Gases & Partial Pressures

Gases are everywhere in medicine. Every breath you take is a carefully controlled delivery of a gas mixture to your lungs. Every blood test that measures oxygen and carbon dioxide levels is measuring dissolved gases. Every anaesthetic works by delivering precisely calculated gas concentrations to keep a patient unconscious during surgery. Understanding how gases behave — how they mix, how they dissolve in liquids, how pressure affects them — is fundamental to understanding breathing, blood gas analysis, altitude sickness, diving medicine, and anaesthesia. The key insight: a gas mixture like air is not a single substance. It is a mixture of separate gases, each behaving independently. Oxygen, nitrogen, and carbon dioxide in the air each exert their own pressure — independently of each other. This concept, called partial pressure, is one of the most important in respiratory physiology.

Before understanding partial pressures, you need to understand how gases behave in general. Boyle's Law — Pressure and Volume At constant temperature, the pressure and volume of a gas are inversely proportional. Double the pressure → volume halves. This is why: - Your chest volume increases when you breathe in — pressure inside the lungs falls below atmospheric pressure, and air rushes in - A sealed syringe gets harder to compress as you push the plunger — you are increasing pressure - Deep-sea divers must ascend slowly — if they rise too fast, dissolved nitrogen comes out of solution as bubbles in the blood (decompression sickness) Charles's Law — Temperature and Volume At constant pressure, gas volume is proportional to temperature. Heating a gas makes it expand. This matters in: - Calibrating spirometers (lung function tests) — they must be corrected for body temperature vs room temperature - Gas cylinders — oxygen cylinders in hospitals feel warm because the gas inside is compressed Dalton's Law of Partial Pressures In a mixture of gases, each gas exerts its own pressure independently, as if the other gases were not present. The total pressure is the sum of all the individual partial pressures. Air at sea level has a pressure of 101.3 kPa (kilopascals). Air is approximately: - 78% nitrogen → partial pressure of nitrogen (pN₂) = 79 kPa - 21% oxygen → partial pressure of oxygen (pO₂) = 21 kPa - 0.04% carbon dioxide → pCO₂ = 0.04 kPa - Remaining small amounts of other gases The partial pressure of a gas is simply its fractional concentration multiplied by the total pressure.

An arterial blood gas (ABG) is one of the most important investigations in emergency medicine. It measures the partial pressures of oxygen and carbon dioxide dissolved in arterial blood, along with pH and bicarbonate. Normal values (arterial blood): - pH: 7.35–7.45 (slightly alkaline) - pO₂: 10.6–13.3 kPa (oxygen) - pCO₂: 4.7–6.0 kPa (carbon dioxide) - HCO₃⁻: 22–26 mmol/L (bicarbonate — the main buffer) What deviations tell us: Low pO₂ (hypoxaemia) — the patient is not getting enough oxygen into the blood. Causes: pneumonia, pulmonary embolism, COPD, high altitude. High pCO₂ (hypercapnia) — the patient is not breathing out enough CO₂. Causes: respiratory failure (the breathing muscles are exhausted), overdose of opioids or sedatives (which suppress the respiratory drive), severe COPD. The pCO₂ is directly controlled by ventilation — breathing faster removes more CO₂, breathing slower allows it to accumulate. This is why anxious people hyperventilate: they blow off CO₂, blood pH rises, and they develop tingling in the hands (respiratory alkalosis). Oxygen dissociation curve: Haemoglobin does not bind oxygen in a simple linear way — it follows an S-shaped curve. At high pO₂ (lungs), haemoglobin saturates quickly and holds oxygen tightly. At low pO₂ (tissues), it releases oxygen efficiently. Temperature, pH, and CO₂ levels shift this curve — fever and acidosis shift it right (haemoglobin releases oxygen more readily, good for working tissues).

Anaesthetic gases work by diffusing from the lungs into the blood and then into the brain, where they suppress consciousness. Their potency is measured by the Minimum Alveolar Concentration (MAC) — the partial pressure of the gas in the alveoli needed to prevent 50% of patients from moving in response to a surgical incision. Common anaesthetic agents include isoflurane, sevoflurane, and desflurane. They are all volatile liquids that evaporate into gas and are delivered through a breathing circuit. Nitrous oxide (N₂O, "laughing gas") — historically the first anaesthetic, still used for pain relief in dentistry and childbirth. A key property: it diffuses rapidly into any gas-containing space in the body. This is a problem if there is a pneumothorax or bowel obstruction — N₂O enters these spaces, expanding them. Oxygen toxicity — breathing 100% oxygen for prolonged periods causes free radical damage to the lungs and brain. This is why supplemental oxygen is titrated to the lowest effective concentration — usually aiming for an oxygen saturation of 94–98%. Carbon monoxide poisoning: Carbon monoxide (CO) from incomplete combustion binds to haemoglobin with 250 times greater affinity than oxygen. Even small amounts of CO displace oxygen from haemoglobin — the patient appears well (or even flushed, due to carboxyhaemoglobin which is cherry-red) but their tissues are severely hypoxic. Treatment: high-flow 100% oxygen, which rapidly displaces CO from haemoglobin by dramatically increasing the pO₂.