Principles of Human Physiology: Cardiovascular System - Cardiac Function

Definition: The cardiac cycle encompasses all events associated with the heart during a single complete heartbeat.
Events Include:
- Electrical signals leading to mechanical actions.
- Pumping actions of the heart.
- Changes in pressure within the atria, ventricles, and aorta.
- Opening and closing of valves.
- Modifications in ventricular volume.
- Heart sounds.

Main Periods of the Cardiac Cycle:
1. Systole: State of contraction.
2. Diastole: State of relaxation.
Valve Functionality:
- AV (Atrioventricular) valves open passively when atrial pressure exceeds ventricular pressure.
- SL (Semilunar) valves open when ventricular pressure exceeds arterial pressure.

Atrial pressure increases gradually due to filling.
Ventricular pressure remains lower than atrial pressure.
Notable Changes:
- A small rise in ventricular pressure occurs at the end of this phase due to atrial contraction.
Diagram Explanation:
- Valves: Atrioventricular valves are open, aortic and pulmonary valves are closed.
- Pressure (mm Hg): Atrial pressure rises while ventricular remains low initially.

Characterized by a rapid increase in ventricular pressure as the ventricles contract.
Atrial pressure decreases.
Diagram Explanation:
- Valves: Atrioventricular valves are closed, aortic and pulmonary valves are also closed.
- Pressure: The pressure in the ventricle rises sharply with no change in volume (isovolumetric).

Ventricular pressure rises and subsequently falls as blood is ejected into the aorta.
Atrial pressure continues to decrease until late ventricular systole.
Diagram Explanation:
- Valves: AV valves remain closed while SL valves are open.
- Pressure Dynamics: Initial rise then drop in ventricular pressure as blood exits.

Ventricular pressure decreases, while atrial pressure increases as they fill with blood.
Notable Changes:
- All valves are closed; no volume change occurs despite pressure change.
Diagram Explanation:
- Valves: All valves remain closed.
- Pressure: Falls sharply as the heart prepares for the next cycle.

The aortic semilunar valve remains closed.
Blood pressure in the aorta falls, leading to diastolic pressure (DP) at a minimum.
Diagrams:
- Shows relationship between aortic pressure and ventricular pressure around closures and openings of valves.

Semilunar valves open, allowing for rapid rise in pressure from the blood being ejected.
Systolic pressure (SP) represents the highest point of aortic pressure during this phase.

Semilunar valves close, producing a dicrotic notch due to brief backward blood flow before complete closure.
Flow Dynamics: Ensures continuous blood flow through the cardiac cycle due to elastic recoil.

Definitions:
- EDV (End-Diastolic Volume): Volume of blood in the ventricle at the end of diastole (130 mL).
- ESV (End-Systolic Volume): Volume of blood in the ventricle at the end of systole (60 mL).
- SV (Stroke Volume): Volume of blood ejected per heartbeat.
- Formula: $SV = EDV - ESV$
- Example Calculation: $SV = 130 mL - 60 mL = 70 mL$
- EF (Ejection Fraction): Fraction of EDV ejected with each heartbeat.
- Formula: $EF = \frac{SV}{EDV}$
- Example Calculation: $EF = \frac{70 mL}{130 mL} = 0.54$ (54% at rest)

Definition: Ability of the heart's ventricles to expand.
- Mathematical Representation: $Compliance = \frac{\Delta V}{\Delta DP}$
- Where:
- $\Delta V$ is the change in volume,
- $\Delta DP$ is the change in distending pressure (pressure inside vs pressure outside).

Phase 2 (Isovolumetric Contraction):
- First heart sound (S1) occurs, characterized as a soft "lubb" sound, caused by turbulence as AV valves close.
Phase 4 (Isovolumetric Relaxation):
- Second heart sound (S2) is louder, characterized as "dupp" or "dubb", caused by turbulence as SL valves close.

Understand how electrial signals trigger mechanical actions during the cardiac cycle.
Key Objectives:
- Describe how phases of the electrocardiogram are related to the cardiac cycle events.