Anatomy & Physiology Endocrine

8/7/2019 Anatomy & Physiology Endocrine

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Chapter V

Anatomy and Physiology

ENDOCRINE SYSTEM



The endocrine system is responsible for maintaining balance within the

body, keeping all the systems under control. Using a series of intricate sensory

mechanisms and quickly reacting to change, it controls the internal environment

of the body allowing normal cellular function to continue despite changes in

environment and requirements, both internal and external.

The role of the endocrine system is to maintain the body in balance

through the release of hormones (chemical signals) directly into the bloodstream.

Hormones transfer information and instructions from one set of cells to another.

Many different hormones move through the bloodstream, but each type of

hormone is designed to affect only certain cells.

A gland is a group of cells that produces and secretes chemicals. A gland

selects and removes materials from the blood, processes them, and secretes the

finished chemical product for use somewhere in the body. The endocrine gland

cells release a hormone into the blood stream for distribution throughout the

entire body. These hormones act as chemical messengers and can alter the

activity of many organs at once.

The parts of the endocrine system are grouped together because they

release hormones into the blood without going through a duct (which is basically

a tube) first. This is different to an exocrine gland, which releases what it creates

through a tube to somewhere other than the blood.



Hormones can act on some specific cells because they themselves do not

actually cause an effect. It is only through binding with a receptor (part of the cell

specifically designed to recognize the hormone) like a key into a lock - that

causes a chain reaction to occur, changing the activity of the cells. If a cell does

not have a receptor for a hormone then there will be no effect. Also, there can be

different receptors for the same hormone, and so the same hormone can have

different effects on different cells.

The Thyroid Gland

The thyroid (meaning 'shield-shaped') gland sits in the center of the neck,

at the front, below the Adam's apple. It is made of two lobes joined in the center.

At 15 to 20 grams it is one of the largest of the endocrine glands.

The thyroid secretes two major hormones called thyroxine (T4) and

triiodothyronine (T3). They cause lots of things, but mostly they increase the rate

of metabolism in the body. Metabolism is the amount of energy used by the body.

An increase means more energy sources like fats and sugars are being broken

down, and the body is using more energy to grow. The thyroid is controlled

mainly by the release of Thyroid Stimulating Hormone (TSH) from the pituitary

gland. The thyroid also secretes a hormone called calcitonin, important in

keeping calcium levels in the body normal.

To create the thyroid hormones, the body needs a substance called

Iodine, which is found mainly in salt.



Low levels of thyroid hormone can result in feelings of tiredness,

excessive sleep, loss of sex-drive, and smaller, less frequent periods in a

woman.

Calcitonin is another thyroid hormone and this assists in the regulation of

calcium concentration in body. Calcitonin lowers plasma calcium levels by

inhibiting the cells which break down bone, and stimulating calcium excretion by

the kidneys.

The Parathyroid Glands

The parathyroid glands are small, ovoid, and lie on the back of the thyroid

gland. Most people have four parathyroid glands, two at the top, and two at the

bottom.

There are two types of cell within the parathyroid gland. While calcitonin is

released from the thyroid when calcium levels are too high, the parathyroids

release their hormone when calcium levels are too low.

The Thymus

The thymus is located in the lower part of the neck, and the front part of

the upper chest. After puberty it is mostly replaced by fat.

While the thymus does not play a big role, it does produce several

hormones important in the development and maintenance of a normal immune

system.



The Adrenal Glands

The adrenal glands (also known as the suprarenal glands) are yellow,

pyramid-shaped glands located at the top of the kidneys. They usually weigh

roughly 7.5g and are heavier in men than women. Each adrenal gland has two

parts: an adrenal medulla (inside), and an adrenal cortex (outside).

The adrenal cortex is the outer layer and secretes corticosteroids and

male sex hormones which are derived from cholesterol and various other fats,

hence their yellowish color. It is divided into three distinct zones, each producing

different hormones.

The adrenal medulla is reddish-brown and the cells here are like nerve

cells and are activated by the nervous system. The cell types of this region are

known as pheochromocytes, or chromaffin cells.

Three major types of hormones are released from the adrenal cortex.

These are mineralocorticoids, glucocorticoids and a small amount of sex

hormones. Mineralocorticoids are called as such due to their effects on the

electrolytes (or minerals) of the body, as well as the level of water.

Glucocorticoids control sugar (glucose) levels. There are two hormones

produced in the cortex of great importance and they are aldosterone, the major

mineralocorticoid and cortisol, the major glucocorticoid.

Cortisol: Cortisol is a 'stress hormone' and is released in times when the

body needs increased energy. It is stimulated for release by Adrenocorticotrophic



hormone (ACTH), mentioned earlier as being released from the pituitary gland

which can be caused by any stressful event.

Cortisol causes the liver to release more sugar, causes breakdown of

muscle and fat for energy and also lowers the amount of energy used by the cells

of the body. It is also anti-inflammatory and lowers the body's ability to protect

itself.

Aldosterone: Aldosterone causes the body to try and keep water and

sodium in the body by acting on the kidney.

The adrenal medulla (the centre) secretes adrenaline, and noradrenaline.

The secretion of these hormones is because of the need for quick bursts of

energy. Their secretion triggers cellular energy use and allows access to the

body's energy reserves. These effects are very rapid and occur within roughly

thirty seconds, and staying there for several minutes. The circulating adrenaline

also causes constriction of virtually every vessel in the body (causing your hands

to go pale), increased activity of the heart (making it beat faster), inhibition of the

gastrointestinal tract (giving you butterflies) and dilation of the pupils of the eyes.

The Pancreas

The pancreas is a, pinkish-grey organ that lies behind to the stomach. The

organ is approximately 15cm in length with a long, slender body connecting the

head and tail segments.



The endocrine pancreas is separate from the exocrine pancreas which is

discussed under the gastrointestinal section. The endocrine pancreas is made up

of small clumps of cells within the pancreas, called pancreatic islets, or the islets

of Langerhans. These account for only 1% of the pancreatic mass. It is

composed of three distinct cell types each producing a different hormone. The

two important hormones are:

Glugagon: Secretion of glucagon is controlled by the level of blood sugar,

being released when levels are too low. This greatly increases the output of

sugar from the liver and returns blood sugar levels to normal.

Insulin: Insulin is designed to lower blood sugar levels when they become

too high and is released in periods when there is a lot of sugar available, like

after a meal. A lack of insulin means the body has to use fat for metabolism

rather than sugar and can lead to a condition known as ketoacidosis.

The Pineal Gland

The pineal gland is a small, red, pinecone-shaped structure in the brain.

The pineal gland secretes a substance called melatonin. Melatonin slows the

maturation of sperm, eggs and reproductive organs by stopping the production of

FSH and LH (mentioned earlier). Melatonin also appears to play a role in

regulating the 'circadian rhythms' of the body, which influence the day-night

cycle. It is also a powerful antioxidant and protects the brain from toxins.



URINARY/RENAL SYSTEM

Our bodies produce several kinds of wastes, including sweat, carbon

dioxide gas, feces (also known as stool), and urine. These wastes exit the body

in different ways. Sweat is released through pores (tiny holes) in the skin. Water

vapor and carbon dioxide are exhaled (breathed out) from the lungs. And

undigested food materials are formed into feces in the intestines and excreted

from the body as solid waste in bowel movements.



Urine, which is produced by the kidneys, contains the by-products of our

body's metabolism - salts, toxins, and water - that end up in our blood. The

kidneys and urinary tract (which includes the kidneys, ureters, bladder, and

urethra) filter and eliminate these waste substances from our blood. Without the

kidneys, waste products and toxins would soon build up in the blood to

dangerous levels.

In addition to eliminating wastes, the kidneys and urinary tract also

regulate many important body functions. For example, the kidneys monitor and

maintain the body's balance of water, ensuring that our tissues receive enough

water to function properly and be healthy.



The urinary tract is composed of the following four structures:

y Kidney

y Ureters

y Urinary bladder

y Urethra

The kidneys balance the excretion of substances against the accumulation

within the body through ingestion or reduction. Consequently, they are a major

controller of fluid and electrolyte homeostasis. The kidney also have several non-

excretory metabolic and endocrine functions, including blood pressure regulation,

erythropoietin production, insulin degradation, prostaglandin synthesis, calcium

and phosphorus regulation and vitamin D metabolism.

Filtration at the renal glomerulus is the first step in urine formation.

Normally, a volume equal to plasma volume is filtered every 45 minutes, and a

volume equal to total body water is filtered every 6 hours. Glomerular filtrate is

similar to plasma but lacks cells and large-molecular proteins. The glomerular

filtrate is facilitated by active transport, diffusion and osmosis as it passes trough

the renal tubules. Reabsorption of renal filtrate components enhances the

conservation of glucose, peptides, and electrolytes. Secretion of plasma

components enhances elimination or organic acids and bases (some drugs).

Remnants of the glomerular filtrate exit the kidney through the ureters.



The ureters conduct urine from the kidney to the bladder by peristaltic

contraction. Then bladder is a distensible chamber the stores the urine until it is

eliminated. The urethra is the exit passage from the bladder, and it carries urine

for elimination from the body.

Structure of the Elimination system

Kidneys

The kidneys are located peritoneally, in the posterior aspect of the

abdomen, on either side of the vertebral column. They lie between the twelfth

thoracic and the third lumbar vertebrae. The left kidney is usually slightly

positioned higher than the right. Adult kidneys average 11 cm in length, 5-7.5 cm

in width, and 2.5 cm thickness. Affixing the kidneys in position behind the parietal

peritoneum is a mass of perirenal fat (adipose capsule) and connective tissue

called Gerota¶s (subserosa) fascia. A fibrous capsule (renal capsule) forms the



external covering of the kidney itself, except the hilum. The kidney is further

protected by layers of muscle of the back, flank, and abdomen as well as by

layers of fat, subcutaneous tissue and skin.

The kidney has a characteristic curved shape, with the convex distal edge

and a concave medial boundary. In the innermost part of the concave section are

the hilus, through which pass the renal artery, renal vein, lymphatics, nerves and

the renal pelvis (the natural upper extension of the ureter). A fibrous capsule

surrounds and adheres to the renal parenchyma. Each kidney id divided into

three major areas: cortex, medulla and pelvis.

The cortex of the kidney lies just under the fibrous capsule, and portions of

it extend down the medullary layer to from the renal columns (columns of Bertin)

or cortical tissue that separates the pyramids. The medulla is divided into 8 cone-

shaped masses of collecting ducts called renal pyramids. The bases of the

pyramids are positioned on the corticomedullary boundary. Their apices extend

toward the renal pelvis, forming papillae. The papillae each have 10 to 25

openings on the surface, through which the urine empties into the renal pelvis. 8

or more groups of papillae are present in each pyramid; each empties into a

minor calyx, and several minor calices join to form a major calyx. The two to

three calices are outpouchings of the renal pelvis. The renal pelvis cavity is lined

with transitional epithelium. The combined volume of the pelvis and calices is

about 8 ml volumes of excess in this amount damage the renal parenchymal



tissue. The renal pelvis narrows as it reaches the hilus and becomes the

proximal end of the ureter.

Within the cortex lies the nephron, the functional unit of the kidneys, which

consists of both the vascular and tubular elements. Filtration begins at the renal

glomerulus. The glomerulus taft contains capillaries and the beginning of the

tubule system, called Bowman¶s capsule. Filtrate from the glomerulus enters

Browman's capsule and then passes through a series of tubule segments that

modify the filtrate as it passes through the renal cortex and medulla and finally

flows into the renal calyces. A secondary capillary bed, peritubular capillaries,

carries reabsorbed water and solute back toward the vena cava.

Although the two kidneys work together to perform many vital functions,

people can live a normal, healthy life with just one kidney. If one kidney is

removed, the remaining one will enlarge within a few months to take over the role

of filtering blood on its own.

Every minute, more than 1 quart (about 1 liter) of blood goes to the

kidneys. About one fifth of the blood pumped from the heart goes to the kidneys

at any one time.



In addition to filtering blood, producing urine, and ensuring that body

tissues receive enough water, the kidneys also regulate blood pressure and the

level of vital salts in the blood. By regulating salt levels through production of an

enzyme called renin (as well as other substances), the kidneys ensure that blood

pressure is regulated.

The kidneys also secrete a hormone called erythropoietin, which

stimulates and controls the body's red blood cell production (red blood cells carry

oxygen throughout the body). In addition, the kidneys help regulate the acid-base

balance (or the pH) of the blood and body fluids, which is necessary for the body

to function normally.

The kidneys are located just under the rib cage in the back, one on each

side. The right kidney is located below the liver, so it's a little lower than the left

one. Each adult kidney is about the size of a fist. Each has an outer layer called

the cortex, which contains the filtering units. The center part of the kidney, the

medulla has 10 to 15 fan-shaped structures called pyramids. These drain urine

into cup-shaped tubes called calyxes. A layer of fat surrounds the kidneys to

cushion and help hold them in place.



Ureters

The ureters form the medial tapering of the renal pelvis at the hilus of the

kidney. Usually 25-35 cm long in the adult, the ureters lie in the extraperitoneal

connective tissue and descend vertically along the psoas muscle toward the

pelvic cavity. After dipping into the pelvic cavity, the ureters course anteriorly to

join the bladder in its posterolateral aspect. At each ureterovesical junction, the

ureter runs obliquely through the bladder wall for about 1.5 to 2 cm before

opening into the lumen of the ladder.

Three points of potential obstruction exist: the ureteropelvic junction, the

pelvic brim (where cross iliac arteries), and the ureterovesical junction. The

ureter is much narrower at these points. This anatomic arrangement usually

functions as a valve that prevents the backflow (reflux) of urine into the kidney.

Because it is difficult for calculi (stones) to traverse these narrow passageways,

they typically lodge at these junctions.



Each ureter has elastic characteristics and is made of three tissue layers:

an inner mucosa (transitional epithelial membrane) lining the lumen, a muscular

layer, and a fibrous outer layer. When the cancer of the bladder or ureter is

diagnosed, the potential for recurrence exist in either structure. The musculature

is generally designated as inner longitudinal and outer circular. Along most of the

ureter, however, the muscle fibers actually run obliquely and blend with one

another to form a mesh-like tissue. The muscle arrangement allows urine to be

propelled down the ureter by peristaltic action. The peristalsis is probably

regulated by a myogenic pacemaker located near the renal calices.

Blood is supplied to the ureters by one or more vessels that run

longitudinally along the tube. The number and assortment of arteries

anastamosing with the ureteric vessels vary with each individual. Because the

ureters travel through several anatomic areas, the ureteral vessels are fed by

several of the following arteries: renal (frequently), testicular or ovarian, aorta and

common iliac, internal iliac (frequently), vesical, umbilical, and uterine.

The innervations of the ureter come form the eleventh thoracic to the first

lumbar nerves. The network of nerves becomes progressively more dense

toward the terminal end of the ureters.



Bladders

The urinary bladder is a hollow, muscular, and distensible (or elastic)

organ that sits on the pelvic floor in mammals. It is the organ that collects urine

excreted by the kidneys prior to disposal by urination. Urine enters the bladder

via the ureters and exits via the urethra.

In males, the bladder is superior to the prostate, and separated from the

rectum by the rectovesical excavation.

In females, the bladder is separated from the rectum by the rectouterine

excavation, and it is separated from the uterus by the vesicouterine excavation.

The bladder wall has several tissue layers. The internal lining of the vesical wall

is transitional and epithelium with some mucus-secreting glands. Then there are

three ill-defined muscle layers: the inner and outer layers (longitudinal) and

middle layer (circular). The fibers from this layer interweave to from a mesh-like

muscle layer called detrusor muscle. This arrangement allows the bladder wall to



be elastic while maintaining strength. Bundles of these smooth muscle layers

some together at the base of the bladder to form the internal sphincter, or

opening into the urethra. The trigone describes the triangular area formed by the

ureterovesical junctions and the internal sphincter.

The superior and lateral aspects of the bladder are served by the superior

vesical artery, which branches from the umbilical; artery and internal iliac artery.

The inferior vesical artery, which supplies the underside of the bladder, may arise

independently or in common with the middle rectal artery. The veins draining the

bladder pass to the internal iliac trunk.

Innervation for the bladder comes from the hypogastric sympathetic, pelvic

parasympathetic and pudendal muscle nerves. Ganglia are most commonly

found in the bladder base and around the urethral orifice. These areas tend to act

in continuity with each other, and their functions seem to be coordinated by both

the sympathetic and parasympathetic nervous systems.

Blood travels to each kidney through the renal artery, which enters the

kidney at the hilus, the indentation in the kidney that gives it its bean shape. As it

enters the cortex, the artery branches to envelope the nephrons - 1 million tiny

filtering units in each kidney that remove the harmful substances from the blood.



Each of the nephrons contains a filter called the glomerulus, which

contain a network of tiny blood vessels known as capillaries. The fluid filtered

from the blood by the glomerulus then travels down a tiny tube-like structure

called a tubule, which adjusts the level of salts, water, and wastes that are

excreted in the urine.

Filtered blood leaves the kidney through the renal vein and flows back to

the heart. The continuous blood supply entering and leaving the kidneys gives

the kidneys their dark red color. While the blood is in the kidneys, water and

some of the other blood components (such as acids, glucose, and other

nutrients) are reabsorbed back into the bloodstream. Left behind is urine. Urine is

a concentrated solution of waste material containing water, urea (, a waste

product that forms when proteins are broken down), salts, amino acids, by-

products of bile from the liver, ammonia, and any substances that cannot be

reabsorbed into the blood. Urine also contains urochrome, a pigmented blood

product that gives urine its yellowish color.

The renal pelvis, located near the hilus, collects the urine flowing from the

calyxes. From the renal pelvis, urine is transported out of the kidneys through the

ureters, tubes that carry the urine out of each kidney to be stored in the urinary

bladder - a muscular collection sac in the lower abdomen.



The bladder expands as it fills and can hold about half a liter (2 cups) of

urine at any given time (an average adult produces about 1 1/2 liters, or 6 cups,

of urine per day). An adult needs to produce and excrete at least one third of this

amount in order to adequately clear waste products from the body. Producing too

much or not enough urine may indicate illness.

When the bladder is full, nerve endings in its wall send impulses to the

brain. When a person is ready to urinate, the bladder walls contract and the

sphincter (a ring-like muscle that guards the exit from the bladder to the urethra)

relaxes. The urine is ejected from the bladder and out of the body through the

urethra, another tube-like structure. The male urethra ends at the tip of the

penis; the female urethra ends just above the vaginal opening.



CARDIOVASCULAR SYSTEM

The heart is a hallow, muscular organ located in the center of the thorax,

where it occupies the space between the lungs (mediastinum) and rest on the

diaphragm. It weighs approximately 300 g (10.6 oz), although heart weight and

size are influence by age, gender, and body weight, extent of physical exercise

and conditioning, and heart disease. The heart pumps blood to the tissues,

supplying them with oxygen and other nutrients.



The pumping action of the heart is accomplished by the rhythmic

contraction and relaxation of its muscular wall. During systole (contraction of the

muscle), the chambers of the heart become smaller as the blood is ejected.

During diastole (relaxation of the muscle), the heart chambers fill with blood in

preparation for the subsequent ejection. A normal resting adult heart beats

approximately 60 to 80 times per minute. Each ventricle ejects approximately 70

ml of blood per beat and has an output of approximately 5L per minute.

The heart is composed of three layers, the inner layer or the endocardium,

consists of endothelial tissue and lines the inside of the heart and valves. The

middle layer, or myocardium, is made up of muscle fibers and responsible for the

pumping action. The exterior layer of the heart is called epicardium

The heart is encased in a thin, fibrous sac called the pericardium, which is

composed of two layers. Adhering to the epicardium is the visceral epicardium.

Enveloping the visceral epicardium is the parietal pericardium, a tough fibrous

tissue that attaches to the great vessels, diaphragm, sternum, and vertebral

column and supports the heart in the mediastinum. The space between these

two layers (pericardial space) is filled with about 30mL of fluid, which lubricates

the surface of the heart and reduces friction during systole.



Heart Chambers

The four chambers of the heart constitute the right-and left-sided pumping

systems. The right side of the heart, made up of the right atrium and right

ventricle, distributes venous blood (deoxygenated blood) to the lungs via

pulmonary artery (pulmonary circulation) for oxygenation. The right atrium

receives blood returning from the superior vena cava (head, neck, and upper

extremities), inferior vena cava (trunk, and lower extremities), and coronary sinus

(coronary circulation). The left side of the heart, composed of the left atrium and

left ventricle, distributes oxygenated blood to the remainder of the body via the

aorta (systemic circulation). The left atrium receives oxygenated blood from the

pulmonary circulation via the pulmonary veins.

The varying thickness of the atrial and ventricular walls relate to the

workload required by each chamber. The atria are thin walled because blood

returning to these chambers generates low pressures. In contrast, the ventricular

walls are thicker because they generate greater pressure during systole. The

right ventricle contracts against low pulmonary vascular pressure and has thinner

walls than left ventricle.

Because the heart lies in a rotated position with in the chest cavity, the

right ventricle lies anteriorly (just beneath the sternum) and the left ventricle is

situated posteriorly. The left ventricle is responsible for the apex beat or the

point of maximum impulse (PMI), which is normally palpable in the left

midclavicular line of the chest wall at the fifth intercostals space.



Heart Valves

The four valves in the heart permit blood flow in only one direction. The

valves, which are composed of thin leaflets of fibrous tissue, open and close in

response to the movement of blood and pressure changes within the chambers.

There are two types of valves: atrioventricular and semilunar.



The valves that separate the atria from the ventricles are termed

atrioventricular valves. The tricuspid valve, so named because it is compose of

three cups or leaflets, separates the right atrium from the right ventricle. The

mitral, or bicuspid (two cups) valve, lies between the left atrium and the left

ventricle.

Normally, when the ventricle contract, ventricular pressure rises, closing

the atrioventricular valve leaflets. Two additional structures, the papillary

muscles and the chordae tendineae, maintain valve closure. The papillary

muscles, located on the sides of the ventricular walls, are connected to the valve

leaflets by thin fibrous bands called chordae tendineae to become taut, keeping

the valve leaflets approximated and closed.

The two semilunar valves are composed of three half-moon-like leaflets.

The valve between the right ventricle and the pulmonary artery is called the

pulmonic valve; the valve between the left ventricle and the aorta is called the

aortic valve.



Coronary Arteries



The left and right coronary arteries and their branches supply atrial blood

to the heart. These arteries originate from the aorta just above the aortic valve

leaflets. The heart has large metabolic requirements, extracting approximately

70% to 80% of the oxygen delivered (other organs consume, on average, 25%).

Unlike other arteries, the coronary arteries are perfused during diastole. An

increase in heart rate shortens diastole and can decrease myocardial perfusion.

Patients, particularly those with coronary artery disease, can develop myocardial

ischemia when the heart rate accelerates.

The left coronary artery has three branches. The artery from the point of

origin to the first major branch is called the left main coronary artery. Two

bifurcations arise off the left main coronary artery. Theses are the left anterior

descending artery, which courses down the anterior wall of the heart, and the

circumflex artery, which circles around to the lateral left wall of the heart.

The right side of the heart is supplied by the right coronary artery, which

progresses around to the bottom or inferior wall of the heart. The posterior wall

of the heart receives its blood supply by an additional branch from the right

coronary artery called the posterior descending artery.

Superficial to the coronary arteries are the coronary veins. Venous blood

from these veins returns to the heart primarily through the coronary sinus, which

is located posteriorly in the right atrium.



Cardiac Muscles

The myocardium is composed of specialized muscle tissue.

Microscopically, myocardial muscle resembles striated (skeletal) muscle, which is

under conscious control. Functionally, however, myocardial muscle resembles

smooth muscle because its contraction is involuntary. The myocardial muscle

fibers are arranged in an interconnected manner (called a syncytium) that allows

for coordinated myocardial contraction and relaxation. The sequential pattern of

contraction and relaxation of individual muscle fibers ensures the rhythmic

behavior of the myocardium as a whole and enables it to function as an effective

pump.

Cerebral Circulation

The cerebral circulation receives approximately 15% of the cardiac output,

or 750mL per minute. The brain does not store nutrients and has a high

metabolic demand that requires high blood flow. The brain¶s pathway is unique

because it flows against gravity; its arteries fill from below and the veins drain

from above. In contrast to other organs that may tolerate decrease in blood flow

because of their adequate collateral circulation, the brain lacks additional blood

flow, which may result in irreversible tissue damage when blood flow is occluded

for even short periods of time.



Arteries

Two internal carotid arteries and two vertebral arteries and their extensive

system of branches provide the blood supply to the brain. The internal carotids

arise from the bifurcation of the common carotid and supply much of the anterior

circulation of the brain. The vertebral arteries branch from subclavian arteries,

flow back and upward on either side of the cervical vertebrae, and enter the

cranium through the foramen magnum. The vertebral arteries join to become the

basilar artery at the level of the brain stem; the basilar artery divides to form two

branches of the posterior cerebral arteries. The vertebrobasilar arteries supply

most of the posterior circulation of the brain.

At the base of the brain surrounding the pituitary gland, a ring of arteries is

formed between the vertebral and internal carotid arterial chains. The ring is

called the circle of Willis and is formed from the branches of the internal carotid

arteries, anterior and middle cerebral arteries, anterior and posterior

communicating arteries. Functionally, the posterior portion of the circulation and

the anterior or carotid circulation usually remain separate. The arteries of the

circle of Willis can provide collateral circulation if one or more of the four vessels

supplying it become occluded or are ligated.



Veins

Venous drainage for the brain does not follow the arterial circulation as in

other body structures. The veins reach the brain¶s surface, join larger veins, then

cross the subarachnoid space and empties into the dural sinuses, which are the

vascular channels lying with in the tough dura matter. The network of the

sinuses carries venous outflow from the brain and empties into the internal

jugular vein, returning the blood to the heart. Cerebral veins and sinuses are

unique because, unlike other veins in the body, they do not have valves to

prevent blood from flowing backward and depend on both gravity and blood

pressure.

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