Cardiac Cycle

Definition: The cardiac cycle is the continuous alternation between the relaxation and contraction of the heart.
Frequency: This cycle repeats itself approximately every second in a healthy human heart.
Scope and Nomenclature:
- The cardiac cycle is divided into two primary stages: diastole and systole.
- Unless explicitly specified otherwise, these stages refer specifically to what the ventricles are doing.
- If a speaker or text refers to "atrial diastole," they are specifically describing the relaxation of the atria, which is an exception to the general naming convention.
Inverse Relationship: The activity of the atria and ventricles can generally be predicted based on one another because they often perform opposite actions.
- While atrial diastole (relaxation) is occurring, the ventricles are typically in systole (contraction).

Systole (Ventricular Contraction):
- This stage involves the contraction of the ventricles, which induces the ejection of blood from the heart.
- The right ventricle contracts to push blood through the pulmonary pump and out through the pulmonary trunk.
- The left ventricle contracts to push blood out through the aorta to the rest of the body.
Diastole (Ventricular Relaxation):
- During this stage, the ventricles relax and begin to fill with blood.
- The ventricles undergo dilation, meaning they get larger in size.
- As the volume of the relaxing ventricles increases, the internal pressure drops.

Directional Flow: Blood moves in the direction of lower pressure, flowing down its pressure gradient.
- When ventricles relax, pressure drops, causing blood to move from the right atrium through the tricuspid valve into the right ventricle.
- Simultaneously, blood moves from the left atrium through the bicuspid (mitral) valve into the left ventricle.
Ideal Gas Law Analogy ( $PV = nRT$ ):
- The speaker applies the principles of the ideal gas law ( $PV = nRT$ , where $P$ is pressure, $V$ is volume, $n$ is the number of particles/moles, $R$ is a constant, and $T$ is temperature) to explain ventricular mechanics.
- If the volume ( $V$ ) of a container (the ventricle) decreases during contraction but the number of particles ( $n$ ) and temperature ( $T$ ) remain constant, the pressure ( $P$ ) must increase.
- Conversely, if the volume ( $V$ ) increases during relaxation, the pressure ( $P$ ) must decrease.
- A smaller container leads to more frequent collisions of particles with the walls, thus increasing pressure.

Isovolumetric Contraction:
- Definition: A phase where the ventricles are contracting harder and harder, but the volume of blood within them does not change.
- Incompressibility: Because blood is largely water, it is an incompressible fluid.
- Mechanism: As the ventricles contract, the tricuspid and bicuspid valves are slammed shut. Pressure builds until it is high enough to "blow open" the doors of the semilunar valves (aortic and pulmonary) to allow ejection.
Isovolumetric Relaxation:
- Definition: A phase immediately following ejection where the ventricular muscle begins to relax and expand, but the volume remains the same because all valves are closed.
- Mechanism: The tricuspid and bicuspid valves remain closed as pressure begins to drop. The volume only begins to change once the pressure drops low enough for the atrioventricular valves to open and allow filling.

Average Heart Rate: The average heart rate is approximately $70\,\text{beats per minute}$ . For simplified calculations, it can be estimated at $60\,\text{beats per minute}$ , which equals $1\,\text{beat per second}$ .
Systole Timing:
- Systole occupies approximately one-third ( $1/3$ ) of the total cardiac cycle.
- At a rate of $1\,\text{beat per second}$ , systole lasts approximately $0.33\,\text{seconds}$ .
- In a real-world scenario of $70\,\text{beats per minute}$ , it is slightly less, around $0.3\,\text{seconds}$ .
Diastole Timing:
- Diastole occupies approximately two-thirds ( $2/3$ ) of the total cardiac cycle.
- At a rate of $1\,\text{beat per second}$ , diastole lasts approximately $0.66\,\text{seconds}$ .
- In a real-world scenario, this is approximately $0.6\,\text{seconds}$ .

Early Diastole:
- The ventricles undergo isovolumetric relaxation.
- The tricuspid and bicuspid valves open, and ventricles begin to fill.
- Atria are simultaneously filling with blood returning from the systemic and pulmonary circuits.
- Closed Circuit Principle: The amount of blood pumped from the right ventricle must equal the amount pumped from the left ventricle because the cardiovascular system is a closed loop (including the systemic, pulmonary, and coronary circuits).
Late Diastole:
- This phase involves "atrial kick" or atrial contraction.
- While the ventricles are finishing their relaxation phase, the atria contract to squeeze the remaining blood out of the auricles (using pectinate muscles) to completely top off the ventricles.

The "Dup" Sound (S2):
- Occurs in early diastole when the ventricles begin to relax.
- Cause: The closing of the semilunar valves (aortic and pulmonary).
- Mechanism: As ventricular pressure drops, the elastic recoil of the great vessels (aorta and pulmonary trunk) presses back on the blood. This backflow catches the valve leaflets and snaps them shut.
- Coronary Charging: This back-flow of blood during the "dup" phase also charges the left and right coronary arteries to provide oxygenated blood to the heart muscle itself.
The "Lubb" Sound (S1):
- Occurs at the start of systole.
- Cause: The closing of the tricuspid and bicuspid (atrioventricular) valves.
- Mechanism: Increasing ventricular pressure slams these valves shut, diverting the blood toward the semilunar valves.
Relative Pressures:
- Systemic Pump (Left): Systolic pressure in the aorta is approximately $120\,\text{mmHg}$ . As the ventricle relaxes, the recoil pressure is approximately $80\,\text{mmHg}$ .
- Pulmonary Pump (Right): Blood is ejected into the pulmonary trunk at approximately $24\,\text{mmHg}$ . During relaxation, the back-pressure is approximately $10\,\text{mmHg}$ .

The Signal Pause: The intrinsic conduction system contains a deliberate delay at the AV bundle.
Duration: The pause lasts for $0.1\,\text{seconds}$ (or $100\,\text{milliseconds}$ ).
Purpose: This pause ensures that the atria have sufficient time to finish their contraction and completely fill the ventricles before the ventricles begin their own contraction (systole).