Cardiac & Circulatory Responses to Exercise

Cardiac Physiology and Exercise

Contractility

• Definition: An increase in contraction strength that is independent of a change in lumen volume or muscle stretch.

• Relationship with Stretch/Volume/Length: These terms ($stretch$, $volume$, $length$) are considered interchangeable. An increase in volume leads to an increase in stretch.

• Causes of Increased Contractility:

  • More calcium availability within the cell.

  • Norepinephrine: Increases the amount of calcium released from the sarcoplasmic reticulum (SR).

Frank-Starling Law of the Heart

• Mechanism: Greater stretch (sarcomere length) leads to greater contraction strength.

• Physiological Range: Sarcomere lengths around $2.2$ emphasize the normal operating range when the heart is fully stretched.

• Calcium Sensitivity: Greater stretch of the sarcomere results in greater calcium sensitivity. This is represented as a leftward shift in the tension-calcium curve.

  • Implication of Leftward Shift: A stronger contraction occurs with the same amount of calcium, meaning the sarcomere becomes more sensitive to calcium and requires less calcium for the same amount of work.

• Calcium Release: Greater stretch also leads to more calcium release.

• Distinction from Contractility: While related to calcium, the cause of these changes (increased calcium sensitivity and release) is the greater stretch, therefore, it is a Frank-Starling mechanism, not contractility (which is stretch-independent).

Staircase Phenomenon (Treppe)

• Description: An increase in contraction strength with an increasing heart rate, observed as successive contractions becoming stronger.

  • Each wave represents a contraction; the height indicates strength.

  • For example, if heart rate increases from $4$ to $8$ seconds, contraction strength also increases.

• Mechanism: Increased heart rate causes calcium influx to be greater than calcium efflux.

  • Calcium Influx vs. Efflux: During normal contractions, calcium influx from outside the cell is balanced by efflux (via the sodium-calcium exchanger, which is slow).

  • Increased Heart Rate: When heart rate increases, the slow sodium-calcium exchanger cannot keep up.

  • SERCA Pump: The sarcoplasmic/endoplasmic reticulum calcium ATPase (SERCA) pump becomes more active, rapidly pumping more calcium into the SR with each contraction. This builds up calcium within the SR.

  • Result: More calcium is released from the SR with subsequent beats, leading to increased contractility.

• Causality: It is crucial to understand that increasing heart rate causes the influx to exceed efflux, which then results in greater contractility. It is not the other way around. Staircase phenomenon is a form of contractility because it's independent of stretch.

Norepinephrine's Multi-faceted Role

• Rate of Contraction (Heart Rate): Affects the SA node, increasing heart rate.

• Strength of Contraction (Contractility): Increases the amount and speed of calcium release from the SR.

• Rate of Relaxation: Paradoxically, norepinephrine also speeds up the rate of cardiac relaxation.

  • Mechanism: Calcium must be removed from the sarcomere for relaxation. Norepinephrine increases SERCA pump activity, accelerating the uptake of calcium back into the SR.

• Beta-Adrenergic Modulation: This term refers to the binding of norepinephrine (or other catecholamines) to beta-adrenergic receptors. "Norepinephrine modulation" is an equivalent term.

Stroke Volume Changes During Exercise

• Rest to Light/Moderate Intensity Exercise:

  • Primary Factor: Enhanced venous return due to skeletal muscle pump and venoconstriction.

  • Effect: Increases end-diastolic volume, which increases preload (stretch on the heart).

  • Mechanism at Low Intensity: Primarily explained by the Frank-Starling Law (increased stretch leads to increased stroke volume).

• Higher Intensity Exercise:

  • Challenge: As heart rate increases significantly, filling time decreases, which would normally reduce stroke volume if only relying on Frank-Starling.

  • Compensatory Mechanism: Increased sympathetic activity leads to increased contractility (e.g., via the staircase phenomenon, more norepinephrine).

  • Overall Effect: Contractility helps maintain or even increase stroke volume despite reduced filling time.

• Afterload Reduction:

  • Mechanism: With some activities, afterload can decrease.

  • Explanation: Active muscles undergo vasodilation, and the greater the active muscle mass, the more widespread the vasodilation. This reduces total peripheral resistance to blood flow.

  • Examples: High-intensity aerobic activity using a large muscle mass (e.g., running, cross-country skiing) causes a significant reduction in afterload compared to activities using smaller muscle groups (e.g., arm ergometry).

  • Benefit: A reduced afterload means the heart pumps blood against less resistance, allowing for a greater stroke volume.

Cardiac Output Max and $VO_2$ Max

• Relationship: Cardiac output max is typically reached when an individual reaches their $VO_2$ max.

• Limiting Factor: For most individuals, the primary limiter of $VO_2$ max (and thus exercise performance) is the heart's ability to pump oxygenated blood, not the ability to take in oxygen (breathing air).

  • Elite endurance athletes might be an exception, where respiratory limitations could play a role.

Bohr Effect and $AVO_2$ Difference

• Definition: The Bohr effect describes the decrease in hemoglobin's affinity for oxygen due to a decrease in pH (increase in $CO_2$ and $H^+$) or an increase in temperature, leading to more oxygen release to tissues.

• Influence on $AVO<em>2$ Difference: The Bohr effect causes the arterial-venous oxygen ($AVO</em>2$) difference to increase during exercise.

• Mechanism: As exercise intensity (oxygen uptake) increases, the venous oxygen ($O_2$) content decreases significantly because tissues extract more oxygen.

• Arterial $O<em>2$ Content: Arterial $O</em>2$ content generally remains about the same. Elite athletes might show a slight decrease, which will be discussed later.

Blood Pressure During Exercise

• Systolic Blood Pressure (SBP):

  • Trend: Increases with increasing exercise intensity.

  • Dependency: Strongly dependent on cardiac output. As cardiac output goes up, SBP goes up.

  • Normal Values: It is normal for SBP to exceed $200$ mmHg during high-intensity exercise in healthy individuals.

• Diastolic Blood Pressure (DBP):

  • Trend: A "wild card"; it can stay the same, increase slightly, or even drop considerably (e.g., to $0$ mmHg).

  • Concern: A large increase in DBP is generally a concern; otherwise, its variability is often not alarming.

• Mean Arterial Pressure (MAP):

  • Definition: The average pressure throughout an entire cardiac cycle (contraction and relaxation).

  • Calculation Note: Tends to be closer to the diastolic pressure at rest because more time is spent in relaxation than contraction.

  • Proportionality: MAP is proportional to cardiac output ($MAP \propto CO$).

  • Total Peripheral Resistance (TPR): MAP is also proportional to TPR ($MAP \propto TPR$).

    • TPR during exercise can increase, decrease, or stay the same.

    • Influence: Depends on the amount of muscle mass being used and environmental factors (e.g., vasodilation in the skin in hot conditions).

      • Large muscle mass aerobic exercise (especially in heat) leads to a large reduction in TPR.

      • Arm ergometry in cold weather might lead to an increase in TPR.

• Resistance Exercise: Blood pressure goes up significantly, especially with activities involving the Valsalva maneuver.

Valsalva Maneuver

• Description: Exhaling against a closed airway, which temporarily increases intrathoracic and intra-abdominal pressure.

• Situational Use: Whether to use or avoid it depends on the individual and the exercise type.

  • Avoid in Most Cases: For older adults or individuals with underlying cardiovascular conditions performing light-to-moderate exercise (e.g., machine work, small muscle group resistance training), avoiding the Valsalva maneuver is recommended due to the strain on the cardiovascular system.

  • Use in Specific Cases: For heavy compound lifts like squats or deadlifts, the Valsalva maneuver is crucial for maintaining the structural integrity and stability of the core and back, preventing injury.

• Conclusion: There's a time and place for both approaches; it's not a universal "always do" or "never do" instruction.

Blood Flow Redistribution During Exercise

• Mechanism: Most of the blood redirected to active muscles comes primarily from the gut (splanchnic circulation).

• Implications for Endurance Athletes:

  • Nutrition: For activities longer than about $2$ hours (e.g., marathons, ultramarathons), consuming easy-to-digest carbohydrates is essential for maintaining performance.

  • Gastrointestinal (GI) Issues: The redirection of blood away from the stomach and intestines to the muscles can lead to GI problems (nausea, vomiting), which is a common reason for withdrawal in ultra-endurance events.

Functional Sympatholysis

• Definition: The local vasodilation that occurs in active muscles, even in the face of widespread sympathetic adrenergic activity that causes vasoconstriction in other parts of the body.

• Mechanism: This local vasodilation is mediated by three primary mechanisms:

  1. Metabolic: Accumulation of local metabolic byproducts (e.g., $H^+$, $CO_2$, adenosine, $K^+$).

  2. Myogenic: Response to changes in muscle wall tension.

  3. Endothelial: Release of vasodilators from the endothelium (e.g., nitric oxide, prostaglandins).