Physiology Week 3: Lecture 2

Cardiovascular Responses to Exercise

Overview

• Focus on aerobic responses of the cardiovascular system during different exercise intensities.

• Key cardiovascular variables: heart rate, stroke volume, cardiac output, blood pressure, total peripheral resistance.

Key Cardiovascular Variables

1. Heart Rate (HR)

   • Normal resting HR: 60-100 bpm.

   • Increases at onset of exercise due to decreased parasympathetic activity.

2. Stroke Volume (SV)

   • Increases due to higher venous return and greater force of contraction (increased sympathetic activity).

3. Cardiac Output (Q)

   • Product of heart rate and stroke volume, increases to meet exercise demands.

   • Initial increase followed by a plateau (steady state) around 3-4 minutes into exercise.

4. Blood Pressure

   • Systolic pressure increases due to higher cardiac output.

   • Diastolic pressure remains constant due to increased vasodilation.

   • Mean arterial pressure is influenced by changes in systolic and diastolic pressures.

5. Total Peripheral Resistance (TPR)

   • Decreases due to vasodilation, allowing more blood flow to working muscles.

Redistribution of Blood Flow

• At rest, cardiac output is approximately 6 liters. During light to moderate exercise, it can rise to nearly double that.

• Majority of blood flows to working muscles (around 50% during light exercise), while skin, brain, and digestive organs receive varying amounts based on needs.

Short Term Light to Moderate Intensity Exercise Responses

• Initial increases in HR, SV, and Q contribute to meeting physical demands.

• After a few minutes, cardiovascular variables reach steady state, allowing efficient blood distribution.

• Intra-exercise physiological shifts lead to increased blood flow to skin for thermoregulation.

Long Term Moderate to Heavy Aerobic Exercise Responses

1. Cardiovascular Drift

   • Stroke volume decreases over prolonged exercise due to increased competition for blood flow (skin vs. working muscle).

   • Heart rate must increase to maintain cardiac output as stroke volume declines.

   • Systolic pressure decreases slightly; diastolic remains unchanged, while mean arterial pressure experiences only modest increases.

2. Blood Flow Redistribution

   • Cardiac output can increase significantly (up to approximately 18 liters during heavy aerobic exercise).

   • Majority of blood redirected to working muscles (up to 75% of cardiac output).

   • Skin receives additional blood for cooling, while less is sent to the digestive system.

   • Brain perfusion remains steady at around 750ml.

Incremental to Maximal Exercise Responses

1. Cardiac Output

   • Initially driven by stroke volume; plateaus at about 50% maximal workload.

   • After this threshold, heart rate increases primarily drive further increases in cardiac output.

2. Blood Pressure

   • Systolic blood pressure shows a linear increase while diastolic remains constant.

   • TPR decreases to manage blood flow to working muscles and maintain output.

3. Total Peripheral Resistance

   • Decreases further under stress to facilitate higher cardiac output.

   • Represents the body's effort to direct blood flow effectively during exercise.

4. Redistribution

   • At maximal exertion, cardiac output may reach 25 liters.

   • Approx. 90% of blood may be directed to skeletal muscle, while brain and coronary blood flow slightly increases to support cognitive function and heart activity.

VO2 Max and Cardiovascular Efficiency

• VO2 max is a crucial indicator involving multi-system interaction (respiratory, cardiovascular, muscular).

• Determined by the Fick equation:

  • VO2 = AVO2 Difference × Cardiac Output.

• Not strictly tied to the muscular system's limits - instead, often constrained by cardiac output.

Limiting Factors of VO2 Max

• Primary limiting factor is inadequate blood flow to working muscle.

• Other possible limitations include respiratory factors (inadequate ventilation) or metabolic issues (mitochondrial limitations).

• VO2 max serves as a cardiovascular measure often linked directly to cardiac output.