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How Fish and Humans breathe

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How fish breathe Fish make use of water as a gas exchange medium instead of air. The water surrounding a fish contains a small percentage of dissolved oxygen and the rate of diffusion of gases in water is therefore slower. Water is denser and more viscous and as a result of this it does not flow as freely as air. In order for fish to breathe specialised gas exchange organs are needed; the gills. Gills are made up of several folds, providing a large surface area over which water can flow and gases can be substituted. In bony fish there are four pairs of gills in the pharynx (throat), they are composed of a gill arch (this provides rigid support to the gill). Along each gill arch is a double row of gill lamellae, each gill lamellae consists of gill plates (gas exchange surfaces). Blood vessels bring deoxygenated blood to the gill lamellae; the blood then passes through tiny capillaries present in each of the gill plates. ...read more.


The counter current system allows the gills of a bony fish to achieve an 80% extraction of oxygen from water. If fish are taken out of water they suffocate this is due to their gill arches collapsing and also there is not enough surface area in order for diffusion to take place. The walking catfish however, can survive out of water this is because they have modified lamellae allowing them to breathe air. Fish may also smother in the water; this could happen if the oxygen in the water has been used up by another biotic source such as bacteria decomposing a red tide. How humans breathe Breathing consists of two phases; inspiration and expiration. During inspiration (breathing in) the external intercostals muscles contract and the internal intercostals muscles relax, raising the ribs upwards and outwards. At the same time, the muscular diaphragm contracts and flattens. Both these actions increase the volume inside the thorax causing the pressure inside the thorax to decrease. Since atmospheric pressure is greater, air rushes into the lungs and they inflate. ...read more.


This blood then flows out of alveolar capillaries, through venuoles and back to the heart via the pulmonary veins. Gas exchange at the alveolus Gas exchange takes place in the lungs at the tiny air sacs or alveoli. Ventilation ensures that air is moved into and out of the air passages of the lungs regularly. This helps to maintain the necessary concentration gradients of oxygen and carbon dioxide. The capillaries around the alveoli provide efficient blood transport of gases, this prevents a build-up of oxygen and carbon dioxide and maintains gas concentration diffusion gradients. The red blood cells contain the respiratory pigment haemoglobin. This has a high affinity for oxygen, making the removal of oxygen from the alveoli even more efficient. Deoxygenated blood enters the capillaries around the alveolus. This blood has less oxygen and more carbon dioxide than the air inside the alveolus. Oxygen diffuses out of the capillary into the air in the alveolus. The lining of the alveolus is moist and gases diffuse in solution. The walls of the alveolus and blood capillary are each only one cell thick, making it easy for diffusion to take place. ...read more.

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