4.1: Introduction

The circulatory system contains several very different components, including the heart, a hollow muscular pump that stands at the operational center of the system that pumps liquid blood throughout the body through three types of flexible tubes, the blood vessels (Figure 4.1). The arteries channel blood from the heart to all parts of the body needing service. Once there, the blood passes through narrow arteries and enters the capillaries, which are the narrowest blood vessels. Many substances and some blood cells pass into and out of the blood by moving through the thin porous capillary walls. The blood is then carried through the veins, which return the blood to the heart. The passage of blood through the vessels in a part of the body is called perfusion of that part.

Figure 4.1 The circulatory system, showing the pathway for blood flow. (Copyright 2020: Augustine G. DiGiovanna, Ph.D., Salisbury University, Maryland. Used with permission.)

Some materials that are carried away from a region of the body do not pass into the blood but are collected by vessels called lymph capillaries (Figure 4.2). The materials in these vessels make up a liquid called lymph, which is carried through the lymph vessels toward the heart. Along the way, the lymph passes through lymph nodes where harmful chemicals and microbes that might have entered it are removed. The lymph is finally added to the blood in the veins shortly before the blood enters the heart.

Figure 4.2 The circulatory system showing pathways for blood flow and lymph flow. (Copyright 2020: Augustine G. DiGiovanna, Ph.D., Salisbury University, Maryland. Used with permission.)