Sports, Exercise and Health Science | Response

Welcome to A.3: The Body's Instant Reaction!

Hello future SEHS experts! This chapter, Response, is one of the most exciting parts of exercise physiology. We are moving away from studying the body at rest and diving into what happens the very *second* you decide to sprint for the bus or lift a heavy weight.

These immediate changes are called acute responses to exercise. Understanding them is key because they determine how well your body can perform and manage different workloads. Don't worry if the terminology seems complex; we will break down the adjustments in the cardiovascular, respiratory, and musculoskeletal systems step-by-step. Let's get moving!

Section 1: Defining the Acute Response

What is an Acute Physiological Response?

The body is a master of maintaining balance (homeostasis). When you start exercising, you challenge this balance dramatically. An acute response is the immediate, temporary adjustment your body makes to cope with the increased demand for energy (ATP) and oxygen.

Think of it like flipping a switch: rest mode instantly changes to high-performance mode.

Key Distinction: Acute vs. Chronic

• Acute Response: Immediate changes during or immediately after a single bout of exercise. (E.g., Your heart rate jumps up.)
• Chronic Adaptation (Training Effect): Long-term structural or functional changes resulting from repeated exercise. (E.g., Your resting heart rate decreases after 6 months of training.)

Section 2: The Cardiovascular System's Response

The cardiovascular system (CV) is the body’s delivery service. When exercise starts, it needs to dramatically increase its output to meet the muscle demands.

1. Heart Rate (HR)

Heart Rate (HR) is the number of times the heart beats per minute.

• Response: HR increases rapidly and linearly with exercise intensity.
• Regulation: This increase is regulated by the Autonomic Nervous System (ANS). Initially, the parasympathetic brake is removed, and then the sympathetic accelerator (via hormones like adrenaline/epinephrine) kicks in.

2. Stroke Volume (SV)

Stroke Volume (SV) is the volume of blood pumped out by the left ventricle per beat.

• Response: SV increases significantly, especially in untrained individuals, often peaking around 40-60% of maximal effort.
• Reasoning: Stronger ventricular contraction and increased venous return (more blood rushing back to the heart).

3. Cardiac Output (CO) (HL Focus: Magnitude)

Cardiac Output (CO) is the total volume of blood pumped by the heart per minute. It is the most important measure of the CV system's acute response.

The relationship is defined by the simple, yet crucial, formula: $$CO = HR \times SV$$

• At Rest: CO is usually around 5 Litres per minute (L/min).
• During Max Exercise: CO can rise to 20-25 L/min (untrained) and up to 35-40 L/min (highly trained endurance athletes).

4. Blood Pressure (BP)

The changes in BP reflect the increase in flow and the body's internal plumbing adjustments.

• Systolic BP (SBP): The pressure during heart contraction. Response: SBP increases linearly with exercise intensity because CO increases significantly.
• Diastolic BP (DBP): The pressure during heart relaxation. Response: DBP remains relatively unchanged or may decrease slightly, as overall resistance in the active muscles decreases due to vasodilation.

5. Blood Flow Redirection (Shunting)

This process ensures oxygen and fuel are delivered only where they are needed—the working muscles—and taken away from non-essential organs.

This is achieved through:

• Vasoconstriction: Blood vessels narrow (constrict) in inactive areas (like the gut, kidneys, and inactive muscles). This is like turning off the water supply to the guest bathroom.
• Vasodilation: Blood vessels widen (dilate) in active muscle tissues. This is like turning the shower on full blast in the main bathroom.

Section 3: The Respiratory System's Response

The lungs must work harder to take in more \(O_{2}\) and, crucially, blow off more metabolic \(CO_{2}\).

1. Ventilation Rate (V)

Ventilation is the amount of air exchanged per minute.

$$V = \text{Tidal Volume (TV)} \times \text{Breathing Frequency (f)}$$

• Response: Ventilation increases immediately, driven by neural stimulation and chemical changes in the blood.
• Tidal Volume (TV): The depth of breathing increases first (you take deeper breaths).
• Breathing Frequency (f): Then, the rate of breathing increases (you breathe faster).

2. Oxygen Consumption (\(V O_{2}\))

\(V O_{2}\) measures the volume of oxygen consumed by the body.

• Response: \(V O_{2}\) increases linearly with exercise intensity, reflecting the increased aerobic demand of the muscles.
• Maximal \(V O_{2}\) (\(V O_{2} \text{max}\)): This is the maximum rate at which an individual can consume oxygen. It is the best measure of cardiovascular fitness.

3. The Role of Chemical Control (HL Depth)

Why does breathing increase so quickly, even before major changes in blood oxygen levels?

• Neural Factors: Motor signals from the brain and sensory feedback from muscle receptors (proprioceptors) trigger an immediate, rapid increase in ventilation.
• Chemical Factors: The most important chemical driver during exercise is the increase in \(CO_{2}\) and lactic acid, which lowers the blood pH. These changes are detected by chemoreceptors (in the aorta, carotid arteries, and brain), which signal the respiratory centre to increase breathing rate and depth.

Section 4: Metabolic and Thermal Responses

1. Fuel Substrate Use

As exercise intensity rises, the body shifts its preferred fuel source:

• Low Intensity: Higher reliance on fats (via aerobic metabolism).
• High Intensity: Increasing reliance on carbohydrates (glycogen/glucose) because they provide ATP much faster, even though they are less efficient (aerobic and anaerobic pathways).
• Enzyme Activity: Enzyme activity increases massively to accelerate the chemical reactions required to break down these fuels into usable energy (ATP).

2. Thermoregulation (Controlling Heat)

Only about 25% of the energy produced during exercise goes to mechanical work; the other 75% is released as heat.

• Response: Core body temperature rises.
• Countermeasure: The body triggers two main responses to cool down:
  1. Sweating: Evaporation of sweat provides a massive cooling effect.
  2. Cutaneous Vasodilation: Blood vessels near the skin widen to allow heated blood to move closer to the surface, where the heat can be radiated away.

3. Oxygen Deficit and EPOC (HL Depth and Essential SL Concept)

Oxygen Deficit

When you start exercising, your aerobic system (the slow, efficient system) takes time to fully ramp up. During this initial period, the demand for oxygen is higher than the current consumption. The body must rely heavily on anaerobic pathways (like the phosphocreatine system and glycolysis) to bridge this gap.

The oxygen deficit is the difference between the required \(V O_{2}\) and the actual \(V O_{2}\) at the start of exercise.

Excess Post-exercise Oxygen Consumption (EPOC) (The Recovery Phase)

After you stop exercising, your body doesn't instantly return to rest. You continue to breathe heavily for a period of time, consuming excess oxygen above resting levels. This is known as EPOC (often informally called the Oxygen Debt).

EPOC is needed to repay the oxygen deficit and restore the body to pre-exercise conditions. It serves two main phases:

1. Rapid Component: Replenishes ATP and PCr (Phosphocreatine) stores used during the initial deficit phase.
2. Slow Component: Converts accumulated lactic acid back into glucose (Cori cycle, primarily in the liver), restores tissue oxygen stores, and continues to fuel the elevated metabolism (due to elevated temperature and hormones).

Section 5: Summary of Acute Responses

Quick Review Checklist

The transition from rest to exercise requires immediate, coordinated changes:

• Heart Rate (HR) and Stroke Volume (SV) both increase, leading to a massive increase in Cardiac Output (CO).
• Blood Flow is shunted (redirected) away from inactive organs towards active muscles (via vasoconstriction and vasodilation).
• Ventilation (V) increases dramatically, driven by faster and deeper breaths, primarily to expel \(CO_{2}\).
• Metabolic Rate increases, shifting fuel preference towards Carbohydrates at higher intensities.
• Temperature regulation begins immediately via sweating and cutaneous vasodilation.
• HL/Depth: Exercise starts with an Oxygen Deficit, which is later repaid during recovery by EPOC.

Keep practicing how these systems interact. Remember, they don't work in isolation—a change in breathing instantly affects circulation, which affects muscle performance! Great job getting through the acute responses!