Summary of adaptations for gas exchange What is gas exchange? The process by which oxygen reaches the cells and carbon dioxide is removed from them. What are the properties of a gas exchange surface? Large surface area to volume ratio Thin – so diffusion pathway is short Permeable – to allow respiratory gases through Moist – so the gases can dissolve before diffusion Able to maintain a favourable concentration gradient How are gases exchanged in unicellular organisms e.g. amoeba? Cell membrane for gas exchange surface Large surface are to volume ratio Membrane is thin, moist, permeable and diffusion pathways are short What extra problems occur in multicellular organisms? The larger the organism the lower the surface are to volume ratio Materials need to be exchanged between different organs as well as the environment Diffusion is too slow to satisfy the metabolic needs of the organism How are gases exchanged in simple multicellular organisms? e.g. Flatworms – aquatic organisms with a flattened shape increased surface are to volume ratio Slow moving so low metabolic rate Oxygen and carbon dioxide diffuse across their skin surface e.g. Earthworms – terrestrial organisms with a tubular shape Live in a damp environment (soil) Elongated shape increases their surface are to volume ratio Keeps skin moist by secreting mucus onto the surface Has a closed blood system consisting of blood within vessels to transport gases to internal cells Blood contains a respiratory pigment (haemoglobin) Favourable concentration maintained as oxygen is carried to the tissues and carbon dioxide carried away Why have larger, more advanced multicellular organisms developed special adaptations for gas exchange? Insects, fish, reptiles and mammals have a high metabolic rate so have a higher requirement for oxygen They have a low surface are to volume ratio and many internal organs Specialised exchange surfaces Terrestrial insects – air filled holes called tracheae Aquatic insects and fish – gills Terrestrial animals (birds, reptiles and mammals) – lungs Gas exchange in the fish – The problems Water contains far less (dissolved) oxygen than air Rate of oxygen diffusion in water is slower than in air Fish are active so have a high metabolic demand The solution – the gill A specialised exchange surface over which a one-way current of water is kept flowing by specialised pumping system The density of water prevents the gill collapsing and lying flat Gills are made of many folds over which gas exchange can take place so has a large surface are to volume ratio What is different about gas exchange in cartilaginous fish e.g. sharks? (mainly sea dwelling) Water enters the mouth and is forced out through gill clefts and slits (which lie at the side of the head) when the floor of the mouth is raised Blood travels in the same direction as water flow – an inefficient parallel flow system What is different about gas exchange in bony fish? (live in salt and fresh water) Gills are covered by a flap called the operculum Gases are exchanged through a counter current flow system where blood flow is in the opposite direction to the flow of water What is the structure of gills in bony fish? pairs of gills in the pharynx (throat) Each gill is supported by a gill arch Each gill arch has pairs of v-shaped gill filaments from which thin plates called gill lamellae protrude In water the gills are supported and provide a large surface area The gill plates have an extensive blood supply through blood capillary vessels What is the counter current flow system? Water passes from the pharynx to the operculum in the opposite direction to blood This improves efficiency as a favourable concentration for diffusion is maintained across the whole length of the gill filament allowing 80% of oxygen to be absorbed from the water (3 times better than exchange in the lungs) High extraction level is essential as water contains 25 times less oxygen than air How have vertebrate groups adapted to gaseous exchange on land? Gills do not function out of water Birds and mammals are very active Amphibians – frogs, toads and newts. Probably the first group of vertebrates to colonise land Lay eggs in water, larvae live in water and have gills Metamorphosis from aquatic larvae to land dwelling adult form involves large body form changes Inactive adult amphibian uses moist skin as a respiratory exchange surface When more active e.g. mating they use lungs as a respiratory surface Reptiles – crocodiles, lizards and snakes Far better adapted to life on land Ribs provide protection and support to internal body organs Ribs are also involved in ventilation of the lungs Lung has a more complex internal structure providing a larger surface area for gas exchange Birds Lungs are similar to mammals Ventilation is more efficient to provide high levels of oxygen needed for flight Air sacs are connected to the lungs and act as bellow Birds do not have a diaphragm and ventilation is brought about by movement of the ribs and the action of flight muscles Gas exchange is very efficient with almost no residual gas remaining in the lung tubes – lungs are continuously filled with fresh air Insects Most insects can fly, flight requires a good supply of oxygen Gas exchange occurs through paired holes called spiracles which run along the side of the insect The spiracles can open and close like valves – reducing water loss The spiracles lead to a system of chitin lined tubes called tracheae At the end of the tracheael branches are tracheoles where gas exchange allows oxygen to pass directly into cells Resting insects rely on diffusion During flight movements of the abdomen ventilate the tracheae The human respiratory system What is the structure of the human respiratory system? An airtight compartment – the thorax A dome shaped sheet of muscle – the diaphragm Air is drawn into the lungs via the trachea The lungs consist of a network of branching tubes called bronchioles spreading from 2 bronchi Describe gas exchange in the alveolus Provide a very large surface area relative to the volume of the body Surfaces are moist for gas exchange Thin walls provide a short diffusion pathway Each alveolus is covered by an extensive capillary network to maintain favourable diffusion gradients The % of oxygen in the alveolus is always less then inspired air because the latter mixes with air already in the lungs which has a lower % of oxygen How do mammals ventilate their lungs? Air always moves down a pressure gradient Inspiration is an active process brought about by the contraction of muscles Expiration is a passive proves caused by recoil of elastic tissues Process of inspiration (breathing in) 1. intercostal muscles contract 2. ribs and sternum move upwards and outwards 3. diaphragm muscles contract 4. diaphragm moves down and flattens o volume of thoracic cavity increases. o pressure in the thoracic cavity and lungs decreases to below atmospheric pressure o air flows into the lungs As the lungs fill with air the stretch receptors send impulses to the expiratory part of the respiration centre to end breathing in. Process of expiration (breathing out) 1. intercostal muscles relax 2. ribs and sternum move downwards and inwards 3. diaphragm muscles relax 4. diaphragm moves up and becomes dome shaped o volume of the thoracic cavity decreases o pressure in the thoracic cavity and lungs increases to above atmospheric pressure o air is flows out of the lungs As the air leaves, the stretch receptors are no longer stimulated. The inhibition of breathing in (via the expiratory part of the centre) stops so breathing in can start again. Where does gas exchange take place? Gas exchange takes place in the alveolus How are alveoli adapted to make gas exchange in lungs easy and efficient? they have really big surface area they have moist, thin walls (just one cell thick) so provide short diffusion pathways they have a lot of tiny blood vessels called capillaries to maintain a favourable concentration gradient for diffusion Oxygen diffuses from the air in the alveoli into the blood. Carbon dioxide diffuses from the blood into the air in the alveoli.
