Cardiovascular mechanics I

Question 1

What is the shape of single ventricular cell?

Answer 1

Rod shaped

Question 2

What is needed for contraction of a single ventricular cell?

Answer 2

External calcium

Question 3

What is the approximate size of a single ventricular cell?

Answer 3

100micrometres long and 15micrometres wide

Question 4

What is the diameter of T-tubule openings?

Answer 4

200nanometres

Question 5

Describe the excitation-contraction coupling of the heart including the receptors involved.

Answer 5

1. Depolarisation causes the opening of L-type calcium channels.
2. Leads to the influx of calcium into the myocyte.
3. Calcium then binds to the Ryanodine receptor( sarcoplasmic reticulum calcium release channel) and leads to the release of calcium from the sarcoplasmic reticulum( store where calcium is inside)
4. After it has had its effects some of the calcium is taken back up into the SR by Ca2+ ATPase channels ( SERCA)
5. The Na/Ca exchanger effluxes out as much calcium as entered the cell in the first place (does not need energy - uses energy from the concentration gradient of sodium to expel calcium from the cell)

Question 6

What is the shape of the force-calcium relationship?

Answer 6

SIGMOIDAL

Question 7

What concentration of calcium is sufficient to generate maximum contraction?

Answer 7

10 micromolar

Question 8

Compare the length-tension relationship in skeletal and cardiac muscle and explain why cardiac muscle has this property

Answer 8

Cardiac muscle is much more resistant to stretch and less compliant. Cardiac muscle exerts a lot more passive force, due to properties of the extracellular matrix and cytoskeleton.

It does NOT OVERSTRETCH as it is encased in the pericardium
It is more RESISTANT to stretch than skeletal so will generate a greater passive force as the length increases (stretched)
It is LESS COMPLIANT than skeletal
It uses both ISOTONIC and ISOMETRIC contraction

Question 9

What are the two types of contraction used by the heart?

Answer 9

Isometric: contraction resists the high pressure, there is no change in length
Isotonic: contraction is the shortening of fibres when blood is ejected from the ventricles.

Question 10

what limb of the cardiac muscle is important in physiological circumstances and why is the other not?

Answer 10

Ascending limb.

The descending limb doesn’t happen in physiological conditions because the pericardium restrict the stretching.

Question 11

What is Preload?

Answer 11

The weight that stretches the muscle before it is stimulated to contract e.g. the filling of the ventricles with blood makes it stretch before it is stimulated

Question 12

What is Afterload?

Answer 12

The weight that is not apparent to the muscle in the resting state and is only encountered once the muscle has started to contract.

-is the weight or mass or pressure that the muscle is trying to overcome- it only becomes apparent after contraction has been stimulated

Question 13

What is the effect of increasing preload ?

Answer 13

Increasing preload increases the force exerted by the muscle fibres. it governs the amount of force a muscle is capable of producing.
-for the same afterload if you stretch the muscle prior to excitation it will generate more force.

Question 14

What are the effects of increasing afterload?

Answer 14

Increasing afterload decreases the amount of shortening of muscle fibres and decreases the velocity of shortening of the fibres.

Question 15

What are the in vivo correlates of preload?

Answer 15

as blood fills the ventricles during diastole it stretched the resting ventricular walls. this streching determines the preload.
so preload is measured: End-diastolic volume,

Question 16

What are the in vivo correlates of afterload?

Answer 16

-pressure in the aorta
the load against which the left ventricle ejects blood after opening of the aortic valve i.e. it is the blood pressure that the heart must overcome to eject blood.

Question 17

What is a simple measure of afterload?

Answer 17

Diastolic arterial blood pressure

Question 18

State Starling’s Law.

Answer 18

Increase in diastolic fibre length increases ventricular contraction.
i.e. increase in preload leads to an increase in shortening and speed of shortening.

Question 19

Starling’s law is caused by what factors?

Answer 19

-Changes in the number of myofilament cross-bridges Change of #crossbridges formed
Too much overlap = very few crossbridges
Too little overlap = very few crossbridges
Optimum overlap = maximal crossbridges

-Change in sensitivity of troponin-C to Ca2+
Longer lengths of sarcomeres causes a conformational change in troponin-C so the sensitivity to Ca2+ increases
Increased length = increased Ca2+ sensitivity

-Change of lattice spacing (space between myosin and actin)
Increased stretch causes decreased space between myosin and actin (lattice spacing) and more corssbridges to form

Question 20

State two possible explanations for increasing calcium sensitivity of the myofilaments.

Answer 20

calcium sensitivity increases when the myofilament are stretched.
-When the muscles are stretched, the lattice spacing (spacing between filaments) decreases meaning that for the same amount of calcium, more cross bridges can be formed. When sarcomere length changes, there is a conformational change in troponin C that gives it a higher affinity for calcium.

Question 21

What is Stroke Work?

Answer 21

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

Question 22

State the equation for stroke work.

Answer 22

Stroke Work = Stroke Volume x Pressure (at which the blood is ejected) Stroke volume is greatly affected by preload, afterload.

Question 23

State the law of Laplace.

Answer 23

When the pressure within a cylinder is kept constant, tension increases with increasing radius. T = PR.

Question 24

What is the physiological relevance of the law of Laplace with regards to the structure of the right and left ventricles?

Answer 24

The left ventricle has a smaller radius of curvature than the right ventricle meaning that the left ventricle is able to generate higher pressures with similar wall tension.

Question 25

What is the clinical significance of the law of Laplace?

Answer 25

In dilated cardiomyopathy, the radius of curvature increases and hence the pressure generated decreases.

Question 26

Explain what happens to active force and passive graphically when muscle is increased to the force produced?

Answer 26

Active force increases more than passive but then starts to decrease.
Passive force increases continuously as muscle length increases