Single and double circulation are types of closed circulatory systems. In a closed circulatory system, blood flows throughout the body in closed blood vessels, i.e., arteries and veins. In single circulation, blood flows to and from the heart through a single pathway, whereas in double circulation there are two separate pathways that are connected to the heart through which oxygenated and deoxygenated blood flows.
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Let us now learn more about single and double circulation and their differences.
Single Circulation
Single circulation is present in fishes. Fishes have a two-chambered heart consisting of an atrium and a ventricle. The heart receives deoxygenated blood from tissues and it is pumped out to gills for purification. From there, oxygenated blood is supplied to different body tissues. From tissues, deoxygenated blood comes back to the heart. As blood flows through a single circulatory pathway, it is called single circulation.
Double Circulation
Double circulation is present in birds and mammals. They have a four-chambered heart consisting of two atria and two ventricles.
The left atrium receives oxygenated blood from the lungs and is transported to the aorta through the left ventricle. It is then returned to the right atrium in deoxygenated state from tissues. This is known as systemic circulation.
The deoxygenated blood is transported to lungs for purification through the pulmonary artery from the right ventricle. After oxygenation in lungs, the oxygenated blood is returned to the left atrium through the pulmonary vein. This is known as pulmonary circulation.
There are two separate pathways and blood comes to the heart twice, hence, it is called double circulation.. The oxygenated and deoxygenated blood is not mixed in the heart.
Difference between Single and Double Circulation
The table below shows the main differences between Single and Double Circulation.
| Single circulation | Double circulation |
| Blood flows through the heart only once to complete the circuit | Blood flows through the heart twice to complete the full circuit |
| Heart is two-chambered with one atrium and one ventricle | Heart is four-chambered with two atria and two ventricles |
| Heart only transports venous blood, i.e., deoxygenated blood | Both, oxygenated and deoxygenated blood circulate through the heart |
| 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 flows in single pathway | Blood flows in two pathways, i.e., pulmonary circulation and systemic circulation |
| 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 |
| Occurs in fishes | Occurs in birds and mammals |
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Frequently Asked Questions
Single circulation is found in which animals?
Single circulation is found in fishes. They have a two-chambered heart which pumps out deoxygenated blood.
What is systemic circulation?
Systemic circulation consists of aorta, arteries, arterioles and veins, venules and vena cava, that carry oxygenated blood from the heart throughout the body and deoxygenated blood back to the heart, respectively. The oxygenated blood from the left ventricle goes to the aorta, from where it goes to various tissues through arteries and arterioles. The deoxygenated blood from tissues comes back to the right atrium through venules, veins and vena cava.
Systemic circulation helps in transporting oxygen and nutrients to tissues, and takes away carbon dioxide and other waste products for elimination.
What is pulmonary circulation?
Pulmonary circulation consists of pulmonary arteries and pulmonary veins, that carry deoxygenated blood from heart to lungs and oxygenated blood back to the heart, respectively. The deoxygenated blood from the right ventricle goes to the pulmonary artery and then to lungs for purification. The oxygenated blood from lungs comes back to the left atrium through the pulmonary vein. This pathway is known as pulmonary circulation.
What is a hepatic portal system?
Hepatic portal system is a system of veins that connect the digestive tract to the liver. It comprises hepatic portal vein and accessory veins. Venous blood from the intestine goes to the liver through hepatic portal vein before entering the systemic circulation.
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