Cardiovascular & Respiratory

Question 1

What does a ventricular cell require for contraction?

Answer 1

Calcium and excitation of cell.

Question 2

Outline the basic process leading up to the contraction of a ventricular cell.

Answer 2

Electrical event (Action Potential) 
Calcium transient (amount of calcium in sarcoplasm has increased for a short period of time). 
Contractile event

Question 3

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

Answer 3

No; the addition of lime or a calcium salt will restore 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 volume?

Answer 5

Myofibrils (46%)

Question 6

What are T-tubules?

Answer 6

Finger-like invaginations of the cell surface.

Question 7

Explain the process of excitation-contraction coupling.

Answer 7

1. L-type Ca2+ channel (LTCCs) on the T-tubules detect the depolarization caused by the AP in the cardiomyocyte, changing conformation to allow the influx of Ca2+ into the cytosol.
2. Extracellular Ca2+ moves across the concentration gradient intracellularly by facilitated diffusion.
3. Minor proportion of Ca2+ activates actin filaments and directly causes contraction.
4. Majority of Ca2+ binds to RyR on SR (Ryanodine receptors or SR Ca2+ release channel); receptor undergoes conformational change (Ligand dated); opening RyR → calcium that is stored in the sarcoplasmic reticulum is released into the cytosol.
5. Ca2+ binds to troponin on actin filaments to stimulate shortening of the sarcomere (sliding filament theory)
6. Relaxation period: Ca2+ actively pumped into a stored position by SR Ca2+ ATPases. The same amount of Ca2+ that came into the cell is effluxed by a Na-Ca exchange system in the T-tubules.

Question 8

Sketch a graph of the relationship between force production and cytoplasmic calcium concentration and then explain it.

Answer 8

Cytoplasmic Ca2+ conc. on x-axis and force on y-axis. Sigmoidal relationship; Intracellular cytoplasmic Ca2+ increase will subsequently result in a greater force exerted by the muscle.

Question 9

Explain the length-tension relation (LTR) in cardiac muscle.

Answer 9

As we increase length, the active force and passive force produced also increases provided we stimulate the cardiac muscle.

Question 10

What are active and passive force?

Answer 10

Passive - Based on the resistance to stretch of the muscle
Active - Dependent on sarcomere shortening, forces act in the direction of the point of muscular attachment towards the center

Question 11

What is the total force?

Answer 11

Passive force + Active force

Question 12

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

Answer 12

Cardiac muscle because of the properties of its extracellular matrix and cytoskeleton.

Question 13

Which limb of the graph (ascending or descending) for cardiac muscle is important for the LTR (length-tension relationship)?

Answer 13

Ascending

Question 14

What are isometric and isotonic contraction?

Answer 14

Isometric: Sarcomeres and muscle fibers don’t change length, but pressures increase in both ventricles. E.g. planque
Isotonic: Shortening of fibers and blood is ejected from ventricles. E.g. Bicep curls, bench press, etc.

Question 15

Define preload in terms of the circulatory system.

Answer 15

The degree to which cardiac muscle cells are stretched from filling of the ventricles prior to contraction.

Question 16

Define afterload in terms of the circulatory system.

Answer 16

Refers to tension/force that ventricles must develop to pump blood effectively against the resistance in the vascular system.

Question 17

Define preload.

Answer 17

A weight that stretches the muscle before it is stimulated to contract.

Question 18

Define afterload.

Answer 18

A weight that isn’t apparent to muscle in the resting state and is only encountered when the muscle has started to contract.

Question 19

What does increased preload do to contractility?

Answer 19

Increased preload → Increased stretching → Increased force, therefore, increased contractility.

Question 20

What is preload (in the circulatory system) dependent on?

Answer 20

Venous return (ventricular filling)

Question 21

What do measures in preload involve?

Answer 21

End-diastolic volume (EDV)
End-diastolic pressure (EDP)
Right atrial pressure

Question 22

What effect does increased afterload have on isotonic contraction?

Answer 22

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

Question 23

What do measures in afterload involve?

Answer 23

Diastolic blood pressure

Question 24

Define the Frank-Starling relationship and what the consequence is.

Answer 24

Definition - Increased diastolic fiber length increases ventricular contraction.
Consequence - Ventricles pump greater stroke volume so that, at equilibrium, cardiac output exactly balances the augmented venous return.
Increased Diastolic fiber length → Increased Ventricular stretching → Greater SV → Increased cardiac output.
(So if we get more venous return we get more stretch on ventricular muscle, therefore, producing a more powerful contraction).

Question 25

What are the 2 factors that are thought to cause the Frank-Starling relationship and then explain each one?

Answer 25

Changes in the number of myofilament cross-bridges that interact
Ventricular stretching subsequently increases
contact between the myosin heads with the myosin-
binding sites presented by the thin actin filaments,
lattice-spacing decreases
Decreasing myofilament lattice spacing, increasing
the probability of forming strong-binding-cross-
bridges; providing more force for the same amount
of activating calcium
Changes in calcium sensitivity of the myofilaments
Ca2+ required for myofilament activation, troponin C
(TnC), is a thin filament protein that binds Ca2+,
subsequently causing tropomyosin to expose
myosin-binding sites, regulating the formation of
cross-bridges between actin and myosin.
At longer sarcomere lengths, the affinity of TnC for
Ca2+ is increased due to conformational change
in protein; thereby less Ca2+ is required for
equivalent amount of force.

Question 26

What is stroke work (include equation)?

Answer 26

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

Question 27

What 3 primary factors affect stroke work?

Answer 27

Preload, afterload, and contractility.

Question 28

Define the Law of Laplace.

Answer 28

When the pressure within a cylinder is held constant, the tension on its wall increases with increasing radius.
Wall tension = Pressure in vessel x Radius of vessel
T = P x R
(Incorporating wall thickness: T = (P x R)/h)

Question 29

How does the left ventricle produce higher pressure than the right ventricle while maintaining an equal wall tension?

Answer 29

The radius of the curvature of the walls of the left ventricle is less than that of the right ventricle, hence the LV can generate higher pressures with similar wall stress.
LV: Lower R therefore greater P; T is constant.

Question 30

What happens to the structure of a failing heart?

Answer 30

Becomes dilated and spherical which increases wall stress.

Question 31

What is meant by end-diastolic volume?

Answer 31

Maximum volume of blood in the heart just before the ventricles contract and at this point are relaxed

Question 32

What is meant by the end-systolic volume?

Answer 32

The residual volume of blood left in the heart following contraction

Question 33

Give a definition of stroke volume and how is it calculated?

Answer 33

The volume of blood expelled by the heart in any one cardiac cycle
Stroke volume = End-diastolic volume - End-systolic volume

Question 34

How is ejection fraction calculated and what is its normal range?

Answer 34

Ejection fraction = SV/EDV x 100

Normal range = 52-72%

Question 35

Define ejection fraction.

Answer 35

The percentage of blood being pumped out of the heart in each contraction.
Stroke volume divided by the end-diastolic volume represented as a percentage

Question 36

What would be the ejection fraction range in a patient with heart failure?

Answer 36

30-35%