Respiration
Respiration covers both the cellular oxidation of fuel molecules and the breathing pathway by which atmospheric O2 enters the body and CO2 leaves it. The AAMC MCAT content outline emphasizes the human respiratory system — alveoli, breathing mechanism, gas transport and respiratory disorders. This is a high-yield area for the Bio/Biochem section.
Human Respiratory System
The human respiratory tract is a continuous tube from the nose to the alveoli. It is split into a conducting zone (no gas exchange) and a respiratory zone (gas exchange occurs).
Pathway of air
Nose / mouth → pharynx → larynx (with vocal cords and epiglottis) → trachea → right and left primary bronchi → secondary & tertiary bronchi → bronchioles → terminal bronchioles → respiratory bronchioles → alveolar ducts → alveoli.
Each lung contains ~300 million alveoli, giving a total surface area of ~70 m2. Features that make them ideal exchange surfaces:
- One-cell-thick squamous epithelium → very short diffusion distance (<1 µm).
- Dense pulmonary capillary network → large area for diffusion and a steep gradient maintained by blood flow.
- Surfactant (made by Type II alveolar cells) lowers surface tension and prevents collapse during expiration. Surfactant deficiency in premature babies causes respiratory distress syndrome.
Breathing Mechanism
Breathing (pulmonary ventilation) brings fresh air to the alveoli and expels stale air. It works on Boyle’s law: change the volume of the thoracic cavity and pressure changes inversely; air flows down the pressure gradient.
The diaphragm contracts and flattens, while the external intercostal muscles contract to lift the ribs upwards and outwards. Thoracic volume increases → intra-pulmonary pressure falls below atmospheric → air rushes in.
The diaphragm and external intercostals relax. Elastic recoil of the lungs and chest wall reduces thoracic volume → intra-pulmonary pressure exceeds atmospheric → air flows out. Forced expiration adds the internal intercostals and abdominal muscles.
Lung volumes and capacities
| Term | Symbol | Volume | Definition |
|---|---|---|---|
| Tidal volume | TV | 500 mL | Air moved per quiet breath in or out |
| Inspiratory reserve volume | IRV | ~3000 mL | Extra air inspired beyond normal inspiration |
| Expiratory reserve volume | ERV | ~1100 mL | Extra air expired beyond normal expiration |
| Residual volume | RV | ~1200 mL | Air remaining after maximal expiration; cannot be measured by spirometry |
| Inspiratory capacity | IC = TV + IRV | ~3500 mL | Max air inhalable from end of normal expiration |
| Functional residual capacity | FRC = ERV + RV | ~2300 mL | Air left after normal expiration |
| Vital capacity | VC = TV + IRV + ERV | ~4600 mL | Max air movable in one breath |
| Total lung capacity | TLC = VC + RV | ~5800 mL | Total volume the lungs can hold |
| Aspect | Inspiration | Expiration (quiet) |
|---|---|---|
| Process type | Active (energy used) | Passive (elastic recoil) |
| Diaphragm | Contracts & flattens | Relaxes & domes up |
| External intercostals | Contract — ribs up & out | Relax — ribs down & in |
| Thoracic volume | Increases | Decreases |
| Intra-pulmonary pressure | Falls below atmospheric | Rises above atmospheric |
| Air flow | Atmosphere → lungs | Lungs → atmosphere |
| Forced version uses | + sternocleidomastoid, scalenes | + internal intercostals, abdominal muscles |
Transport of Gases
Once O2 diffuses into pulmonary capillaries it must travel to tissues; CO2 moves the other way.
About 98.5% of O2 is carried bound to hemoglobin in red blood cells; only ~1.5% is dissolved in plasma. Each hemoglobin molecule has 4 heme groups and binds 4 O2. Binding is cooperative — the first O2 makes binding the next ones easier — which produces the characteristic S-shaped (sigmoid) oxyhemoglobin dissociation curve.
An increase in CO2, H+ (low pH) or temperature shifts the oxyhemoglobin curve to the right — hemoglobin releases O2 more readily. This is exactly what tissues need: actively respiring tissues are warm, acidic and CO2-rich, so hemoglobin unloads more O2 there. In the lungs the opposite shift loads O2 efficiently.
Carbon dioxide transport
CO2 travels in three forms:
- ~70% as bicarbonate (HCO3−) in plasma. CO2 + H2O ⟶carbonic anhydrase H2CO3 → H+ + HCO3−. The H+ is buffered by hemoglobin.
- ~23% as carbamino-hemoglobin — CO2 bound to amino groups of globin chains.
- ~7% dissolved in plasma.
Respiratory Disorders
- Asthma
- Chronic inflammatory airway disease. Bronchospasm, mucus and airway edema cause reversible narrowing — wheezing, shortness of breath. Triggered by allergens, exercise, cold air. Treated with bronchodilators (salbutamol) and inhaled corticosteroids.
- COPD (Chronic Obstructive Pulmonary Disease)
- Umbrella term for chronic bronchitis (productive cough > 3 months/year, 2 years) and emphysema (destruction of alveolar walls → reduced surface area, loss of elastic recoil). Strongly linked to smoking. Largely irreversible.
- Tuberculosis (TB)
- Bacterial infection by Mycobacterium tuberculosis. Forms granulomas (tubercles) in lungs. Symptoms: chronic cough, blood-stained sputum, night sweats, weight loss. Treated with multi-drug regimens (rifampicin, isoniazid, pyrazinamide, ethambutol). TB remains a major global disease burden — BCG vaccination is given at birth in many countries.
- Pneumonia
- Acute infection (often bacterial) that fills alveoli with fluid and pus, impairing gas exchange.
- Lung cancer
- Most strongly associated with cigarette smoking; squamous cell and small cell carcinomas are commonest in smokers.
Worked MCQs
Five MCQs covering the high-yield testing patterns for human respiration.
Q1. The volume of air inspired or expired during a single normal quiet breath is called the:
Tidal volume is about 500 mL in a healthy adult. Vital capacity is the maximum movable volume in one breath; residual volume is what remains after maximal expiration; IRV is extra air inspired beyond a normal breath.
Q2. Most CO2 is transported in the blood as:
Roughly 70% of CO2 is carried as bicarbonate. Inside red blood cells, carbonic anhydrase converts CO2 + H2O to H2CO3, which dissociates to H+ and HCO3−. The bicarbonate diffuses into plasma in exchange for chloride (chloride shift).
Q3. A right shift in the oxyhemoglobin dissociation curve indicates:
A right shift means at any given partial pressure hemoglobin is less saturated — it gives up O2 more readily. Caused by increased CO2, H+ (low pH) and temperature: the Bohr effect. This delivers more O2 to active tissues.
Q4. During quiet inspiration, which muscles contract?
Quiet inspiration is active — the diaphragm flattens and the external intercostals lift the ribs up and out, expanding the thorax. Internal intercostals and abdominal muscles are accessory muscles for forced expiration.
Q5. Tuberculosis is caused by:
TB is a bacterial infection caused by Mycobacterium tuberculosis, which forms granulomas (tubercles) in the lungs. Treatment uses a multi-drug regimen for at least 6 months. The BCG vaccine, made from a related attenuated bacterium, gives partial protection.
Quick Recap
- Air pathway: nose → trachea → bronchi → bronchioles → alveoli (~300 million, surface area ~70 m2).
- Surfactant from type II cells lowers alveolar surface tension.
- Inspiration: diaphragm + external intercostals contract; expiration: passive elastic recoil.
- Tidal volume ~500 mL; residual volume ~1200 mL; vital capacity ~4600 mL.
- O2 mostly carried as oxyhemoglobin; sigmoid dissociation curve.
- Bohr effect: high CO2/H+/temperature → right shift → O2 released.
- CO2 transport: ~70% bicarbonate, ~23% carbamino-Hb, ~7% dissolved.
- Asthma (reversible), COPD (chronic), TB (M. tuberculosis) — key disorders.