In considering the role of distribution of blood volume in filling the heart, it is useful to divide the blood volume into central (or intrathoracic) and extrathoracic portions. The central blood volume includes the blood in the superior vena cava and intrathoracic portions of the inferior vena cava, right atrium and ventricle, pulmonary circulation, and left atrium, this constitutes approximately 25% of the total blood volume. The central blood volume can be decreased or increased by shifts in blood to and from the extrathoracic blood volume. From a functional standpoint, the most important components of the extrathoracic blood volume are the veins of the extremities and abdominal cavity. Depending on several factors to be discussed below, blood shifts readily between these veins and the vessels containing the central blood volume. Although a part of the extrathoracic blood volume, the blood in the neck and head is less important because there is far less blood in these regions, and the blood volume inside the cranium cannot change much because the skull is rigid. Blood in the central and extratho-racic arteries can be ignored because the low compliance of these vessels means that little change in their volume occurs. The volume of blood in the veins of the abdomen and extremities is about equal to the central blood volume,- therefore, about half of the total blood volume is involved in shifts in distribution that affect the filling of the heart.
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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.