In an organism, circulation is the flowing of sap or blood through the veins or channels, by means of which the perpetual and simultaneous movements of composition and decomposition manifested in organic life are carried on. Although Galen, who had observed the opposite directions of the blood in the arteries and veins, may be said to have been upon the very point of discovering the circulation, the discovery was reserved for William Harvey, who in 1628 pointed out the continuity of the connections between the heart, arteries, and veins, the reverse directions taken by the blood in the different vessels, the arrangements of valves in the heart and veins so that the blood could flow only in one direction, and the necessity of the return of a large proportion of blood to the heart to maintain the supply.
In 1661 Malpighi exhibited microscopically the circulation in the web of a frog's foot, and showed that the blood passed from arteries to veins by capillaries or intermediate vessels. This finally established the theory with regard to animals, but the movements of sap in vegetables were only traced with difficulty and after numerous experiments.
Many physiologists were reluctant to ascribe the term 'circulation' to this portion of the economy of plants; but though sap, unlike the blood, does not exhibit movements in determinate vessels to and from a common centre, a definite course is observable. In the stem of a dicotyledonous tree, for example, the sap describes a sort of circle, passing upwards from the roots through the newer woody tissue to the leaves, where it is elaborated under the action of air and light; and thence descending through the bark towards the root, where what remains of it is either excreted or mixed with the new fluid, entering from the soil for a new period of circulation.
In infusorial animalcules the movement of the fluids of the body is maintained by that of the animal itself and by the disturbing influence of nutritive absorption. In the Coelentera (zoophytes, etc) the movement receives aid besides from the action of cilia on the inner walls of the body. The Annelids, as the earth-worm, possess contractile vessels traversing the length of the body. The Insects, Crustaceans, Myriapods, and Spiders have a dorsal tube, a portion of which may be specially developed as a heart. The blood is driven to the tissues, in some cases along arterial trunks, being distributed not in special vessels, but simply through the interstices of the tissues. From the tissues it is conveyed, it may be by special venous trunks to a venous sinus which surrounds the heart and opens into it by valvular apertures. The Mollusca have the heart provided with an auricle and a ventricle, as in the snail and whelk; two auricles, one on either side of the ventricle, as in the fresh-water mussel; or two auricles and two ventricles, as in the ark-shells. Among the ascidians, which stand low in that division of animals to which the molluscs belong, the remarkable phenomenon is encountered of an alternating current, which is rhythmically propelled for equal periods in opposite directions.
All vertebrated animals (except Amphioxus) have a heart, which in most fishes consists of an auricle and ventricle, but in the mud-fishes (Lepidosiren) there are two auricles and one ventricle; and this trilocular heart is found in the amphibians, and in most reptiles except the crocodiles, which, like birds and mammals, have a four-chambered organ consisting of two auricles and two ventricles. In these two last-named classes the venous and arterial blood are kept apart; in the trilocular hearts the two currents are mixed in the ventricle. Research Circulation
The alveoli are the tiny sacs at the ends of the bronchial tree. Each small bronchiole divides into half a dozen or so alveolar ducts, which are the narrow inlets into alveolar sacs. Each alveolar duct subdivides, leading into three or more alveolar sacs. Each large alveolar sac is like a grapecluster which contains ten or more alveoli. Because the membrane separating the alveolus and the capillary network which carries blood over them is very thin and semi-permeable, oxygen can transfer from the air into the blood cells within the capillaries. Likewise, carbon dioxide and other waste gases can transfer out of the blood and into the air to be exhaled from the lungs. The alveoli are particularly susceptible to infection, as they provide bacteria and viruses a perfect place to grow. This accounts for the tendency for a chest cold or other lung problem to advance into pneumonia and pneumonitis, both potentially dangerous conditions in which the innermost parts of the lungs become infected and inflamed, diminishing air flow and oxygen transport. Research Alveoli
Arteries are muscular and elastic-walled vessels that form a network to carry oxygen-rich blood from the heart to all parts of the body. Smaller branches called arterioles extend from the arteries and connect to even smaller branches called metarterioles which deliver the blood to the capillaries. The exchange of oxygen and carbon dioxide between blood and body cells takes place through the thin walls of the capillaries.
There are two principal arteries or arterial trunks: the aorta, which rises from the left ventricle of the heart and ramifies through the whole body, sending off great branches to the head, neck, and upper limbs, and downwards to the lower limbs, etc; and the pulmonary artery, which conveys venous blood from the right ventricle to the lungs, to be purified in the process of respiration. Research Artery
The bronchial arterioles and venules supply blood to the alveolar sacs for regeneration and carry the regenerated blood back to the heart, respectively. The arterioles branch off of the pulmonary artery, which originates at the heart. These arterioles lead to smaller vessels called metarterioles which, in turn, lead to tiny capillaries in the alveolar tissue. The semipermeable membrane of the capillary wall allows oxygen to transport from the air to the blood cells (binding to the hemoglobin in blood), while allowing excess carbon dioxide and other waste gases to transport from the blood to the air to be exhaled. The capillaries then carry the blood cells to larger vessels, called metavenules, which lead to venules and then to the pulmonary vein. The pulmonary vein returns this regenerated blood to the heart to be pumped throughout the body. It is worthwhile to note that, in most graphic representations, as in the body itself, oxygen-poor blood is blue or dark purple, while oxygen-rich blood is bright red. In the lungs, however, the reverse is true. Blood passing through the pulmonary artery and arterioles is oxygen-poor, while the blood passing back to the heart through the pulmonary vein and venules is oxygen-rich. Research Bronchial Arterioles
About ten billion capillaries lace all body tissues, bringing blood within reach of every cell. They are the smallest blood vessels, microscopic in size, and contain less than five percent of the total circulating blood volume at any one time. Capillaries branch off from the metarterioles which connect arterioles with venules. The capillaries have thin walls, only one cell thick, across which oxygen and metabolic exchanges take place. As blood flows through the capillaries in the lungs, it changes from venous blood to arterial blood by unloading carbon dioxide and picking up oxygen. Its colour changes in the process from a deep crimson to a bright scarlet. As blood flows through tissue capillaries, it changes back from arterial blood to venous blood. The oxygen leaves the blood to enter cells, and the carbon dioxide leaves the cells and enters the blood. Research Capillaries
In the female, once a graafian follicle discharges its mature ovum, the cavity once occupied by the egg is replaced by luteal cells made of a yellow lipoid material. Together, the erupted graafian follicle and its clot of luteal cells compose the corpus luteum. If the ovum is fertilized, the corpus luteum will eventually create hormones which regulate the development of the placenta, the suppression of menstruation, the growth of the mammary glands, and the eventual development of more mature ova. If the ovum is not fertilized, the corpus luteum will become interpenetrated by bloodcapillaries and will eventualy disintegrate to leave a small scartissue called the corpus albicans. Research Corpus Luteum
The process by which tissues respond to injury or infection is known as inflammation. The term arose of course as a graphic description of the 'fieriness' of the tissues in this condition. Everyone is familiar with classical symptoms and signs of inflammation. It is important to realise that the tissues respond both to injury and infection in a similar manner, although the response to infection is usually more dramatic and accompanied by a greater general effect on the patient. Infection and inflammation are not synonymous. The underlying change in inflamed tissue is the great increase in the amount of blood flowing into the area involved. The chemicals released by the damaged tissues produce dilatation of the arteries, veins and capillaries to such an extent that serum exudes from the capillaries into the tissues, producing oedema. This distension of the tissues with fluid - the inflammatory exudate - is the main factor producing pain in inflammation. Pain and swelling together result in loss of function. This local inflammatory reaction is the beginning of the healing process and is at the same time a means of defence
against infection. White blood cells appear at the site of inflammation in great numbers and are responsible for the local control of bacteria which may have gained entrance. The invading organisms or the poisons which they have produced are carried away from the site of inflammation, partly by the veins and partly by the lymphatic system. As infection travels up the lymphatic vessels, these too become involved in the inflammatory process (lymphangitis). The lymph glands to which these lymphatics drain will become similarly inflamed, large and tender (lymphadenitis). Where there has been injury without infection, in addition to the inflammation, poisonous substances are released from the damaged tissues and these circulate in the bloodstream, producing a state of shock. Research Inflammation
The portal vein is a large vein (a little over eight centimeters in length) responsible for carrying the oxygen and nutrient poor blood from the organs of the abdomen to the liver, where wastes will be eliminated. The portal vein enters the underside of the liver and divides into a network of capillaries. The capillaries then reconverge and form the hepatic veins that carry the blood to the inferior vena cava. The veins of the liver are relatively large to accommodate the great volume of blood passing through it. The liver receives 28% of the body's total cardiac output of blood, which equates to 1.4 liters of blood circulating through an average adult at rest every minute. Research Portal Vein
One quarter of the total blood output from the heart comes to the kidneys along the renal artery. Two renal arteries arise from the abdominal section of the aorta, each artery supplies a lobe of the kidney. The incoming artery divides into four or five branches, eventually forming arterioles, each of which leads to the compact ball of capillaries called the glomerulus. Research Renal Artery
The Probert Encyclopaedia was designed, edited and programed by
Matt and Leela Probert
Abbreviations
Research Circulation
