Cardiovascular mechanics (26)

Question 1

How is the cardiac muscle contraction different to skeletal muscle contraction mechanism?

Answer 1

you need calcium extracellularly for cardiac contraction

Question 2

What is the purpose of T-tubules in ventricular cells?

Answer 2

• finger-like invaginations from the cell surface
• spaced so that they coincide w/ each Z-line on every myofibril
• carry surface depolarisation deep into cell
• wrapped partly in sarcoplasmic reticulum

Question 3

What are the contents of a single cardiac cell?

Answer 3

• myofibrils 46%
• mitochondria (bc require lots of ATP) 36%
• sarcoplasmic reticulum
• nucleus
• other

Question 4

What is the role of L-type Ca2+ channels?

Answer 4

channel senses depolarisation–> opens up (normally closed) in response to incoming AP
–> calcium moves down conc. gradient into cell–> most Ca2+ binds to SR Ca2+ release channel

Question 5

What is the role of SR in calcium-induced calcium release?

Answer 5

calcium binds to SR calcium release channel–> receptor undergoes conformational change–> opens up channel–> calcium flows from SR stores into cytosol–> binds to myofilaments and produces contraction

Question 6

How is relaxation brought about by SR ATPase?

Answer 6

calcium pumped back up into sarcoplasmic reticulum store by SR ATPase (uses ATP bc against Ca2+ conc. gradient)

Question 7

What is the function of the sodium-calcium exchange system on the membrane during relaxation?

Answer 7

takes calcium out of cell and brings sodium in, so maintains calcium balance
(uses downhill energy gradient of sodium)

Question 8

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

Answer 8

sigmoidal

Question 9

How is length related to tension in cardiac muscle in isometric contraction?

Answer 9

as we stretch the cardiac muscle, we get more force produced (active and passive)

Question 10

Why is cardiac muscle more resistant to stretch and less compliant than skeletal muscle?

Answer 10

• due to different properties of extracellular matrix and cytoskeleton
• more passive force produced

Question 11

What is isotonic contraction?

Answer 11

shortening of muscle fibres

- blood is ejected from ventricles

Question 12

What is isometric contraction?

Answer 12

no shortening of muscle fibres

- pressure inc. in both ventricles

Question 13

What type of contraction does the heart use?

Answer 13

isometric AND isotonic

Question 14

What is cardiac preload?

Answer 14

ventricular wall stress at the end of diastole

Question 15

What is cardiac after load?

Answer 15

ventricular wall stress during systole/ejection

Question 16

What determines preload in the heart?

Answer 16

• dependent on venous pressure and return, atrial contraction, and valve resistance
• as blood fills heart during diastole, it stretches the resting ventricular walls–> this stretching determines preload on ventricles before ejection
  N.B. measures of preload: end-diastolic volume, end-diastolic pressure and right atrial pressure

Question 17

What determines after load in the heart?

Answer 17

• the load against which the left ventricle ejects blood after opening the aortic valve
• any inc. in afterload dec. the amount of isotonic shortening
  N.B. measure of after load: diastolic bp

Question 18

What is the Frank-Starling relationship?

Answer 18

• as filling of the heart inc.–> the force of contraction also inc.
• ventricles pump greater stroke volume, so that, at equilibrium–> cardiac output exactly balances an augmented venous return

Question 19

How does the end-diastolic volume affect the number of myofilament cross bridges that interact and therefore stroke volume?

Answer 19

• at low end diastolic volume, there is not much stretching of the muscle wall, and so the sarcomeres are squeezed close together–> few myosin heads bind to actin filaments–> weak contractions–> low stroke volume
• as volume of blood returning to ventricles inc.–> sarcomeres stretch–> more myosin-actin binding–> inc. strength of contraction–> inc. stroke volume UNTIL end-diastolic volume too high, so myosin and actin too far apart–> dec. binding–> dec. contraction and stroke volume

Question 20

What is the relevance of changes in the calcium sensitivity of the myofilaments as a cause of the Frank-Starling relationship?

Answer 20

• calcium is required for myofilament activation
• troponin C (TnC) is a thin filament protein that binds calcium
• TnC regulates formation of cross-bridges between actin and myosin
• at longer sarcomere lengths, the affinity of TnC for calcium inc. due to conformational change in protein
• so less calcium needed for same amount of force

Question 21

What is stroke work?

Answer 21

work done by heart to eject blood under pressure into aorta and pulmonary artery
- volume of blood ejected during each stroke (SV) X pressure at which blood ejected (P)

Question 22

What is the law of LaPlace?

Answer 22

when the pressure w/in a cylinder is constant, the tension on its wall inc. as radius inc.
- wall tension = pressure in vessel (P) x radius of vessel (Including thickness, divide equation by wall thickness)

Question 23

How does the LV generate higher pressures with similar wall stress/tension?

Answer 23

radius of curvature of LV walls less than RV curvature, LV can generate higher pressures w/ similar wall stress/tension
(N.B. if a heart fails and becomes dilated, radius inc., so inc. wall stress)