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f -TINTtrODUCTION

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The domestic pig, JuJ scrofa, is a member of theclass Mammal ia - the mammals. Al l mammalshave two characterist ics that set them apart fromother animals - hair and marnmary glands. Mod-ern mammals are also dist inguished from other vertebrates by the presence or absence of scveral addil iun,r l . lru, lur.-. On eJ n ,rde u, the lohar I.r\^.mammals have a single dcntary bone, the mandible,which art iculates with the squamosal bone. Thcreare three bones in the middle ear cavity the mal-leus (hammer), the incus (anvil), and the stapes(stirrup). Between the thoracic cavity and thc ab-dominal cavity there is a muscular diaphragm thataids in respiration. Most mammals have sweatglands. There is no cloaca present in thc adult,except in the monotremes (the platypus has a cloacaand lays eggs). Al l mammals, except the toothedwhales, have heterodont dentit ion; that is, theyhave two or more type-r ofteeth, for example, rnolarsand canines. There are usually two -sets ofteeth, a setof milk teeth and a set of permanent teeth. Themammalian bone marrow produces enuc/eatec/(without a nucleus), round, red blood cells (camelsand l lamas do not have round RBCS). The right,fourth aort ic arch of mammals is much reuucecr;what l i t t le remains is part of the r ight subctavtanartery. Nearly aJl mammals have a speciat souno-collectin8 structure known as the pinna or outer ear.The larynx is highly special ized. Perhaps most nora-ble of al l , at least from a human viewpoint, is theenlarSement of the cerebral cortex, al lowing thedevelopment of what we loosely term ,, intel l i

8ence.Pigs are members of the order Artiodactyla, the

even toed ungulates, which also includes cows anddeer. Pigs are omnivores theyeatboth plantsandanimals. Thedomestic pigcan l ive 15 20years,butmost become pork chops, bacon, and sausage longbefore that. Adults may reach weights of up to 400k8 (approximately 900 lbs.).

The length ofthe estrous cycle in pigs is 21 days. l f

a

conception occurs, the gestation period, or ttme oidevelopment of a pig fetus within the uterus, isbetween 112 1 15 days- At least 40 percent of theembryos die before birth and are usually reabsorbedby the body. Sows normally givc birth ro 7 '12 oifsprinB per l i f ter.

Pigs are useful sLrbjects for anatomy and physiology experiments because of their similari t ies to man-Many drugs used in modern rnedicine have bcentested on pigs. Likewise, many sLfgical techniqueswere perfected on pigs before being performed onhumans. In some instances, piB body parts havebeen used in humans. The valves used in surgicalneart vaive replacement were, at one t ime, pigvalves. Pig skin was often used as a temporary skingraftfor human burn vict ims. The pig is an exceitentexample of a typical mammal and exhibits manysimilari t ies to human anatomy.

Orientation

Betore you begin the actual dissection process,you need to become famil iar wjth some commonlyused anatomical terms (Fjg. 1).

Anterior lctanial) toward the head.

Posteriot (caudal) toward the tail.

Dorsal (supetior) toward the back.

Ventral (infetior) toward the lower surface orbelly.

[atera/ - toward the side.

Medla/ - toward the midl ine.

Proximal - near ortoward a point of reference, e.g.the midl ine of the body.

Dista/ - ah/ay from a point of reference, e.g. themidl ine-

Transverse p/ane - perpendiculaf to the lorlg axis.

sagitta/p/ane - a lonB section that divides the bodyinto r iBht and lelt port ions. A mid-sa8ittal section has equal r ight andleft halves.

Fronta/p/anc-alongsect ionthatdiv idesthebodyinto dorsal and ventral port ions.

Gland ducl

Sebaceous gland

Questions

1. What are some characterist ics shared by al lmammals?

2. How are domestif'trlgs related to tBiwild

;5***-ff*$;;How long in humans?

5. Which is more distal to the elbow - thewrist or the hoof?

Epidermis

Taclile nerue ending

pl l imuscle

Blood vessel

Hair buLbPapilla

Figure 2. Diagram of rhe ,k in.

Press!re-sensilive nerve end ngSweal gland

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+ Junit.{ 't f EXrErtfiAL ffArunrsf1 r

Take some time to study carefully the exterior ofthe pig. (To properly learn about your pig, you needto examine the exterior as well as the interior-)

How old is your pi8? To f ind out, measure it fromthe t ip of i ts nose to the base of i ts tai l . (Str ing worksbetter than a ruler since it bends and fol lows thecurves of the body. Measu re the str ing with a metricruler.) Use the chart provided below to determine itsage.

Aq?ro\imat' d8( inlength of specimen days from fertilization

f.1

1 cm (% inch)

2 cm (7a inch)

3 cm (1 7a inches)

4 cm (l 7, inches)

20 cm (77a inches)

25 cm (9% inches)

30 cm (1 1 % inches) ' l 12 1 '15 days (ful l term)

Perhaps the most often overlooked organ of thebody is the most obvious one. In fact, i f you look atyour pig, you are looking right at i t . l fyou look in themirror, you' l l see the same or8an. l t 's the largestorgan of the human body as well. l t is the skin.

The skin performs an enormous job- l t is a barierto ultraviolet radiation, keeps out harmful sub-stances and disease-producinB organisms, helps toprevent the loss of water from the body, aids inregulating the body temperature, is an excretoryorgan (sweat, etc.), serves as a receptor for externalst imulations through the many nerve endings andsensory structures embedded there, provides stor-a8e for reserve nutrients, and is the site of vitamin Dproduction. The skin is separated into two layers

an outer layer known as the epidelmis and a deeperlayer known as the dermi5 (Fig. 2). Also found in theskin are hair {ol l icles, several kinds o{ glands, var-rou\ \en.e organ\. blood r"..e1.. and nerte'.

The hooves ofthe piB (and the nails of your handsand feet) are modif ications of the epidermis. In hu-mans, the whit ish crescent-shaped area at the baseof the nail is the active growing port ion.

The pi8 can be divided into four sectionshead, neck, trunk, and tai l - each ofwhich has itsown external features (Fig. 3).

The Head

First locate the snout. This structure ends in a f lat,plate l ike t ip containing cart i lage. The paired ex-ternal nares (nostri ls) located on the end of the snoutserve as the passa8eway for air into the nasal cavity,where it is warmed, f i l tered, and moistened. Youcan easily f ind the mouth with i ts f/eshy / ips. Theupper l ip is thick-rimmed while the lower l ip issmaller and more pointed- Look into the mouth andfind the tongue and possibly, depending on the ageof your specimen, teeth. The rool of the mouth, thehaldpalate, consists of a thin layer oft issue coveringa base of bone and cart i lage that separates themouth cavity from the nasal cavity. This al lows foodto be chewed while breathing continues. Examinethe eyes and the upper and lower eyelids. In addi-t ion, there is a third eyelid, the nict i tat ingmembranethat covers the anterior portion of the eye and helpsto clean it . You need to make a small incision andopcn thp othpr l \^o i id. to spe this. The inr i . iorshould be forward from the inner corner of the eye.Only a small vestige of the nict i tat ing membrane ispresent in the inner corner of the human eye.

The externa/ ears, ot pinnae, sefte to channelsound waves into the small opening known as theexternal auditory meatus. A short passage, the auditory cara/, leads to the eardrum ot tympanrc mem-

20 days

35 40 days

,19 50 days

75 days

100 days

Urnbi l icalcod

Forelegs

Figure 3. External anatomy of the pig.

brane, from which sound vibrations are transmittedto special ized nerve endings in the cochlea.

The Neck

The neck of the pig is short and th ick. No externalfeatures are noticeable.

The Trunk

The trunk of mammals is divided into the thoraxand the abdomen. The thorax is the region thatincludes the ribs- Below the neck, a single pair ofappendages, the forelegs, are connected at theshoulders. Note that the pig has four toes, and that i twalks on the middle two. On the foreleS, the footbends posteriorly at the w/ist. Moving proximallyalong the l imb, you see that i t bends anteriorly at the

Hind egs

On the abdomen aretwotowsol mammaty papil/ae, or teats. How many pairs are there? There arenormally five to seven pairs of teats. Both male andfemale have theseteats, butthey become iunctionalonly in the female.

Ihe umbilical cord provided the attachment tothe p/acenta. The placenta is a feta/ orSan that servesas an area of interaction between the circulatorysystems of mother and fetus. Cut the umbil ical cordabout 2 cm (% inch)from the abdomen. Close ex-amination of the cut cord reveals openings of threeblood vessels ._ one umbilicalvein and rwo umbilicalartefies. They are embedded in gelatinous con-nective t issue. The umbil ical vein carries blood richin food and oxyBen from the placenta to the de-velLrping letu. . lhe l \^o umbi l i (d l dr ler ie. (drrvwaste laden blood to the placenta, where thewastespass across into the mother's blood; normally thereis no mixing of blood betlveen the mother and fetus.

(The latex in your pi8's circulatory system was rnjected into the umbil ical arteries and vein.)

Behind the umbil ical cord are the hitd /-"9s. Theseare very similar to the forelegs. The /eet bend an-teriorly at the ank/e and the leg bends posteriorly atthe kree.

Beneath the ta// is the anus, the caudal opening ofthe diSestive tract.

l f you've wondered what sex your pig is, now'slhe l im* lu l rnd uul r l - i8. 4 . In mdl. . . lhere i . . rnopening immediately caudal to the umbil ical cord.This is the uroge, i ta / crpen in8 and is the opening iorthe penis. Also in males, you should f ind the scrotum, a sac of skin bcneath the an us and betu,een thehind le8s. In mature malcs, the scrotum contains thetestes. lf you have a young pig, the testes rfay nothave ful ly descended from the abdominal cavityinto the scrotum.

Females also have a urogenita/ opcning thatserves both the reproductive and urinary organs.The /abra are small folds on either side oi thc open

ing. These come together bcneath the an us to forrn asmall spike-l ike projection, the genita/papii la. Be-neath the papil la is the c/i toris, an erecti le organhomoluguu\ lo lhe mdle peni.. B..Lrre ro F\Jminepigs of both sexes.

lJrcgeni lalopening

Genilal papilla

FiSure 4. View of external genital ia of male and female.

rTTT'' 'Unit 4

TNTERNAT ANATO'-ll-L

Slomach

Pancrca6

F

'JfThyroid gland

f r l_,brf t F

r f - r

Lungs

Oiaphragm

To view the internal anatomy, i t is necessary toopen your pig (Fig. 15). Lay your pig on its back andtie the legs with str inS as you did for skinninS. Takeyour scalpel and make an incision alon8 the midventral l ine of the body, start ing at the neck andworking toward the tai l .

As you cut through the muscles ofthe neck, uselight pressure. Be careful not to plunge the scalpelinto the pig or you could cut structures you need tosee later. In fact, whenever possible you should useupward pressure when cuft ing, by tLrrning the bladeover. Careful ly cut through the muscles of the neckunti l you see a whit ish tube the size of a pencil . Thisis the trachea or windpipe. The thymus, an endocrine gland involved in the immune response duringinfancy, l ies on either side of the trachea and extends over the heart. You wil l probably see a brown,oval structure lyinS on top of the trachea. This is thethyroid. Remove the thymus and the thyroid at thisttme.

Now lenSthen the incision to include the thoraciccavity. You immediately encounter resistance. Thestructures causing the problem are the ribs and ster-num. As this is a feta/ pig, i ts r ibs and sternum aresti l l composed ofcart i lage. Cart i lage can be cut by asharp scalpel or a Bood pair ol di. \e(t inB \( i \ \or\.

Use either of these to cut throuSh the rib cage andexpose the organs in the chest cavity. The f irst organyou see is the heart, enclosed by a whit ish, sac-l ikemembrane, the pericadium. You can also see thelungs.

Ftee Ihe diaphragm, a bell-shaped muscle thatseparates the thoracic and the abdominal cavit ies,by cutt ing around its edge near the body wall. Thiswil l keep it intact for further study. Make a shallowincision al l the way to the umbil ical cord, thenaround the cord (making the "U"-shaped cut formales and cuft in8 a circle for females) to the pubicregion (the area immediately between the legs) andthe anus. Cut about 3mm (7s")deep unti l you reachthe pubis. As you are makin8 this cut, use l iShtpressure. The layer of muscle over the abdomen isvery thin and you can easily cut into the internalorgans. You have cut deeply enouBh when yoLl seethe white, shiny semi-transparenl peritoneum.When you reach the pubic region, make your inci-sion sl ightly oftcenter and deeper. The musclesoverlying the pubic region are roughly the samethickness as those ofthe neck region, so you havetocut mLrch deeper than in the abdominal cavity. Youalso need to cut through the pubis to expose structures that lie beneath it.

26

mUnit 5' J-

sPrRAroRt EphtFf

JRlght apicat lobe

Right diaphragmatic lobe

: ieur. 16 ni" . . ' - ^r .^-^,-- .-- .

Lett antgdor tobe

Lett diaphragmattc tobe

Intormediate tobe

Bronchiole

Capitaries

Figure 17. Exploded view of respiratory sysiem.

To study the respiratory system, begin in the neckwith the /arynx. This is the hard, enlarged region ofthe trachea. Inside the larynx are f ibrous cords thaivibrate when air passes over them, producingsounds. The trachea is the tube that connects thelun8s with the nasopharynx. l t is re'nforced withrinSs of cart i lage that keep it from collapsing and isl ined with ci l iated cells that trap mucus and foreignpart icles and pass them back to the oral cavity. justdorsal to the trachea l ies a second tube, the esopha-gus, a part of the digestive system.

Follow the trachea to the /urgs, where it branchesinto two tubes (Fig. 16). Each ofthese is known as abronchus (pl. : bronchi). lnside the lungs, the bron-chial tubes further divide into seconclary hrcrnchi,

then into bronchio/es, and f inal ly into very f inetubes that give r ise to microscopic air sacs known asalveoli \FiB. 17). Caseous exchange takes place inthe alveoli . The lunSs are a pair of highly vascular-ized orBans. Each lung is divided into sectionsknown as /obest there are four on the right side andthree on the left.

The lungs appear to "cradle" the heart. Ljf t thehear-t and you wil l see a junction where the lungsand blood vessels seem to merge. This is where thepulmonary ve/ri and arterier enter and leave thelungs.

The diaphragm is a bell,shaped muscle that sepa-rates the abdominal cavity from the thoracic cavity.In the fetal pig, i t is a sheet of muscle attached

t

2B

around its perimeter to the body wall. The lungs andheart are anterior to it, and the stomach, liver, andother abdominal viscera are posterior to it. The dia-phragm is responsible for the physical movementsthat we term breathing. When the diaphragrrr con-tracts, it moves downward, increasingthe size of thethoracic cavity. When it relaxes, it moves backto itsoriginal posit ion and the thoracic cavity decreasesin size. This increase and decrease in the size of thethoracic cavity acts as a bellows, pull ing air in whenthe cavity increases in size and forcing air out whenit decreases.

trachea colla

5. Why isa

S,{+e exhaling if fterecartilage in itc \afis?

2. How does heblood?

3. Why mustthe lublood vessels i

4. What

29

Righl medial lobe

Gall bladder

Righl laleral

blood supply to the l iver is a system of veins knownas the po{a/ system. In the portal circulation, bloodfrom the digestive organs, the stomach, and bothlarge and small intestines enters the l iver throuBh thepolta/ vein; this can be identif ied i f the l iver is l i f ted.The portal vein branches repeatedly into smallerand smaller vessels that f inal ly feed into t iny vesselscal led capi l lar ies. The capi l lar ies come backtogether to form hepatic veins that then join thepostcava/ vern, which returns the blood to the heart.This system of veins-capil laries-veins enables thecells of the l iver to perform many o{ their functions.

Located in a depression on the dorsal sudace ofthe right medial lobe is the ga// b/adder, a smali,sacl ike structure. The gall bladder, which appearsas a green sac in the adult, serves as a storage organfor bi le, the digestive juice that is produced by theliver and that emulsif ies fat. Emer8in8 from one end

Figure 19. View of the liver and gall bladder removed from the body cavity.

Lefi medial lobe

Lell laleral lobe

of the gall bladder is the cystic duct, which serves asthe passageway for bi le. The cystic duct joins thehepatic duct as it emerges from the liver. Togetherthey form the common brle duct, which carries bi leto the small intestine.

The l iver has other functions besides digestion. l tfilters out and breaks down the toxins ingested orproduced durin8 protein metabolism and produces,stores, and releases BlycoSen, a carbohydrate usedby muscles as a source of energy. The l iver is also amajor storage site for nutrients. Remove the l iver bycutt inB the postcaval vein, the portal vein, the com-mon bi le derct, and the connective t issue holdinB theliver. Be careful not to cut through the esophaBusleading to the stomach.

When you remove the l iver, you wil l see thestomach (Fi8. 20) lyinS in the upper left side of theabdomen. Clands in the l ininB o{ the stomach pro-

3)

duce and secrete digest ive ju ices, pr incipal lyhydrochloric acid and pepsin, which are involved inbreaking down food. Food firstenters the stomach inthe cardiac region (Fig.21). At the junction of thestomach and the esophagus is a r ing of muscle, thecardiac sphincter. This contracts to prevent foodf rom passing back up the esophagus. The stomach isa large, bean-shaped, muscular, sac-l ike orSan- Thewall of the stomach is composed of a l ining thatproduces the digestive juices, surounded by threelayers of muscle. Each layer is oriented in a differ-ent direction: the outer layer runs longitudinally,the middle layer is circular, and the inner layer is

Diaphragm

Stomach

inlestine

oblique. On the dorsal surface of the stomach youmay see a pocket. This is cal led a divert iculum anclis common in pigs but rare in humans. The stomachof your pig may seem to be ful l of food- l f you openthe stomach, you' l l f ind that i t is ful l of f luid andgreen debr is. Fetal mammals are constant lyswallowin8 amniotic //uid. This, along with mucLrsand cells from the skin and digestive tract, is what isin your pig's stomach. The combined materials arecalled meconium.

At the lower end of the stomach, the pyloric end,is another sphincter, the pyloric sph/ncter. Thissphincter serves as the junction between the stom

Descending aorta

Smal l in lesl ine

Figure 20. Abdominal organ5 with l iver removed.

Umbilical vein

lJmbilical arleries

33

Part of diaphragm

Esophagus

Cardiac sphincler

ach and the small intestine and al lows food to passthrouBh for further digestion.

Next to the stomach are the pancreas and thesp/een (FiB. 20). The spleen is a brown, oblong, f irmorgan found along the greater curvature ofthe stomach. l t is a part of the lymphatic system, and is not adigestive organ. This hiBhly vascularized organ isdesigned to f i l ter the blood. The outer surface of thespleen is covered by a capsule of connective t issueand smooth muscle. The interior of the spleen isf i l led with a complex network of connective t issue,nerves, blood vessels, and the highly vascularizedsplenic pulp. After birth, the spleen functions in theproduction of white blood cells in removing old redblood cells, and in the production of antibodies. Inthe fetus, the spleen f unctions as the major srte oi redblood cell production. Remove the stomach and

py oric region

Figwe 21. Ventral view ol ihe stomach removed frorn the body .avity

Cardiac region

spleen by cutt ing the esophagus above the stomachand cutt inS the small intestine just below the pyloricsphincter.

Thepancreasfitsintothe"c shaped"curveoithesmall intestine where it joins the stomach (FiB. 20).The pancreas is an elongated, granular structure thatis l ighter in color than either the intestines or thestomach. Lift the main port ion of the small intestinein order to view the pancreas closely. The pancreasis a dualjunction organ. l t is both an endocrineorgdn dnd Jn c\u' r ine or8Jn. A. dr c\o, rrnc orgdnit is concerned with the secretion of pancreaticjuice. This juice contains a nLrmber of enzymesessentialforthe digestion offood and is released intothe small intestine through the pancreatic duct. Asan endocrine organ, the pancreas prodLrces threehormones that regulate blood sugar levels. / t5u/in is

l4

essential for normal carbohydrate metabolism.When the level of sugar in the blood rises, insulin issecreted into the blood stream, resulting In an In-crease in the uptake of blood sugar by muscle cel lsand a decrease in the level ofblood su8ar. C/ucagothas the opposite effect. When there is a decline inthe level of blood sugar, 8lucagon is secreted, caus-inB an immediate but short- l ived increase in bloodsugar. Insulin and Slucagon together regulate thelevel of blood su8ar. The third hormone, somato-

stal in, inhibits the release of both glucagon andinsul in.

The sma// intestine is a slender tube that emer8esfrom the pyloric end of the stomach (Fig. 20). l tcontinues as a long, coi led tube unti l i t reaches thelarge intestine. In the fetal pi8, the small intestinemay be bet\,veen one and tlvo meters (three to sixfeet) long. There are three divisions the duode-num, the lelunum, and the i/eum. The duodenum isthe port ion of the small intestine that is adjacent to

Small inlesline {ileum) Spkal colon

Descending colon

Figure 22. Diagram of the regions of the large intestine.

l5

Longitudina muscle

Figure 23. Scction oi the small intcstine showing thecircular io ds and vi l l i .

the stomach. l t is the shortest port ion of the smalJintestine. Following the duodenum is the longestport ion of the small intestine, the jejunLrm. Theterminal port ion of the srnall intestine is the i leum,which joins the large iniestine (Fig. 22)- Material isprevented from passing prematurely into the largeintestine by a sphincter known as the i/eocecal va/veat the junction of the i leum and the large rntestrne.

Take a moment to cut open a section of the smallintestine and look inside. Notice that i t has a velvetyappearance. This is due to many thousands of circu-lar folds, and the hundreds of thousands of vi l / i onthe folds (Fig. 23). These structures provide a verylarge surface area for absorption of digested lood,which orr ur ' L l r l ) in lhp -mdl l Inte. t ine.

Most digestion also occurs in the smail Intesttne.The stomach provides for mixing of gastric juiceswith the salivary juices and food. Fats, proteins, andstarches are broken down predominately by thedigestive iuices of the l iver, the pancreas, and thesmall intestine.

lust beyond the junction of the small intestinewith the /afge irtestine is the cecum (Fi8. 22)- lnherbivorous mammals, the cecum often piays an

important role in digestion, but in the pig this is notthe case. In man the cecum tcrminates In rneappendix-

Remove the small intestine by cutt in€i i t at i tsjunci ion with the large intestine above the cecum,careful ly c lear ing away the connect ive t issue(mesentery) holding the intestine in place, and l i f t-ing the small intestine and pancreas as a unit fromthe abdomen. The large intestine wil l be clearlyvis ib le.

The large intestine is divided into the ipira/ coror,the ascendirig co/on, the descending co/on, the rec-tum, and the anuJ. The spiral colon is the longestport ion of the large intestine and resernbles a coi ledsprinS. l t is found on the left side of the abdomen.The next port ion is the ascending colon, found justto the right of the midl ine of the abdomen. As thename suggests, the ascendinS colon runs anteriorly.lust beneath the stomach h turns posteriorly andbecomes the descending colon, which connects tothe rectum in the most caudal region of the abdomen. The rectum is a dark, straight tube that opensto the outside through the anus. The large intesttne tspr imari ly concerned with consol idat ing wastemdler id l . In doinB.o. r l remu\ e. mo. lof lhe r .mdrn-in8 water. A IarBe variety of bacteria are found In thelarSF inte<rine. lhe\e nor onl\ fcrmFnl dn) remdtr-ing carbohydrates, but also produce vitamin K,which is absorbed. The compacted waste, nowknown as feces, leaves the body through the anus.

Queslions

L What arections of the teeth?2. Why doeffTlfie.tomd( h digesl il\elrl .d,J. Whv i\ i l imponrnt that there be many viff

' -fii:$:+,n':"T,il[i,r',#:il{th.rt-run at different angler in the.ton!

5. Whv i. i t ddngeror.,,or rn inf[ hou"diarrhea?

!f

36

i""r frunit"z h +THE CIRCULATORY SYSTEMT J

The circulatory system consists of a four-chambered heart and an elaborate system ot arteries,veins, and capil laries. We wil l discuss the heart f irst-

Look in the thoracic cavity and locate the heart(Fig. 24). Notice that i t is sunounded by an opaquemembrane, the pericatdium. Contained within thepericardium is a fluid that Jubricates and protects theheart. Careful ly peel away the pericardium. Theheaft is a muscular pump and is composed ot cardiac muscJe. In essence, the heart is tvo separatepumps ioined together. The left side of the heartpumps blood throu8hout the systemic circulation,

Lefl subclavian adery

Brachiocephalic artery

Righl auricle

Righl al um

Righl venticle

while the right side ofthe heart pumps blood through-out the pulmonary circulation. (These circuits willbe discussed later.)

There are four chambers in the hea(: h^r'o atriaand tlvo ventricles. We will d'scuss each area of theheart by following the course of blood as it passesthrough the organ (Fig. 25). We wil l describe theflow as il occurs in the animal after birth (see belowfor fetal modifications).

Venous blood enters the heart through the rBht.rt l ium (Fig. 26), a thin walled, sac-l ike structurethat receives blood from the large veins. Most of the

Left atrium

Coronary arlery

Figure 24. Diagram showing an anter ior v iew of the hca(.

37

To lungs

figure 25. Diagfam of the head ,howing the flow of blood throughout the entire hcalt.

Figure 26. Blood flow through the right side of the hea(.

38

blood enterinS the atr ium (some 70%) passes im-mediately into the right ventfic/e. The rest of theblood is forced into the ventricle as the atnum con-tracts. As the atr ium relaxes and the ventricle beginsto contract, a three-leafed valve, the tricuspid va/ve,slams shut, preventing blood from flowing back intothe atr ium. The right ventricle is a muscular cham-ber that generates enough force to push bloodthrouBhout the pulmonary circulation. When theriBht venkicle contracts, blood is forced out throughthe pulmonary artery. Blood is prevented from flowing back into the ventricle by the pulmonafy semi/unar valve, a set of three pocket like flaps that faceinto the artery.

After passinS through the pulmonary circulation,blood repnter. the hearl I hroLrBh the pLrimond 0 \ erninto the /efi at/ lum (Fig. 27)- Most of the bloodenterinS the atrium flows freely into the /eft verit/!c/e. As in the riBht side of the heart, the rest is iorceclinto the ventricle during atr ial contraction. Blood isprevented from passing back into the atr ium by avalve with two leaflets, the brcuspid va/ve. The leftvent. icle is the strongest of the heart chambers andhas the thickest walls. l t must generate enough pres

sure durinS contraction to force blood throughoutthe systemic circulation. As the left ventricle con-tracts and blood is forced into the muscular aorta, i tis prevented from reentering the ventricle by theaortic -semi/unar valve.

Between the left atr ium and the right atr ium is theinteratr ial septum (Fig. 28). Mammalian fetuseshave a hole in the septum, the foramen ova/e, thatal lows blood to pass from the right atr ium into theleft and bypass the pulmonary circulation. Thisopening closes after birth. They also have anembryonic vessel, the ductui arteriosus, that shuntsblood from the pulmonary adery to the aorta,bypassing the pulmonary circulation.

The vessels through which blood f lows.rre theartefies and the veins- Arteries have smooth muscleand elastic f ibers in their walls. This combinationallows for the maintenance of a relatively constantblood pressure by expandinS sl iShtly as the contraction ofthe heaftforces bloodthrough them andthensprinBinB back to their ori8inal shape as the heartrelaxes.

Veins have very few elastic f ibers and l i t t lcsmooth muscle; they can expand to accept blood

r igure 27. Blood t lor rhrou8,h rhe |" f t r id" uf lhc hedn

l9

To body

To lunqs

Figure 28. Blood flow through the fetal hcalt.

but cannot recoil . Blood is prevented from tlowingbackward in the veins by one way valves. Betweenlhe .rr leries Jnd \ein. i . J ncl lrorl of ni, roi opi(vessels cal led capil laries. l t is at this level that theexchange of nutrients and wastes beiween the bloodand the cells takes place. The network oi arteries,veins, and capil laries can be divided into two sys-tems, the pulmonary circu/atior and the systemic

The pulmonary circulation begins with the pul-monary a/tery as i t leaves the f ight ventricle. At thispoint, the blood is r ich in carbon dioxide and low inoxygen. This blood enters the lungs, whefe i t givesup the COr and takes on Or. l t then returnsto the leftatr ium through the pulmonary vein.

The systemic circulation is a bit more complex(Fig. 29). lt starts as blood leaves the left ventriclethrough the aorta. As the aorta leaves the heart, itarches to the pig's left (this arching to the left ischaracterist ic of al l mammals). A number of arteriesbranch off the aorta as it leaves the heart. These are

the coronary arteri€s and supply the heart itself withfreshly oxygenated blood. At the top ofthe arch, twoarteries branch otf. The f irst is the brachiocephalictrunk, which soon branches to form the right sub-c/avian arlery and the bicarotid trurk. The bicarotidtrunk forks to produce the left and riSht carotrdarter;es, which supply blood to the head and ncck.The subclavian arteries supply blood to the thoraxand forel imbs.

The aorta turns posteriorly and runs alon8 thedorsal surJace of the body cavity as the dol-sa/aorta.Arteries a r ise at various points along the dorsal aoftato supply the organs in the body cavity. One pair ofbranches, the rena/arteries, supply the kidneys withblood. The dorsal aorta f inal ly divides into a pair of// iac arteries, which supply the pelvis and the hindlegs. A major branch of the i l iac is thc iamora/artery, the main artery of the leg.

In the fetal piB, there is an addit ional pair ofbranches from the dorsal aorta, the umbl/ ica/ arteries that carry blood to the placenta where it is oxy-genated and where nutrients are absorbed (remem-ber that the lungs are not functional in the feta I piB).After birth, the umbii ical arteries close and thoseport ions distal to the bladder become a part of theventral l igament of the bladder.

Blood returns to the heart via the venous system(Fig. 30). Blood from those port ions of the bodyposterior to the heart reenters the right atf iumthrough the posterior vera cava. In the letal pig,oxySenated blood flows from the placenta throughthe umbil ica/ vein, passes throuSh the IVer, anothen into the posterior vena cava. Blood from thehind legs enters the posterior vena cava through the/emora/ veins, which run paral lel to the iemoralarteries. As blood f lows up the posterior vena cava,it passes the kidneys, and blood i i l tered by the kid-neys enters through the fenal veins. Following theposterior vena cava anteriorly, we see that there area number of smaller veins that receive blood fromlhe abdominal r is, "rr and "rter lhe po.leriur ! cn.rcava. As this vessel passes the l iver, blood from theporta/sy5tem enters it through the hepatic veins. Theportal system is a network of veins that col lect bloodfrom the intestine, stomach, pancreas, and spleenand then pass it through a capil lary network in thel iver, al lowing the blood to be f i l tered.

Anterior to the heart, blood f lows through the/trguhr and \ub, /ar ian r pin' Inro lhp dnrpr ior t endcava and then into the right atr ium.

40

Common carolid aa.

S!bscapular a

Subclavran a.

Brachrocephahc a.

Superior mesenleric a

Gooadal aa,

lliolumbar a.

Descending aoda

Coeliac a,

Umbilical aa.

Exlerna iliac a.

FiBure 29. Diagram of systemic ci fculat ion thc al tcr ies.

4l

In lernal jugular v

Externaljugular v.

Brachiocephalic v.

Superior mesenle c v.

Subscapular v.

Subclavian v.

Coronary v.

Umbilical v.

Exlernal iliac v.

lnlernal iliac v.

Sacralv. Caudal v.

Figure 30. Diagram of systemic circulat ion theveins.

collect on the

elasticity?3

4

{.::5

6