Cardiovascular Mechanics

Question 1

What does a ventricular cell require for contraction?

Answer 1

Calcium and excitation of the cell

Question 2

Outline the basic process leafing up to the contraction of a ventricular cell

Answer 2

Electrical event (AP)
Calcium transient (amount of Ca 2+ in sarcoplasm increased for short period of time)
Contractile event

Question 3

Can the hearts contractility be sustained by saline solution with bicarbonate of soda and potassium chloride?

Answer 3

No
Addition of lime or calcium salt will resort good contractility

Question 4

Does skeletal muscle need external calcium for contraction?

Answer 4

No

Question 5

What component of a ventricular cell takes up the largest voume?

Answer 5

Myofibrils (46%)

Question 6

What are T-tubules?

Answer 6

Finger-like invaginations of the cell surface

Question 7

Outline the dimensions of a ventricular cell?

Answer 7

Ventricular cell length is 100 micrometers and width is 15 micrometers

Question 8

Outline the dimensions of T tubule

Answer 8

T tubule is 200nm in diameter
T tubules are separated by 2 micrometers, intermediate between each z line of myofibril transmitting surface depolarisation deep into the cell

Question 9

First step of excitation contraction coupling

Answer 9

Depolarisation excites the L-type Ca 2+ ion channel, and opens it up

Question 10

Second step of excitation-contraction coupling

Answer 10

Extracellular Ca 2+ diffuses into the cell

Question 11

Step 3 of Excitation contraction coupling

Answer 11

Minor proportion of Ca 2+ activates actin filaments causing contraction

Question 12

Step 4 of Excitation contraction coupling

Answer 12

Ca 2+ binds to Ryanodine receptor on SR.
Receptor undegoes conformational change (Ligand dated) opening RyR

Question 13

Step 5 of excitation contraction coupling

Answer 13

Ca 2+ binds to TnC on actin filaments to stimulate shortening of sarcomere

Question 14

Step 6 of excitation-contraction coupling

Answer 14

Ca 2+ actively pumped into stored position by Ca 2+ ATPase channels of SR
Same amount of Ca 2+ that entered the cell during depolarisation effluxed by Na-Ca exchanger

Question 15

Graph showing relationship between force production and intracellular signalling

Answer 15

Sigmoidal curve
Intracellular cytoplasmic Ca 2+ subsequently results in greater force exerted by muscle

Question 16

Explain the length-tension relation in cardiac muscle

Answer 16

As we increase length the active force produced also increases provided we stimulate the cardiac muscle

Question 17

What are active and passive force?

Answer 17

Active- dependant on sarcomere shortening (forces act in direction of points muscular attachment towards to the centre)
Passive- based on the resistance to the stretch of the muscle

Question 18

What is total force?

Answer 18

Addition of passive and active force

Question 19

Which type of muscle is more resistant to stretch and less compliant and why?

Answer 19

Cardiac muscle as it is surrounded by the pericardium so it is not able to stretch further- properties of its extracellular matrix and cytoskeleton

Question 20

Which limb (ascending or descending ) for cardiac muscle is important for the LTR?

Answer 20

only ascending

Question 21

What is isometric contraction

Answer 21

Isometric contraction is the generation of force from a muscle without the shortening of the muscle fibres or sarcomeres
Pressure increases in both ventricles

Question 22

What is Isotonic contraction

Answer 22

Shortening of fibres to generate force
Blood is ejected from ventricles

Question 23

Define preload

Answer 23

Degree to which cardiac muscle cells are stretched from the filling of ventricles prior to contraction

Question 24

Define afterload

Answer 24

The force that the ventricle must develop to pump blood effectively against the resistance in the vascular system.

Question 25

What does an increased preload do to contractility

Answer 25

Increased preload-> increased stretching -> increased force therefore increased contractility

Question 26

What is preload dependant on?

Answer 26

Venous return

Question 27

What do measures in preload involve?

Answer 27

End Diastolic pressure End Diastolic volume Right Atrial Pressure

Question 28

What effect does increased afterload have on isotonic contraction

Answer 28

Increased afterload leads to decreased amount of isotonic shortening and decreases velocity of shortening

Question 29

What do measures in afterload involve>

Answer 29

Diastolic blood pressure

Question 30

Define the Frank-Starling Relationship

Answer 30

Increased diastolic fibre length increases the ventricular contraction

Question 31

What is the consequence of the Frank-Starling Relationship

Answer 31

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

Question 32

What are the 2 factors that cause the Frank-Starling Relationship

Answer 32

Changes in the number of myofilament cross-bridges that interact. Changes in Calcium sensitivity of the myofilaments

Question 33

How does the number of cross bridges affect the Frank-Starling relationship

Answer 33

Ventricular stretching increases contact between myosin heads with myosin binding sites presented by the thin actin filaments-lattice spacing decreases Decreasing myofilament lattice spacing, increasing probability of forming strong binding cross-bridges provides more force for the same amount of activating calcium/

Question 34

How do changes in calcium sensitivity affect the Frank Starling relationship

Answer 34

Ca 2+ needed for myofilament activation. Troponin C (TnC) binds Ca2+, moving tropomyosin off the myosin binding sites on actin. At longer sarcomere lengths, affinity for TnC for Ca 2+ is increased because of conformational change in protein; less Ca 2+ required for equivalent amount of force

Question 35

What is stroke work?

Answer 35

Work done by heart to eject blood under pressure into aorta and pulmonary artery. Stroke work = Stroke volume x presure

Question 36

What 3 factors affect stroke work?

Answer 36

Preload Afterload Contractility

Question 37

Define the Law of Laplace

Answer 37

When pressure within a cylinder is held constant, tension on its wall increases with increasing radius. Wall tension = Pressure in vessel x Radius of Vessel incorporating thickness divide PxR by thickness

Question 38

Which ventricle of the heart generates greater pressure and why?

Answer 38

Radius of curvature of walls of left ventricle less than that of right so it can generate higher pressure with similar stress Lower R therefore greater P as T is constant

Question 39

What happens to the structure of a failing heart/

Answer 39

Becomes dilated and spherical- increases wall stress.