Circulatory system :
The human circulatory system (simplified). Red indicates
oxygenated blood carried in arteries, blue indicates deoxygenated blood carried
in veins. Capillaries, which join the arteries and veins .
The circulatory system, also called the cardiovascular
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, These are two
separate systems. The passage of lymph for example takes a lot longer than that
of blood. 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, 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. The more primitive, diploblastic animal phyla lack
circulatory systems.
The Circulation of
Blood :
The human circulatory system is really a two-part system
whose purpose is to bring oxygen-bearing blood to all the tissues of the body.
When the heart contracts it pushes the blood out into two major loops or
cycles. In the systemic loop, the blood circulates into the body’s
systems, bringing oxygen to all its organs, structures and tissues and
collecting carbon dioxide waste. In the pulmonary loop, the blood
circulates to and from the lungs, to release the carbon dioxide and pick up new
oxygen. The systemic cycle is controlled by the left side of the heart, the
pulmonary cycle by the right side of the heart. Let’s look at what happens
during each cycle:
The systemic loop begins when the oxygen-rich blood coming
from the lungs enters the upper left chamber of the heart, the left atrium. As
the chamber fills, it presses open the mitral valve and the blood
flows down into the left ventricle. When the ventricles contract during a
heartbeat, the blood on the left side is forced into the aorta. This
largest artery of the body is an inch wide. The blood leaving the aorta brings
oxygen to all the body’s cells through the network of ever smaller arteries and
capillaries. The used blood from the body returns to the heart through the
network of veins. All of the blood from the body is eventually collected into
the two largest veins: the superior vena cava, which receives blood from
the upper body, and the inferior vena cava, which receives blood from the
lower body region. Both vena cava empty the blood into the right atrium of the
heart.
From here the blood begins its journey through the pulmonary
cycle. From the right atrium the blood descends into the right ventricle
through the tricuspid valve. When the ventricle contracts, the blood is
pushed into the pulmonary artery that branches into two main parts: one going
to the left lung, one to the right lung. The fresh, oxygen-rich blood returns
to the left atrium of the heart through the pulmonary veins.
Although the circulatory system is made up of two cycles,
both happen at the same time. The contraction of the heart muscle starts in the
two atria, which push the blood into the ventricles. Then the walls of the
ventricles squeeze together and force the blood out into the arteries: the aorta
to the body and the pulmonary artery to the lungs. Afterwards, the heart muscle
relaxes, allowing blood to flow in from the veins and fill the atria again. In
healthy people the normal (resting) heart rate is about 72 beats per minute,
but it can go much higher during strenuous exercise. Scientists have estimated
that it takes about 30 seconds for a given portion of the blood to complete the
entire cycle: from lungs to heart to body, back to the heart and out to the
lungs.
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.
Location of the Heart :
The center of the circulatory system is the heart,
which is the main pumping mechanism. The heart is made of muscle. The heart is
shaped something like a cone, with a pointed bottom and a round top. It is hollow
so that it can fill up with blood. An adult’s heart is about the size of a
large orange and weighs a little less than a pound.
The heart is in the middle of the chest. It fits snugly
between the two lungs. It is held in place by the blood vessels that carry the
blood to and from its chambers. The heart is tipped somewhat so that there is a
little more of it on the left side than on the right. The pointed tip at the
bottom of the heart touches the front wall of the chest. Every time the heart
beats it goes “lub - dubb ” against the chest wall. You can feel the thumps if
you press there with your hand. You can also listen to them with your ear.
Structure of the Heart :
Heart wall of muscle divides it down the middle, into a left
half and a right half. The muscular wall is called a septum. The septum is
solid so that blood cannot flow back and forth between the left and right
halves of the heart. Another wall separates the rounded top part of the heart
from the cone-shaped bottom part. So there are actually four chambers (spaces)
inside the heart. Each top chamber is called an atrium (plural:
atria). The bottom chambers are called ventricles. The atria are often
referred to as holding chambers, while the ventricles are called pumping
chambers. Thus, each side of the heart forms its own separate system, a right
heart and a left heart. Each half consists of an atrium and a ventricle, and
blood can flow from the top chamber to the bottom chamber, or ventricle, but
not between the two sides.
The Valves:
Blood can flow from the atria down into the ventricles
because there are openings in the walls that separate them. These openings are
called valves because they open in one direction like trapdoors to
let the blood pass through. Then they close, so the blood cannot flow backwards
into the atria. With this system, blood always flows in only one direction
inside the heart. There are also valves at the bottom of the large arteries
that carry blood away from the heart: the aorta and
the pulmonary artery. These valves keep the blood from flowing backward
into the heart once it has been pumped out.
Branching Blood Vessels :
The heart is a pump whose walls are made of thick muscle. They can
squeeze (contract) to send blood rushing out. The blood does not spill all over
the place when it leaves the heart. Instead, it flows smoothly in tubes
called blood vessels. First, the blood flows into tubes
called arteries. The arteries leaving the heart are thick tubes. But the
arteries soon branch again and again to form smaller and smaller tubes. The
smallest blood vessels, called capillaries, form a fine network of tiny
vessels throughout the body. The capillaries have extremely thin walls so that
the blood that they carry can come into close contact with the body tissues.
The tiny red blood cells can then pass easily through the walls of the
capillaries to deliver the oxygen they carry to nearby cells. As the blood
flows through the capillaries, it also collects carbon dioxide waste from the
body cells. The capillaries containing carbon dioxide return this used blood to
the heart through a different series of branching tubes: The capillaries join
together to form small veins. The veins, in turn, unite with each other to
form larger veins until the blood from the body is finally collected into the
large veins that empty into the heart. So the blood vessels of the body carry
blood in a circle: moving away from the heart in arteries, traveling to various
parts of the body in capillaries, and going back to the heart in veins. The
heart is the pump that makes this happen.
Cardiovascular system:
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.
The essential components of the human cardiovascular system
are the heart, blood and blood vessels. 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 macro circulation and amicro circulation.
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. Also, the digestive system works with the
circulatory system to provide the nutrients the system needs to keep
the heart pumping.
Capillaries:
Arteries branch into small passages called arterioles and
then into the capillaries. The capillaries merge to bring blood into
the venous system.
Arteries:
Oxygenated blood enters the systemic circulation when
leaving the left ventricle, through the aortic . 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.
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.
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 The besian
valve. The smallest cardiac veinsdrain directly into the heart chambers.
Pulmonary circulation
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
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
valve into the pulmonary artery to the lungs. Gas exchange occurs in the
lungs, whereby is released from the blood, and oxygen is absorbed. The
pulmonary vein returns the now oxygen-rich blood to the left atrium.
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.
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.
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.
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 of Fetal circulation
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.
Arterial development
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.
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
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 is chaemical 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.
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.
Two-chambered heart of a fish :
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 assingle cycle circulation.
The heart of fish is, therefore, only a single pump (consisting of two
chambers).
The circulatory systems of all vertebrates, as well as
of annelids (for example, earthworms)
and cephalopods (squids, octopuses and relatives)
are closed, just as in humans. Still, the systems
of fish, amphibians, reptiles, and birds show various
stages of the evolution of the circulatory system.
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.
Birds, mammals, 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.
Open circulatory system:
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 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).
Hemolymph fills all of the interior hemocoel of the body and
surrounds all cells. Hemolymph is composed of water, inorganic salts (mostly sodium, chlorine, potassium, magnesium,
and calcium), and organic
compounds (mostly carbohydrates,proteins, and lipids). The
primary oxygen transporter molecule is hemocyanin.
There are free-floating cells, the hemocytes, within
the hemolymph. They play a role in the arthropod immune system.
An open
circulatory system is made up of a heart, vessels, and hemolymph. This diagram
shows how the hemolymph, fluid present in most invertebrates that is equivalent
to blood, is circulated throughout the body of a grasshopper. The hymolymph is
first pumped through the heart, into the aorta, dispersed into the head and
throughout the hemocoel, then back through the ostium that are located in the
heart, where the process is repeated.
Absence of circulatory system
Circulatory systems are absent in some animals,
including flatworms (phylum Platyhelminthes ). 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.
TAGS :
COMMENTS