Heart and Circulatory System Function

Week 1: The Heart

Purpose of the Cardiovascular System

To provide adequate blood flow to all tissues/organs according to immediate needs.

Heart Contractions

• Systole: Periods of contraction
• Diastole: Periods of relaxation

Note: The atria contract simultaneously, followed by simultaneous ventricular contraction.

Blood Vessels

• Arteries: Carry blood away from the heart.
• Veins: Carry blood towards the heart.

Intrinsic Conduction System

1. Sinoatrial (SA) Node (Pacemaker): Generates impulses in the right atrial wall. Depolarizes 80-100 times per minute, modified by the autonomic nervous system to approximately 75 times per minute.
2. Atrioventricular (AV) Node: Impulses pause here for 0.1 seconds, allowing the atria to complete contraction. Generates impulses 40-60 times per minute. Serves as a backup pacemaker if the SA node fails.
3. Atrioventricular (AV) Bundle: Connects the atria to the ventricles.
4. Bundle Branches: Conduct impulses through the interventricular septum.
5. Purkinje Fibers: Depolarize the contractile cells of both ventricles.

The Cardiac Cycle

1. Diastole: All four chambers are relaxed. Ventricles are partially filled with blood (approximately 70%).
2. Atrial Systole: Atria contract, completely filling the relaxed ventricles.
3. Atrial Diastole: Atria relax. This continues until the next cycle.
4. Ventricular Systole (Phase 1 - Isovolumetric Contraction): Ventricular contraction begins at the apex, pushing blood upwards and closing the AV valves. Pressure is not yet high enough to open the semilunar (SL) valves.
5. Ventricular Systole (Phase 2 - Ventricular Ejection): Ventricular pressure increases, forcing the SL valves open and pushing blood out of the ventricles.
6. Ventricular Diastole (Early): As ventricles relax, arterial blood flows backwards, closing the SL valves.
7. Ventricular Diastole (Late): All chambers are relaxed. AV valves are open. Blood passively flows from atria to ventricles, filling them to approximately 70% of their final volume (approximately 400 msec).

Week 2: The Circulatory System

Key Measurements

• Heart Rate (HR): Number of beats per minute.
• Stroke Volume (SV): Volume of blood ejected from the left (or right) ventricle per beat.
• Cardiac Output (CO): Volume of blood pumped by the systemic circuit per minute (L/min).

Formula for Cardiac Output

CO = HR x SV