Cardiovascular Mechanics

Question 1

What are ventricular cells?

Answer 1

100 μm long and 15 μm wide

Question 2

What are T tubules?

Answer 2

T-tubules (transverse tubules) are finger-like invaginations from the cell surface

Question 3

How large are T-tubule openings? How far spaced apart are they?

Answer 3

Spaced (approx. 2 μm apart) so that a T-tubule lies alongside each Z-line of every myofibril
Carry surface depolarisation deep into the cell

Question 4

What makes up cardiac cells?

Answer 4

• Myofibrils (mainly 1)

- Mitochondria (mainly 2)

Question 5

What is the excitation-contraction coupling in the heart?

Answer 5

1. L type Ca2+ channel opens in response to depolarisation Ca2+ enters due to conc gradient
2. Some goes to activate myofiliament and most goes to bind to SR Ca2+ release channel
3. As a result channel undergoes confrontational change as ligand gated
4. Therefore Ca2+ allowed to flow fromm stores into cytosol and bind to myofiliaments
5. For relaxation, calcium is pumped back up into SR but SR Ca2+ ATPase and calcium taken up against conc gradient so calcium’s restored in SR, ready to be related for next beat

Question 6

How do you stop too much calcium as stuff coming in and stuff being stored?

Answer 6

• Exchange system on membrane takes calcium out of cell
• Na+/Ca2+ changer and same amount that enters leaves
• Does not use ATP

Question 7

What happens as gradually increase cytosolic calcium ?

Answer 7

• More and more force produced
• As more and more myofiliaments activated
• Sigmoidal relationship with calcium intracellular and force production

Question 8

What is relationship between length-tension and force in cardiac muscle?

Answer 8

• Active force and baseline (passive) force increase as muscle length increase but happens when isometric (no shortening) contraction
• More stretch muscle more force produced up to certain point
• So total force is active and passive force combined

Question 9

Difference between skeletal and cardiac muscle?

Answer 9

-don’t need calcium outside as calcium causes opening
1. Cardiac muscle is more resistant to reach and less compliant than skeletal muscle
2. Due to properties of the extracellular matrix and cytoskeleton
3 .Only ascending limb of the relation is important for cardiac muscle
-Heart does not get overstretch

Question 10

What is isometric contraction?

Answer 10

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

Question 11

What is isotonic contraction?

Answer 11

Shortening of fibres and blood is ejected from ventricles

Question 12

What is preload?

Answer 12

-Weight that stretches muscle before it is stimulated to contract

Question 13

What is after load?

Answer 13

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

Question 14

If large load how much shortening?

Answer 14

LESS shortening

Question 15

What happens to shortening if longer muscle length?

Answer 15

• MORE shortening
• Decrease in sarcomere length, less preload, less stretch potential so less tension so less actin myosin cross bridges so less contraction potential
• If increase in preload, increase in active tension
• If long sarcomere length more active tension

Question 16

What is the preload of the heart?

Answer 16

1. As blood fills the heart during diastole, it stretches the resting ventricular walls
2. This stretch (filling) determines the preload on the ventricles before ejection
   - More stretch, so more force
   - Deteminate of preload is amount of blood that comes back to heart

Question 17

What is preload dependent upon?

Answer 17

venous return

Question 18

What are the measures of preload?

Answer 18

end-diastolic volume, end-diastolic pressure (amount of blood in heartbefore contraction) and right atrial pressure (determines venous return)

Question 19

What is afterload in the heart?

Answer 19

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

Question 20

What are measures of afterload?

Answer 20

diastolic blood pressure

Question 21

What factors affect contraction of heart?

Answer 21

• Isometric contraction (ventricular filling)

- Isontinic contraction (shooter lengths, longer lengths)

Question 22

What is the frank starling relationship?

Answer 22

Observations by Frank (1895) and later by Starling (1914) showed that as filling of the heart increased, the force of contraction also increased

Question 23

What is the definition of FS relationship?

Answer 23

Increased diastolic fibre length increases ventricular contraction

Question 24

What are the consequences of the FS relationship?

Answer 24

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

Question 25

What is one factor causing FS relationship?

Answer 25

1. Changes in number of my-filament cross bridges that interact 2. At shooter lengths than optimal the actin filaments overlap on themselves so redoing he number of myosin cross bridges that can be made

Question 26

What is another factors causing FS relationship?

Answer 26

Changes in Ca2+ sensitivity of the myofilaments

Question 27

What is the first hypothesis?

Answer 27

1. Ca2+ required for myofilament activation 2. Troponin C (TnC) is thin filament protein that binds Ca2+ 3. TnC regulates formation of cross-bridges between actin and myosin 4. At longer sarcomere lengths the affinity of TnC for Ca2+ is increased due to conformational change in protein 5. Less Ca2+ required for same amount of force

Question 28

What is the second hypothesis?

Answer 28

1. With stretch the spacing between myosin and actin filaments (so-called “lattice spacing”) decreases 2. With decreasing myofilament lattice spacing, the probability of forming strong binding cross-bridges increases 3. This produces more force for the same amount of activating calcium

Question 29

What is the definition of stroke work?

Answer 29

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

Question 30

What is the equation for stroke work?

Answer 30

volume of blood ejected during each stroke (SV) multiplied by the pressure at which the blood is ejected (P) Stroke work =SV xP

Question 31

What affects SV?

Answer 31

preload and aferload

Question 32

What affects P

Answer 32

Cardiac structure

Question 33

What is the law of LaPlace?

Answer 33

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

Question 34

What is equation for La place?

Answer 34

wall tension = pressure in vessel x radius of vessel T = P x R Incorporating wall thickness (h) T= (PxR)/h

Question 35

How is Law place different?

Answer 35

1. Radius of curvature of walls of LV less than that of RV allowing LV to generate higher pressures with similar wall stress 2. Wall stress kept low in the giraffe by long, narrow, thick-walled ventricle 3. In frogs, where pressures are low the ventricle is almost spherical 4. Failing hearts often become dilated which increases wall stress