Cardiovascular mechanics 1

Question 1

What events lead up to a contractile event in the heart

Answer 1

1. Electrical event
2. Calcium influx
3. calcium release
4. contractile event

Question 2

Where must calcium enter from

Answer 2

Outside of the cell

Question 3

What are T-tubules

Answer 3

Invaginations of cardiac cells spaced at each Z line that carry surface depolarisation deep into cells

Question 4

What is one of the most important organelles for cardiac excitation-contraction

Answer 4

Sarcoplasmic reticulum for calcium storage

Question 5

Describe the excitation process in the heart

Answer 5

1. Depolarisation causes the L-type Ca2+ to open to allow Ca2+ to enter the cell down its concentration gradient
2. Calcium binds to the ryanodine receptor to cause a conformational change (some is used in contraction)
3. Calcium stored inside the sarcoplasmic reticulum can move into the sarcoplasm
4. Contraction

Question 6

How are calcium levels reduced after excitation

Answer 6

Calcium ATPase pumps calcium against its concentration gradient from cytoplasm to the SR. Additionally use of the sodium calcium exchanger (no ATP needed)

Question 7

Describe the length-tension relationship in the heart

Answer 7

As the length of cardiac muscle increases the active force production increases, as well as base line passive force

Question 8

What differentiates cardiac from skeletal muscle

Answer 8

Cardiac muscle is more resistant to stretch/less compliant due to its ECM and cytoskeleton. Only the ascending limb is important for cardiac muscle

Question 9

Why can you not overstretch cardiac muscle

Answer 9

Containment in the pericardial sac

Question 10

Where does isotonic contraction occur in the heart

Answer 10

increasing the pressure in the ventricles

Question 11

Where does isometric contraction occur in the heart

Answer 11

ejecting the blood from the ventricles

Question 12

Define preload

Answer 12

The weight that stretches muscle before it is stimulated to contract

Question 13

Define afterload

Answer 13

The weight not apparent to muscle in the resting state, only encountered when muscle has started to contract

Question 14

Describe preload in the heart

Answer 14

The amount of blood returning in diastole, stretching the resting ventricular walls. Determines the preload on the ventricles before ejection

Question 15

How can one measure preload in the heart

Answer 15

End-diastolic volume
End-diastolic pressure
Right atrial pressure

Question 16

Describe after load in the heart

Answer 16

the load against the left ventricle ejects blood after opening of the aortic valve

Question 17

How does after load relate to hypertension

Answer 17

Afterload is greater and requires the ventricle to work harder in order to expel blood

Question 18

How does after load affect velocity

Answer 18

Increasing afterload decreases the amount of isotonic shortening and decreases the velocity of shortening

Question 19

What is Starlings law of the heart

Answer 19

Increased diastolic fibre length increases ventricular contraction

Question 20

What are the causal factors of starlings law

Answer 20

Changes in the number of myofilament cross bridges that interact or changes in the calcium sensitivity of the myofilaments

Question 21

Define stroke work

Answer 21

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

Question 22

How is stroke work calculated

Answer 22

stroke volume x pressure

Question 23

Which factors affect stroke work

Answer 23

stroke volume - preload and after load

pressure - cardiac structure

Question 24

How is tension affected by an increase in radius (pressure of cylinder is the same)

Answer 24

tension of the walls increases with increasing radius

Question 25

How is wall tension calculated

Answer 25

wall tension = pressure x radius

Question 26

Why can the left ventricle generate greater pressures with similar wall stress

Answer 26

radius of the curvature of the walls in the left is less than that of the right + thicker muscular walls

Question 27

What happens to the ventricles in heart failure

Answer 27

They dilate to increase wall stress

Question 28

How is preload measured

Answer 28

End-diastolic volume
End-diastolic pressure
Right atrial pressure