Cardiovascular Adaptations to Exercise

Understanding Cardiac Output and Cardiovascular Changes During Exercise

• Cardiac Output (CO): Total volume of blood pumped by the heart per minute.

  • ext{Cardiac Output} = ext{Stroke Volume} imes ext{Heart Rate}

• Stroke Volume (SV): Volume of blood ejected from each ventricle during a single contraction.

  • Normal resting value: 60-80 ext{ mL} (average around 70 ext{ mL}).

• Heart Rate (HR): Number of heart contractions per minute.

  • Normal resting value: around 70 beats per minute; can range from 60-200 beats during various activities.

• Mechanisms of Cardiac Output:

  • Atrial contraction contributes about 10% of ventricular filling at rest but can increase to 30-40% during exercise.

  • Uncoordinated contraction such as in atrial fibrillation can affect overall cardiac function.

  • Increased atrial contractility during sympathetic stimulation (fight or flight response) can enhance performance during physical exertion.

Relationship Between Heart Rate and Stroke Volume During Exercise

• Exercise Induced Changes:

  • Increased heart rate typically increases stroke volume in healthy individuals.

  • In untrained individuals, stroke volume may plateau despite an increase in heart rate.

  • Endurance athletes often show a linear relationship between increasing heart rate and stroke volume.

• Physiological Implications:

  • Enhanced blood flow through improved cardiac output is vital for meeting metabolic demands during physical activity.

Ventricular Contraction and Cardiac Function

• Coordinated Contraction:

  • Proper ventricular contraction is essential for effective cardiac output.

  • Aconitine, a sodium channel opener, can disrupt ventricular and aortic pressures and causes changes in the ECG, resulting in dangerous conditions like ventricular tachycardia.

• Effects of Tachycardia:

  • Can lead to ineffective pumping and increased risk of thrombi formation due to stagnant or poorly circulated blood.

  • Septal anatomy influences the consequences of atrial vs ventricular tachycardia, where ventricular fibrillation is particularly dangerous as it directly affects cardiac output.

Cardiovascular Adjustments During Exercise

• Immediate Cardiovascular Changes:

  • Increased cardiac output and blood pressure.

  • Redistributed blood flow to working muscles while reducing flow to non-essential areas (e.g., gut, kidneys).

• Regulation Mechanisms:

  • Neural controls like sympathetic nervous system adjustments.

  • Local metabolic factors such as increased metabolites during muscle activity promoting vasodilation in working muscles.

• Central Mechanisms:

  • Central mechanisms refer to the neural and hormonal responses that originate from the brain and central nervous system, crucial in regulating cardiovascular functions.

  • Control of heart rate and blood pressure is achieved through the autonomic nervous system (ANS), specifically through sympathetic (increases heart rate and contractility) and parasympathetic (decreases heart rate) branches.

  • The cardiovascular center in the medulla oblongata processes sensory information and adjusts cardiac output accordingly.

  • Hormonal influences like adrenaline and noradrenaline released from the adrenal medulla can enhance heart rate and myocardial contractility during stress or exercise.

  Peripheral Mechanisms:

  • Peripheral mechanisms involve local adjustments in blood flow and vascular resistance within the tissues themselves, often in response to metabolic demands.

  • During physical activity, skeletal muscle vasodilation occurs due to the accumulation of metabolic byproducts (e.g., carbon dioxide, lactate), promoting increased blood flow to working muscles.

  • The role of baroreceptors located in blood vessels helps monitor and regulate blood pressure by adjusting vascular resistance and heart rate based on changes in blood volume or pressure.

Post-Exercise Recovery and Its Importance

• Post-Exercise Hypotension:

  • A drop in blood pressure post-exercise is common and can signal cardiovascular health.

  • Mechanism includes decreased total peripheral resistance and muscle sympathetic activity, which may help manage hypertension.

• Heart Rate Recovery:

  • Faster recovery of heart rate indicates better cardiovascular fitness.

  • Normal recovery rates: 12-15 bpm reduction in the first minute.

  • The rate of recovery can indicate cardiovascular health status and is linked to exercise intensity and regularity.

Importance of Hydration During Exercise

• Fluid Loss & Its Effects:

  • During exercise, fluid loss occurs primarily due to sweating, which can range from 0.5 to 1 ext{ L/hour}.

• Consequences of Dehydration:

  • Decreased plasma volume leads to reduced venous return and stroke volume.

  • Increased heart rate due to compensatory mechanisms.

  • Risks include increased blood viscosity, cramps, cardiac arrhythmias, and heat-related illnesses.

• Rehydration Strategy:

  • It's essential to consume isotonic fluids to maintain electrolyte balance and prevent osmotic changes in cells during and after exercise.