B3.1 Gas exchange

• Gas exchange supplies respiratory gases between organisms and environment

• Animals take in oxygen and release carbon dioxide for aerobic respiration

• Larger or active organisms need specialized exchange surfaces because SA:V decreases

• Efficient gas-exchange surfaces are permeable, thin, moist, and large

• Large surface area and steep concentration gradients increase diffusion

• Short diffusion paths speed movement of oxygen and carbon dioxide

• Dense capillary networks provide large exchange area and rapid transport

• Continuous blood flow carries oxygen away and carbon dioxide toward the surface

• Ventilation refreshes air or water to maintain steep gas gradients

• Many small alveoli provide large surface area and thin squamous walls

• Capillary beds maintain gradients and bring red blood cells close to alveolar air

• Surfactant reduces surface tension, while elastic fibres support ventilation and recoil

• Diaphragm and intercostal muscles change thoracic volume and pressure

• Inspiration uses diaphragm and external intercostal contraction to draw air in

• Expiration reduces thoracic volume; internal intercostals and abdominal muscles aid forced exhalation

• Spirometry records breathing patterns, ventilation rate, and lung volumes

• Tidal volume is air moved during normal relaxed breathing

• Vital capacity equals tidal volume plus inspiratory and expiratory reserve volumes

• Stomata allow carbon dioxide, oxygen, and water vapour to diffuse

• Guard cells open and close stomata by changes in turgor

• Waxy cuticle, lower epidermal stomata, air spaces, mesophyll, and veins balance gas exchange with water loss

• Dicot leaves have waxy cuticle, upper and lower epidermis, palisade and spongy mesophyll

• Vascular bundles contain xylem and phloem for water and assimilate transport

• Spongy mesophyll air spaces and stomata create internal diffusion routes

• Transpiration is evaporation from mesophyll walls followed by diffusion of water vapour through stomata

• Temperature, humidity, wind, and light alter concentration gradients and stomatal opening

• Potometers estimate transpiration indirectly by measuring water uptake

• Stomatal density is the number of stomata per unit leaf area

• Higher density can increase carbon dioxide uptake and water loss

• Leaf casts or micrographs allow stomatal counts and comparisons between species