Cardiac Cycle of the Heart

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What is the Cardiac Cycle

What is the Cardiac Cycle

The cycle of mechanical events occurring during one single heart beat

One Heart Beat

1 Heart Beat = 1 cycle = 2 atrial contractions and 2 ventricular contractions

How long does One Cycle Last?

About 0.8 seconds or 72 bpm

How long does the Diastole (relaxation) portion of the cycle take?

About 0.5 seconds

How long does the Systole (contraction) portion take?

About 0.3 seconds

What are the Four Stages of the Cardiac Cycle?

1. Passive filling (of atrial)

2. Atrial contraction (ventricles fill with blood)

3. AV valves close (no backflow)

4. Semilunar valves open and ventricles contract

The first two steps are Diastole while steps 3,4 are the contraction phase

Cardiac Cycle (in general)

Blood flows from high to low pressures

Contractions of heart chambers increase pressure

AV vales open when the atrial pressure is high

SL valves open when the ventricular pressure is high

If not specified, systole and diastole refer to the ventricles

Isovolumetric means that the volume stays the same

3 phases but 5 steps

How many phases of heart contraction are there?

3

Phase 1: Mid to late diastole/ Atrial depolarization/ contraction

Blood flows into ventricles; AV vales close; 1st heart sound; beginning of ventricular systole

Phase 2: Ventricular systole/ Isovolumertric contraction

SL valves remain closed; vetnricles contract within a closed system; blood pressure builds in ventricles; SL valves open; blood is ejected; ventricular pressure drops ; SL valves close; 2nd heart sound, S2: end of ventricular systole

Phase 3: Early diastole

Ventricles relax; all valves closed; ventricular pressure is low; atrial finish re-filling, atrial pressures rise; ventricular filling starts

What are the Two Heart Sounds?

The Lubb (1st) and Dupp (2nd)

Lubb: Long, booming sound from the AV valves closing

Dupp: Short, sharp shound from the semilunar valves closing at the end of ventricular contraction

Cardiac Output (CO)

Amount of blood pumped by the heart in one minute

Heart rate x stroke volume

Heart rate

Number of times the heart beats in one minute

Stroke Volume

The volume of blood pumped during each cardiac cycle

Stroke volume usually remains constant

Starling’s law of the Heart

The more that the cardiac muscle is stretched, the stronger the contraction and the larger the stroke volume; physiologically important mechanism in making sure that the output of blood by both the right and left ventricles is the same

Changing heart rate

Most common way to change cardiac output

Chemoreceptors

These signal the glossopharyngeal nerve; this nerve goes back to the medulla oblongata

Carotid Sinus

This is where Chemoreceptors for CO2 and pH are

What does the Cardiac Nerve do?

Affects heart rate when working out as well as ventricular contractions

Electrocardiogram (EKG)

Traces the flow of electrical currents through the heart

Picked up by electrodes placed on the skin

P phase of EKG

Atrial contractions

QRS phase of EKG

Results from depolarization of ventricles & signals ventricular contraction

Repolarization of atria also occurs

T phase of EKG

Repolarization of the ventricles