The cardiovascular system is comprised of the heart and blood vessels. The heart is the pump and the vessels are the delivery and return system for the blood. Every tissue in the body, directly or indirectly, receives oxygen and nutrients from blood supplied by the cardiovascular system.

The heart is a muscle. The muscle is a modified form of skeletal muscle called cardiac muscle. The muscle cells are composed of individual filaments. The muscle cells for a syncytium--that is, they are connected to each other, separated by thin intercalated disks. The nuclei of cardiac muscle are located centrally in the muscle fibers. The heart muscle is arranged into four chambers that make up the pump.

Within the heart are specialized myocardial fibers that conduct electrical impulses more readily and compose the cardiac conducction system. There is a sinoatrial (SA) node near the nodal branch of the right coronary artery in the right atrium that initiates atrial contractions. There is an atrioventricular (AV) node below the membranous septum that sends a bundle of His down the intraventricular septum, conducting impulses to the ventricles to initiate contractions.

The two ventricles (right and left) provide the force, while the two atria (right and left) are mainly chambers that fill and store the blood to be pumped. The right atrium and ventricle supply blood under low pressure to the lungs and require less force to do so than the left atrium and ventricle, which must pump blood at higher pressure systemically throughout the remainder of the body.

The vascular system into which the blood is pumped can be divided into arteries, veins, and capillaries.

• Arteries: the arteries must be strong and resilient to bear the force of the blood under pressure from the heart. The pumped blood has a "systolic" pressure generated by the force of the left ventricle and a "diastolic" pressure that remains between beats.

  Every artery has three layers: intima, media, and adventitia. In between these are the internal and external elastic lamina, which are a thin layer of elastic tissue. The intima is normally quite thin and is just an endothelium with some underlying connective tissue and ground substance. It is the intima that mainly increases with atherosclerosis. The media contains smooth muscle. The adventitia that surrounds the artery contains connective tissue.

  The blood is conducted from the heart by the aorta, the largest artery in the body. The aorta has a circumference of about 1.5 to 2.5 cm. The aorta is distinguished from other arteries by having a very thick media that has parallel layers of elastic fibers as well. This is what gives the aorta its strength. The aorta is also so large that it requires its own blood supply -the vasa vasora.

  The aorta gives off branches that further divide. These are the "muscular" arteries that conduct the blood to various parts of the body. These arteries are so named because they contain a significant media.

  Eventually, the muscular arteries branch to smaller and smaller segments, eventually giving rise to arterioles. An arteriole has little smooth muscle and no elastic laminae. Many arterioles can be controlled by the nervous system or by hormonal factors that determine the amount of blood that can travel through them. This helps to regulate the amount of blood flowing to various parts of the body as it is needed, and helps to regulate blood pressure.

• Capillaries: between the arteries that conduct blood to the tissues and the veins that conduct blood away from the tissues are the capillaries. A capillary consists of just an endothelial layer. The endothelial cells are so thin that oxygen and nutrients can diffuse out to the tissues while CO2 and waste products can diffuse inward. Capillaries are so small that red blood cells must squeeze single file through them.

• Veins: beyond capillaries are the first component of the venous system, called venules. The venules collect blood emanating from capillaries. Venules have an important role to play, because they can dilate to reduce the speed of blood flow and increase intravascular pressure, leading to a net outflow of fluid and inflammatory cells into the tissues. This is a key component of the process of inflammation.

  Venules conduct blood to ever-increasing sizes of veins. Veins of the extremities have a key feature--valves. It is these one-way valves that keep blood from flowing backward or increasing intravascular pressure that would cause fluid to leak out. This is particularly important if you are a giraffe. Veins that become dilated over time--so-called "varicose veins"--have non-functional valves.

  The great veins, particularly the inferior and superior vena cava that conduct blood back to the right atrium, can store a considerable amount of the blood volume in the body, thus insuring a steady supply of blood to the heart.

Lymphatics: in addition to arteries, veins, and capillaries, there is an additional vascular component--the lymphatics. Lymphatics are vascular channels composed mainly of just an endothelium. Lymphatics are located in many tissues, particularly soft tissues. The job of the lymphatics is to "scavenge" extravascular fluid that accumulates and return it to the vascular system. Normally, the veins return almost all of the fluid, but veins have some pressure within them. The lymphatics have no pressure, and they contain lymph--a protein-rich fluid--that tends to draw tissue fluid into them. The lymphatics drain back toward groups of lymph nodes strategically placed to filter the flow--inguinal nodes filtering lymph from lower extremities, axillary nodes with drainage from upper extremities, cervical nodes with drainage from neck organs, etc. The antigens in the lymph alert the lymphoid tissues to activate the immune system to fight potential infections or other inflammatory conditions.