3.2.3. Regulation of cardiac function

Question 1

Define heart rate:

Answer 1

• determined by time required for one complete period of diastolic filling and systolic ejection
• completion of one full loop

Question 2

Define: stroke volume

Answer 2

volume of blood pumped into the aorta during one cardiac cycle (normal is 60-100 mL/beat)

Question 3

Define contractility:

Answer 3

• power developed by the muscle for any given sarcomere length independent of preload and afterload
• determines rate and magnitude of force generation
• determines end systolic volume

Question 4

Define ejection fraction:

Answer 4

-the clinical index of contractility
-fraction of EDV that is ejected during systole, expressed as percentage
Ejection fraction=(EDV-ESV)/EDV = SV/EDV
-normal is 50-70%

Question 5

Define cardiac output (CO):

Answer 5

• volume of blood that is pumped into aorta per unit time (L/min) (normal is 5 L/min)
• CO=heart rate x stroke volume

Question 6

Define cardiac index:

Answer 6

• cardiac output (CO) adjusted for differences n body surface area (BSA)
• cardiac index=CO/BSA (m^2)
• normal values are 2.5-4.0 L/min/m^2

Question 7

Define stroke work:

Answer 7

amount of energy that the heart converts to work during a single cardiac cycle (normal values are 45-75 mg-m/m^2/Beat)

Question 8

Define preload:

Answer 8

• degree of tension on the muscle when it begins to contract

- equal to end diastolic pressure at end diastolic volume

Question 9

Define afterload

Answer 9

• tension or force acting on the myocardium during ventricular ejection
• equals systolic pressure in artery outside ventricle (generally)

Question 10

What is intrinsic regulation of cardiac function?

Answer 10

-adjustment of cardiac function through properties inherent to the muscle itself (Frank-Starling mechanism and Right Atrial stretch)

Question 11

What is the Frank-Starling Mechanism?

Answer 11

-greater force of contraction with increased stretch during filling (intrinsic)

Question 12

What are the factors effecting end diastolic volume?

Answer 12

• effective filling pressure (preload)
• filling time
• wall distensibility or compliance

Question 13

What is extrinsic regulation of cardiac function?

Answer 13

-adjustment of cardiac function through neural or hormonal signaling, which increases cardiac function beyond that available through intrinsic regulation (sympathetic and parasympathetic nerves)

Question 14

What are the major effects of sympathetic stimulation?

Answer 14

1. increased heart rate
2. increased force and magnitude of atrial and ventricular contractions
3. end systolic volume is decreased

Question 15

What are the major effects of parasympathetic stimulation?

Answer 15

1. decreased heart rate

2. decreased force of contraction

Question 16

How is stroke volume affected by preload?

Answer 16

-increased preload causes larger EDV and larger stroke volume through the Frank-Starling mechanism

Question 17

How is stroke volume affected by afterload?

Answer 17

-increase in afterload causes a larger ESV and smaller stroke volume

Question 18

How is stroke volume affected by contractility?

Answer 18

-increased contractility results in decreased ESV and a larger stroke volume

Question 19

How is stroke volume affected by afterload when preload is NOT constant?

Answer 19

• increase in afterload causes larger ESV and smaller stroke volume on FIRST beat
• when venous return is added to this larger ESV, a larger EDV results for the next beat (increased preload) and stroke volume increases

Question 20

What are the 4 main determinants of cardiac function?

Answer 20

1. preload
2. afterload
3. contractility
4. heart rate

Question 21

What is the function of the cardiovascular system?

Answer 21

-to maintain arterial pressure in order to meet the demands over a wide variety of condition, while maintaining regulatory flexibility and minimizing cardiac work.

Question 22

What does contractility depend on?

Answer 22

1. number of crossbridges formed between actin and myosin
2. rapidity with which the crossbridges are formed, broken and reformed (crossbridge recycling)
3. availability of intracellular calcium and/or increased calcium sensitivity