1

MODULE 1

THE EYE

The function of the eyes and adnexa (accessory structures) is to provide an individual with the sense of vision

by capturing light rays and focusing them on the retina to produce an image. The interpretation of these images

is the function of the nervous system. With certain visual disorders, the image may be correctly imaged by the

eye, but misinterpreted by the brain.

The eye can be divided into the ocular adnexa—the structures that surround and support the function of the

eyeball—and the structures of the globe of the eye itself: the eyeball. Our binocular vision sends two slightly

different images to the brain in order to produce depth of vision. The Latin term for the right eye is oculus

dextra, with the left eye termed oculus sinistra. Please note that the combining forms dextr/o and sinistr/o refer

to right and left; not right and evil! The term for “each eye” is oculus uterque.

Ocular Adnexa

Each of our paired eyes is encased in a protective, bony socket called the orbit or orbital cavity.

Within the orbit, the eyeball is protected by a cushion of fatty tissue. The eyebrows mark the supraorbital area

and provide a modest amount of protection from perspiration and sun glare. Further protection is provided by

the upper and lower eyelids and the eyelashes that line their edges.

The corners of the eyes are referred to as the canthi (sing. canthus); the inner canthus is termed medial (towards

the middle of the body), and the outer canthus is lateral (towards the side of the body). The area where the upper

and lower eyelids meet is referred to as the palpebral fissure. This term is related to the function of blinking,

called palpebration.

The eyelids are lined with a protective, thin mucous membrane called the conjunctiva (pl. conjunctivae) that

spreads to coat the anterior surface of the eyeball as well. The conjunctival sacs (also referred to as the upper

and lower fornix of the eye) are the folded extensions of this membrane that provide the looseness necessary for

movement of the eye.

2

Also surrounding the eye are two types of glands. Sebaceous glands in the eyelids called meibomian glands, or

tarsal glands, secrete oil to lubricate the eyelashes, and lacrimal glands above the eyes produce tears to keep

the eyes moist. These glands can become blocked or infected. The lacrimal gland, or tear gland, provides a

constant source of cleansing and lubrication for the eye. The process of producing tears is termed lacrimation.

The lacrimal glands are located in the upper outer corners of the orbit. The constant blinking of the eyelids

spreads the tears across the eyeball. The tears then drain into two small holes called the lacrimal puncta in the

upper and lower eyelids in the medial canthus, then into the lacrimal ducts (also called the lacrimal canals or

canaliculi [tiny canals]), next into the lacrimal sacs, and finally into the nasolacrimal ducts, which carry the

tears to the nasal cavity. Normally, there are few tears that need draining, but when an individual cries, the

excess tears exit down the cheeks and through the nose.

The extraocular muscles attach the eyeball to the orbit and, on impulse from the cranial nerves, move the eyes.

These six voluntary (skeletal) muscles are made up of four rectus (straight) and two oblique (diagonal) muscles.

The origin of five of these muscles is in a ringlike structure surrounding the optic nerve behind the eyeball

called the annulus of Zinn (also referred to as the annular tendon). This is only mentioned because later, when

the lens of the eye is described, another structure in the lens called a zonule of Zinn will be named. Note that

the muscle to raise the eyelids, the levator palpebrae superior muscle, is also labeled. “Levator” is used for any

muscles whose function it is to elevate a structure. When this muscle is dysfunctional, it can result in an eyelid

that droops (ptosis). The orbicularis oculi are the sphincter (ring like) muscles that close the eye.

The Eyeball

The anatomy of the eyeball itself is traditionally explained in three layers or tunics. The outer layer, or fibrous

tunic, consists of the sclera and cornea. The middle layer, or vascular tunic, is composed of the choroid, ciliary

body, and iris. The inner layer, or nervous tunic, consists of the retina.

3

The outermost lateral and posterior portion of the eye, the white of the eye, is called the sclera, which means

“hard.” Its three sections are the episcleral layer (literally the layer on top of the sclera), Schlemm’s canal (also

called the scleral venous sinus) which is a ring like tube that returns excess fluid to the bloodstream collected

from the final layer of the sclera, the trabecular network. The deepest layer of the sclera, the trabecular network

is spongy porous tissue that serves to drain fluid from the eye in order to maintain healthy intraocular pressure.

The cornea is the anterior, transparent continuation of the sclera. The combining forms for the cornea (corne/o

and kerat/o) refer to the tough nature of this part of the outer layer of the eye. The border of the cornea, between

it and the sclera, is called the limbus. The combining form limb/o refers to an edge, as in the margin between

two structures. The cornea is where refraction (the bending of light) begins as light enters the eye.

The iris (pl. irides) is a smooth muscle that contracts and relaxes to moderate the amount of light that enters the

eye. In most individuals, this is the colored part of the eye (brown, gray, hazel, blue) because of its

pigmentation. Individuals with albinism, however, have reddish-pink irides because a lack of pigment makes

the blood cells visible as they travel through the vessels supplying the iris.

The pupil is the opening in the center of the iris (that appears as a dark area) where the light continues its

progress through to the lens. Although not technically a part of the vascular tunic, the crystalline lens is a

biconvex, transparent, avascular structure made of protein and covered by an elastic capsule.

Between the lens and the inner layer, the retina, is a transparent jellylike substance called the vitreous humor

(also called the vitreous body), which holds the choroid membrane against the retina to ensure an adequate

blood supply. The combining form vitre/o means “glass or glassy” which may refer to its appearance, although

it is not especially helpful to define the term.

4

The inner layer of the eye, called the retina, is composed of several parts. The pars optica retinae contain the

sensory receptors (rods and cones), the optic disk, the ora serrata, the macula lutea, and the fovea centralis. This

layer is nourished by the retinal vessels that radiate from the optic nerve.

The sensory receptors for the images carried by the light rays are named for their appearance. They are the

rods, which appear throughout the retina and are responsible for vision in dim light and the cones, which are

concentrated in the central area of the retina and are responsible for color vision. Three types of cones, termed

L, M, and S (for long, medium and short) cones, are endowed with photo pigments that react to different

wavelengths of light that produce the perception of red, green, and blue vision. Those individuals who have

difficulty with their color vision (through inheritance or trauma) have deficiencies in one or more of these

cones.

The optic disk is the small area in the retina where the optic nerve enters the eye. Also called the optic papilla

for its nipple like appearance, it is referred to as the blind spot of the eye because of its lack of light receptors.

The ora serrata (ora is the plural of os, meaning an opening, while serrata refers to its “notched” appearance) is

the jagged border between the retina and the ciliary body of the choroid. The macula lutea (literally meaning a

yellow spot) is the area of central vision in the retina, while the fovea centralis, or simply fovea, is the

depression in the middle of the macula that is the area of sharpest vision because of its high density of cones

(color receptors). The term fovea means a “small pit,” so the fovea centralis is literally a small pit in the middle

of a yellow spot.

5

The ocular adnexa and the fibrous, vascular and nervous layers, or tunics, are essential to vision. All parts work

together with impressive harmony. The eye muscles coordinate their movements with one another; the cornea

and pupil control the amount of light that enters the eye; the lens focuses the image on the retina; and the optic

nerve transmits the image to the brain through an opening in the skull termed the optic foramen.

Two important mechanisms contribute to the ability to see. As light hits the eye, it passes first through the

cornea, which bends the rays of light (refraction) so that they are projected properly onto the receptor cells in

the eye. The muscles in the ciliary body adjust the shape of the lens to aid in this refraction. The lens flattens to

adjust to something seen at a distance, or thickens for close vision, a process called accommodation. Errors of

refraction are the most common reason for lens prescriptions.

THE EAR

The ears provide an individual with the sense of hearing and balance, or equilibrium. The ear is regionally

divided into the outer, middle, and inner ear. Sound travels through air, bone, and fluid across these divisions. The

middle and inner ear are contained within the harder, protective petrous portion of the temporal bone. The

mastoid process is a hard, small projection of the temporal bone full of air cells. Located behind the opening of

the auditory canal, the air cells of the mastoid are connected to the middle ear through a cavity termed the

mastoid antrum. This connection is the conduit for infections from the middle ear to the mastoid process.

The Outer Ear

6

Sound waves are initially gathered by the flesh-covered elastic cartilage of the outer ear called the pinna, or auricle. The

auricular cartilage is folded into several distinct structures with separate names. The helix is the upper outer rim of the

auricle, while the antihelix is the inner curve that is parallel to the helix. The antihelix has two “legs” or crura (sing. crus)

that divide to form a shallow depression between them referred to as the triangular fossa. The tragus is the fleshy tag of

tissue with a tuft of hair on its underside that covers the opening of the external auditory canal.

The antitragus is the small raised prominence that is opposite to the tragus. It is important to remember that elastic

cartilage is covered by a layer of connective tissue called perichondrium. When this is separated from the cartilage by

trauma, deformities of the pinna may occur. The lobule, usually referred to as the ear lobe, is the only noncartilaginous

part of the external ear. This fleshy protuberance is composed of adipose tissue.

The gathered sound is then funneled into the external auditory canal. The opening of the auditory canal is

termed the auditory (acoustic) meatus. Earwax, or cerumen, is secreted by modified sweat glands within the

external auditory canal and protects the ear with its antiseptic property and its stickiness, trapping foreign debris

and moving it out of the ear.

The tympanic membrane (TM), or eardrum, marks the end of the external ear and the beginning of the

middle ear. This concave membrane of the eardrum is attached to an almost complete ring of bone called the

tympanic annulus. The membrane is composed of a thick, taut part (the pars tensa) and a thin, flexible part (the

pars flaccida). The center of the membrane is pulled inward, forming a shallow depression termed the umbo.

Because the membrane is extremely delicate and vulnerable to perforation and infection, the additional terms

naming structures of the eardrum are necessary to specify where the injury occurs.

The Middle Ear

The eardrum conducts sound to the air-filled tympanic cavity of the middle ear. The Eustachian tube, also

called the auditory tube or the pharyngotympanic tube, is a mucous membrane-lined connection between the

middle ear and the throat. It functions to supply air for sound conduction and pressure equalization.

The three tiny bones in the middle ear are called the ossicles, or the ossicular chain, and are named for their

shapes: the malleus, or hammer; the incus or anvil; and the stapes ( pl. stapedes), or stirrup. The ossicles

transmit the sound to the oval window through the stapes. The main cavity of the middle ear, opposite the

7

tympanic membrane, is termed the tympanic cavity proper. Above the level of the eardrum is a separate space

called the epitympanic recess, or attic.

The Inner Ear

Once sound is conducted to the oval window, it is transmitted to a structure called the labyrinth, or the inner

ear. A membranous labyrinth is enclosed within a bony labyrinth. Between the two, and surrounding the inner

labyrinth, is a fluid called perilymph. Within the membranous labyrinth is a fluid called endolymph. Hair cells

within the inner ear fluids act as nerve endings that function as sensory receptors for hearing and equilibrium.

Tiny calcium carbonate crystals called otoliths are attached to these hair cells and act as receptors to aid in

balance. The outer, bony labyrinth is composed of three parts: the vestibule, the semicircular canals, and the

cochlea. The vestibule and semicircular canals function to provide information about the body’s sense of

equilibrium, whereas the cochlea is an organ of hearing. Within the vestibule, two structures called the utricle

and the saccule function to determine the body’s static (nonmoving) equilibrium (Fig. 14-1 inset). A specialized

patch of epithelium, called the macula, found in both the utricle and the saccule, provides information about the

position of the head and a sense of acceleration and deceleration. The semicircular canals detect dynamic

equilibrium or a sense of sudden rotation through the function of a structure called the crista ampullaris.

The cochlea receives the vibrations from the perilymph and transmits them to the cochlear duct, which is filled

with endolymph. The transmission of sound continues through the endolymph to the organ of Corti, where the

hearing receptor cells (hairs) stimulate a branch of the eighth cranial nerve, the vestibulocochlear nerve, to

transmit the information to the temporal lobe of the brain.

THE NOSE, PARANASAL SINUSES, AND THROAT

(The Upper Respiratory System)

The upper respiratory system encompasses the area from the nose to the larynx. Air can enter the body through

the mouth, but for the most part, it enters the body through the two nares (nostrils) of the nose that are separated

by the nasal septum. The nasal turbinates (also called nasal conchae) are three scroll-shaped bones (inferior,

middle, and superior) that increase the surface area that air must pass over on its way to the lungs. The vibrissae

(the coarse hairs in the nose) serve to filter out large particulate matter, and the mucous membrane and cilia

(small hairs) of the respiratory tract provide a further means of keeping air clean, warm, and moist as it travels

to the lungs. The cilia continually move in a wavelike motion to push the sticky mucus and debris out of the

respiratory tract. The air then travels up and backward, where it is filtered, warmed, and humidified by the

environment in the upper portion of the nasal cavity. Damage to the cilia keeps the germ-laden mucus from

leaving the body and consequently provides a hospitable environment for infection.

The receptors for olfaction are located in the nasal cavity, which is connected to the paranasal sinuses,

collectively named for their proximity to the nose.

The paranasal sinuses, divided into the frontal, maxillary (the largest sinus, also referred to as the antrum of

Highmore), sphenoid, and ethmoid cavities, acquire their names from the bones in which they are located. The

paired ethmoid sinuses are divided into anterior, middle, and posterior air cells. The function of sinus cavities in

the skull is to warm and filter the air taken in and to assist in the production of sound. They are lined with a

mucous membrane that drains into the nasal cavity and can be the site of painful inflammation. The ethmoid

bone cradles the olfactory bulb in the cribriform plate. This sieve-like bone has numerous openings through

which olfactory nerves descend into the nasal cavity.

8

Air continues to travel from the back of the nasal cavity to the nasopharynx, a part of the throat (pharynx)

behind the nasal cavity. The Eustachian tubes (also called the auditory or pharyngotympanic tubes) connect the

ears with the throat at this point and serve to equalize pressure between the ears and the throat. The nasopharynx

is the site of the pharyngeal tonsils (adenoids), which are made of lymphatic tissue and help to protect the

respiratory system from pathogens. The next structure, the oropharynx, is the part of the throat posterior to the

oral cavity. It is the location of more lymphatic tissue, the palatine tonsils, so named because they are

continuous with the roof of the mouth (the palate). The lingual tonsils, located on the posterior aspect of the

tongue, also serve a protective function. Note that the oropharynx is part of the digestive system as well as the

respiratory system; both food and air pass through it. Below the oropharynx is the part of the throat referred to

as the laryngopharynx because it adjoins the opening of the larynx.

The larynx, commonly referred to as the voice box, is the main organ of sound production. It is a short tube that

is composed of nine sets of supportive, protective cartilaginous structures, and two sets of vocal folds, one true

and one false. The false vocal folds, also called the vestibular folds for their location at the entrance to the

larynx, do not function in the production of speech. Speaking and singing are controlled by the true vocal folds

(also termed vocal cords), which are composed of the glottis, two muscular folds, and the space between them

(the rima glottidis). The pitch of one’s voice is determined by the degree to which the vocal cords are stretched

as they vibrate. Loudness of speech is determined by the force of the exhaled air that travels out through the

larynx.

9

One of the cartilages, the epiglottis, is an oval-shaped structure that covers the trachea (windpipe) when an

individual swallows to prevent food from being pulled into the windpipe instead of the esophagus. When

looking at the anatomy of the neck, it is useful to note the proximity of the thyroid gland, which is anterior and

inferior to the larynx. Although the thyroid gland will be described more fully in the chapter on the endocrine

system, one can see that its location allows for a shared connection, the thyroid cartilage. Normally larger and

more angular in the male than in the female, the thyroid cartilage consists of a pair of thin plates called laminae.

These plates cover the anterior surface of the larynx, and are attached to the hyoid bone on either side with the

thyrohyoid ligament. The area where the two plates join is the laryngeal prominence, commonly called the

Adam’s apple. The cricoid cartilage, named for its ringlike appearance, forms the lower part of the larynx,

attaching it to the trachea. The paired arytenoid cartilages, located in the back upper border of the cricoid

cartilage, are attached to the vocal folds and function to close them.

THE GASTROINTESTINAL SYSTEM

The digestive system provides the nutrients needed for cells to replicate themselves continually and build new

tissue. This is done through several distinct processes: ingestion, the intake of food; digestion, the mechanical

and chemical breakdown of food; absorption, the process of extracting nutrients; and elimination, the excretion

of any waste products. Other names for this system are the gastrointestinal (GI) tract, which refers to the two

main parts of the system (the stomach and intestines) and the alimentary canal, which refers to the function of

the tubelike nature of the majority of the digestive system, which starts at the mouth and continues in varying

diameters to the anus.

The digestive system begins in the oral cavity, passes through the thoracic cavity in the mediastinum, crosses

the diaphragm into the abdominopelvic cavity, and finally exits at the anus. Several glands and organs located

in the oral and abdominopelvic cavity are instrumental in carrying out the functions of the digestive system.

Food normally enters the body through the mouth, or oral cavity. The digestive function of this cavity is to

break down the food mechanically by chewing (mastication) and lubricate the food to make swallowing

(deglutition) easier.

The oral cavity begins at the lips, the two fleshy structures surrounding its opening. The upper lip is termed

the labium superioris and the lower lip, the labium inferioris. The vermilion borders of each are the margins

between the lip and surrounding skin. The term “vermilion” shares a combining form with the vermiform

appendix. Here verm/o refers to the dark red color of a worm, while in the appendix, the term is used to describe

its shape. The frenulum of each lip is the small fold of tissue on the inside of each lip that restrains its

10

movement: the superior and inferior labial frenula. (The term frenulum is derived from Latin, meaning a

“bridle” as one would use to restrain the movement of a horse.) The small vertical depression above the upper

lip (and under the nose) is called the philtrum.

11

The sides of the face and inside of the mouth are bounded by the cheeks which are covered by skin on the

outside, a mucous membrane on the inside, and muscles, fat, nerves, and glands in between. Several glands

secrete mucus in the oral cavity: buccal, molar, palatine, and labial. The buccal glands are located throughout

the inner cheek wall, while the molar glands are on the cheek near the back teeth. The labial glands are located

inside the lips and surrounding the mouth, and the palatine glands surround the soft roof in the back of the

mouth.

The tongue, the muscular organ in the oral cavity, is responsible for tasting, chewing, swallowing, and speaking.

It is attached in the front to the floor of the mouth by the frenulum lingua, a small fold of tissue under the

tongue and in the back to the hyoid bone. The tongue is coated in a mucous membrane studded with thousands

of tiny projections called papillae. In between the papillae are nervelike cells called taste buds that have

receptors for the five known tastes: sour, sweet, salty, bitter, and savory (umami). The lingual tonsil is

lymphatic tissue located at the base of the tongue that serves a protective function against pathogens attempting

to enter via the mouth. The anterior hard palate and posterior soft palate form the roof of the mouth. The

uvula is a tag of flesh that hangs down from the medial surface of the soft palate. It has a role in the production

of speech and the initiation of the gag reflex.

The three pairs of salivary glands provide saliva, a substance that moistens the oral cavity and aids in chewing

and swallowing. Saliva begins the chemical digestive process by initiating the digestion of starches. The glands

are named for their locations: parotid, near the ear; submandibular, under the lower jaw; and sublingual,

under the tongue.

12

The esophagus is a muscular, mucus-lined, approximately 12-inch tube that extends from the throat to the

stomach. It carries a masticated lump of food, a bolus, from the oral cavity to the stomach by means of

peristalsis. The glands in the lining of the esophagus produce mucus, which aids in lubricating and easing the

passage of the bolus to the stomach. The muscle that must relax before the food enters the stomach is known by

three names: the lower esophageal sphincter (LES), the gastroesophageal sphincter, or the cardiac

sphincter, which gets its name because of its proximity to the heart. Sphincters are ringlike muscles that appear

throughout the digestive and other body systems. These muscles may be either voluntary or involuntary in their

action.

The peritoneum is a double-sided membrane that holds many of the organs inside the abdominopelvic cavity.

The outer side of the membrane, near the body wall, is termed the parietal peritoneum, whereas the inner side,

near the organs, is the visceral peritoneum. The visceral peritoneum is the serosal layer that coats the

abdominopelvic viscera with its serous fluid facilitating movement between the organs. Ascites, for example, is

an abnormal accumulation of this peritoneal fluid in the abdominopelvic cavity.

Not all of the organs in the abdominopelvic cavity lie within the peritoneum. Some, for example, the aorta,

kidneys, ureters, duodenum, and pancreas are outside and behind the peritoneum in the retroperitoneum.

The organs that are within the peritoneum (intraperitoneal), however, have additional structures that serve to

support and supply them: mesenteries, (visceral) ligaments, and folds. The mesenteries are extensions of the

visceral peritoneum that stretch out to hold many of the abdominal organs and serve as a channel for blood

vessels, nerves and lymphatic vessels traveling to and from the organs in question.

The Stomach

The stomach, an expandable saclike vessel located between the esophagus and the small intestines, has three

main functions. It begins the process of digesting proteins by storing the swallowed food and mixing it with

gastric juices and hydrochloric acid to further the digestive process chemically. This mixture is called chyme.

The smooth muscles of the stomach contract to aid in the mechanical digestion of the food. A continual coating

of mucus protects the stomach and the rest of the digestive system from the acidic nature of the gastric juices.

Finally, the partially digested mixture is moved to the small intestines.

The stomach is divided into three main sections: the fundus, the body, and the pylorus (also called the gastric

antrum).

The fundus is the area of the stomach that abuts the diaphragm. This section of the stomach has no acid-

producing cells, unlike the remainder of the stomach. The body, or corporis, is the central part of the stomach,

and the pylorus (pl. pylori) is at the distal end of the stomach, where the small intestine begins. The pylorus is

divided into the pyloric antrum, the pyloric canal, and the pyloric sphincter. The pyloric sphincter regulates the

gentle release of food from the stomach into the small intestine. The portion of the stomach that surrounds the

esophagogastric connection is the cardia (so named because of its proximity to the heart). When the stomach is

empty, it has an appearance of being lined with many ridges. These ridges, or wrinkles, are called rugae (sing.

ruga).

13

The Small Intestine

Once the chyme has been formed in the stomach, the pyloric sphincter relaxes a bit at a time to release portions

of it into the first part of the small intestine, called the duodenum. The small intestine gets its name, not

because of its length (it is about 20 feet long), but because of the diameter of its lumen (a tubular cavity within

the body). The second part of the small intestine is the jejunum and the distal part is the ileum. The

duodenojejunal flexure is the border between the first two sections of the small intestines.

Multiple circular folds in the small intestines, called plicae, contain thousands of tiny projections called villi

(sing. villus), which contain blood capillaries that absorb the products of carbohydrate and protein digestion.

The villi also contain lymphatic vessels, known as lacteals that absorb lipid substances from the chyme.

The suffix -ase is used to form the name of an enzyme. It is added to the name of the substance upon which the

enzyme acts: for example, lipase, which acts on lipids, or amylase, which acts on starches. -ose is a chemical

suffix indicating that a substance is a carbohydrate, such as glucose.

The Large Intestine

In contrast to the small intestine, the large intestine (Fig. 5-8) is only about 5 feet long, but it is much wider in

diameter. The primary function of the large intestine is the elimination of waste products from the body. Some

synthesis of vitamins occurs in the large intestine, but unlike the small intestine, the large intestine has no villi

and is not well suited for absorption of nutrients. The ileocecal valve is the exit from the small intestine and the

entrance to the colon. The first part of the large intestine, the cecum, has a wormlike appendage, called the

vermiform appendix (pl. appendices), dangling from it. Although this organ does not seem to have any direct

function related to the digestive system, it is thought to have a possible immunologic defense mechanism.

No longer called chyme, whatever has not been absorbed by the small intestines is now called feces. The feces

pass from the cecum, to the ascending colon, bending at the hepatic flexure to cross the abdomen at the trans-

verse colon, bending downward at the splenic flexure to become the descending colon, and then on to the S-

shaped sigmoid colon. The teniae coli are the muscular bands that contract lengthwise and form the haustra—

14

the bulges in the colon. The rectosigmoid junction marks the beginning of the last straight part of the large

intestine, the rectum and its junction with the anus (the anorectal junction), where the feces are held until

released from the body completely through the external and internal anal sphincters. The internal sphincter is

an involuntary muscle, while the external sphincter is voluntary. The process of releasing feces from the body

is called defecation or a bowel movement (BM).

The

Accessory Organs

The accessory organs are the gallbladder (GB), liver, and pancreas. These organs secrete fluid into the GI tract

but are not a direct part of the tube itself. Sometimes accessory structures are referred to as adnexa.

The four lobes that form the liver virtually fill the right upper quadrant of the abdomen and extend partially

into the left upper quadrant directly inferior to the diaphragm.

The liver forms a substance called bile, which emulsifies, or mechanically breaks down, fats into smaller

particles so that they can be chemically digested. Bile is composed of bilirubin, the waste product formed by

the normal breakdown of hemoglobin in red blood cells at the end of their life spans, and cholesterol, a fatty

substance found only in animal tissues. Bile is released from the liver through the right and left hepatic ducts,

which join to form the hepatic duct. The cystic duct carries bile to and from the gallbladder. When the hepatic

and cystic ducts merge, they form the common bile duct, which empties into the duodenum. Collectively, all of

these ducts are termed bile vessels. Bile is stored in the gallbladder, a small sac found on the underside of the

right lobe of the liver. When fatty food enters the duodenum, a hormone called cholecystokinin is secreted,

causing a contraction of the gallbladder to move bile out into the cystic duct, then the common bile duct, and

finally into the duodenum.

15

The pancreas is a gland located in the upper left quadrant. It is involved in the digestion of the three types of

food molecules: carbohydrates, proteins, and lipids. The pancreatic enzymes are carried through the pancreatic

duct, which empties into the common bile duct. Pancreatic involvement in food digestion is an exocrine

function because the secretion is into a duct.

16

THE RESPIRATORY SYSTEM

(The Lower Respiratory System)

• Delivering oxygen (O2) to the blood for transport to cells in the body.

• Excreting the waste product of cellular respiration, carbon dioxide (CO2).

• Filtering, cleansing, warming, and humidifying air taken into the lungs.

• Regulating the pH of the blood.

• Helping the production of sound for speech and singing.

• Providing the tissue that receives the stimulus for the sense of smell, olfaction.

The respiratory system partners with the circulatory system to deliver oxygen to and remove carbon dioxide

from the cells of the body. Breathing in (inspiration or inhalation) pulls air containing oxygen into the lungs

where it passes into the circulatory system. Carbon dioxide is released when air is pushed out of the lungs

(expiration or exhalation). When one dies, one breathes out and no longer breathes in again—hence the

expression the patient has “expired.” Using the combining form “spir/o” meaning “to breathe”, we can see that

respiration means to breathe again (and again). The term “ventilation” is used to describe the movement of air

into the lungs, whether it is a natural or an artificial activity.

Filtering, cleansing, warming, and humidifying air are important processes that protect the lungs from disease

and allow for an optimal environment for gas exchange. The acidity and alkalinity (pH) of the blood are

accomplished through mechanisms that control the rate of breathing to keep the blood pH within a narrow

range. The function of producing sound (phonation) for speech and singing is accomplished by the interaction

of air and the structures of the voice box, the larynx, and the hollow cavities, the sinuses, connected to the nasal

passages.

Finally, although the sense of smell, olfaction, is not strictly a function of respiration, it is accomplished by the

tissue in the nasal cavity, which receives the stimulus for smell and routes it to the brain through the nervous

system.

17

The lower respiratory tract begins with the trachea (or windpipe), which extends from the larynx into the chest

cavity. The trachea lies within the space between the lungs called the mediastinum. Air travels into the lungs as

the trachea bifurcates (branches) at the carina, a keel-shaped cartilage where the right and left airways called

bronchi (sing. bronchus) divide into smaller branches. The metaphor of an upside-down tree makes sense here,

as one can imagine the trachea as the trunk and the bronchi and bronchioles as branches.

Each lung is composed of sections called lobes, which correspond to the secondary bronchi that supply these

areas within each lung. The right lung is made up of three lobes, whereas the left has only two. The

abbreviations for the lobes of the lungs are RUL (right upper lobe), RML (right middle lobe), RLL (right lower

lobe), LUL (left upper lobe), and LLL (left lower lobe). Within each of these lobes, the secondary bronchi

branch out to tertiary bronchi, and the areas that each supplies are referred to as segments. Each segment is

supplied with blood from a segmental artery that branches off the pulmonary arteries. The segments are named

by their location, e.g., anterior, posterior, apical, basal, medial, or lateral. The lingula is the area where the

superior and inferior segments appear on the left lung. Although named for its tongue-like shape, it has been

postulated that it may represent the remnants of a left middle lobe. At the end of the segmental bronchi are still

smaller branches called bronchioles. These bronchioles end in terminal bronchioles that branch to respiratory

bronchioles. The respiratory bronchioles extend into microscopic ducts capped by air sacs called alveoli (sing.

alveolus). Each alveolus is in contact with a blood capillary to provide a means of exchange of gases. It is at this

point that O2 is diffused across cell membranes into the blood cells, and CO2 is diffused out to be expired.

The cells that line the respiratory tract include goblet cells (that produce mucus) and ciliated basal (also

termed “stem”) cells (that help cleanse the lining). As the bronchial tree progressively divides into smaller and

smaller branches, the shapes of the cells that line it change from a thicker to a thinner appearance. Tall, simple

columnar cells in the primary bronchi give way to squat, simple cuboidal cells in the terminal bronchioles. The

cells in the terminal bronchioles are still equipped with ciliated cells to remove debris from the airways, but the

presence of goblet cells (with their secretion of mucus) is missing. As gas exchange becomes the most

important function, simple squamous (scaly, flat) cells appear in the alveoli. The epithelial lining of the alveoli

is composed of type I and type II cells. Type I cells are responsible for gas exchange, while type II cells produce

a substance called surfactant that keeps the lung from collapsing.

Primary, secondary, and tertiary bronchi that correspond to lung, lobe, and segment.

18

Each lung is also enclosed by a double-folded, serous membrane called the pleura (pl. pleurae). The side of

the membrane that coats the lungs is the visceral pleura; the side that lines the inner surface of the rib cage is

the parietal pleura. The two sides of the pleural membrane contain a serous (watery) fluid that facilitates the

expansion and contraction of the lungs with each breath.

The muscles responsible for normal, quiet respiration are the dome-shaped diaphragm and the muscles

between the ribs (intercostal muscles). On inspiration, the diaphragm is pulled down as it contracts and the

intercostal muscles expand, pulling air into the lungs because of the resulting negative pressure. On expiration

the diaphragm and intercostal muscles relax, pushing air out of the lungs.

THE CIRCULATORY SYSTEM

19

The primary function of the circulatory system is to provide a means of transportation for nutrients, water,

oxygen, hormones, and body salts (to) and wastes (from) the cells of the body. It also serves a protective role by

dispatching specialized defensive cells through the lymphatic system. Both of these tasks require anatomic

structures and mechanisms that direct these “highways” to every cell of the body without stopping. As will be

discussed, any disruption to these functions may result in a disease or disorder of the circulatory system.

Pulmonary Circulation

Pulmonary circulation begins with the right side of the heart, sending blood to the lungs to absorb oxygen (O2)

and to release carbon dioxide (CO2). Once the oxygen is absorbed, the blood is considered oxygenated, or

oxygen rich. The blood then progresses back to the left side of the heart, where it is pumped out to begin its

route through the systemic circulatory system.

Systemic Circulation

Systemic circulation carries blood from the heart to the cells of the body, where nutrient and waste exchange

takes place. Certain organs of the body are key to the process of waste removal. During systemic circulation,

blood passes through the kidneys. This part of systemic circulation is known as renal circulation. In this phase,

the kidneys filter much of the waste from the blood to be excreted in the urine. Blood also passes through the

small intestine during systemic circulation. This phase is known as portal circulation. Here, the blood from the

small intestine collects in the portal vein, which passes through the liver. The liver serves to filter sugars from

the blood and store them for use as needed. Upon return to the right side of the heart, the blood is pushed out to

the lungs to dispose of its CO2, absorb O2, and repeat the cycle.

In systemic circulation, the blood traveling away from the heart first passes through the largest artery in the

body called the aorta. From the aorta, the vessels branch into conducting arteries, then into smaller arterioles,

and finally to the capillaries. This is the site of exchange between the cells’ fluids and the plasma of the

circulatory system. Oxygen and other substances are supplied, and carbon dioxide is collected, along with a

number of other wastes. Once the blood begins its journey back to the heart, it first goes through venules, then

veins and finally into one of the two largest veins, the superior or inferior vena cava. The great vessels

include the pulmonary arteries and veins, the superior vena cava, and the thoracic aorta. The inferior vena cava

is classified as a lower vein.

All of the vessels of the cardiovascular system (including the heart) share a lining of endothelial cells. While all

carry blood, each type of vessel has a slightly different, but significant structure. The muscular, thick nature of

arteries is composed of three tunics, or coats: the outer layer called the tunica externa (adventitia), the muscle

layer and elastic layer called the tunica media, and the inner layer called the tunica interna (intima). Compare

the thickness and structure of an artery to the thinner, valvular nature of veins. Veins do not have the thick

muscle coat of the arteries to propel the blood on its journey through the circulatory system but instead rely on

one-way valves that prevent the backflow of blood. In addition, skeletal muscle contraction provides pumping

action. The capillaries have no coats, and their diameters are so tiny that only one blood cell at a time can pass

through them.

The Heart

The human heart is about the size of a fist. It is located in the mediastinum of the thoracic cavity, slightly left of

the midline. Its pointed tip, the apex, rests just above the diaphragm. The area of the chest wall anterior to the

heart and lower thorax is referred to as the precordium because of its location “in front of” the heart.

20

Inside, the heart has four chambers. The upper chambers are called atria (sing. atrium). The ear-shaped

pouch that is connected to each atrium is called the auricular appendage. Clinically, the right auricular

appendage is associated with a rapid heartbeat (tachycardia), while the left auricular appendage is associated

with blood clots from atrial fibrillation (an extremely rapid and irregular heartbeat). The lower chambers are

called ventricles, which are composed of fleshy, beam-shaped structures called trabeculae carneae. Between

the atria and ventricles, and between the ventricles and vessels, are valves that allow blood to flow through in

one direction. The tissue wall between the top and bottom chambers is called the atrioventricular septum ( pl.

septa).

The great vessels include the superior and inferior venae cavae, the pulmonary arteries and veins, and the

aorta.

The heart wall is constructed of three distinct layers. The endocardium is composed of endothelial cells and

connective tissue that act as a lining for each of the chambers and valves. The myocardium is the cardiac

muscle surrounding each of these chambers. The pericardium is the double-folded layer of connective tissue

that surrounds the heart. The pericardial cavity holds a serous fluid that protects the heart from friction. The

inner layer of this double fold is called the visceral pericardium, and the outer membrane, closest to the body

wall, is the parietal pericardium. Another name for the visceral pericardium is the epicardium because it is the

structure on top of the heart. The term “transmural” is used to describe a heart disorder that is through the wall

of the heart. An example would be a transmural infarct, which is tissue death (an infarct) that extends through

the entire thickness of the heart wall from the endocardium to the epicardium.

21

The Blood Flow Through the Heart

Deoxygenated blood is returned to the heart through the venae cavae. The superior vena cava returns blood

from the upper body, whereas the lower body is drained by the inferior vena cava. Blood is squeezed from the

right atrium (RA) to the right ventricle (RV) through the tricuspid valve (TV). Valves are considered to be

competent (capable) if they open and close properly, letting through or holding back an expected amount of

blood. Once in the right ventricle the blood is squeezed out through the pulmonary semilunar valve into the

pulmonary arteries (PA), which carry deoxygenated blood to the lungs from the heart. These are the only

arteries that carry deoxygenated blood. The main pulmonary artery (pulmonary trunk) divides into right and

left arteries to supply each lung. The conus arteriosus is the cone-shaped extension of the right ventricle into the

pulmonary trunk. In the capillaries of the lungs, the CO2 is passed out of the blood and O2 is taken in. The

now-oxygenated blood continues its journey back from the lungs to the left side of the heart through the

pulmonary veins (PV). These are the only veins that carry oxygenated blood. The blood then enters the heart

through the left atrium (LA) and has to pass the mitral valve (MV), also termed the bicuspid valve, to enter the

left ventricle (LV). When the left ventricle contracts, the blood finally pushes out through the aortic semilunar

valve into the aorta (the largest artery in the body) and begins yet another cycle through the body. The first part

of the aorta, the ascending aorta, rises toward the head, then bends into the aortic arch and continues downward

through the chest as the descending thoracic aorta. Once it passes the diaphragm, it is termed the abdominal

aorta.

Each valve has a fibrous ring at its base called the annulus. The bicuspid valve has two leaflets (cusps) that are

attached to two nipple-like papillary muscles by the chordae tendineae, cordlike tendons. The papillary

muscles open and close the heart valves. The tricuspid valve has three leaflets attached to three papillary

muscles, connected again by chordae tendineae. When a writer refers to heartstrings being tugged at in

sentimental situations, he/she is referring to the chordae tendineae.

The heart muscle has its own dedicated system of blood supply, the coronary arteries. The two main coronary

arteries are called the left and right coronary arteries (LCA, RCA).

The Conductive Mechanism of the Heart

Systemic and pulmonary circulations occur as a result of a series of coordinated, rhythmic pulsations, called

contractions and relaxations, of the heart muscle. The cardiac muscle is controlled by the autonomic nervous

system, so (thankfully) the heart beats involuntarily. The normal rate of these pulsations in humans is 60 to 100

bpm and is noted as a patient’s heart rate. Blood pressure (BP) is the resulting force of blood against the

arteries. The contractive phase is systole, and the relaxation phase is diastole. Blood pressure is recorded as

systolic pressure over the diastolic pressure. Optimum blood pressure is a systolic reading less than 120 and a

diastolic reading less than 80. Normal blood pressure is represented by a range. See the table below for blood

pressure guidelines.

22

The cues for the timing of the heart beat come from the electrical pathways in the muscle tissue of the heart

termed the conductive mechanism. The heartbeat begins in the right atrium at the sinoatrial (SA) node, called

the natural pacemaker of the heart. The initial electrical signal causes the atria to undergo electrical changes that

signal contraction. This electrical signal is sent to the atrioventricular (AV) node, specialized cardiac tissue,

which is located at the base of the right atrium proximal to the interatrial septum. From the AV node, the signal

travels next to the bundle of His (also called the atrioventricular bundle), which carries the electrical impulse

from the top to the bottom chambers. This bundle is in the interatrial septum, and its right and left bundle

branches transmit the impulse to the Purkinje fibers, in the right and left ventricles. Once the Purkinje fibers

receive stimulation, they cause the ventricles to undergo electrical changes that signal contraction to force blood

out to the pulmonary arteries and the aorta. If the electrical activity is normal, it is referred to as a normal sinus

rhythm (NSR) or heart rate. Any deviation of this electrical signaling may lead to an arrhythmia, an abnormal

heart rhythm that compromises an individual’s cardiovascular functioning by pumping too much or too little

blood during that segment of the cardiac cycle.

23

When the SA node (the pacemaker of the heart) fires, the voltage of both atria decreases (referred to as

depolarization). This drop in voltage appears as an upward tracing (termed a deflection) called a P wave. The P

wave represents the atria contracting to push blood through their respective valves and into the ventricles.

The impulse next travels to the AV node, the AV bundle and the Purkinje fibers. Once these fibers are

activated, this sequence causes the ventricles to relax and contract. This depolarization appears as the QRS

segment on an EKG.

The repolarization (the T wave) represents the recovery time when the atria fill with blood from the venae

cavae and pulmonary veins.

The ST segment is an indicator of heart muscle function.

24

Arterial Map

Upper Arteries

25

Lower Arteries

26

Venous Map

Upper Veins

27

Lower Veins