Yes, sharks possess a heart, much like other vertebrates. This organ circulates blood throughout their bodies, sustaining life. While performing the same basic function as hearts in other animals, a shark’s cardiac system has unique structural and functional characteristics adapted to its aquatic environment.
The Shark’s Cardiac System
A shark’s heart is a two-chambered organ, consisting of a single atrium and a single ventricle, arranged in an S-shaped tube. Deoxygenated blood from the shark’s body first enters a collection sac called the sinus venosus. From the sinus venosus, blood flows into the atrium, which then pumps it into the ventricle. The muscular ventricle, the primary pumping chamber, propels deoxygenated blood into a specialized structure called the conus arteriosus. The conus arteriosus, equipped with a series of valves, directs blood into the ventral aorta.
From the ventral aorta, blood travels through afferent branchial arteries to the gills. Within the gill filaments, oxygen from the surrounding water diffuses into the blood, and carbon dioxide is released. The now oxygenated blood is collected by efferent branchial arteries, which merge to form the dorsal aorta. The dorsal aorta then distributes this oxygen-rich blood throughout the shark’s body before returning to the heart to complete the cycle.
How Shark Circulation Differs
The shark’s circulatory system operates as a single loop, distinct from the double-loop system found in mammals. In a single-loop system, blood passes through the heart only once per complete circuit of the body. This arrangement results in lower blood pressure after blood has passed through the capillaries of the gills, which can reduce oxygen delivery to tissues. Consequently, this lower pressure is associated with a slower metabolic rate compared to animals with higher pressure, double-loop systems.
Sharks have developed several adaptations to compensate for the lower blood pressure inherent in their single-loop system. Many active shark species employ ram ventilation, swimming continuously with open mouths to force water over their gills, ensuring a constant oxygen supply. Other species, particularly bottom-dwelling sharks, utilize buccal pumping, actively drawing water over their gills without constant movement. The rigid walls of the pericardium, the sac enclosing the heart, also help create suction that aids in maintaining blood flow. Some fast-swimming sharks possess a specialized network of blood vessels called the rete mirabile, which allows them to retain metabolic heat and maintain a higher body temperature than the surrounding water, supporting their active lifestyles.