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The development of convection systems What needs convecting?
Convection is the act or process of conveying. Things in the body that need to be conveyed include: o Gases eg oxygen or carbon dioxide o Water and salts o Food components (fatty acids, glucose, vitamins and trace elements o Waste products (ammonia, urea) o Heat o Hormones The respiratory system design The design of an animal's respiratory system depends on a number of things. Firstly is the animal's oxygen demand, which may depend on its size, activity levels, whether it is an endotherm or an ectotherm and whether it is a conformer or a regulator. Aquatic and terrestrial lifestyles also require different systems. But whatever the design, the basic principle is always the same - gas exchangers need to be thin and have a large surface area. There are two main strategies that animals adopt in response to their environmental oxygen. They may be a oxygen conformer or an oxygen regulator. Conformers oxygen consumption decreases in proportion to ambient oxygen concentration, whereas oxygen regulators are able to maintain a constant O2 consumption at below normal ambient PO2. Regulators include most protozoans, annelids, molluscs and many crustaceans. How is oxygen consumed?
There are a number of ways that an animal may facilitate the uptake of ambient oxygen. These can include: o Diffusion: this only really is possible in animals less than 1mm in size. This is found in most single-called organisms, nematodes and platyhelminthes. o Skin: semi-permeable skin can uptake oxygen in many aquatic invertebrates such as freshwater leeches, as well as some frogs and toads, these creatures typically have very efficient internal circulation. o Gills: found in most aquatic animals o Book-lung: this is like gills arranged into a chamber. It is found in arachnids and is also known as book gills. o Simple lung: found in fish, terrestrial crabs and molluscs o True lungs: Found in chordates (amphibians, reptiles, birds and mammals).
Gills Gills can come in three forms:
1. Tuft gills These are raised, thin areas of skin, an example of which is the 'tube feet' found in sea urchins and starfish.
2. Filament gills These are a series of feather-like structures, with more elaborate flow structure. These are found in aquatic arthropods and early stages of vertebrates (tadpoles).
3. Lammellar gills These are flat platelets with orientation to water flow. These are found in fish, and are illustrated in the diagram. The evolution of lungs Freshwater fish arose in the Silurian period (443million years ago). The earliest lungs were thought to be sacks used to hold gulped air in response to low oxygen levels in stagnant pools. Drought was thought to be common in the Devonian period (416million years ago) making these back-up systems advantageous. There are some fish that still use this strategy today. There are species of actinopterygii (ray-finned fish) that live in water where temporary depletion of O2 occurs that demonstrate adaptations to airbreathing. The fish holds the gasped air bublles against gills, or gulps the air and takes it up across the mouth. The famous South American lungfish (sarcopterygii - bony-fish) has lungs that are sub-divided into numerous smaller air sacs, greatly increasing surface area for gas exchange.
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