CIRCULATION (Lat. rircu la t io, circular course, from circa/us, circle, dim. of cirrus, circle). _k term used in anatomy and physiol ogy to designate the course of the blood through the blood-1.essels. A knowledge of the heart, arteries, capillaries, and veins ((q.v.) is, of course, essent in I to a. complete understanding of the subject of circula tion; hut by means of a diagram (Fig. I), we can indicate the circula tion of the blood in a general way. The shaded part of Fig. I represents the vessels carrying the impure or venous. blood, which has already given Up its oxygen to the body and taken in exeliange the ea rhonie-a cid gas. The unshaded diagram represent the vessels filled with intre blood, which is freely :supplied with oxygen (arterial blood). The heart isshown here as composed of four chambers, of which the two right ones belong to the circulation of venous blood and the two left to that of ar terial blood. Now the blood from the whole body is brought to the right auricle of the heart (c) pry t \vo large veins, the superior vena eava and the inferior vena cava, both of which are here represented by d. By the contraction of this chamber. the blood is forced through the right aurieulo-ventrieular opening into the second chamber of the right side of the heart., the right ventricle (v), and this by its contraction drives the blood to the lungs in the direction of the arrow pointing to f. The blood i, prevented from returning into the auricle by the tricuspid valve, which completely closes the auriculo-ventrieular opening (luring the contrac tion of the ventricle. In its through the lungs the blood is purified and oxygenated, and then is brought back to the heart again by the four pulmonary veins, entering the auricle on tic left side. \Viten this contracts, the blood is forced onward into the left ventricle, and then by ventricular contraction into the aorta for general arterial distribution. The mitral valve prevents regurgitation into the left auricle. and the semi-lunar valve at the beginning of the aorta stops any reflux into the ventricle. (Sim
ilar valves are present in the pulmonary artery.) The aorta divides into branches, and these in turn into smaller ones, until the whole body is supplied by a minute arterial plexus, or network; the smallest arteries divide into a finer network of still more minute vessels, the eapillaries, whialt have extremely delicate walls, so that the blood can enure int 0 the closest relation with the cells of the body-tissues. It is in these capil varies that the oxygen is given off, the nourish ment furnished to the body elements, and the waste products taken up into the blood. The capillaries then unite to form a venous plexus, and later small veins which unite with each other to form larger ones until we have all the blood collected into the superior and inferior venue eava-, and thus brought to the heart again.
We see, from this description, that there are in reality two (dreulations—one, a short cir cuit, from the right side of the heart through the lungs to the left side of the heart ; and the seeond, a longer circuit, from the left side of the In-art through the body hack to the right side of the heart. In the heart the two circula tions connect with each other, and become con tinuous. In addition to the pyintollary and sys tonnir described above, we have another subsidiary to the venous system, and known as the portal rireulution. This is not indicated in the diagram. A certain amount of the blood of the intestines is collected into the portal vein and carried to the liver, where it traverses a capillary network in intimate rela tion with the liver-cells. Bile is formed and other important changes are effected in the blood, which is highly charged with foodstuffs recently absorbed in its passage through the intestinal capillaries. The blood is collected a second time into veins, and carried to the inferior vena eava, where it. again joins the general circulation. In its passage through other spacial organs, the blood undergoes further modifications. See KID NEY: SPLEEN; etc.