The teleost heart has four chambers. The generalization you learned in freshman biology, that fish have a two-chambered heart, means that they have only two pumping chambers, the atrium and the ventricle, but they also have a sinus venosus and a bulbous arteriosus. Blood returning from the fish's body enters the sinus venosus, a thin-walled sac where the major veins coalesce. Expansion of the weakly muscular atrium pulls blood from the sinus venosus. Blood then flows from the atrium to the ventricle, strong contractions of the ventricle's thick muscular wall send the blood under pressure into the elastic bulbous arteriosus. From there, the blood flows into the ventral aorta and on through the gills. There are three valves in the heart to prevent back flow during the expansion (diastole) of the pumping chambers.
Fish have a very low pressure circulatory system. There is very little blood pressure in the venous system and return to the heart is aided in all species by skeletal muscular contraction and in some species by accessory hearts. By the time the blood reaches the sinus venosus, pressure is essentially zero. Contractions of the atrium draw the blood from the sinus venosus and help fill the ventricle. Ventricular contractions generate the pressure to move the blood through the body.
The bulbous arteriosus is neither contractile nor valved, but elastic. It expands with each ventricular contraction as it fills with blood and maintains aortal pressure during ventricular diastole. In terms of pressure, the gills are somewhat restrictive, with blood cells meeting resistance within the lamellae. When the ventricle contracts, it sends a charge of blood into the bulbous, when the ventricle expands, the valve between the bulbous and the ventricle keeps the blood from going back into the ventricle. Coupled with the resistance of the gills, this causes the elastic bulbous to expand, then as the blood continues to flow through the gills the bulbous begins to "deflate", then comes another charge of blood from the ventricle. The bulbous functions to average out the pressure extremes and keep a steadier flow of blood going through the gills.
If teleosts did not have a bulbous, then the blood would strongly pulse over the gills. It appears to be adaptive for the fish to move the blood across the gills at a more constant rate. However, there is some pulsing even with the bulbous, and fish actually synchronize their heartbeat with their opercal movements in order to match peak blood flow with the water pulses associated with the buccal pump. This is especially evident when fish are subjected to hypoxia.
In elasmobranchs, agnathans, and holosteans, the fourth chamber, termed conus arteriosus, is not elastic, but fairly rigid, and its wall contains a series of valves to prevent back flow of blood. Since the conus is a more primitive condition, we can think of teleosts having the conus reduced to one valve (between bulbous arteriosus and ventricle) with the bulbous arteriosus evolved from the ventral aorta. In lungfish and amphibians, there is a septum dividing the atrium into two chambers, but not the ventricle.
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