The cardiovascular system is the body's principal chemical and cellular transportation network. It supplies nutrients and O2 to tissues to support their metabolism by continuously circulating blood through the intestines and lungs. It plays a fundamental role in homeostasis by maintaining the composition of water and electrolytes in extracellular fluid and removing metabolic wastes by circulating blood through the kidneys. It is the delivery route for immune system cells that defend the body against pathogens that manage to breech barrier epithelia. The cardiovascular system also distributes the vast array of signaling molecules, such as hormones and cytokines, which orchestrate much of our physiology and influence our behavior.
The cardiovascular system is comprised of the heart, which is a pump, and the vessels, which carry blood on circuits around the body. Both the heart and the large conducting vessels are comprised of a concentric, three-layered structure. The corresponding layers of the heart and vessels share similar composition and functions. Endothelial cells line the innermost layer of the heart and vessels and are supported by a layer of loose connective tissue. The middle layer of both structures is comprised principally of muscle; cardiac muscle powers the pumping of the heart, and smooth muscle controls the diameter of vessels. The outer layer of the heart and vessels is constructed from connective tissue, and each serves to facilitate proper interactions with surrounding structures. In contrast, the exchangers (capillaries and postcapillary venules) consist mainly of a single layer of endothelial cells surrounded by a basal lamina. Endothelial cells also line the lymphatic vessels, which return a portion of the fluid that escapes from capillaries and venules back to the circulatory system.
This section begins with a description of the heart followed by components in the order taken by circulating blood: arteries, arterioles, capillaries, venules, and veins.
The major function of the heart is to provide the propulsive force to circulate blood. However, the heart also functions as an endocrine organ; for example, cardiac muscle cells located in the atria produce the hormone atrial natriuretic peptide (ANP). The release of this hormone is stimulated by high blood pressure in the atria. ANP binds to receptors found at many sites in the body and produces several effects, the sum of which results in the reduction of blood pressure and the increase of sodium excretion into the urine.
The heart contains four chambers consisting of two atria and two ventricles.
Atria. Receive blood from the body (right atrium) or the lungs (left atrium). The atria then contract to force the blood into the more muscular ventricles to which they connect (Figure 7-1A).
Ventricles. Receive blood from the two atria. Output of the right ventricle is directed to ...