Gas exchange — the alveolus and its capillary
Diffusion across the 0.5 µm blood–gas barrier drawn with the partial pressures on both sides: O₂ down a 60 mmHg gradient from alveolar 100 to venous 40, CO₂ back down only 6 mmHg because it is twenty times more soluble, equilibrium reached in 0.25 s of a 0.75 s transit, and the V̇/Q̇ ratio that decides the arterial result.
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{
"nodes": [
{ "id": "inspired", "type": "cloud", "label": "Inspired air — PO₂ 159 mmHg when dry", "x": 40, "y": 40, "width": 150, "height": 104, "fill": "#ede9fe", "stroke": "#6d28d9", "bold": true },
{ "id": "airways", "type": "cylinder-h", "label": "Conducting airways — 150 mL dead space", "x": 215, "y": 40, "width": 150, "height": 104, "fill": "#fef3c7", "stroke": "#b45309" },
{ "id": "alveolus", "type": "sphere", "label": "Alveolus — PO₂ 100, PCO₂ 40 mmHg", "x": 390, "y": 40, "width": 150, "height": 104, "fill": "#dbeafe", "stroke": "#1d4ed8", "bold": true },
{ "id": "membrane", "type": "layers", "label": "Blood–gas barrier — 0.5 µm thick", "x": 565, "y": 40, "width": 150, "height": 104, "fill": "#e0f2fe", "stroke": "#0369a1", "bold": true },
{ "id": "capillary", "type": "honeycomb", "label": "Pulmonary capillary — 0.75 s transit", "x": 740, "y": 40, "width": 150, "height": 104, "fill": "#fef3c7", "stroke": "#b45309", "bold": true },
{ "id": "arterial", "type": "water-drop", "label": "Arterial blood — PO₂ 100, SaO₂ 98%", "x": 915, "y": 40, "width": 150, "height": 104, "fill": "#fce7f3", "stroke": "#be185d", "bold": true },
{ "id": "humidified", "type": "water-drop", "label": "Warmed and humidified — PO₂ falls to 149", "x": 40, "y": 200, "width": 150, "height": 104, "fill": "#fef3c7", "stroke": "#b45309" },
{ "id": "surfactant", "type": "petri-dish", "label": "Surfactant from type II pneumocytes", "x": 215, "y": 200, "width": 150, "height": 104, "fill": "#e0f2fe", "stroke": "#0369a1", "bold": true },
{ "id": "area", "type": "bar-chart", "label": "70 m² of surface — half a tennis court", "x": 390, "y": 200, "width": 150, "height": 104, "fill": "#ccfbf1", "stroke": "#0f766e" },
{ "id": "fick", "type": "clipboard", "label": "Fick: rate ∝ area × ΔP ÷ thickness", "x": 565, "y": 200, "width": 150, "height": 104, "fill": "#f1f5f9", "stroke": "#475569", "bold": true },
{ "id": "venous", "type": "water-drop", "label": "Mixed venous — PO₂ 40, PCO₂ 46 mmHg", "x": 740, "y": 200, "width": 150, "height": 104, "fill": "#ede9fe", "stroke": "#6d28d9", "bold": true },
{ "id": "haemoglobin", "type": "sphere", "label": "Haemoglobin carries 20 mL O₂ per dL", "x": 915, "y": 200, "width": 150, "height": 104, "fill": "#dbeafe", "stroke": "#1d4ed8", "bold": true },
{ "id": "o2flux", "type": "arrow-right", "label": "O₂ diffuses down 60 mmHg of gradient", "x": 40, "y": 360, "width": 150, "height": 104, "fill": "#fef3c7", "stroke": "#b45309", "bold": true },
{ "id": "co2flux", "type": "arrow-left", "label": "CO₂ leaves down only 6 mmHg", "x": 215, "y": 360, "width": 150, "height": 104, "fill": "#fef3c7", "stroke": "#b45309", "bold": true },
{ "id": "solubility", "type": "gauge-chart", "label": "CO₂ is 20× more soluble, so 6 is enough", "x": 390, "y": 360, "width": 150, "height": 104, "fill": "#f1f5f9", "stroke": "#475569", "bold": true },
{ "id": "equilibrium", "type": "clock", "label": "Equilibrium reached in 0.25 s", "x": 565, "y": 360, "width": 150, "height": 104, "fill": "#ccfbf1", "stroke": "#0f766e", "bold": true },
{ "id": "vq", "type": "scatter-chart", "label": "V̇/Q̇ ≈ 0.8 — matching sets the PaO₂", "x": 740, "y": 360, "width": 150, "height": 104, "fill": "#ccfbf1", "stroke": "#0f766e", "bold": true },
{ "id": "thickening", "type": "cross", "label": "Oedema or fibrosis thickens the barrier", "x": 915, "y": 360, "width": 150, "height": 104, "fill": "#fee2e2", "stroke": "#b91c1c", "bold": true },
{ "id": "shunt", "type": "cross", "label": "Shunt — perfused but never ventilated", "x": 390, "y": 520, "width": 150, "height": 104, "fill": "#fee2e2", "stroke": "#b91c1c" },
{ "id": "curve", "type": "line-chart", "label": "Dissociation curve shifts with pH and CO₂", "x": 740, "y": 520, "width": 150, "height": 104, "fill": "#ccfbf1", "stroke": "#0f766e" }
],
"edges": [
{ "id": "e1", "source": "inspired", "target": "humidified", "thickness": 2.5 },
{ "id": "e2", "source": "humidified", "target": "airways", "thickness": 2.5 },
{ "id": "e3", "source": "airways", "target": "alveolus", "label": "only 350 mL of each breath arrives", "thickness": 2.5 },
{ "id": "e4", "source": "alveolus", "target": "membrane", "thickness": 2.5 },
{ "id": "e5", "source": "membrane", "target": "o2flux", "thickness": 2.5 },
{ "id": "e6", "source": "o2flux", "target": "capillary", "thickness": 2.5 },
{ "id": "e7", "source": "venous", "target": "capillary", "label": "arrives desaturated at 75%", "thickness": 2.5 },
{ "id": "e8", "source": "capillary", "target": "arterial", "thickness": 2.5 },
{ "id": "e9", "source": "capillary", "target": "co2flux", "thickness": 2.5 },
{ "id": "e10", "source": "co2flux", "target": "alveolus", "label": "breathed out", "thickness": 2.5 },
{ "id": "e11", "source": "surfactant", "target": "alveolus", "label": "stops it collapsing at low volume", "style": "dashed" },
{ "id": "e12", "source": "area", "target": "membrane", "label": "the A in Fick's law", "style": "dashed" },
{ "id": "e13", "source": "fick", "target": "membrane", "style": "dashed" },
{ "id": "e14", "source": "equilibrium", "target": "capillary", "label": "two-thirds of the transit is spare", "style": "dashed" },
{ "id": "e15", "source": "haemoglobin", "target": "arterial", "label": "98.5% of the O₂ travels bound" },
{ "id": "e16", "source": "curve", "target": "haemoglobin", "style": "dashed" },
{ "id": "e17", "source": "vq", "target": "alveolus", "style": "dashed" },
{ "id": "e18", "source": "shunt", "target": "vq", "label": "V̇/Q̇ = 0 — oxygen cannot fix it", "style": "dashed" },
{ "id": "e19", "source": "thickening", "target": "membrane", "label": "diffusion distance up, rate down", "style": "dashed" },
{ "id": "e20", "source": "solubility", "target": "co2flux", "style": "dashed" }
]
}