Physiologic Medications and Cardiac Output

• Physiologic medications are known to enhance heart function.

  • They allow the heart to pump stronger and more efficiently.

  • Increased maximum cardiac output can be achieved with training.

  • Maximum heart rate does not dramatically change with training.

  • Ability to pump more blood per beat (stroke volume) increases.

Discussion on Training and Heart Rate

• The scenario discussion with Moe about training habits highlights the effects of training on max heart rate and stroke volume.

  • Moe has an assumed maximum heart rate of 190 bpm (beats per minute).

  • With training, maximum stroke volume is increased to 125 ml (milliliters per beat).

• It’s emphasized that maximum cardiac output increases, while resting cardiac output remains unchanged.

Role of the Nervous System in Heart Rate Regulation

• The central nervous system significantly influences heart functions, especially through the SA (sinoatrial) node.

  • SA node is the heart's natural pacemaker with an intrinsic rate of about 60 bpm.

• Trained individuals may exhibit lower resting heart rates, potentially down to 45 bpm.

• Two main components of heart rate regulation:

  • Parasympathetic Nervous System (PNS): Acts as a brake, influencing the SA node to keep heart rate lower.

  • Sympathetic Nervous System (SNS): Acts as the accelerator. After a heart rate of approximately 100 bpm, the SNS takes control to increase heart rate and meet exercise demands.

Considerations with Beta Blockers

• Beta blockers are commonly prescribed for various cardiac conditions, influencing the SA node’s activity.

  • They reduce heart rate both at rest and during exercise, important for individuals with high blood pressure and atherosclerosis.

• Implications of exercising on beta blockers include:

  • Attenuated adaptation of the heart to exercise due to regulated heart rates.

  • Heart rate responses during exercise may be lower than expected at the same intensity; demanding more effort results in fatigue and increased risk of arrhythmia.

Frank-Starling Mechanism

• The Frank-Starling law describes how an increase in the heart's end-diastolic volume (EDV) leads to a stronger contraction due to increased stretch of cardiac muscle fibers.

  • Essential concept: More blood filling the heart leads to stronger contractions.

  • The relationship can be illustrated with a water balloon: more fluid stretches the balloon, resulting in more significant force upon release.

• To achieve higher EDV, sympathetic nervous system activation facilitates venous return, ensuring adequate blood flow back to the heart.

  • Constriction of veins aids in rolling blood toward the heart.

Interplay Between Heart Rate and Stroke Volume During Exercise

• Importance of matching blood flow to oxygen demand during heavy exercise:

  • If heart rate increases significantly without corresponding venous return, oxygen delivery may falter.

  • Maintaining an optimal relationship between heart rate and stroke volume is critical for sufficient oxygenation of working tissues.

Vascular Dynamics During Exercise

• During exercise, the body can increase blood flow to working muscles while restricting blood supply to lesser-priority organs (like kidneys).

  • This mechanism is crucial for meeting the metabolic demands of active muscles.

• The effect of exercise on blood pressure:

  • Systolic blood pressure tends to rise due to increased heart output, while diastolic pressure is more stable under exercise.

Changes in Blood Pressure and Heart Rate During Exercise

• Understanding blood pressure responses:

  • Increased systolic blood pressure during exertion is a response to heightened demand.

  • The majority of resting blood flow is directed to internal organs, but during exercise, flow is redirected to working muscles.

Concept of Blood Flow Redistribution

• Mechanisms of redistribution:

  • Through vasoconstriction and vasodilation, blood is directed preferentially towards active muscle tissues.

• The distinction between absolute and relative blood flow during exercise:

  • Absolute values may remain high; percentages can decrease in less prioritized organs yet still account for adequate supply.

Oxygen Extraction and Delivery

• The arteriovenous oxygen difference (a-vO2 difference) refers to the difference in oxygen content between arterial and venous blood.

  • Increased a-vO2 difference reflects greater oxygen extraction at higher exercise intensities.

• Oxygen consumption leads to adaptations in the oxyhemoglobin dissociation curve, which shifts to the right with increased exercise:

  • This enhances oxygen delivery and extraction from hemoglobin to muscle tissues.

Factors Influencing VO2 Max

• VO2 max is defined as the maximum amount of oxygen the body can utilize during intense exercise, and it relates to the supply (cardiac output) and demand (a-vO2 difference).

  • The equation linking VO2 max, cardiac output, and a-vO2 difference:
    (VO2{max} = CO imes (a-vO2){difference}).