Cardiorespiratory System
The Cardiovascular System
The cardiovascular system consists of the heart, blood, and blood vessels. Its primary role during exercise is to transport oxygen and nutrients to working muscles and remove carbon dioxide and waste products.
Structure of the Heart
The heart is a muscular pump divided into four chambers:
| Chamber | Location | Role |
|---|---|---|
| Right atrium | Top right | Receives deoxygenated blood from the body via the vena cava |
| Right ventricle | Bottom right | Pumps deoxygenated blood to the lungs via the pulmonary artery |
| Left atrium | Top left | Receives oxygenated blood from the lungs via the pulmonary vein |
| Left ventricle | Bottom left | Pumps oxygenated blood to the body via the aorta |
The left ventricle has a thicker muscular wall because it must pump blood around the entire body at high pressure.
Valves (tricuspid, bicuspid/mitral, and semi-lunar) prevent the backflow of blood.
Blood Vessels
| Vessel | Function | Structure |
|---|---|---|
| Arteries | Carry blood away from the heart | Thick, muscular, elastic walls; small lumen; carry oxygenated blood (except pulmonary artery) |
| Veins | Carry blood back to the heart | Thinner walls; larger lumen; contain valves; carry deoxygenated blood (except pulmonary vein) |
| Capillaries | Exchange gases and nutrients at tissues | One cell thick; allow diffusion of O₂, CO₂, and nutrients |
Double Circulatory System
The human body uses a double circulatory system:
1. Pulmonary circuit — right ventricle → pulmonary artery → lungs (gas exchange) → pulmonary vein → left atrium
2. Systemic circuit — left ventricle → aorta → body (delivers O₂) → vena cava → right atrium
Key Cardiac Definitions
- Heart rate (HR) — number of beats per minute (bpm). Resting HR is typically 60–80 bpm.
- Stroke volume (SV) — volume of blood pumped per beat, typically ~70 ml at rest.
- Cardiac output (Q) — total volume of blood pumped per minute. Q = HR × SV
- Blood pressure — the force exerted by blood on artery walls. Measured as systolic/diastolic (e.g. 120/80 mmHg).
Redistribution of Blood (Vascular Shunting)
During exercise, blood is redirected away from inactive organs (e.g. digestive system) towards working muscles and the skin (for cooling). This process is controlled by vasodilation (widening of blood vessels to increase flow) and vasoconstriction (narrowing to reduce flow). The pre-capillary sphincters control blood flow into capillary beds.
The Respiratory System
Structure and Function
Air enters through the mouth/nose, passes through the trachea (windpipe), which divides into two bronchi (one to each lung). The bronchi subdivide into smaller bronchioles, which end in tiny air sacs called alveoli.
Gas exchange occurs at the alveoli. Oxygen diffuses from the alveoli into the blood (capillaries), and carbon dioxide diffuses from the blood into the alveoli to be exhaled. This is efficient because alveoli have:
- A huge surface area (roughly the size of a tennis court)
- Walls that are one cell thick
- A moist lining to dissolve gases
- A rich blood supply (dense capillary network)
Key Respiratory Definitions
| Term | Definition | Typical Resting Value |
|---|---|---|
| Tidal volume (TV) | Volume of air breathed in or out per breath | ~0.5 litres |
| Vital capacity (VC) | Maximum volume of air that can be exhaled after a maximum inhalation | ~4.8 litres |
| Breathing rate (BR) | Number of breaths per minute | 12–20 breaths/min |
| Minute ventilation | Total volume of air breathed per minute: MV = TV × BR | ~6 litres/min |
| Residual volume | Air remaining in the lungs after maximal exhalation | ~1.2 litres |
Mechanics of Breathing
Inspiration (breathing in):
- Intercostal muscles contract → ribs move up and out
- Diaphragm contracts and flattens
- Chest cavity volume increases → pressure decreases → air rushes in
Expiration (breathing out):
- Intercostal muscles relax → ribs move down and in
- Diaphragm relaxes and domes upwards
- Chest cavity volume decreases → pressure increases → air is pushed out
During exercise, expiration becomes active — the abdominals and internal intercostals contract forcefully to push air out faster.
Anaerobic and Aerobic Respiration
- Aerobic respiration: glucose + oxygen → energy + CO₂ + water. Used during low to moderate intensity exercise (e.g. jogging).
- Anaerobic respiration: glucose → energy + lactic acid. Used during high intensity exercise when oxygen supply cannot meet demand (e.g. sprinting).
Excess Post-Exercise Oxygen Consumption (EPOC) — after intense exercise the body continues to consume elevated levels of oxygen to break down lactic acid, replenish energy stores, and return the body to its resting state. This is sometimes called the oxygen debt.
Exam Tips
- Learn the equation Q = HR × SV and practise calculations (e.g. "A performer has a heart rate of 140 bpm and a stroke volume of 80 ml. Calculate cardiac output").
- Remember: pulmonary artery carries deoxygenated blood (the exception to the arteries rule) and pulmonary vein carries oxygenated blood.
- When explaining vascular shunting, name specific muscles and organs (e.g. "blood is redirected from the digestive system to the quadriceps during running").
- For gas exchange, always mention diffusion down a concentration gradient.