Cardiac Mechanics

Question 1

What is the structure of ventricular cells?

Answer 1

Rectangular —> 100 x 15 µm
- Mitochondria - near plasma membrane —> ATP
supply
- 36%
- Sarcoplasmic reticulum - surround myofibrils (46%)
—> Ca2+ supply
- 4%
- T-tubules - at each Z-line —> Ca2+ in
- finger-like invaginations of membrane
- 200nm diameter

Question 2

How does Ca2+ release occur in a cardiomyocyte for contraction?

Answer 2

1. Ca2+ enters cell via L-type Ca2+ channels along T-
   tubule
2. Ca2+ enters sarcoplasmic reticulum via ryanodine
   receptors
3. Calcium-induced calcium release from SR
4. Ca2+ stimulates contraction
5. Ca2+ —> back into SR via Ca2+ ATPase channels
   —> out cell via Na+/Ca2+ exchanger

Question 3

What is the relationship between cytoplasmic Ca2+ conc and force?

Answer 3

More Ca2+ —> more forceful contraction

Question 4

How does cardiac muscle affect the force of contraction?

Answer 4

Longer muscle —> more forceful contraction
- passive force: inc continuously
- active force: inc then dec

Question 5

What are the 2 differences between cardiac and skeletal muscle and why?

Answer 5

Cardiac:
1. More resistant to stretch
2. Less compliant
- ECM and cytoskeleton properties

Question 6

What are the 2 types of cardiac contraction?

Answer 6

1. Isometric = fibres don’t shorten —> exert force
   - inc pressure in ventricles
2. Isotonic = fibres shorten
   - eject blood from ventricles

Question 7

What is preload?

Answer 7

‘Load’ the heart experiences during diastole (when blood fills chambers)
- preload inc —> isometric contraction force inc
(before peak)
- measure —> end-diastolic volume
—> end-diastolic pressure
—> right atrial pressure

Question 8

What is afterload?

Answer 8

‘Load’ the heart works against during systole (to eject blood)
- afterload inc —> isotonic shortening dec
- measure —> diastolic blood pressure

Question 9

What is Starling’s Law?

Answer 9

Increased diastolic fibre length increases ventricular contraction
- inc blood into heart —> stretch muscle fibres more
—> inc contraction —> inc blood out heart

Question 10

What are the 2 factors causing Starling’s law?

Answer 10

1. Number of myofilament cross bridges
   
   • shorter fibres —> actin filaments overlap —> less
     actomyosin cross bridges —> less contraction
2. Ca2+ sensitivity of myofilaments
   
   • longer sarcomere —> conformational change to
     troponin C —> higher affinity for Ca2+ —> more
     contraction

Question 11

What is the equation for stroke work?

Answer 11

Stroke work = stroke volume x pressure
- stroke volume —> preload and afterload
- pressure —> cardiac structure

Question 12

What is the Law of LaPlace?

Answer 12

T = (P x R) / h
- T —> wall tension
P —> pressure
R —> radius
h —> wall thickness
- eg. left ventricle —> (same T x inc h) / dec R = inc P