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DIVISIONS OR COMPONENTS OF THE CIRCULATORY
SYSTEM
The circulatory system consists of the cardiovascular and
lymphatic components. The cardiovascular portion includes the
heart, blood, and vessels that transport the blood.
The lymphatic element of the circulatory system is composed of
a clear, watery fluid called lymph, along with lymphatic vessels
and lymphatic nodes. The cardiovascular and lymphatic
components differ in their function and method of transporting
their respective fluids within the vessels.
The cardiovascular, or blood circulatory, division may be divided
further into the cardio (circulation within the heart) and vascular
(blood vessel) components.
The heart is the major organ of the cardiovascular system; it functions as
a pump to maintain circulation of blood throughout the body. The
vascular component is composed of a network of blood vessels that
carry blood from the heart to body tissues and back to the heart again.
Functions of the cardiovascular system include the following:
1.Transportation of oxygen, nutrients, hormones, and chemicals
necessary for normal body activity
2.Removal of waste products through the kidneys and lungs
3.Maintenance of body temperature and water and electrolyte
balance. These functions are performed by the following blood
components: red blood cells, white blood cells, and platelets
suspended in plasma.
SYSTEMIC CIRCULATION
Arteries
Vessels that transport oxygenated blood
from the heart to tissues are called
arteries. Arteries that originate directly
from the heart are large, but they subdivide
and decrease in size as they extend from
the heart to various parts of the body. The
smaller arteries are termed arterioles. As
the blood travels through the arterioles, it
enters the tissues through the smallest
subdivision of these vessels, known as
capillaries.
Veins
The deoxygenated blood returns to the
heart through the venous system. The
venous system extends from venous
capillaries to venules to veins, increasing
in size as it nears the heart.
General Systemic Circulation
HEART
The heart is a muscular organ that pumps blood throughout the various parts of the
body. Anatomically, the heart lies within the mediastinum and rests on the
diaphragm . Cardiac tissue differs from other muscle tissues of the body in its
construction and is termed myocardium. The left side of the heart is responsible
for the extensive systemic circulation; thus the left muscle wall is about three times
as thick as the right side.
The heart itself is divided into four chambers: the right and left atria and the right
and left ventricles. Each chamber functions to receive and/or pump blood. The
blood circulation is a closed system by which unoxygenated blood enters the right
atrium from all parts of the body, is reoxygenated in the lungs, and is returned to
the body by the left ventricle.
Blood returning to the heart enters the right atrium through the superior and
inferior venae cavae. Blood in the superior vena cava originates from the head,
chest, and upper limbs. The inferior vena cava serves to deliver blood into the right
atrium from the abdomen and lower limbs.
The aorta is the major artery leaving the left ventricle of the heart.
Three major branches arise from the arch of the aorta and include
the following:
1.Brachiocephalic artery
2.Left common carotid artery
3.Left subclavian artery
The brachiocephalic trunk is a short vessel that bifurcates into the
right common carotid artery and the right subclavian artery.
This bifurcation occurs directly posterior to the right
sternoclavicular joint. The right and left vertebral arteries are
branches of the subclavian arteries on each side as described
above. Because the left common carotid artery rises directly from
the arch of the aorta, it is slightly longer than the right common
carotid artery.
In the cervical region, the two common carotids resemble one
another. Each common carotid artery passes cephalad from its
origin along either side of the trachea and larynx to the level of the
upper border of the thyroid cartilage. Here, each common carotid
artery divides into external and internal carotid arteries. The site
of bifurcation for each common carotid is at the level of the fourth
cervical vertebra.
BLOOD SUPPLY TO THE BRAIN
The brain is supplied with blood by major arteries of the
systemic circulation. The four major arteries that supply the
brain are as follows:
1.Right common carotid artery
2.Left common carotid artery
3.Right vertebral artery
4.Left vertebral artery
INTERNAL CAROTID ARTERY
Each internal carotid artery ascends to enter the carotid
canal in the petrous portion of the temporal bone. Within
the petrous pyramid, the artery curves forward and
medially. Before supplying the cerebral hemispheres,
each internal carotid artery passes through a collection of
venous channels around the sella turcica. Each internal
carotid artery passes through the dura mater, medial to
each anterior clinoid process, to bifurcate into the cerebral
branches.
The S-shaped portion of each internal carotid artery is
termed the carotid siphon and is studied carefully by the
radiologist.
VERTEBROBASILAR ARTERIES
The two vertebral arteries enter the cranium through the
foramen magnum and unite to form the single basilar artery.
The vertebral arteries and the basilar artery and their branches
form the vertebrobasilar system. By omitting much of the
occipital bone, these arteries are shown along the base of the
skull. Several arteries arise from each vertebral artery before
their point of convergence to form the basilar artery. These
branches supply the spinal cord and the hindbrain. The basilar
artery rests on the clivus, a portion of the sphenoid bone, and
the base of the occipital bone anterior to the foramen magnum
and posterior to the dorsum sellae.
ARTERIAL CIRCLE (CIRCLE OF WILLIS)
The blood to the brain is supplied by the internal carotid and
vertebral arteries. The posterior brain circulation
communicates with the anterior circulation along the base of
the brain in the arterial circle or circle of Willis The five
arteries or branches that make up the arterial circle are (1)
the anterior communicating artery, (2) the anterior
cerebral arteries, (3) branches of the internal carotid
arteries, (4) the posterior communicating artery, and (5)
the posterior cerebral arteries.
Cerebral Veins
GREAT VEINS OF THE NECK
The three pairs of major veins that drain the head, face, and
neck region include the following:
1.Right and left internal jugular veins
2.Right and left external jugular veins
3.Right and left vertebral veins
Each internal jugular vein drains the meninges and brain. In
addition, many smaller veins join each internal jugular vein as
it passes caudad to eventually become the brachiocephalic
vein on each side. The right and left brachiocephalic veins join
to form the superior vena cava, which returns blood to the right
atrium of the heart.
The pair of external jugular veins are more superficial trunks that
drain the scalp and much of the face and neck. Each external jugular
vein joins the respective subclavian vein.
The right and left vertebral veins form outside the cranium and drain
the upper neck and occipital region. Each vertebral vein enters the
transverse foramen of C1, descends to C6, and then enters the
subclavian vein.
THORACIC ARTERIES
The aorta and pulmonary arteries are the major arteries located within the
chest. The pulmonary arteries supply the lungs with deoxygenated blood.
The aorta extends from the heart to about the fourth lumbar vertebra and is
divided into thoracic and abdominal sections. The thoracic section is
subdivided into the following four segments
1.Aortic bulb (root)
2.Ascending aorta
3.Aortic arch
4.Descending aorta
The bulb, or root portion, is at the proximal end of the aorta and is the area from
which the coronary arteries originate. Extending from the bulb is the ascending
portion of the aorta, which terminates at approximately the second sternocostal
joint and becomes the arch. The arch is unique from the other segments of the
thoracic aorta because three arterial branches arise from it: the brachiocephalic
artery, the left common carotid artery, and the left subclavian artery.
Abdominal Circulatory System
ABDOMINAL ARTERIES
The abdominal aorta is the continuation
of the thoracic aorta. The abdominal
aorta is anterior to the vertebrae and
extends from the diaphragm to
approximately L4, where it bifurcates
into the right and left common iliac
arteries. Five major branches of the
abdominal aorta exist that are of
greatest interest in angiography. Any
one of these branches may be
selectively catheterized for study of a
specific organ.
The trunk of the celiac artery arises from the anterior aspect of the aorta just below
the diaphragm and about 1.5 cm above the origin of the superior mesenteric artery.
Organs supplied with blood by the three large branches of the celiac trunk are the
liver, spleen, and stomach.
The superior mesenteric artery supplies blood to the pancreas, most of the small
intestine, and portions of the right side of the large intestine (cecum, ascending, and
about one half of the transverse colon). It originates from the anterior surface of the
aorta at the level of the first lumbar vertebra about 1.5 centimeters below the celiac
artery.
The inferior mesenteric artery originates from the aorta at about the third lumbar
vertebra (3 or 4 cm above the level of the bifurcation of the common iliac arteries).
Blood is supplied to portions of the large intestine (left half of transverse colon,
descending colon, sigmoid colon, and most of the rectum) by the inferior mesenteric
artery.
The right and left renal arteries supplying blood to the kidneys originate on each
side of the aorta just below the superior mesenteric artery at the level of the disk
between the first and second lumbar vertebrae
The distal portion of the abdominal aorta bifurcates at the level of the fourth
lumbar vertebra into the right and left common iliac arteries. Each common
iliac artery then divides into the internal and external iliac arteries. The
internal iliac arteries supply the pelvic organs (urinary bladder, rectum,
reproductive organs, and pelvic muscles) with blood.
The lower limbs receive blood from the external iliac arteries. The external
iliac artery is significant in angiography and is used to study each lower
limb.
ABDOMINAL VEINS
Blood is returned from structures below the diaphragm (the trunk and lower
limbs) to the right atrium of the heart by the inferior vena cava. Several
radiographically important tributaries to the inferior vena cava exist. These veins
include the right and left common iliacs, internal iliacs, external iliacs, renal
veins and the hepatic portal system. The iliac veins drain the pelvic area and
lower limbs, and the renal veins return blood from the kidneys.
The superior and inferior mesenteric veins return blood from the small and
large intestine through the hepatic portal vein and the hepatic veins, and into
the inferior vena cava.
The hepatic portal system includes all veins that drain blood from the
abdominal digestive tract and from the spleen, colon, and small intestine.
From these organs, this blood is conveyed to the liver through the hepatic
portal vein. While in the liver, this blood is “filtered” and is returned to the
inferior vena cava by the hepatic veins. Several major tributaries to the
hepatic veins exist. The splenic vein is a large vein with its own tributaries,
which return blood from the spleen.
The inferior mesenteric vein, which returns blood from the rectum and from
parts of the large intestine, usually opens into the splenic vein, but in about
10% of cases, it ends at the angle of union of the splenic and superior
mesenteric veins. The superior mesenteric vein returns blood from the
small intestine and parts of the large intestine. It unites with the splenic vein to
form the portal vein.
UPPER LIMB ARTERIES
The arterial circulation of the upper limb is generally considered to
begin at the subclavian artery. The origin of the subclavian artery
differs from the right side to the left side. On the right side, the
subclavian arises from the brachiocephalic artery, whereas the
left subclavian originates directly from the aortic arch.
The subclavian continues to become the axillary artery, which
gives rise to the brachial artery. The brachial artery bifurcates into
the ulnar and radial arteries at approximately the level of the neck
of the radius. The radial and ulnar arteries continue to branch until
they join together to form two palmar arches (deep and
superficial). Branches of these arches supply the hand and fingers
with blood.
LOWER LIMB ARTERIES
The arterial circulation of the lower limb begins at the external iliac artery
and ends at the veins of the foot . The first artery to enter the lower limb is
the common femoral artery. The common femoral artery divides into the
femoral and deep femoral arteries. The femoral artery extends down the
leg and becomes the popliteal artery at the level of the knee. Major
branches of the popliteal are the anterior tibial and posterior tibial
arteries.
The anterior tibial artery continues as the dorsalis pedis artery, with
branches to the ankle and foot. The posterior tibial artery supplies the
calf and plantar surface of the foot.
Lymphatic System
LYMPH DRAINAGE
The lymphatic system serves to drain interstitial fluid (fluid in the spaces between
the cells) and return it to the venous system. The fluid from the left side of the
body, the lower limbs, pelvis, and abdomen enters the venous system by the
thoracic duct (largest lymph vessel in the body), which drains into the left
subclavian vein near its junction with the left jugular vein.
The upper right side of the body, upper limb, and head and neck region drain lymph
fluid into the venous system at the junction of the right jugular and right subclavian
veins by the right lymph duct.
FUNCTIONS
Functions of the lymphatic portion of the circulatory system are as follows:
1.Fights disease by producing lymphocytes and macrophages
2.Returns proteins and other substances to the blood
3.Filters the lymph in the lymph nodes
4.Transfers fats from the intestine to the thoracic duct and hence to the blood