CIRCULATION. - By c. is meant that complex of structures and functions suited to make a current of liquid circulate incessantly from the surfaces of absorption of nutrients (respiratory surfaces, intestine) to the place of their utilization in the tissues, and from there to the surfaces of elimination of waste substances (respiratory surfaces, kidneys), with the purpose of rapidly transporting these substances and thus permitting an intense material and energy exchange in the organism.
This current is effected 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 one-way, and, in lower vertebrates (selachians), the blood leaves the heart and reaches the gills in arterial vessels, where the gaseous exchange of oxygen and carbon dioxide takes place (v. RESPIRAZIONE); from the gills it is distributed to the various organs, among them the digestive tract and the kidney, and from these it returns, in venous vessels, to the heart. In higher vertebrates (birds, mammals), on the other hand, there are two separate circuits, the lesser circulation, or pulmonary circulation, and the greater circulation, or somatic circulation, and two pumps, the right heart and the left heart. The blood leaves the right heart and through the pulmonary artery reaches the lungs, where the gaseous exchanges take place, and from them
it returns through the pulmonary veins to the left heart; from here the blood is driven into the greater circulation and reaches the various organs, among them 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, the oxygenation of the blood does not take place in the lungs but at the level of the placenta. The placental circulation with the umbilical artery and the umbilical vein are branches of the greater circulation. The oxygenated blood coming from the umbilical vein reaches the right heart and from here passes, in part, directly into the left heart through the foramen ovale. The pulmonary circulation is considerably reduced not only because of the presence of the foramen ovale but also because part of the blood expelled through the pulmonary artery reaches the aorta through the ductus arteriosus, avoiding the passage through the lungs.
The exchanges between blood and the external environment (gills, lungs, intestine, kidney) and between blood and tissues take place at the level of the extreme branches of the circulatory tree, the capillary vessels, surrounded by a simple endothelial wall, permeable to water, gases and crystalloids.
The circulation is able to adapt itself to the varying needs of the organism. In the case of intense muscular work, since a greater quantity of oxygen and nutritive substances is required for the muscles that contract, the blood flow is increased, and this is obtained by an increase in the velocity of the blood in the vascular bed through an intensified activity of the heart, and by an increase in the caliber of the muscular vascular bed through a dilation of the vessels already open and the opening of capillaries previously impassable. At the same time the visceral and cutaneous vascular bed narrows so that the great mass of the blood can pour entirely into the muscles. In the case of increased intestinal work (digestion), on the other hand, there is dilation of the visceral vascular bed and narrowing of other vascular districts, so that a certain quantity of blood is diverted toward the digestive apparatus.
These adaptations are the work of reflexes (v. FISIOLOGIA) which have as their starting point excitations arising within the organism and as their effectors the heart and the vessels themselves.