November 7th

Overview of the Heart and Cardiac Cycle

  • Discussion focused on structures of the heart and their role in controlling heart rate

  • Emphasized importance of understanding cardiac physiology in relation to exercise

Cardiac Muscle vs. Skeletal Muscle

  • Cardiac Muscle Characteristics:

    • Involuntary and homogeneous (all muscle fibers are similar)

    • Closely resembles Type I muscle fibers:

    • Highly oxidative (aerobic metabolism)

    • Abundant mitochondria

    • High caloric density

  • Differences from Skeletal Muscle:

    • Cardiac muscle contracts as one unit due to structural features.

    • Key structural features that allow this:

      1. Desmosomes: Structural junctions that hold cells together.

      2. Intercalated Discs: Facilitates communication and contraction between cardiac cells.

      3. Gap Junctions: Permits the passage of ions and electrical impulses between cells.

Cardiac Cycle During Exercise

  • Sinoatrial Node (SA Node):

    • The primary pacemaker of the heart; located in the upper right atrium.

    • Sets a baseline heart rate of about 100 beats per minute but is regulated by the autonomic nervous system in a resting state.

    • Establishes sinus rhythm, which is crucial for normal heart function.

  • Atrioventricular Node (AV Node):

    • Delays electrical conduction from the atria to ventricles (approximately 1300ths of a second).

    • This delay allows for complete filling of the ventricles before contraction, known as the atrial kick.

Cardiac Conduction System

  • Purkinje Fibers:

    • Transmit impulses quickly through the ventricles, allowing rapid contraction.

    • Essential for synchronized ventricular contraction to effectively push blood throughout the body.

Definitions of Cardiac Cycle Phases

  • Cardiac Cycle Components:

    • Two phases: Systole and Diastole

    • Diastole:

      • Period of heart filling; characterized by low pressure, allowing for optimal filling of the heart.

      • Significant for atrial filling that occurs passively until the last moment (atrial kick).

    • Systole:

      • Period of heart contraction and blood ejection.

      • Pressure is high as blood is pushed out to systemic circulation and pulmonary circulation.

  • Pressure Readings:

    • Normal blood pressure should be less than 120/80 mmHg at rest.

    • Systolic pressure increases during exercise while diastolic pressure may remain the same or slightly decrease due to vasodilation.

  • Implications on Health:

    • Higher blood pressure linked to conditions such as atherosclerosis and elevated cholesterol.

    • Aging populations, and genetic predisposition, are common factors contributing to hypertension.

Effects of Exercise on Cardiac Function

  • Acute Effects of Exercise:

    • Systolic pressure increases due to higher stroke volume and demand for oxygenated blood.

    • Heart rate and cardiac output will rise to meet oxygen needs.

  • Chronic Effects of Regular Exercise:

    • Resting blood pressure tends to decrease as cardiovascular fitness improves.

    • Improved efficiency in oxygen delivery and metabolic capabilities.

Ejection Fraction and Stroke Volume

  • Ejection Fraction:

    • A key measurement of heart health; defined as Stroke Volume / End Diastolic Volume.

    • Normal ejection fraction at rest is about 60%. This can improve with exercise due to enhanced stroke volume.

  • Stroke Volume:

    • Defined as the volume of blood pumped per beat, determined by two factors:

    • End Diastolic Volume: The amount of blood in the ventricles at the end of diastole (should be high for effective pumping).

    • End Systolic Volume: The amount of blood remaining in the ventricles after contraction (should be low to indicate efficient pumping).

  • Stroke Volume is calculated as follows:
    Stroke Volume = End Diastolic Volume - End Systolic Volume

Torsional Control (Diastolic Suction)

  • Torsional Control:

    • Describes the twisting motion of the heart that enhances filling of the ventricles during diastole.

    • Increased heartbeats enhance the rapid change in pressure, aiding blood flow from atria to ventricles.

    • Most diastolic filling remains passive but can be assisted by active mechanisms, especially during exercise.

Homework Assignment

  • Outline Chapter 7 of your textbook for further review, specifically focusing on the heart’s function and the cardiac cycle, as well as the detailed physiological processes involved during exercise.

  • Note: Blood flow in the heart has already been reviewed earlier and does not need to be included in your outline.