Cardiovascular mechanics

Question 1

How does contraction occur in a ventricular cells?

Answer 1

Electrical event –> Ca ion influx –> contractile event

AP increases first, then calcium, then contraction occurs ( graph on slide )

• calcium transient effect.

Question 2

What are the characteristics of ventricular cells and t-tubules?

Answer 2

100 micrometer long, 15 wide.

T tubules create projections causing invaginations of the cell surface

T tubules open to 200nm in diameter.

*Each T-tubule lies alongside each Z-line

Question 3

How does the sarcoplasmic reticulum look surrounding Ventricular cells?

Answer 3

It binds around each t -tubule and creates a network across the muscle filaments.

Question 4

How does Excitation-contraction coupling occur in the heart?

Answer 4

L-type Calcium channels in T-tubules allow Ca2+ into cytosol of cell.

Binds to SR Ca2+ ligand release channels. And stored calcium in SR is released.

Binds to troponin.

Question 5

How to relax ventricular cells?

Answer 5

Calcium ions actively pumped into SR with SR Ca2+ ATPase

and

Na+/Ca2+ exhange system in T-tubule to remove calcium.

Allows a calcium balance in the cell itself

Question 6

Is there a relationship between force production and intracellular Ca2+?

Answer 6

Sigmodial relationship

Question 7

Is there a relationship between length of cell and force produced?

Answer 7

Active force production increase

There is also a passive force due to the recoil of elastic

Question 8

What comparisons can be made between skeletal muscle and cardiac muscle length-tension relationship

Answer 8

Cardiac muscle is less resistant to stress due to the properties of extracellular matrix and cytoskeleton. So, it has a higher passive forc.

Both have similar active force

So cardiac muscle has higher total force

Question 9

What are the characteristics of isometric contraction?

Answer 9

Muscle fibres do not change length by pressures increase in both ventricles.

Question 10

What are the characteristics of Isotonic contraction?

Answer 10

Shortening of fibres and blood is ejected from ventricles

*heart uses both types

Question 11

What is Preload?

What does a small preload?

Answer 11

Weight that stretches muscle before it is stimulated to contract

Small preload (shorter muscle lengths) – less force can be produced and opposing for larger preload.

• Isometric contraction, preload also known as ventricular filling before isotonic contraction expels blood

Question 12

What is Afterload?

Answer 12

Weight not apparent to muscle in resting state; only encountered when muscle has started to contract

Larger afterload would mean less shortening and speed of shortening of muscle tissue.

*Isotonic contraction, afterload also known as pressure in the aorta

Question 13

What is preload dependent upon?

How can we measure preload?

Answer 13

Dependent on venous blood return

Measures of preload include end-diastolic volume, end-diastolic pressure and right atrial pressure

Question 14

How to measure of afterload?

Answer 14

Diastolic blood pressure

Question 15

What is the frank-starling relationship?

Answer 15

( filling of the heart increased, the force of contraction also increased )

Increased diastolic fibre length increases ventricular contraction

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

Question 16

What are the two factors that affect the Frank-Starling relationship?

( cross-bridges )

Answer 16

Changes in the number of myofilament cross bridges that interact as muscle is stretched.

( at shorter lengths the actin are overlapping reducing the number of cross bridges that could be made at optimal length. )

Question 17

What are the two factors that affect the Frank-Starling relationship?

( Calcium ion )

2 hypothesis

Answer 17

Changes in the Ca2+ sensitivity of the myofilaments

Hypothesis 1
At longer sarcomere lengths the affinity of Troponin C for Ca2+ is increased due to conformational change in protein
Less Ca2+ required for same amount of force ( graph shift to left hand side )

Hypothesis2

With decreasing myofilament lattice spacing ( gets thinner width wise ), the probability of forming strong binding cross-bridges increases

This produces more force for the same amount of activating calcium

Question 18

What is stroke work?

Answer 18

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

influenced by preload and afterload.

*And cardiac structure

Question 19

Formula for Stroke work?

Answer 19

Stroke work = SV x P

Volume of blood ejected during each stroke (SV) multiplied by the pressure at which the blood is ejected (P)

Question 20

What is the Law of LaPlace?

Answer 20

When the pressure within a cylinder is held constant, the tension on its walls increases with increasing radius

Question 21

How to calculate wall tension?

Answer 21

Wall Tension = Pressure in vessel x Radius of vessel

• Incorporating wall thickness (h), this can be amended to:
  T= (PxR)/h

Question 22

If right heart and left heart walls are working at different pressures how is wall tension constant?

Answer 22

T = P x R

Need to change Radius.

Radius of curvature of walls of LV less than that of RV allowing LV to generate higher pressures with similar wall stress

Question 23

What happens in Failing hearts related to the law of LaPlace?

Answer 23

Failing hearts often become dilated which increases wall stress