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Chapter 3 / Body Systems
111
Nervous System Organization
The nervous system is structurally organized in two distinct
systems: the CNS and the PNS. The CNS consists of the
brain and spinal cord. The PNS includes everything in the
nervous system outside the brain and spinal cord: the nerves
that exit the brain, called cranial nerves, and the nerves that
exit the spinal cord, called spinal nerves.
Central Nervous System
The brain and the spinal cord are the structures that make
up the CNS. This is the body’s main control center for all
of the body’s functions, receiving sensory input from everywhere in the body, processing the information, and directing the body to make appropriate responses. Within the
brain are four ventricles, or cavities, and outside the brain
and spinal cord are three distinct layers of connective tissue
coverings.
The Brain
The four main sections of the brain are the cerebrum (sehREE-bruhm), the cerebellum (SAIR-eh-BEHL-uhm), the
brainstem, and the diencephalon (DAHY-ehn-SEHF-uhlahn) (Fig. 3-51).
The cerebrum is the largest portion, divided into two
halves by a deep fissure called the longitudinal fissure. Each
cerebral hemisphere is divided into four different sections,
Diencephalon
Thalamus
Cerebrum
Hypothalamus
Pineal gland
called the parietal, occipital, temporal, and frontal lobes.
The central fissure divides the brain into anterior and posterior sections, and just posterior to the central fissure is the
parietal lobe. The parietal lobe receives information from
the somatosensory receptors (touch, pressure, pain, temperature) and integrates information from all of the senses. The
occipital lobe, the most posterior section of the cerebrum,
processes visual input. The temporal lobe is located laterally and is responsible for hearing and the sense of smell.
Anterior to the central fissure is the frontal lobe, which processes voluntary movement of our skeletal muscles, including speech. The cerebral hemispheres are connected by a
mass of nervous tissue called the corpus callosum, which
helps us coordinate movements that require the left and
right side of our bodies to work together in activities such as
crawling and walking.
The cerebellum is a cauliflower-like structure toward
the back and base of the brain. It acts as the center for equilibrium and helps us maintain our balance. The cerebellum
also controls coordination of skeletal muscles, maintains
muscle tone, and allows movements to be fluid instead
ofjerky.
The brainstem, located at the central base of the brain,
consists of the midbrain, pons, and medulla oblongata. The
brainstem controls such activities as hearing, vision, breathing, sleep cycles, and organ activity.
The diencephalon sits above the brainstem, deep within
the cerebrum, and includes the hypothalamus and thalamus.
The hypothalamus plays a significant role in homeostasis.
It regulates hunger, thirst, pain, sexual behavior, body temperature, and emotions. The activity of the thalamus is less
specific, processing sensory input and redirecting it to the
cerebrum.
The Spinal Cord
The spinal cord runs through the bony vertebral column,
starting at the base of the brainstem and ending around the
first or second lumbar vertebra. The spinal cord acts as a telegraph wire for sending signals to and from the brain. The
inner core of the spinal cord is gray matter: nerve cell bodies
without myelin covering. The outer periphery is white matter made of myelinated axons and dendrites (see Fig. 3-15).
Pituitary
gland
Brainstem
Midbrain
Pons
Medulla
oblongata
Figure 3-51. Brain.
Cerebellum
Spinal cord
Reflex Arcs
The spinal cord acts as the control center for reflexes, which
are instantaneous, automatic responses that require very
few nerve cells. The communication path allows the body
to respond automatically and predictably to stimuli that
are potentially dangerous. Reflex arcs are the specific nerve
cell paths from stimulus to response, or receptor to effector. Receptors are located at the end of dendrites to detect
stimuli. The information from a stimulus travels along a
sensory neuron to the CNS. In the CNS, the impulses are
112 INTRODUCTION TO MASSAGE THERAPY
coordinated and processed into an automatic, involuntary
response. Once the response is determined, the motor
neuron carries the signal away from the CNS to the effector. The effector is the muscle, organ, or gland that reacts
or responds according to the information received. Motor
reflexes, or somatic reflexes, activate skeletal muscles.
Autonomic reflexes stimulate organs and glands to react.
Figure 3-52 illustrates a reflex arc.
The simplest reflex arc only requires two neurons—one
sensory and one motor. A nerve impulse travels along a sensory receptor to the spinal cord and back out to the effector
along a motor neuron. Also called a spinal reflex, this reflex
arc does not travel to the brain for coordination.
Stretch Reflexes
Stretch reflexes are protective muscle contractions that
occur when the tissues are stretched too far and/or too
fast. They prevent the muscle from being torn. The familiar test in which a doctor strikes a person’s knee and the
person responds with a kicking action is actually a stretch
reflex that evaluates the patellar tendon. This tendon, found
just inferior to the patella, is the tendon for the quadriceps
femoris muscles that extend the knee joint and flex the hip
joint. When the tendon is tapped, the muscle spindles in
the quadriceps femoris sense that the muscle is being pulled
too quickly. The proprioceptors send an impulse to the spinal cord, and the spinal cord sends an impulse back along
a motor neuron to the quadriceps femoris, causing it to
contract quickly in a protective mechanism. Stretch reflex
contraction of the quadriceps femoris quickly extends the
knee with a kicking action.
Stretch reflexes come into play in massage therapy
when you move a client’s body. If a muscle is being stretched
too fast and beyond its comfort zone, it can respond with
a protective contraction that the client feels as a cramp.
Therefore, always move the client’s body carefully and
knowledgeably to prevent protective muscle cramps.
Tendon Reflexes
Tendon reflexes occur when a muscle and its tendon are
subjected to slow and gentle tension. The nerve impulse
travels to the CNS, which determines that the muscle is
not in danger of being torn and sends a nerve impulse that
reflexively lengthens the muscle, allowing the stretch to go
a bit further. Because the tendon reflex causes the opposite
result from the stretch reflex, it is also important to a massage therapist. It can be used to encourage clients’ tissues to
stretch and create more space for circulation.
Protecting the body is the purpose of reflexes. When
a stretch is potentially harmful to the muscle tissue, the
body reflexively responds to the stretch with a contraction.
A stretch that does not threaten the integrity of the tissues
initiates the tendon reflex, which relaxes the muscle. The
tendon reflex allows the connective tissue to stretch farther;
the stretch reflex does not. For that reason, athletes can
stretch more effectively by slowly and steadily increasing
the stretch instead of by bouncing. Bouncing can induce
the stretch reflex.
Flexor
muscles
Receptor
Figure 3-52. Reflex arc. (Reprinted with permission from Bear
MF, Connors BW, Paradiso MA. Neuroscience: Exploring the
Brain. 2nd ed. Baltimore: Lippincott Williams & Wilkins, 2001.)
Stimuli
Pain
afferent
Effector
Motor neuron
Synapse
Excitatory
interneurons
Axon of
sensory
neuron
Cell body
of sensory
neuron
Dendrite of
sensory neuron
Flexor Reflexes
The flexor reflex, or withdrawal reflex, is another type of spinal reflex. The flexor reflex is activated usually in response to
a harmful or painful stimulus, such as stepping on a tack or
touching a hot pan. Unlike the stretch reflex that is activated
by proprioceptors, the flexor reflex is activated by sensory
receptors in the integument. The body uses three neurons
to accomplish the safety mechanism of pulling away. A sensory neuron receives the input that is sent as an impulse to
the spinal cord. There, an interneuron in the spinal cord
transmits the impulse to a motor neuron. The impulse travels along the motor neuron to the appropriate muscles that
must contract to move the body out of harm’s way.
Ventricles
Enclosed inside the brain are four ventricles, or cavities.
There are networks of blood capillaries in the ventricles
that filter the blood and add cellular secretions to produce CSF. The fluid is constantly generated and circulated
throughout the CNS to provide nutrients to and remove
waste from the brain and spinal cord. It also acts as a cushion against impact and other trauma. The CSF eventually
returns to the venous blood through the connective tissue
covering of the brain.
Chapter 3 / Body Systems 113
Lateral column
Ventral horn
Ventral column
Spinal pia mater
Subarachnoid
space
Spinal arachnoid
mater
Spinal dura mater
Dorsal horn
Dorsal columns
Spinal canal
DORSAL
Lateral horn
Dorsal root filaments
Dorsal root
Dorsal root ganglion
Spinal nerve
Ventral root
VENTRAL
Figure 3-53. Meninges.
Meninges
The brain and spinal cord are supplied with three layers of connective tissue coverings called meninges (Fig. 3-53). The dura
mater (DUHR-uh MAH-ter) is the toughest and outermost
layer that provides a strong, protective covering for the structures of the CNS. The dura mater covering the spinal cord is
sometimes referred to as the dural tube. The arachnoid mater
(ah-RAK-noyd MAH-ter) is the middle layer with a structure
like a spider web that allows CSF to flow through the meninges. The pia mater (PEE-ah MAH-ter) lies closest to the brain.
It is delicate and carries most of the blood supply for the brain.
Peripheral Nervous System
The PNS consists of all the nerve tissue outside the CNS.
Its function is to transmit information to and from the
CNS. Again, nerves are made up of organized bundles
Ventral root
filaments
(6X)
containing nerve cells, connective tissue coverings, and
blood vessels. The nerves that branch out from the brain
are called cranial nerves, and the nerves that branch out
from the spinal cord are called spinal nerves. Functionally,
the PNS can be divided into the somatic and autonomic
nervous systems. The somatic nervous system is responsible for voluntary skeletal muscle contractions. The
autonomic nervous system (ANS) controls the involuntary smooth muscles of the organs, the cardiac muscles
in the heart, and the activity of glands. The cranial and
spinal nerves of the ANS are separated into the sympathetic and parasympathetic divisions, each with its own
set of responses.
The PNS can be classified structurally, by the location
of the nerves. There are cranial nerves, spinal nerves, and
nerves in the extremities.
There are 12 pairs of cranial nerves originating from the
brain (Fig. 3-54). The cranial nerves are identified by names
114 INTRODUCTION TO MASSAGE THERAPY
Figure 3-54. Cranial nerves.
Olfactory nerve (l)
Optic nerve (ll)
Oculomotor
nerve (lll)
Trochlear nerve (lV)
Abducens
nerve (Vl)
Glossopharyngeal
nerve (lX)
Vagus nerve (X)
Trigeminal
nerve (V)
Facial nerve (Vll)
Vestibulocochlear
nerve (Vlll)
Hypoglossal
nerve (Xll)
and roman numerals, starting at the superior end. Most of
them serve the head and neck region, but the vagus nerve
(cranial nerve X) extends to the thoracic and abdominal
cavities.
The 31 pairs of spinal nerves extend out from the spinal
cord. They are identified according to where they exit the
spinal cord, named for the closest vertebrae. For example,
C8 exits the spinal cord just inferior to the seventh cervical
vertebra (Fig. 3-55). Spinal nerves are mixed nerves, carrying
sensory and motor neurons. Each spinal nerve is connected
to the spinal cord by two roots. The dorsal root contains the
sensory neurons that transmit nerve impulses to the spinal
cord. The ventral root is made of the motor neurons that
transmit the nerve impulses from the spinal cord out to the
effectors.
Dermatomes are zones of the skin supplied by a specific spinal nerve root. The illustration in Figure 3-56 is an average representation of dermatomes, but the zones vary from
person to person, and some dermatomes overlap. Massage
therapists may encounter clients who suffer from a condition that affects one or more dermatomes.
The nerves in the extremities are located in the anterior
and posterior arms and legs as shown in Figure 3-57. These
are important for massage because they innervate the skeletal muscles of the body.
The PNS can also be classified by functions of the different tissues. The PNS is responsible for receiving sensory
input and delivering nerve impulses that control bodily
activities. The functions of the PNS can be separated into
voluntary and involuntary activities. Voluntary activity is
controlled by the somatic (soh-MAT-ik) nervous system. It
Accessory
nerve (Xl)
serves all the skeletal muscles, allowing us to move muscles
when we want to. Involuntary activities, including those of
organs and glands, are controlled by the ANS.
Somatic Nervous System
The effectors of the somatic nervous system are our skeletal muscles, which are discussed in the section above on
the muscular system. Recall that a motor unit is one motor
neuron and all of the muscle cells that it controls. Precision
movement is created by motor units with very few muscle
cells. Strength is a function of the quantity of actin and
myosin filaments within a muscle cell.
Neurological Memory
“Practice makes perfect,” as the old saying goes. This is the
basis for neurological memory. Repetition of a movement
or holding the body’s position in space reinforces the body’s
ability to produce that movement or position over time. The
same activity, practiced over and over, creates a worn path
in the brain and nervous system, sometimes called a nerve
track. Very similar to a reflex arc, this figurative “groove”
involves chemical and anatomical changes that reinforce
learning. An association area of the brain handles the ability
to remember movements and positions, but the nerve track
promotes this ability. One theory of neurological memory
suggests that the neurons that store the memories grow in
size. Another theory is that the repetition increases the neuron’s output of memory-enhancing proteins. The repetitions
may also strengthen the connections between neurons, facilitating the transmission of impulses along a specific path.
Chapter 3 / Body Systems 115
Brain
C1
Radial nerve
Median nerve
Ulnar nerve
Intercostal
nerves
Phrenic nerve
Femoral nerve
Sciatic nerve
Spinal cord
S5
C2
C3
C4
C5
C6
C7
C8
T1
T2
T3
T4
T5
T6
T7
T8
T9
T10
T11
T12
L1
L2
L3
L4
L5
CO1
S4
S3
S1
S2
Cervical
plexus
Brachial
plexus
Lumbosacral
plexus
Brain stem
Cervical
enlargement
Spinal
cord
Lumbar
enlargement
Cervical
nerves
(C1-C8)
Thoracic
nerves
(T1-T12)
Lumbar
nerves
(L1-L5)
Sacral
nerves
(S1-S5)
Coccygeal
nerve
AB
Figure 3-55. Spinal nerves. (A) Posterior view. (B) Lateral view.
The reinforcement of nerve tracks occurs with repetition,
so repeating an activity correctly will reinforce the correct
movement, and repeating an activity incorrectly will reinforce
the incorrect movement.
Unlearning an incorrect process and
relearning it correctly is much more difficult than simply
learning it properly from the beginning. For example, consider how children learn to hold a crayon or pencil. Those
who learn to hold a pencil “incorrectly” will probably hold
a pencil the same way for the rest of their lives despite
efforts to hold it the “right” way. Once a nerve track is established, the body tends to respond predictably with the same
pattern, just like in a reflex response. Repatterning undesirable actions or behaviors requires effort and repetition of
the desired action or behavior.
This concept is commonly seen in massage clients.
When people get hurt, they tend to favor the injury and
develop compensation patterns. Consider a person who
stepped on a piece of glass and cut her foot. She might favor
the injured foot with a limp or an abnormal posture, and
her muscles and body will acquire a new “normal” position in space. The longer a client maintains the new position, the more the brain and nervous system reinforce the
116 INTRODUCTION TO MASSAGE THERAPY
Trigeminal
nerve (V)
C5
T1
C6
C7
C8
L4
C2
C3
C4
T2
T3
T4
T5
T6
T7
T8
T9
T10
T11
T12
S3L1 L1
L2 L2
L3 L3
L5 L5
L4
C2
C3
C4
C5
C6
T6
T7
T8
T9
T10
T11
T12
L1 L1
S3S4S3
L2 L2
S2 S2
T2
T3
C7
C8
T1
C6
L4
L5
S1 S1
Figure 3-56. Dermatomes.
nerve track, and the more difficult it is for the client to
return to the “normal” posture. Theoretically, a client who
has an acute injury can return to a balanced posture more
quickly than one who has allowed an injury to go untreated
for months or years. Understanding this concept of neurological memory and educating your clients about it can help
them understand that it may take more than one massage
session to rid them of their aches andpains.
Autonomic Nervous System
The ANS controls the smooth muscles, cardiac muscles,
organs, and glands, allowing them to function without our
conscious effort. The ANS is divided into two systems that
work together to maintain homeostasis: the sympathetic
and parasympathetic divisions. When one of these two systems is too active or not active enough, homeostasis is disrupted, and the whole body suffers.
Sympathetic Nervous System
The sympathetic nervous system is the stimulatory division of the ANS. It is also known as the thoracolumbar
division because it includes spinal nerves T1 through L2. It
S1
activates the sympathetic response , sometimes called the
fight or flight response, in which the body prepares for a
stressful situation. Even a thought or perception of a threat
can stimulate the sympathetic nervous system to release its
neurotransmitters, including epinephrine (adrenaline) and
norepinephrine (noradrenaline). When stimulated, the sympathetic nervous system affects many structures and organs,
preparing them for an emergency situation. For example,
the heart pumps faster to provide more oxygen, the skeletal
muscles contract, the pupil of the eye dilates to allow more
light in, the sweat glands are stimulated to perspire, and
digestive activity slows down (Table 3-5).
Parasympathetic Nervous System
The parasympathetic nervous system is the relaxing,
restorative division of the ANS. It is also known as the
craniosacral (KRAY-nee-oh-SAY-kruhl) system because the
motor pathways arise from the cranial nerves and sacral
portions of the spinal nerves. The primary neurotransmitter of the parasympathetic nervous system is acetylcholine. When the parasympathetic nerves are triggered, the
organs and glands have a response opposite to the sympathetic nervous response—the heart slows down, the
Chapter 3 / Body Systems 117
Axillary nerve
Radial nerve
Ulnar nerve
Superficial branch
of radial nerve
Posterior
interosseous
nerve
Obturator nerve
Sciatic nerve
Common fibular
(peroneal) nerve
Tibial nerve
Superficial fibular
(peroneal) nerve
Deep fibular
(peroneal) nerve
Medial plantar nerve
Lateral plantar nerve
Musculocutaneous
nerve
Median nerve
Radial nerve
Ulnar nerve
Deep branch of
radial nerve
Superficial branch
or radial nerve
Ulnar nerve
Median nerve
Femoral nerve
Saphenous nerve
Common fibular
(peroneal) nerve
Superficial fibular
(peroneal) nerve
Deep fibular
(peroneal) nerve
AB
Figure 3-57. Nerves in the extremities. (A) Posterior view. (B) Anterior view.
skeletal muscles relax, the pupils constrict, sweat glands
are not activated, and normal digestion occurs (Table 3-5).
Typically, massage evokes the parasympathetic response ,
which is a relaxation response that encourages the body to
“rest and digest.”
These divisions of the nervous system work in balance. Too much stress or too much excitement can result in
exhaustion. Likewise, too much rest or not enough activity
has negative effects on the body. The body works best when
structures and functions are balanced, including the activity
of the ANS.
Functions of the Nervous
System
There are three different responsibilities for the nervous system.
Together, the functions of the nervous system help monitor
input from both inside and outside the body and regulate the processes within our bodies to keep cellular metabolism in balance.
Monitor
The nervous system detects changes that occur within the
body or outside the body. Sensory input of pain, pressure,
118 INTRODUCTION TO MASSAGE THERAPY
Sympathetic Effector Parasympathetic
Dilation Pupils of the eyes Constriction
Inhibition Digestive glands Stimulation
Vasoconstriction Blood supply to digestive
system
Decrease peristalsis Smooth muscles of
digestive system
Increase strength and
rate of contractions
Dilation Bronchioles Constriction
Stimulates epinephrine
and norepinephrine release
Decrease activity Kidneys None
Relaxation Urinary bladder Contraction for urination
Release more glucose Liver None
Ejaculation Penis Erection
Vasodilation Blood supply to skeletal
Vasoconstriction Blood supply to skin None
Stimulates perspiration Sweat glands in skin None
Heart Decrease strength and rate
Adrenal gland None
muscles
Vasodilation
Increase peristalsis
of contractions
None
and temperatures both inside and on the surface of our bodies are monitored by the nervous system. Monitoring our
body positions in space so we know where we are and sensing scalding hot water on our skin are part of the nervous
system’s responsibilities.
Integrate
Once the nervous system has detected a change or has
received sensory input, it processes the signal for an appropriate response. By monitoring body position, the nervous system
can help us know if we are about to bump into something or
fall over. When scalding hot water is detected on the skin, the
nervous system knows that it is a dangerous situation that must
be avoided. Conversely, when we receive a massage in a professional and safe environment, the sensory input is integrated by
the nervous system to determine how much we will relax.
Respond
Finally, the nervous system takes the information it has
detected and integrated and activates the appropriate
response, or motor output. Sometimes the nervous system activates a muscular contraction and other times it
activates a gland to secrete hormones. Scalding hot water
would cause the nervous system to respond with a motor
output that contracts the muscles that can pull the body
away from the hot water. In a frightening or emergencytype situation, the nervous system will activate the adrenal
glands to release adrenaline and noradrenaline to prepare
the body for impending physical exertion. A trusting and
comfortable sensation during a massage can trigger the
nervous system to send motor output signals to relax the
skeletal muscles.
Effects of Massage on the
Nervous System
All of the sensory input of your massage environment
can affect the nervous system as well as the mental condition of the client, so be aware of your surroundings and
be sensitive to client responses. Initial contact with the skin
Chapter 3 / Body Systems
119
reflexively stimulates a sympathetic nervous response to
prepare us for flight or fight in case the contact turns out
to be a real or perceived threat. When the body has determined that the sustained touch does not pose any danger,
it shifts the balance from a primarily sympathetic nervous
response to a primarily parasympathetic response. Massage
usually causes physiological changes associated with the
parasympathetic nervous response of relaxation, changing
the blood levels of several neurochemicals and hormones
associated with pain:
• Increases dopamine (DOH-pah-meen)—a painrelieving chemical involved in voluntary movement
and clear thinking
• Increases endorphins (ehn-DOR-finz)—very strong
pain-relieving chemicals that suppress all nerve functions to some degree
• Increases enkephalins (ehn-KEHF-uh-lihnz)—strong
pain relievers involved in sensory integration
• Increases oxytocin (AHK-sih-TOH-sihn)—a chemical
that increases the pain threshold, stimulates smooth
muscle contractions, decreases sympathetic nervous
response, and has sedative effects
• Increases serotonin (SAIR-uh-TOH-nihn)—a chemical that generally diminishes pain and appetite,
regulates moods and sleep patterns, and stimulates
smooth muscle contraction
• Decreases cortisol (KOR-tih-sohl)—a natural antiinflammatory produced in response to stress that
can accelerate the breakdown of tissues and prevent
tissue repair, both of which can cause pain
• Decreases substance P—a neurotransmitter that triggers the pain response
Structures of the
Cardiovascular System
The cardiovascular system consists of the blood, the blood
vessels, the capillaries and the heart. These structures create
separate pathways for blood, including the pulmonary circuit and the systemic circuit.
Blood
A single drop of blood contains about 10 million separate
blood cells. Blood is a liquid connective tissue whose cells
are suspended in an extracellular fluid matrix called plasma.
Within the blood plasma are various components including
RBCs, white blood cells, platelets, and proteins.
Blood Plasma
Plasma is a clear, straw-colored matrix that is similar in composition to cytosol. Mostly water, it also contains proteins,
glucose, salts, vitamins, hormones, antibodies, and wastes. It
acts as the transport system for delivering gases and nutrients
throughout the body. Fibrinogen is a protein manufactured in
the liver that resides in the plasma to help with hemostasis,
the process of controlling blood loss and stopping blood flow.
Alpha and beta globulins are plasma proteins that act as transport molecules for lipids and hormones; they are also made in
the liver. Gamma globulins, or immunoglobulins, are antibodies made in the lymphoid tissues that float in the plasma and
are one of the primary components of the immune system.
Erythrocytes
Erythrocytes (ee-RITH-roh-sahytz), also called red blood
cells (RBCs), are biconcave, rounded structures with a central depression (Fig. 3-58). Erythrocytes are the only cells that
Research has shown that massage can cause brain wave
patterns of relaxation and alertness that were also associated
with better performance on math computations.
Cardiovascular System
The cardiovascular system is a circulatory system that provides a link between the external environment and the internal fluid environment of the body by carrying nutrients
and gases to all cells, tissues, organs, and organ systems and
removing metabolic wastes. This exchange is necessary to
maintain homeostasis within the body. Massage promotes
the mechanical movement of fluids and thereby enhances
the delivery of vital ingredients and removal of wastes.
In addition to the primary structures of the cardiovascular system (blood, blood vessels, and heart), the spleen,
liver, bone marrow, and thymus gland also have circulatory
functions, producing and storing blood components and differentiating immune cells.
Erythrocytes
Basophil
Platelets
Neutrophil
Lymphocyte
Monocyte
Eosinophil
Leukocytes
Figure 3-58. Erythrocytes, leukocytes, and platelets.
120 INTRODUCTION TO MASSAGE THERAPY
do not have a nucleus. Instead, these cells are full of hemoglobin molecules that transport oxygen and buffer the pH of
blood. Erythrocytes are enclosed by a highly permeable, elastic membrane that allows gases to diffuse through. Oxygen
moves in and out of RBCs via diffusion. The high concentration of oxygen in the lungs causes oxygen to diffuse into
the blood cells and onto the hemoglobin molecule. As the
blood cells move through the body, oxygen diffuses out of
the RBCs to tissues with lower concentrations of oxygen.
A tiny drop of blood contains over 5 million RBCs that
circulate through the body 300,000 times in about 4 months
before they break down and are actively destroyed by macrophages in the liver and spleen. Homeostasis is maintained
via hematopoiesis in the red bone marrow, which generates
approximately 3 million new RBCs each second.
Leukocytes
Leukocytes (LOO-koh-sahytz) are sometimes called white
blood cells because they have a clear, colorless appearance
(Fig. 3-58). They differ from erythrocytes because they are
larger, they have a nucleus, they do not carry hemoglobin,
and they have the special ability to squeeze through the
cells in the capillary membranes to reside in the interstitial fluids (Fig. 3-59). There are five different kinds of leukocytes—neutrophils, basophils, eosinophils, lymphocytes,
and monocytes—and they all function to defend the body
against disease and foreign substances by destroying pathogens, which are bacteria, viruses, fungi, and other harmful
agents that can cause disease.
A drop of blood only contains about 5,000 leukocytes, in contrast to 5 million erythrocytes. Leukocytes are
Cells of capillary wall
Erythrocytes
(red blood cells)
Leukocytes
Figure 3-59. Leukocytes passing through a membrane wall.
Pathogens
produced by the bone marrow. They may circulate within
the tissues for less than a day or reside in the tissues for
months or years, depending on the severity of the infection or injury, serving as sentinels even in healthy tissues. In
response to an injury or infection, leukocytes are produced
at a higher rate and are much more abundant, so white
blood cell counts can be useful tools for determining the
presence of infection.
Platelets
Platelets, also called thrombocytes, are small, irregular,
non-nucleated fragments of cells. Formed in the bone
marrow as extensions of megakaryocytes that break off,
these components are half the size of RBCs (Fig. 3-58).
There are normally somewhere between 150,000 to
400,000platelets in a drop of blood. Their main function is
to aid in blood clotting, also known as hemostasis, and they
can respond within 15 seconds to 2 minutes of the injury.
The Heart
The heart is the main structure of the cardiovascular system.
It is about the size of a fist and is located between the lungs
in the middle of the thoracic cavity. The heart has four separate chambers separated by muscle walls and valves. The
two upper chambers are called atria (AY-tree-uh) and are
encased by thin walls of cardiac muscle. The left and right
atria are receiving chambers for incoming blood. The two
ventricles have thicker walls and are located below the atria.
The ventricles are the discharging chambers responsible for
pumping blood from the heart to deliver it to the rest of the
body (Fig. 3-60).
The myocardium, or cardiac muscle tissue, varies in
thickness and is arranged in spiral bundles that wrap around
the heart chambers. The spiral bundles contract with a
wringing action that squeezes blood out of the chambers.
Cardiac muscle tissue contracts spontaneously and independently and, unlike skeletal muscle, can contract even if all
the nerve connections are severed. Cardiac muscle fibers
contain electrical impulses that exchange charges back and
forth to create a rhythmic contraction. These rhythmic contractions allow the heart to push approximately 6,000 quarts
of blood through the body each day. The ANS controls
the heart rate, which varies depending on the demands of
the body for oxygen. The heart rate accelerates when the
sympathetic nervous system is in control, and it decelerates
under the parasympathetic response.
The valves in the heart are one-way gates that allow
blood to flow in only one direction. Atrioventricular valves
(AV valves) sit between the atrium and ventricle and are
forced shut when the ventricles contract to prevent blood
from leaking into the atria. The semilunar valves are located
at the exits of the ventricles. When the ventricle contracts,
the semilunar valves are forced open and allow blood to
only flow out of the heart. When the ventricle relaxes, the