2 - Cardiovascular Mechanics 1

Question 1

What is the shape of the force-calcium relationship?

Answer 1

sigmoidal

Question 2

What concentration of calcium is sufficient to generate maximum contraction?

Answer 2

10 micromolar

Question 3

What are the features of a single ventricular cell?

Answer 3

• requires Ca2+ for contraction
• rod shaped
• 100 μm x 15 μm

Question 4

What is isometric contraction, with an example?

Answer 4

• muscle fibres don’t change length but pressures increase in both ventricles
• isovolumetric contraction

Question 5

What is isotonic contraction, with an example?

Answer 5

• shortening of fibres and blood is ejected from ventricles

- ventricular contraction ejecting blood out

Question 6

What is the length-tension relationship?

Answer 6

• increase in muscle length causes increase in force
• as muscle keep stretching, point reached where no more force is generated regardless of stretch (not enough overlap between filaments to produce force)

Question 7

Which 3 factors increase pre-load?

Answer 7

• decreased HR (more time to fill)
• increased central venous pressure
• increased venous return

Question 8

How is cardiac muscle affected by the length-tension relationship?

Answer 8

• more resistant to stretch and less compliant than skeletal muscle
• doesn’t overstretch as encased in pericardium

Question 9

What is preload?

Answer 9

• weight that stretches muscle before it is stimulated to contract
• more preload = more force

Question 10

How is preload used in the heart?

Answer 10

• blood filling heart during diastole stretches resting ventricular walls
• stretch (filling) determines preload on ventricles before ejection
• preload dependent on venous return

Question 11

What are some measures of preload?

Answer 11

• end-diastolic volume
• end-diastolic pressure
• right atrial pressure

Question 12

What is afterload?

Answer 12

weight not apparent to muscle in resting state, only encountered when muscle has started to contract

Question 13

What is the result of more afterload?

Answer 13

• less shortening

- lower velocity of ejection (as more pressure so there’s a lowered stroke volume)

Question 14

What is a measure of afterload?

Answer 14

diastolic blood pressure

Question 15

How is afterload used in the heart?

Answer 15

• load against which left ventricle ejects blood after opening of aortic valve
• pressure in aorta that left ventricle needs to overcome

Question 16

What is the Frank-Starling relationship?

Answer 16

• increased diastolic fibre length increases ventricular contraction
• as filling of heart increases, force of contraction also increases

Question 17

What is an example of the mechanism of the Frank-Starling relationship?

Answer 17

• increased venous return leads to increased preload
• more receptors of ventricle fibres
• increased force of contraction
• increased stroke volume
• CO = HR x SV (so CO also increases)

Question 18

What 2 factors affect the Frank-Starling relationship?

Answer 18

• changes in no. of myofilament cross bridges that overlap

- changes in Ca2+ sensitivity of myofilaments (2 hypotheses)

Question 19

What is stroke work?

Answer 19

• work done by heart to eject blood under pressure into aorta and pulmonary artery
• stroke work = stroke volume x pressure

Question 20

What is the stroke volume?

Answer 20

volume of blood ejected during each stroke

Question 21

What is the law of LaPlace?

Answer 21

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

Question 22

What are the equations for wall tension?

Answer 22

• wall tension = pressure x radius (T = P x r)

- T = (P x r)/h (h is wall thickness)

Question 23

How is the law of LaPlace used in relation to the heart?

Answer 23

• radius of curvature LV walls less than RV
• LV can generate higher pressure with similar wall stress
• failing hearts become dilated so increases wall stress

Question 24

How are pressure-volume loops used in conjunction with preload and afterload?

Answer 24

• if preload (length) is increased then for given afterload (force to overcome) amount of force created will increase
• increasing preload will increase muscle shortening

Question 25

How does the number of myofilament cross bridges affect the Frank-Starling relationship?

Answer 25

• at shorter lengths than optimal, actin filaments overlap
• no. of myosin cross bridges reduced
• more stretch = more optimum interdigitation of actin and myosin filaments that can be achieved
• too much overlap can also reduce no. of cross bridges

Question 26

How do changes in calcium sensitivity of myofilaments affect the Frank-Starling relationship?

Answer 26

• at longer sarcomere lengths, affinity of troponin c for Ca2+ is increased due to conformational change in protein (increased length = increased Ca2+ sensitivity)
• spacing between myosin and actin filaments decreased with stretch (decreased lattice spacing means probability of forming strong binding cross-bridges increases)