cardiovascular mechanics 1

Question 1

how do you measure internal calcium

Answer 1

add dye that fluoresces in the presence of calcium

Question 2

how was the importance of calcium outside the cell identified

Answer 2

Ringer
heart cells didn’t contract with distilled water
tap water in London - 1mmol Ca - same as needed outside the cell

Question 3

describe the myocyte structure

Answer 3

ventricular cells 100um long, 15um wide
transverse tubules - cell surface invaginations - open up to 200nm in diameter, run along each z line
carry depolarisation deep into cell
SR under T tubules

Question 4

describe the composition of a myocyte by cell volume

Answer 4

46% myofibrils

36% mt

Question 5

describe the relationship between T tubules and myocytes

Answer 5

receptors on surface of both

Question 6

excitation coupling in the heart

Answer 6

depolarisation is sensed by L type Ca2+ channels on the plasma membrane
calcium enters down a concentration gradient
some Ca activates the myocyte or
Ca bind to ryanodine receptors/SR Ca2+ release channels – conformational change - calcium efflux into the cytoplasm
calcium binds to troponin and causes a contraction
Ca from the SR enters SR after contraction using ATP pump
Ca leaves the cell by a sodium calcium exchange - no ATP use the downhill energy of sodium
in the relaxation period – same amount of Ca in and out of cell as before

Question 7

describe the relationship between force production and intracellular Ca2+

Answer 7

force depends on amount of Ca supplied
complex relationship
log scale
sigmoidal relationship based on log scale

Question 8

describe the length tension relation

Answer 8

If the tissue is stretched, when stimulated it produces a higher force and base line increases
the passive force is a measure of base line from diastole
isometric contraction - where muscle cells don’t stretch they just produce force

Question 9

compare the length tension relation in cardiac and skeletal muscle

Answer 9

cardiac muscle more resistant to stretch as less compliant - means skeletal passive force increases less
this is because of the ECM and cytoskeleton
cardiac - only the ascending limb of the relationship is important - pericardial sac means muscle can’t over stretch

Question 10

describe isometric contraction in the heart

Answer 10

muscle fibres don’t change length
pressure increases in both ventricles
initial contraction where valves are closed

Question 11

describe isotonic contraction in the heart

Answer 11

shortening of fibres and blood is ejected from heart

when valves open because high pressure

Question 12

what is preload

Answer 12

weight that stretches muscle before it is stimulated to contract - not visible to muscle
will contract more than normal when it does, because of stretch

Question 13

what is afterload

Answer 13

weight apparent now muscle contracts
heavier weight
means muscle stretches less
velocity of shortening behave in similar way
same amount of afterload but larger preload it can produce the same amount of force

Question 14

describe preload in the heart

Answer 14

blood fills in diastole - stretches ventricular walls
determines the preload on ventricles
it is dependant on venous return
measures of preload include end diastolic vol - sets set on preparation, end diastolic pressure and right atrial pressure

Question 15

describe afterload in the body

Answer 15

load against which the LV ejects blood after opening the aortic valve
increase reduces isotonic contraction
measure - diastolic pressure
high Bp mean need to work harder to expel blood

Question 16

describe factors that affect the contraction of the heart

Answer 16

ventricular filling - preload

pressure in aorta - amount have to overcome - afterload

Question 17

what is the Frank-Starling relationship

Answer 17

filling of heart increase, force of contraction increase
‘increased diastolic fibre length increases ventricular contraction’
ventricle produces the right force so that the correct stroke volume is pumped balancing the venous return

Question 18

how does changes in the number of myofilament cross bridges that interact cause the F-S relationship

Answer 18

at less stretch actin overlaps - can’t optimise crosss bridges

Question 19

how does changes in the calcium sensitivity affect the F-S relationship

Answer 19

1 - longer length TnC has higher affinity for calcium increase due to conformational change of protein - less Ca required for same force - Ca doesn’t increase but force does
2 - when stretched - space between actin and myosin (lattice spacing) decreases - probability of forming strong cross-bridges increases = more force for the same amount of calcium

Question 20

definition of stroke work

Answer 20

work done by the heart to eject blood under pressure to aorta and pulmonary artery

Question 21

calculation for stroke work and what affects it

Answer 21

stroke work = stroke volume * pressure that blood is ejected
preload and afterload affect SV
cardiac structure affect P

Question 22

what is the definition for the law of Laplace

Answer 22

when the pressure within a cylinder is constant, the tension in its walls increases with increasing radius

Question 23

calculation for law of laplace

Answer 23

wall tension = pressure in vessel * radius of vessel

Question 24

calculation for law of laplace incorporating thickness

Answer 24

T=(p*R)/h

Question 25

describe the Law of Laplace in the heart

Answer 25

wall of LV smaller radius of curvature - higher pressure with similar wall stress
failing hearts dilated - increased radius = more wall stress