Circulatory system
Anatomical terminology

Thecirculatory system, also called thecardiovascular system or thevascular system, is anorgan system that permitsblood to circulate and transportnutrients (such asamino acids andelectrolytes),oxygen,carbon dioxide,hormones, andblood cells to and from thecells in the body to provide nourishment and help infighting diseases,stabilize temperature andpH, and maintainhomeostasis.

The circulatory system includes thelymphatic system, which circulateslymph.^[1] The passage of lymph takes much longer than that of blood.^[2] Blood is a fluid consisting ofplasma,red blood cells,white blood cells, andplatelets that is circulated by theheart through the vertebrate vascular system, carrying oxygen and nutrients to and waste materials away from all body tissues. Lymph is essentially recycled excess blood plasma after it has beenfiltered from theinterstitial fluid (between cells) and returned to the lymphatic system. The cardiovascular (from Latin words meaning "heart" and "vessel") system comprises the blood, heart, andblood vessels.^[3] The lymph,lymph nodes, andlymph vessels form the lymphatic system, which returns filtered blood plasma from the interstitial fluid (between cells) as lymph.

The circulatory system of the blood is seen as having two components, a systemic circulation and a pulmonary circulation.^[4]

While humans, as well as othervertebrates, have a closed cardiovascular system (meaning that the blood never leaves the network ofarteries,veins andcapillaries), someinvertebrate groups have an open cardiovascular system. The lymphatic system, on the other hand, is an open system providing an accessory route for excess interstitial fluid to be returned to the blood.^[5] The more primitive,diploblastic animalphyla lack circulatory systems.

Many diseases affect the circulatory system. This includescardiovascular disease, affecting the cardiovascular system, andlymphatic disease affecting the lymphatic system.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 other parts of the circulatory system.

Structure

Cardiovascular system

Depiction of the heart, major veins and arteries constructed from body scans

Cross section of a human artery

The essential components of the human cardiovascular system are theheart,blood andblood vessels.^[6] It includes thepulmonary circulation, a "loop" through thelungs where blood is oxygenated; and the systemic circulation, a "loop" through the rest of the body to provideoxygenated blood. The systemic circulation can also be seen to function in 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.^[7] Blood consists ofplasma,red blood cells,white blood cells, andplatelets. Also, thedigestive system works with the circulatory system to provide the nutrients the system needs to keep theheart pumping.^[8]

The cardiovascular systems of humans are closed, meaning that the blood never leaves the network ofblood vessels. In contrast, oxygen and nutrients diffuse across the blood vessel layers and enterinterstitial fluid, which carries oxygen and nutrients to the target cells, and carbon dioxide and wastes in the opposite direction. The other component of the circulatory system, thelymphatic system, is open.

Arteries

Oxygenated blood enters the systemic circulation when leaving theleft ventricle, through theaortic semilunar valve. The first part of the systemic circulation is theaorta, 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. Later it descends down and supplies branches to abdomen, pelvis, perineum and the lower limbs. The walls of aorta are elastic. This elasticity helps to maintain theblood pressure throughout the body. When the aorta receives almost five litres of blood from the heart, it recoils and is responsible for pulsating blood pressure. Moreover, as aorta branches into smaller arteries, their elasticity goes on decreasing and their compliance goes on increasing.

Capillaries

Arteries branch into small passages calledarterioles and then into thecapillaries.^[9] The capillaries merge to bring blood into the venous system.^[10]

Veins

Capillaries merge intovenules, which merge intoveins. The venous system feeds into the two major veins: thesuperior vena cava – which mainly drains tissues above the heart – and theinferior vena cava – which mainly drains tissues below the heart. These two large veins empty into theright atrium of theheart.

Portal veins

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.

Heart

The heart pumps oxygenated blood to the body and deoxygenated blood to the lungs. In the humanheart 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.

Coronary vessels

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 theaorta 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. Back flow of blood through its opening duringatrial systole is prevented byThebesian valve. Thesmallest cardiac veins drain directly into the heart chambers.^[8]

Lungs

Thecirculatory system of the lungs is the portion of the cardiovascular system in whichoxygen-depletedblood is pumped away from the heart, via thepulmonary artery, to thelungs and returned, oxygenated, to the heart via thepulmonary vein.

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, wherebyCO
2 is released from the blood, and oxygen is absorbed. The pulmonary vein returns the now oxygen-rich blood to theleft atrium.^[8]

A separate system known as thebronchial circulation supplies blood to the tissue of the larger airways of the lung.

Systemic circulation

Systemic circulation is the portion of the cardiovascular system which transports oxygenated blood away from the heart through the aorta from the left ventricle where the blood has been previously deposited from pulmonary circulation, to the rest of the body, and returns oxygen-depleted blood back to the heart.^[8]

Brain

The brain has a dual blood supply that comes from arteries at its front and back. These are called the "anterior" and "posterior" circulation respectively. The anterior circulation arises from theinternal carotid arteries and supplies the front of the brain. The posterior circulation arises from thevertebral arteries, and supplies the back of the brain andbrainstem. The circulation from the front and the back join together (anastomise) at theCircle of Willis.

Kidneys

Therenal circulation receives around 20% of the cardiac output. It branches from theabdominal aorta and returns blood to the ascendingvena cava. It is the blood supply to thekidneys, and contains many specialized blood vessels.

Lymphatic system

Thelymphatic system is part of the circulatory system. It is a network oflymphatic vessels andlymph capillaries,lymph nodes andorgans, andlymphatic tissues and circulatinglymph. One of its major functions is to carry the lymph, draining and returninginterstitial fluid back towards the heart for return to the cardiovascular system, by emptying into thelymphatic ducts. Its other main function is in theadaptive immune system.^[11]

Development

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 which 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.^[12]

Heart

Arteries

The human arterial system originates from the aortic arches and from thedorsal aortae starting from week 4 of embryonic life. The first and second aortic arches regress and forms 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 theaorta 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.

Veins

The human venous system develops mainly from thevitelline veins, theumbilical veins and thecardinal veins, all of which empty into thesinus venosus.

Function

Cardiovascular system

About 98.5% of theoxygen in a sample of arterial 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 inmammals and many other species.

Lymphatic system

Clinical significance

Many diseases affect the circulatory system. These include a number ofcardiovascular diseases, affecting the cardiovascular system, andlymphatic diseases affecting the lymphatic system.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 other parts of the circulatory system.

Cardiovascular disease

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.

Investigations

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.

Other more invasive means can also be used. Acannula orcatheter inserted into an artery may be used to measurepulse pressure orpulmonary wedge pressures. Angiography, which involves injecting a dye into an artery to visualise an arterial tree, can be used in the heart (coronary angiography) or brain. At the same time as the arteries are visualised, blockages or narrowings may be fixed through the insertion ofstents, and active bleeds may be managed by the insertion of coils. An MRI may be used to image arteries, called anMRI angiogram. For evaluation of the blood supply to the lungs aCT pulmonary angiogram may be used.

Vascular ultrasonography include for example:

• Intravascular ultrasound
• Ultrasonography of deep venous thrombosis
• Ultrasonography of chronic venous insufficiency of the legs

Surgery

There are a number of surgical procedures performed on the circulatory system:

• Coronary artery bypass surgery
• Coronary stent used inangioplasty
• Vascular surgery
• Vein stripping
• Cosmetic procedures

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.^[13]

Society and culture

In Ancient Greece, the heart was thought to be the source of innate heat for the body.The circulatory system as we know it was discovered byWilliam Harvey.

Other animals

While humans, as well as othervertebrates, have a closed cardiovascular system (meaning that the blood never leaves the network ofarteries,veins andcapillaries), someinvertebrate groups have an open cardiovascular system. The lymphatic system, on the other hand, is an open system providing an accessory route for excess interstitial fluid to be returned to the blood.^[5] The more primitive,diploblastic animalphyla lack circulatory systems.

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 ancestralvertebrate some 540–510 million years ago.^[14]

Open circulatory system

Inarthropods, the open circulatory system is a system in which a fluid in a cavity called the hemocoel bathes the organs directly with oxygen and nutrients and there is no distinction betweenblood andinterstitial fluid; this combined fluid is calledhemolymph or haemolymph.^[15] Muscular movements by the animal duringlocomotion can facilitate hemolymph movement, but diverting flow from one area to another is limited. When theheart relaxes, blood is drawn back toward the heart through open-ended pores (ostia).

Hemolymph fills all of the interior hemocoel of the body and surrounds allcells. Hemolymph is composed ofwater,inorganicsalts (mostlysodium,chloride,potassium,magnesium, andcalcium), andorganic compounds (mostlycarbohydrates,proteins, andlipids). The primary oxygen transporter molecule ishemocyanin.

There are free-floating cells, thehemocytes, within the hemolymph. They play a role in the arthropodimmune system.

Closed circulatory system

The circulatory systems of allvertebrates, as well as ofannelids (for example,earthworms) andcephalopods (squids,octopuses and relatives) areclosed, just as in humans. Still, the systems offish,amphibians,reptiles, andbirds show various stages of theevolution of the circulatory system.^[16]

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).

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.

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 theleft 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.

Birds, mammals, 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.^[17]

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 surroundingwater into the cells, andcarbon 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

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.

In the 6th century BCE, the knowledge of circulation of vital fluids through the body was known to theAyurvedic physicianSushruta inancient India.^[18] He also seems to have possessed knowledge of thearteries, described as 'channels' by Dwivedi & Dwivedi (2007).^[18] Thevalves of the heart were discovered by a physician of theHippocratean school around the 4th century BCE. 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.

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 entered by very small vessels between veins and arteries. Thus he apparently postulated capillaries but with reversed flow of blood.^[19]

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.

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.

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.^{[20][verification needed]} 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."^[21]

In 1242, theArabian physician,Ibn al-Nafis, became the first person to accurately describe the process ofpulmonary circulation, for which he is sometimes considered the father ofcirculatory physiology.^{[22][failed verification]} Ibn al-Nafis stated in hisCommentary on Anatomy in Avicenna's Canon:

"...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 would become 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.^[23] Ibn al-Nafis' theory, however, 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"^[24][25] (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.

Better known discovery of pulmonary circulation was byVesalius's successor atPadua,Realdo Colombo, in 1559.

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."^[26] This work, with its essentially correct exposition, slowly convinced the medical world. However, Harvey was not able to identify the capillary system connecting arteries and veins; these were later discovered byMarcello Malpighi in 1661.

In 1956,André Frédéric Cournand,Werner Forssmann andDickinson W. Richards were awarded theNobel Prize in Medicine "for their discoveries concerningheart catheterization and pathological changes in the circulatory system."^[27]In his Nobel lecture, Forssmann credits Harvey as birthing cardiology with the publication of his book in 1628.^[28]

In the 1970s,Diana McSherry developed computer-based systems to create images of the circulatory system and heart without the need for surgery.^[29]

See also

References

External links

Look upcirculatory system in Wiktionary, the free dictionary.

• Circulatory PathwaysArchived 2021-08-28 at theWayback Machine inAnatomy and Physiology byOpenStax
• The Circulatory System
• Michael Servetus Research Study on the Manuscript of Paris by Servetus (1546 description of the Pulmonary Circulation)

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