Biology » The Circulatory System » Overview of the Circulatory System

Summarizing Overview of the Circulatory System


In most animals, the circulatory system is used to transport blood through the body. Some primitive animals use diffusion for the exchange of water, nutrients, and gases. However, complex organisms use the circulatory system to carry gases, nutrients, and waste through the body. Circulatory systems may be open (mixed with the interstitial fluid) or closed (separated from the interstitial fluid). Closed circulatory systems are a characteristic of vertebrates; however, there are significant differences in the structure of the heart and the circulation of blood between the different vertebrate groups due to adaptions during evolution and associated differences in anatomy.

Fish have a two-chambered heart with unidirectional circulation. Amphibians have a three-chambered heart, which has some mixing of the blood, and they have double circulation. Most non-avian reptiles have a three-chambered heart, but have little mixing of the blood; they have double circulation. Mammals and birds have a four-chambered heart with no mixing of the blood and double circulation.



(plural: atria) chamber of the heart that receives blood from the veins and sends blood to the ventricles

closed circulatory system

system in which the blood is separated from the bodily interstitial fluid and contained in blood vessels

double circulation

flow of blood in two circuits: the pulmonary circuit through the lungs and the systemic circuit through the organs and body

gill circulation

circulatory system that is specific to animals with gills for gas exchange; the blood flows through the gills for oxygenation


cavity into which blood is pumped in an open circulatory system


mixture of blood and interstitial fluid that is found in insects and other arthropods as well as most mollusks

interstitial fluid

fluid between cells

open circulatory system

system in which the blood is mixed with interstitial fluid and directly covers the organs


(plural: ostia) holes between blood vessels that allow the movement of hemolymph through the body of insects, arthropods, and mollusks with open circulatory systems

pulmocutaneous circulation

circulatory system in amphibians; the flow of blood to the lungs and the moist skin for gas exchange

pulmonary circulation

flow of blood away from the heart through the lungs where oxygenation occurs and then returns to the heart again

systemic circulation

flow of blood away from the heart to the brain, liver, kidneys, stomach, and other organs, the limbs, and the muscles of the body, and then the return of this blood to the heart

unidirectional circulation

flow of blood in a single circuit; occurs in fish where the blood flows through the gills, then past the organs and the rest of the body, before returning to the heart


(heart) large inferior chamber of the heart that pumps blood into arteries

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