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The Fish Heart
Most fishes have never solved this problem, which is probably why most of them are "cold-blooded".Blood collected from throughout the fish"s body enters a thin-walled receiving chamber, the atrium.As the heart relaxes, the blood passes through a valve into the thick-walled, muscular ventricle.Contraction of the ventricle forces the blood into the capillary networks of the gills where gas exchange occurs.The blood then passes on to the capillary networks that supply the rest of the body where exchanges with the tissues occur.Then the blood returns to the atrium.While obviously adequate to the fish"s needs, this is not a very efficient system. The pressure generated by contraction of the ventricle is almost entirely dissipated when the blood enters the gills.
The Squid HeartsThis group of marine invertebrates has solved the problem by having separate pumps:two gill hearts to force blood under pressure to the gills anda systemic heart to force blood under pressure to the rest of the body.
Three Chambers: the Frog and Lizard
The Frog HeartThe frog heart has 3 chambers: two atria and a single ventricle.The atrium receives deoxygenated blood from the blood vessels (veins) that drain the various organs of the body.The left atrium receives oxygenated blood from the lungs and skin (which also serves as a gas exchange organ in most amphibians).Both atria empty into the single ventricle.While this might appear to waste the opportunity to keep oxygenated and deoxygenated bloods separate, the ventricle is divided into narrow chambers that reduce the mixing of the two blood. So when the ventricle contracts, oxygenated blood from the left atrium is sent, relatively pure, into the carotid arteries taking blood to the head (and brain);deoxygenated blood from the right atrium is sent, relatively pure, to the pulmocutaneous arteries taking blood to the skin and lungs where fresh oxygen can be picked up.Only the blood passing into the aortic arches has been thoroughly mixed, but even so it contains enough oxygen to supply the needs of the rest of the body.Note, that in contrast to the fish, both the gas exchange organs and the interior tissues of the body get their blood under full pressure.
The Lizard HeartLizards have a muscular septum which partially divides the ventricle. When the ventricle contracts, the opening in the septum closes and the ventricle is momentarily divided into two separate chambers.This prevents mixing of the two bloods.The left half of the ventricle pumps oxygenated blood (received from the left atrium) to the body.The right half pumps deoxygenated blood (received from the right atrium) to the lungs.
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Four Chambers: Birds, Crocodiles, and MammalsThe septum is complete in the hearts of birds, crocodiles, and mammals providing two separate circulatory systems:pulmonary for gas exchange with the environment andsystemic for gas exchange (and all other exchange needs) of the rest of the body.The efficiency that results makes possible the high rate of metabolism on which the endothermy ("warm-bloodedness") of birds and mammals depends.
|Link to these illustrated discussions of the human (mammalian) circulatory system.the human circulatory system: its anatomy and physiologycontrol of the human heart|