Circulatory System
Andrea Cesalpino, working in the fourteenth century, was one of the first scientists to use the term "circulation," and to discuss the concept of a closed circulatory system. William Harvey did a great deal of work which cemented the concept of the closed circulatory system, combining data he obtained from dissections, experiments, and calculations. Our current understanding of the circulatory system was born of Harvey's work.
The circulatory system is a complex network of organs and tubes which serve to pump and carry blood throughout the body. The purpose of carrying blood throughout the body is two-fold: to deliver oxygen and other necessary substances to all of the tissues and organs throughout the body; to remove waste products, including carbon dioxide, from all of the tissues and organs.
Some species of organisms have open circulatory systems, meaning that the blood is not entirely enclosed in a system of tubes. In these organisms (including some mollusks and arthropods), the blood leaves the blood vessels and flows directly into large spaces called sinuses. All vertebrate organisms (organisms with backbones) have closed circulatory systems, meaning that the blood never leaves the blood vessels. In fact, the circulatory systems of all of the vertebrates are remarkably similar.
The human circulatory system consists of a pump (the heart); a medium which is pumped through the vessels (the blood); a network of vessels to take blood from the heart to the tissues of the body (arteries, arterioles, and capillaries); and a network of vessels to return blood from the body's tissues back to the heart (veins, venules).
The right side of the heart processes unoxygenated blood coming from the tissues and organs of the body. This blood flows into the top of the right heart (right atrium), and is pumped into the bottom of the right heart (right ventricle). The right ventricle pumps this unoxygenated blood into the pulmonary artery, the only artery in the body which carries blood devoid of oxygen. The pulmonary artery carries blood to the lungs, where it loads up on oxygen and gets rid of carbon dioxide. The pulmonary vein (the only vein in the body carrying blood rich in oxygen) takes the blood to the top of the left heart (the left atrium), which pumps into the bottom of the left heart (the left ventricle). The left ventricle then pumps the blood into the body's largest artery, the aorta. The aorta has many major arteries which come off, and lead to all of the major organs and tissues of the body.
The arteries branch further into smaller vessels, called arterioles, which themselves branch into the tiny vessels called capillaries. Capillaries are only one-cell thick, allowing oxygen, nutrients, hormones and other substances to diffuse across into the tissues and organs. Capillaries exist as networks of tiny tubes, which receive blood from arteries, and deliver blood to venules. Venules are small veins which collect the newly de-oxygenated blood, delivering it to the larger veins. Large veins from the lower body empty into the inferior vena cava; large veins from the upper body empty into the superior vena cava. Both venae cavae empty directly into the right atrium, where the cycle begins again.
The circulatory system can be broken down loosely into a number of important circuits. These include the pulmonary circuit (most important for oxygen delivery, and carbon dioxide removal); the hepatic portal circuit (which allows the blood to pick up various nutrients from the digestive organs, as well as waste products from the liver); the renal circuit (which plays an important role in regulating the quantity of water within the body, as well as removing various waste products); and the general body circuit (which includes muscle, bone, glands, and brain). The lymphatic system, which filters harmful substances out of the fluid surrounding body tissues, include lymphatic fluid, lymph nodes, lymph vessels, and the thoracic duct. Lymphocytes, a type of white blood cell, are highly concentrated in lymphatic fluid. Originating in the bloodstream, lymphatic fluid bathes organs and tissues, and returns to the bloodstream after passing through lymphatic filters, which function as part of the body's defense system.
This is the complete article, containing 681 words
(approx. 2 pages at 300 words per page).
