Mechanical properties of the heart 1

Question 1

What are the shape of ventricular cells?

Answer 1

rod shaped

Question 2

What are the dimension of the ventricular cells?

Answer 2

length - 100 micrometers
width - 15 micrometers
T tubule - 200 nm

Question 3

How are T tubules spread around Z lines?

Answer 3

They are at every Z line

Question 4

What is the importance of calcium in cardiac muscle contraction?

Answer 4

External calcium is needed

Question 5

What is the process of excitation contraction coupling?

Answer 5

1. Depolarisation is detected by L-type calcium channel and external calcium enters the cell.
   a. Some calcium directly causes contraction
   b. The rest binds to ryanodine receptors (RyR – also called Sarcoplasmic Reticulum Calcium Release Channel) which causes release of calcium from SR
2. After the effect has taken place, some calcium is taken back up into the SR by CaATPase channels (also called SERCA – Sarco/Endoplasmic Reticulum Calcium ATPase).
3. Calcium that came into the cell is then effluxed by a Sodium-Calcium Exchanger (no energy needed)

Question 6

What is the relationship between force produced and calcium?

Answer 6

Sigmoidal
Around a 10 micromol intracellular concentration of calcium is needed to produce maximal force
More calcium = greater force made

Question 7

What does passive force depend on?

Answer 7

The resistance to stretch of the muscle.

Question 8

What is isometric contraction?

Answer 8

No change in length but change in tone

Question 9

What is isotonic contraction?

Answer 9

Change in length but no change in tone

Question 10

What is preload?

Give an in vivo correlate of preload

Answer 10

Weight that stretched the muscle before it is stimulated to contract

The filling of the ventricles with blood makes the muscle stretch. The preload determines the stretching of the cardiac cells and is defined by blood pressure/volume.

Question 11

What is afterload?

Give an in vivo correlate of afterload

Answer 11

The weight that is not apparent to muscle in the resting state, only encountered when the muscle begins to contract

Pressure in the aorta which must be overcome to force blood from the ventricles. The afterload determines the contraction/shortening of the cardiac cells and is defined as the resistance of the blood flow.

Question 12

What is a measure of preload?

Answer 12

end diastolic volume
end diastolic pressure
right atrial pressure

Question 13

What is a measure of afterload?

Answer 13

diastolic arterial blood pressure

if the person is hypertensive it means the heart has to work harder ti eject blood

Question 14

If afterload is greater, what does it mean?

If preload is greater, what does it mean?

Answer 14

Less shortening, lower velocity
More stretching, more force generated

For the same afterload, if you have more preload then more force is generated.

Question 15

What does preload depend on?

Answer 15

The venous return - more blood means more preload

Question 16

What is the Frank Starling relationship (Starling’s Law)?

Answer 16

Increased diastolic fibre length increases ventricular contraction - increasing stretching/preload leads to an increase in shortening and speed of shortening

Question 17

What is a consequence of the Frank Starling relationship?

Answer 17

With a larger preload, the ventricles have to pump a greater stroke volume so that at equillbrium, CO exactly balances the venous return - blood coming in determines the strength of contraction and this determines the volume of blood leaving ventricles.

Question 18

What 2 factors is the Frank Starling relationship due to?

Answer 18

1) changes in the number of myofilament cross bridges that interact
2) changes in the calcium sensitivity of the myofilament

Question 19

What do the changes in the number of crossbridges mean for contraction?

Answer 19

At shorter lengths than optimal, the actin filaments overlap thus reducing the number of myosin cross-bridges that can be made so less force is generated.
The more you stretch a muscle the more optimum the lining of actin and myosin so more crossbridges and a stronger contraction. If you stretch too much, the myofilaments are too far and fewer crossbridges so weaker force is generated.

Question 20

What are the two possibilities for why calcium sensitivity changes when myofilaments are stretched?

Answer 20

Calcium sensitivity increases when myofilaments are stretched.

1) Troponin C is a thin filament that binds to calcium and regulates cross bridge formation. At longer sarcomere lengths the affinity of troponin C for calcium increases due to a conformational change in TnC. Then less calcium is needed for the same force generated.
2) With stretch, the space between (lattice spacing) myosin and actin filaments decrease. With decreasing myofilament lattice spacing the probability of forming strong cross bridges increases so more force is generated for the same amount of calcium.

Question 21

What is stroke work?

Answer 21

The work done by the heart to eject blood under pressure into the aorta and pulmonary artery (in one contraction).
WORK = SV * P
stroke volume is affected by preload and afterload
pressure is affected by the structure of the heart

Question 22

What is the law of laplace?

Answer 22

When the pressure within a cylinder is held constant, the tension on its walls increases with increasing radius.
Greater radius means more tension.
The force around the circumference is equal to pressure*radius.
T = PR/h

Question 23

How does the curvature differ between the left and right ventricle and why?
Wall stress is giraffes
Pressures in frogs
Cardiomyopathy and wall stress

Answer 23

• The radius of curvature of the walls of the left ventricle is less of that of the right ventricle.
• This allows the LV to generate high pressures with similar wall tension as T = PR.
• In giraffes, wall stress is kept low by the long, narrow, thick-walled ventricle which has a small radius to generate high pressure (blood to the head).
• In frogs, pressures are kept lower so the ventricles are almost spherical which makes a large radius so a low pressure.
• Failing hearts (dilated cardiomyopathy) display an enlarged radius which increases wall stress.

Question 24

What is the difference between passive force in skeletal and cardiac muscle?

Answer 24

Skeletal muscle produces less passive force - if you overstretch muscle you get a lower force (known as pulling a muscle)
Cardiac muscle is more resilient to stretching so greater passive forces are generated

Question 25

Which part of the graph in the length tension graph is important?

Answer 25

The ascending limb as the descending limb doesn’t occur because the pericardium restricts overstretching

Question 26

What is the shape of the force preload graph?

Answer 26

Preload causes stretching so the force-preload graph is the same as the force-length graph

More preload = more stretch = more force produced (so same graph shape)

Question 27

In isometric force experiments what are the two parts?

Answer 27

active force production (cross bridge formation)

passive force production (recoil after stretching)

Question 28

When muscle is stretched beyond a limit what happens?

Answer 28

More force is not produced as not enough overlap between filaments

Question 29

What is passive force due to?

Answer 29

Cytoskeletal proteins such as titin. Titin joins the Z line to the M line.

Question 30

Why is cardiac muscle more resistant to stretch than skeletal?

Answer 30

extracellular matrix and cytoskeleton