Lecture 30: Heart Control

Question 1

Cardiac output (Q) averages

Answer 1

• At rest: 4-6 L/min in average adult
• Strenuous exercise: 15+ L/min
• Increases occur with minimal changes in blood pressure

Question 2

Cardiac output is determined by 2 parameters

Answer 2

• Stroke volume
• Heart rate
• Both intrinsic and extrinsic mechanisms can alter parameters

Question 3

Normal adult heart rate

Answer 3

• About 70 beats/min
• Much higher in children
• Sleep decreases heart rate by 10-20 beats/min
• Anxiety, activity and exercise increase it

Question 4

Heart rates of well-trained athletes

Answer 4

• Lower resting heart rates
• About 50 beats/min
• Resting cardiac outputs are the same for sedentary and well-trained athletes

Question 5

Intrinsic factors altering heart control

Answer 5

• Relatively minor importance, evokes subtle effects
• Atrial stretch (distension)
• Temperature
• Ionic Changes

Question 6

Atrial stretch or distension (intrinsic)

Answer 6

• Stretch of SA node pacemaker cells
• Increase excitability
• Increases HR by 15%

Question 7

Temperature (intrinsic)

Answer 7

• Increase in temperature increases HR by increasing ionic permeabilities

Question 8

Ionic changes (intrinsic)

Answer 8

• Alterations in electrolyte balance
• Particularly K+ and Ca2+
• Effects excitability of pacemaker cells
• High Ca2+o or high K+o decreases HR

Question 9

Extrinsic control of the heart

Answer 9

• Neurohormones
• Sympathetics
• PNS
• Bainbridge reflex
• Baroreceptor reflex
• Higher center control
• Respiratory
• Chemoreceptors

Question 10

Sympathetics (extrinsic)

Answer 10

• Accelerate

- Norepinephrine effects on SA node

Question 11

PNS (extrinsic)

Answer 11

• Decelerate

- Acetylcholine effects on SA node

Question 12

Bainbridge reflex

Answer 12

• Stretch of atrial receptors in veno-atrial junctions of left and right heart
• Sensory impulses via vagus to cardiovascular center
• Increase sympathetic and decrease parasympathetic output to heart
• Therefore, an increase in HR

Question 13

Bainbridge reflex stretching

Answer 13

• Accounts for 50-60% of stretch effect (along with SA nodal stretch effect)

Question 14

Baroreceptor reflex relationship

Answer 14

• Inverse relationship between blood pressure and heart rate

Question 15

In hypotension, such as that induced by hemorrhage, in response to the loss of a moderate amount of blood,

Answer 15

• Vagal tone diminishes
• Sympathetic activity increases
• Vagal activity ceases after BP declines to 20-30 mmHg below normal level
• Further acceleration of heart in response to declines in BP is mediated exclusively by increases in sympathetics

Question 16

Higher center control (extrinsic)

Answer 16

• Cerebral cortex, hypothalamus, thalamus
• First two play roles in HR changes seen in excitement, anxiety
• Hypothalamic responses seen with temperature changes

Question 17

Cortical and diencephalic centers

Answer 17

• Initiate cardiovascular reactions that occur during excitement, anxiety, and other emotional states
• Also during certain febrile disease states

Question 18

Respiratory (extrinsic)

Answer 18

• Increase HR during inspiration
• Decrease HR during expiration
• Changes more pronounced in children
• Mediated via stretch receptors in lungs, via changes in venous return (i.e. Bainbridge reflex and baroreceptor reflex)

Question 19

Chemoreceptors (extrinsic)

Answer 19

• Stimulation of receptors produces slight effects on HR
• Primary reflex effect of carotid chemoreceptor stimulation on SA node is inhibitory
• Secondary effects via changes in pulmonary ventilation (small increase, small increased HR; large increase, marked increased HR)

Question 20

Hormones (extrinsic)

Answer 20

• Epinephrine (like sympathetic norepinephrine) produces a positive chronotropy
• Atrial natriuretic peptide (ANP), produced by atrial cells when stretched, affects blood volume, but probably has no effect on heart rate

Question 21

Stretch effect (intrinsic control of SV)

Answer 21

• Starling’s Law of Heart (heterometric autoregulation)

- In simple terms cardiac output matches venous return

Question 22

Increased venous return in Starling effect leads to an increase in

Answer 22

• EDV
• Stretch of ventricular fibers (increased sarcomere length)
• Force of contraction
• SV

Question 23

Stretch effect is measured in

Answer 23

• Heart lung preparation
• Family of ventricular function curves are used as other parameters are changed
• These other parameters are related to changes in contractility (inotropism)

Question 24

Changes in contractility

Answer 24

• Inherent changes due to alteration of Cai, Ki, other ions

Question 25

Homeometric autoregulation

Answer 25

• Pressure induced (Anrep effect)
• Increase in afterload
• Decrease in SV
• Seen only at blood pressures above 160 mmHg

Question 26

Rate induced changes

Answer 26

• Treppe/staircase
• After period of rest
• Ventricle responds to repetitive stimulation by increase in force of contraction

Question 27

Alteration of blood gases

Answer 27

• Decreased Po2
• Increased Pco2
• Decreased pH
• Last two probably via decrease in pHi (since acidosis of cardiac muscle depresses contractility)

Question 28

Inhibitory interactions between vago-sympathetic divisions

Answer 28

• Direct and indirect vagal effects via sympathetic inhibition

Question 29

Increase in cyclic GMP causes

Answer 29

• Decrease SV (direct)

Question 30

Decrease in cyclic AMP causes

Answer 30

• Decrease SV (indirect)

Question 31

Increase in cyclic GMP due to

Answer 31

• Increased breakdown of cyclic AMP

Question 32

Presynaptic inhibition of sympathetic terminals by vagal terminals effects

Answer 32

• ACh from vagal terminals inhibits sympathetic NE release

Question 33

Presynaptic inhibition of parasympathetic terminals by sympathetic terminals effects

Answer 33

• NPY from sympathetic terminals inhibits vagal ACh release