--- module: 013-01 language: en chapter: 13 title: "Ventilation, Perfusion, Diffusion, and Gas Transport" module_title: "Foundations" source_sha256: 0a9a28e1d3a1130f112e06e20ca0fb73a169af0c8c0d301bc0a7827b8ee2da8a --- # Ventilation, perfusion, diffusion, and gas transport ## Orientation and functional anatomy ### Oxygenation and ventilation are distinct #### Alveolar ventilation determines carbon dioxide removal ### Conducting airways warm, humidify, filter #### Cartilage in large, smooth muscle in small ### Type one cells form the thin surface ### Type two cells make surfactant and repair ### Interface: epithelium, interstitium, endothelium, plasma, red cell ### Pulmonary circulation is low pressure, low resistance #### Bronchial arteries add small physiological shunt ## Ventilation and dead space ### Minute ventilation: tidal volume times frequency ### Anatomical dead space stays in conducting airways ### Alveolar dead space: ventilated, barely perfused ### Alveolar ventilation subtracts dead space first #### Twelve by five hundred: four point two litres #### Twenty-four by two fifty: two point four litres #### Same minute volume, less carbon dioxide cleared #### Mechanism explained, not a ventilator prescription ### Dependent alveoli smaller but more compliant ## Alveolar gas and partial pressure ### Humidification lowers inspired oxygen pressure ### Alveolar oxygen: inspired gas, pressure, carbon dioxide ### Altitude lowers barometric pressure, not fraction ### Hypoventilation raises carbon dioxide, lowers oxygen #### Coupled change separates pure hypoventilation ## Diffusion ### Rises with area, coefficient, pressure gradient ### Falls with membrane thickness ### Carbon dioxide more soluble, diffuses readily ### Emphysema destroys exchange surface area ### Fibrosis and oedema lengthen diffusion distance ### Carbon monoxide keeps capillary pressure low #### Low in emphysema, fibrosis, anaemia, vascular disease #### Alveolar haemorrhage can raise the value ## Ventilation-perfusion matching ### Whole-lung average hides regional variation ### Apex: perfusion falls more than ventilation ### Bases: perfusion predominates, ratio lower ### Hypoxic vasoconstriction diverts blood from poor units #### Widespread hypoxia raises pulmonary pressure ### Shunt is perfusion without ventilation #### Collapse, fluid or pus, intracardiac flow #### Incomplete response to supplemental oxygen ### Dead space: ventilation without effective perfusion ### Alveolar-arterial difference widens with mismatch or shunt ## Oxygen transport ### Content: haemoglobin, saturation, dissolved oxygen ### Pressure drives diffusion and saturation ### Sigmoidal curve from cooperative binding #### Plateau protects saturation in the lung #### Steep region promotes tissue unloading ### Right shift: carbon dioxide, acid, heat, bisphosphoglycerate ### Anaemia lowers content despite normal saturation ### Carbon monoxide occupies sites, raises remaining affinity ### Methaemoglobin iron cannot bind oxygen normally ## Carbon dioxide and acid-base ### Carried dissolved, as carbamino, mainly bicarbonate ### Carbonic anhydrase speeds red-cell hydration ### Haemoglobin buffers hydrogen, chloride shift follows ### Lung oxygenation reverses the reactions ### Acute hypoventilation raises carbon dioxide, lowers pH ### Hyperventilation lowers carbon dioxide, raises pH ## Exercise, altitude, and fetal circulation ### Output, ventilation, perfusion, diffusing capacity rise ### Mixed venous oxygen falls, arterial maintained ### Breathlessness alone does not localise failure ### Altitude hypoxaemia drives respiratory alkalosis #### Kidneys excrete bicarbonate over days #### High-altitude pulmonary and cerebral oedema #### Descent and oxygen in severe illness ### Fetal exchange occurs at the placenta #### Fetal haemoglobin has higher affinity #### Birth: expansion, separation, shunt closure ## Applied interpretation and oximetry limits ### Hypoxaemia has five mechanisms ### Hypoventilation is the main hypercapnia mechanism ### Saturation on oxygen does not prove ventilation ### A normal carbon dioxide can be inappropriate ### Oximetry reads pulsatile light absorption #### Perfusion, movement, dyshemoglobins distort readings ### Measures no carbon dioxide, pH, or content ### A reassuring number never overrides deterioration