02 Mechanical Properties 1

Question 1

What is the purpose of T tubules. How are they distributed throughout the ventricular cells

Answer 1

T tubules carry surface depolarisation deep into the cells. They are aligned with Z line which is approximately 2um apart

*ventricular cells are about 100 um long and 15 um wide

Question 2

What are the channels involved for the increase in intracellular calcium

Answer 2

L type calcium channel

Ryanodine receptor on SR

Question 3

What are the transporters involved for repolarisation of ventricles

Answer 3

SERCA (sarco/endoplasmic reticulum Ca2+ ATPase)

Ca2+/Na+ exchanger (primary active transport) on ventricular cell

Question 4

Describe the relationship between calcium concentration and force of contraction

Answer 4

Non linear - sigmoid shape

Calcium concentration in Log

Question 5

Differentiate between the length - tension relations between skeletal and cardiac cells

Answer 5

Cardiac cells are:

1) more resistant to stretch and less compliant
2) only the ascending limb of the relation is important due to the properties of extracellular matrix and cytoskeleton

Question 6

Describe the relationship between amount of shortening, velocity of shortening and afterload/preload

Answer 6

The higher the afterload, the lower the shortening and velocity of shortening. Increased preload improves both shortening and velocity of shortening(although fibre shortening is linear, but velocity of shortening is not)

Question 7

Define preload

Answer 7

The stretch or filling of resting ventricular walls during diastole
Measured from: end diastolic pressure/volume and right atrial pressure

Question 8

Define afterload

Answer 8

It is the load against which the left ventricle ejects blood after opening of the aortic valve
Measured from diastolic arterial blood pressure

Question 9

Define Frank-Starling relationship and explain the hypothesis behind this effect

Answer 9

Increased diastolic fibre length increases ventricular contraction. This means there is increased stroke volume for cardiac output to balance augmented venous return

Hypotheses:

1) increased number of myofilament cross bridges (no overlap between actions and no wasted myosin heads)
2) increased Ca2+ sensitivity
- increased affinity of TnC to Ca2+
- decreases myofilament lattice spacing to form more strong binding cross bridges

Question 10

Define stroke work

Answer 10

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

Stroke work = SV x P
(SV influenced by preload and afterload)
(P influenced by the structure of heart)

Question 11

Define law of Laplace

Answer 11

T = Pr/h (tension = pressure x radius / wall thickness)