Cardiovascular mechanics

Question 1

What is excitation-contraction coupling?

Answer 1

Process whereby the excitatory event turns into the contractile event via a series of processes

Question 2

Describe a single ventricular event

Answer 2

Electrical event
Ca2+ influx
Ca2+ release
Contractile event

Question 3

How was the importance of calcium for the heart beat discovered?

Answer 3

Sydney Ringer

Saline solution with pipe water allowed the heart to contract

Question 4

How big are ventricular cells?

Answer 4

100 μm long and 15 μm wide

Question 5

What are t-tubules?

Answer 5

Finger-like invaginations from the cell surface

Carry surface level depolarisation into the heart of the cell

Question 6

How big are t-tubules?

Answer 6

T-tubule openings up to 200 nm in diameter

Spaced (approx. 2 μm apart) so that a T-tubule lies alongside each Z-line of every myofibril

Question 7

What percentage of cell volume do myofibrils occupy?

Answer 7

46%

Question 8

Why are there so many mitochondria in muscle cells?

Answer 8

Provide ATP for muscle contraction

Question 9

What key calcium handling proteins are in the t-tubules?

Answer 9

L-type calcium channel

Question 10

What causes the L-type calcium channels to open?

Answer 10

Response to the action potential arriving

Calcium moves into cell down concentration gradient

Question 11

Where does calcium go after entering the cell?

Answer 11

Binds to SR Ca2+ (ryanodine receptor) release channel

Question 12

What does Ca2+ binding result in?

Answer 12

Conformational change

Allows calcium to flow from the sarcoplasmic reticulum into the cytosol

Question 13

How is relaxation induced?

Answer 13

ATP is used to restore calcium into the sarcoplasmic reticulum

Question 14

What is the sodium/calcium exchange system?

Answer 14

Doesn’t use ATP

Uses the downhill energy gradient of Sodium to expel calcium out of the cell

Question 15

What happens as more and more calcium enters the cell?

Answer 15

More force produced
Activation of more myofilaments
Sigmoidalish relationship

Question 16

What happens when you lengthen a muscle?

Answer 16

More force produced with increasing length
Baseline force also increases
‘Active force production line”

Question 17

What happens to elastic component when lengthened?

Answer 17

Elastic components stretch

Passive force produced

Question 18

What is isometric contraction?

Answer 18

Non-shortening force production

Muscle fibres do not change length but pressures increase in both ventricles

Question 19

How is cardiac and skeletal muscle different?

Answer 19

Cardiac muscle more resistant to stretch and less compliant than skeletal muscle
Less passive force in skeletal muscle

Question 20

What causes this difference in skeletal and cardiac properties?

Answer 20

Due to properties of the extracellular matrix and cytoskeleton

Question 21

What is unique about cardiac muscle?

Answer 21

It cannot be overstretched as the heart is contained in the pericardial sac that does not allow it to overstretch

Therefore Only ascending limb of the relation is important for cardiac muscle

Question 22

What is isotonic contraction?

Answer 22

Shortening of fibres and blood is ejected from ventricles

Question 23

What contraction is involved in the cardiac cycle?

Answer 23

Under normal circumstances: both isometric and isotonic

Question 24

Define preload

Answer 24

Weight that stretches muscle before it is stimulated to contract

Question 25

Define after load

Answer 25

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

Question 26

What is the relationship between shortening and load in isotonic contraction?

Answer 26

If load is great the shortening the muscle can do is very small

Question 27

What is the relationship between muscle length (preloaded) and stretch?

Answer 27

More interdigitation of the fibres occur, more stretch with length

Question 28

What is the in vivo correlate of preload?

Answer 28

As blood fills the heart during diastole, it stretches the resting ventricular walls This stretch (filling) determines the preload on the ventricles before ejection Preload is dependent on venous return

Question 29

What are measures of preload?

Answer 29

End-diastolic volume End-diastolic pressure Right atrial pressure

Question 30

What are the in vivo correlates of after load?

Answer 30

Afterload is the load against which the left ventricle ejects blood after opening of the aortic valve Any increase in afterload decreases the amount of isotonic shortening that occurs and decreases the velocity of shortening.

Question 31

What is the measure of after load?

Answer 31

Diastolic blood pressure

Question 32

Why is it important to keep diastolic pressure low?

Answer 32

That is the pressure against the heart to work to eject blood If diastolic blood pressure is high then the heart has to do more work

Question 33

What happens as filling of the heart increases?

Answer 33

Force of contraction increases

Question 34

Define the frank-starling relationship?

Answer 34

Increased diastolic fibre length increases ventricular contraction

Question 35

What is the consequence of the F-S relationship?

Answer 35

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

Question 36

What are the factors causing the F-S relationship?

Answer 36

Changes in the number of myofilament cross bridges that interact Changes in the Ca2+ sensitivity of the myofilaments

Question 37

How does the number of cross bridges interacting affect force?

Answer 37

As the muscle stretches Allow for more cross bridges to form Causes greater force

Question 38

What happens at shorter lengths?

Answer 38

At shorter lengths than optimal the actin filaments overlap on themselves so reducing the number of myosin cross bridges that can be made

Question 39

How does Ca2+ sensitivity affect force? | Hypothesis 1

Answer 39

Ca2+ required for myofilament activation Troponin C (TnC) is thin filament protein that binds Ca2+ TnC regulates formation of cross-bridges between actin and myosin At longer sarcomere lengths the affinity of TnC for Ca2+ is increased due to conformational change in protein Less Ca2+ required for same amount of force

Question 40

How does Ca2+ sensitivity affect force? | Hypothesis 2

Answer 40

With stretch the spacing between myosin and actin filaments (so-called “lattice spacing”) decreases With decreasing myofilament lattice spacing, the probability of forming strong binding cross-bridges increases This produces more force for the same amount of activating calcium

Question 41

Define stroke work

Answer 41

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

Question 42

What is stroke work equal to?

Answer 42

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

Question 43

What affects stroke volume?

Answer 43

Preload and afterload

Question 44

What affects pressure?

Answer 44

Cardiac structure

Question 45

What is the law of LaPlace?

Answer 45

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

Question 46

What is law tension equal to?

Answer 46

Pressure in vessel x Radius in vessel T= P x R Incorporating wall thickness (h), this can be amended to: T= (PxR)/h

Question 47

What is the significance of radius of curvature?

Answer 47

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