What is Single Circulation?

Single circulation is a circulatory system in which blood passes through the heart only once in the entire cycle. Single circulation is exhibited by fish where the heart pumps deoxygenated blood to the gills for oxygenation. The oxygenated blood then flows to the rest of the body and back to the heart.

Single circulation and double circulation differences lie in the number of times blood passes through the heart in each cycle. In this article, we will cover single circulation examples, diagrams and its features.

What is Single Circulation?

Single circulation is characterized by the fact that blood passes through the heart only once each cardiac cycle. Single circulation is found in fishes where the blood is pumped by the heart to the gills for oxygenation, after that it flows to the rest of the body and back to the heart.

Furthermore, their hearts are two-chambered, with only one atrium and one ventricle. As a result, single circulation has a significant disadvantage: the oxygen delivery rate is slower, and the rate of metabolism is reduced as a result. Humans have a double circulatory system, where blood travels in two separate circuits.

Single Circulation Examples

The following are some of the examples of single circulation:

Single Circulation in Amphibians

Nearly all amphibians, especially frogs and toads, have one circulatory system. However, their hearts are slightly more complex than those of fish, with three chambers. Through deoxygenated blood flow, the body is brought into two atria, and oxygenated blood from the lungs is put into them. The next step after the mixing of the blood is the pumping of the blood by the ventricle to the body, where some of the oxygen is absorbed by the tissues before the circulation of the blood starts again.

Single Circulation in Reptiles

Almost all reptiles, among them lizards and snakes alike, have one-way circulation with a heart pumping blood in a three-chambered heart. Meanwhile, differences exist within reptiles as their heart has a partial separation of the ventricle, which permits at least some separation between oxygenated and deoxygenated blood. During this condition, there is a blood shift from body organs to the lungs, which helps in heightened oxygen delivery.

What are the Main Features of Single Circulation?

The following are the main features of single circulation:

• The movement is termed single circulation and consists of blood that passes through the heart only once in a single path.
• There is a single system of flow (single circulatory system), which is present only in animals that are in a group of animals having two-chambered or three-chambered (two-chambered and three-chambered) hearts. 
• The circulatory system alone has been developed in aquatics. In a heart system, the blood, which contains no oxygen, yields the only transfer.

What are the Differences Between Single Circulation and Double Circulation?

The following table lists the differences between single circulation and double circulation:

Features	Single Circulation	Double Circulation
Blood Flow	Blood flows through the heart only once to complete the circuit.	Blood flows through the heart twice to complete the full circuit.
Heart Chambers	Heart is two-chambered with one atrium and one ventricle.	Heart is four-chambered with two atria and two ventricles.
Blood Type Transported	Heart only transports venous blood, i.e., deoxygenated blood.	Both, oxygenated and deoxygenated blood circulate through the heart.
Return Path after Oxygenation	Blood, after oxygenation in gills, does not return to the heart and directly goes to body tissues.	Blood after oxygenation in lungs, comes back to the heart and then is pumped to different body parts through systemic circulation.
Blood Pathways	Blood flows in single pathway.	Blood flows in two pathways, i.e., pulmonary circulation and systemic circulation.
System Efficiency	It is a less efficient system and blood flows at a low pressure.	It is a more efficient system and blood flows at a high pressure.
Occurrence	Occurs in fishes.	Occurs in birds and mammals.

Single Circulation Diagram

The diagram given below shows the single circulation:

Single circulation

Single Circulation in Fish

Similar to the circulatory system of the human body, a single system is responsible for nourishing and transporting oxygen to all fish. Only blood reaches the heart in a whole cycle of one circulatory system, except just after going through the body of this fish. The process of gases exchange happens in the gills, thus through them the blood from the heart is delivered (carbon dioxide is released and oxygen is absorbed).

The product of the circulatory system is the oxygenated blood which supplies all the organ of the body through the gills and returns to the heart again using the same organs. The blood is in a single circulation since it flow only once time through the heart.

Conclusion – Single Circulation

In conclusion, the single circulation flowing blood contains one circuit via which the heart and the body complete the process within a single pass. This single circulation is seen in the fish as their hearts comprise only two chambers—the atrium and ventricle. Although this system is not very efficient with oxygen regulation, it works with a simple exchange mechanism and process. However, it does have a problem or a drawback of oxygen getting delivered at a lower rate, which in turn affects metabolic efficiency.

FAQs on Single Circulation

How does Single Circulation Differ from Double Circulation?

Circulation can be either single or double, where single circulation is only one passage by blood through the heart per circuit, while double circulation two comprises two passages by the blood through the heart in every circuit.

Which Animals have a Single Circulation System?

Fish, hagfish, torrent fish, amphibians, amians, and some reptiles, snakes have a single system of circulation.

What is the Role of the Heart in Single Circulation?

In the single circulation process, the heart pushes blood with low oxygen contents to the gills or lungs so that it gets oxygenated. Oxygen extracts from the air, then transports into the blood, which in turn circulates to the body and returns to the heart.

How does Blood Oxygenation Occur in Fish with Single Circulation?

Fish supply their blood with oxygen through the compression of the blood that is not oxygenated into the fish gills. Most importantly, at the gills, oxygen gets into the blood and carbon dioxide is released.

Do Amphibians have a Similar Circulatory System to Fish?

Although frog and toad circulatory systems like the lizard’s are single circulation, their heart’s chamber is further developed, making them a three-chambered heart.

What is the Significance of the Heart’s Chambers in Single Circulation?

The heart, with its number of chambers and not its circulation capabilities, is the one that is effective in circulating the blood. An example of this is that a fish must have two chambers, while an amphibian can have three.

Are there Variations in Single Circulation among Reptiles?

To an extent, yes, reptiles also share some of those changes. The heart of a few reptiles, such as those whose species include lizards and snakes, is divided into three chambers equipped with a partial division of the ventricle.