Cardiovascular mechanics

Question 1

Explain what is needed in order for a single ventricular cell to contract

Answer 1

Action potential

Calcium ion influx and release

contractile event

Question 2

Describe the parameters (length and width of a single ventricular cell)

Answer 2

100uM in length

15uM wide

Question 3

Describe the structure of T tubules and how much do they open up due when theres an action potential? Where do that carry AP to?

Answer 3

Finger like invagination from cell surface

opens up to 200nm in diameter

it is spaced so that T tubule lies alongside each Z line of every myofibril

carries surface depolarisation deep into cell

Question 4

Describe the 3D component of a single cardiac cell structure in terms of organelles distribution

describe significance of mitochondria

Answer 4

Myofibrils, mitochondria, nucleus, SR

mitochondria - 36%; need large ATP

Question 5

Describe the excitation- contraction coupling in the heart?

Answer 5

• Ca²⁺ moves into sarcoplasm via L-type calcium channel
• stimulate ryanodine receptor in SR to release Ca ions
• same amount of Ca²⁺ that entered now leave sarcoplasm via Na+/Ca²⁺ transporter to avoid bulid up of Ca²⁺.. this doesnt use ATP; only electrical gradient.

Question 6

What’s the relationship w=etwwen intracellular Ca ions and force production

Answer 6

There’s a threshold needed but it’s SIGMOIDAL

Question 7

What is the relationship between length and tension? what other factors increase

Answer 7

The longer the muscle the higher the maximum contractility force (iSOMETRIC contraction)

There’s also more PASSIVE FORCE

Question 8

Contrast skeletal and cardiac muscle

what causes these differences

Answer 8

• Cardiac muscle more resistant to stretch
• LESS compliant than skeletal muscle

this is due to differences in properties of ECM and cytoskeleton

Question 9

in the relationship between length and tension, What limb is only important/relavant for cardiac muscle

Answer 9

Ascending limb of graph; heart muscle doesnt pull as it is contained in pericardial sac unlike skeletal muscle

Question 10

What are two forms of contraction that the heart uses ? which occurs earlier?

Answer 10

• Isometric- muscle fibres doesnt change in length but pressure increases
• Isotonic- shortening of fibres and blood is ejected

theres isometric FIRST then isotonic

Question 11

What is the law of LaPlace

Answer 11

When pressure within a cylinder is led constant, the tension on it’s walls increases with increasing radius

wall tension = (P X R)/h

Pressure in vessel, R= radius of vessel

Question 12

what is preload?

Answer 12

Blood fills the heart during diastoles and stertches resting ventricular walls.

this fills ventricles and the stretching determines the pre-load

allows heart to adapt its force to the volum of blood it recieves

Dependent on venous return

Question 13

what are the measures of pre-load?

Answer 13

• END-DIASTOLIC VOLUME
• END-DIASTOLIC PRESSURE
• RIGHT ATRIAL PRESSURE

Question 14

what is after load?

what si the signifcance of increase in afterload?

Answer 14

THE PRESSURE in which the ventricles MUST WORK against to OPEN the aortic valve.

high afterload means there’s less amount of isotonic shortening. Also less velocity of shortening

Question 15

what are the measures of afterload?

Answer 15

Diastolic blood pressure

Question 16

what does an increase in pre load affect?

Answer 16

increase in ISOMETRIC CONTRACTION

Question 17

what does an increase in after load affect?

Answer 17

• shorter ISOTONIC contraction
• lower VELOCITY of contraction

note the change in gradient of velocity

Question 18

describe the Frank-Starling relationship?

Answer 18

Increased diastolic fibre length (due pre-load) will increase ventricular contraction.

hence ventricles pump greater stroke volume; at equilibrium, cardiac output exactly balances augemnted venous return

Question 19

what are the potential causes of the frank-starling relationship

Answer 19

• Changes in number of myofilament crossbridges formed- the more stretched, the more cross bridges can form
• Changes in Ca²⁺sensitivity of the myofilaments

Question 20

descirbe hypothesis 1 for increase in Ca senstivity

Answer 20

at longer sarcomere length, the affinity of troponin C for Ca²⁺ increases due to change in protein structure

Question 21

describe the other hypothesis for the starling relationship

Answer 21

With higher length, there’s decreasing myofilament lattice spacing, the probabilty of forming strong bidning cross-bridges increases

Question 22

what is stroke work?

give equations? what affects each value?

Answer 22

Stroke work= Stroke volume x Pressure at which blood is ejected

SV; affected by pre-load AND after load

Pressure- affetced by cardiac structure

hence stroke work is work done by heart to pump blood under pressure into aorta or pulmonary artery

Question 23

How does radius of curvature affect Tension using Law of Laplace?

Answer 23

Lower radius of curvature increases pressure as thickness is taken into consideration

hence HIGHER PRESSURE; Tension is the same

those with failing heart have heart with HIGHER radius of curvature; more work and pressure on heart

Question 24

Does skeletal muscle need influx of extracellular calcium in order to contract?

Answer 24

No; ONLY CARDIAC muscle cells do

Question 25

Contrast the structure of T-tubules to SR, between skeletal muscle and caridac muscle, in terms of recptors and connections

Answer 25

Skeletal muscle; theres a DIRECT mechanical link between L-type receptor and ryanodine recptor on SR; hence depolarisation in L type receptor directly opens up ryanodine receptor CARDIAC- no connection; there's a space for Ca²⁺ to enter and activate ryanodine receptor

Question 26

what is responsible for passive force?

Answer 26

Cytoskelton stretching, NOT MYOFILAMENT SLIDING

Question 27

what is the correct answer?

Answer 27

B

Question 28

what is the correct answer

Answer 28

C hardening of Pulmonary artery affect AFTERLOAD NOT PRELOAD

Question 29

whar is the correct order for this?

Answer 29

F,C,A,H,E,B,G,D