**Blood Circulation in Fishes**

Blood circulation in fishes is a vital process that ensures the distribution of oxygen, nutrients, and waste products throughout their bodies. Unlike mammals and birds, fishes have a single circulation system called a single-loop circulation. This means that blood only passes through the heart once during each complete cycle.

**Heart Structure:**

The fish heart consists of four chambers: sinus venosus, atrium, ventricle, and bulbus arteriosus. The sinus venosus receives deoxygenated blood from the veins and acts as a pacemaker to maintain the heart's rhythm. From there, the blood moves into the atrium, which contracts, forcing the blood into the ventricle. The ventricle is the main pumping chamber that propels blood to the gills for oxygenation. Finally, the oxygenated blood is pumped out of the ventricle into the bulbus arteriosus, which acts as a pressure chamber to regulate blood flow.

**Gill Respiration:**

In fishes, the gills are responsible for respiration. As deoxygenated blood enters the gill filaments, it comes into close contact with water, allowing oxygen to diffuse into the blood while carbon dioxide diffuses out. The oxygenated blood then travels to the bulbus arteriosus, where it is pumped to the rest of the fish's body.

**Single-Loop Circulation:**

In a single-loop circulation, blood flows through the fish's heart and gills in a specific sequence. Here's a step-by-step breakdown of the process:

1. Deoxygenated blood enters the heart's sinus venosus from the systemic veins.
2. The blood then moves into the atrium, which contracts, forcing the blood into the ventricle.
3. The ventricle contracts, propelling the deoxygenated blood to the gills via the bulbus arteriosus.
4. Within the gills, oxygen is taken up from the surrounding water, and carbon dioxide is released.
5. Oxygenated blood is collected in the bulbus arteriosus.
6. The bulbus arteriosus contracts, pushing the oxygenated blood to the rest of the fish's body through a series of arteries.
7. Oxygen is delivered to the body tissues, and carbon dioxide is collected.
8. Deoxygenated blood returns to the heart through the systemic veins, restarting the cycle.

**Advantages of Single-Loop Circulation:**

The single-loop circulation in fishes is efficient for their aquatic lifestyle due to several reasons:

1. It allows for a continuous flow of oxygenated blood to the gills for efficient respiration.
2. The heart pumps blood at a relatively high pressure, ensuring effective oxygen delivery to the body tissues.
3. The single-loop circulation is well-suited to the fish's streamlined body shape, minimizing resistance and maximizing swimming efficiency.

In conclusion, blood goes through the fish's heart only once in each cycle due to their single-loop circulation system. This design allows for efficient oxygenation of blood in the gills and effective delivery to the fish's body tissues, ensuring their survival in an aquatic environment.

Blood goes once in one cycle through heart in fishes because fish has two chambered heart . The blood is pumped to the gills for oxygenation from where it is passed directly to the rest of the body .