The
circulatory system, also called the
cardiovascular system or the
vascular system, is an
organ system that permits blood to circulate and transport nutrients (such as amino acids and electrolytes), oxygen, carbon dioxide, hormones, and blood cells to and from the cells in the body to provide nourishment and help in fighting diseases, stabilize temperature and pH, and maintain homeostasis. The study of the blood flow is called hemodynamics. The study of the properties of the blood flow is called hemorheology.
The circulatory system is often seen to comprise both the
cardiovascular system, which distributes blood, and the
lymphatic system, which circulates
lymph.[1] These are two separate systems. The passage of lymph for example takes a lot longer than that of blood.[2] Blood is a fluid consisting of plasma, red blood cells, white blood cells, and platelets that is circulated by the heart
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 been filtered from the interstitial fluid
(between cells) and returned to the lymphatic system. The
cardiovascular (from Latin words meaning 'heart' and 'vessel') system
comprises the blood, heart, and blood vessels.[3] The lymph, lymph nodes, and lymph vessels form the lymphatic system, which returns filtered blood plasma from the interstitial fluid (between cells) as lymph.
While humans, as well as other
vertebrates, have a closed cardiovascular system (meaning that the blood never leaves the network of arteries, veins and capillaries), some invertebrate
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.[4] The more primitive, diploblastic animal phyla lack circulatory systems.
Structure
Cardiovascular system
Depiction of the heart, major veins and arteries constructed from body scans.
Cross section of a human artery
Relative percentages of cardiac output delivered to major organ systems
The essential components of the human cardiovascular system are the
heart,
blood and
blood vessels.
[5] It includes the
pulmonary circulation, a "loop" through the
lungs where blood is oxygenated; and the systemic circulation, a "loop" through the rest of the body to provide
oxygenated blood. The systemic circulation can also be seen to function in 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.
[6] Blood consists of
plasma,
red blood cells,
white blood cells, and
platelets. Also, the
digestive system works with the circulatory system to provide the nutrients the system needs to keep the
heart pumping.
[7]
The cardiovascular systems of humans are closed, meaning that the blood never leaves the network of
blood vessels. In contrast, oxygen and nutrients diffuse across the blood vessel layers and enter
interstitial 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, the
lymphatic system, is open.
Arteries
Oxygenated blood enters the systemic circulation when leaving the
left ventricle, through the
aortic semilunar valve. The first part of the systemic circulation is the
aorta,
a massive and thick-walled artery. The aorta arches and branches into
major arteries to the upper body before passing through the diaphragm,
where it branches further into arteries which supply the lower parts of
the body.
Capillaries
Arteries branch into small passages called arterioles and then into the
capillaries.
[8] The capillaries merge to bring blood into the venous system.
[9]
Veins
After their passage through body tissues, capillaries merge once again into
venules, which continue to merge into
veins. The venous system finally coalesces into two major veins: the
superior vena cava (roughly speaking draining the areas above the heart) and the
inferior vena cava (roughly speaking from areas below the heart). These two great vessels empty into the
right atrium of the
heart.
Coronary vessels
The heart itself is supplied with oxygen and nutrients through a small "loop" of the systemic circulation.
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 the
hepatic portal vein which combines from capillaries around the
gut
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 the
liver.
Heart
The heart pumps oxygenated blood to the body and deoxygenated blood to the lungs. In the human
heart there is one
atrium and one
ventricle for each circulation, and with both a systemic and a pulmonary circulation there are four chambers in total:
left atrium,
left ventricle,
right atrium and
right 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.
The
coronary circulation system provides a blood supply to the
heart muscle itself. The coronary circulation begins near the origin of the
aorta by two 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. Back flow of blood through its opening during
atrial systole is prevented by the
Thebesian valve. The
smallest cardiac veins drain directly into the heart chambers.
[7]
Lungs
The
circulatory system of the lungs is the portion of the cardiovascular system in which
oxygen-depleted
blood is pumped away from the heart, via the
pulmonary artery, to the
lungs and returned, oxygenated, to the heart via the
pulmonary vein.
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 is released from the blood, and oxygen is absorbed. The pulmonary vein returns the now oxygen-rich blood to the
left atrium.
[7]
A separate system known as the
bronchial circulation supplies blood to the tissue of the larger airways of the lung.
Systemic circulation
The systemic circulation is the circulation of the blood to all parts
of the body except the lungs. 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.
[7]
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 the
internal carotid arteries and supplies the front of the brain. The posterior circulation arises from the
vertebral arteries, and supplies the back of the brain and
brainstem. The circulation from the front and the back join together (
anastomise) at the
Circle of Willis.
Kidneys
The
renal circulation receives around 20% of the cardiac output. It branches from the
abdominal aorta and returns blood to the ascending
vena cava. It is the blood supply to the
kidneys, and contains many specialized blood vessels.
Lymphatic system
The
lymphatic system is part of the circulatory system. It is a network of
lymphatic vessels and
lymph capillaries,
lymph nodes and
organs, and
lymphatic tissues and circulating
lymph. One of its major functions is to carry the lymph, draining and returning
interstitial fluid back towards the heart for return to the cardiovascular system, by emptying into the
lymphatic ducts. Its other main function is in the
immune system.
Physiology
An animation of a typical human red blood cell cycle in the circulatory
system. This animation occurs at real time (20 seconds of cycle) and
shows the red blood cell deform as it enters capillaries, as well as
changing color as it alternates in states of oxygenation along the
circulatory system.
About 98.5% of the
oxygen in a sample of arterial blood in a healthy human, breathing air at sea-level pressure, is chemically combined with
hemoglobin
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
mammals and many other species.
Development
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 which develop on the upper part of the embryo. The
venous system arises from three bilateral veins during weeks 4 – 8 of
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
nutrients) from the mother through the
placenta and the
umbilical cord.
[10]
Arterial development
Main article:
Aortic arches
The human arterial system originates from the aortic arches and from the
dorsal aortae starting from week 4 of embryonic life. The first and second aortic arches regress and forms only the
maxillary arteries and
stapedial arteries respectively. The arterial system itself arises from aortic arches 3, 4 and 6 (aortic arch 5 completely regresses).
The dorsal aortae, present on the
dorsal 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 the
intercostal arteries,
arteries of the arms and legs, lumbar arteries and the lateral sacral
arteries. Branches to the sides of the aorta will form the definitive
renal,
suprarenal and
gonadal arteries. Finally, branches at the front of the aorta consist of the
vitelline arteries and
umbilical arteries. The vitelline arteries form the
celiac,
superior and
inferior mesenteric arteries of the gastrointestinal tract. After birth, the umbilical arteries will form the
internal iliac arteries.
Venous development
The human venous system develops mainly from the
vitelline veins, the
umbilical veins and the
cardinal veins, all of which empty into the
sinus venosus.
Clinical significance
Many diseases affect the circulatory system. This includes
cardiovascular disease, affecting the cardiovascular system, and
lymphatic disease affecting the lymphatic system.
Cardiologists are medical professionals which specialise in the heart, and
cardiothoracic 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 called
cardiovascular 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 small
atheromatous 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 for
acute coronary syndromes,
which are diseases which are characterised by a sudden deficit of
oxygenated blood to the heart tissue. Atherosclerosis is also associated
with problems such as
aneurysm formation or splitting ("dissection") of arteries.
Another major cardiovascular disease involves the creation of a
clot, 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 may
embolise, meaning travel to another location in the body. The results of this may include
pulmonary embolus,
transient ischaemic attacks, or
stroke.
Cardiovascular diseases may also be congenital in nature, such as
heart defects or
persistent 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 are
anatomical variations.
Measurement techniques
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 the
cardiovascular examination, including the taking of a person's
pulse as an indicator of a person's
heart rate, the taking of
blood pressure through a
sphygmomanometer or the use of a
stethoscope to listen to the heart for
murmurs which may indicate problems with the
heart's valves. An
electrocardiogram can also be used to evaluate the way in which electricity is conducted through the heart.
Other more invasive means can also be used. A
cannula or
catheter inserted into an artery may be used to measure
pulse pressure or
pulmonary 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 of
stents, and active bleeds may be managed by the insertion of coils. An MRI may be used to image arteries, called an
MRI angiogram. For evaluation of the blood supply to the lungs a
CT pulmonary angiogram may be used.
Ultrasound can also be used, particularly to identify the health of blood vessels, and a
Doppler ultrasound of the
carotid arteries or
Doppler ultrasound of the lower limbs can be used to evaluate for narrowing of the carotid arteries or
thrombus formation in the legs, respectively.
Surgery
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This section requires expansion. (March 2015) |
There are a number of surgical procedures performed on the circulatory system:
Cardiovascular procedures are more likely to performed in the
inpatient setting than in an ambulatory care setting; in the United
States, only 28% of cardiovascular surgeries were performed in the
ambulatory care setting.
[11]
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