Metabolism and exercise

Welcome to Metabolism and Exercise!

Ever wondered why your heart thumps against your ribs during a sprint, or why you keep huffing and puffing long after you’ve crossed the finish line? In this chapter, we explore the fascinating way our bodies turn into high-performance machines during physical activity. We’ll look at how your heart and lungs adapt, how your muscles actually move at a microscopic level, and what "fitness" really means in biological terms. This is a key part of the Energy, reproduction and populations section of your OCR A Level course.

Don’t worry if some of the graphs or chemical names seem tricky at first—we’ll break them down step-by-step!

1. How the Body Responds to Exercise

When you start exercising, your body has to switch gears immediately. This involves the cardiovascular system (heart and blood vessels), the respiratory system (lungs), and your skeletal muscles.

Immediate (Short-term) Effects

• Heart Rate Increases: To pump more oxygenated blood to the working muscles.
• Breathing Rate & Depth Increase: To take in more \(O_2\) and remove the extra \(CO_2\) produced by respiration.
• Vasodilation: Small arteries (arterioles) supplying the muscles widen to allow more blood flow.

Long-term Effects (Training Adaptations)

If you exercise regularly, your body "levels up." This is often called physiological adaptation.

• Cardiac Hypertrophy: The heart muscle becomes stronger and the ventricles larger. This leads to a higher stroke volume (more blood pumped per beat) and a lower resting heart rate.
• Increased Capillary Density: More tiny blood vessels grow around the muscle fibers, making oxygen delivery more efficient.
• Muscle Changes: Muscles become better at using oxygen and storing energy (glycogen).

Factors Affecting Aerobic Fitness

Not everyone has the same starting line. Fitness is influenced by:

• Age: Generally, our maximum heart rate and oxygen capacity decrease as we get older.
• Gender: Men typically have larger hearts and higher hemoglobin levels, though training can narrow this gap significantly.
• Participation in Exercise: The more you train, the more your body adapts!

The F.I.T.T. Principle

To improve fitness, athletes use the F.I.T.T. factors to design training programs:

• Frequency: How often you train.
• Intensity: How hard you work (e.g., % of max heart rate).
• Type: The kind of exercise (e.g., running vs. weightlifting).
• Time: How long each session lasts.

Quick Review: Immediate effects are about surviving the workout; long-term adaptations are about making the body better at it for next time.

Key Takeaway: Exercise forces the body to adapt both instantly (higher heart rate) and over time (stronger heart muscle) to meet the increased demand for ATP.

2. \(VO_2\) Max: The Ultimate Fitness Metric

\(VO_2\) Max is the maximum rate at which your body can take in, transport, and use oxygen during intense exercise. Think of it as the "size of your engine."

Did you know? Elite cross-country skiers often have the highest \(VO_2\) max scores because they use almost every muscle group in their body at once!

Common Mistake: Students often think \(VO_2\) max is just about lung capacity. It’s actually about three things: how well the lungs take in \(O_2\), how well the heart pumps it, and how well the muscles "grab" it from the blood.

3. Oxygen Transport and the "Dissociation Curve"

Oxygen is carried in the blood by haemoglobin (Hb). To understand how Hb releases oxygen to the muscles, we use an oxygen dissociation curve.

The Bohr Effect

When you exercise, your muscles produce \(CO_2\), become more acidic (lower pH), and get hotter. These changes cause the dissociation curve to shift to the right. This is a good thing! A shift to the right means haemoglobin has a lower affinity for oxygen—it "unloads" it more easily so the hard-working muscles can use it.

Memory Aid: "CADET, face Right!"
The curve shifts Right when these increase: CO2, Acid, 2,3-DPG (a chemical in blood), Exercise, and Temperature.

Haemoglobin vs. Myoglobin

• Haemoglobin (Hb): The transport molecule in the blood. It’s like a delivery truck.
• Myoglobin: Found inside muscle cells. It has a very high affinity for oxygen. It acts like a "storage tank" that only gives up its oxygen when levels in the muscle get dangerously low.
• Fetal Haemoglobin: Babies in the womb have Hb with a higher affinity than their mother’s. This allows the baby to "steal" oxygen from the mother’s blood.

Key Takeaway: The body uses different types of haemoglobin and the Bohr Effect to ensure oxygen goes exactly where it is needed most (the muscles) during exercise.

4. Oxygen Deficit and Oxygen Debt (EPOC)

When you start running, your heart can't immediately supply enough oxygen for aerobic respiration. This creates a "gap."

• Oxygen Deficit: The volume of oxygen that would have been consumed if the body could have reached a steady state of aerobic respiration immediately. During this time, the body uses anaerobic systems.
• Oxygen Debt (EPOC): After you stop exercising, you keep breathing hard. This is Excess Post-exercise Oxygen Consumption. You are "paying back" the oxygen to:
  1. Replenish ATP and phosphocreatine stores.
  2. Convert lactic acid back into glucose (in the liver).
  3. Re-oxygenate myoglobin and haemoglobin.

Analogy: Oxygen Deficit is like buying something on a credit card because you don't have the cash (oxygen) right now. Oxygen Debt is paying back that credit card bill with interest after you get home.

5. Skeletal Muscle: The Machine

To understand how we move, we have to look deep inside the muscle at its histology (tissue structure).

Muscle Structure Hierarchy

1. Muscle (the whole organ)
2. Muscle Fibre (a single long cell with many nuclei)
3. Myofibril (long strands inside the fibre)
4. Sarcomere (the functional unit of contraction)

The Sliding Filament Theory

This is the "how-to" of muscle contraction. Muscles don't shorten by shrinking; they shorten because protein filaments slide past each other.

Key Players:

• Actin: The thin filament.
• Myosin: The thick filament with "heads" that look like golf clubs.
• Troponin & Tropomyosin: The "guards" that prevent contraction when the muscle is at rest.
• Calcium Ions (\(Ca^{2+}\)): The "key" that unlocks the guards.
• ATP: The "fuel" that provides energy for the movement.

Step-by-Step Contraction:

1. An impulse arrives, causing calcium ions to be released into the muscle fibre.
2. Calcium binds to troponin, which pulls tropomyosin away, exposing binding sites on the actin.
3. Myosin heads bind to the actin, forming a cross-bridge.
4. The myosin head tilts (the power stroke), pulling the actin filament along. ADP and Pi are released.
5. A new ATP molecule binds to the myosin head, causing it to detach from the actin.
6. ATP is hydrolysed (broken down) to provide energy to "reset" the myosin head for the next pull.

Quick Review: Calcium unlocks the muscle; ATP moves and resets it. Without ATP, the muscle stays locked (this is why rigor mortis happens!).

Key Takeaway: Muscle contraction is a cycle of grabbing, pulling, and releasing driven by Calcium and ATP.

6. Enhancing Performance: Science and Ethics

Athletes often look for ways to push their biological limits. Some are legal (dietary), and some are banned (doping).

• Carbohydrate Loading: Eating lots of complex carbs days before an event to maximize glycogen stores in the muscles. (Legal and effective!)
• RhEPO (Recombinant Erythropoietin): A hormone that stimulates the body to make more red blood cells. More cells = more oxygen delivery. (Banned/Illegal)
• Blood Doping: Removing your own blood, storing it, and re-injecting it before a race to boost red blood cell count. (Banned/Illegal)
• Anabolic Steroids: Synthetic versions of testosterone that increase muscle mass and speed up recovery. (Banned/Illegal, with many health risks)

Common Mistake: Don't confuse health with fitness. Enhancing performance with drugs might make an athlete "fitter" (able to run faster), but it usually makes them less "healthy" due to side effects like heart damage.

Key Takeaway: Performance enhancement focuses on increasing oxygen delivery or muscle power, but artificial methods carry significant ethical and health risks.