Cardiovascular Disease

Redistribution of Blood Flow During Exercise

• During exercise, significant changes occur in cardiac output and blood flow distribution.

• Cardiac output (Q) at rest is approximately 5 L/min, but increases to 25 L/min during heavy exercise.

  • Blood flow to skeletal muscles increases dramatically:

    • 20% during rest contributes to 1 L/min in muscles.

    • 80-85% during heavy exercise contributes to ~20 L/min in muscles.

• Blood flow to less active organs (liver, kidneys, gastrointestinal tract) decreases significantly.

Regulation of Cardiovascular Adjustments to Exercise

• Key factors influencing cardiac output and blood flow include:

  • Metabolic vasodilation in muscles increases blood flow.

  • Increased cardiac stroke volume and heart rate (HR) during exercise.

  • The sympatho-adrenal system enhances heart rate and improves venous return.

  • Skeletal muscle activity prompts sympathetic vasoconstriction in viscera.

  • Deeper breathing also contributes to cardiovascular adjustments.

Meeting Increased Oxygen Demands during Exercise

• The equation related to cardiac output and physical activity is:

  • Q (cardiac output) = SV (stroke volume) x HR (heart rate).

• The redistribution of blood flow primarily facilitates the increased oxygen demands by muscles during exercise.

Changes in Cardiovascular Variables: Exercise Intensity

• During exercise, there is:

  • An increase in heart rate (HR) and stroke volume (SV).

  • In untrained subjects, SV does not increase beyond workloads of 40% VO2max.

  • Consequently, the rise in cardiac output at higher intensities is mainly due to an increase in HR.

Changes in Cardiovascular Variables: Exercise Duration

• Cardiac output is maintained during prolonged exercise, but:

  • Stroke volume gradually decreases due to dehydration and reduced plasma volume.

  • Heart rate gradually increases, termed "cardiovascular drift."

Increased Blood Flow to Skeletal Muscles

• At rest, 15-20% of cardiac output is directed to muscles.

• This increases to 80-85% during maximal exercise.

• The redistribution of blood flow is dependent on exercise intensity and metabolic rate.

Mechanisms for Meeting Muscle's Oxygen Demands

• Oxygen demand during exercise is 15-25 times greater than at rest.

• This demand is met by:

  1. Increased cardiac output and redistribution of blood flow.

  2. Enhanced extraction/uptake of O2 by muscle tissues.

Fick Equation: Oxygen Consumption (VO2)

• VO2 is expressed as:

  • VO2 = Q x (a - vO2Δ)

  • Where a-vO2Δ is the arteriovenous oxygen difference, representing the difference in oxygen content between arterial and venous blood.

• Not all O2 delivered to muscles is consumed, necessitating the efficient extraction of O2 to maximize VO2.

a-vO2 Difference During Activity

• The a-vO2 difference reflects the oxygen extracted from 100 mL of blood during its circulatory journey.

  • Example:

    • At rest: 20 mL O2 – 15 mL O2 = 5 mL extracted.

    • During exercise: 20 mL O2 – 10 mL O2 = 10 mL extracted.

• A higher a-vO2 difference during exercise indicates better oxygen utilization.

Cardiovascular Adaptations to Endurance Training

• Endurance training involves continuous, dynamic exercises, such as running or cycling, for 20-60 mins at >50% VO2max, 3 times a week over 2-3 months.

• Such training enhances:

  • Cardiac output delivery (Q) and oxygen extraction (a-vO2Δ).

  • The VO2max, which indicates the maximum capacity of oxygen utilization during exercise.

Effects of Training on Cardiac Output

• Qmax largely increases due to a rapid rise in stroke volume (SV).

  • An 11% increase in SV can occur within the first 6 days of training.

• Following training, it has been observed that resting and submaximal heart rates decrease without affecting HRmax.

Factors Influencing Stroke Volume Max

• Stroke volume increases (SVmax) with:

  1. Increased preload (end-diastolic volume).

  2. Increased plasma volume (fluid retention).

  3. Greater filling time due to a decreased resting heart rate.

• There is also an increase in ventricular chamber size allowing for maximal filling and decreased afterload.

Types of Hypertrophy from Different Training Regimens

• Aerobic Training:

  • Results in eccentric hypertrophy (volume overload), characterized by chamber dilation and elongated myocyte length.

• Resistance Training:

  • Leads to concentric hypertrophy (pressure overload), marked by increased myocyte width without chamber dilation.

Training Effects on a-vO2Δ

• The maximum a-vO2Δ improves due to:

  • Increased mitochondrial size and enzyme activity.

  • Enhanced capillary density and blood flow in muscles through vessel dilation.