WJEC Biology for AS: Study and Rev Guide

Parallel flow : blood and water flow in the same direction at the gill lamellae, maintaining the concentration gradient for oxygen to diffuse into the blood only up to the point where its concentration in the blood and water is equal. Key Term Pointer If you find the concept of surface area : volume ratio difficult, just remember that as an organism gets bigger whilst keeping the same shape, the distance to the centre of the organism increases. QUI QUI q Why do multicellular animals require specialised exchange surfaces? QUI w Name three different gas exchange surfaces. Multicellular animals In larger organisms the surface area to volume ratio decreases, so diffusion across the body surface is insufficient to meet the needs of the organism. A number of adaptations have evolved to solve these problems, which become more specialised the larger the organism. Where animals are very active and therefore have a higher metabolic rate, their oxygen requirements cannot be supplied by the body surface alone. This is often solved by the presence of a specialised gas exchange surface with a ventilation mechanism that ensures that the concentration gradient is maintained across the respiratory surface. One consequence of maintaining a moist respiratory surface in terrestrial animals is water loss: this is minimised by having internal gas exchange surfaces, called lungs. Organism Adaptations Flatworm Has a flattened body to reduce the diffusion distance between the surface and the cells inside and to increase the overall surface area (we saw this as an adaptation to cylindrical shaped mitochondria see page 23). Earthworm Secretes mucus to maintain a moist surface and has a well developed capillary network under the skin. Has a low metabolic rate to reduce oxygen requirements. Has a network of blood vessels and blood containing haemoglobin for the transport of oxygen. Carbon dioxide is transported largely in the blood plasma. Amphibians, e.g. frogs and newts Moist and permeable skin with a well developed capillary network beneath the surface. Have lungs that are used when more active. Reptiles, e.g. snakes and crocodiles Have internal lungs like amphibians, but these are more complex and have a larger surface area. Birds Flight generates a very high metabolic rate and hence oxygen requirement. To meet this, birds have an efficient ventilation mechanism to increase concentration gradient across the lung surface. Gas exchange in fish Fish have developed a specialised internal gas exchange surface called gills that are made up of numerous gill filaments containing gill lamellae at right angles to the filaments. These greatly increase the surface area for the exchange of oxygen and carbon dioxide. Fish ventilate their gills in two different ways: 1. Cartilaginous fish, e.g. the shark: blood and water flow in the same direction over the gill ( parallel flow ). Gas exchange is only possible over part of the gill filament surface as an equilibrium is reached which prevents further diffusion and reduces the oxygen that can be absorbed into the blood. The ventilation mechanism in cartilaginous fish is basic: as they swim, they open their mouth allowing water to pass over the gills. 79 2.2 Adaptations for gas exchange