Pig Anatomy: Labeled Diagrams and Interactive 3D Model

A pig is an even-toed hoofed mammal (order Artiodactyla) with a simple, single-chambered stomach, a long intestine with a spiral colon, a snout stiffened by a unique rostral bone, four toes on each foot and a bicornuate uterus that carries large litters. Its heart, kidneys, skin and digestive tract are close enough to ours that pigs are a major model in medicine.

This page describes the domestic pig and uses a fetal pig for the 3D model, because that is what most students dissect. The fetus has all the adult organs plus the umbilical cord and fetal blood shunts. Where the adult differs, the text says so. Directions follow veterinary usage: cranial (towards the head), caudal (towards the tail), dorsal (back) and ventral (belly); left and right are the pig's own.

The external anatomy of a pig includes the snout with its flat rostral disk and nostrils, the eyes and eyelids, the ears, the jowl, neck, shoulder, back, loin, rump and tail, the ham, flank and belly, two rows of teats, and four legs ending in cloven hooves with dewclaws.

Inside a pig, the chest holds the lungs, the heart in its pericardium and, in young pigs, a large thymus. The dome-shaped diaphragm separates the chest from the abdomen. Behind it lie the lobed liver with the gallbladder, the stomach and spleen on the left, the pancreas along the duodenum, the long coiled small intestine, the caecum and the cone-shaped spiral colon, the kidneys against the roof of the abdomen, and the bladder and reproductive organs at the back.

• Lungs: Spongy organs on both sides of the heart. The right lung has cranial, middle, caudal and accessory lobes; the left has cranial and caudal lobes. Exchange oxygen and carbon dioxide after birth. Before birth they are not inflated.
• Pericardium: The thin membrane sac around the heart. Holds the heart in place and lets it beat with little friction.
• Left ventricle: The lower left chamber, forming the apex of the heart. Pumps oxygenated blood into the aorta for the whole body.
• Thymus: A large, soft, pale gland in the neck and over the heart. It is much bigger in a fetus than in an adult. Trains T lymphocytes (immune cells) early in life.
• Liver: The largest organ in the abdomen: dark brown, deeply divided into lobes, and very large in the fetus. Makes bile, stores nutrients and processes blood from the gut. In the fetus it also makes blood cells.
• Stomach: A large, J-shaped, single-chambered (monogastric) stomach lying mostly on the left. Stores and churns food and begins protein digestion with acid and pepsin.
• Spleen: A long, flat, tongue-shaped red-brown organ along the stomach. Filters and stores blood and fights infection. It is not part of the digestive tract.
• Pancreas: Pale, bumpy, granular tissue lying under the stomach and in the loop of the duodenum. Makes digestive enzymes for the duodenum and the hormones insulin and glucagon.
• Jejunum and ileum: The long, coiled rest of the small intestine, held together by mesentery. Finishes digestion and absorbs nutrients into blood vessels of the mesentery.
• Spiral colon: The ascending colon, wound into a cone of tight coils: inward (centripetal) coils, then outward (centrifugal) coils. Absorbs water and salts and ferments fibre with its gut bacteria.
• Kidneys: Two bean-shaped organs on the roof of the abdomen, outside the peritoneum. Filter blood and make urine.

To find these organs in a real specimen, step by step, use our fetal pig dissection guide.

This is the text version of the 3D model: every labeled structure of the fetal pig, grouped by body system. Male-only and female-only structures are marked.

Yes, more than most farm animals. Pigs share with humans similar anatomy and physiology of the cardiovascular, urinary, skin and digestive systems, which is why they are one of the main animals used in surgical training, translational research and drug safety testing. (Swindle et al. (2012), Swine as models in biomedical research and toxicology testing, Veterinary Pathology.)

Both species are true omnivores with a single-chambered stomach, and the small intestine is built in much the same way. The pig's intestine is longer in absolute terms, but its length per kilogram of body weight is about the same as ours (roughly 0.1 m per kg). (Gonzalez, Moeser & Blikslager (2015), Porcine models of digestive disease, Translational Research.)

The liver looks different from the outside, with deeper lobes, but its internal segments, blood supply and bile ducts are arranged remarkably like the human liver, so pigs are used to practise liver resections and transplants. (Court et al. (2003), Segmental nature of the porcine liver, British Journal of Surgery.)

Pig kidneys, like human kidneys, are multi-lobed inside, with many renal papillae. Many other mammals, such as rats and mice, have a single papilla. (Mammalian kidney, Wikipedia.)

The similarity has limits. Pigs walk on four legs, their colon is coiled into a spiral, their caecum is large, they have a snout supported by an extra rostral bone, more thoracic and lumbar vertebrae than we have, cloven hooves, a bicornuate uterus carrying large litters, and a placenta in which six tissue layers separate mother and fetus. (Bazer et al. (2012), Uterine biology in pigs and sheep, Journal of Animal Science and Biotechnology.)

Pigs and humans have the same organ systems, but posture, diet and reproduction have shaped them differently. These are the differences students are most often asked about.

Sources: intestine lengths from Gonzalez et al. (2015); heart from Crick et al. (1998); vertebrae from IMAIOS; the tracheal bronchus from Radiopaedia; placenta from Bazer et al. (2012); teeth, teats and sweat glands from the Wikipedia pig article. All are listed in the references.

A pig heart has the same four chambers, four valves and great vessels as a human heart and is similar in size, but a detailed comparison found clear differences. The pig heart has a rounded 'Valentine' shape, while the human heart is more trapezoidal, a difference linked to four-legged versus upright posture. The pig's right auricle is tubular, its two venae cavae enter the right atrium at right angles, a large left azygos vein drains into the coronary sinus, and the left atrium usually receives only two pulmonary vein openings rather than the four usually seen in humans. The ventricles also have coarser muscular ridges (trabeculations) near the apex. (Crick et al. (1998), Anatomy of the pig heart: comparisons with normal human cardiac structure, Journal of Anatomy.)

Because the hearts are so close in size and design, pig tissue is used in medicine. Bioprosthetic heart valves are made mostly from cow pericardium, but some are made from pig tissue. (Artificial heart valve, Wikipedia.)

In 2022 surgeons at the University of Maryland transplanted a heart from a genetically modified pig with 10 gene edits into a 57-year-old man with end-stage heart failure who could not receive a human heart. The heart worked without obvious rejection for weeks; it failed suddenly on day 49, and life support was withdrawn on day 60. The cause was still being studied when the case was published. (Griffith et al. (2022), Genetically modified porcine-to-human cardiac xenotransplantation, NEJM.)

To find the parts of a real heart, a sheep or pig heart dissection follows the same steps; our 3D model shows the fetal pig heart with its fetal shunts.

For chambers, valves and coronary vessels in an adult heart, see the Sheep and pig heart dissection guide.

Air passes from the nostrils and larynx down the trachea, which divides into two main bronchi. Pigs have an extra branch: the bronchus to the right cranial lung lobe leaves the trachea itself, before it divides. In human medicine this rare variant is even called a "pig bronchus" (bronchus suis).

• Larynx: The voice box: a short tube of cartilage at the top of the trachea. Guards the airway and produces the pig's squeals and grunts.
• Trachea: The windpipe, stiffened by C-shaped rings of cartilage. Carries air between the larynx and the bronchi.
• Bronchi (with tracheal bronchus): The branches of the trachea into the lungs, including a separate bronchus to the right cranial lobe. Carry air into each lung lobe.
• Lungs: Spongy organs on both sides of the heart. The right lung has cranial, middle, caudal and accessory lobes; the left has cranial and caudal lobes. Exchange oxygen and carbon dioxide after birth. Before birth they are not inflated.
• Diaphragm: The dome-shaped sheet of muscle between the thorax and the abdomen. Contracts to enlarge the thorax and draw air in; it is the main breathing muscle.

An adult pig has about 15–22 m of small intestine and about 4–6 m of large intestine. An adult human has about 5.5–7 m and 1.5 m. The ratio of intestine length to body weight is similar in the two species. (Gonzalez, Moeser & Blikslager (2015), Porcine models of digestive disease, Translational Research.)

The pig's large intestine begins at a large, clearly marked caecum, and its ascending colon is coiled into a spiral. Like the human colon, it has pouches (sacculations) and bands of longitudinal muscle (taeniae) for most of its length. (Gonzalez, Moeser & Blikslager (2015), Porcine models of digestive disease, Translational Research.)

The spiral colon is shaped like a cone on the left side of the abdomen. Food runs inward along the outer centripetal coils to the tip of the cone, turns, and comes back out along centrifugal coils tucked inside them, before the colon continues as the transverse and descending colon to the rectum. In a near-term fetal pig the small intestine alone can be about 3.5 m long when freed from its mesentery. (Whitman College, Virtual Pig Dissection.)

The pig has a simple, single-chambered stomach, like a human and unlike a cow. It is large and J-shaped and lies mostly on the left behind the liver. Folds called rugae line its inside, the cardiac sphincter guards its entry and the pyloric sphincter its exit to the duodenum. (Biology Corner, Fetal pig dissection and lab guide.)

In many non-human mammals, part of the stomach next to the oesophagus is lined with oesophagus-like (non-glandular) epithelium rather than with gastric glands, and the regions of gastric glands do not match the human pattern. (Stomach: other animals, Wikipedia.)

In the pig this non-glandular area (the pars oesophagea) is a small patch around the entrance of the oesophagus. Pigs also have an unusually large cardiac gland region and a small blind pouch, the gastric diverticulum, at the top of the stomach.

School lab guides often say 'five lobes', and you will see that number in many worksheets. Veterinary and surgical descriptions are more precise: the pig liver has four main lobes (left lateral, left medial, right medial and right lateral), plus a small quadrate lobe around the gallbladder and a caudate lobe near the caudal vena cava. Depending on whether the small lobes are counted, a source may give four, five or six. (Nykonenko, Vávra & Zonča (2017), Anatomic peculiarities of pig and human liver, Experimental and Clinical Transplantation.)

The lobes are separated by deep fissures, and pig liver has unusually obvious lobules because of the connective tissue between them. Despite the different outer shape, it has the same number of functional segments as the human liver. (Nykonenko, Vávra & Zonča (2017), Anatomic peculiarities of pig and human liver, Experimental and Clinical Transplantation.)

Food passes from the mouth through the pharynx and oesophagus to the stomach, then the duodenum, jejunum and ileum, the caecum, the spiral colon and the rest of the colon, the rectum and the anus. The liver, gallbladder and pancreas add bile and enzymes in the duodenum.

• Oesophagus: The muscular tube that carries food from the pharynx to the stomach. Moves swallowed food to the stomach by waves of muscle contraction (peristalsis).
• Stomach: A large, J-shaped, single-chambered (monogastric) stomach lying mostly on the left. Stores and churns food and begins protein digestion with acid and pepsin.
• Pyloric sphincter: The ring of muscle at the exit of the stomach. Controls the passage of food from the stomach into the small intestine.
• Duodenum: The first, fairly straight part of the small intestine. Receives bile from the liver and enzymes from the pancreas.
• Jejunum and ileum: The long, coiled rest of the small intestine, held together by mesentery. Finishes digestion and absorbs nutrients into blood vessels of the mesentery.
• Caecum: A blind-ended pouch where the small intestine joins the large intestine. Holds microbes that ferment fibre, which pigs digest better than humans.
• Spiral colon: The ascending colon, wound into a cone of tight coils: inward (centripetal) coils, then outward (centrifugal) coils. Absorbs water and salts and ferments fibre with its gut bacteria.
• Rectum: The last straight part of the large intestine, running to the anus. Stores faeces before they leave through the anus.
• Gallbladder: A small, greenish sac tucked under the liver. Stores bile and releases it into the duodenum.

The usual vertebral formula of the pig is 7 cervical, 14–15 thoracic, 6 lumbar, 4 sacral and 20–23 caudal vertebrae (C7 T14–15 L6 S4 Cd20–23). Other references give 6–7 lumbar vertebrae. As in nearly all mammals there are seven neck vertebrae. (IMAIOS vet-Anatomy: vertebral column (vertebral formulae of domestic mammals).)

The count really does vary. Wild boar have 19 thoracic plus lumbar vertebrae, while European commercial breeds have 21–23, probably because breeders selected for longer bodies. A gene variant in NR6A1 accounts for part of this difference. (Mikawa et al. (2007), Fine mapping of a swine QTL for number of vertebrae, Genome Research.)

Each thoracic vertebra carries a pair of ribs, so pigs usually have 14 or 15 pairs. The snout is stiffened by a special prenasal (rostral) bone, which lets pigs dig with it. Adult pigs have 44 teeth (dental formula 3.1.4.3 in each half of each jaw), and the canines of boars grow continuously into tusks. (Pig, Wikipedia.)

Pigs have no clavicle: the forelimb is held to the trunk by muscles. The 3D model shows C7 T14 L6 S4 and about 20 caudal vertebrae.

Each pig foot has four hoofed toes. The two large central toes, digits III and IV, carry most of the weight and form the cloven hoof; the two outer toes, digits II and V, are the dewclaws, which sit higher and help on soft ground. (Pig, Wikipedia.)

Underneath, the toe bones follow the same pattern: four metacarpals or metatarsals, each with its phalanges. The joint people call a pig's 'knee' on the front leg is really the carpus (wrist), and the hock on the hind leg is the tarsus (ankle).

The sow has a bicornuate uterus with two long uterine horns, where the litter develops, and pregnancy only continues if embryos are present in both horns. Her cervix has a series of interlocking pads that grip the boar's corkscrew-shaped penis during mating. Sows typically have 12–14 teats, and gestation lasts about 114 days (roughly 112–120). (Pig, Wikipedia.)

The pig placenta is diffuse and epitheliochorial: the fetal membranes contact a large area of the uterine lining, but six tissue layers separate maternal and fetal blood. Term is around day 114 of pregnancy. (Bazer et al. (2012), Uterine biology in pigs and sheep, Journal of Animal Science and Biotechnology.)

In the boar, the testes lie in a scrotum high in the perineum, just below the anus. The sperm ducts (ductus deferentia) run to the urethra, where the vesicular glands, prostate and very large bulbourethral glands add fluid. The penis runs forward under the belly skin to the preputial opening just behind the navel.

Use the Female/Male switch on the 3D model to compare the two reproductive tracts.

A fetal pig has the same organ systems as an adult, so it is a good guide to pig anatomy, but several features exist only before birth or change size after it.

• Umbilical cord with two umbilical arteries, one umbilical vein and the allantoic duct, and an elongated allantoic bladder.
• Fetal shunts: the foramen ovale, ductus arteriosus and ductus venosus, which close soon after birth.
• Lungs are fluid-filled and solid; the thymus and liver are relatively huge.
• The skeleton is partly cartilage, and the testes may still be on their way through the inguinal canal.
• The head is large and the legs short compared with an adult; the gut contains meconium instead of food.

Butchers first divide a pig into four primal cuts: the shoulder, the loin, the belly and the leg. Pork butt (Boston butt) comes from the upper shoulder, and only the hind leg is properly called a ham. Spare ribs are cut from the ribs with the meat between them. (Cuts of pork, Wikipedia.)

Pork tenderloin is the psoas major muscle, which lies under the lumbar vertebrae. Because it does little heavy work, it is the most tender cut. (Pork tenderloin, Wikipedia.)

The loin chops and loin roast are mainly the longissimus muscle along the back. The ham is the large muscle mass of the thigh (biceps femoris, semimembranosus, semitendinosus and quadriceps). The shoulder includes the triceps and other muscles around the shoulder blade, and bacon comes from the belly: the layered abdominal wall with its fat.

• Masseter: The thick chewing muscle on the cheek. Closes the jaw.
• Triceps brachii: The large muscle mass behind the upper forelimb. Extends (straightens) the elbow so the leg can carry weight.
• Longissimus dorsi: The long back muscle beside the spine: the pork loin. Extends and stiffens the back.
• Psoas major: A long, soft muscle under the lumbar vertebrae: the pork tenderloin. Flexes the hip.
• Ham muscles (biceps femoris): The large muscles of the rump and thigh, led by the biceps femoris. Extend the hip and drive the pig forward.
• Abdominal wall muscles: Thin layered muscles of the belly: external and internal obliques, transversus and rectus abdominis. Hold in and compress the abdominal organs.

No. The guinea pig, Cavia porcellus, is a rodent (order Rodentia, family Caviidae) from South America. Despite its name it is not native to Guinea and is not closely related to pigs. Its anatomy is a rodent's: ever-growing incisors, clawed feet instead of hooves, and a large caecum for digesting plants. (Guinea pig, Wikipedia.)

Use the model or diagrams to answer these. Answers are in the FAQ and on the dissection page.

1. How many toes does a pig have on each foot, and which ones carry the weight?
2. Which bone supports the pig's snout disk?
3. Where in the abdomen is the spiral colon, and what is its job?
4. About how long is the large intestine of an adult pig?
5. Name the four main lobes of the pig liver.
6. What is unusual about the bronchus to the right cranial lung lobe?
7. How many thoracic vertebrae does a pig usually have?
8. Name two differences between the pig heart and the human heart.

Full lab questions with an answer key are in the fetal pig dissection guide.