Why do reptiles have incomplete heart chambers
Answers
Except for crocodilians, which have a four-chambered heart, all reptiles have a three-chambered heart consisting of two atria and one ventricle. ... The oxygenated blood from the lungs returns to the left atrium and once again enters the same ventricle, from which it is pumped to the body tissues.
Even in the three-chambered heart, however, as recent research has shown in contrast to earlier beliefs, there is little mixing of oxygenated and deoxygenated blood. This has been achieved by the development within the ventricle of interconnected "subchambers" within which a sequence of changes in blood pressure takes place. Several anatomical variations occur among the reptiles, but a simplified description of the lizard heart will serve to illustrate one way in which this circulatory efficiency was accomplished.
The ventricle of the lizard heart is incompletely partitioned into two subchambers by a muscular ridge that descends from the roof of the heart almost to the floor. The right subchamber is called the right ventricle, or cavum pulmonale; it leads to the lungs. The left subchamber is called the left ventricle, or cavum venosum; it receives blood from the right atrium and leads to the body. The two subchambers are connected not only beneath the partition but also across its incomplete rearward end.
A third subchamber, called the cavum arteriosum, is situated in the upper wall of the right ventricle; it receives blood from the left atrium and is connected through a valve-controlled opening to the left ventricle.
When the two atria contract, oxygenated blood from the left atrium enters the third subchamber, or cavum arteriosum, pressing against and shutting the valves controlling the opening into the left ventricle; this closes off the third subchamber and temporarily holds its contained blood in storage. At the same time the deoxygenated blood in the right atrium has been pumped into the left ventricle, filling it to overflowing, the excess blood moving across the open posterior end of the muscular partition into the right ventricle. The three ventricular subchambers are now filled with blood; oxygenated blood in the third subchamber (cavum arteriosum) and deoxygenated blood in the two ventricles.
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