CVS 2 - Mechanical Properties of the Heart 1

Question 1

Describe the process of cardiac contraction for a single ventricular cell.

Answer 1

1. Electrical event
2. Ca transient (increase in Ca conc in sarcoplasm temporarily)
3. Contractile event

Question 2

Describe a single ventricular cell.

Answer 2

1. T-tubules roughly 200nm

2. Spread about 2 um apart- each T-tubule lies alongside Z-line of every myofibril

Question 3

What is different about cardiac contraction and skeletal muscle contraction?

Answer 3

Heart won’t beat without extracellular calcium whereas skeletal muscle can contract without external calcium

Question 4

Describe excitation-contraction coupling in the heart.

Answer 4

1. L-type calcium channel senses depolarisation; external calcium enters the cell.
2. Some Ca directly causes contraction
3. The rest of the Ca binds to Ryanodine receptors (aka SRCRC). Causes release of Ca from SR.
4. After Ca has had its effect, some Ca taken up into SR b Ca ATPase channels (aka SERCA).
5. Same amount of Ca that entered the cell is effluxed by a Sodium/Calcium Exchanger - THIS PROCESS DOES NOT REQUIRE ENERGY

Question 5

What kind of relationship is the Force-Ca conc relationship?

Answer 5

Sigmoidal

Question 6

Describe Length-Tension relationship in cardiac muscle in isometric contraction.

Answer 6

Isometric - muscle doesn’t shorten, it just pulls on force transducer

Increase in muscle length causes an increase in force, UPTO A POINT.

AFTER THIS POINT, FURTHER STRETCHING DOESNT GENERATE MORE FORCE - because not enough overlap between filaments to produce force.

Question 7

What does overstretching the muscle lead to?

Answer 7

Decrease in force.

This is what happens in skeletal muscle when you pull a muscle

Question 8

What is passive force?

Does skeletal muscle or cardiac muscle exert more passive force?

Answer 8

Passive force = based on resistance to stretch of the muscle

Cardiac muscle more resilient to stretch so exerts more passive force.

Question 9

Why is cardiac muscle more resistant to stretch?

Answer 9

Due to properties of the ECM and Cytoskeleton

Question 10

Why is cardiac muscle very unlikely to be overstretched?

Answer 10

Due to the pericardium, which restricts stretching.

Therefore, only the ascending limb of the length-tension graph is important

Question 11

Which 2 forms of contraction does the heart use?

Answer 11

1. Isometric - resists high pressure, no change in length but change in tone.
2. Isotonic - fibres shorten when blood ejected from ventricles.

Question 12

What is preload?

Answer 12

Weight that stretched the muscle before it is stimulated to contract (i.e. filling of ventricles)

It is dependent on venous return

Question 13

What is after load?

Answer 13

Weight that is not apparent to muscle in resting state - encountered only once muscle has started to contract.

“The load against which the LV ejects blood after opening of the aortic valve”

Basically BP

Question 14

What does more after load mean?

Answer 14

More Afterload = Less shortening

More after load = less velocity of shortening

Question 15

What does preload govern?

Answer 15

Amount of force muscle is capable of producing

Question 16

What are measures of preload?

Answer 16

1. End diastolic volume
2. End diastolic pressure
3. Right atrial pressure

(Preload is the stretching of the ventricular walls as blood fills)

Question 17

How can afterload be measured?

Answer 17

Diastolic arterial blood pressure

Question 18

2 results of increased after load.

Answer 18

Decreased shortening + decreased velocity of shortening

Question 19

Describe 2 features of increased aortic pressure (increased afterload)

Answer 19

1. Increase in aortic pressure = decrease in shortening

2. At the same aortic pressure, more ventricular filling (more preload) means increased shortening

Question 20

Describe Starling’s Law.

Answer 20

Increased diastolic fibre length increases ventricular contraction.

aka - increased preload leads to increased shortening and speed of shortening

Consequence: Increased diastolic fibre length causes ventricles to pump an increased stroke volume so that cardiac output exactly balances increased venous return (at eqm)

aka - amount of blood coming into ventricles determines strength of ventricular contraction and so determines amount of blood leaving ventricles.

Question 21

Which 2 factors cause the Frank-Starling relationship (Starlings Law).

Answer 21

1. Changes in number of myofilament cross bridges that interact
2. Changes in Ca sensitivity of myofilaments

Question 22

Describe how changes in the number of cross bridges affects Frank-Starling relationship.

Answer 22

1. At sub-optimal lengths, actin filaments overlap which reduces the number of myosin cross bridges that can form.
2. Increased stretching of muscle allows more optimum interdigitation of actin and myosin filaments.

Question 23

What happens when myofilaments are stretched?

Answer 23

Ca sensitivity increases

Question 24

Explain the 2 possibilities for increased Ca sensitivity with stretched myofilaments.

Answer 24

1. Increased Troponin C affinity for calcium due to conformation change in protein (at increased sarcomere lengths) - less Ca needed for same amount of force
2. Stretching causes space between myosin and actin filaments to decrease. This increases the probability of forming strong cross bridges - more force for same Ca

Question 25

What is stroke work?

Answer 25

Work done by the heart to eject blood under pressure into the aorta and pulmonary artery

SW = SV x P

Question 26

What factors greatly affect stroke volume?

Answer 26

Preload and afterload

Question 27

What greatly affects pressure?

Answer 27

Structure of the heart

Question 28

What is the Law of Laplace?

Answer 28

When the pressure within a cylinder is held constant, the tension on its walls increases with increasing radius.

Increased radius = increased tension (increased force around the sides)

Force around sides (T) = P x r

Question 29

What is the physiological relevance of the Law of Laplace?

Answer 29

1. Same wall tension must be maintained.
2. T =P x r
3. Tension must be same, but LV requires much higher pressure, meaning that it has a lower radius of curvature.

IT ALLOWS LV TO GENERATE HIGH PRESSURES WITH SAME WALL TENSION

Question 30

What happens as muscle length increases?

Answer 30

Passive force increases continuously