Cardiovascular Physiology Part C: Cardiac Output

Question 1

Cardiac Output

Answer 1

volume of blood ejected by each ventricle in 1 minute

=(HRxStroke Volume)

Question 2

Stroke Volume

Answer 2

volume ejected by each ventricle per beat

-is equal to the difference b/w EDV and ESV (EDV-ESV)

Question 3

End Diastolic Volume (EDV)

Answer 3

volume of blood in each ventricle at end of ventricular diastole (=preload)
-max ventricular volume ~ 120ml

Question 4

End Systolic Volume (ESV)

Answer 4

volume of blood in each ventricle at the end of ventricular systole (what’s left after ejection)
~50ml

Question 5

What is the heart rate set and modified by?

Answer 5

• Set by SA node = intrinsic control
• Modified = extrinsic control
  
  • change pacemaker potential (NOT AP)

Question 6

Types of CO Extrinsic Control

Answer 6

1. Neural
2. Hormonal
3. Other

Question 7

Neural Extrinsic Control (CO):

SNS (Thoracic nerves)

Answer 7

Na+ channels open wider ∴ ⇑ Na+ permeability at SA node ∴ ⇑ slope of pacemaker potential ∴ reach threshold faster ∴ ⇑ HR

Question 8

Neural Extrinsic Control (CO):

PSNS (Vagus nerve)

Answer 8

• keeps resting HR lower than pace set by SA node alone (sends continuous impulses)
• ⇑ K+ permeability at SA node ∴ more –ve on repol. ∴ further to go to get to threshold ∴ takes longer ∴ ⇓ HR

Question 9

Hormonal Extrinsic Control (CO)

Answer 9

• epinephrine, NE - ⇑ HR - same mechanism as SNS
• thyroid hormone - directly ⇑ HR (but slow, so takes days)
  - also ⇑ # of epi receptors ∴ more sensitive to epi

Question 10

Other Extrinsic Control (CO):

Ions - High K+ in ISF

Answer 10

• MP more +ve than normal ⇒ pacemaker Na+ channels may not open
• also slows repol.
• ∴ ⇓ HR ⇒ may lead to cardiac arrest

Question 11

Other Extrinsic Control (CO):

Ions - Low K+ in ISF

Answer 11

-evidence that K+ channels in some cells change specificity: allow Na+ through instead of K+ ∴ depolarizes membrane ⇒ ⇑ HR - feeble beat, abnormal rhythms

Question 12

Other Extrinsic Control (CO):

Fever

Answer 12

⇑ temp - ⇑ HR

Question 13

Other Extrinsic Control (CO):

Age

Answer 13

newborn = high

Question 14

Other Extrinsic Control (CO):

Fitness

Answer 14

⇑ fitness = ⇓ HR

Question 15

Intrinsic Control of Stroke Volume

• increased venous return:
• relationship b/w EDV and SV
• get increased venous returnn due to:

Answer 15

1. ⇑ venous return ⇒ ⇑ EDV ⇒ ⇑ heart muscle stretch ⇒ ⇑ force of contraction (at rest, cardiac fibers are at less than optimal length ∴ stretch ⇒ approach optimal length = more cross bridges attach = more force) ⇒ ⇑ SV (within physiological limits)
   i. e. more blood in ⇒ more blood out
2. relationship between EDV and SV:
   
   • Frank-Starling’s Law of the Heart: ⇒ force of ejection is directly proportional to length of ventricular contractile fibers (within physiological limits).
3. Get ⇑ venous return due to:
   
   • exercise – venous return speeded up
   • lower HR - has longer to fill ⇒ less of an effect than exercise

Question 16

Extrinsic Controls of Stroke Volume:

ANS-SNS

Answer 16

1. ⇑ force of contraction (for a given EDV) ∴ ⇑ SV (SNS stimulation ⇑ opening of Ca++ channels ⇒ ⇑ Ca++ into cytosol ∴ more cross bridges ∴ ⇑ force)
2. BUT SNS also ⇑ HR = less time to fill ∴ have ⇓ EDV at higher HR. However, ⇑ force ⇒ ⇓ ESV - compensates for ⇓ in EDV
3. By ⇑ both force + HR, allows at least maintenance of SV even at high HR (usually an increase)
4. PSNS – no significant effect
5. Overall: SNS ⇑ CO; PSNS ⇓ CO

Question 17

Extrinsic Controls of Stroke Volume:

Hormones

Answer 17

1. Epi, NE - same mechanism as SNS - ⇑ force

2. thyroid hormone - ⇑ force (+ ⇑ epi receptor #)

Question 18

Other Extrinsic Controls of Stroke Volume

Answer 18

1. force ⇑ by:
   
   • ⇑ external Ca++ (more Ca++ moves in on AP)
   • digitalis (drug) - ⇑ Ca++ inside
2. force ⇓ by:
   
   • acidosis
   • ⇑ external K+
   • Ca++ channel blockers (drugs) e.g. verapamil