2. Mechanical Properties of the Heart I Flashcards by Kieron Young

What is required for contraction?

Calcium

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What is the shape of a single ventricular cell?

Rod shaped

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Describe the process of contraction

An electrical event produces a calcium transient that causes a contractile event

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Define calcium transient

The amount of calcium in the sarcoplasm has increased for a short period of time

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What allows contraction to take place?

The rise and fall of calcium

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What is the function of T-tubules?

T tubules allow excitation from the surface to be conducted down into the middle of the cell

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What does the sarcoplasmic reticulum store?

Calcium

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Where do T tubules lie?

They lie 2 micrometers apart so that a T-tubule lies alongside each Z-line of every myofibril

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What is the name of a cardiac muscle cell?

cardiomyocyte

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What makes up 50% of myocytes

Myofibrils

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Where do the sarcoplasmic reticulum lie?

On the ends of the T-tubules

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Difference between cardiac muscle and skeletal muscle?

The heart needs external calcium to contract whereas skeletal muscle can contract without external calcium

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What are ryanodine receptors?

These are sarcoplasmic reticulum calcium release channels which cause the release of calcium stores in the SR when calcium binds

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Describe the contraction phase of the excitation-contraction coupling process in the heart

Depolarisation is sensed by L-type calcium channels in the T-tubules which open. This allowing calcium from the outside to enter. Some calcium bind to troponin causing contraction while some bind to RyR. The influx of calcium activates the myofilaments.

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Describe the relaxation phase of the excitation-contraction coupling process in the heart

Calcium is taken back into the SR by Ca ATPase channels. This movement of calcium is against its concentration gradient which brings about relaxation. The same amount of calcium that entered the cell is effluxed by a sodium calcium exchanger; requires no ATP

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Describe the relationship between force and calcium

This is a sigmoidal relationship. 10 micromolar intracellular concenration of calcium is required to produce maximum force

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The longer the muscle length…

The greater the force. However at one point further stretching will not generate more force because there is not enough overlap between the filaments.

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What is active force?

The force produced from the cross bridges forming

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Define isometric contraction

This is contraction where the muscle doesn’t shorten but there is a change in tone. Isometric contraction resists the high pressure

What is passive force?

Passive force production is the stretch of the cardiac muscle cell. Is based of the resistance to stretch of the muscle. The more you stretch the greater the passive force.

Which has a greater passive force, cardiac or skeletal muscle?

Cardiac muscle as it is more resistant to stretch due to its extracellular matrix and cytoskeleton

Why does only the ascending limb of the length tension curve important in cardiac muscle?

The descending limb does occur since you cannot overstretch cardiac muscle. This is because it is limited by the pericardium which restricts the stretching

What is the total force?

Sum of the active and passive force

Define isotonic contraction

This contraction involves the shortening of fibres (no change in tension) when blood is ejected from the ventricles

Describe a contraction of the heart

When the heart fills with blood the valves are closed. Initially contraction starts up by the ventricles against the closed valves when the ventricles fill with blood. Through isometric contraction the ventricles resist the high pressure caused by the blood filling the ventricles. The blood doesn’t leave because of the valves. Eventually the pressure in the ventricle overcomes the pressure in the aorta and the aortic valve opens allowing the blood to flow out of the ventricle. Isotonic contraction then occurs.

Define Preload

The weight that stretched the muscle before it is stimulated to contract. (i.e. the filling of the ventricles with blood makes it stretch before it is stimulated to contract)

Define Afterload

Weight that is not apparent to the muscle in the resting state - only encountered once muscle has started to contract (i.e the back pressure on the aortic valves). This is the weight or mass or pressure that the muscle is trying to overcome.

The more preload...

The more stretch so the more force is produced

The more afterload...

The less shortening + decrease in the velocity of shortening

In vivo correlates of preload =

The amount of ventricular filling that occurs

What is preload dependent on?

The venous return to the heart. The greater the preload, the greater the volume of blood the greater the total force of contraction

What are the measures of preload?

End diastolic volume End diastolic pressure Right atrial pressure

In vivo correlates of afterload =

the pressure in the aorta. Basically the afterload is the blood pressure.

What is a measure of afterload?

Diastolic arterial blood pressure; the pressure the ventricles have to pump against

What is Starling's Law?

Increased diastolic fibre length increases ventricular contraction

What is the consequence of Starling's Law?

When diastolic fibre length increases, ventricles pump a greater stroke volume so that, at equilibrium, cardiac output exactly balances the augmented venous return. The heart can modulate its output depending on what blood comes back to it

What factors affect Starling's Law?

1) Changes in the number of myofilament cross bridges that interact 2) Changes in the calcium sensitivity of the myofilaments

How does the number of myofilament cross bridges affect the force produced?

The more cross bridges that can be formed the greater the force of the muscle produced by the muscle

What two possibilities exist for why there is an increased calcium sensitivity when the myofilaments are stretched?

Possibility one: The longer the sarcomere length, the affinity of troponin C increased due a conformational change in the protein. Possibility two: When stretched there is decreased lattice spacing meaning the probability of forming strong binding cross bridges increases.

What is lattice spacing?

The space between myosin and actin filaments

What is the definition of stroke work?

Work done by the heart to eject blood under pressure into the aorta and pulmonary artery. Essentially the work done by the heart in one contraction.

Stroke work =

SV x P

What is the definition of SV

Stroke volume is the volume of blood ejected during each stroke

What is the law of Lapace?

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

What is the physiological relevance of the Law of Lapace?

The aim is to keep wall tension the same throughout the heart. This is achieved by varying radius size. The radius of the LV is less than the RV which allows the LV to generate high pressure with similar wall stress (tension)

At rest what is the typical cardiac output of a human?

3.65 litres/min

What is the normal end diastolic volume?

130ml