Principles of Human Physiology: Cardiovascular System - Cardiac Function

Explain the following events in the cardiac cycle:
- Changes in ventricular pressure
- Changes in aortic pressure
- Changes in atrial pressure
- Changes in ventricular volume
- Heart sounds
Trace the path of action potentials through the conduction system of the heart
Relate the heart's electrical activity to its pumping action.

Autorhythmicity:
- The heart generates its own rhythm through autorhythmic cells.
- These cells spread excitation throughout the heart.
- This feature is termed myogenic as the heart muscles control their own contractions.
Key components in the conduction system include:
- Pacemaker Cells: Initiate action potentials.
- Conduction Fibers: Distribute electrical impulses through the heart muscle.

Mechanism of Excitation Spread:
- Direction of excitation: Atria contract before ventricles contract.
- Coordination is facilitated by:
- Gap Junctions: Allow electrical communication between cells.
- Intercalated Disks:
  - Contain Desmosomes: Resist mechanical stress
  - Contain Gap Junctions: Enable electrical coupling.

Action Potentials:
- Initiation of action potentials occurs at the SA node (sinoatrial node).
- Impulses travel through:
- AV Node (Atrioventricular Node)
- Bundle of His
- Purkinje Fibers

Autorhythmic Cells:
- Action Potential Phases:
1. Pacemaker Potential: Slow depolarization
  - Initiated by open funny channels.
2. Rapid Depolarization:
  - Driven by L-type Ca$^{2+}$ channels opening.
3. Repolarization:
  - Involves potassium channels opening.
Summary Table of Autorhythmic Cell Action Potential
- Movement of ions during phases:
- Sodium (Na$^+$) moves in > Potassium (K$^+$) moves out > Calcium (Ca$^{2+})$ moves in.

Excitation-Contraction Coupling:
- Similarities to skeletal muscle:
- T-Tubules: Invaginations that facilitate excitation.
- Sarcoplasmic Reticulum: Stores calcium ions.
- Similarities to smooth muscle:
- Gap junctions allow for coordinated contractions.

Recording Heart Activity:
- Einthoven's Triangle is used to record electrical activity with bipolar limb leads:
- Lead I: Left Arm to Right Arm
- Lead II: Left Leg to Right Arm
- Lead III: Left Leg to Left Arm
Key ECG Components:
- P wave: Atrial depolarization
- QRS Complex: Ventricular depolarization and atrial repolarization
- T wave: Ventricular repolarization
Component Characteristics:
- Amplitudes and durations:
- P wave: Amplitude: 0.2 mV; Duration: 0.10 sec
- QRS complex: Amplitude: 1.0 mV; Duration: 0.08-0.12 sec
- T wave: Amplitude: 0.2-0.3 mV; Duration: 0.16-0.27 sec
- P-Q Interval: Duration: 0.12-0.21 sec
- Q-T Interval: Duration: 0.30-0.43 sec

Heart Rhythm Types:
- Sinus Rhythm: Normal pace generated by the SA node.
- Sinus Arrhythmias: Variations in heart rhythm.
- Tachycardia: Heart rate exceeds 100 bpm (beats per minute).
- Bradycardia: Heart rate drops below 50 bpm.

First-degree block:
- Slowed conduction through the AV node increases the P-Q interval.
Second-degree block:
- Loss of 1-to-1 relationship between P wave and QRS complex, leading to missed atrial breaths.

Premature Contractions:
- Premature Atrial Contraction (PAC): Extra heartbeat occurs.
- Premature Ventricular Contraction (PVC): Extra ventricular contraction.
- While these conditions may cause an extra systole, they are not necessarily serious unless frequent.

Understanding the electrical activity responsible for cardiac function is essential for recognizing normal heart rhythms and diagnosing various heart conditions.