47.1
Gas Exchange Across Respiratory Surfaces
pp. 1045–1046- The universal problem: get enough O₂ for mitochondria and dump the CO₂ that cellular respiration produces. Watch The Respiratory System on YouTube, from Bozeman Science
- Respiration has three distinct phases. External respiration (ventilation) runs the medium — air or water — past the respiratory membrane. Internal respiration carries O₂ and CO₂ in the blood between the respiratory organ and the tissues. Cellular respiration is the actual ATP-making chemistry. Watch Respiratory System on YouTube, from Amoeba Sisters
- Plasma membranes need water to stay stable, so the exchange environment is always aqueous. Even in a lung, oxygen must first dissolve in the fluid layer coating the epithelium.
- Diffusion here is entirely passive — driven by the concentration difference and the gases' solubility in the membrane. For dissolved gases, concentration is expressed as pressure. Watch Fick's law of diffusion | Respiratory system physiology | NCLEX-RN | Khan Academy on YouTube, from khanacademymedicine
- Fick's Law of Diffusion: R = (D · A · Δp) / d, where R is rate, D is a molecule-specific diffusion constant (size, membrane permeability, temperature), A is area, Δp is the pressure difference, and d is distance. Watch Fick's law of diffusion | Respiratory system physiology | NCLEX-RN | Khan Academy on YouTube, from khanacademymedicine
- That equation gives exactly three ways to improve gas exchange: raise A, raise Δp, or shrink d. Every respiratory adaptation in animals does one of these. Watch Fick's law of diffusion | Respiratory system physiology | NCLEX-RN | Khan Academy on YouTube, from khanacademymedicine
- Pure diffusion cannot supply oxygen further than about 0.5 mm, which caps the body size of anything without a respiratory system. Bacteria, archaea, and protists get by; larger animals do not.
- Many invertebrates lack specialized organs but beat cilia to keep fresh water flowing over the surface. This constantly replaces oxygen-depleted water, holding Δp high.
- Mollusks, arthropods, echinoderms, and vertebrates evolved actual respiratory organs — gills, tracheae, lungs — which raise A and cut d by bringing the outside medium close to circulating internal fluid. Watch The Respiratory System on YouTube, from Bozeman Science
47.2
Gills, Cutaneous Respiration, and Tracheal Systems
pp. 1046–1049- Gills are tissue extensions projecting into water, from simple echinoderm papulae to the highly folded gills of fish. They give aquatic animals far more exchange area than bare skin could. Watch Fish Gills and Counter Current Exchange on YouTube, from Jay Hosler
- External gills (larval fish and salamanders, and adult axolotls) sit outside the body. Two drawbacks: they must be waved constantly to meet fresh oxygenated water, and their thin epithelium damages easily. That is why most animals tuck gills into a protected chamber.
- Branchial chambers pump water past stationary gills. In mollusks the muscular mantle cavity pulls water in through an inhalant siphon and pushes it out an exhalant siphon. In crustaceans the chamber lies between body and exoskeleton, ventilated by limb movement.
- Bony fish hold gills between the buccal (oral) cavity and the opercular cavities, covered by the operculum. The two cavities act as alternating pumps: mouth opens and jaw drops to draw water in, then mouth closes and operculum opens to push water across the gills. Watch Fish Gills and Counter Current Exchange on YouTube, from Jay Hosler
- Continuous swimmers like tuna have nearly immobile opercula and use ram ventilation — swimming with the mouth ajar. The remora rides sharks and switches between ram ventilation and opercular pumping depending on whether the shark is moving.
- Gill anatomy: three to seven gill arches per side, each bearing two rows of gill filaments, each filament carrying thin plates called lamellae that stick into the water flow. Watch Fish Gills and Counter Current Exchange on YouTube, from Jay Hosler
- Countercurrent flow is the key: water moves one direction across the lamella while blood moves the opposite direction inside it. A positive oxygen gradient is therefore maintained along the entire diffusion path, keeping Δp favorable everywhere. Blood leaves at roughly 85% saturation while incoming water is 100% and outflowing water is down to 15%. Watch Fish Gills and Counter Current Exchange on YouTube, from Jay Hosler
- If flow were concurrent, the gradient would collapse quickly and net diffusion would stop once both sides reached about 50%. Countercurrent exchange makes fish gills the most efficient respiratory organ known. Watch Fish Gills and Counter Current Exchange on YouTube, from Jay Hosler
- Fish gills carry neuroepithelial cells that sense water oxygen, which varies with depth, temperature, and flow. Falling oxygen triggers behavioral change, faster respiration, and recruitment of more capillary beds. Hence oxygen is the limiting blood gas in fish.
- Cutaneous respiration — exchange across thin, highly vascularized skin — needs constant moisture. It supplements lungs in most amphibians, and plethodontid salamanders rely on it exclusively. Watch L4: Respiratory system & Types of Respiration in Frog (cutaneous, pulmonary & buccopharyngeal) on YouTube, from Ozone Classes by Vipin Sharma Sir
- Terrestrial reptiles have dry scaly skin that blocks cutaneous respiration, but aquatic ones exploit it: soft-shelled turtles exchange gas across throat lining, pond sliders stay submerged for days in winter, and some Australian turtles pump water into the cloaca to use skin papillae there.
- Terrestrial arthropods have no single respiratory organ. Branched, cuticle-lined tracheae subdivide into tracheoles that touch individual cells, so oxygen diffuses straight across the plasma membrane with no blood transport needed. Watch A-Level Biology - Gas Exchange in Insects - Spiracles | Tracheae | Tracheoles (2026/27 exams) on YouTube, from Cognito
- Air enters through spiracles, exoskeletal openings with valves. Being able to close them to stop water loss was a key adaptation for life on land. Watch A-Level Biology - Gas Exchange in Insects - Spiracles | Tracheae | Tracheoles (2026/27 exams) on YouTube, from Cognito
47.3
Lungs
pp. 1049–1051- Gills were abandoned on land for two reasons. First, air does not support the delicate lamellae — a fish out of water suffocates surrounded by oxygen because its gills collapse into a clump. Second, water evaporates, so a gill's huge surface area would be a fatal dehydration risk. Watch The Respiratory System on YouTube, from Bozeman Science
- A lung solves both: it is internal, so the body provides structural support, and the branched tubing saturates incoming air with water vapor before it reaches the thin exchange membrane.
- Unlike the one-way flow across gills, air in a mammalian lung moves in and out through the same passages — a two-way system. Birds are the exception. Watch How do Birds Breathe? | Avian Respiration Animation on YouTube, from CableMuseum
- Dry air is 78.09% N₂, 20.95% O₂, 0.93% argon and inert gases, 0.03% CO₂, and stays that composition up to at least 100 km, even though the number of molecules falls with altitude. Watch Effect of Altitude on Atmospheric Pressure and Oxygen Partial Pressure on YouTube, from Vivo Phys - Evan Matthews
- At sea level a barometer reads 760 mm Hg = 1.0 atm. Each gas contributes its own partial pressure in proportion to its share: P_N₂ = 593.5, P_O₂ = 159.2, P_CO₂ = 0.2 mm Hg. Watch Effect of Altitude on Atmospheric Pressure and Oxygen Partial Pressure on YouTube, from Vivo Phys - Evan Matthews
- Above about 6000 m humans cannot survive long. Air is still 20.95% oxygen, but total pressure has fallen to ~380 mm Hg, so P_O₂ ≈ 80 mm Hg — half the sea-level value. Watch Effect of Altitude on Atmospheric Pressure and Oxygen Partial Pressure on YouTube, from Vivo Phys - Evan Matthews
- Amphibian lungs are simple saclike outpouchings of the gut with limited folding. Each connects to the pharynx through a valve, the glottis. Watch L4: Respiratory system & Types of Respiration in Frog (cutaneous, pulmonary & buccopharyngeal) on YouTube, from Ozone Classes by Vipin Sharma Sir
- Amphibians use positive pressure breathing: fill the mouth with air, seal mouth and nostrils, then raise the floor of the oral cavity to force air into the lungs — like mouth-to-mouth resuscitation. Watch L4: Respiratory system & Types of Respiration in Frog (cutaneous, pulmonary & buccopharyngeal) on YouTube, from Ozone Classes by Vipin Sharma Sir
- Most reptiles use negative pressure breathing: expand the rib cage, lower the internal pressure, and let atmospheric pressure push air in — like drinking through a straw. Watch Respiratory | Mechanics of Breathing: Pressure Changes | Part 1 on YouTube, from Ninja Nerd
- Endotherms (birds and mammals) run high metabolic rates and need more oxygen, so they evolved far more elaborate lungs than ectothermic amphibians and reptiles.
- Mammalian lungs pack millions of alveoli in grapelike clusters. Alveolar epithelium is one cell thick and the surrounding capillary wall is also one cell thick, so diffusion distance d is only 0.5–1.5 µm. Watch Respiratory System, Part 1: Crash Course Anatomy & Physiology #31 on YouTube, from CrashCourse Watch Fick's law of diffusion | Respiratory system physiology | NCLEX-RN | Khan Academy on YouTube, from khanacademymedicine
- Air path: mouth and nose → pharynx → larynx (voice box) → through the glottis between the vocal cords → trachea, held open by C-shaped cartilage rings → splits into right and left bronchi → bronchioles → alveoli. Watch Respiratory System, Part 1: Crash Course Anatomy & Physiology #31 on YouTube, from CrashCourse
- Each human lung holds about 300 million alveoli, giving up to 80 m² of diffusion surface — roughly 42 times the body's surface area. Watch Respiratory System, Part 1: Crash Course Anatomy & Physiology #31 on YouTube, from CrashCourse
- Birds have the most efficient respiration of any land vertebrate. Their lungs do not end in blind alveoli; air flows one way through tiny parabronchi. Watch How do Birds Breathe? | Avian Respiration Animation on YouTube, from CableMuseum
- Unidirectional flow comes from anterior and posterior air sacs. Over two cycles: inhale → fresh air fills the posterior sacs while the anterior sacs draw air out of the lungs; exhale → posterior sac air is pushed into the lungs while anterior sac air leaves the body. So fresh air never mixes with stale air. Watch How do Birds Breathe? | Avian Respiration Animation on YouTube, from CableMuseum
- Blood in the avian lung runs at 90° to the airflow — crosscurrent exchange. Less efficient than the 180° countercurrent of fish gills, but better at extracting oxygen than a mammalian lung. Watch How do Birds Breathe? | Avian Respiration Animation on YouTube, from CableMuseum
- The payoff: a sparrow can be active at 6000 m, where a mouse of similar mass and metabolic rate would quickly die of hypoxia. Watch Effect of Altitude on Atmospheric Pressure and Oxygen Partial Pressure on YouTube, from Vivo Phys - Evan Matthews
47.4
Structures, Mechanisms, and Control of Ventilation in Mammals
pp. 1052–1055- A mammalian lung holds billions of capillaries, roughly 100 per alveolar sac — effectively an air bubble entirely bathed in blood.
- The gradient that drives loading: blood returning from the body has P_O₂ ≈ 40 mm Hg while alveolar gas is ≈ 105, a Δp of 65 mm Hg. Blood leaving the lungs reaches about 100 mm Hg — very effective, not quite perfect.
- Full partial-pressure picture: alveolar gas 105 / 40 (O₂/CO₂); blood entering the lungs 40 / 46; blood leaving the lungs 100 / 40; blood after unloading at the tissues back to 40 / 46.
- Each lung is wrapped in the visceral pleural membrane; the parietal pleural membrane lines the thoracic wall. Fluid in the thin pleural cavity between them makes the two adhere, coupling lung to chest wall. Because each lung is packaged separately, one can collapse while the other keeps working. Watch Respiratory | Mechanics of Breathing: Pressure Changes | Part 1 on YouTube, from Ninja Nerd
- Inhalation is active: the external intercostal muscles raise the ribs and the diaphragm — a convex striated sheet between thorax and abdomen — contracts and flattens. Thoracic volume rises, pressure falls, and air is pulled in: negative pressure ventilation. Watch Respiratory | Mechanics of Breathing: Pressure Changes | Part 1 on YouTube, from Ninja Nerd
- Quiet exhalation is passive: relaxing those muscles lets the elastic tension stored during inhalation recoil. Forceful exhalation adds active abdominal muscle contraction, as when blowing up a balloon. Watch Respiratory | Mechanics of Breathing: Pressure Changes | Part 1 on YouTube, from Ninja Nerd
- Pressure numbers across the cycle: at rest intrapulmonary pressure equals atmospheric and no air moves; on inspiration intrapulmonary pressure drops about 3 mm Hg below atmospheric; on expiration it rises about 3 mm Hg above. Intrapleural pressure stays below intrapulmonary throughout. Watch Respiratory | Mechanics of Breathing: Pressure Changes | Part 1 on YouTube, from Ninja Nerd
- Tidal volume — air moved per resting breath — averages 500 mL. About 150 mL of that never reaches an alveolus; it sits in the anatomical dead space of trachea, bronchi, and bronchioles, and mixes with fresh air on the next breath. That mixing is a major reason mammals are less efficient than birds. Watch Respiratory | Spirometry: Lung Volumes & Capacities on YouTube, from Ninja Nerd
- Inspiratory reserve volume (IRV) is what you can inhale beyond a normal breath; expiratory reserve volume (ERV) is what you can force out beyond it. Watch Respiratory | Spirometry: Lung Volumes & Capacities on YouTube, from Ninja Nerd
- Vital capacity = tidal volume + IRV + ERV, averaging 4.6 L in young men and 3.1 L in young women. Abnormally low vital capacity signals alveolar damage or pulmonary disease. Watch Respiratory | Spirometry: Lung Volumes & Capacities on YouTube, from Ninja Nerd
- Residual volume is air that can never be exhaled; it keeps the lungs from collapsing. Total lung capacity = vital capacity + residual volume. Watch Respiratory | Spirometry: Lung Volumes & Capacities on YouTube, from Ninja Nerd
- Hypoventilation is breathing too little, so blood P_CO₂ rises — the rise in P_CO₂ is the best single indicator. Hyperventilation is breathing beyond metabolic need, so P_CO₂ falls abnormally. Heavy breathing during exercise is not hyperventilation, because it matches a genuinely higher metabolic rate and blood gases stay normal. Watch Respiratory | Regulation of Breathing: Central & Peripheral Chemoreceptors: Part 2 on YouTube, from Ninja Nerd
- Every breath is initiated by neurons in the respiratory control center of the medulla oblongata and pons. They fire the diaphragm and intercostals to inhale; when they stop, the muscles relax and exhalation follows. Though these are skeletal muscles, control is automatic — but can be voluntarily overridden briefly. Watch Respiratory | Regulation of Breathing: Central & Peripheral Chemoreceptors: Part 2 on YouTube, from Ninja Nerd
- In mammals carbon dioxide is the limiting blood gas, because atmospheric oxygen barely varies. Rising P_CO₂ produces more carbonic acid (H₂CO₃), lowering blood pH. Watch Respiratory | Regulation of Breathing: Central & Peripheral Chemoreceptors: Part 2 on YouTube, from Ninja Nerd
- Falling pH is detected by peripheral chemoreceptors in the aortic and carotid bodies and by central chemoreceptors in the brain that monitor cerebrospinal fluid pH. Both signal the medullary control center to increase ventilation. Watch Respiratory | Regulation of Breathing: Central & Peripheral Chemoreceptors: Part 2 on YouTube, from Ninja Nerd
- You cannot voluntarily hyperventilate for long: dropping plasma and CSF P_CO₂ suppresses the drive to breathe. Hyperventilating before a breath-hold buys time by lowering CO₂, not by storing extra oxygen. Watch Respiratory | Regulation of Breathing: Central & Peripheral Chemoreceptors: Part 2 on YouTube, from Ninja Nerd
- Low P_O₂ only becomes a major breathing stimulus at altitude. Mountain sickness — weakness, headache, nausea, vomiting, impaired thinking — comes from low P_O₂ and often resolves with supplemental oxygen. Watch Effect of Altitude on Atmospheric Pressure and Oxygen Partial Pressure on YouTube, from Vivo Phys - Evan Matthews
- COPD (chronic obstructive pulmonary disease) is any long-term airflow obstruction; the major forms are asthma, chronic bronchitis, and emphysema. In asthma an allergen triggers histamine release and intense bronchial constriction.
- In emphysema alveolar walls break down, leaving fewer but larger alveoli, and the lung becomes fibrotic and less elastic. Airways open on inhalation but collapse on exhalation, trapping air. Sufferers spend three to four times the normal energy just breathing. 80–90% of emphysema deaths trace to cigarette smoking.
- Lung cancer kills more people than any other cancer. Over 90% of tumors start in the mucous membranes of the large bronchi. Because it metastasizes before symptoms alarm anyone — coughing being routine for smokers — only about 3% of patients survive five years.
47.5
Transport of Gases in Body Fluids
pp. 1056–1059- Oxygen is barely soluble: plasma alone can hold only about 3 mL of O₂ per liter. Whole blood carries nearly 200 mL per liter, because almost all of it rides on hemoglobin inside red blood cells. Watch Respiratory | Oxygen-Hemoglobin Dissociation Curve on YouTube, from Ninja Nerd
- Hemoglobin is four polypeptide chains — two α and two β — each paired with a heme group whose central iron atom binds one O₂. So one hemoglobin carries up to four oxygen molecules. Watch Respiratory | Oxygen-Hemoglobin Dissociation Curve on YouTube, from Ninja Nerd
- Loaded in the lungs it becomes bright red oxyhemoglobin; after releasing oxygen in the tissues it becomes darker deoxyhemoglobin, which gives tissues a bluish cast.
- Two other oxygen carriers exist. Hemocyanins are copper-based and found in Mollusca and Arthropoda; they float free in the circulating fluid, form enormous complexes with up to 100 binding sites, and show extremely high cooperativity — excellent in low-oxygen settings. Hemerythrins are iron-based and found mainly in several worm phyla.
- All these carriers predate the Cambrian. They likely arose as free O₂ from photosynthesis rose, first to sequester a dangerously reactive molecule and later to transport it once metabolism began using it as the final electron acceptor.
- Saturation numbers to know: at arterial P_O₂ 100 mm Hg, hemoglobin is about 97% saturated. At resting venous P_O₂ 40, it is 75% — so only 22% of the oxygen was unloaded, leaving four-fifths in reserve. Watch Respiratory | Oxygen-Hemoglobin Dissociation Curve on YouTube, from Ninja Nerd
- During exercise venous P_O₂ can fall to 20 mm Hg, where saturation is only 35%, so 62% is unloaded. This is what the oxyhemoglobin dissociation curve is showing you. Watch Respiratory | Oxygen-Hemoglobin Dissociation Curve on YouTube, from Ninja Nerd
- That reserve keeps the blood oxygenated enough to sustain life for 4 to 5 minutes if breathing or the heartbeat stops.
- Myoglobin is a second reserve inside muscle cells: a single polypeptide with one iron atom and a higher oxygen affinity than hemoglobin, so it holds oxygen until hemoglobin supply is exhausted. Deep-diving mammals like elephant seals carry huge myoglobin stores.
- The Bohr effect: CO₂ from working tissue forms carbonic acid, which dissociates and lowers pH; H⁺ binding to hemoglobin reduces its oxygen affinity and shifts the dissociation curve to the right, dumping more oxygen. Watch Respiratory | Oxygen-Hemoglobin Dissociation Curve on YouTube, from Ninja Nerd
- Higher temperature shifts the curve right too. Since exercising muscle makes both extra CO₂ and extra heat, it automatically pulls more oxygen out of the blood exactly where it is needed. Watch Respiratory | Oxygen-Hemoglobin Dissociation Curve on YouTube, from Ninja Nerd
- CO₂ travels three ways: about 8% dissolved in plasma, 20% bound to hemoglobin's protein portion as carbaminohemoglobin, and 72% as bicarbonate. Watch Transport of Carbon Dioxide and Chloride Shift on YouTube, from Andrey K
- Crucially, CO₂ binds the protein, not the iron, so it does not compete with oxygen for binding sites — but it does change hemoglobin's shape and lower its oxygen affinity. Watch Transport of Carbon Dioxide and Chloride Shift on YouTube, from Andrey K
- In red blood cells, carbonic anhydrase catalyzes CO₂ + H₂O → H₂CO₃, which dissociates into HCO₃⁻ and H⁺. Removing CO₂ this way keeps the diffusion gradient steep so more CO₂ keeps entering from tissues. Watch Transport of Carbon Dioxide and Chloride Shift on YouTube, from Andrey K
- The chloride shift: HCO₃⁻ leaves the red blood cell through a transporter that brings one Cl⁻ in for each HCO₃⁻ out. Bicarbonate then serves as the major buffer of blood plasma. Watch Transport of Carbon Dioxide and Chloride Shift on YouTube, from Andrey K
- In the lungs the low alveolar P_CO₂ runs the carbonic anhydrase reaction backwards, regenerating CO₂ gas that diffuses into the alveoli and leaves on the next exhalation. Watch Transport of Carbon Dioxide and Chloride Shift on YouTube, from Andrey K
- Hemoglobin also carries nitric oxide (NO), important in vessel dilation. Carbon monoxide (CO) binds hemoglobin far more tightly than oxygen — the reason CO poisoning is lethal, and why victims often have bright red skin.