Circulatory system
- This is an article about circulation in animals. For transport in plants, see Vascular tissue. For the band, see Circulatory System.
The circulatory system (also called the cardiovascular system, in vertebrates) is an organ system that moves blood and lymph to and from the cells of the body through a network of hollow vessels. The circulation of blood brings oxygen and food molecules to cells, and removes carbon dioxide and the waste products of metabolism from them. Blood circulation also helps stabilize the body temperature and pH (part of homeostasis). The circulatory system provides for the widespread transportation ofimmune cells and antibodies that fight germs and toxins, as well as the transport of hormones and other signal molecules that have functions throughout the body. The circulation of lymph is particularly important for the body's absorption of fat from the small intestine, and the movement of excess fluid as well as white cells and proteins. The lymph and blood circulations are connected and the contents of both sets of vessels are ultimately moved by the pumping of the heart.
In this article, the human circulatory system is taken as a model.
Closed circulatory system: overview in vertebrates
The main components of the circulatory system are the heart, the blood, and the blood vessels, the lymph and the lymphatic vessels. The blood and lymphatic circulations are connected, and between them, account for the entire systemic circulation.
There are some parts of the body that have circulating fluids that are not in direct contact with the systemic circulation, but are more protected. These include the cerebrospinal fluid of the central nervous system that supplies the brain and spinal cord, and the fluid of the anterior chamber of the eye.
The circulatory systems of all vertebrates, as well as of annelids (for example, earthworms) and cephalopods (squid and octopus) are closed, meaning that the blood never leaves the system of blood vessels consisting of arteries, capillaries and veins.
Arteries carry blood away from the heart , and veins bring blood back to the heart. The heart is a hollow muscular organ that is always pumping, and is responsible for the movement of the blood. Blood passes from arteries to veins through capillaries, which are the thinnest and most numerous of the blood vessels. The capillaries are a netweok of blood vessels that are something like the low pressure tributaries that irrigate the soil, it is through the capillaries that nutrients and oxygen pass out to cells, and it is through the capillaries that carbon dioxide and wastes pass from the cells into the blood.
Now the arteries bring blood not just to tissues in general, but to each organ. Some of the organs have special functions that make the blood leaving them in veins quite different than the blood that enters them in arteries. For example the lungs inhale air that passes through to the alveoli, small sacs of pulmonary tissue with a very rich capillary network. These alveoli are where the actual exchange of gasses takes place in the lungs, oxygen being taken in by the body, and carbon dioxide given off with exhalation. The arteries that bring blood to the lungs are called the pulmonary arteries and the veins that drain blood from the lungs are called the pulmonary veins. When blood enters that alveolar capillary network from the arteriole end, it is pretty much depleted of oxygen but is full of carbon dioxide. The capillary blood gives up that carbon dioxide to the lung, and takes on oxygen from the air sac of the alveolus. The blood that collects from the venule end of the alveolar capillary bed is oxygen rich and quite low in carbon dioxide. This bright red blood drains into the heart through the large pulmonary veins and is pumped from the heart into the systemic circulation through the largest artery of the body, the aorta.
The systems of fish, amphibians, reptiles, birds and mammals show various stages of evolution.
In fish, the system has only one circuit, with the blood being pumped through the capillaries of the gills and on to the capillaries of the body tissues. This is known as single circulation. The heart of fish is therefore only a single pump (consisting of two chambers).
In amphibians and most reptiles, a double circulatory system is used, but the heart is not always completely separated into two pumps. Amphibians have a three-chambered heart.
Birds and mammals show complete separation of the heart into two pumps, for a total of four heart chambers; it is thought that the four-chambered heart of birds evolved independently of that of mammals.
Human circulatory system
Poorly oxygenated blood collects in two major veins: the superior vena cava and the inferior vena cava. The superior and inferior vena cava empty into the right atrium. The coronary sinus which brings blood back from the heart itself also empties into the right atrium. The right atrium is the larger of the two atria although it recieves the same amount of blood. The blood is then pumped through the tricuspid atrioventricular valve into the right ventricle. From the right ventricle, blood is pumped through the pulmonary semi-lunar valve into the pulmonary trunk. This blood leaves the heart by the pulmonary arteries and travels through the lungs (where it is oxygenated) and into the pulmonary veins. The oxygenated blood then enters the left atrium. The blood then travels through the bicuspid valve, also called mitral valve, into the left ventricle. The left ventricle is thicker and more muscular than the right ventricle because it pumps blood at a higher pressure. From the left ventricle, blood is pumped through the semi-lunar valve into the aorta. Once the blood goes through systemic circulation, peripheral tissues will extract oxygen from the blood, which will again be collected inside the vena cava and the process will continue. Peripheral tissues do not fully deoxygenate the blood, thus venous blood does have oxygen, only in a lower concentration as arterial blood. By: Hector Medina
Measurement techniques
Health and disease
History of discovery
The valves of the heart were discovered by a physician of the Hippocratean school around the 4th century BC. However their function was not properly understood then. Because blood pools in the veins after death, arteries look empty. Ancient anatomists assumed they were filled with air and that they were for transport of air.
Herophilus distinguished veins from arteries but thought that the pulse was a property of arteries themselves. Erasistratus observed that arteries that were cut during life bleed. He ascribed the fact to the phenomenon that air escaping from an artery is replaced with blood that entered by very small vessels between veins and arteries. Thus he apparently postulated capillaries but with reversed flow of blood.
The 2nd century AD Greek physician, Galen knew that blood vessels carried blood and identified venous (dark red) and arterial (brighter and thinner) blood, each with distinct and separate functions. Growth and energy were derived from venous blood created in the liver from chyle, while arterial blood gave vitality by containing pneuma (air) and originated in the heart. Blood flowed from both creating organs to all parts of the body where it was consumed and there was no return of blood to the heart or liver. The heart did not pump blood around, the heart's motion sucked blood in during diastole and the blood moved by the pulsation of the arteries themselves.
Galen believed that the arterial blood was created by venous blood passing from the left ventricle to the right by passing through 'pores' in the interventricular septum, air passed from the lungs via the pulmonary artery to the left side of the heart. As the arterial blood was created 'sooty' vapors were created and passed to the lungs also via the pulmonary artery to be exhaled.
Ibn Nafis in 1242 was the first person to accurately describe the process of blood circulation in the human body. Contemporary drawings of this process have survived. In 1552, Michael Servetus described the same, and Realdo Colombo proved the concept. All these results were not widely accepted however.
Finally William Harvey, a pupil of Hieronymus Fabricius (who had earlier described the valves of the veins without recognizing their function), performed a sequence of experiments and announced in 1628 the discovery of the human circulatory system as his own and published an influential book about it. This work with its essentially correct exposition slowly convinced the medical world. Harvey was not able to identify the capillary system connecting arteries and veins; these were later described by Marcello Malpighi.
See also
External links
- Cardiovascular Topics
- Studies on Hemodynamics and Coronary Circulation
- The Circulatory System, a comprehensive overview
- The InVision Guide to a Healthy Heart An interactive website
References
- Iskandar, Albert Z. "Comprehensive Book on the Art of Medicine by Ibn al-Nafis". Retrieved May 2 2005.