DOUBLE AND SINGLE CIRCULATORY SYSTEM

a.Bio Learning Journal: Transport in Living Things (3): text, images ...b.

The circulatory system shown in figure b. is a double circulatory system. This means that the blood passes through the heart twice on one complete circuit of the body. We can think of the circulatory system being made up of two parts - the blood vessels that take the blood to the lungs and back, called pulmonary system, and the blood vessels that take the blood vessels that take the blood to the rest of the body and back, called the systemic system.
     
        Double circulatory system are found in all mammals, and also in birds and reptiles. However, fish have a circulatory system in which the blood passes through the heart only once on a complete circuit. This is called a single circulatory system, and is shown in figure a.
     
        Double circulatory systems have some advantages over single circulatory systems. When blood flows through the tiny blood vessels in a fish's gills, or a mammal's lungs, it loses a lot of the pressure that was given to it by the pumping of the heart. In a mammal, this low-pressure blood is delivered back to the heart, which raise its pressure again before sending itv off to the rest of the body.
     
        In a fish, though, the low-pressure blood just carries on around the fish's body. This means that blood travels much more slowly to a fish's body organs than it does in a mammal.
     
        This is particularly important when you think about the delivery of oxygen for respiration. Any tissues that are metabolically very active need a lot of oxygen delivered to them as quickly as possible, and this delivery is much more effective in a mammal than in a fish.

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