Evolution of Vertebrate Heart – Fish

Fish have a unique heart with four chambers that work together to pump their blood and keep them alive. These chambers are called:

Sinus Venosus: This is a thin-walled chamber that receives blood from the tissues and pumps it into the atrium.

Atrium: The atrium is the next chamber that receives blood from the sinus venosus. It contracts and pumps the blood into the ventricle.

Ventricle: The ventricle is a chamber with a thick muscular wall. It receives blood from the atrium and then pumps it out to the next chamber, called the conus arteriosus.

Conus Arteriosus: The conus arteriosus is an elastic chamber that does not contract. It receives blood from the ventricle and then allows it to flow into the aorta, which carries the blood to the rest of the body.

 

How Blood Flows in Fish

In fish, blood flows through a single circuit, which means it passes through the gills where it gets oxygenated, and then flows through capillaries in other organs of the body. The oxygenated blood is carried by coronary arteries from the dorsal aorta to the heart, and then is returned by coronary veins back to the heart. Interestingly, the fish's heart never receives oxygenated blood. Only deoxygenated blood flows through its chambers.

 

Valves Control Blood Flow

The fish's heart has valves that ensure blood flows in one direction, preventing any backflow. These valves help maintain the proper flow of blood through the different chambers of the heart, ensuring that the blood keeps circulating in a single circuit.

Evolution of Vertebrate Heart – Fish

The Evolution of Fish Hearts: From Simple to Complex

The hearts of vertebrates, including fish, have evolved over time to become more complex and efficient. Let's take a closer look at how fish hearts have evolved to meet the unique challenges of their aquatic environment.

Early Fish: Simple Circulation

Early fish, such as jawless fish like lampreys and hagfish, have a simple tubular heart with just two chambers - an atrium and a ventricle. The blood flows in a single circuit, with deoxygenated blood coming into the atrium and then being pumped out to the gills for oxygenation, and then distributed to the rest of the body.

The Evolution of Four Chambers

As fish evolved, some species developed a more advanced heart with four chambers, which provided greater efficiency in oxygenation and circulation. This four-chambered heart consists of a sinus venosus, atrium, ventricle, and conus arteriosus, as mentioned earlier.

The sinus venosus receives deoxygenated blood from the tissues and pumps it into the atrium. The atrium then contracts, pushing the blood into the ventricle. The ventricle, with its thick muscular wall, pumps the blood into the conus arteriosus, which is an elastic chamber that helps regulate blood flow into the aorta.

Efficient Oxygenation in Gills

Fish have a unique challenge of obtaining oxygen from water as it flows through their gills. In fish with a four-chambered heart, blood flows in a single circuit, passing through a capillary network in the gills where it becomes oxygenated before flowing through capillaries in other organs of the body. This allows for efficient oxygenation of the blood and ensures that oxygen-rich blood is distributed to the rest of the fish's body.

Valves for Effective Blood Flow

Fish hearts also developed valves to control the direction of blood flow and prevent backflow. These valves ensure that blood flows in a single direction, from the sinus venosus to the atrium, then to the ventricle, conus arteriosus, and finally to the aorta. This helps maintain efficient blood flow and ensures that oxygen-poor blood is properly oxygenated in the gills before being circulated to the rest of the body.

The evolution of fish hearts has resulted in a specialized four-chambered heart that is adapted to the unique challenges of oxygenation and circulation in an aquatic environment. This complex heart structure, along with the presence of valves, allows fish to efficiently oxygenate their blood and distribute oxygen-rich blood to different parts of their body, enabling them to thrive in their underwater habitat.

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