2. Cardiovascular mechanics 1

Question 1

What is needed for contraction of a single ventricular cell?

Answer 1

Extracellular calcium

Due to Calcium induced calcium release mechanism

Question 2

Size of ventricular myocyte

Answer 2

100 μm long

15 μm wide

Question 3

What are T tubules?

Answer 3

Finger-like invaginations from the cell surface

Carry surface depolarisation deep into cell

Question 4

Describe the excitation-contraction coupling of the heart

Answer 4

Depolarisation causes the opening of L-type Ca2+ channels.
Influx of Ca2+ into the myocyte
Some Ca2+ activates myofilaments
Most Ca2+ binds to the Ryanodine receptor causing conformational change and efflux of Ca2+ from the sarcoplasmic reticulum into cell.
Ca2+ binds to troponin and activates actin-myosin interaction

Question 5

How is Ca2+ returned to the SR?

Answer 5

Use ATP to pump Ca2+ against concentration gradient from cytoplasm back into SR via Ca2+ ATPase

Question 6

How is Ca2+ removed from cell?

Answer 6

Na+/Ca2+ exchange system

Doesn’t use ATP, uses energy gradient of Na+ to efflux Ca2+ from cell to relax

Question 7

What is the shape of the force production-intracellular calcium relationship?

Answer 7

SIGMOIDAL

Question 8

What concentration of calcium is sufficient to generate maximum contraction?

Answer 8

10 micromolar

Question 9

Length-tension relationship

Answer 9

As muscle is stretched up to an optimum point, more force is produced

Question 10

Equation for total force

Answer 10

Active force + Passive force

Question 11

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

Answer 11

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

Question 12

What are the 2 types of contraction used by the heart?

Answer 12

Isometric: Muscle fibres don’t change length, but pressures increase in both ventricles
Isotonic: Shortening of fibres, blood is ejected from ventricles

Question 13

What is Preload?

Answer 13

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

Question 14

What is Afterload?

Answer 14

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

Question 15

What is the effect of increasing preload (and stretch)?

Answer 15

Increases the force exerted by the muscle fibres

Question 16

In vivo preload

Answer 16

Amount of blood that returns to the heart during diastolic interval determines how much heart fills with blood and stretches

Question 17

Measures of preload

Answer 17

End-diastolic volume: amount of blood at end of diastole (relaxation), sets stretch
End-diastolic pressure
Right atrial pressure

Question 18

In vivo afterload

Answer 18

Load against which left ventricle ejects blood after opening of the aortic valve

Question 19

Measure of afterload

Answer 19

Diastolic blood pressure

Question 20

What are the effects of increasing afterload?

Answer 20

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

Question 21

Why is hypertension a risk factor for cardiac disease?

Answer 21

The higher the afterload (diastolic BP) the harder the ventricle has to work to expel blood

Question 22

State Starling’s Law.

Answer 22

Increase in diastolic fibre length increases ventricular contraction.

Question 23

What is the consequence of starlings law?

Answer 23

Ventricles pump greater stroke volume so that, at equilibrium, cardiac output exactly balances the augmented venous return

Question 24

Starling’s law is caused by what 2 factors?

Answer 24

Changes in the number of myofilament cross-bridges

Changes in the calcium sensitivity of the myofilaments

Question 25

Describe the effects of changes in the number of myofilament cross-bridges

Answer 25

At shorter lengths than optimal, actin filaments overlap on themselves so reducing the number of myosin cross bridges that can be made.
As stretched, more and more, optimal interdigitation between thick and thin filaments until maximum

Question 26

State 2 possible explanations for increasing calcium sensitivity of the myofilaments.

Answer 26

1. With stretch, lattice spacing (spacing between filaments) decreases, meaning for the same amount of Ca2+, more cross bridges can be formed.
2. At longer sarcomere lengths, affinity of TnC for Ca2+ is increased due to conformational change in protein
   Less Ca2+ required for same amount of force

Question 27

What is Stroke Work?

Answer 27

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

Question 28

Equation for stroke work

Answer 28

Stroke Volume x Pressure (at which the blood is ejected)

=SV x P

Question 29

What greatly affect stroke volume and pressure?

Answer 29

SV: Preload and afterload
P: Cardiac structure

Question 30

State the law of Laplace.

Answer 30

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

Question 31

Equation of Law of Laplace

Answer 31

Wall tension = Pressure in vessel x Radius of vessel

T = P x R

Question 32

Law of Laplace equation incorporating wall thickness (h)

Answer 32

T= (PxR) / h

Question 33

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

Answer 33

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 34

What is the clinical significance of the law of Laplace?

Answer 34

In dilated cardiomyopathy, radius of curvature increases

Hence pressure generated decreases.

Question 35

How is wall stress kept low in giraffes?

Answer 35

They have long, narrow, thick-walled ventricles