Download Lab Exer 1 Anatomy of the Digestive System

Laboratory Exercise 1: Anatomy of the Digestive System
The digestive system physically and chemically converts complex food molecules into smaller, simpler
molecules. These molecules then are absorbed, transported by the circulatory system and used by the
cells of the body. The primary organ is the alimentary canal, which is a continuous tube extending
from mouth to anus. It is specialized in various regions to carry out specific functions. The accessory
organs – salivary glands, pancreas, liver and gall bladder – attached to the alimentary canal by way of
ducts contribute secretions that digest the food molecules and allow the simpler molecules to be used
by the cells.
A. Gross Examination of the Oral Cavity
The oral cavity (mouth) acquires food, physically (mechanically) prepares it and begins the chemical
digestion of carbohydrates through enzymes from the salivary gland. The chewed food mass that
leaves the mouth is the bolus.
B. Gross Examination of the Alimentary Canal and Accessory Organs of Digestion
Salivary glands – produce saliva consists of water, salts and hydrolytic enzymes.
Parotid glands – located in cheek just anterior to the external auditory meatus. The duct opens into the
vestibule of the oral cavity at the level of the second upper molar.
Sublingual gland – located under the tongue. Its many small ducts open under the tongue.
Submandibular gland – located below the mandible behind the sublingual gland. The submandibular
gland has two ducts. They open on either side of the tongue’s frenulum.
Pharynx – a common passage for food and air. The pharynx is lined by a mucous membrane.
Swallowing is aided by constrictor muscles in the wall of the pharynx. The reflex contractions called
peristalsis, which propel food forward.
Esophagus – a tube between the pharynx and stomach. External muscle layer – upper 1/3 skeletal,
middle 1/3 mixed skeletal and smooth and lower 1/3 smooth. At distal end smooth muscle organized
into the gastroesophageal (cardiac) sphincter. Sphincters are circular arranged smooth muscle which
closes off a body opening.
Stomach – a J-shaped dilated segment of the alimentary canal. The areas of the stomach: cardiac
region, fundus, pyloric region and pyloric sphincter. Chemical digestion of protein begins in the
stomach. The stomach stores food and by peristalsis and other smooth muscle contractions mixes the
food bolus with the gastric juice to reduce it to a semisolid mass, the chime. There is no digestion per
se in the pharynx and esophagus.
Small Intestine – duodenum, jejunum and ileum. There is complete digestion of all food and
absorption of the simple food molecules.
1
Large Intestine – ileocecal valve, cecum, vermiform appendix, ascending colon, transverse colon,
descending colon, sigmoid colon, rectum and anus. The large intestine absorbs water and salts,
prepares undigested material for removal from the body (egestion).
Liver – right lobe has 2 sublobes – quadrate lobe, inferior and caudate lobe, posterior.
Left lobe
Ducts of the Liver – bile is drained from the right and left lobes by a pair of ducts which unite to form
the common hepatic duct. The bile is stored in the gall bladder; the bile leaves the gall bladder by way
of the cystic duct, which merges with the common hepatic duct to form the common bile duct. The
common bile duct along with the pancreatic duct pierces the duodenum.
Pancreas – the head fits within the C-shaped curvature of the duodenum. The body of the pancreas
extends horizontally to the left and tapers to a tail. The pancreas is retroperitoneal dorsal to the
stomach. The pancreatic digestive fluids are conveyed by the pancreatic duct. This duct joins the bile
duct to form the ampulla. The ampulla opens into the duodenum. The opening is marked by an
elevation of the duodenal mucosa, duodenal papilla. The location of the opening permits bile and
pancreatic fluid to be delivered anterior to where the digestion takes place in the small intestine.
Membranes of the Abdominopelvic Cavity – the peritoneum line the abdominopelvic cavity as the
parietal peritoneum and cover the viscera as visceral peritoneum. The mesentery, a continuation of the
peritoneum, suspends the small and large intestines from the dorsal body wall.
Greater omentum – a double layer of peritoneum attaches the greater curvature of the stomach to the
dorsal body wall.
Lesser omentum – attaches the duodenum and lesser curvature of the stomach to the liver.
C. Microanatomy of the Alimentary Canal
The alimentary canal has the same basic arrangement of tissues or histology from the esophagus
through the large intestine.
There are four basic layers of the digestive tract:
From within to out:
Mucosa – epithelial and areolar connective tissue (lamina propria with lymphatic modules and glands)
muscular mucosa
Submucosa – areolar connective tissue, nerve tissue (submucosal plexus) and blood vessels
glands are located in the submucosa connected to the lumen by way of ducts
Muscularis externa – inner circular, outer longitudinal smooth muscle, nerve tissue (myenteric plexus))
and blood vessels
Serosa or adventitia – connective tissue, epithelial tissue (serosa only)
Esophagus – Mucosa – stratified squamous epithelial tissue
Submucosa – submucosal mucus secreting glands connected to the lumen by ducts.
The mucus coats the food bolus to ensure smooth swallowing.
Muscularis externa – thick muscle layer which changes from striated at the proximal end
to smooth at the distal end.
2
Adventitia – fibrous connective tissue attaches esophagus to surrounding tissues. There
is no serosa.
Stomach - The internal lining of the stomach is arranged in grossly visible longitudinal folds, the
rugae.
Mucosa – is indented by gastric pits lined with mucus secreting simple columnar
epithelium. From the bottom of the pits gastric glands project. The secretory epithelial
cells of the glands are parietal cells found towards the top of the gland and chief cells
found towards the bottom of gland. The parietal cells produce HC1, the chief cells produce
pepsinogen.
Muscularis externa has 3 well-developed layers of smooth muscle.
Small intestine – the internal lining of the small intestine is arranged in grossly visible horizontal folds,
the plicae circulares.
Projecting from the folds to further increase mucosal surface area are the intestinal villi.
The villi project into the lumen of the small intestine. The villi are covered with simple
columnar epithelium with scattered mucus-producing goblet cells. The core of each villus
is connective tissue containing a central lacteal (a lymphatic capillary). Between the villi
and projecting down into the mucosa are tubular intestinal glands (crypts of Lieberkuhn).
To further increase absorptive surface area, the cell membrane of each columnar cell on its
free surface is highly folded, as the microvilli.
The microvilli are seen in the light microscope as the brush or striated border.
Large intestine – the wall of the large intestine the four basic tissue layers. The large intestine lacks
digestive glands, plicae circulares and villi. The intestinal glands contain simple
columnar epithelium to absorb water and electrolytes and goblet cells to supply
mucus.
Muscularis externa – inner circular smooth muscle.
Outer longitudinal smooth muscle is organized into 3 longitudinal bands, the
teniae coli. The bands are shorter than the colon create small pouches or
haustra. The mucosa between the haustra is folded, increasing the absorptive
surface area.
3