CARDIAC CYCLE

Definition: The cardiac cycle consists of events during the contraction and relaxation of the heart, encapsulating what happens during one heartbeat.

Valves in the Heart
- Atrioventricular Valves: Separate the atria from the ventricles.
- Tricuspid Valve: Located between the right atrium and right ventricle.
- Mitral Valve: Located between the left atrium and left ventricle.
- Semilunar Valves: Located at the exits of the ventricles.
- Aortic Valve: Opens into the aorta.
- Pulmonary Valve: Opens into the pulmonary artery.
Function of Valves:
- Valves open and close based on pressure differentials.
- Increased pressure on one side of the valve causes it to open, allowing blood flow.
- The contraction of the ventricles creates pressure to open the aortic and pulmonary valves.
- A brief period of blood flow may occur despite equalized pressures due to inertia.
- Valves close when pressure on the opposite side becomes greater than on the ventricle side.
- The primary function ensures correct blood flow direction in the heart.

Description: Blood enters the ventricles from the atria.
Mechanism:
- Atrioventricular valves are open due to higher atrial pressure compared to ventricular pressure.
- Blood flow starts rapidly, marking the rapid phase of filling.
- Pressure begins to equalize between atria and ventricles, leading to a slower filling rate.
Atrial Contraction: Following the filling, atrial contraction (indicated by P wave on EKG) occurs, pushing additional blood into the ventricles (referred to as atrial kick).
Valve Status: The aortic valve remains closed due to higher pressure in the aorta compared to the ventricle.

Definition: The ventricle contracts with no change in volume (all valves closed).
Initial Event: Ventricular depolarization occurs, leading to a contraction.
Pressure Changes: Ventricular pressure rises and surpasses the atrial pressure, closing the atrioventricular valve, which generates the first heart sound (S1).
Volume Note: Volume remains constant during this phase as no blood enters or exits the ventricles.

Description: Blood is ejected from the ventricles into the aorta and pulmonary artery.
Mechanism:
- Ventricular pressure exceeds aortic and pulmonary artery pressures, opening the semilunar valves.
- The volume of blood in the ventricles decreases sharply as blood is discharged.
- Even as pressures may equalize momentarily, there is still inertia allowing for continued blood flow.
Valve Status: The atrioventricular valve remains closed, confirming that pressure in the ventricle is greater than in the atria.

Definition: Ventricle relaxes without change in volume (all valves closed).
Events: Following ejection, ventricular pressure drops and becomes insufficient to keep semilunar valves open, causing them to close, which generates the second heart sound (S2).
Valve Status: All valves are closed, maintaining a constant volume until the atrioventricular valve opens again due to lower atrial pressure compared to the ventricle.

S1 (First Heart Sound): Marks the closure of atrioventricular valves during isovolumetric ventricular contraction.
S2 (Second Heart Sound): Marks the closure of semilunar valves during isovolumetric ventricular relaxation.

Both the left and right sides of the heart undergo these phases simultaneously:
- Ventricular filling, isovolumetric ventricular contraction, ventricular ejection, and isovolumetric ventricular relaxation occur in both the left and right ventricles and atria.

Systole: The phase when cardiac muscle (specifically ventricular muscle) is contracting, which includes isovolumetric ventricular contraction and ventricular ejection.
Diastole: The phase when ventricular muscle is relaxing, encompassing isovolumetric ventricular relaxation and ventricular filling.