Cardiovascular physiology (The cardiac cycle and output)

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List the 4 phases of the ventricular cycle.

(1) Ventricular filling. (2) Isovolumetric contraction. (3) Ventricular ejection. (4) Isovolumetric relaxation.

What are the three waves and two descents of the atrial cycle?

Waves = a, c, v. Descents = X, Y.

What does the "a" wave represent?

Pressure rise in the atrium as a result of atrial contraction.

What does the "c" wave represent?

A rise in atrial pressure arising from the closure of the AV valve, which bulges into the atrium as the underlying ventricle contracts.

What does the "v" wave represent?

A rise in atrial pressure as the atrium refills with venous blood.

What does the X descent represent?

A fall in atrial pressure as the atrium relaxes and the AV valve is pulled downwards by the contracting ventricle. This stretches the atria, reducing the pressure inside them and sucking blood into them.

What does the Y descent represent?

The sharp fall in atrial pressure as the AV valve reopens and the atrium empties into the ventricle.

Which atrial pressure waves may be seen as rapid pulsations in the JVP?

The a and v waves.

How do murmurs arise?

From stenosed (narrowed) valves accelerating forward flow, or from incompetent valves leaking blood backwards.

List 4 methods of measuring cardiac output.

(1) Fick's principle. (2) The Hamilton dye-dilution method. (3) Thermodilution. (4) Pulsed Doppler method.

Name the two main categories by which CO is controlled.

Changes in HR and changes in SV.

How does the autonomic NS influence HR?

Parasympathetic nerve fibers (Vagus nerve) run to the SA and AV nodes - SLOW HR. Sympathetic nerve fibers (Thoracic segments I to V) innervate the SA and AV nodes BUT ALSO heart muscle - INCREASE HR.

How does the baroreceptor reflex influence ANS control of the HR?

An increase in BP stimulates the baroreceptors, causing the heart to slow by INCREASING parasympathetic activity and DECREASING sympathetic activity. (OPPOSITE for drop in BP).

Which two opposing factors determine CO?

(1) The arterial pressure against which the heart ejects blood ["afterload"]. (2) Work of contraction by the heart.

Describe the Frank-Starling law of the heart.

Increased filling pressure stretches the ventricle more during diastole; this increased stretch stimulates the ventricle to do more work during systole.

What 3 things tend to increase the energy of contraction of the heart?

(1) Increases in filling pressure of the heart. (2) Stimulation by sympathetic nerves. (3) Circulating catecholamines.

Why does stretching cardiac muscle fibers make them stretch more energetically?

(1) Stretch makes cardiac muscle more sensitive to calcium. (2) Stretch changes the overlap between actin and myosin filaments in the cardiac muscle - reducing interference between myosin heads that pull in opposite directions at very short fiber lengths.

What is the importance of the Frank-Starling law?

It equalizes the output of the right and left side of the heart.

Describe the role of sympathetic innervation and noradrenaline on SV.

Sympathetic innervation moves teh Frank-Starling curve to the left - increasing the energy of contraction for any given filling pressure. They release noradrenaline, which binds to beta-1 receptors, causing the release of intracellular Ca2+, thereby increasing the force of contraction.

What is a Guyton curve and what does it show?

A Guyton curve is a graphic representation of the combination of a vascular function curve and a cardiac function curve. The cardiac function curve describes how the heart responds to changes in central venous pressure. The vascular function curve describes how the vascular system responds to changes in CVP.

Why does early heart failure lead to oedema of the lower body?

In right sided HF, the increased Central Venous Pressure (CVP) backs up and increases BP inside the systemic capillaries (because capillaries drain into venules which drain into veins). This increased BP inside systemic capillaries makes more fluid filter out of them per unit time.

How does salt and water retention by the kidney's in Chronic HF partially restore CO?

This retention increases the volume of the ECF, including blood. Increased blood volume inflates the central veins, and moves the vascular function curve upwards, partially restoring CO - but increasing filling pressure.

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