Circulatory system: Difference between revisions

The blood '''circulatory system''' is a [[organ system|system of organs]] that includes the [[heart]], [[blood vessel]]s, and [[blood]] which is circulated throughout the entire body of a human or other vertebrate. It includes the '''cardiovascular system''', or '''vascular system''', that consists of the heart and blood vessels (from Greek ''kardia'' meaning ''heart'', and from Latin ''vascula'' meaning ''vessels''). The circulatory system has two divisions, a [[systemic circulation|systemic circulation or circuit]], and a [[pulmonary circulation|pulmonary circulation or circuit]]. Some sources use the terms ''cardiovascular system'' and ''vascular system'' interchangeably with the ''circulatory system''.

In vertebrates, the [[lymphatic system]] is complementary to the circulatory system. The lymphatic system carries excess plasma ([[Capillary filtration|filtered]] from the circulatory system [[capillaries]] as [[interstitial fluid]] between cells) away from the body tissues via accessory routes that return excess fluid back to blood circulation as [[lymph]]. 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. The circulation of lymph takes much longer than that of blood and, unlike the closed (blood) circulatory system, the lymphatic system is an open system. Some sources describe it as a ''secondary circulatory system''.

The circulatory system includes the [[heart]], [[blood vessel]]s, and [[blood]]. The '''cardiovascular system''' in all vertebrates, consists of the heart and blood vessels. The circulatory system is further divided into two major circuits – a [[pulmonary circulation]], and a [[systemic circulation]]. The pulmonary circulation is a circuit loop from the [[right heart]] taking deoxygenated blood to the [[lung]]s where it is oxygenated and returned to the [[left 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 via [[Great vessels|large veins]] known as the [[venae cavae]]. The systemic circulation can also be defined as two parts – a ''macrocirculation'' and a ''[[microcirculation]]''. 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. Blood consists of [[Blood plasma|plasma]], [[red blood cells]], [[white blood cells]], and [[platelets]]. The [[digestive system]] also works with the circulatory system to provide the nutrients the system needs to keep the [[heart]] pumping.

The human circulatory system is [[closed circulatory system|closed]], meaning that the blood is contained within the [[blood vessels|vascular network]]. Nutrients travel through tiny blood vessels of the microcirculation to reach organs. The [[lymphatic system]] is an essential subsystem of the circulatory system consisting of a network of [[lymphatic vessel]]s, [[lymph nodes]], [[lymphatic organs|organs]], [[lymphatic tissues|tissues]] and circulating [[lymph]]. This subsystem is an [[open circulatory system|open system]]. A major function is to carry the lymph, draining and returning [[interstitial fluid]] into the [[lymphatic duct]]s back to the heart for return to the circulatory system. Another [[Lymphatic system#Function|major function]] is working together with the [[immune system]] to provide defense against [[pathogen]]s.

Oxygen-deprived blood from the superior and inferior [[vena cava]] enters the right atrium of the heart and flows through the [[tricuspid valve]] (right atrioventricular valve) into the right ventricle, from which it is then pumped through the [[pulmonary semilunar valve]] into the pulmonary artery to the lungs. [[Gas exchange]] occurs in the lungs, whereby {{CO2|link=yes}} is released from the blood, and oxygen is absorbed. The pulmonary vein returns the now oxygen-rich blood to the [[left atrium]].

The systemic circulation is a circuit loop that delivers oxygenated blood from the left heart to the rest of the body through the [[aorta]]. Deoxygenated blood is returned in the systemic circulation to the right heart via two large veins, the [[inferior vena cava]] and [[superior 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 a [[microcirculation]].

The [[blood vessel]]s of the circulatory system are the [[arteries]], [[veins]], and [[capillaries]]. The large arteries and veins that take blood to, and away from the heart are known as the [[great vessels]].  

Oxygenated blood enters the systemic circulation when leaving the left ventricle, via the [[Aortic valve|aortic semilunar valve]]. 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. Later, it descends down and supplies branches to abdomen, pelvis, perineum and the lower limbs.

The walls of the aorta are elastic. This elasticity helps to maintain the [[blood 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. As the aorta branches into smaller arteries, their elasticity goes on decreasing and their compliance goes on increasing.

Arteries branch into small passages called [[arteriole]]s and then into the [[Capillary|capillaries]]. The capillaries merge to bring blood into the venous system.

Capillaries merge into [[venule]]s, which merge into veins. The [[venous 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.

The coronary circulation system provides a blood supply to the [[myocardium|heart muscle]] itself. The coronary circulation begins near the origin of the aorta by two [[coronary arteries]]: the [[right coronary artery]] and the [[left coronary artery]]. After nourishing the heart muscle, blood returns through the coronary veins into the [[coronary sinus]] and from this one into the right atrium. Backflow of blood through its opening during [[atrial systole]] is prevented by the [[Thebesian valve]]. The [[smallest cardiac veins]] drain directly into the heart chambers.

The brain has a dual blood supply, an ''anterior'' and a ''posterior circulation'' from arteries at its front and back. The anterior circulation arises from the [[Internal carotid artery|internal carotid arteries]] to supply the front of the brain. The posterior circulation arises from the [[Vertebral artery|vertebral arteries]], to supply the back of the brain and [[brainstem]]. The circulation from the front and the back join ([[anastomise]]) at the [[circle of Willis]]. The [[neurovascular 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.

The development of the circulatory system starts with [[vasculogenesis]] in the [[embryo]]. The human arterial and venous systems develop from different areas in the embryo. The arterial system develops mainly from the [[aortic 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 of [[Human embryogenesis|embryogenesis]]. [[Fetal circulation]] begins within the 8th week of development. Fetal circulation does not include the lungs, which are bypassed via the [[truncus arteriosus]]. Before birth the [[fetus]] obtains [[oxygen]] (and [[nutrient]]s) from the mother through the [[placenta]] and the [[umbilical cord]].

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.

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.

The blood vascular system first appeared probably in an ancestor of the [[triploblasts]] over 600 million years ago, overcoming the time-distance constraints of diffusion, while [[endothelium]] evolved in an ancestral vertebrate some 540–510 million years ago.

In [[arthropod]]s, the open circulatory system is a system in which a fluid in a [[body cavity|cavity]] called the '''hemocoel''' bathes the organs directly with oxygen and nutrients, with there being no distinction between blood and interstitial fluid; this combined fluid is called [[hemolymph]] or haemolymph. Muscular movements by the animal during [[Animal locomotion|locomotion]] 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).

The circulatory systems of all vertebrates, as well as of [[annelid]]s (for example, [[earthworm]]s) and [[cephalopod]]s ([[squid]]s, [[octopus]]es and relatives) always keep their circulating blood enclosed within heart chambers or blood vessels and are classified as ''closed'', just as in humans. Still, the systems of [[fish]], [[amphibian]]s, [[reptile]]s, and [[bird]]s show various stages of the [[evolution]] of the circulatory system. Closed systems permit blood to be directed to the organs that require it.

Mammals, birds and [[crocodilia]]ns 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. Double circulatory systems permit blood to be repressurized after returning from the lungs, speeding up delivery of oxygen to tissues.

In the 6th century BCE, the knowledge of circulation of vital fluids through the body was known to the [[Ayurveda|Ayurvedic]] physician [[Sushruta]] in [[History of India|ancient India]]. He also seems to have possessed knowledge of the arteries, described as 'channels' by Dwivedi & Dwivedi (2007). The [[Heart valve|valves of the heart]] were discovered by a physician of the [[Hippocrates|Hippocratean]] 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 the transport of air.

In 1025, ''[[The Canon of Medicine]]'' by the [[Ancient Iranian Medicine|Persian 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 the [[cardiac cycle]]s and valvular function", and "had a vision of blood circulation" in his ''Treatise on Pulse''. 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."

In addition, Ibn al-Nafis had an insight into what would become a larger theory of the [[capillary]] 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. 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" (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.

Finally, the English physician [[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 published his ''[[Exercitatio 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." 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 by [[Marcello Malpighi]] in 1661.

In 1956, [[André Frédéric Cournand]], [[Werner Forssmann]] and [[Dickinson W. Richards]] were awarded the [[List of Nobel laureates in Physiology or Medicine|Nobel Prize]] in Medicine "for their discoveries concerning [[heart catheterization]] and pathological changes in the circulatory system."

In his Nobel lecture, Forssmann credits Harvey as birthing cardiology with the publication of his book in 1628.

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