Cardiac mechanics

Question 1

What are the dimensions of ventricular cells?

Answer 1

100 μm long and 15 μm wide

Question 2

What are T tubules (transverse tubules)?

Answer 2

finger-like invaginations of the cell surface

Question 3

What is the diameter of T tubule openings?

Answer 3

Up to 200 nm

Question 4

Describe the positioning of t tubules

Answer 4

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

Question 5

What is the purpose of t tubules?

Answer 5

To carry surface depolarisation deep into the cell

Question 6

What is the relationship between force production (% max) and cytoplasmic Ca2+?

Answer 6

Sigmoid curve

Question 7

Which is more resistant to stretch, cardiac or skeletal muscle?

Answer 7

Cardiac

Question 8

Which is more compliant, cardiac or skeletal muscle?

Answer 8

Skeletal

Question 9

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

Answer 9

Due to properties of the extracellular matrix and cytoskeleton

Question 10

What are the names of the two forms of contraction the heart uses?

Answer 10

Isometric and isotonic

Question 11

Describe isometric contraction

Answer 11

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

Question 12

Describe isotonic contraction

Answer 12

Shortening of fibres and blood is ejected from ventricles

Question 13

What is preload?

Answer 13

Weight that stretches muscle before it is stimulated to contract

Question 14

What is afterload?

Answer 14

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

Question 15

What are in vivo correlates of preload?

Answer 15

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
Measures of preload include end-diastolic volume, end-diastolic pressure and right atrial pressure

Question 16

What are in vivo correlates of afterload?

Answer 16

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
Measures of afterload include diastolic blood pressure

Question 17

Define fs relationship

Answer 17

Increased diastolic fibre length increases ventricular contraction

Question 18

What are the consequences of the fs relationship?

Answer 18

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

Question 19

What is the fs relationship thought to be caused by?

Answer 19

1. Changes in the number of myofilament cross bridges that interact
2. Changes in the Ca2+ activity of the myofilaments

Question 20

Define stroke work

Answer 20

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

Question 21

How do you calculate stroke work?

Answer 21

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

Question 22

What greatly influences SV?

Answer 22

Preload and afterload

Question 23

What greatly affects pressure (P)?

Answer 23

Cardiac structure

Question 24

Define law of LaPlace

Answer 24

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

Question 25

What is the formula for wall tension?

Answer 25

T=PxR
or
T= (PxR)/h

P= pressure
R= radius of vessel
h= wall thickness

Question 26

Why is wall stress increased in failing hearts?

Answer 26

increased dilation