Invertebrates, thecirculatory system is asystem of organs that includes theheart,blood vessels, andblood which is circulated throughout the body.[1][2] It includes thecardiovascular system, orvascular system, that consists of the heart and blood vessels (from Greekkardia meaningheart, and Latinvascula meaningvessels). The circulatory system has two divisions, asystemic circulation or circuit, and apulmonary circulation or circuit.[3] Some sources use the termscardiovascular system andvascular system interchangeably withcirculatory system.[4]
In vertebrates, thelymphatic system is complementary to the circulatory system. The lymphatic system carries excess plasma (filtered from the circulatory systemcapillaries asinterstitial fluid between cells) away from the body tissues via accessory routes that return excess fluid back to blood circulation aslymph.[5] The lymphatic system is a subsystem that is essential for the functioning of the blood circulatory system; without it the blood would become depleted of fluid.
The lymphatic system also works with the immune system.[6] The circulation of lymph takes much longer than that of blood[7] and, unlike the closed (blood) circulatory system, the lymphatic system is an open system. Some sources describe it as asecondary circulatory system.
The circulatory system can be affected by manycardiovascular diseases.Cardiologists are medical professionals which specialise in the heart, andcardiothoracic surgeons specialise in operating on the heart and its surrounding areas.Vascular surgeons focus on disorders of the blood vessels, and lymphatic vessels.
Structure
Blood flow in the pulmonary and systemic circulations showing capillary networks in the torso sections
The circulatory system includes theheart,blood vessels, andblood.[2] Thecardiovascular system in all vertebrates, consists of the heart and blood vessels. The circulatory system is further divided into two major circuits – apulmonary circulation, and asystemic circulation.[8][1][3] The pulmonary circulation is a circuit loop from theright heart taking deoxygenated blood to thelungs where it is oxygenated and returned to theleft heart. The systemic circulation is a circuit loop that delivers oxygenated blood from the left heart to the rest of the body, and returns deoxygenated blood back to the right heart vialarge veins known as thevenae cavae. The systemic circulation can also be defined as two parts – amacrocirculation and amicrocirculation. An average adult contains five to six quarts (roughly 4.7 to 5.7 liters) of blood, accounting for approximately 7% of their total body weight.[9] Blood consists ofplasma,red blood cells,white blood cells, andplatelets. Thedigestive system also works with the circulatory system to provide the nutrients the system needs to keep theheart pumping.[10]
Diagram of the humanheart showing blood oxygenation to the pulmonary and systemic circulation
The heart pumps blood to all parts of the body providingnutrients andoxygen to everycell, and removing waste products. The left heart pumps oxygenated blood returned from the lungs to the rest of the body in thesystemic circulation. The right heart pumps deoxygenated blood to the lungs in thepulmonary circulation. In the human heart there is oneatrium and oneventricle for each circulation, and with both a systemic and a pulmonary circulation there are four chambers in total:left atrium,left ventricle,right atrium andright ventricle. The right atrium is the upper chamber of the right side of the heart. The blood that is returned to the right atrium is deoxygenated (poor in oxygen) and passed into the right ventricle to be pumped through the pulmonary artery to the lungs for re-oxygenation and removal of carbon dioxide. The left atrium receives newly oxygenated blood from the lungs as well as the pulmonary vein which is passed into the strong left ventricle to be pumped through the aorta to the different organs of the body.
Oxygen-deprived blood from the superior and inferiorvena cava enters the right atrium of the heart and flows through thetricuspid valve (right atrioventricular valve) into the right ventricle, from which it is then pumped through thepulmonary semilunar valve into the pulmonary artery to the lungs.Gas exchange occurs in the lungs, wherebyCO2 is released from the blood, and oxygen is absorbed. The pulmonary vein returns the now oxygen-rich blood to theleft atrium.[10]
A separate circuit from the systemic circulation, thebronchial circulation supplies blood to the tissue of the larger airways of the lung.
Systemic circulation
Capillary bedDiagram of capillary network joining the arterial system with the venous system
The systemic circulation is a circuit loop that delivers oxygenated blood from the left heart to the rest of the body through theaorta. Deoxygenated blood is returned in the systemic circulation to the right heart via two large veins, theinferior vena cava andsuperior vena cava, where it is pumped from the right atrium into the pulmonary circulation for oxygenation. The systemic circulation can also be defined as having two parts – a macrocirculation and amicrocirculation.[10]
Depiction of the heart, major veins and arteries constructed from body scans
Oxygenated blood enters the systemic circulation when leaving the left ventricle, via theaortic semilunar valve.[15] The first part of the systemic circulation is the aorta, a massive and thick-walled artery. The aorta arches and gives branches supplying the upper part of the body after passing through the aortic opening of the diaphragm at the level of thoracic ten vertebra, it enters the abdomen.[16] Later, it descends down and supplies branches to abdomen, pelvis, perineum and the lower limbs.[17]
The walls of the aorta are elastic. This elasticity helps to maintain theblood pressure throughout the body.[18] When the aorta receives almost five litres of blood from the heart, it recoils and is responsible for pulsating blood pressure. As the aorta branches into smaller arteries, their elasticity goes on decreasing and their compliance goes on increasing.[18]
Capillaries
Arteries branch into small passages calledarterioles and then into thecapillaries.[19] The capillaries merge to bring blood into the venous system.[20] The total length of muscle capillaries in a 70 kg human is estimated to be between 9,000 and 19,000 km.[21]
Capillaries merge intovenules, which merge into veins.[22] Thevenous system feeds into the two major veins: the superior vena cava – which mainly drains tissues above the heart – and the inferior vena cava – which mainly drains tissues below the heart. These two large veins empty into the right atrium of the heart.[23]
The general rule is that arteries from the heart branch out into capillaries, which collect into veins leading back to the heart.Portal veins are a slight exception to this. In humans, the only significant example is thehepatic portal vein which combines from capillaries around thegastrointestinal tract where the blood absorbs the various products of digestion; rather than leading directly back to the heart, the hepatic portal vein branches into a second capillary system in theliver.
The heart itself is supplied with oxygen and nutrients through a small "loop" of the systemic circulation and derives very little from the blood contained within the four chambers.The coronary circulation system provides a blood supply to theheart muscle itself. The coronary circulation begins near the origin of the aorta by twocoronary arteries: theright coronary artery and theleft coronary artery. After nourishing the heart muscle, blood returns through the coronary veins into thecoronary sinus and from this one into the right atrium. Backflow of blood through its opening duringatrial systole is prevented by theThebesian valve. Thesmallest cardiac veins drain directly into the heart chambers.[10]
The brain has a dual blood supply, ananterior and aposterior circulation from arteries at its front and back. The anterior circulation arises from theinternal carotid arteries to supply the front of the brain. The posterior circulation arises from thevertebral arteries, to supply the back of the brain andbrainstem. The circulation from the front and the back join (anastomise) at thecircle of Willis. Theneurovascular unit, composed of various cells and vasculature channels within the brain, regulates the flow of blood to activated neurons in order to satisfy their high energy demands.[24]
Renal circulation
Therenal circulation is the blood supply to thekidneys, contains many specialized blood vessels and receives around 20% of the cardiac output. It branches from theabdominal aorta and returns blood to the ascendinginferior vena cava.
The development of the circulatory system starts withvasculogenesis in theembryo. The human arterial and venous systems develop from different areas in the embryo. The arterial system develops mainly from theaortic arches, six pairs of arches that develop on the upper part of the embryo. The venous system arises from three bilateral veins during weeks 4 – 8 ofembryogenesis.Fetal circulation begins within the 8th week of development. Fetal circulation does not include the lungs, which are bypassed via thetruncus arteriosus. Before birth thefetus obtainsoxygen (andnutrients) from the mother through theplacenta and theumbilical cord.[25]
Animation of a typical human red blood cell cycle in the circulatory system. This animation occurs at a faster rate (~20 seconds of the average60-second cycle) and shows the red blood cell deforming as it enters capillaries, as well as the bars changing color as the cell alternates in states of oxygenation along the circulatory system.
The human arterial system originates from theaortic arches and from thedorsal aortae starting from week 4 of embryonic life. The first and second aortic arches regress and form only themaxillary arteries andstapedial arteries respectively. The arterial system itself arises from aortic arches 3, 4 and 6 (aortic arch 5 completely regresses).
The dorsal aortae, present on thedorsal side of the embryo, are initially present on both sides of the embryo. They later fuse to form the basis for the aorta itself. Approximately thirty smaller arteries branch from this at the back and sides. These branches form theintercostal arteries, arteries of the arms and legs, lumbar arteries and the lateral sacral arteries. Branches to the sides of the aorta will form the definitiverenal,suprarenal andgonadal arteries. Finally, branches at the front of the aorta consist of thevitelline arteries andumbilical arteries. The vitelline arteries form theceliac,superior andinferior mesenteric arteries of the gastrointestinal tract. After birth, the umbilical arteries will form theinternal iliac arteries.
About 98.5% of the oxygen in a sample ofarterial blood in a healthy human, breathing air at sea-level pressure, is chemically combined withhemoglobin molecules. About 1.5% is physically dissolved in the other blood liquids and not connected to hemoglobin. The hemoglobin molecule is the primary transporter of oxygen in vertebrates.
Diseases affecting the cardiovascular system are calledcardiovascular disease.
Many of these diseases are called "lifestyle diseases" because they develop over time and are related to a person's exercise habits, diet, whether they smoke, and other lifestyle choices a person makes.Atherosclerosis is the precursor to many of these diseases. It is where smallatheromatous plaques build up in the walls of medium and large arteries. This may eventually grow or rupture to occlude the arteries. It is also a risk factor foracute coronary syndromes, which are diseases that are characterised by a sudden deficit of oxygenated blood to the heart tissue. Atherosclerosis is also associated with problems such asaneurysm formation or splitting ("dissection") of arteries.
Another major cardiovascular disease involves the creation of aclot, called a "thrombus". These can originate in veins or arteries.Deep venous thrombosis, which mostly occurs in the legs, is one cause of clots in the veins of the legs, particularly when a person has been stationary for a long time. These clots mayembolise, meaning travel to another location in the body. The results of this may includepulmonary embolus,transient ischaemic attacks, orstroke.
Cardiovascular diseases may also be congenital in nature, such asheart defects orpersistent fetal circulation, where the circulatory changes that are supposed to happen after birth do not. Not all congenital changes to the circulatory system are associated with diseases, a large number areanatomical variations.
The function and health of the circulatory system and its parts are measured in a variety of manual and automated ways. These include simple methods such as those that are part of thecardiovascular examination, including the taking of a person'spulse as an indicator of a person'sheart rate, the taking ofblood pressure through asphygmomanometer or the use of astethoscope to listen to the heart formurmurs which may indicate problems with theheart's valves. Anelectrocardiogram can also be used to evaluate the way in which electricity is conducted through the heart.
Cardiovascular procedures are more likely to be performed in an inpatient setting than in an ambulatory care setting; in the United States, only 28% of cardiovascular surgeries were performed in the ambulatory care setting.[26]
Other animals
The open circulatory system of the grasshopper – made up of a heart, vessels and hemolymph. The hemolymph is pumped through the heart, into the aorta, dispersed into the head and throughout the hemocoel, then back through the ostia in the heart and the process repeated.
While humans, as well as othervertebrates, have a closed blood circulatory system (meaning that the blood never leaves the network of arteries, veins and capillaries), someinvertebrate groups have an open circulatory system containing a heart but limited blood vessels. The most primitive,diploblastic animalphyla lack circulatory systems.
An additional transport system, the lymphatic system, which is only found in animals with a closed blood circulation, is an open system providing an accessory route for excess interstitial fluid to be returned to the blood.[5]
The blood vascular system first appeared probably in an ancestor of thetriploblasts over 600 million years ago, overcoming the time-distance constraints of diffusion, whileendothelium evolved in an ancestral vertebrate some 540–510 million years ago.[27]
Inarthropods, the open circulatory system is a system in which a fluid in acavity called thehemocoel orhaemocoel bathes the organs directly with oxygen and nutrients, with there being no distinction between blood and interstitial fluid; this combined fluid is calledhemolymph or haemolymph.[28] Muscular movements by the animal duringlocomotion can facilitate hemolymph movement, but diverting flow from one area to another is limited. When the heart relaxes, blood is drawn back toward the heart through open-ended pores (ostia).
There are free-floating cells, thehemocytes, within the hemolymph. They play a role in the arthropodimmune system.
Flatworms, such as thisPseudoceros bifurcus, lack specialized circulatory organs.
Closed circulatory system
Two-chambered heart of a fish
The circulatory systems of all vertebrates, as well as ofannelids (for example,earthworms) andcephalopods (squids,octopuses and relatives) always keep their circulating blood enclosed within heart chambers or blood vessels and are classified asclosed, just as in humans. Still, the systems offish,amphibians,reptiles, andbirds show various stages of theevolution of the circulatory system.[29] Closed systems permit blood to be directed to the organs that require it.
In fish, the system has only one circuit, with the blood being pumped through the capillaries of thegills and on to the capillaries of the body tissues. This is known assingle cycle circulation. The heart of fish is, therefore, only a single pump (consisting of two chambers).[citation needed]
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.[citation needed]
In reptiles, theventricular septum of the heart is incomplete and thepulmonary artery is equipped with asphincter muscle. This allows a second possible route of blood flow. Instead of blood flowing through the pulmonary artery to the lungs, the sphincter may be contracted to divert this blood flow through the incomplete ventricular septum into the left ventricle and out through theaorta. This means the blood flows from the capillaries to the heart and back to the capillaries instead of to the lungs. This process is useful toectothermic (cold-blooded) animals in the regulation of their body temperature.[citation needed]
Mammals, birds andcrocodilians 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 and crocodilians evolved independently from that of mammals.[30] Double circulatory systems permit blood to be repressurized after returning from the lungs, speeding up delivery of oxygen to tissues.[citation needed]
No circulatory system
Circulatory systems are absent in some animals, includingflatworms. Theirbody cavity has no lining or enclosed fluid. Instead, a muscularpharynx leads to an extensively brancheddigestive system that facilitates directdiffusion of nutrients to all cells. The flatworm's dorso-ventrally flattened body shape also restricts the distance of any cell from the digestive system or the exterior of the organism.Oxygen can diffuse from the surrounding water into the cells, and carbon dioxide can diffuse out. Consequently, every cell is able to obtain nutrients, water and oxygen without the need of a transport system.
Some animals, such asjellyfish, have more extensive branching from theirgastrovascular cavity (which functions as both a place of digestion and a form of circulation), this branching allows for bodily fluids to reach the outer layers, since the digestion begins in the inner layers.
History
Human anatomical chart of blood vessels, with heart, lungs, liver and kidneys included. Other organs are numbered and arranged around it. Before cutting out the figures on this page,Vesalius suggests that readers glue the page onto parchment and gives instructions on how to assemble the pieces and paste the multilayered figure onto a base "muscle man" illustration. "Epitome", fol.14a. HMD Collection, WZ 240 V575dhZ 1543.
The earliest known writings on the circulatory system are found in theEbers Papyrus (16th century BCE), anancient Egyptian medical papyrus containing over 700 prescriptions and remedies, both physical and spiritual. In thepapyrus, it acknowledges the connection of the heart to the arteries. The Egyptians thought air came in through the mouth and into the lungs and heart. From the heart, the air travelled to every member through the arteries. Although this concept of the circulatory system is only partially correct, it represents one of the earliest accounts of scientific thought.[citation needed]
In the 6th century BCE, the knowledge of circulation of vital fluids through the body was known to theAyurvedic physicianSushruta inancient India.[31] He also seems to have possessed knowledge of the arteries, described as 'channels' by Dwivedi & Dwivedi (2007).[31] The first major ancient Greek research into the circulatory system was completed by Plato in theTimaeus, who argues that blood circulates around the body in accordance with the general rules that govern the motions of the elements in the body; accordingly, he does not place much importance in the heart itself.[32] Thevalves of the heart were discovered by a physician of theHippocratic school around the early 3rd century BC.[33] 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 the transport of air.[citation needed]
TheGreek physician,Herophilus, distinguished veins from arteries but thought that thepulse was a property of arteries themselves. Greek anatomistErasistratus 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 enters between veins and arteries by very small vessels. Thus he apparently postulated capillaries but with reversed flow of blood.[citation needed]
In 2nd-century ADRome, theGreek physicianGalen 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.[citation needed] 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.[citation needed]
In 1025,The Canon of Medicine by thePersian physician,Avicenna, "erroneously accepted the Greek notion regarding the existence of a hole in the ventricular septum by which the blood traveled between the ventricles." Despite this, Avicenna "correctly wrote on thecardiac cycles and valvular function", and "had a vision of blood circulation" in hisTreatise on Pulse.[34] While also refining Galen's erroneous theory of the pulse, Avicenna provided the first correct explanation of pulsation: "Every beat of the pulse comprises two movements and two pauses. Thus, expansion : pause : contraction : pause. [...] The pulse is a movement in the heart and arteries ... which takes the form of alternate expansion and contraction."[35]
In 1242, theArabian physician,Ibn al-Nafis described the process ofpulmonary circulation in greater, more accurate detail than his predecessors, though he believed, as they did, in the notion of vital spirit (pneuma), which he believed was formed in the left ventricle. Ibn al-Nafis stated in hisCommentary on Anatomy in Avicenna's Canon:[36]
...the blood from the right chamber of the heart must arrive at the left chamber but there is no direct pathway between them. The thick septum of the heart is not perforated and does not have visible pores as some people thought or invisible pores as Galen thought. The blood from the right chamber must flow through the vena arteriosa (pulmonary artery) to the lungs, spread through its substances, be mingled there with air, pass through the arteria venosa (pulmonary vein) to reach the left chamber of the heart and there form the vital spirit...
In addition, Ibn al-Nafis had an insight into what later became a larger theory of thecapillary circulation. He stated that "there must be small communications or pores (manafidh in Arabic) between the pulmonary artery and vein," a prediction that preceded the discovery of the capillary system by more than 400 years.[36] Ibn al-Nafis' theory was confined to blood transit in the lungs and did not extend to the entire body.
Michael Servetus was the first European to describe the function of pulmonary circulation, although his achievement was not widely recognized at the time, for a few reasons. He firstly described it in the "Manuscript of Paris"[37][38] (near 1546), but this work was never published. And later he published this description, but in a theological treatise,Christianismi Restitutio, not in a book on medicine. Only three copies of the book survived but these remained hidden for decades, the rest were burned shortly after its publication in 1553 because of persecution of Servetus by religious authorities.[citation needed]
Finally, the English physicianWilliam Harvey, a pupil ofHieronymus Fabricius (who had earlier described the valves of the veins without recognizing their function), performed a sequence of experiments and published hisExercitatio Anatomica de Motu Cordis et Sanguinis in Animalibus in 1628, which "demonstrated that there had to be a direct connection between the venous and arterial systems throughout the body, and not just the lungs. Most importantly, he argued that the beat of the heart produced a continuous circulation of blood through minute connections at the extremities of the body. This is a conceptual leap that was quite different from Ibn al-Nafis' refinement of the anatomy and bloodflow in the heart and lungs."[39] This work, with its essentially correct exposition, slowly convinced the medical world. However, Harvey did not identify the capillary system connecting arteries and veins; this was discovered byMarcello Malpighi in 1661.[citation needed]
^abHall, John E. (2011).Guyton and Hall textbook of medical physiology (Twelfth ed.). Philadelphia, Pennsylvania. p. 4.ISBN9781416045748.{{cite book}}: CS1 maint: location missing publisher (link)
^abSaladin, Kenneth S. (2011).Human anatomy (3rd ed.). New York: McGraw-Hill. p. 520.ISBN9780071222075.
^abSaladin, Kenneth S. (2011).Human anatomy (3rd ed.). New York: McGraw-Hill. p. 540.ISBN9780071222075.
^Alberts, B.; Johnson, A.; Lewis, J.; Raff, M.; Roberts, K.; Walters, P. (2002).Molecular Biology of the Cell (4th ed.). New York and London: Garland Science.ISBN978-0-8153-3218-3.Archived from the original on 17 August 2006. Retrieved30 August 2017.
^Standring, Susan (2016).Gray's anatomy : the anatomical basis of clinical practice (Forty-first ed.). [Philadelphia]: Elsevier Limited. p. 1024.ISBN9780702052309.
^SeeTimaeus 77a–81e. For a scholarly discussion, see Douglas R. Campbell, "Irrigating Blood: Plato on the Circulatory System, the Cosmos, and Elemental Motion,"Journal of the History of Philosophy 62 (4): 519–541. 2024. See also Francis Cornford,Plato's Cosmology: The Timaeus of Plato, Indianapolis: Hackett, 1997.
^The central text here is the Hippocratic textOn The Heart, which Elizabeth Craik argues was written between 300 and 250 BC. See Craik, Elizabeth. 2015.The ‘Hippocratic’ Corpus: Content and Context. New York: Routledge.
^Shoja, M.M.; Tubbs, R.S.; Loukas, M.; Khalili, M.; Alakbarli, F.; Cohen-Gadol, A.A. (2009). "Vasovagal syncope in the Canon of Avicenna: The first mention of carotid artery hypersensitivity".International Journal of Cardiology.134 (3):297–301.doi:10.1016/j.ijcard.2009.02.035.PMID19332359.
^Gonzalez Etxeberria, Patxi (2011)Amor a la verdad, el – vida y obra de Miguel servet [The love for truth. Life and work of Michael Servetus]. Navarro y Navarro, Zaragoza, collaboration with the Government of Navarra, Department of Institutional Relations and Education of the Government of Navarra.ISBN84-235-3266-6 pp. 215–228 & 62nd illustration (XLVII)
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