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Circulatory System: Facts, Function & Diseases

The circulatory system is a vast network of organs and vessels that is responsible for the flow of blood, nutrients, hormones, oxygen and other gases to and from cells. Without the circulatory system, the body would not be able to fight disease or maintain a stable internal environment — such as proper temperature and pH — known as homeostasis.

Description of the circulatory system
While many view the circulatory system, also known as the cardiovascular system, as simply a highway for blood, it is made up of three independent systems that work together: the heart (cardiovascular); lungs (pulmonary); and arteries, veins, coronary and portal vessels (systemic), according to the U.S National Library of Medicine (NLM).

In the average human, about 2,000 gallons (7,572 liters) of blood travel daily through about 60,000 miles (96,560 kilometers) of blood vessels, according to the Arkansas Heart Hospital. An average adult has 5 to 6 quarts (4.7 to 5.6 liters) of blood, which is made up of plasma, red blood cells, white blood cells and platelets. In addition to blood, the circulatory system moves lymph, which is a clear fluid that helps rid the body of unwanted material.

The heart, blood, and blood vessels make up the cardiovascular component of the circulatory system. It includes the pulmonary circulation, a "loop" through the lungs where blood is oxygenated. It also incorporates the systemic circulation, which runs through the rest of the body to provide oxygenated blood, according to NLM.

The pulmonary circulatory system sends oxygen-depleted blood away from the heart through the pulmonary artery to the lungs and returns oxygenated blood to the heart through the pulmonary veins, according to the Mayo Clinic.

Oxygen-deprived blood enters the right atrium of the heart and flows through the tricuspid valve (right atrioventricular valve) into the right ventricle. From there it is pumped through the pulmonary semilunar valve into the pulmonary artery on its way to the lungs. When it gets to the lungs, carbon dioxide is released from the blood and oxygen is absorbed. The pulmonary vein sends the oxygen-rich blood back to the heart, according to NLM.

The systemic circulation is the portion of the circulatory system is the network of veins, arteries and blood vessels that transports blood from the heart, services the body's cells and then re-enters the heart, the Mayo Clinic noted.

Diseases of the circulatory system
According to the American Heart Association, cardiovascular disease is the leading cause of death in the United States. Because of its vastness and critical nature, it is one of the systems of the body most prone to disease.

One of the most common diseases of the circulatory system is arteriosclerosis, in which the fatty deposits in the arteries causes the walls to stiffen and thicken the walls. For example, 2.6 million people in the United Kingdom suffer from narrowing of the heart arteries. According to the Mayo Clinic, the causes are a buildup of fat, cholesterol and other material in the artery walls. This can restrict blood flow or in severe cases stop it all together, resulting in a heart attack or stroke.

Stroke involves blockage of the blood vessels to the brain and is another major condition of the circulatory system, according to Mitchell Weinberg of the North Shore-LIJ Health System. "Risk factors include smoking, diabetes and high cholesterol," he noted.

Another circulatory disease, hypertension — commonly called high blood pressure — causes the heart to work harder and can lead to such complications as a heart attack, a stroke, or kidney failure, the NLM noted. Around 75 million American adults, or one in every three adults, have high blood pressure, according to the Centers for Disease Control and Prevention.

Aortic aneurysm:
An aortic aneurysm occurs when the aorta is damaged and starts to bulge or eventually tear, which can cause severe internal bleeding. This weakness can be present at birth or the result of atherosclerosis, obesity, high blood pressure or a combination of these conditions, according to Weinberg.

Peripheral arterial disease:
Peripheral arterial disease (also known as PAD) typically involves areas of narrowing or blockage within an artery, according to Jay Radhakrishnan, an interventional radiologist in Houston, Texas. In addition, chronic venous insufficiency (also known as CVI) involves areas reflux (or backward flow) within the superficial veins of the lower extremities.

PAD is diagnosed with noninvasive testing including ultrasound, CT scan and/or MRI. Ultrasound is the least expensive of these methods, but also gives the least amount of detail, as CT and MRI show a much higher degree of anatomic detail when identifying areas of narrowing/blockage within an artery. CVI is diagnosed with ultrasound as the venous reflux can be measured accurately by ultrasound, which ultimately guides treatment.

Study of the circulatory system
Cardiologists are specialists who are certified to diagnose, treat and prevent disease of the heart, arteries and veins. Cardiologists are certified by the American Board of Internal Medicine (ABIM) after meeting educational and practice requirements. Before being certified as cardiologists, those aspiring to the specialty must be certified in internal medicine.

Then cardiologists can become certified in one of several cardiology subspecialties, including transplant cardiology, cardiovascular disease, clinical cardiac electrophysiology and interventional cardiology.

Some milestones in the history and study of the circulatory system include:

16th century B.C.: The Ebers Papyrus, an ancient Egyptian medical document, provides some of the earliest writing on the circulatory. It describes the connection of the heart to the arteries.
6th century B.C.: Ayurvedic physician Sushruta in ancient India describes how vital fluids circulate through the body.
2nd century A.D.: the Greek physician Galen documents how blood vessels carry blood, identifies venous (dark red) and arterial (brighter and thinner) blood and notes that each has a separate functions.
1628: William Harvey, an English physician, first describes blood circulation.
1706: Raymond de Vieussens, a French anatomy professor, first describes the structure of the heart's chambers and vessels.
1733: Stephen Hales, an English clergyman and scientist, measures blood pressure for the first time.
1816: Rene T.H. Laennec, a French physician, invents the stethoscope.
1902: American physician James B. Herrick first documents heart disease resulting from hardening of the arteries.
1903: Dutch physiologist Willem Einthoven invents the electrocardiograph.
1952: The first successful open-heart surgery takes place by F. John Lewis, an American surgeon.
1967: South African surgeon Christiaan Barnard performs the first transplant of a whole heart from one person to another.
1982: American physician Robert Jarvik designs the first artificial heart, and American surgeon Willem DeVries implants it.
2016: Study finds that 45 percent of all heart attacks in the United States may not have any symptoms, according to the study, published in the journal Circulation.
2017: Researchers find that marijuana may reduce the likelihood of A-fib among heart failure patients. "I was very surprised that it was actually a reduced association I found," said study lead author Dr. Oluwole Adegbala, a medical resident at Englewood Hospital and Medical Center in New Jersey. Adegbala presented the findings this month at the American Heart Association's Scientific Sessions meeting in Anaheim, California.
2018: A type of virus called a bacteriophage, which was procured from a lake, saves an elderly man who had an antibiotic resistant infection in his heart. [A Man with a Life-Threatening Heart Infection Was Saved by a Virus Plucked from a Lake]
2018: Google scans 300,000 patients retinas to train AI to detect heart disease.

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