The terms 'circulatory system' and 'cardiovascular system' are often used interchangeably. Although the heart and blood vessels (the cardiovasculature) comprise the core components of the circulatory system, we'll see that several other organs (in particular the lungs, liver, kidneys, and adrenal glands) play vital roles in the operation of the circulatory system as well.
Most people are vaguely aware that humans and other mammals possess a four chambered heart. In physiological terms, it is more useful to think of the heart as a dual pump mechanism. The left side of the heart is the high pressure side that pumps oxygenated blood throughout the arterial/systemic circulation. In contrast, the right side of the heart is the low pressure side whose job is to move deoxygenated venous blood through the pulmonary circulation, where gas exchange occurs.
Before discussing the finer nuances of blood circulation, follow the path of a red blood cell starting at the left side of the heart. Oxygenated blood enters the left atrium (LA) through the relatively short pulmonary veins. From the LA, blood passes through the mitral valve into the left ventricle (LV), the most muscular chamber of the heart. With each contraction, the LV ejects approximately 2/3 of its blood volume into the aorta, the largest artery in the body.
An adult's aorta is roughly the same diameter as a garden hose. Numerous arteries branch off the aorta, starting with the coronary arteries, which provide blood to the heart itself. Next comes the aortic arch, where the brachiocephalic, common carotid, and subclavian arteries originate. These arteries provide blood to the head, neck, arms, and much of the spinal column. Once the aorta descends below the diaphragm into the abdomen, the celiac, superior mesenteric, and inferior mesenteric arteries branch off. These provide arterial blood to the so called splanchnic circulation, a term encompassing the liver, most of the gastrointestinal tract, and the spleen. Finally, the renal arteries branch above the aortic bifurcation, basically a fork in the road where the aorta gives rise to the arteries of the pelvis and legs.
With a few exceptions, arteries branch into ever smaller vessels called arterioles which subdivide into capillaries, the smallest of all blood vessels, whose walls are a single cell layer thick. This diameter is so small that red blood cells must move through capillaries in a single file line. Tissues throughout the body depend on capillary beds to provide them with oxygen and nutrients and remove carbon dioxide and other waste products.
After blood moves through the capillaries, it begins its return journey to the heart through the venous system as well as other small vessels called lymphatics, which serve as spillways. Small venules merge into veins. Above the diaphragm, venous blood enters the right atrium (RA) of the heart through the superior vena cava. Venous blood from structures below the diaphragm flows into the RA through the inferior vena cava. The lymphatic network, present everywhere except the spleen and the brain, transports lymph fluid into the thoracic duct, which in turn empties into the superior vena cava.
From, the RA, blood passes through the tricuspid valve into the right ventricle (RV). The RV is far less muscular than the LV. Whereas the LV generates enough force to pump blood several feet, the RV need only move blood 6 inches or so through the pulmonary circulation.
The RV pumps blood into the pulmonary artery (PA), the only artery in the body that carries deoxygenated blood. The PA branches numerous times within the lungs until it gives rise to capillary beds surrounding the millions upon millions of air sacs called alveoli. Gas exchange takes place here. After absorbing more oxygen and releasing carbon dioxide, the red blood cells return to the left side of the heart.
Beyond their essential role in gas exchange, the lungs also help regulate blood pressure as part of the renin-angiotensin system, discussed shortly.
Although the heart affects blood pressure by altering its rate and force of contractility, several non-cardiac mechanisms come into play.
First, the liver produces many blood proteins, one of which, albumin, acts as a sponge to minimize fluid leakage out of the blood vessels. The liver also produces angiotensinogen. This protein plays a key role in raising blood pressure. When pressure sensor cells in the kidneys detect low blood flow, the kidneys release an enzyme called renin, which cleaves angiotensinogen into a more active form called angiotensin I. Cells in the kidneys and lungs contain an enyme called ACE (angiotensin converting enzyme), which cleaves angiotensin I into its most active form, angiotensin II (AII).
AII raises blood pressure in three ways: First, it causes arterial smooth muscle to contract, which raises blood pressure in a matter of seconds. Second, AII stimulates cells in the adrenal cortex to produce a hormone called aldosterone. Aldosterone is a steroid hormone that increases sodium and water reabsorption in the kidneys. This effectively raises blood pressure by expanding blood volume. Finally, AII acts on certain parts of the brain, producing the sensation of thirst.
The interplay of the circulatory system with the rest of the body in moving blood and maintaining blood pressure is arguably one of the most complicated yet elegant feats in all of physiology.