The types of circulatory systems
There exist two types of circulatory systems in the animal kingdom, the open circulatory systems and the closed circulatory systems.
In the open circulatory systems, the blood passes through the heart only once each cycle, and it leaves the blood vessels momentarily, mixing with the intersticial fluid (a fluid made of blood plasma and some types of white blood cells). This mixture is called Hemolymph. The hemolymph then performs direct nutrient and gas trades with the cells. This is the slowest type of circulatory systems, as the hemolymph has to leave and rejoin the vessels.
This type of circulatory systems can be found in arthropods (most insects and some sea organisms) and some molluscs.
The closed circulatory systems are systems that, as the name suggests, have a closed circuit, which means the blood never leaves the vessels.
The closed circulatory systems can be divided in two categories: single and double.
In the single circulatory systems, the blood only passes once through the heart, heading to the respiratory structures, where it exchanges O2 and CO2 with the surrounding environment. Then, the blood heads to the rest of the body, performing the usual nutrient and gas trades with the cells, after which it goes back to the heart. These circulatory systems, although faster than the open circulatory systems, are still quite slow. They can be found in fish.
In the double circulatory systems, the blood passes two times through the heart in each cycle. It can be divided in two sections, the pulmonary circulation and the systemic circulation. In the pulmonary circulation, blood leaves the heart, heads to the lungs, exchanging gases, then it heads back to the heart. It is then pumped to the rest of the body, starting the systemic circulation, and performs the usual nutrient and gas trades with the cells, after which it returns to the heart. The cycle then re-begins.
This is the fastest type of circulatory systems, as the blood is pumped twice each cycle. It can be found in birds, reptiles, amphibians and mammals.
Open and Closed circulatory systems
Single and Double circulatory systems
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