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II.   ANATOMY AND PHYSIOLOGYThe Circulatory System 

The Circulatory System is designed to deliver oxygen andnutrients to all parts of the body and pick up waste materials

and toxins for elimination. This system is made up of the heart,

the veins, the arteries, and the capillaries.

Circulation is achieved by a continuous one-way movement of

blood throughout the body. The network of blood vessels that

flow through the body is so extensive that blood flows within

close proximity to almost every cell.

Heart

The heart is a muscular pump that propels blood throughout the

body. The heart is located between the lungs, slightly to the

left of center in the chest. The heart is broken down into four

chambers including:

  The right atrium, which is a chamber which receives oxygen-

poor blood from the veins.

  The right ventricle which pumps the oxygen-poor blood from

the right atrium to the lungs.

  The left atrium which receives the now oxygen-rich blood

that is returning from the lungs.

  The left ventricle, which pumps the oxygenated blood

through the arteries to the rest of the body.

This process occurs about 72 times per minute, every day of our

lives.

Blood Vessels

Blood vessels are broken down into three groups: the

arteries which carry blood out of the heart to the capillaries,

the veins which transport oxygen-poor blood back to the heart,

and the capillaries which transfer oxygen and other nutrients

into the cells and removes carbon dioxide and other metabolic

waste from these body tissues.

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Blood Pressure

Blood pressure is the force exerted by the blood against

the walls of the blood vessels. The output or direct pumping of

the heart and the resistance to blood flow in the vessels

determines blood pressure. Resistance is determined by blood

viscosity and by friction between the blood and the wall of the

blood vessel.

Blood pressure = blood flow x resistance. 

HUMAN CARDIOVASCULAR SYSTEM 

The main components of the human cardiovascular system arethe 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 provideoxygenated blood. An average adult

contains five to six quarts (roughly 4.7 to 5.7 liters) of

blood, which consists of plasma, red blood cells, white blood

cells, and platelets. Also, thedigestive system works with the

circulatory system to provide the nutrients the system needs to

keep the heart pumping.

Pulmonary circulation

The pulmonary circulatory system is the portion of the

cardiovascular system in whichoxygen-depleted blood is pumped

away from the heart, via the pulmonary artery, to the lungsand

returned, oxygenated, to the heart via the pulmonary vein.

Oxygen deprived blood from the 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

heart.

Systemic circulation

Systemic circulation is the portion of the cardiovascular

system which transports oxygenated blood away from the heart, to

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the rest of the body, and returns oxygen-depleted blood back to

the heart. Systemic circulation is, distance-wise, much longer

than pulmonary circulation, transporting blood to every part of

the body.

Coronary circulation

The coronary circulatory system provides a blood supply to

the heart. As it provides oxygenated blood to the heart, it is

by definition a part of the systemic circulatory system.

The Blood Supply of the Brain and Spinal Cord 

The entire blood supply of the brain and spinal

cord depends on two sets of branches from the dorsal aorta.

The vertebral arteries arise from the subclavian arteries, and

the internal carotid arteries are branches of the common carotid

arteries. The vertebral arteries and the ten medullary

arteries that arise from segmental branches of the aorta provide

the primary vascularization of the spinal cord. These medullary

arteries join to form the anterior and posterior spinal

arteries. If any of the medullary arteries are obstructed or

damaged (during abdominal surgery, for example), the blood

supply to specific parts of the spinal cord may be compromised.

The pattern of resulting neurological damage differs according

to whether the supply to the posterior or anterior artery is

interrupted. As might be expected from the arrangement of

ascending and descending neural pathways in the spinal cord,

loss of the posterior supply generally leads to loss

of sensory functions, whereas loss of the anterior supply more

often causes motor deficits.

The brain receives blood from two sources: the internal

carotid arteries, which arise at the point in the neck where the

common carotid arteries bifurcate, and the vertebral arteries.

The internal carotid arteries branch to form two major cerebral

arteries, the anterior and middle cerebral arteries. The right

and left vertebral arteries come together at the level of

the pons on the ventral surface of the brainstem to form the

midline basilar artery. The basilar artery joins the blood

supply from the internal carotids in an arterial ring at the

base of the brain (in the vicinity of the hypothalamus and

cerebral peduncles) called the circle of Willis.

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The posterior cerebral arteries arise at this confluence, as do

two small bridging arteries,

the anterior and posterior communicating arteries. Conjoining

the two major sources of cerebral vascular supply via the circle

of Willis presumably improves the chances of any region of the

brain continuing to receive blood if one of the major arteries

becomes occluded

The major arteries of the brain. (A) Ventral view (compare

with Figure 1.13B). The enlargement of the boxed area shows the

circle of Willis. Lateral (B) and (C) midsagittal views showing

anterior.

The major branches that arise from the internal carotid

artery — the anterior and middle cerebral arteries — form

the anterior circulation that supplies the forebrain. These

arteries also originate from the circle of Willis. Each gives

rise to branches that supply the cortex and branches that

penetrate the basal surface of the brain, supplying deep

structures such as the basal ganglia, thalamus, and internal

capsule. Particularly prominent are the lenticulostriate

arteries that branch from the middle cerebral artery. These

arteries supply the basal ganglia and thalamus.

The posterior circulation of the brain supplies

the posteriorcortex, the midbrain, and the brainstem; it

comprises arterial branches arising from

the posterior cerebral,basilar, and vertebral arteries. The

pattern of arterial distribution is similar for all the

subdivisions of thebrainstem: Midline arteries

supply medial structures, lateral arteries supply the

lateral brainstem, and dorsal-lateral arteries supply dorsal-

lateral brainstem structures and the cerebellum. Among the most

important dorsal-lateral arteries (also called long

circumferential arteries) are the posterior inferior cerebellar

artery (PICA) and the anterior inferior cerebellar

artery (AICA), which supply distinct regions of

the medulla and pons. These arteries, as well as branches of the

basilar artery that penetrate the brainstem from its ventral and

lateral surfaces (called paramedian and short

circumferential arteries), are especially common sites of

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occlusion and result in specific functional deficits of

cranial nerve, somatic sensory, and motor function.

Blood supply of the three subdivisions of the brainstem.

(A) Diagram of major supply. (B) Sections through different

levels of the brainstem.

The physiological demands served by the blood supply of the

brain are particularly significant because neurons are more

sensitive to oxygen deprivation than other kinds of cells with

lower rates of metabolism. In addition, the brain is at risk

from circulating toxins, and is specifically protected in this

respect by the blood-brain barrier. As a result of the high

metabolic rate of neurons, brain tissue deprived of oxygen and

glucose as a result of compromised blood supply is likely to

sustain transient or permanent damage. Brief loss of blood

supply (referred to as ischemia) can cause cellular changes,

which, if not quickly reversed, can lead to cell death.

Sustained loss of blood supply leads much more directly to death

and degeneration of the deprived cells. Strokes — an anachronistic

term that refers to the death or dysfunction of brain tissue due

to vascular disease — often follow the occlusion of (or hemorrhage

from) the brain's arteries. Historically, studies of the

functional consequences of strokes, and their relation to

vascular territories in the brain and spinal cord, provided

information about the location of various brain functions. The

location of the major language functions in the left hemisphere,

for instance, was discovered in this way in the latter part of

the nineteenth century. Now, noninvasive functional imaging

techniques based on blood flow have largely supplanted the

correlation of clinical signs and symptoms with the location of

tissue damage observed at autopsy.