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Cardiac Cycle

defined as the complete cycle of contraction and relaxation.


the period of cardiac contraction.


the period of cardiac relaxation.

Systole and Diastole

The sequence of changes in blood pressure and volumes in the left atrium, left ventricle, and aorta are the same as those in the right atrium, right ventricle, and pulmonary trunk EXCEPT that the pressure changes in the right ventricle and pulmonary artery are much less than those of the left ventricle and aorta and they are pumping into a lower pressure system than the systemic system.


Peak pressure in the right ventricle and pulmonary artery is about 20mm Hg. Peak pressure on the left side of the heart reaches 120mm Hg.


is the relaxation of a heart chamber just PRIOR TO AND DURING filling of that chamber. During early ventricular diastole (prior to the onset of filling), all 4 heart valves are closed.

Atrial systole

The next event after ventricular diastole occurs. "tops off" the almost full ventricles with another small amount of blood. At the end, the atria begin to relax and the ventricles prepare to contract and there is a momentary back flow of blood which closes AV valves.

End of Ventricular diastole

the back flow of ventricular blood and closure of the AV valves

End- diastolic volume (EDV)

The amount of blood contained in each ventricle at the end of diastole

Ejection fraction

The percentage of EDV that is ejected by the ventricle during systole. Provides a measure of pumping effectiveness of the ventricles. During each contraction, each ventricle ejects only a percentage of its EDV, typically 50-60%.


The period of contraction which drives blood from the heart chamber. Pressure in the ventricles increases almost as soon as the begin to contract. When ventricular pressure exceed that in the respective atria, the pressure difference forces the AV valves closed.


closure of the AV valves is associated with the first heart sound. The "lub" of the "lug dub". Lower pitched and longer in duration than S2. Heard in all species.

Isovolumetric Contraction period of the systole

The period of systole when all 4 valves are closed. This means that the blood volume of each ventricle remains constant or is unchanging.

Rapid ejection phase of systole

the semilunar valves are forced open and blood is ejected


closure of the semilunar valves associated with generation of the second heart sound. The "dub" of the "lub dub". Higher pitched and shorter in duration than S1. Heard in all species.

End- systolic Volume

The ventricle does not completely empty during systole. ESV is the volume of blood which remains in the ventricle following systole.

Cardiac output

volume of blood pumped by a ventricle of the heart into its respective vessel per unit time. "Stroke volume X heart rate"


S1, correlates with closure of the AV valves.


S2, correlates with the closure of the semilunar valves.


Normal in horses. In dogs and cats it indicates myocardial disease.


typically not associated with any abnormalities in the equine heart. It's presence is abnormal in dogs and cats and indicates that the ventricular wall is too stiff or non- compliant.

Cardiac muscle

Form a functional syncytium. Contain intercalated disks. Do not require a nerve impulse to stimulate an action potential.

The SA node

the "pacemaker" tissue of the heart. Located in the right atrium near the entry of the cranial vena cava. Can initiate its own action potential because its cell membranes are very unstable and have increased permeability for sodium. Most rapid rate of depolarization/ repolarization in the heart.


spontaneous depolarization

Conduction system

SA node-> Atrioventricular (AV) node -> Bundle of His -> Bundle of branches in each ventricle -> Myocardial cells

SA node

Impulses leave the SA node by the internodal pathways. The internodal impulses converge to the AV node.

Atrioventricular node

Located in the inter- atrial septum and divides to form the R &L bundle branches

Bundle of His

passes the impulse to the myocardial cells of the ventricles

Purkinje Fibers

specialized conducting fibers in the ventricles which facilitate the rapid spread of the action potential throughout all parts of the ventricle. Large and speed up conduction by a factor of 2-3X

Intercalated disks

areas of low electrical resistance. The basis for the concept of the functional syncytium.


Arrangement of muscle fibers in which the fibers form a functionally interconnected mass of fibers.


innervation is provided by the vagus nerve. Vagal stimulation slows the heart rate and slows conduction of impulses through the AV node.


nerves increase the heart rate and force contraction.

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