Blood vessels make up a closed system, since there is no place in the system where whole blood can leave.
a. Direction of Flow of Arteries and Veins. Arteries carry blood from the chambers of the heart to the tissue of the body. Veins carry blood from the tissues to the chambers of the heart. (Coronary arteries carry blood from the chambers of the heart inside to the walls of the heart outside.)
b. Two-Cycle System. It is also a two-cycle system (Figure 10-2). It involves both the pulmonary cycle and the systemic cycle. Blood circulates through two circuits. In the pulmonary cycle, blood circulates from the heart to the lungs and back to the heart. In the systemic cycle, blood circulates from the heart to the rest of the body and back to the heart.
c. Fetal Circulation. Since the fetus is located within the uterus, its lungs do not take in air. Therefore, the pulmonary cycle does not function in the fetus. Essentially, fetal blood flows to and from the placenta. There are certain bypasses in the heart to avoid the pulmonary cycle. At the time of birth, the fetal circulation is changed to the normal pattern.
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This ebook provides an introductory explanation of the workings of the human body, with an effort to draw connections between the body systems and explain their interdependencies. A framework for the book is homeostasis and how the body maintains balance within each system. This is intended as a first introduction to physiology for a college-level course.