Many people wonder about the internal workings of aquatic animals, particularly fish, and often ask if they possess two hearts, similar to some complex circulatory systems found in other animals. The animal kingdom exhibits a wide array of circulatory designs, each uniquely adapted to its environment and lifestyle.
The Fish Heart: A Single Pump
Fish have a single heart, not two. This single organ efficiently pumps blood. It is a two-chambered heart that effectively performs the entire pumping cycle. These two primary chambers are an atrium and a ventricle, which work in sequence to move blood through the body.
Anatomy of a Fish Heart
The fish heart is composed of four serially arranged parts. Deoxygenated blood returning from the body first enters a thin-walled collecting sac called the sinus venosus. From the sinus venosus, blood flows into the atrium, which is a muscular chamber that weakly contracts to push blood into the next chamber. The ventricle, a highly muscular chamber, receives blood from the atrium and is responsible for generating the main pumping force to propel blood out of the heart. Finally, blood is pushed into the bulbus arteriosus, an elastic chamber that helps to smooth out blood flow and maintain pressure before blood enters the gills.
How Blood Circulates in Fish
Fish utilize a “single circulation” system, meaning blood passes through their heart only once during each complete circuit of the body. Deoxygenated blood from the body first enters the heart. The heart then pumps this deoxygenated blood forward to the gills. At the gills, gas exchange occurs; oxygen diffuses into the blood, and carbon dioxide diffuses out. After being oxygenated in the gills, the blood then travels directly to the rest of the body, delivering oxygen and nutrients to tissues and organs. Once the blood has circulated through the body and delivered its oxygen, it becomes deoxygenated and returns to the heart, completing the single loop.
Why This Design Suits Aquatic Life
The single circulatory system and two-chambered heart design are well-suited for fish living in an aquatic environment. This system is energy-efficient because blood is pumped through only one circuit, requiring less energy compared to the double circulation found in mammals and birds. Fish have lower metabolic rates than terrestrial animals, and this lower-pressure system is sufficient to meet their oxygen delivery needs. The gills, where oxygen uptake occurs, are also the site where blood pressure is reduced due to the resistance of the gill capillaries. This design ensures efficient gas exchange in water and effective distribution of oxygenated blood throughout the fish’s body.