Cardiovascular Mechanics

Question 1

All muscle cells when isolated can contract on their own. TRUE or FALSE and explain answer

Answer 1

False - Cardiac muscle needs an influx of calcium to contract

Question 2

When does contraction happen relative to Calcium influx

Answer 2

Contraction happens after calcium influx

Calcium is released from sarcoplasmic reticulum after the ryanodine receptors are activated and then binds to troponin to move tropomyosin which exposes the myosin binding filaments on the actin

Question 3

To what part of the microfiliament are the T tubules aligned with

Suggest why?

Answer 3

The Z line

Question 4

Describe how calcium is moved around within the cardiac muscle cell

Answer 4

Enters via channels in T tubules called LT calcium channels

some of the calcium attaches directly to the troponin, but most acts on ryanodine to open the calcium Chanel in the sarcoplasmic reticulum which increases the amount of calcium which binds to troponin

Some of the calcium is taken back into the sarcoplasmic reticulum by active transport

Some of it is pumped back out by the T tubule using Na/Ca co transporter

Question 5

What’s the main job of T tubules

Answer 5

To carry surface depolarisation deep into the cell

Question 6

What are the dimensions of a ventricular cell

Answer 6

About 100 micrometers long and 15 micrometers wide

Question 7

What are the dimensions of T tubules

Answer 7

200nm in

Question 8

How far apart are T tubules spaced

Answer 8

2 micrometers apart

Question 9

What happens to calcium in the steady state

Answer 9

There is no net gain/loss of calcium due to:

Loss of calcium from the sarcoplasmic by the SR calcium ATPase

AND loss of calcium by Na/Ca exchanger

Question 10

What is the name of the channel that allows Calcium to enter the sarcoplasm from the T tubule

Answer 10

L type Calcium channel

Question 11

What’s another name for ryanodine

Answer 11

SR Calcium Channel

Question 12

How can you change the amount of calcium in the sarcoplasm

Answer 12

Sympathetic stimulation

Phosphorylation of certain proteins

Question 13

Draw a diagram of the relationship between force and calcium concentration in the cell

Answer 13

S shaped up to about 100 milimoles

Question 14

Describe the tension length experiment in cardiac muscles and draw diagram

Answer 14

As muscle length increases, so does the force it can bear

Passive force ( the ‘recoil’) always increases but the active force decreases after a certain point

Question 15

1)Draw force (% max) vs muscle length (% max) of cardiac muscle showing:

Total force
Active force
Passive force

2) Which part of this is clinically relevant

Answer 15

1) Total force increases to a max just before 100% of stretching length and then decreases and plateaux

Active force increases to a max just before 100% and continuously goes down to nearly 0 after 200%

Passive force continually goes up

2) Only relevant up to 100%

Question 16

What are the 2 forms of contraction that the heart muscles undergo

And give examples

Answer 16

Isometric - muscle fibres stay the same length but pressure increases in both ventricles eg ventricles filling up with blood

Isotonic - muscle fibre shortens and ejects blood out of the ventricle eg ventricles contracting to push blood into aorta

Question 17

What’s preload

Answer 17

Weight that stretches muscle before it is stimulated to contract

Question 18

What is after load

Answer 18

Weight not apparent to muscle in resting state

Only when muscle has started to contract is it encountered

Question 19

What correlations of preload and after load are clinically important

Answer 19

Preload = filling up of ventricles

After load = contraction of ventricles

Question 20

Draw a graph of force vs preload (stretch) for an isometric muscle

Answer 20

Curved and increases to a point then in a curve decreases

Question 21

Draw graph of shortening vs after load for an isotonic contraction for both with a smaller preload(blue) and a larger preload(green )

Answer 21

Straight line to 0 but the one with a larger preload has a higher line

Question 22

What correlates to preload

Answer 22

Stretching of ventricles as it fills with blood

So the venous drainage into the entry lees dtermines the preload

Question 23

State the Franklin Starling relationship diastolic

Answer 23

Increased fibre length increases ventricular constriction

Question 24

Give 2 reasons for the Franklin Starling relationship

Answer 24

1) As it stretches the actin gets further and further apart, the actin is further away from each other so can bind to more myosin
(At shorter lengths the actin is closer to each other and so sub optionally binds to myosin as they are in each other’s way )

2) Troponin C undergoes a conformational change to bind more favourably to calcium when it is stretched
3) stronger cross bridges between myosin and actin when further away

Question 25

What is the definition of stroke work

Answer 25

Work done by ventricles under pressure to pump blood into aorta and pulmonary artery

Stroke work = Volume of blood pumped * pressure

Question 26

What is the law of La Place

Answer 26

When the pressure on a wall is held the same, the tension on its walls increases with increasing radius

T = R*P

Tension = readius* pressure

Question 27

apply the law of LaPlace to the structure of the heart

Answer 27

Tension = Pressure *radius

You want a higher pressure in the aorta than the pulmonary artery

BUT….

You want the tension to be the same (as you want the Heart muscles to use the same force)

So the left ventricle has a smaller radius than the righ ventricle