Cardiovascular System 2 - Cardiovascular Mechanics 1

Question 1

Describe the process of excitation contraction coupling in the heart.

Answer 1

• Action potential travels down the T tubule and causes the L-type Ca2+ channel to open
• Ca2+ ions bind to the ryanodine receptor and cause a conformational change, which opens the channel and calcium ions flood out and bind to troponin
• Calcium ions concentration is then returned to the original concentration by action of Ca2+ ATPase in the sarcoplasmic reticulum, and a Na+ Ca2+ exchanger in the cell membrane. This does not use ATP, instead it uses the chemical gradient of Na+

Question 2

How does cardiac muscle length change the force of contractions?

Answer 2

• If a piece of muscle is stimulated with electricity, there is a small force starting from baseline 0
• If this muscle is stretched and stimulated again, the baseline force increases as does the amount of force at the peak.

Question 3

In the length-tension relationship of cardiac muscle, what are the two types of force?

Answer 3

• The baseline force is the passive force, which requires no energy to produce
• The highest force reached is the active force.

Question 4

Compare the length-tension relation between cardiac and skeletal muscle.

Answer 4

• Cardiac muscle is more resistant to stretch and less compliant than skeletal muscle.
• This is due to properties of the ECM and cytoskeleton

Question 5

What are the two types of muscle contraction used in the heart?

Answer 5

• Isometric, where the muscle fibres don’t change length but the pressures increase in both ventricles
• Isotonic, where the fibres shorten and blood is ejected from the ventricles

Question 6

What is preload?

Answer 6

Weight that stretches muscle before it is stimulated to contract.

Question 7

What is afterload?

Answer 7

Weight not apparent to muscle in the resting state, only encountered when muscle has started to contract.

Question 8

How does the force change as preload increases in isotonic contraction?

Answer 8

It increases to a point then decreases

Question 9

How does increased afterload affect shortening in isotonic contraction?

Answer 9

The shortening decreases.

Question 10

What determines preload in the heart?

Answer 10

• As blood fills the heart, it stretches the ventricular walls
• The stretch determines the preload in the ventricles.
• Preload is dependent on venous return

Question 11

How can preload be measured?

Answer 11

• End diastolic volume
• End diastolic pressure
• Right atrial pressure

Question 12

What generates afterload in the heart?

Answer 12

• The load against which the left ventricle ejects blood after opening the aortic valve
• Increase in afterload decreases isotonic shortening and the velocity of shortening

Question 13

What is a measure of preload?

Answer 13

Diastolic pressure

Question 14

How does afterload affect velocity of shortening?

Answer 14

As afterload increases velocity of shortening decreases

Question 15

Describe the frank starling relationship.

Answer 15

• As filling of the heart increases, the force of contraction also increases
• The consequence of this is that ventricles pump a greater stroke volume so that at equilibrium cardiac output balances the increased venous return.

Question 16

Define the frank-starling relationship

Answer 16

Increased diastolic fibre length increases ventricular contraction

Question 17

What two factors cause the frank starling relationship?

Answer 17

• Changes in the number of myofilament cross bridges

- Changes in the Ca2+ sensitivity of the myofilaments

Question 18

How do changes in the number of actin myosin cross bridges cause the frank-starling relationship?

Answer 18

At shorter lengths then optimum, fewer actin myosin cross bridges can be made due to overlap.

Question 19

Describe hypothesis 1 for how changes in Ca2+ sensitivity cause the frank-starling relationship.

Answer 19

• Ca2+ is required for myofilament activation
• Troponin C binds to Ca2+
• At longer sarcomere lengths the affinity of troponin C for Ca2+ increases due to a conformational change in the protein
• Less Ca2+ is required for the same amount of force

Question 20

Describe hypothesis 2 for how changes in Ca2+ sensitivity cause the frank-starling relationship.

Answer 20

• With stretch spacing between myosin and actin filaments decreases
• The probability of forming strong binding cross bridges increases
• This produces more force for the same amount of acting calcium

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

What is the equation for stroke work?

Answer 22

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

SV x P

Question 23

Define the LaPlace.

Answer 23

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

Question 24

What is the equation for the Law of LaPlace?

Answer 24

Wall tension = pressure x radius
T = P x R

Or, including wall thickness (h)
T= (P x R)/h

Question 25

How does the law of LaPlace apply to heart mechanics?

Answer 25

• The heart has the same wall tension all around
• The pressure in the right ventricle lower, so as a result, the radius of curvature is larger to increase the wall tension. The radius of curvature is smaller in the left ventricle, as the pressure is higher.
• In dilated hearts, wall stress is increased as tension increases.
• Giraffes have a narrow radius of curvature, to allow them to withstand high pressures.