CVS 2 - Mechanical properties of the heart 1

Question 1

What is needed for contraction of a single ventricular cell?

Answer 1

External calcium as you have CICR

Skeletal muscle can contract in solution which has no calcium

Question 2

What is the approximate size of a ventricular myocyte and T tubules?

Answer 2

15 x 100 micrometres for cell

T tubule 200 nm diameter

Question 3

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

Answer 3

Depolarisation causes the opening of L-type calcium channels situated on the myocyte membrane. This leads to influx of calcium into the myocyte. The calcium then binds to the Ryanodine receptor on the SR membrane and leads to the release of calcium from the SR –> this is Calcium Induced Calcium Release (CICR). This calcium causes the myofilament contraction.
SERCA then takes the calcium used for the contraction from the cytoplasm back into the SR. The Na/Ca exchanger pumps out as much calcium as entered the cell in the first place.

Question 4

What is the shape of the force-calcium relationship?

Answer 4

Sigmoidal

Question 5

What concentration of calcium is sufficient to generate maximum contraction?

Answer 5

10 micromolar

Question 6

Compare the length-tension relationship in skeletal and cardiac muscle.

Answer 6

Cardiac muscle is much more resistant to stretch and exerts a lot more passive force

Question 7

What happens to force when muscle is stretched?

Answer 7

Amount of active force increases as muscle gets longer. Relationship is called the active force production
Active force is parabolic relationship (kind of)
If overstretched, active force declines

Question 8

What is passive force?

Answer 8

Passive recoil force from stretching due to elastic nature of the tissue - linear relationship

Question 9

What are the two forms of contraction of the heart?

Answer 9

Isometric - no shortening but pressure in both ventricles increases
Isotonic - shortening of fibres and blood ejected from ventricles

Question 10

What is Preload?

Answer 10

The weight that stretches the muscle before it is stimulated to contract

Question 11

What is afterload?

Answer 11

The weight that the muscle encounters after it contracts. Not visible when the muscle is not contracting

Question 12

What is the effect of increasing preload?

Answer 12

Increasing preload increases the force exerted by the muscle fibres

Question 13

What are the effects of increasing afterload?

Answer 13

Increasing afterload decreases the amount of shortening of muscle fibres so their force and decreases the velocity of shortening of the fibres.
–> weaker and slower contraction

Question 14

What are the in vivo correlates of preload?

Answer 14

End diastolic volume (this is the venous return to the heart that stretches the muscle fibres) as blood fills the ventricules during the relaxation phase of the cardiac cycle (diastole) it stretches the resting ventricular walls –> stretch=preload
Dependent on venous return to the heart
More blood filles–> more preload –> more force

Question 15

What are the in vivo correlates of afterload?

Answer 15

The load against which the left ventricle ejects blood after opening of the aortic valve –> pressure agains the aortic valve us not seen until it opens = afterload = diastolic pressure
So simple measure of afterload is DBP
Any increase in afterload decreases the amount of shortening that occurs which decreases force and velocity of contraction

Question 16

What is Frank-Starling’s law?

Answer 16

Increase in diastolic fibre length increases ventricular contraction.

Question 17

What is the consequence of Starling’s law?

Answer 17

Ventricles pump greater stroke volume so that at equilibrium CO exactly balances the augmented venous return
(blood going in balanced by blood going out)

Question 18

Starling’s law is caused by what two factors?

Answer 18

Changes in the number of myofilament cross-bridges (if too short they overlap and too stretched there fewer cross bridges)
Changes in the calcium sensitivity of the myofilaments with increased length

Question 19

What is Stroke Work?

Answer 19

Work done by heart to eject blood under pressure into aorta and pulmonary artery
Stroke work = SV x P
Volume of blood ejected with each stroke x pressure at which blood is ejected

Question 20

State the law of Laplace

Answer 20

When the pressure within a cylinder is held constant, the tension on its walls (wall stress) increases with increasing radius
T = P x R
E.g Left and right ventricle have similar wall stress (=T). However LV has smaller radius than RV (smaller R) meaning it can generate higher pressures (P has is bigger because T is the same but R is smaller). Generating high pressure in aorta is essential to go to whole body.
Wall stress is like ‘pulling pressure’ what pulls the walls apart
So a heart failing often becomes dilated which increases wall stress
Also thickness of wall has an impact: the bigger, the less tension there is so LV has thicker walls to reduce tension and hence can increase pressure.