Cardiac Cycle

Classification: Cardiovascular
Figure 1 The main phases of the cardiac cycle: 1) atrial systole, 2) isovolumetric contraction/systole, 3) ventricular systole/ejection, and 4) isovolumetric relaxation. Valvular activity is denoted by C (closed) or O (opened).

Systole: contraction of the myocardium, inducing blood flow from one chamber to another (or to the pulmonary/aortic systems)

Diastole: relaxation of the myocardium, chambers fill with blood

The relationship between systole and diastole is controlled by the relative pressures of the atria, ventricles, and arterial pulmonary and aortic systems – see Figure 1.

Physiological Blurb: Blood Pressure

Blood pressure (BP) is defined as the force exerted by blood flow against your arteries, and is typically measured using both systolic and diastolic pressure values, giving one value of SP/DP (e.g. 120/80). BP is especially relevant when ascertaining conditions such as hyper- or hypotension.

It is also calculated through the formula BP = CO (cardiac output) * SVR (systemic vascular resistance), indicating that BP depends on both the total volume of blood the heart pumps, as well as arterial blood flow resistance.

BP is commonly regulated by baroreceptors, as well as the kidney-mediated RAAS system.

While BP is now measured using a sphygmomanometer, BP may still be measured by listening to Korotkoff sounds using a stethoscope.

Atrial systole

On EKG: P wave

  • aP > vP (see Figure 1), so the AV valves open → 70-80% of blood will passively flow to ventricles under ventricular diastole
  • The remaining 20% is pushed out by the contraction of the atria following depolarization by the SA node

Isovolumetric contraction & systole

On EKG: QRS complex

  • Ventricular pressure rises (vP > aP), so the AV valves close
    • The AV valves closing produces the sound S1 or “lub”
  • Blood in the ventricles rises to the pulmonary and aortic valves, but since vP < artP, the semilunar valves stay closed, so no blood leaves the ventricles

Ventricular systole & ejection

On EKG: ST segment

  • Both ventricles reach a pressure that exceeds the aortic and pulmonary systems (vP > artP), so the semilunar valves open and blood is ejected from the ventricles
  • The AV valves remain closed

Isovolumetric relaxation

On EKG: T wave

  • The aortic and pulmonary arteries fill with blood quickly, increasing pressure significantly → artP > vP, which closes the semilunar valves
    • The closure of the semilunar valves in this phase is S2 or “dub”
  • The pressure changes from minimal backflow of blood from the aorta before the semilunar valves are closed to when they are closed causes the dicrotic notch on the EKG

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