[Photo credit: Mariana Ruiz Villareal]
Types of Circulatory Systems
There are three major types of circulatory systems in animals. They are:
- single and double circulatory systems
- pulmonary and systematic circulatory systems
- closed and opened circulatory systems
1. Single and Double Circulatory Systems
(i) Single circulatory system: In single circulatory system, each time the blood makes one complete movement round the body, it passes through the heart once. It is common in fish, which has only two-chambered heart - one auricle and one ventricle.
(ii) Double circulatory system: In double circulatory system, each time the blood makes one complete movement round the body, it passes through the heart twice. It is found in mammals.
Double circulatory system gives rise to pulmonary and systematic circulatory system - two-pathways involved in double circulatory system.
2. Pulmonary and Systematic Circulatory System
(i) Pulmonary circulatory system: In pulmonary circulatory system, blood moves between the heart and lungs. The blood moves from the heart to the lungs for oxygenation.
(ii) Systematic circulatory system: In systematic circulatory system, the blood moves between the heart and all other parts of the body except the lungs. In one complete circulation, for the first time blood moves from any part of the body to the heart, it is sent to the lungs for oxygenation. After oxygenation, it is brought back to the heart for the second time. It is then distributed to all parts of the body. This is the sequence involved in double circulation. Mathematically, we can represent pulmonary and systematic circulation as:
Pulmonary circulation = Heart + Lungs
Systematic circulation = Heart + Body
The above equations also represent also represent double circulation of blood in mammals.
3. Closed and Opened Circulatory Systems
(i) Closed circulatory system: This is made up of blood vessels called arteries, from the heart which branch many ties into small units called capillaries, but eventually join up with other vessels called veins that are connected to the heart. By this design, blood is therefore always confined within the cavities of the vessels and the heart and never comes into direct contact with the cells of the body.
It is found in some invertebrates like annelids and also in all vertebrates. In mammals, there is no mixing of oxygenated and deoxygenated blood in the heart. Oxygenated is confined to the left part of the heart while deoxygenated blood is confined to the right side.
(ii) Opened circulatory system: In this system, the blood vessels lead out of the heart, but end in blood spaces called haemocoels within the body cavity. In these spaces, the blood comes into direct contact with the cells after which it is returned to the heart. It is found in arthropeds and some mollusces.
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