CIRCULATION. — By "circulation" is meant that complex of structures and functions designed to keep a continuous current of liquid circulating from the upper sites of food absorption (respiratory surfaces, intestine) to the places where it is utilized in the tissues, and from there to the surfaces where waste substances are eliminated (respiratory surfaces, kidneys), with the purpose of rapidly transporting these substances and thus allowing an intense material and energy exchange within the organism.
This current is carried entirely, in vertebrates, in closed vessels with elastic walls and is maintained by a suction-and-pressure pump, the heart. The direction of the current is unidirectional, and in lower vertebrates (fish), the blood leaves the heart and reaches the gills through arterial vessels, where gaseous exchange of oxygen and carbon dioxide takes place (see *RESPIRATION*); from the gills it is distributed to various organs, including the digestive tract and the kidney, and from these it returns, through venous vessels, to the heart. In higher vertebrates (birds, mammals), on the other hand, there are two separate circulations: the lesser or pulmonary circulation, and the greater or somatic circulation, along with two pumps, the right and left heart. Blood leaves the right heart and, via the pulmonary artery, reaches the lungs where gaseous exchanges occur, and from there it returns through the pulmonary veins to the left heart; from here the blood is forced into the greater circulation and reaches the various organs, including the intestine and the kidney, through the aorta and its branches, and returns to the right heart through the venae cavae.
During the fetal stage in mammals, blood oxygenation does not occur in the lungs but at the level of the placenta. The placental circulation, with the umbilical artery and umbilical vein, are branches of the greater circulation. The oxygenated blood coming from the umbilical vein reaches the right heart and from there passes, in part, directly into the left heart through the foramen ovale. The pulmonary circulation is notably reduced, not only because of the foramen ovale but also because part of the blood expelled through the pulmonary artery reaches the aorta through the ductus arteriosus, bypassing the lungs.
Exchanges between the blood and the external environment (gills, lungs, intestine, kidney) and between the blood and the tissues occur at the level of the finest branches of the circulatory tree, the capillary vessels, which are surrounded by a simple endothelial wall permeable to water, gases, and crystalloids.
The circulation can adapt to the varying needs of the organism. In cases of intense muscular work, since a greater quantity of oxygen and nutrients is required for the contracting muscles, blood flow is increased; this is achieved by an increase in the speed of the blood in the vascular bed due to intensified cardiac activity, and by an increase in the lumen of the muscular vascular bed through dilation of already open vessels and the opening of previously impervious capillaries. At the same time, the vascular beds of the viscera and skin are constricted so that the bulk of the blood can be diverted entirely to the muscles. In cases of increased intestinal work (digestion), on the other hand, there is dilation of the visceral vascular bed and constriction of other vascular districts, so that a certain quantity of blood is diverted toward the digestive apparatus.
These adaptations are the result of reflexes (see physiology) whose starting point is stimuli arising within the organism and whose effectors are the heart and the vessels themselves.