Sharks are marine animals with adaptations for diverse ocean environments. Understanding their internal systems, particularly their circulatory system, provides insight into how these predators sustain their movements and predatory lifestyles. The design of their hearts and the path blood takes through their bodies are specialized features for effective underwater function.
Shark Heart Structure
Sharks possess a two-chambered heart, a characteristic feature among fish. This heart is an S-shaped tube located in the head region, positioned near the gills. It consists of two primary chambers: an atrium and a ventricle. The atrium is the receiving chamber, collecting deoxygenated blood as it returns from the body. The ventricle, a muscular chamber, is responsible for pumping this blood forward.
Beyond these two main chambers, a shark’s heart also includes a sinus venosus, which collects deoxygenated blood before it enters the atrium, and a conus arteriosus, which helps regulate blood flow as it exits the ventricle. This structure forms the basis of what is known as a “single-circuit” circulatory system.
Blood Flow in Sharks
The circulatory system in sharks operates as a single-circuit system, meaning blood passes through the heart only once during each complete circuit of the body. Deoxygenated blood, returning from the shark’s body tissues, first enters the sinus venosus, a thin-walled collecting chamber. From there, it flows into the atrium, which then pumps the blood into the muscular ventricle. The ventricle propels the deoxygenated blood into the conus arteriosus and then into the ventral aorta.
This deoxygenated blood is then directed to the gills via afferent branchial arteries. Within the gill capillaries, gas exchange occurs, where oxygen is absorbed from the surrounding water and carbon dioxide is released. Once oxygenated, the blood does not return to the heart. Instead, it flows directly from the gills through efferent branchial arteries into the dorsal aorta, which distributes the oxygen-rich blood throughout the rest of the shark’s body and tissues.
Evolutionary Context of Vertebrate Hearts
The two-chambered heart found in sharks, and most fish, represents a fundamental design in vertebrate circulatory systems. This simpler structure, consisting of one atrium and one ventricle, efficiently supports their aquatic lifestyle where blood is oxygenated at the gills and then circulates directly to the body. This single-circuit pathway differs from the more complex systems seen in terrestrial vertebrates.
As vertebrates evolved, their circulatory systems adapted to different environments and metabolic demands, leading to an increase in the number of heart chambers. Amphibians, for instance, possess a three-chambered heart with two atria and a single ventricle, allowing for some mixing of oxygenated and deoxygenated blood. Reptiles share this three-chambered design. Mammals and birds, with their higher metabolic rates and endothermic nature, have evolved a four-chambered heart, featuring two atria and two completely separated ventricles. This advanced design ensures a complete separation of oxygenated and deoxygenated blood, providing efficient oxygen delivery for their active lifestyles.