L17 Determinants of cardiac output

Question 1

what are the 4 factors that affect cardiac output

Answer 1

Preload
Contractility
Afterload
Heart ratet

All affect stroke volume which affects CO ( = SV x HR)

Question 2

What are the 5 ways in which preload is influenced by:

Answer 2

1. muscle pump
2. Respiratory pump
3. Blood volume
4. Venous tone
5. Posture

Question 3

How does muscle pump influence preload as a determinant of cardiac output

Answer 3

muscle squeezes veins. Valves dont allow back flow therefore blood is forced back to the heart

Question 4

How does respiratory pump influence preload as a determinant of cardiac output

Answer 4

Reduction in intrathoracic pressure + rise in intraabdominal pressure increases flow to the heart during inspiration

Question 5

How does blood volume influence preload as a determinant of cardiac output

Answer 5

Starling hypothesis of capillary filtration
There is a net recovery of plasma from vasoconstricted areas but loss to active areas.
Vasodilation in active muscle means higher mean pressure in that capillary bed. This causes greater net fluid loss to interstitial space. Metabolic activity of muscle cells produce more osmotically active molecules (larger storage molecules broken down into smaller units

Question 6

How does venous tone influence preload as a determinant of cardiac output

Answer 6

Since flow through the capillary bed the veins will not be distended and they recoil passively to smaller volume.
Increases sympathetic nervous system drive to veins constricts them too

Question 7

How does posture influence preload as a determinant of cardiac output

Answer 7

Influences venous return
In supine position, end diastolic volume is greater at each intensity compared to upright.
The end systolic volume also remains the same where as, in an upright position, end diastolic volume will decrease as exercise intensity increases

Question 8

Explain how plasma volume changes during exercise. How and why may this may differ between arm and leg exercise?

Answer 8

• water is drawn into the muscle cells and they swell - higher intensity exercise causes a greater shift in this.

There is a larger loss in arms than in legs due to higher capillary pressure in arms tending to force more fluid

Question 9

What is the Frank-Starling Law of the heart

Answer 9

SV is increased when venous return increases

Question 10

Graph the relationship between ventricular end-diastolic volume and stroke volume at rest and during sympathetic stimulation.

Answer 10

look it up

Question 11

List some of the factors which can influence afterload during exercise

Answer 11

Sympathetic nerve activity 
Static muscle contraction
Anatomical impedance 
Intrathoracic pressure 
Blood pressure

Question 12

Explain how changes in heart rate may affect stroke volume

Answer 12

Increasing heart rate will decrease stroke volume when working at the same cardiac output.
This is because as HR increases, the time for the heart to fill with blood will decrease so theres less for the heart to pump out, thus decreasing SV