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Exercise Physiology
Acute Responses

Explore how your body immediately adapts to physical activity: from heart rate and blood pressure to ventilation and energy systems.

Anatomy & Physiology

What Are Acute Responses?

Acute responses are the immediate, short-term physiological changes that occur in your body the moment you begin exercising. Unlike chronic adaptations that develop over weeks of training, acute responses happen within seconds to minutes and return to baseline once exercise stops.

Key Systems Involved

Cardiovascular
Heart rate, stroke volume, cardiac output, and blood pressure all change rapidly
Respiratory
Breathing rate and tidal volume increase to boost oxygen delivery and \(\text{CO}_2\) removal
Muscular
Blood flow to muscles increases, temperature rises, and energy substrates are mobilized

Heart Rate During Exercise

60 100 140 180 200 HR (bpm) Rest Warm-up Exercise Peak Recovery ~195 bpm ~70 bpm

The Cardiac Output Equation

The volume of blood pumped by the heart per minute is described by:

At Rest
\( Q = 72 \times 70 = 5{,}040 \) mL/min ≈ 5 L/min
Max Exercise
\( Q = 195 \times 130 = 25{,}350 \) mL/min ≈ 25 L/min

Blood Pressure Response

60 100 140 180 220 mmHg Rest Light Moderate Intense SBP DBP 120/80 ~200/80
Systolic BP
Increases linearly with intensity due to greater cardiac output. Can reach ~200+ mmHg
Diastolic BP
Stays relatively constant or decreases slightly due to vasodilation in working muscles

Ventilation and Gas Exchange

At Rest
\( \dot{V}_E \approx 12 \times 0.5 = 6 \) L/min
Max Exercise
\( \dot{V}_E \approx 50 \times 3.0 = 150 \) L/min

Minute ventilation can increase 20-25 times during maximal exercise. Initially, tidal volume increases more; at higher intensities, breathing frequency becomes the primary driver.

Acute vs. Chronic Responses

Acute Response Chronic Adaptation
Onset Seconds to minutes Weeks to months
Duration Only during exercise Persistent at rest
Heart Rate Increases during bout Lower resting HR
Stroke Volume Increases during bout Higher resting SV
Muscle Increased blood flow More capillaries, mitochondria
Example Elevated HR during a run Cardiac hypertrophy

Blood Redistribution During Exercise

At Rest Muscles 20% Organs 50% Brain 15% Skin 5% → Max Exercise Muscles 80-85% Organs 3-5% Brain 3-4% Skin 10% Vasoconstriction in non-essential organs Vasodilation in working muscles = "Vascular Shunt Mechanism"

Energy Systems and Acute Demand

ATP-PC System
Immediate energy for 0-10 seconds. Anaerobic, no oxygen needed. Sprints, jumps.
Anaerobic Glycolysis
Dominant for 10-90 seconds. Breaks down glucose without oxygen. Produces lactic acid.
Aerobic System
Dominant after ~2 minutes. Uses oxygen to break down carbs, fats, proteins. Sustained energy.

Analyzing Acute Responses Step-by-Step

1
Identify the exercise type: Is it aerobic (running, cycling) or anaerobic (sprinting, weightlifting)? Duration and intensity matter.
2
Determine the dominant energy system: ATP-PC (<10s), anaerobic glycolysis (10-90s), or aerobic (>2 min).
3
Predict cardiovascular changes: Calculate \( Q = HR \times SV \). Both HR and SV rise, but SV plateaus at moderate intensity.
4
Predict respiratory changes: Calculate \( \dot{V}_E = f \times V_T \). Both rate and depth increase.
5
Consider blood redistribution: Blood shifts from organs to muscles via the vascular shunt mechanism.

Example 1: Jogging for 30 Minutes

A student begins jogging at moderate intensity. Their resting HR is 72 bpm and resting SV is 70 mL. After 10 minutes of jogging, HR is 145 bpm and SV is 110 mL.

Example 2: 100m Sprint

An athlete performs a maximal 100m sprint lasting ~12 seconds. How do acute responses differ from jogging?

Energy System
ATP-PC dominates for the first 10 seconds, then anaerobic glycolysis contributes for remaining seconds
HR Response
HR rises very rapidly, may approach \( HR_{\max} \) within seconds. Peak occurs after the sprint ends

Oxygen Deficit and EPOC

\(\text{VO}_2\) (L/min)
 Time Rest Exercise Recovery
\(\text{O}_2\) Demand
\(\text{VO}_2\)
\(\text{O}_2\) Deficit
 EPOC
Oxygen Deficit
Gap between \( \text{O}_2 \) demand and actual \( \text{O}_2 \) consumption at exercise onset. Covered by anaerobic energy.
EPOC
Excess Post-exercise Oxygen Consumption: elevated \( \text{O}_2 \) use after exercise to restore homeostasis.

Thermoregulation During Exercise

Only about 25% of metabolic energy produces movement: the remaining ~75% is released as heat. The body must dissipate this heat to prevent dangerous core temperature rises.

1
Vasodilation of skin blood vessels: Increases blood flow to the skin surface for heat radiation and convection.
2
Sweating: Eccrine glands secrete sweat. Evaporation is the primary cooling mechanism, removing ~580 kcal per liter of sweat.
3
Increased ventilation: Some heat is lost through expired air, especially in cold environments.

Common Mistakes

✕ Confusing Acute and Chronic
Saying "exercise lowers resting heart rate" is a chronic adaptation, not an acute response. Acute = during/immediately after a single bout.
✕ Assuming SV Always Rises
Stroke volume plateaus at ~40-60% max intensity. Beyond that, only HR drives further increases in cardiac output.
✕ Diastolic BP Rises Too
During dynamic (aerobic) exercise, diastolic BP stays constant or decreases slightly. Only systolic BP rises significantly. (Exception: isometric exercise raises both.)

Summary of Acute Responses

Variable At Rest During Exercise Direction
Heart Rate \(60-80 \text{ bpm}\) Up to \(\sim200 \text{ bpm}\) ↑↑↑
Stroke Volume \(\sim70 \text{ mL}\) \(\sim130 \text{ mL}\) (plateaus) ↑↑
Cardiac Output \(\sim5 \text{ L/min}\) \(\sim25 \text{ L/min}\) ↑↑↑
Systolic BP \(120 \text{ mmHg}\) \(\sim200+ \text{ mmHg}\) ↑↑↑
Diastolic BP \(80 \text{ mmHg}\) \(\sim80 \text{ mmHg}\) →
Ventilation \(\sim6 \text{ L/min}\) \(\sim150 \text{ L/min}\) ↑↑↑
Core Temperature \(\sim37^\circ\text{C}\) \(\sim39-40^\circ\text{C}\) ↑↑

Question 1 of 5

During moderate aerobic exercise, what happens to diastolic blood pressure?

Question 2 of 5

A runner has a heart rate of 160 bpm and a stroke volume of 120 mL during exercise. What is their cardiac output?

Question 3 of 5

Which energy system would primarily fuel a 5-second maximal vertical jump?

Question 4 of 5

The vascular shunt mechanism during exercise redistributes blood primarily by:

Question 5 of 5

EPOC (Excess Post-exercise Oxygen Consumption) refers to: