The arcuate fasciculus is a white matter tract that runs parallel to the superior longitudinal fasciculus.
Due to their proximity, they are sometimes referred to interchangeably.
They can be distinguished by the location and function of their
endpoints in the frontal cortex. The arcuate fasciculus terminates in Broca's area (specifically BA 44) which is linked to processing complex syntax. However, the superior longitudinal fasciculus ends in the premotor cortex which is implicated in acoustic-motor mapping.
Connection
Historically, the arcuate fasciculus has been understood to connect two important areas for language use: Broca's area in the inferior frontal gyrus and Wernicke's area in the posterior superior temporal gyrus.
It is mostly considered to be an oversimplification, but this model is
still utilized because a satisfactory replacement has not been
developed.
The topographical relationships between independent measures of white
matter and gray matter integrity suggest that rich developmental or
environmental interactions influence brain structure and function. The
presence and strength of such associations may elucidate
pathophysiological processes influencing systems such as language and motor planning.
As the technique of diffusion MRI
has improved, this has become a testable hypothesis. Research indicates
more diffuse termination of the fibers of the arcuate than previously
thought. While the main caudal source of the fiber tract appears to be
posterior superior temporal cortex, the rostral terminations are mostly
in premotor cortex, part of Brodmann area 44.
Developmental differences
Myelination is a process by which axons are covered with a protective substance called myelin that drastically increases the signaling efficiency of the neuron.
The arcuate fasciculus is heavily myelinated in healthy adult brains.
The density of this myelination has been found to predict the accuracy
and speed to which one can comprehend sentences. However, the arcuate
fasciculus of newborns is unmyelinated. The myelination process occurs
gradually during childhood; myelin density has been shown to increase
between the age of 3 and 10. A study comparing a group of 6-year-olds to
a group of 3-year-olds found that the 6-year-olds had stronger
functional connectivity of the arcuate fasciculus. The arcuate
fasciculus is similarly undeveloped in non-human primates such as
chimpanzees and macaques. This supports the theory that the arcuate
fasciculus is a critical component in language.
Dorsal stream
The two-streams hypothesis of language proposes that there are two streams by which the brain processes language information: the dorsal and ventral streams. The basis of this model is generally accepted, however the details of it are highly contentious.
The dorsal pathway consists of multiple fiber tracts, one of which is
the arcuate fasciculus. The dorsal pathway as a whole is implicated in sensory-to-motor mapping and processing complex syntax.
Role in language
Syntax
Syntax
refers to a set of rules by which we order words within a language.
Some researchers argue that syntax is what distinguishes language as a
uniquely human capacity. Though the exact function of the arcuate
fasciculus is still debated, the predominant theory is that it is
involved with processing complex sequences of syntax. Studies indicate
that as the arcuate fasciculus matures and undergoes myelination, there
is a corresponding increase in the ability to process syntax.
Furthermore, lesions in the arcuate fasciculus often result in
difficulties with syntax. Researchers have found that when subjects are
confronted with difficult syntactic structures, there is high
synchronicity between the left frontal and parietal regions due to their
connection by the arcuate fasciculus. This research further supports
the arcuate fasciculus as the key component of human language.
Lateralization
The arcuate fasciculus is a bilateral structure; this means that it is present in both the right and left hemispheres
of the brain. These fiber tracts are asymmetrical; the left arcuate
fasciculus is stronger than the right. While the left arcuate fasciculus
is thought to be the one involved with syntax processing, the right
arcuate fasciculus has been implicated in prosody processing.
Studies further suggest that the right arcuate fasciculus is involved
with the ability to read emotion from human facial expression.
Clinical significance
Conduction aphasia
Historically the arcuate fasciculus has been linked to conduction aphasia, which is usually the result of damage to the inferior parietal lobule that extends into the subcortical white matter and compromises the arcuate fasciculus. This type of aphasia is characterized by difficulty with repetition and prevalent phonemic paraphasias. Patients otherwise exhibit a relatively normal control of language. The symptoms of conduction aphasia
suggest that the connection between the posterior temporal cortex and
frontal cortex plays a vital role in short-term memory of words and
speech sounds that are new or have just been heard. The arcuate
fasciculus is the main connection between these two regions. Studies
that challenge the claim that the arcuate fasciculus is responsible for
repetition cite that in some cases lesions to the arcuate fasciculus nor
total agenesis produce conduction aphasia.
Progressive aphasia
Progressive aphasia is a type of aphasia
that slowly worsens over time. It can affect both the production and
comprehension of language. Progressive aphasic patients that have lesions
in their arcuate fasciculus were especially deficient in their syntax
processing abilities. Worsened syntax processing correlated with the
degree of degradation in the arcuate fasciculus.
Tone deafness
In nine out of ten people with tone deafness,
the superior arcuate fasciculus in the right hemisphere could not be
detected, suggesting a disconnection between the posterior superior
temporal gyrus and the posterior inferior frontal gyrus. Researchers
suggested the posterior superior temporal gyrus was the origin of the
disorder.
Stuttering
In stutterers, the arcuate fasciculus appears to have bilateral deficits that reduce it by one-third or more relative to non-stutterers.
However, there is ongoing debate concerning the contribution of each
hemisphere. Diffusion-based evidence of differences between stutterers
and controls is not isolated to the arcuate fasciculus.
Specific language impairment
Specific language impairment
is a disorder that prevents children from developing language normally.
These children particularly have difficulty with the syntactic and
hierarchal structures of language. Damage to the arcuate fasciculus is
implicated as a possible cause of specific language impairment, however
further data is required to validate this claim.
Dyslexia
Dyslexia
is a disorder that is primarily characterized by reading deficits.
Research has shown that decreases in the integrity of the arcuate
fasciculus coincide with worsened reading ability in dyslexic subjects.
In 1977, Witleson reported that developmental dyslexia
may be associated with (i) bi-hemisphere representation of spatial
functions, in contrast to the unitary right hemisphere control of these
functions observed in normal individuals. The bilateral neural
involvement in spatial processing may interfere with the left hemisphere's processing of its own specialized functions and result in deficient linguistic, sequential cognitive processing and in overuse of the spatial, holistic cognitive mode, reflecting a functional disconnection syndrome in these individuals confirmed by Leisman in the 1980s and in the 2000s.
The concept of functional disconnection developed further with Stachowiak and Poeck in 1976. who reported on a case in 1976 of a 67-yr-old male with hemianopia resulting from a cerebrovascular accident resulting in pure alexia
and a color naming deficit that he suggested was due to a functional
disconnection mechanism. He noted that the underlying disconnection
mechanism is improved by the facilitating effect of unblocking methods
(in the tactile, somesthetic, auditory, and visual systems), so that pathways other than the one impaired by the brain lesion are used.
In 1998, Fritson
presented a mechanistic account of how dysfunctional integration among
neuronal systems arises, based on the central role played by synaptic plasticity in shaping the connections. He hypothesized that the pathophysiology of schizophrenia
is expressed at the level of modulation of associative changes in
synaptic efficacy; specifically the modulation of plasticity in those
brain systems responsible for emotional learning and emotional memory
in the postnatal period. This modulation is mediated by ascending
neurotransmitter systems that: (i) have been implicated in
schizophrenia; and (ii) are known to be involved in consolidating
synaptic connections during learning. The pathophysiology results in a disruption of the reinforcement of adaptive behavior consistent with the disintegrative aspects of the disorder. Kim and colleagues in 2003 further described the disconnection hypothesis in schizophrenia as the result of a prefrontal-parietal lobe functional disconnection, particularly prefrontal dissociation and abnormal prefrontal-parietal interaction during working memory processing.
The concept of functional disconnection developed still further when it was applied to the understanding of the nature of autistic spectrum disorder. Geschwind and Levitt in 2007 suggested a model of the symptoms of autism in which higher-order association areas of the brain (that normally connect to the frontal lobe) are partially disconnected during development, thereby explaining the heterogeneity of autism etiology. The autism group at Cambridge University provided evidence that the functional connectivity of medial temporal lobe structures specifically is abnormal in people with Asperger’s syndrome,
at least during fearful face processing. Melillo and Leisman have
similarly concluded that a functional disconnection syndrome is a basis
for explaining the symptoms of autistic spectrum disorder.
Disconnection syndrome is a general term for a collection of neurological symptoms caused – via lesions to associational or commissuralnerve fibres – by damage to the white matter axons of communication pathways in the cerebrum (not to be confused with the cerebellum), independent of any lesions to the cortex. The behavioral effects of such disconnections are relatively predictable in adults.
Disconnection syndromes usually reflect circumstances where regions A
and B still have their functional specializations except in domains that
depend on the interconnections between the two regions.
Callosal syndrome, or split-brain, is an example of a disconnection syndrome from damage to the corpus callosum between the two hemispheres of the brain. Disconnection syndrome can also lead to aphasia, left-sided apraxia,
and tactile aphasia, among other symptoms. Other types of disconnection
syndrome include conduction aphasia (lesion of the association tract
connecting Broca’s area and Wernicke’s), agnosia, apraxia, pure alexia, etc.
Anatomy of cerebral connections
Theodore Meynert,
a neuroanatomist of the late 1800s, developed a detailed anatomy of
white matter pathways. He classified the white matter fibers that
connect the neocortex into three important categories – projection fibers, commissural fibers and association fibers.
Projection fibers are the ascending and descending pathways to and from
the neocortex. Commissural fibers are responsible for connecting the
two hemispheres while the association fibers connect cortical regions
within a hemisphere. These fibers make up the interhemispheric
connections in the cortex.
Callosal disconnection syndrome is characterized by left ideomotor apraxia and left-hand agraphia and/or tactile anomia, and is relatively rare.
Hemispheric disconnection
Many studies have shown that disconnection syndromes such as aphasia, agnosia, apraxia, pure alexia
and many others are not caused by direct damage to functional
neocortical regions. They can also be present on only one side of the
body which is why these are categorized as hemispheric disconnections.
The cause for hemispheric disconnection is if the interhemispheric
fibers, as mentioned earlier, are cut or reduced.
An example is commissural disconnect in adults which usually
results from surgical intervention, tumor, or interruption of the blood
supply to the corpus callosum or the immediately adjacent structures.
Callosal disconnection syndrome is characterized by left ideomotor
apraxia and left-hand agraphia and/or tactile anomia, and is relatively rare.
Other examples include commissurotomy,
the surgical cutting of cerebral commissures to treat epilepsy and
callosal agenesis which is when individuals are born without a corpus callosum.
Those with callosal agenesis can still perform interhemispheric
comparisons of visual and tactile information but with deficits in
processing complex information when performing the respective tasks.
Sensorimotor disconnection
Hemispheric
disconnection has impacted behaviors relating to the sensory and motor
systems. The different systems affected are listed below:
Olfaction
– The olfactory system is not crossed across hemispheres like the other
senses, which means that left input goes to the left hemisphere and
right input goes to the right hemisphere. Fibers in the anterior commissure control the olfactory regions in each hemisphere. A patient who lacks an anterior commissure
cannot name odors entering the right nostril or use the right hand to
pick up the object corresponding to the odor because the left
hemisphere, responsible for language and controlling the right hand, is
disconnected from the sensory information.
Vision
– Information from one visual field travels to the contralateral
hemisphere. Therefore, with a commissurotomy patient, visual information
presented in the left visual field travelling to the right hemisphere
would be disconnected from verbal output since the left hemisphere is
responsible for speech.
Somatosensory
– If the two hemispheres are disconnected, the somatosensory functions
of the left and right parts of the body become independent. For example,
when something is placed on the left hand of a blindfolded patient with
the two hemispheres disconnected, the left hand can pick the correct
object within a set of objects but the right hand cannot.
Audition
– Though most of the input from one ear would go through the same ear,
the opposite ear also receives some input. Therefore, the disconnection
effects seems to be reduced in audition compared to the other systems.
However, studies have shown that when the hemispheres are disconnected,
the individual does not hear anything from the left and only hears from
the right.
Movement – Apraxia and agraphia
may occur where responding to any verbal instructions by movement or
writing in the left hand is inhibited because the left hand cannot
receive these instructions from the right hemisphere,
History
The
concept of disconnection syndrome emerged in the late nineteenth
century when scientists became aware that certain neurological disorders
result from communication problems among brain areas. In 1874, Carl Wernicke introduced this concept in his dissertation when he suggested that conduction aphasia
could result from the disconnection of the sensory speech zone from the
motor speech area by a single lesion in the left hemisphere to the arcuate fasciculus.
As the father of the disconnection theory, Wernicke believed that
instead of being localized in specific regions of the brain, higher
functions resulted from associative connections between the motor and
sensory memory areas.
Lissauer, a pupil of Wernicke, described a case of visual agnosia as a disconnection between the visual and language areas.
Dejerine in 1892 described specific symptoms resulting from a lesion to the corpus callosum that caused alexia without agraphia. The patient had a lesion in the left occipital lobe, blocking sight in the right visual field (hemianopia),
and in the splenium of the corpus callosum. Dejerine interpreted this
case as a disconnection of the speech area in the left hemisphere from
the right visual cortex.
In 1965, Norman Geschwind,
an American neurologist, wrote ‘Disconnexion syndromes in animals and
man’ where he described a disconnectionist framework that revolutionized
neurosciences and clinical neurology. Studies of the monkey brain led
to his theory that disconnection syndromes were higher function
deficits. Building on Wernicke and previously mentioned psychologists’
idea that disconnection syndromes involved white matter
lesion to association tracts connecting two regions of the brain,
Geschwind was more detailed in explaining some disconnection syndromes
as lesions of the association cortex itself, specifically in the
parietal lobe. He described the callosal syndrome, an example of a
disconnection syndrome, which is a lesion in the corpus callosum that
leads to tactile anomia in just the patient’s left hand.
Though Geschwind made significant advances in describing
disconnection syndromes, he was not completely accurate. He didn’t think
the association cortex had any specialized role of its own besides
acting as a relay station between the primary sensory and motor areas.
However, in the 1960s and 1970s, Mesulam and Damasio incorporated
specific functional roles for the association cortex. With Mesulam and
Damasio’s contributions, Geschwind’s model has evolved over the past 50
years to include connections between brain regions as well as
specializations of association cortices.
More recently, neurologists have been using imaging techniques such as diffusion tensor imaging (DTI) and functional magnetic resonance imaging (fMRI) to visualize association pathways in the human brain to advance the future of this disconnection theme.
Hemispherectomy is a surgery that is performed by a neurosurgeon where an unhealthy hemisphere of the brain is disconnected or removed. There are two types of hemispherectomy. Functionalhemispherectomy
refers to when the diseased brain is simply disconnected so that it can
no longer send signals to the rest of the brain and body. Anatomical hemispherectomy
refers to when not only is there disconnection, but also the diseased
brain is physically removed from the skull. This surgery is mostly used
as a treatment for medically intractable epilepsy, which is the term used when anti-seizure medications are unable to control seizures.
History
The first anatomical hemispherectomy was performed and described in 1928 by Walter Dandy. This was done as an attempt to treat glioma, a brain tumor.
The first known anatomical hemispherectomy performed as a treatment for
intractable epilepsy was in 1938 by Kenneth McKenzie, a Canadian
neurosurgeon.
Krynaw, a neurosurgeon from South Africa, was one of the first to
perform and report a case series on hemispherectomies in 1950. He
performed the surgery on pediatric patients with infantile hemiplegia,
specifically as a treatment for their seizures and cognitive impairment. His hemispherectomy technique removed the damaged hemisphere except the thalamus and caudate
structures. Krynaw reported good outcomes overall, although there was
one post-operative death. Specifically, there was an overall theme of
improvement in weakness, spasticity and cognition. Amazingly, ten out of the twelve patients had seizures prior to the operation and none of the patients had seizures afterwards.
Other neurosurgeons began performing hemispherectomies as well,
primarily for the treatment of seizures. For the most part, the
surgeries would go well initially, but there was a general theme of
subsequent deterioration and even death years after the surgery. As a
result of the complication risk and the introduction of new anti-seizure
medications, the popularity of the procedure began to decline in the
1950s.
Oppenheimer and Griffith were one of the first to describe the
potential complications, and they reported their findings in 1966,
describing superficial hemosiderosis, granular ependymitis and obstructive hydrocephalus. They posited a theoretical solution to this problem, a surgery that is now known as a functional hemispherectomy.
Rasmussen was one of the first neurosurgeons to develop and apply a
functional hemispherectomy in practice. He initially made modifications
to the original hemispherectomy by preserving the least epileptogenic
quarter or third of the hemisphere, hoping this would ameliorate the
known complications of the original anatomic hemispherectomy. Although
this modification seemed to solve this issue, patients undergoing the
modified hemispherectomy continued to have seizures, which was
problematic. Therefore, he further modified his surgery to functionally
sever residual portions of the frontal and parieto-occipital lobes.
This surgery, the functional hemispherectomy, has been further modified
over the years by several different neurosurgeons, and to this day
there is not a consensus as to which exact technique should be used.
Hemispherotomy refers to some of the more recently developed approaches
to disconnect the epileptic hemisphere while minimizing brain removal
and the risk for complications.
Nomenclature
There are two main types of hemispherectomy: Anatomical and Functional.
Anatomical hemispherectomy refers to the resection and removal of an entire hemisphere of the brain, which includes all four lobes, with or without the removal of basal ganglia and thalamus.
Functional hemispherectomy refers to surgeries that
disable the function of one hemisphere, while maintaining its blood
supply and without physically removing the entire hemisphere from the
skull.
Functional hemispherectomies are performed more frequently than
anatomical hemispherectomies due to their lower complication rates.
However, they do carry a risk of incomplete disconnection, which refers
to when the surgeon inadvertently leaves remnants of fibers that
continue to connect the hemisphere to the brain and body. These
remaining fibers can be problematic, as they may lead to seizure
recurrence.
Another term that falls under the hemispherectomy umbrella includes hemidecortication,
which is the removal of the cortex from one half of the cerebrum, while
attempting to preserve the ventricular system by maintaining the
surrounding white matter. Hemidecortication was originally developed as a
possible strategy to mitigate some of the complications seen with
complete anatomical hemispherectomy.
The term hemispherotomy refers to a surgery that is akin
to a functional hemispherectomy in that it functionally severs the
damaged hemisphere from the other and leaves some of the severed
hemisphere within the skull, but the difference is that it removes even
less tissue from the skull.
The term hemispherotomy is now used as an umbrella term to describe the
group of modern techniques and procedures that predominate at most
contemporary epilepsy centers.
There is no statistically significant difference in seizure-free
rates between the four different types of surgeries: Hemispherotomy,
functional hemispherectomy, anatomical hemispherectomy and
hemidecortication. The overall rate of seizure freedom is estimated to
be 73.4%. However, hemispherotomy procedures may be associated with a more favorable complication profile.
Candidates
The
typical candidates for hemispherectomy are pediatric patients who have
intractable epilepsy due to extensive cerebral unilateral hemispheric
injuries.
In addition, the seizures should ideally be emanating from that same
hemisphere. In some situations, a hemispherectomy may still be performed
if there are seizures from both hemispheres, as long as the majority
come from one side. In order to assess the patient’s epilepsy
completely, patients undergo extensive testing, including EEG and MRI. Most patients also undergo other studies including functional MRI (fMRI), positron emission tomography (PET) or magnetoencephalography (MEG).
Today, hemispherectomy is performed as a treatment for severe and
intractable epilepsy, including for young children whose epilepsy has
been found to be drug-resistant. The most common underlying etiologies include malformations of cortical development (MCD), perinatal stroke and Rasmussen’s encephalitis. MCD is an umbrella term for a wide variety of developmental brain anomalies, including hemimegalencephaly and cortical dysplasia. Other less common underlying etiologies include hemiconvulsion-hemiplegia epilepsy syndrome and Sturge-Weber syndrome.
Procedure
Patients often shave the area of the scalp that will be involved with the surgery. Patients undergo general anesthesia
and are unconscious for the procedure. The surgical site is sterilized,
after which the skin is incised. A substantial portion of the bone is
removed, followed by incision of the dura,
which is the outer covering of the brain. There are several blood
vessels that have connections with both sides of the brain, and these
are carefully identified and clipped in such a way that spares the
healthy hemisphere. Ultimately, a bundle of fibers that connect both of
the cerebral hemispheres, the corpus callosum,
is removed which results in the functional separation of one hemisphere
from the other. Portions of the cerebral lobes from the damaged side of
the brain are removed, depending on the specific procedure being
performed. The surgeon may leave some brain tissue, such as the thalamus or choroid plexus.
After completing the resection, the surgical site is irrigated with
saline, the brain covering called the dura is sutured back together, the
bone that was removed is replaced and the skin is sutured. This surgery
often takes four to five hours.
Patients often spend a few nights in the hospital post-operatively, and
they undergo physical and occupational therapy soon after the surgery.
Potential complications
The most common complication from surgery is hydrocephalus,
a condition in which fluid accumulates within the brain, and this is
often treated with a shunt to divert the fluid away. The rate of shunts
following surgery ranges from 14–23%. Other complications include wound complications, epidural hemorrhages, subdural hemorrhages, intraparenchymal hemorrhages, intracranial abscesses, meningitis, ventriculitis and venous thrombosis. Additional epilepsy surgery following hemispherectomy is rare (4.5%),[7]
but may be recommended if there is a residual connection between the
two hemispheres that is causing frequent seizures. Mortality rates are
low and estimated to be <1% to 2.2%. Most patients do not experience changes in cognition, but some individuals may be at risk.
A visual deficit called contralateral homonymous hemianopsia is
expected to occur in most patients, where the entire visual field
contralateral to the removed hemisphere is lost. There is a risk of motor deficits, and this is variable. Other possible complications include infection, aseptic meningitis, hearing loss, endocrine problems and transient neurologic deficits such as limb weakness.
Outcomes
Since
seizures are the most common indication for hemispherectomy surgery,
most research on hemispherectomy analyzes how the surgery affects
seizures. Many patients undergoing surgery obtain good surgical
outcomes, some obtaining complete seizure freedom (54–90%) and others
having some degree of improvement in seizure burden.
A recently developed scoring system has been proposed to help predict
the probability of seizure freedom with more accuracy: HOPS
(Hemispherectomy Outcome Prediction Scale).
Although it cannot definitively predict surgical outcome with exact
precision, some physicians may use it as a guide. The scoring system
takes certain variables into consideration including age at seizure
onset, history of prior brain surgery, seizure semiology and imaging
findings.
There is also data pertaining to how hemispherectomy affects the
body in other ways. After surgery, the remaining cerebral hemisphere is
often able to take over some cognitive, sensory and motor functions. The
degree to which the remaining hemisphere takes on this additional
workload often depends on several factors, including the underlying
etiology, which hemisphere is removed and the age at which the surgery
occurs.
In terms of postoperative motor function, some patients may have improvement or no change of their weaker extremity, and many can walk independently. Most patients postoperatively have minimal to no behavioral problems, satisfactory language skills, good reading capability, and only a minority of patients have a decline in IQ. Predictors of poor outcome may include seizure recurrence and structural abnormalities in the intact hemisphere.
Ultimately, risks and benefits should be weighed on an individual
basis and discussed in detail with the neurosurgeon. Many patients have
excellent outcomes, and the International League Against Epilepsy
(ILAE) reports that “about one-fifth of hemispherectomy patients are
gainfully employed and even fewer live independently.”
The Brain Recovery Project
The Brain Recovery Project is a non-profit corporation which funds new research and is based in the United States.
This corporation hosts an annual two-day conference for patients who
have had hemispherectomies and their families. There are several
purposes to this reunion. The main goal is to educate patients and their
families on the surgery and its necessary subsequent rehabilitation. It
also serves as a way for patients and families to connect with one
another, learn from specialists in the field and often offers research
enrollment.
The circulatory system is a system of organs that includes the heart, blood vessels, 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 or circuit, and a pulmonary circulation or circuit. Some sources use the terms cardiovascular system and vascular system interchangeably with circulatory system.
In vertebrates, the lymphatic system is complementary to the circulatory system. The lymphatic system carries excess plasma (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 can be affected by many cardiovascular diseases. 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 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 the heart, blood vessels, 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 lungs 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 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 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.
Diagram of the human heart showing blood oxygenation to the pulmonary and systemic circulation
The heart pumps blood to all parts of the body providing nutrients and oxygen to every cell, and removing waste products. The left heart pumps oxygenated blood returned from the lungs to the rest of the body in the systemic circulation. The right heart pumps deoxygenated blood to the lungs in the pulmonary circulation. 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 pulmonary circulation is the part of the circulatory 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.
A separate circuit from the systemic circulation, the bronchial 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 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.
Depiction of the heart, major veins and arteries constructed from body scans
Oxygenated blood enters the systemic circulation when leaving the left ventricle, via the 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.
Capillaries
Arteries branch into small passages called arterioles and then into the capillaries. The capillaries merge to bring blood into the venous system. The total length of muscle capillaries in a 70 kg human is estimated to be between 9,000 and 19,000 km.
Capillaries merge into venules, 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 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 gastrointestinal 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 the liver.
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 the 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 arteries to supply the front of the brain. The posterior circulation arises from the 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.
Renal circulation
The renal circulation is the blood supply to the kidneys, contains many specialized blood vessels and receives around 20% of the cardiac output. It branches from the abdominal aorta and returns blood to the ascending inferior vena cava.
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 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.
Animation
of a typical human red blood cell cycle in the circulatory system. This
animation occurs at a faster rate (~20 seconds of the average 60-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 the aortic arches and from the dorsal aortae starting from week 4 of embryonic life. The first and second aortic arches regress and form 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.
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 vertebrates.
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 that 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.
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.
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.
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 other vertebrates,
have a closed blood circulatory system (meaning that the blood never
leaves the network of arteries, veins and capillaries), some invertebrate groups have an open circulatory system containing a heart but limited blood vessels. The most primitive, diploblastic animal phyla 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.
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 arthropods, 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, with there being
no distinction between blood and interstitial fluid; this combined fluid
is called hemolymph or haemolymph. Muscular movements by the animal during 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).
There are free-floating cells, the hemocytes, within the hemolymph. They play a role in the arthropod immune system.
Flatworms, such as this Pseudoceros bifurcus, lack specialized circulatory organs.
Closed circulatory system
Two-chambered heart of a fish
The circulatory systems of all vertebrates, as well as of annelids (for example, earthworms) and cephalopods (squids, octopuses 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, amphibians, reptiles, and birds show various stages of the evolution of the circulatory system. 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 the gills and on to the capillaries of the body tissues. This is known as single 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, the ventricular septum of the heart is incomplete and the pulmonary artery is equipped with a sphincter 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 the aorta.
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 to ectothermic (cold-blooded) animals in the regulation of their body temperature.
Mammals, birds and crocodilians
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.
No circulatory system
Circulatory systems are absent in some animals, including flatworms. Their body cavity has no lining or enclosed fluid. Instead, a muscular pharynx leads to an extensively branched digestive system that facilitates direct diffusion
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 as jellyfish, have more extensive branching from their gastrovascular 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 the Ebers Papyrus (16th century BCE), an ancient Egyptian medical papyrus containing over 700 prescriptions and remedies, both physical and spiritual. In the papyrus,
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 the Ayurvedic physician Sushruta in ancient India. He also seems to have possessed knowledge of the arteries, described as 'channels' by Dwivedi & Dwivedi (2007). 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. The valves of the heart were discovered by a physician of the Hippocratic school around the early 3rd century BC.
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.
The Greek physician, Herophilus, distinguished veins from arteries but thought that the pulse was a property of arteries themselves. Greek anatomist Erasistratus
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.
In 2nd-century AD Rome, the Greek physician Galen
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 the 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 cycles 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 1242, the Arabian physician, Ibn al-Nafis described the process of pulmonary 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 his Commentary 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 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.
A better known discovery of pulmonary circulation was by Vesalius's successor at Padua, Realdo Colombo, in 1559.
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.
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