Partial pressures you should memorize (mm Hg)
6 rows| Location | P_O₂ | P_CO₂ |
|---|---|---|
| Dry air at sea level | 159 | 0.2 |
| Alveolar gas | 105 | 40 |
| Blood entering the lungs (from body) | 40 | 46 |
| Blood leaving the lungs (to body) | 100 | 40 |
| Blood after unloading at tissues | 40 | 46 |
| Air at 6000 m altitude | ~80 | — |
Lung volumes
7 rows| Measure | Value / definition |
|---|---|
| Tidal volume | ~500 mL per resting breath |
| Anatomical dead space | ~150 mL — conducting airways, no exchange |
| Inspiratory reserve volume | Extra air you can inhale beyond tidal |
| Expiratory reserve volume | Extra air you can force out beyond tidal |
| Vital capacity | Tidal + IRV + ERV = 4.6 L men, 3.1 L women |
| Residual volume | Never exhaled; prevents collapse |
| Total lung capacity | Vital capacity + residual volume |
How each group breathes
6 rows| Group | Organ | Mechanism | Efficiency note |
|---|---|---|---|
| Bony fish | Gills under an operculum | Buccal + opercular pumping, or ram ventilation | Countercurrent (180°) — most efficient of all |
| Arthropods | Tracheae and tracheoles | Air enters valved spiracles | Air reaches cells directly, no blood transport |
| Amphibians | Simple saclike lungs + skin | Positive pressure — pushes air in | Low surface area; skin supplements |
| Reptiles | Lungs | Negative pressure via rib cage | Dry skin blocks cutaneous exchange |
| Mammals | Alveolar lungs | Negative pressure via diaphragm + intercostals | Huge area (80 m²) but two-way flow mixes stale air |
| Birds | Parabronchi + air sacs | One-way flow over two cycles | Crosscurrent (90°) — best of the land vertebrates |
How carbon dioxide is carried
3 rows| Form | Share | Detail |
|---|---|---|
| Bicarbonate (HCO₃⁻) | 72% | Made in red blood cells by carbonic anhydrase; the main plasma buffer |
| Bound to hemoglobin | 20% | Carbaminohemoglobin — binds protein, not iron, so no competition with O₂ |
| Dissolved in plasma | 8% | Simple physical solution |
Hemoglobin saturation
3 rows| Condition | P_O₂ | Saturation | Unloaded |
|---|---|---|---|
| Arterial blood | 100 mm Hg | 97% | — |
| Venous blood at rest | 40 mm Hg | 75% | 22% |
| Venous blood during exercise | 20 mm Hg | 35% | 62% |
Exam traps
10 pairsQuestions get built out of near-misses. If you can state each difference in one sentence, you will not lose those points.
Oxygen is limiting in FISHvsCarbon dioxide is limiting in MAMMALS
Water oxygen swings with depth, temperature, and flow, so fish monitor O₂. Air oxygen is essentially constant, so mammals monitor CO₂ instead. Expect this comparison on the exam.
Countercurrent (fish, 180°)vsCrosscurrent (birds, 90°)
Countercurrent is the most efficient arrangement that exists. Crosscurrent is second — better than a mammal lung, not as good as a gill.
Positive pressure breathingvsNegative pressure breathing
Amphibians PUSH air in with the mouth floor. Reptiles, birds, and mammals PULL air in by expanding the chest. Only amphibians push.
Light on percentage vs pressure at altitudevsThe oxygen fraction never changes
Air is 20.95% oxygen at sea level AND on Everest. What falls is total pressure, and therefore P_O₂. Do not say "there is less oxygen in the air."
Bohr effect shifts the curve RIGHTvsA left shift means tighter binding
Lower pH and higher temperature = shift RIGHT = hemoglobin releases oxygen more easily. Exercising muscle creates both conditions on purpose.
HemoglobinvsMyoglobin
Hemoglobin: 4 chains, 4 O₂, in blood. Myoglobin: 1 chain, 1 O₂, in muscle, and it has HIGHER affinity — which is why it holds oxygen back as a last reserve.
CO₂ binds hemoglobin proteinvsO₂ binds the heme iron
Different sites, so they do not compete. But CO₂ binding changes hemoglobin's shape and lowers its oxygen affinity anyway.
HyperventilationvsHeavy breathing during exercise
Hyperventilation means P_CO₂ drops abnormally low. Exercise breathing matches a real metabolic demand and keeps blood gases normal — that is not hyperventilation.
Vital capacityvsTotal lung capacity
Vital capacity is everything you can move. Total lung capacity adds the residual volume you can never exhale.
Trachea (vertebrate)vsTracheae (arthropod)
Same word, unrelated structures — not homologous. One is a windpipe, the others are air tubes reaching cells directly.