The blood vessels, together with the four chambers of the heart, form a closed system in which blood is carried to and from the tissues. Although whole blood does not leave the vessels, components of the plasma and tissue fluids can be exchanged through the walls of the tiniest vessels, the capillaries. The vascular system is easier to understand if you refer to the appropriate illustrations in this page as the vessels are described. When this information is added to what you already know about the blood and the heart, a picture of the cardiovascular system as a whole will emerge.

Blood Vessels

Blood vessels may be divided into five groups, named below according to the sequence of blood flow from the heart:
* Arteries carry blood away from the heart and toward the tissues. The heart’s ventricles pump blood into the arteries.
* Arterioles are small subdivisions of the arteries. They carry blood into the capillaries.
* Capillaries are tiny, thin-walled vessels that allow for exchanges between systems. These exchanges occur between the blood and the body cells and between the blood and the air in the lung tissues. The capillaries connect the arterioles and venules.
* Venules are small vessels that receive blood from the capillaries and begin its transport back toward the heart.
* Veins are vessels formed by the merger of venules. They continue the transport of blood until it is returned to the heart.

Blood Circuits
The vessels together may be subdivided into two groups, or circuits: pulmonary and systemic. Figure 11-1 shows the vessels in these two circuits; the anatomic relation of the circuits to the heart is shown Figure 10-4.

The Pulmonary Circuit The pulmonary circuit delivers blood to the lungs where carbon dioxide is eliminated and oxygen is replenished. The pulmonary vessels that carry blood to and from the lungs include the following:
* The pulmonary artery and its branches, which carry blood from the right ventricle to the lungs;
* The capillaries in the lungs, through which gases are exchanged;
* The pulmonary veins, which carry blood back to the left atrium.
   The pulmonary vessels differ from those in the systemic circuit in that the pulmonary arteries carry blood that is low in oxygen, and the pulmonary veins carry blood that is high in oxygen. All the remaining arteries carry highly oxygenated blood, and all remaining veins carry blood that is low in oxygen.
Blood flow in a closed system of vessels
Figure 11-1 Blood flow in a closed system of vessels. Oxygen content changes as blood flows through the capillaries.
The heart as a double pump
Figure 10-4 The heart as a double pump. The right side of the heart pumps blood through the pulmonary circuit to the lungs to be oxygenated; the left side of the heart pumps blood through the systemic circuit to all other parts of the body.

The Systemic Circuit

The systemic circuit serves the rest of the body. These vessels supply nutrients and oxygen to all the tissues and carry waste materials away from the tissues for disposal. The systemic vessels include the following:
* The aorta, which receives blood from the left ventricle and then branches into the systemic arteries carrying blood to the tissues;
* The systemic capillaries, through which materials are exchanged;
* The systemic veins, which carry blood back toward the heart. The venous blood flows into the right atrium of the heart through the superior vena cava and inferior vena cava.

Vessel Structure

The arteries have thick walls because they must be strong enough to receive blood pumped under pressure from the heart’s ventricles (Fig. 11-2). The three tunics (coats) of the arteries resemble the three tissue layers of the heart:
Sections of small blood vessels
Figure 11-2 Sections of small blood vessels. Drawings show the thick wall of an artery, the thin wall of a vein, and the singlelayered wall of a capillary. A venous valve also is shown. The arrow indicates the direction of blood flow.
* The innermost membrane of simple, flat epithelial cells makes up the endothelium, forming a smooth surface over which the blood flows easily.
* The middle and thickest layer is made of smooth (involuntary) muscle, which is under the control of the autonomic nervous system.
* An outer tunic is made of a supporting connective tissue.
     Elastic tissue between the layers of the arterial wall allows these vessels to stretch when receiving blood and then return to their original size. The amount of elastic tissue diminishes as the arteries branch and become smaller. The small subdivisions of the arteries, the arterioles, have thinner walls in which there is little elastic connective tissue but relatively more smooth muscle. The autonomic nervous system controls this involuntary muscle. The vessels become narrower (constrict) when the muscle contracts and widen (dilate) when the muscle relaxes. In this manner, the arterioles regulate the amount of blood that enters the various tissues at a given time. Change in the diameter of the arterioles is also a major factor in blood pressure control. The microscopic capillaries that connect arterioles and venules have the thinnest walls of any vessels: one cell layer. The capillary walls are transparent and are made of smooth, squamous epithelial cells that are a continuation of the lining of the arteries. The thinness of these walls allows for exchanges between the blood and the body cells and between the lung tissue and the outside air. The capillary boundaries are the most important center of activity for the entire circulatory system. The smallest veins, the venules, are formed by the union of capillaries, and their walls are only slightly thicker than those of the capillaries. As the venules merge to form veins, the smooth muscle in the vessel walls becomes thicker and the venules begin to acquire the additional layers found in the larger vessels.
Cross-section of an artery and vein
Figure 11-3 Cross-section of an artery and vein. The smooth muscle and connective tissue of the vessels are visible in this photomicrograph.
The walls of the veins have the same three layers as those of the arteries. However, the middle smooth muscle tunic is relatively thin in the veins. A vein wall is much thinner than the wall of a comparably sized artery. These
vessels also have less elastic tissue between the layers. As a result, the blood within the veins is carried under much lower pressure. Because of their thinner walls, the veins are easily collapsed. Only slight pressure on a vein by a tumor or other mass may interfere with return blood flow. Most veins are equipped with one-way valves that permit blood to flow in only one direction: toward the heart
(see Fig. 11-2). Such valves are most numerous in the veins of the extremities. Figure 11-3 is a cross-section of an artery and a vein as seen through a microscope.
The heart in position in the thorax
Figure 10-1 The heart in position in the thorax (anterior view).
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