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Understanding the 'cardiac cycle'

Posted by Erling 
Understanding the 'cardiac cycle'
January 08, 2011 01:01PM
"The cardiac cycle is a term referring to all or any of the events related to the flow or blood pressure that occurs from the beginning of one heartbeat to the beginning of the next.[1] The frequency of the cardiac cycle is described by the heart rate. Each beat of the heart involves five major stages [phases]"

"The standard model used to understand the cardiac action potential is the action potential of the ventricular myocyte. The action potential has 5 phases (numbered 0-4). Phase 4 is the resting membrane potential, and describes the membrane potential when the cell is not being stimulated."


-- potential = electric potential = voltage [en.wikipedia.org]

-- membrane potential = voltage measured across the cell membrane = ~95 mV (~1/10th volt, negative inside). Picture a flashlight battery projecting through the cell membrane, negative flat end to the inside [en.wikipedia.org]

-- resting membrane potential = phase 4 of the cardiac cycle. At this phase the cell-battery is fully charged, ready to be stimulated for ‘action’.

-- action potential = "a short-lasting event in which the electrical membrane potential of a cell rapidly rises and falls, following a stereotyped trajectory." i.e. phases 4 to 0 to 1 to 2 to 3 and back to 4, about 1/5th second, if the heart rate is 60 beats per minute. At 1 second per beat (cardiac cycle), the ‘action potential’ is ~1/5th of the full cardiac cycle. [en.wikipedia.org]

-- refractory period = RP [en.wikipedia.org]
-- effective refractory period = ERP
-- absolute refractory period = ARP
-- atrial effective refractory period = AERP

-- polarized = cell-battery is charged

-- depolarized = cell-battery is discharged

Using the diagram ‘Phases of the cardiac action potential’ at [en.wikipedia.org]:

-- phase 4 = cardiac muscle cell-battery is fully charged, ready for stimulation to 'act'.
-- phase 0 = rapid depolarization = QRS complex on the ECG.
-- phase 1 = termination of depolarization
-- phase 2 = depolarized (zero volts) = roughly the same as ‘refractory period’ = roughly the S-T segment on the ECG.
-- phase 3 = gradual repolarization and return to fully polarized phase 4

This is intended as clarification for understanding the trans-membrane movement of electrolytes throughout the cardiac cycle as shown in the diagram. It is a basis for understanding the significance of a high K/Na ratio in the heart muscle cells.

Excellent for further information: 'Structure & Function. Electrical Properties of The Heart' [www.sant.anzca.edu.au]

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