Circulatory System

I

Introduction

Circulatory System, in anatomy and physiology, the course taken by the blood through the arteries, capillaries, and veins and back to the heart. In humans and the higher vertebrates the heart is made up of four chambers: the right and left auricles, or atria, and the right and left ventricles. The right side of the heart pumps oxygen-poor blood from the cells of the body back to the lungs for new oxygen; the left side of the heart receives blood rich in oxygen from the lungs and pumps it through the arteries to the various parts of the body. Circulation begins early in foetal life. It is estimated that a given portion of the blood completes its course of circulation in approximately 30 seconds.

II

Pulmonary Circulation

Blood from the entire body is transported to the right auricle through two large veins: the superior vena cava and the inferior vena cava. When the right auricle contracts, it forces the blood through an opening into the right ventricle. Contraction of this ventricle drives the blood to the lungs. Blood is prevented from returning into the auricle by the tricuspid valve, which completely closes during contraction of the ventricle. In its passage through the lungs, the blood is oxygenated, that is, saturated with oxygen; it is then brought back to the heart by the four pulmonary veins, which enter the left auricle. When this chamber contracts, blood is forced into the left ventricle and thence by ventricular contraction into the aorta. The bicuspid, or mitral, valve prevents the blood from flowing back into the auricle, and the semi-lunar valves at the beginning of the aorta stop it from flowing back into the ventricle. Similar valves are present in the pulmonary artery.

III

Branching

The aorta divides into a number of main branches, which in turn divide into smaller ones until the entire body is supplied by an elaborately branching series of blood vessels. The smallest arteries divide into a fine network of still more minute vessels, the capillaries, which have extremely thin walls; thus, the blood is enabled to come into close relation with the fluids and tissues of the body. In the capillaries, the blood performs three functions: It releases its oxygen to the tissues, it furnishes to the body cells the nutrients and other essential substances that it carries, and it takes up waste products from the tissues. The capillaries then unite to form small veins. The veins, in turn, unite with each other to form larger veins until the blood is finally collected into the superior and inferior venae cavae from which it goes to the heart, completing the circuit.

IV

Portal Circulation

In addition to the pulmonary and systemic circulations described above, a subsidiary to the venous system exists, known as portal circulation. A certain amount of blood from the intestine is collected into the portal vein and carried to the liver. There it enters into the open spaces called sinusoids, where it comes into direct contact with the liver cells. In the liver important changes occur in the blood, which is carrying the products of the digestion of food recently absorbed through the intestinal capillaries. The blood is collected a second time into veins, where it again joins the general circulation through the right auricle. In its passage through other organs, the blood is further modified.