Respiratory Physiology — Why It’s More Than Just Breathing In and Out🫁

Hey first-years! As we dive into the respiratory system, it’s time to go beyond the basics of “inhale, exhale” and explore what’s really happening every time you breathe.

Understanding respiratory physiology gives you a foundation for clinical reasoning  — so here’s your guided tour of the essentials:

🔄 VENTILATION: Moving air in and out

Breathing is driven by pressure gradients. During inspiration, the diaphragm and external intercostal muscles contract, increasing thoracic volume and decreasing intrapulmonary pressure — drawing air in. Expiration is normally passive at rest, but can become active with effort or disease.

🧠 Controlled by the respiratory centres in the brainstem (medulla and pons), ventilation is primarily regulated by chemoreceptors responding to CO₂, not O₂! Central chemoreceptors in the medulla detect pH changes in CSF, while peripheral chemoreceptors in the carotid and aortic bodies respond to low O₂ and high H⁺.

💨 LUNG VOLUMES AND CAPACITIES: It’s all in the measurements

• Tidal Volume (TV): Air in and out during normal breathing (~500 mL in adults)
• Residual Volume (RV): Air left after full exhalation — never fully emptied
• Vital Capacity (VC): The max air you can exhale after a max inhale
• Functional Residual Capacity (FRC): Volume in lungs after normal exhale — important for gas exchange between breaths

🧪 You’ll meet these again in spirometry, which is key in diagnosing obstructive vs restrictive lung diseases.

🌬️ GAS EXCHANGE: Getting O₂ in, CO₂ out

Takes place in the alveoli, where a thin respiratory membrane allows rapid diffusion of gases.

• O₂ diffuses from alveoli into capillaries (high → low gradient)
• CO₂ moves from capillaries into alveoli to be exhaled

This relies on: 

✔️ Adequate alveolar ventilation

✔️ Intact alveolar-capillary membrane

✔️ Haemoglobin to carry O₂ in the blood

🩸 Oxygen transport is mostly via binding to haemoglobin. Only ~2% is dissolved in plasma. The oxygen-haemoglobin dissociation curve tells you how readily Hb picks up or releases O₂ — affected by pH, CO₂, temperature, and 2,3-BPG.

⚖️ V/Q MATCHING: The ventilation-perfusion dance

Ventilation (V) = air reaching alveoli

Perfusion (Q) = blood reaching alveoli

Ideal V/Q ratio ≈ 0.8

🔻 Low V/Q → poor ventilation (e.g. asthma, pneumonia)

🔺 High V/Q → poor perfusion (e.g. pulmonary embolism)

The body adapts with hypoxic pulmonary vasoconstriction — redirecting blood to better-ventilated areas.

🧮 CONTROL OF BREATHING: It’s not all voluntary

While you can control your breath (e.g. holding it), your body takes over if CO₂ rises or O₂ falls.

Hypercapnia (↑CO₂) is the main driver of respiration

In chronic respiratory disease, some patients rely on hypoxic drive — low O₂ as a stimulus to breathe

💡 Clinical tip: Giving too much O₂ to a person with COPD may reduce their drive to breathe — use oxygen carefully and per guidelines.

🩺 CLINICAL SCENARIO

You’re on a hospital placement and your supervisor asks you to review a 72-year-old patient with COPD who is receiving oxygen via nasal prongs.

You note:

• Respiratory rate: 10 breaths per minute
• Oxygen saturation: 98% on 4 L/min
• Drowsy, but rousable
• PaCO₂ on recent ABG: elevated

🧠 Your supervisor asks:

“Why might this patient’s respiratory rate be falling, even though their oxygen levels look good?”

💬 What do you think?

Comment your answer below before scrolling! 👇👇👇

📝 Answer & Discussion:

In patients with chronic hypercapnia (like COPD), the central chemoreceptors become less sensitive to CO₂. These patients may rely more on hypoxic drive (low O₂) to stimulate breathing. By administering high-flow oxygen, you may reduce that hypoxic stimulus — causing hypoventilation, CO₂ retention, and drowsiness (CO₂ narcosis) - this theory is unclear and not well elucidated, and certainly not seen in all patients. 

⚠️ Clinical pearl: In COPD, oxygen therapy should be titrated carefully to target SpO₂ 88–92%, unless otherwise guided. Don't fear O2 in COPD patients, just handle with care. 

📊 WHY THIS MATTERS CLINICALLY

Respiratory physiology underpins:

• Diagnosing and managing asthma, COPD, pneumonia, and PE
• Understanding ABGs (arterial blood gases) and interpreting acid-base disturbances
• Safe use of oxygen in hospital settings
• Monitoring ventilation during anaesthesia and in ICU 

📚 Your Takeaway?

Respiratory physiology isn’t just a set of numbers and diagrams. It’s how your body adapts to exercise, altitude, illness, and injury — and it’s vital for every future clinical decision you’ll make about breathing, gas exchange, and oxygenation.

Ask questions, work through examples, and keep revisiting the physiology when you hit the clinical years. It will absolutely pay off.