Overview of the Digestive Tract

Question 1

Parts that relate to Prehension and Mastication

Answer 1

Lips
•Teeth
•Tongue
• Salivary glands 
• Musculature

Question 2

tongue

Answer 2

The tongue is an elongated muscular organ with the top surface covered with specialized little mushroom-shaped structures called papillae. These papillae contain tiny holes or pores that lead to taste buds. The bulk of the tongue consists of muscle bundles mixed with connective (strong/tough) and adipose (fat) tissue. It has many blood vessels and bleeds profusely when lacerated. The tongue is surrounded by the openings of the ducts of the salivary glands, which pour their secretions (saliva) into the oral cavity.

The tongue is used mainly for guiding food and water into the mouth and throat. The tongue assists in the chewing and swallowing of food. It serves as a ladle for lapping water and other liquids into the mouth during drinking. The taste buds of the tongue are important in the detection and sense of taste. The tongue also helps reduce body temperature in the dog. Air passing back and forth over a panting tongue is cooled, and this cooling is enhanced as saliva evaporates. Dogs also use the tongue as a tool to clean reachable areas on the body, and wounds. The dog uses it tongue to groom and to stimulate urination and defecation in puppies, especially by licking the abdomen and genital areas.

Question 3

Teeth

Answer 3

Each tooth consists of four types of tissue: pulp, dentin, enamel and cementum. Connective tissue surrounds the root of the tooth. This tissue, called the periodontal ligament, holds the root in the bony socket in the jaw.

The teeth are used to tear apart and process the food. The food is broken down into smaller pieces by the teeth. Each type of tooth serves a different function in the chewing process. There are four types of teeth. Incisors are the primary biting teeth. The canine teeth bite and tear food. The premolars shear, grind and mash food. Molars are responsible for the most rigorous chewing.

Question 4

Salivary glands

Answer 4

The mouth itself has several important functions. It manufactures and secretes saliva. Saliva lubricates the food, helps hold food together as a bolus that can be swallowed, and contains substances that begin the digestion of the food. Saliva also cleanses the tongue.

Question 5

Mastication

Answer 5

is the process whereby food is broken down by mechanical digestion in the oral cavity. The cheeks and tongue function to position food over the teeth, where grinding can occur. Mastication requires correct muscle movements and jaw articulation.

Question 6

Rumination

Answer 6

allows food to undergo mastication more than once. This is also called ‘chewing the cud’, it allows greater nutrients to be extracted and absorbed from the food particles.

Question 7

Jaw Opening Muscles

Answer 7

The Digastricus muscle is the ‘jaw opening’ muscle. Its origin is the paracondylar process of the occipital bone. It inserts at the angle of the mandible. The muscle has two bellies; The caudal half from the second visceral arch innervated by the facial nerve (CN VII) and the cranial half from the first visceral arch, innervated by the mandibular branch of the trigeminal nerve (CN V3).

Question 8

Jaw Closing Muscles

Answer 8

masseter muscle
lateral pterygoid muscle
medial pterygoid muscle
temporal muscle

All jaw closing muscles are derived from the first visceral arch and are innervated by the mandibular branch of the trigeminal nerve

Question 9

masseter muscle

Answer 9

originates from the maxillary region of the skull and the zygomatic arch. It inserts on the wide area on the caudal side of the mandible. It has several divisions and causes unilateral and bilateral contraction. It also protrudes the jaw.

Question 10

lateral pterygoid muscle

Answer 10

originates from the pterygopalatine region of the skull. It inserts on the lateral aspect of the mandible. It also protrudes the jaw (one-sided contraction).

Question 11

medial pterygoid muscle

Answer 11

originates from the pterygopalatine region of the skull. It inserts on the medial aspect of the mandible. It causes one-sided contraction to close the jaw.

Question 12

temporal muscle

Answer 12

originates from the lateral surface of the cranium. It inserts on the coronoid process. It pulls the mandible dorsally and also pulls the mandible rostrally (overbite) and caudally (underbite).

Question 13

Lateral Translation of the Mandible

Answer 13

The masseter muscle and the contralateral medial and lateral pterygoids are involved in the lateral translation of the mandible.

Question 14

Simple monogastric stomach

Answer 14

The larger part of the stomach lies to the left of the midline, under cover from the ribcage and in contact with the liver and diaphragm. The oesophagus opens into it at the cardiac sphincter. The smaller part of the stomach has thicker walls and passes to the right of the midline into the duodenum at the pyloric sphincter. The angular point between the two parts of the stomach is called the angular notch (incisura).

Contractions start near the cardia and spread distally, accelerating and becoming more vigorous as they reach the pyloric region. The pyloric sphincter is open for 1/3 of the time during contractions. The empty stomach lies completely within the rib cage and does not contact the abdominal floor. Little secretion is produced and only small peristaltic contractions occur. Once food is offered or anticipated, the secretions begin.

Question 15

Gastrophrenic ligament

Answer 15

from the greater curvature of the stomach to the crura of the diaphragm

Question 16

Lesser omentum

Answer 16

connecting the lesser curvature of the stomach and the initial segment of duodenum to the liver in the region of the hepatic porta

Question 17

Gastrosplenic ligament

Answer 17

connecting the greater curvature of the stomach to the spleen by a double fold of peritoneum

Question 18

Greater omentum

Answer 18

connecting the greater curvature of the stomach to the duodenum and dorsal body wall

Question 19

coeliac artery

Answer 19

a branch of the dorsal aorta). The coeliac artery splits into the hepatic artery supplying the liver, pancreas and stomach (right gastric and right gastro-epiploic arteries). The coeliac artery also splits into the splenic artery which supplies the spleen and the stomach (left gastro-epiploic artery), it also splits into the left gastric artery supplying the stomach.

Question 20

gastro-epiploic arteries

Answer 20

supply the greater curvature of the stomach

Question 21

gastric arteries

Answer 21

s supply the lesser curvature of the stomach

Question 22

Histology of the stomach

Answer 22

The monogastric stomach has a columnar epithelium. The folded mucosa of the stomach forms longitudinal rugae. The folds form invaginations called gastric pits which are continuous with gastric glands.

The pyloric sphincter is a thickened tunica muscularis from the middle circular smooth muscle layer. In the fundic region, the tunica muscularis is thinner, the glands are straight and the gastric pits are shallow. There is also an abundance of parietal and chief cells in the gland. In the pyloric region, the tunica muscularis is thicker, the glands are coiled and the gastric pits are deep. The cardia is a narrow muscle strip. Lymphatic vessels are present in the submucosa.

Question 23

Layers of the stomach

Answer 23

The 4 layers of the stomach wall are:
Serosa/adventitia
Tunica muscularis
Submucosa
Mucosa

Question 24

The 3 layers of muscularis

Answer 24

the outer longitudinal, middle circular and inner oblique

Question 25

Small Intestine

Answer 25

Duodenum
Jejunum and Ileum
The mucosa is arranged into villi that provide a large surface area for absorption.
Epithelium is simple columnar - ideal for absorption.
Membrane bound enzymes and transport proteins are also within the epithelium.
Each villus houses a generous blood supply of capillaries that transport amino acids, monosaccharides and other digestive products and lacteals that transport triacylglycerides. Lacteals drain into the lymphatic system.
Two types of secretion are produced in the small intestine; together they are called the succus entericus. The first type of secretion is from the crypts, the second is from the Brunner’s glands.
Crypts are present at the base of each villus in the mucosa

Question 26

Cell types in mucosal crypts (from luminal to basal):

Answer 26

goblet at the tip of the crypt. Produce mucous by exocytosis.
entero-endocrine in the middle of the crypt. Produce many important substances for the regulation of GIT motility. See Regulation Control.
paneth at the base of the crypt. Function unknown. Contain eosinophilic granules.

Question 27

Brunner’s glands

Answer 27

present in the submucosa of the duodenum.
Secrete a protective mucous.
Produce an alkaline secretion which neutralises stomach acid.
Open into the crypts in the mucosa above.
Contraction of smooth muscle shortens the villus. This helps to pump out absorbed products of digestion. Relaxation of smooth muscle lengthens the villus which increases surface area, facilitating absorption.

Question 28

Liver

Answer 28

The liver is located in the cranial part of the abdomen. It is immediately caudal to the diaphragm and cranial to the stomach and intestines.

It is divided into lobes by fissures. Cranially the liver is convex, called the diaphragmatic surface. Caudally the liver is concave, called the visceral surface. The caudate lobe has a renal impression from the right kidney. The gastric impression occupies the whole of the left half of the visceral face. The duodenal impression at the junction of the right and quadrate lobes continues onto the right lateral and caudate lobes. Passages or notches on the median plane allow the caudal vena cava and oesophagus to pass by. The gall bladder is located between the right medial and quadrate lobes.

Question 29

Lobes of the Liver

Answer 29

The lobes of the liver include the left lateral, left medial, right lateral, right medial, quadrate, caudate and papillary.

Question 30

Function of the liver

Answer 30

Nearly all the blood circulated around the abdomen flows back through the portal vein to the liver where it comes in contact with the liver cells, ensuring the products of digestion are presented to the hepatic cells before entering the general circulation. Other functions include carbohydrate metabolism, glycogenesis, glyconeolysis, gluconeogenesis and the breakdown of insulin and other hormones. Protein metabolism produces soluble mediators of the clotting cascade, Albumin and hormone transporting globulins. The liver is also involved in lipid metabolism, lipogenesis and the synthesis of cholesterol.

The liver has a role in hormonal control of the following; Insulin and glucagon, Glucocortocoids, Catecholamines and the synthesis of other important hormones (see Endocrine System). It also has a role in immunoregulation via kupfer cells and the complement synthesis and metabolism.
The liver is important in storage of water soluble vitamins, fat soluble vitamins, iron, triglyceride and glycogen.
The liver breaks down haemoglobin and toxic substances through drug metabolism. It converts ammonia to urea and allows the management of endogenous waste, e.g haem (Hb, cytochromes, Mb) and ammonia (amino acids).

Question 31

Liver Histology

Answer 31

The larger liver cells are called lobules. Each lobule contains an opening for the central vein and contains portal areas. The lobules are composed of liver cords called hepatocytes. Sinusoids are present between hepatocytes containing red blood cells. There is a connective tissue capsule around each liver lobule. A thin mesothelium covers the connective tissue layer.
The portal area present in the lobules contains the hepatic artery, which has thick walls and a small diameter and the hepatic vein, which has thin walls and a large and irregular shape. It also contains bile ducts, with cuboidal or columnar epithelium and lymphatics that are small and delicate.
Hepatocytes are the smaller liver cells in the lobules. They contain glycogen granules and have a spherical nucleus. They form cords called branching plates (lamellae). The upper and lower margins are tight junctions. They have 3 functioning surfaces. Kupfer macrophages are present near the lining of the sinusoids. The hepatocytes stain pink as they are eosinophilic.

Question 32

Pancreas

Answer 32

The pancreas is a tubuloalveolar gland and has exocrine and endocrine tissues. The exocrine is the larger of the two parts and secretes pancreatic juice; a solution containing enzymes for carbohydrate, protein and triacylglycerol digestion. Pancreatic juice drains into the small intestine where it is functional. The endocrine part secretes hormones for the regulation of blood glucose concentration, including insulin, glucagon and somatostatin. The functional units of the endocrine part are the islets of Langerhans.

Question 33

Pancreas structure

Answer 33

The pancreas is located in the craniodorsal part of the abdomen in close association with the duodenum. It can be divided into three parts; a body and left and right lobes. The lobes are loosely united by interlobular connective tissue. Connective tissue contains blood vessels, nerves and lymphatics.
The pancreas is roughly “V” shaped in all species.
there are two ducts present in the pancreas. Their presence reflects the convergent development pattern of the pancreas, however in some species one or other of the ducts may atrophy. The pancreatic duct is the biggest of the two and opens into the duodenum with the bile duct at the major duodenal papilla. The accessory duct opens on the opposite aspect of the duodenum at the minor duodenal papilla.

Question 34

Pancreas histology

Answer 34

The exocrine pancreas consists of acini, which resemble bunches of grapes. Each acinus consists of a single layer of 40 - 50 pyramidal epithelial cells surrounding a lumen. The epithelial cells produce the secretion (pancreatic juice) containing enzymes, ions and water. The apex of the cells is abundant with secretory granules containing the zymogen precursors of the pancreatic enzymes. The number of secretory granules increases after fasting, and decreases after a meal.

In the endocrine pancreas, the islets of Langerhans are embedded in the exocrine tissue. Each islet is composed of 2 - 3 thousand epithelial cells. The epithelial cells are arranged in a compact structure that is pervaded by a capillary network. A thin layer of reticular fibres separates the islets from the surrounding exocrine tissue.

Question 35

Caecum & Colon (Large Intestine)

Answer 35

Caecum > Ascending Colon > Transverse Colon > Descending Colon > Rectum
• Absorb water and ions
• compaction of undigested material and microflora
• Expel faeces via rectum and anus
The large intestine extends from the ileum of the small intestine to the anus. Water, electrolytes and nutrients are absorbed which concentrates the ingesta into faeces. Faeces are stored prior to defeacation. There is no secretion of enzymes and any digestion that takes place is carried out by microbes. All species have a large microbial population living in the large intestine, which is of particular importance to the hindgut fermenters. For this reason, hindgut fermenters have a more complex large intestine with highly specialised regions for fermentation.

Question 36

Histology of Caecum & Colon (Large Intestine)

Answer 36

The mucosa of the large intestine is smooth; there are no villi or microvilli. The mucosal glands are much longer and straighter. The number of goblet cells in the mucosa is increased compared to the small intestine, as mucus is very important for lubrication of the ingesta as it passes through the intestine, particularly as more water is absorbed from the lumen making chyme drier.
There are numerous scattered lymph nodules in the large intestine. The number of lymph nodules increases compared to the small intestine. The submucosa is also much reduced in thickness.
Taenia may be present. These are concentrations of the longitudinal muscle layer into long bands. When the taenia contract, they cause shortening of the large intestine, which produces saccualtions, or haustra. Haustra help to mix the content and to slow the transit time. Many glands are present in the mucosa and skin of the anal region.

Question 37

Mesenteric Artery

Answer 37

The superior mesenteric artery (SMA) is a major artery of the abdomen. It arises from the abdominal aorta, and supplies arterial blood to the organs of the midgut – which spans from the major duodenal papilla (of the duodenum) to the proximal 2/3 of the transverse colon
The superior mesenteric artery then gives rise to various branches that supply the small intestines, cecum, ascending and part of the transverse colon:
Inferior Pancreaticoduodenal Artery- supplies the inferior region of the head of the pancreas, the uncinate process, and the duodenum.
Jejunal and Ileal Arteries
Middle colic artery – supplies the transverse colon.
Right colic artery – supplies the ascending colon.
Ileocolic Artery- branches to the ascending colon, appendix, cecum, and ileum.

Question 38

Portal Vein

Answer 38

The portal vein or hepatic portal vein (HPV) is a blood vessel that carries blood from the gastrointestinal tract, gallbladder, pancreas and spleen to the liver. This blood contains nutrients and toxins extracted from digested contents.

Question 39

Nerve Supply

Answer 39

Somatic innervation to body wall
• Autonomic Innervation to viscera
• Parasympathetic – increases digestive processes
• Sympathetic – Decreases digestive processes