Every time we breathe we take in life-supporting oxygen from the surrounding air. Oxygen is extremely important for life : we can survive for weeks without food and for days without water but only a few minutes without oxygen.
In our bodies oxygen is used to release energy stored in the chemical fuel we consume as food. This energy is vital for powering our life process. Broadly, the sequence of processes leading to the release of energy in living cell is called respiration. In most multicellular animals, it involves
*the intake and absorption of oxygen from the surrounding environment ;
*the transport of oxygen to the individual cell in the body and
*using oxygen to release energy as (ATP) from energy - rich substance via the kreb's cycle.
The above metabolic process which uses oxygen to release energy at ATP in living cell is known as cellular respiration, and has been discussed in chapter 11 in this chapter, we shall discuss the respiratory system that are concerned with the intake of oxygen and it entry into the circulatory system for distribution to the individual cell in the bodies of various organisms.
The same system also get rid of carbon dioxide and water, the waste products of cellular respiration in fact, the intake and absorption of oxygen Is automatically accompanied by the elimination of carbon dioxide and water in the surrounding. Thus, these respiratory system are also known as Gaseous exchange systems.
TYPES OF ANIMAL RESPIRATORY SYSTEMS
well - developed respiratory system are found in higher animals. Simple organisms, like monerans, protists, fungi and some multicellular animals and plant, do not have any particular respiratory system. The games simple diffuse through their cell or body surfaces.
The following are important Gaseous exchange or respiratory structures found in protists and animals : body surface, gills, trachea and lungs.
DIFFUSION AND RESPIRATORY STRUCTURE.
gases enter and leave the cell of an organism by diffusion. The gases, however must first be dissolved in water before they can diffuse across cell membrane. Hence respiratory structure must have Gaseous exchange surface that are bathed in water or Moist.
Respiratory structure must also have ways of increasing the rate of Gaseous diffusion to do this
*the Gaseous exchange surface must be large
*the barrier (I.e the membrane) across which the gases diffuse must be thin;
*there must be ventilation mechanisms to maintain a big difference in the concentration of the gas across the barrier (I.e maintain a steep diffusion gradient) and
* the Gaseous exchange surface must be closely linked to the transport system.
Note by removing and distributing the diffusing gas, the transport system would also help to maintain a steep diffusion gradient across the Gaseous exchange surface.
BODY SURFACE
unicellular organisms like amoeba, Gaseous exchange take place through the plasma membrane the size of such an organism is small to ensure the sufficient oxygen reaches all parts of the organisms to maintain its life processes. In amoeba, all parts of it protoplasm are within a distance of 0.5mm from the plasma membrane. Thus there is no necessity for a special respiratory structure.
MULTICELLULAR ANIMALS
multicellular animals can have larger body surface for Gaseous exchange than unicellular organisms. However, these organisms too cannot grow beyond a certain size since the body surface area does not increase as rapidly as body size or volume thus only few simple multicellular animals carry out Gaseous exchange thorough their body surface or skin without using specialized respiratory structures. Such animals are fairly inactive so that their energy requirement and as a result, their oxygen demands are low
GASEOUS EXCHANGE BODY SURFACE IN SOME SIMPLE ANIMALS
Sponge and coelenterates in these simple animals all their body cells are in direct contact with the surrounding aquatic medium Thus cell can get it own supply of oxygen
FLATWORM
in flatworm the body is extremely flattened and elongated in the free living planaria the body is 10mm long and only 0.6mm thick this is the body form
* increases the surface area to volume ratio and
*brings the innermost cells very close to the body surface.
It is therefore, possible for these worms to meet their oxygen requirements through diffusion over their body surface. In addition free living flatworms are found in well aerated flowing waters so that oxygen concentration in the water surrounding them is high.
Note parasitic flatworms are anaerobes as the oxygen concentration in their environment is very low.
ANNELIDS
In this more advanced worms, Gaseous exchange occur by diffusion over their body surface this is possible because
*their cylindrical shape give a high surface area to volume ratio so that the rate at which oxygen diffuse is sufficient to meet the worms requirements and
* the cell in their body surface (epidermis) have rich supply of blood capillaries.
AMPHIBIANS
these more advanced group of animals live on land and water. They have lungs which are not properly developed to meet their oxygen demands. As a result many amphibians, like frog, obtain much of their oxygen through their Moist skin. This is known as cutaneous respiration. It is possible for the skin of a frog to carry out Gaseous exchange because the skin is
* richly supplied with blood capillaries and
*kept Moist by mucus secreted by glands.
AQUATIC RESPIRATORY STRUCTURE:
THE GILL Gill are specially respiratory organ used for absorbing dissolved oxygen from an aquatic medium. Basically they are outgrowth from the body which projects into the external environment.
They are very closely linked with circulatory system gills range from very simple form found in certain sea slugs to very complex ones, enclosed in chambers, found in bony fishes
SIMPLE GILLS
external gills are very simple respiratory structures just outgrowth of skin which projects into the water they are however
*highly branced and convoluted exposing a large Gaseous exchange surface to water and
*richly supplied with blood capillaries
External gills are found in certain sea Slugs and many fish and amphibians larvae water only circulate over the gills when these organisms move. Movement on the other hand is hampered by the highly branched gills
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