Cram sheet
30 points- Three phases of respiration: external respiration (= ventilation, moving air or water past the respiratory membrane), internal respiration (transporting O₂ and CO₂ in blood between organs and tissues), and cellular respiration (using O₂ to make ATP). Watch Respiratory System on YouTube, from Amoeba Sisters
- Fick's Law of Diffusion: R = D·A·Δp / d. Rate goes UP with surface area and pressure difference, and DOWN with distance. Every respiratory adaptation in the animal kingdom is one of those three levers. Watch Fick's law of diffusion | Respiratory system physiology | NCLEX-RN | Khan Academy on YouTube, from khanacademymedicine
- Diffusion alone can only supply oxygen across about 0.5 mm. That limit is why anything bigger than a protist needs a respiratory system. Watch Fick's law of diffusion | Respiratory system physiology | NCLEX-RN | Khan Academy on YouTube, from khanacademymedicine
- Gas exchange is always aqueous — even in your lungs, oxygen dissolves in a thin fluid film before crossing the membrane. Watch The Respiratory System on YouTube, from Bozeman Science
- Fish gills: gill arch → gill filaments → lamellae. Water flows one way, blood flows the opposite way — countercurrent flow, which keeps a positive gradient the whole length. Blood leaves at ~85% saturation. Watch Fish Gills and Counter Current Exchange on YouTube, from Jay Hosler
- If flow were concurrent (same direction), both would equalize at 50% and diffusion would stop. Countercurrent is why fish gills are the most efficient respiratory organ that exists. Watch Fish Gills and Counter Current Exchange on YouTube, from Jay Hosler
- Oxygen is the limiting blood gas in fish (water O₂ varies a lot). Carbon dioxide is the limiting blood gas in mammals (air O₂ is basically constant). This contrast is a favorite exam question. Watch Respiratory | Regulation of Breathing: Central & Peripheral Chemoreceptors: Part 2 on YouTube, from Ninja Nerd
- Insects use tracheae → tracheoles delivering air directly to cells, entering through valved spiracles. Closable spiracles prevented water loss and helped arthropods colonize land. Watch A-Level Biology - Gas Exchange in Insects - Spiracles | Tracheae | Tracheoles (2026/27 exams) on YouTube, from Cognito
- Gills fail on land for two reasons: air does not support the lamellae (they collapse), and they would lose enormous amounts of water to evaporation. Watch The Respiratory System on YouTube, from Bozeman Science
- Air is 78.09% N₂, 20.95% O₂, 0.03% CO₂. At sea level total pressure = 760 mm Hg = 1 atm, so P_O₂ = 159 mm Hg. Watch Effect of Altitude on Atmospheric Pressure and Oxygen Partial Pressure on YouTube, from Vivo Phys - Evan Matthews
- At 6000 m the air is still 20.95% oxygen, but total pressure is only ~380 mm Hg, so P_O₂ ≈ 80 mm Hg — half of sea level. The percentage never changes; the pressure does. Watch Effect of Altitude on Atmospheric Pressure and Oxygen Partial Pressure on YouTube, from Vivo Phys - Evan Matthews
- Amphibians use positive pressure breathing — they push air in by raising the floor of the mouth. Reptiles, birds, and mammals use negative pressure breathing — they expand the chest and air gets pulled in. Watch L4: Respiratory system & Types of Respiration in Frog (cutaneous, pulmonary & buccopharyngeal) on YouTube, from Ozone Classes by Vipin Sharma Sir Watch Respiratory | Mechanics of Breathing: Pressure Changes | Part 1 on YouTube, from Ninja Nerd
- Mammalian air path: nose/mouth → pharynx → larynx → glottis → trachea (C-shaped cartilage rings) → bronchi → bronchioles → alveoli. Watch Respiratory System, Part 1: Crash Course Anatomy & Physiology #31 on YouTube, from CrashCourse Watch Respiratory System on YouTube, from Amoeba Sisters
- Each human lung has about 300 million alveoli, total surface ~80 m² — roughly 42× your body surface. Diffusion distance is only 0.5–1.5 µm. Watch Respiratory System, Part 1: Crash Course Anatomy & Physiology #31 on YouTube, from CrashCourse
- Birds are the most efficient land breathers: air moves one way through parabronchi using anterior and posterior air sacs, over two breathing cycles, so fresh air never mixes with stale air. Blood runs at 90° to airflow — crosscurrent (better than mammals, not as good as fish countercurrent). Watch How do Birds Breathe? | Avian Respiration Animation on YouTube, from CableMuseum
- Bird cycle: inhale → fresh air into posterior sacs; exhale → that air into the lungs; next inhale → into anterior sacs; next exhale → out the trachea. Watch How do Birds Breathe? | Avian Respiration Animation on YouTube, from CableMuseum
- Inhalation = diaphragm contracts and flattens + external intercostals raise the ribs → thorax expands → pressure drops → air flows in. Quiet exhalation is passive elastic recoil. Watch Respiratory | Mechanics of Breathing: Pressure Changes | Part 1 on YouTube, from Ninja Nerd
- The pleural cavity fluid between visceral and parietal pleura glues the lungs to the chest wall. Each lung is packaged separately, so one can collapse and the other still works. Watch Respiratory | Mechanics of Breathing: Pressure Changes | Part 1 on YouTube, from Ninja Nerd
- Tidal volume ≈ 500 mL, of which 150 mL sits in the anatomical dead space (trachea, bronchi, bronchioles) where no exchange happens. Watch Respiratory | Spirometry: Lung Volumes & Capacities on YouTube, from Ninja Nerd
- Vital capacity = tidal volume + IRV + ERV ≈ 4.6 L (young men) / 3.1 L (young women). Total lung capacity = vital capacity + residual volume (the air you can never exhale, which keeps the lungs from collapsing). Watch Respiratory | Spirometry: Lung Volumes & Capacities on YouTube, from Ninja Nerd
- Partial pressures to memorize: alveolar P_O₂ 105 / P_CO₂ 40; blood arriving from the body 40 / 46; blood leaving the lungs 100 / 40.
- Breathing is driven by neurons in the medulla oblongata and pons. Rising CO₂ → more carbonic acid → lower pH → sensed by central chemoreceptors (CSF pH) and peripheral chemoreceptors in the aortic and carotid bodies → breathe more. Watch Respiratory | Regulation of Breathing: Central & Peripheral Chemoreceptors: Part 2 on YouTube, from Ninja Nerd
- Hyperventilating before a breath-hold works by lowering CO₂, not by adding oxygen — it just takes longer for CO₂ to build back up. Watch Respiratory | Regulation of Breathing: Central & Peripheral Chemoreceptors: Part 2 on YouTube, from Ninja Nerd
- Hemoglobin = four polypeptide chains (2 α, 2 β), each with a heme group containing one iron atom → up to 4 O₂ per molecule. Plasma alone could only carry ~3 mL O₂/L; whole blood carries ~200 mL/L. Watch Respiratory | Oxygen-Hemoglobin Dissociation Curve on YouTube, from Ninja Nerd
- Saturation: arterial 97%; resting venous 75% (so only 22% is unloaded — the rest is a reserve); exercising venous can fall to 35% (62% unloaded). That reserve keeps you alive 4–5 minutes if breathing stops. Watch Respiratory | Oxygen-Hemoglobin Dissociation Curve on YouTube, from Ninja Nerd
- Myoglobin is a single chain with higher O₂ affinity than hemoglobin — a second reserve inside muscle. It is why diving seals stay under so long.
- Bohr effect: lower pH (more CO₂) and higher temperature shift the dissociation curve RIGHT, meaning hemoglobin lets go of oxygen more easily — exactly where exercising muscle needs it. Watch Respiratory | Oxygen-Hemoglobin Dissociation Curve on YouTube, from Ninja Nerd
- CO₂ transport, three ways: 8% dissolved in plasma, 20% bound to hemoglobin protein as carbaminohemoglobin (not to the iron, so it does not compete with O₂), and 72% as bicarbonate made by carbonic anhydrase in red blood cells. Watch Transport of Carbon Dioxide and Chloride Shift on YouTube, from Andrey K
- The chloride shift: for every HCO₃⁻ leaving the red blood cell, one Cl⁻ moves in. Bicarbonate is also the main buffer of blood plasma. Watch Transport of Carbon Dioxide and Chloride Shift on YouTube, from Andrey K
- Carbon monoxide binds hemoglobin far more tightly than oxygen — hence the danger, and the bright red skin of victims.