Gas Exchange Essay

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Describe and contrast the gas exchange system of fish and mammals The process of respiration is vital to the survival off both mammals and fish. The demand for oxygen and the need to expel carbon dioxide is overcome by very intricate organs and systems. I will explain the main principles and how these organisms meet the requirements for gas exchange. For gas exchange to take place across a membrane, it needs to be permeable, moist and as thin as possible. Both lungs and gills tackle these problems with differing levels of efficiency. I will firstly start with the mammalian invaginations that are the lungs. These are a complex network of vessels that decrease in size from the trachea down to the 150 million alveoli in an average human pair of lungs. The air supply is created by changes in volume of the thoracic cavity. This cavity consists of a hemispherical base called a diaphragm and sides that are made up of ribs and both internal and external intercostal muscles. Inhalation occurs when the external intercostal muscles and the diaphragm contract. This forces the ribs out and the pressure within the cavity drops. Air is subsequently drawn in. For exhalation the internal intercostal muscles contract and the diaphragm relaxes. This reduces the cavity volume and the rise in pressure pushes air out. The first problem overcome is the lack of moisture in the air. Goblet cells in the trachea produce a constant supply of mucus and this ensures the pulmonary system never desiccates. The oxygen rich air works its way to the alveoli and then diffuses across the thin membrane to the capillary network that surrounds each one, this distance is between 0.5-2.5µm. As the diffusion gradient changes, carbon dioxide diffuses into the lungs and is then expelled via exhalation. This is tidal ventilation as the flow of air is bi-directional through the lungs. This system has a

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