29DIGESTIVE AND RESPIRATORY SYSTEMS The dig estiv e system is re spon sible fo r obtaining and processing food for the all of the ce lls in an… [601764]

29DIGESTIVE AND RESPIRATORY SYSTEMS
The dig estiv e system is re spon sible fo r obtaining and processing food for the all of the ce lls in an
organism . Structu rally it is closely asso ciated with the res piratory system so the tw o will be studied
together. Each system is composed of a series of tubular structures throug h which materials p ass. In the
case of the digestiv e system the raw materials are food particles tha t are broken in to sim ple m olecules as
they pass thro ugh the system . Material that is no t brok en down is re moved fro m the system . In the case
of the respira tory system the major materials m oving throu gh it are o xygen and the w aste p roduc t carbo n
dioxid e.
The dig estiv e and resp iratory system share som e common spaces. The digestiv e system is composed of
the m outh, pharynx, esophag us, stom ach, small intestin e, large intes tine, rectum and anus. There are also
a number of glands that lie outside the system that contribute to its function. These include the salivary
glands, liver an d pancreas. The respiratory system is composed of the m outh or ex ternal n ares, the
pharynx, glott is, trachea, bron chi, bron chioles and alveol i. The la tter three structures ar e in t he lu ng. As
you study these two system s consider how the material m oving throu gh the m is being altered .
Saliva ry gland s
Three paired saliv ary glands lie in the h ead and neck region (F ig. 4.1). Loca te the follow ing:
Parotid g land . These triangular glands lie ventral to
the ear and extend along the lateral surface of the neck.
Mand ibula r gla nd. These large oval glands cover the
ventral portion of the neck.
Sublingu al gla nd. These light colored glands are
closely asso ciated with the m andibu lar gland.
The ducts of the saliv ary glands enter th e mouth. The
glands produ ce m ucus to lub ricate foo d. In humans,
they produce amylase fo r the break down of starch .
Lymp h nodes . These are darker an d less globular
than the sa livary glands. They help initiate the im mune
respon se an d are the site of lymphoc yte replica tion.
Extraorb ital lacrimal gland . This g land lies cranial to the paro tid. It is o ne of sev eral g lands that
produce tears in the rat. The others lacrimal glands lie deeper in the orbit.
Mouth and Phar ynx
The dig estiv e and resp iratory system s are closely asso ciated in the mouth. Cut throug h the angle o f the
jaw on each side of the mouth. Push down on the ja w to exp ose the oral cavity. The tong ue attaches at
the rea r of the o ral cav ity. Lift the tong ue and note that it is attach ed anteriorly by a sheet of tissu e
known as the frenu lum. Also note the location of the two types of teeth, incisors for cutting and molars
for g rinding . The relationsh ip be tween the spaces associated with the oral ca vity can be confu sing. Try
Figure 4.1 Glands of the neck regi on.

30tracing the path food and air w ould take th roug h this region
(Fig. 4.2 – 4.4).
Hard p alate. The bo ny roof of the o ral cav ity.
Soft palate . A continu ation of the tissue lining the roof o f the oral ca vity.
Pharynx . A space at the back of the o ral cav ity divided into:
• Nasopharyn x. Dorsal to the soft p alate the nasopha rynx
receives air from the external nares and is not directly in
the o ral cav ity,
• Oropha rynx . The space ventral to the soft p alate.
• Laryngopharynx. The space posterior to the soft palate
and anterior to the esophag us. Both fo od and air pas s
through the oral cavity, ho wever their pathways diverge
in the laryngopha rynx. Dorsally food moves into th e
esop hagus and the d igestiv e system . Ventrally air m oves
into th e larynx and the respiratory system.
Glottis. (Fig. 4.2) A slit-like opening into the larynx (1, Fig.
4.4), lea ding via the trach ea (2, Fig. 4.4) into the lungs. Un like
the es opha gus, the trac hea has rings of cartilag e to keep it ope n.
Epiglottis. (Fig 4.2) A flap of tissu e that block s the larynx when food or fluid is in the laryngopha rynx.
Thyroid & Parath yroid glan ds (3, Fig 4.4). Posterior to the larynx, paired thyroid g lands control
metabolism. A p arathyroid gland is embedded at the anterior end of each. It regulates blood calcium.
Body C avities, Pleura and Peritoneu m
The bod y cavity of mammals is divided into a n anterior
and posterior region by a muscular diap hrag m. The
anterior region is further divided into two lateral pleur al
cavities that house the lungs and a central perica rdial
cavity that hous es the heart.
Posterio r is th e abdominal (periton eal) cav ity which
contains the digestive, reproductive and excretory
organs. The w alls of the b ody cavities and the surfaces
of the organs are co vered with an epithelium , derived
from mesode rm. Parietal epithelium lines the body wall
and covers the surface of the diaphragm. Visceral
Figure 4.3. Oral cavity. Inset: view into the slit-
like glottis and triang ular ep iglottis.
Figure 4.3. Sag ittal section through the head of a rat
showing the path taken by air and food.
Figure 4.4. Upper respiratory tra ct.
Figure 4.5. Sagittal secti on at the level of the lungs.

31epithelium covers the surface of the internal organs. These two types of epithelium are further divided into
pleura, pericardium or peritoneu m depending on the body cavity in whic h the y are found . Ventral to the
heart th e walls of the tw o plural ca vities m eet to form the mediastina l sep tum. The heart lies in a space
within the septum . The perica rdial sac surroun ding the heart is form ed by the parietal pericardium and
the su rface o f the heart is covered with viscera l per icardiu m (Fig. 4.5).
The abdo minal ca vity is lin ed w ith parieta l peritone um and the surface of the abdominal structures are
covered with viscera l per itoneu m. The org ans in this region a re attach ed to the body wall by exten sions
form ed by the passag e of the p eritoneu m to and from the body wall. These exten sions are k nown as
mesenteries or ligaments.
As you begin your dissection note the extent of the mesenteries and ligaments. They help keep the in ternal
organs in pla ce. Also n ote th at the majority of the m esenteries arise from the dorsal body wall.
Openin g the Body Cavities
To open the body cavities you will need to make the five incis ions show n in figure 4.6. The skin should
alread y have been loosen ed on both s ides a long the m idventra l leng th of the b ody and into th e groin a rea.
Use care not to damage major blood vesse ls.
1. Severa l centim eters to the right of the midline c ut thr ough
the cla vicle a nd posterio rly to the d iaphra gm. Repeat on
the left side u sing care not to damage the mediastinal
septum that divides the th oracic cavity into tw o sid es.
2. Cut laterally throug h the ribs and along the edge of the
diaph ragm toward the ve rtebral colum n. Avoid c utting
the d iaphra gm.
3. Make a cut slightly to the righ t of the m idline a nd follow
it pos teriorly until ju st ab ove the g enital a rea.
4. If the skin h as not be en removed continu e cut three
laterally throug h only th e skin to the ca udal margin o f the
ischium . Use care so th e underlying reprodu ctive
structure s are not dam aged .
5. In the abdominal c avity c ut later ally a long the ed ge of the
diaph ragm toward the ve rtebral colum n.
Sprea d the flaps of body wall to reveal the in ternal orga ns.
Gently bre ak th e ribs near the spinal colum n to help hold the
pleura l cavity open .
Carefully lift th e midventra l flap of skin and bone. Note the
mediastina l sep tum attached to this flap. It divides the thoracic
cavity into two pleural cavities. .
Rela tionsh ips b etween structu res in the thoracic cavity.
Note the position of the m ulti- lobe d lun gs on either side o f the heart a nd the p osition of the diap hragm and
ribs. The hear t lies within the pericardial cavity form ed fro m parieta l perica rdium (Fig . 4.5). The surface
of the heart is co vered with visceral p ericardium . Mak e sure you can disting uish betw een these two types
of pericardium . Also make sure that you can disting uish betw een the p arietal an d visceral p leura. See
Figure 4 .5 for clarificatio n of the relationsh ip be tween the various types of tissu es.
Figure 4.6. Sequence o f cuts for opening
the body cav ities.

32Thymus – (Fig . 4.7) this g land is part of
the en docrine system . In young rats the
thymus gland lies w ithin th e mediastinal
septum and over the anterior part o f the
heart. This g land is part of the immune
system and functions in the maturation of
T-lymphoc ytes. It is quite large in
young anim als an d gets sm aller as the
anim als ag e. It prod uces thymosin whic h
stimulates the im mune respon se.
Resp iratory System
Return to the p haryn x and tra ce the pathwa y air would tak e from the naso phar ynx into th e
laryng opha rynx, thro ugh the g lottis, larynx, trachea, and bron chi into the lu ngs. Gently reflect the h eart
and lung and try to find where the bronchi enter the lungs. Avoid damage to any of the blood vessels in
this reg ion. Cut off a piece of the lung and note a ll the small cha nnels. Within the lu ng the bronc hi div ide
repeated ly form ing thin walled spaces, alveoli, where gas ex chang e occurs. The m ovement o f air in to an d
the lungs is controlled by increasing the size of the pleural cavity as the muscular diap hrag m contracts
enlarg ing this space. Wh en th e diap hragm relaxe s, it curv es into the pleural cavity redu cing the size of the
space and forcing air o ut of the lu ngs.
1. Left cran ial ve na cava
2. Azygou s vein – drains the back.
3. Descending aorta – carries blood to
organs in the abdominal cavity
4. Esophagus – this tube is only
expan ded when filled with food
5. Trachea – this tube contain s ring s of
cartilage to keep it from co llapsing.
Dige stive S ystem
Now trace the path food would tak e
throug h the dige stive o rgan s( Fig 4.7-
4.12, Tabl e 4.1). Deflect the h eart a nd
lungs to the righ t and trace the
esophag us through t he di aphr agm.
As you continu e your dissection note the m embranes that s uppo rt the orga ns. Try to preserve them as well
as all of th e major blood vesse ls. Loca te all the stru ctures listed in table 4.1.
Figure 4.7. Lateral View of the thoracic cavity.
Figure 4. 8. Thoracic cavity with the lung reflected to the right to
expose the deep stru ctures.

33Table 4.1. Major structures of the digestiv e system listed in seq uence .
Structure Function
Mouth (oral cavity) Initial processing of food.
Phary nx
Nasop hary nx
Orophary nx
Lary ngophary nxPathway f or onl y air
Pathway for air and food
Point whe re fo od meets air t hat has entere d through the n ose
Esophagus Connect s the p hary nx and stomach.
Stomach Produces mucu s, hydrochl oric acid, and pep sin (a protease). Toget her they
initiate the b reakd own o f proteins. The hi ghly acid stomach d eacti vates the
salivary enz ymes that we re init iating the breakdown of carbohydrates.
Pyloric valve Regulates movemen t of mat erial out of the stomach
Small intestine (duo denum
jejunum , ileum)Receives ducts from gall bladder and pancreas. B reakdown of fats,
carb ohydrates and p rotei ns is completed in this organ.
Caecum A large bli nd pouch l ocated between t he small intestine and the co lon. It
contains bacteria that produce cell ulase, whi ch facilitates the b reakd own o f the
cellulo se found in p lant material. Breakdown products are then absorbed into
the bloodstream.
Colon (ascending, transverse,
descend ing)Reabs orption of ions and wat er and product ion of mucu s to lubricate mat erial as
it passes towards the rectum.
Rectum A muscul ar po rtion of the d igestive tract that completes wat er reabsorp tion.
Anus Controls the removal of feces.
Structures accessory to the digestive tract
Liver Proc esses glucose and stores it as glycogen, detoxifies other products delivered by
the circulatory system, and produces bile.
Bile Duct* Transports bile from duct s in the liver to the deuodenum. Bile helps neutralize
the partially digested material entering the deuodenum and the bile sa lts he lp to
emulsify fats. In hum ans t he b ile is stored i n the gal bladd er before t ransport to the
deuodenum.
Pancreas Lies in the mese ntary near the deuodenum an d stomach. This gland pr oduces
enzymes responsible for pr otein digestion. It is also an e ndocri ne organ that
relea ses insuli n and gluca gon int o the circulato ry system to regulate b lood gluco se
levels.
Spleen This organ is responsible for the production of lymph ocytes and the breakdown of
old red and white b lood cells.
* Deflect the liver anteriorly and try to find th e clea r bile duct as it leaves the liver in route to the
duod enum (Fig. 4.10).

34Abdomina l stru ctures
The various regions of the coiled small
intestine are co nnected by a layer of
tissue k nown as the mesentery . Gently
spread the intestine to see the extent of
this m esentery . Also note the band of
tissue e xtendin g from the stom ach to the
liver. This is the lesser omentum . Not
visible in this photo is th e greate r
omentum which attaches the spleen to
the greater curvature of the stomach. All
of the se mesenteries help keep the o rgans
in pla ce.
Figure 4.9. Abdominal cavity.
Figure 4.12. Relationship between t he caecum and
the rest of the digestive tract.
Figure 4.10. Relationship
between t he vari ous regions of the
digestive system.
Figure 4.11. Relationship between the liver, bile
duct (1), pancreas (2) and duodenum.