Regulation of stroke volume and heart rate

Question 1

What is stroke volume?

Answer 1

Volume of blood pumped per ventricle per contraction

Question 2

What is End diastole volume?

Answer 2

Volume of blood in the ventricles at the beginning of contraction.

Question 3

What does End Diastole volume determine?

Answer 3

The length of the muscle

Question 4

What is stroke volume directly related to?

Answer 4

The force generated by cardiac muscle during contraction

Question 5

What is force of ventricular contraction affected by?

Answer 5

1. Length of muscle fibres at the beginning of contraction

2. Contractility of the heart

Question 6

What determines the length of cardiac muscle fibres at the beginning of contraction?

Answer 6

End diastole volume- how full the ventricle is of blood at the beginning of contraction

Question 7

What is contractilty of the heart determined by?

Answer 7

Function of Ca interaction with the contractile filaments.

Question 8

As contraction force increases, _______ increases

Answer 8

stroke volume

Question 9

What is the force created by a muscle fibre directly related to?

Answer 9

The sarcomere length, AS INDICATED BY THE INITial length of the muscle fibre

Question 10

Relate the sarcomere length and how the initial length of a muscle fibre creates a bigger force to the HEART.

Answer 10

As stretch of the ventricle wall increases it increases the stroke volume.

-If additional blood flows into the ventricle, the muscle fibres stretches and contracts more forcefully, ejecting more blood.

Question 11

What is preload?

Answer 11

The degree of myocardial stretch before contraction begins

Question 12

What is End diastole volume a measure of?

Answer 12

Measure of stretch in the ventricles

Question 13

What does the measure of stretch in the ventricles determine?

Answer 13

Sarcomere length

Question 14

What does the X axis of the Starling curve show?

Answer 14

End diastolic volume which is a measure of stretch of the ventricles which in turn determines sarcomere length.

Question 15

What does the Y axis of the Starling curve show?

Answer 15

Stroke volume, which indicates the force of contraction

Question 16

What does the Starling curve show?

Answer 16

Stroke volume is proportional to End Diastolic Volume ie stroke volume increases as end diastolic volume increases.

Question 17

What is venous return affected by?

Answer 17

1. Sympathetic innervation of veins
2. Skeletal muscle pump (its contractions compress/contract the veins, particularly in the legs, to squeeze more blood out of them and towards theheart)
3. Respiratory pump (pressure changes in the abdomen and thorax during breathing)

Question 18

How can you affect contractility?

Answer 18

Via the nervous and endocrine system.

• As the amount of calcium svailable for contraction increases, contractility increases.
• Norepinephrine and epinephine bind to B1 receptors.
• This phosphoryltes voltage gated Ca channels, and opens them for longer, increasing Ca entry from ECF.
• Increases Ca stores in Sarcoplasmic reticulum, increases the amount of Ca being released, giving a stronger strength of contraction.
• Ca is removed from cytosol faster, shortening the Ca-troponin binding time, shorter duration of contraction

Question 19

If left ventricle is damaged, what effect does this have on cardiac output?

Answer 19

Ventricular myocardium has weakened therefore decreased sstroke volume, in turn decreased cardiac outut

Question 20

CO = ?

Answer 20

HR * SV

Question 21

What is afterload?

Answer 21

Combined EDV and arteriolar resistance during ventricular contraction.

Load placed on the ventricle as it contracts

Question 22

What determines afterload?

Answer 22

End diastolic volume + Arterial blood pressure

Question 23

When do we see afterload and what is its effect over a long period of time?

Answer 23

• High arteriolar blood pressure and loss of stretchability (compliance) in the Aorta
• Myocardial hypertrophy because to maintain constant stroke volume the heart muscle has to work harder and ventricle increases its force of contraction, increases the uscles need for oxygen and ATP production

Question 24

What is heart rate affected by? What does heart rate determine?

Answer 24

• Autonomic divisions of the nervous system i.e. parasympathetic (acetylcholine acting on muscarinic receptors, increases K efflux and decreases Ca release, hyperpolarising the cell) and sympathetic nervous system (norepinephrine on B1 receptors, increases K and Ca influx, depolarising the cell)
• Epinephrine

Question 25

What does afterload reflect?

Answer 25

• The preload and the effort required to push the blood out into the arterial system.

Question 26

What is a clinical indicator of Afterload?

Answer 26

Mean arterial pressure

Question 27

What does a)Norepinephrine and epinephrine act on and b)Acetylcholine act on?

Answer 27

a) Norepinephrine and epinephrine acts on B1 receptors to speed up the rate of pacemaker depolrisation.
b) Acetylcholine acts on muscarinic receptors to hyperpolaise the pacemaker cells.

Question 28

What does echocardiography allow?

Answer 28

Ejection fraction (stroke volume/end diastole volume) to be calcualted. Indicates ventricualr function