Exchange surfaces

• Exchange surfaces are specialised areas that are adapted to make it easier for molecules to cross from one side of the surface to the other.
• Unicellular organisms can rely on diffusion alone to meet their metabolic requirements, due to their large SA:V ratio.
• Multicellular organisms have a smaller SA:V ratio so diffusion alone cannot meet their metabolic requirements. There are large distances between the cells and the external environment due to layers of tissues and cells.
• Some features of a good exchange surface include: large SA:V ratio, large surface area, thin, permeable barrier, good blood supply to maintain concentration gradient and also a good ventilation system.
• The trachea consists of rings of muscle that are C-shaped, which enable flexibility when swallowing as the esophagus is situated right behind it.
• The bronchi have a similar structure to the trachea but have thinner walls and a smaller diameter, and a full ring of cartilage as they don't require any flexibility.
• Bronchioles are tubes that aren't supported by cartilage. They posses elastic fibres and smooth muscle that can adjust airflow.
• Alveoli are a single layer of epithelial cells and contain elastic fibres, so that the alveoli can expand during inhalation.
• Surfactant forms a thin film covering the alveoli to reduce surface tension and reduces the tendency to collapse. It overall helps with expiration and inhalation.
• The pleural cavity surrounds each lung which secrete plural fluid into the cavity and helps to lubricate and prevent friction during breathing movements.
• There are 4 types of tissue in the lungs: goblet cells, ciliated epithelial cells, elastic fibres, smooth muscle and cartilage.
• Goblet cells produce mucus which is secreted onto the respiratory tract to trap dust particles and bacteria. The mucus then moves upwards towards the pharynx where it is swallowed or coughed out.
• Ciliated epithelial cells move mucus along the respiratory tract using their hair-like projections called cilia. This movement prevents the buildup of mucus and allows for efficient removal of trapped particles.
• Elastic fibres allow the lungs to stretch and recoil during breathing. During inspiration, the diaphragm contracts causing the ribcage to rise and the volume of the chest increases. As a result, pressure inside the lungs decreases allowing air to enter through the nose/mouth. Elastic fibres within the lungs cause them to expand and fill with air. When we exhale, the diaphragm relaxes and the ribcage falls back down, increasing pressure inside the lungs and forcing air out.
• Smooth muscles contract around bronchioles to control the rate and depth of breathing.
• Elastic fibres allow the lungs to stretch when they fill with air (inspiration) and recoil when they empty (expiration).
• Cartilage provides support to the walls of the trachea and bronchi. Without this structure, the airway would collapse under its own weight.
• Inhalation- high concentration of O2 in alveoli, low concentration in blood therefore O2 diffuses down the concentration gradient into the blood.
• Exhalation- high concentration of CO2 in blood, low concentration in alveoli therefore CO2 diffuses down concentration gradient into alveoli.
• In inspiration, the external intercostal muscles contract and the internal intercostal muscles relax. This pulls the ribcage upwards and outwards so the volume of the thorax increases. The diaphragm also contracts and flattens which also increases volume.
• In expiration, the external intercostal muscles relax and the internal intercostal muscles contract, which pulls the ribcage downwards and inwards so that the volume of the thorax decreases. The diaphragm relaxes and domes also decreasing the volume.
• When the volume increases in the thorax, the pressure decreases which causes air to rush into the lungs as the atmospheric pressure is greater.
• When the volume in the thorax decreases, the pressure increases, so rushes out of the lungs as the atmospheric pressure is smaller.
• Condition that make the human respiratory system efficient: many alveoli- so a large overall surface area, large SA:V ratio increases the rate of diffusion, good ventilation system and blood supply help to maintain the steep concentration gradient, the layer of surfactant enables the gases to dissolve and the single layer of squamous epithelial cells allows efficient exchange.
• Vital capacity is the maximum volume of air that can be breathed in or out in one breath.
• Tidal volume is the volume of air that is breathed in or out during normal breathing.
• Breathing rate is the number of breaths taken in one minute.
• Oxygen uptake is the volume of oxygen used up by someone in a given time.
• Residual volume is the volume of gas remaining in the lungs after forced expiration which cannot be expelled.
• In spirometers, soda lime absorbs the exhaled CO2 which can cause the volume of oxygen inhaled to gradually decrease.
• Precautions for spirometer- ensure it is airtight and that nose is clipped so air exhaled isn't lost to the atmosphere. Use medical grade oxygen and sufficient volume in chamber. Disinfect mouthpiece between each subject. Check health of subject. Ensure CO2 is being absorbed by scavenger (soda lime).
• The tracheal system in insects is not linked to the circulatory system.
• Exoskeletons are impermeable to gases.
• Spiracles are openings in the exoskeleton, and can open and close to prevent air from leaving or entering.
• Tracheae come from the spiracles and have rings chitin to keep them open. They branch into tracheoles which are filled with fluid to allow oxygen to dissolve before entering the muscle fibres.
• Air sacs in the insects provide a reserve of air, for aquatic insects as well as desert insects who need to conserve water by shutting their spiracles.
• The concentration gradient is maintained in insects due to the respiring cells in constant need of oxygen and releasing CO2.
• More active/ larger insects need a larger supply of oxygen, therefore can create mass flow by using their abdominal muscles to create a pumping movement for ventilation.
• In the tracheal system of fish, their are a series of gills on each side of the head, each attached to two stacks of filaments, where the surfaces have rows of lamellae which provide a large surface area.
• Lamellae consist of a single layer of squamous epithelial cells that cover a vast network of capillaries.