B3.1 Gas exchange Notes

B3.1 Gas exchange

• Blood vessels form a characteristic "tree" pattern as they branch out into a network to feed tissues with oxygen and nutrients, while removing waste products. A fundamental challenge for multicellular organisms is distributing nutrients and removing wastes to cells that are interior and not in direct contact with the environment.
• Both animals and plants have evolved adaptations that combine a respiratory gas exchange system with a fluid transport system. A common example is lungs for gas exchange coupled with a circulatory system for transport. Plants also exchange atmospheric gases for respiration and photosynthesis.
• Guiding questions: How are multicellular organisms adapted to carry out gas exchange? What are the similarities and differences in gas exchange between a flowering plant and a mammal?

B3.1.1 Gas exchange as a vital function in all organisms

• The challenge of delivering gases increases with organism size because the surface area‑to‑volume ratio decreases as size increases, and the distance from the exterior to interior tissues increases.
• Most organisms are aerobic: they require oxygen to metabolize energy from organic substances (e.g., glucose) and must remove metabolic waste products such as carbon dioxide.
• Some small organisms (e.g., many single-celled life forms) can exchange O2 and CO2 directly with the environment across their plasma membranes. Larger multicellular organisms cannot rely on direct exchange due to interior, metabolically active tissues.
• These organisms have evolved complex adaptations to exchange respiratory gases between the atmosphere or water and their tissues.
• Example: axolotl (Ambystoma mexicanum) – an endangered freshwater salamander with six external gills for gas exchange with water.
• Key implication: surface area-to-volume constraints drive the need for specialized exchange surfaces and transport systems.

B3.1.2 Gas exchange surfaces

• Gas exchange surfaces must be:
  • relatively permeable to respiratory gases
  • thin (often a single cell layer) to minimize diffusion distances
  • moist to facilitate diffusion
  • large in surface area to maximize diffusion capacity
• Specialized tissues for gas exchange are found in the skin (some small organisms), gills (aquatic organisms), and lungs (larger terrestrial organisms).
• Gas exchange can occur between air and living tissue (lungs) or between water and living tissue (gills).
• In many organisms, respiratory gases are exchanged directly with blood vessels (often capillaries) for circulation to body tissues.

B3.1.3 Concentration gradients at exchange surfaces in animals

• Gas exchange relies on diffusion, which requires maintained concentration gradients: O2 diffuses into blood, CO2 diffuses out.

• To maintain gradients, animals need:

  • dense networks of blood vessels and continuous blood flow
  • ventilation: air movement for lungs or water movement for gills

• Example concept (gills): two fluids are involved – environmental water over the gill tissue and blood in capillaries. Water O2 concentration is kept relatively high compared to blood O2, enabling diffusion into the blood; CO2 diffuses from blood into the water.

• Blood leaving body tissues is high in CO2 and low in O2, so when it reaches the gills, gas exchange occurs again to restore gradients.

• Note: animals that use gills are generally ectothermic (cold-blooded). The relatively high metabolic rate required for constant internal temperature would not be supported by the low O2 levels in water.

• The only vessels that permit exchange are capillaries (one cell thick).

B3.1.4 Gas exchange in mammalian lungs

• Key adaptations for gas exchange in mammalian lungs (alveolar lungs):
  • surfactant to reduce surface tension and prevent alveolar collapse
  • a branched network of bronchioles to distribute air
  • extensive capillary beds surrounding alveoli for rapid diffusion
  • a very large surface area via millions of alveoli
• Structure and function:
  • Lungs subdivide into lobes, which further subdivide into millions of alveoli connected by bronchioles.
  • Each alveolus is lined by a thin film and is surrounded by dense capillaries; diffusion occurs across two thin cell layers: alveolar epithelium and capillary endothelium.
  • The inner surface of each alveolus contains surfactant produced by specialized alveolar cells to lower surface tension and prevent collapse during expiration.
  • The alveolar-capillary membrane is extremely thin (two cell layers) to facilitate diffusion of O2 into blood and CO2 into the alveolar air.
  • The air in alveoli is refreshed with each inhalation/exhalation; the capillary network around alveoli is dense to maintain gradients and enable efficient gas exchange.
• Alveolar count note: Each lung contains about 300 million alveoli.

B3.1.5 Lung ventilation

• Breathing involves the diaphragm, intercostal muscles, abdominal muscles, and ribs. The lungs themselves are passive; they do not actively move.
• Mechanics rely on the inverse relationship between pressure and volume (Boyle's law): P ext{ is inversely proportional to } V ext{ (at fixed temperature)} or more explicitly, P1 V1 = P2 V2.
• Process for inspiration (inhalation):
  1. The diaphragm contracts, increasing the volume of the thoracic cavity.
  2. External intercostal muscles and abdominal muscles contract to raise the rib cage, further increasing thoracic volume.
  3. Increased thoracic volume lowers intrathoracic pressure, reducing pressure on the lungs.
  4. Lungs expand, lowering intrapulmonary pressure and creating a partial vacuum.
  5. Air flows into the respiratory tract to equalize pressure, filling the alveoli.
• During exercise, these steps occur more rapidly and with greater amplitude to achieve deeper breathing.
• Expiration is largely a passive process at rest but involves active muscle action during heavy exercise: the diaphragm relaxes and internal intercostals contract to increase intrathoracic pressure and expel air.
• Challenge: Outline the steps of expiration using the five-step framework; Step 1 is given: The diaphragm relaxes, decreasing the volume of the thoracic cavity. Other steps involve contraction of internal intercostal muscles and abdominal muscles to decrease thoracic volume and increase pressure, forcing air out.

B3.1.6 Lung volume

• Spirometry measures lung volumes: tidal volume, inspiratory reserve volume, expiratory reserve volume, and vital capacity.
• Definitions:
  • Tidal volume: the volume of air breathed in or out during a typical resting breath.
  • Inspiratory reserve volume: maximum volume of air that can be inhaled beyond the tidal volume.
  • Expiratory reserve volume: maximum volume of air that can be exhaled beyond the tidal volume.
  • Vital capacity: the sum of inspiratory reserve volume, tidal volume, and expiratory reserve volume.
• Practical activity: Use a spirometer to measure these volumes and follow the stepwise procedure to determine tidal volume, inspiratory reserve volume, expiratory reserve volume, and vital capacity. An example exercise uses measurements shown in a labeled figure (Figure 4).
• Health context: According to the Institute for Health Metrics and Evaluation (IHME), 13% of deaths worldwide are directly due to smoking, with an additional 2% due to second-hand smoke. Lung cancer and COPD are among the principal diseases linked to these fatalities.

B3.1.7 Gas exchange in leaves

• Leaves are adapted for gas exchange with the atmosphere, balancing cell respiration and photosynthesis.
• Core processes:
  • Cell respiration: ext{glucose} + ext{O}2 ightarrow ext{CO}2 + ext{H}_2 ext{O}
  • Photosynthesis: ext{CO}2 + ext{H}2 ext{O}
    ightarrow ext{glucose} + ext{O}_2
• During daylight, photosynthesis can outpace respiration, leading to net production of oxygen and consumption of CO2.
• Leaf adaptations facilitating gas exchange:
  • Wax cuticle to reduce water loss
  • Upper epidermis secreting the cuticle
  • Palisade mesophyll: densely packed cells with many chloroplasts for light capture
  • Spongy mesophyll: loosely packed with air spaces to maximize diffusion surface area
  • Veins containing xylem and phloem for water transport and sugar distribution
  • Lower epidermis containing stomata guarded by guard cells; stomata regulate gas exchange and water loss
  • Stomata allow CO2 intake and release of O2 and water vapor; stomata tend to close at night to limit water loss
• Stomatal distribution and function depend on environmental conditions to balance CO2 uptake with water conservation.

B3.1.8 Leaf tissue distribution

• Students should be able to draw and label a plan diagram of the distribution of tissues in a transverse section of a dicotyledonous leaf (Figure 5).
• Note: The IB expects a plan diagram showing epidermis, palisade and spongy mesophyll, veins, and stoma distribution; detailed cellular morphology is not required for the plan diagram.

B3.1.9 Transpiration

• Transpiration is the evaporation of water from open stomata as a consequence of gas exchange for photosynthesis.
• Open stomata permit CO2 entry for photosynthesis, but also allow water vapor and O2 to diffuse out.
• Factors affecting the rate of transpiration (Table 1):
  • Environmental factor: Increased light → increases transpiration because light stimulates guard cells to open stomata and increases photosynthesis, which requires CO2 intake.
  • Increased temperature → increases transpiration (higher molecular movement and diffusion rates).
  • Increased wind speed → increases transpiration by removing water vapor from stomatal openings, increasing the internal-external water vapor gradient.
  • Increased humidity → decreases transpiration by reducing the water vapor gradient.
• Note: If there is no light, stomata tend to close and transpiration may be zero, limiting the other factors from having an effect.
• Challenge: A stomatal density calculation exercise (Figure 6) prompts calculating density in stomata per mm^2, using scale bar measurements and image dimensions, and to report as a whole number.
• Scientific note: Natural variation in stomatal density is expected; replicates increase reliability of measurements.

B3.1.10 Stomata

• Stomata are microscopic openings on the lower surface of leaves, each consisting of two guard cells.
• Guard cells regulate opening and closing, thus controlling gas exchange and water loss.
• Stomata density varies among species and within a species depending on long-term environmental factors. Measurements can be expressed as either stomata per mm^2 or stomata per μm^{-2}.
• Figure 6 shows a micrograph with a scale bar for estimating stomatal density.
• Practical notes: For optical measurements, consider area calculations (field of view) and use micrometry or scale bars to convert to actual densities.

B3.1.11 Haemoglobin and oxygen transport

• Haemoglobin (Hb) is the protein in red blood cells responsible for carrying most of the oxygen in the bloodstream. Each erythrocyte contains haemoglobin, which reversibly binds oxygen and carbon dioxide.
• Haemoglobin structure:
  • Four polypeptides form a quaternary structure (refer to Chapter B1.2).
  • Each polypeptide contains a haem group with an iron atom at its centre.
  • Each haem group can reversibly bind to one O2 molecule; with four haem groups, Hb can carry up to four O2 molecules when saturated: ext{Hb} + 4 ext{O}2 ightarrow ext{Hb(O}2)_4.
• Saturation refers to the state where all four haem groups are bound to oxygen.
• Cooperative binding: Oxygen binding to one haem group increases the affinity of the remaining haem groups for O2, creating a sigmoidal oxygen-hemoglobin dissociation curve (conceptual, see Chapter B1.2).
• Allosteric binding of carbon dioxide: CO2 binding can influence Hb conformation and oxygen affinity (foetal vs. adult haemoglobin illustrate adaptations for efficient oxygen transport in varying conditions).
• Foetal haemoglobin has different oxygen affinity to optimize transfer of O2 from the maternal to the fetal circulation.

Connections and implications

• Gas exchange and transport systems are tightly integrated: diffusion at exchange surfaces is only effective when combined with ventilation and circulation to maintain concentration gradients.
• Structure–function relationships are central: thin, moist, highly vascular surfaces with large surface areas maximize diffusion; alveolar surfaces and stomata are examples where structure supports function.
• Ethical/practical relevance: health statistics (e.g., smoking-related deaths) highlight the real-world impact of altered gas exchange on human health (lung cancer, COPD).
• Environmental and evolutionary context: leaf gas exchange and transpiration reflect plant adaptations to water availability, light, and CO2; animal gas exchange systems reflect ecological and metabolic demands (endothermy vs ectothermy, aquatic vs terrestrial habitats).
• Quantitative skills: measurements of lung volumes via spirometry and stomatal density demonstrate how quantitative data underpin physiological understanding and cross-species comparisons.

Key formulas and constants

• Boyle's law (ventilation mechanism): P1 V1 = P2 V2
• Cellular respiration in leaves (balanced form):
  • Respiration: ext{glucose} + ext{O}2 ightarrow ext{CO}2 + ext{H}_2 ext{O} + ext{energy}
  • Photosynthesis: ext{CO}2 + ext{H}2 ext{O}
    ightarrow ext{glucose} + ext{O}_2
• Haemoglobin oxygen-carrying capacity: ext{Hb} + 4 ext{O}2 ightarrow ext{Hb(O}2)_4

Glossary of key terms

• Gas exchange surface, diffusion, concentration gradient, ventilation, perfusion, capillary, alveolus, surfactant, diffusion distance, surface area-to-volume ratio, transpiration, stomata, guard cell, stomatal density, spirometer, tidal volume, inspiratory reserve volume, expiratory reserve volume, vital capacity, haemoglobin, erythrocyte, cooperative binding, allosteric binding, foetal haemoglobin, capillary endothelium, alveolar epithelium, xylem, phloem