34

Explain the events B –> C in this diagram involving heart failure

Answer 34

B –> C = ANS activates to attempt restoration of BP so shift curve to the right (decreased peripheral C)

Question 35

Explain events C –> D in this diagram involving heart failure

Answer 35

Kidneys retain fluid, so VR shift due to increased V

Question 36

Explain the net effect of events A –> D in this diagram involving heart failure

Answer 36

D reaches the same CO as A, however P_RA is increased, so more susceptible to pulmonary congestion since fillling pressure in increased

Question 37

Define decompensated heart failure

Answer 37

Non-viable heart (i.e. EF of 5 - 10%) that cannot surpass the critical point, so essentially drowning due to too much fluid retention

Question 38

NYHA clasification for decompensated heart failure

Answer 38

NYHA class 4

Question 39

Critical CO for normal fluid balance

Answer 39

5 L/min