FLG 222: Gastrointestinal tract and
nutrition
Basic Anatomical Structure (Histology)
- Energy – required for physiological activities
- Mammals need to consume nutrients from the
external environment to meet the energy
requirements
- Gastrointestinal tract - organ system responsible
for the processing and absorption of consumed
nutrients
- The GIT system is specifically engineered to meet
its functional needs.
Organization of The Digestive System
Organs digestive system 2 main groups: the
gastrointestinal tract (GI tract) and accessory structures.
1. GI tract
- continuous tube extending through the ventral
cavity from the mouth to the anus
- it consists of the mouth, oral cavity, oropharynx,
esophagus, stomach, small intestine, large intestine , rectum , and
anus
2. Accessory structures
- teeth, tongue (in oral cavity) , salivary glands , liver , gallbladder,
and pancreas.
Anatomy
- The digestive system begins with the oral cavity (mouth and pharynx) ->
receptacle for food.
- Swallowed food enters the gastrointestinal tract (GIT) -> esophagus, stomach,
small intestine, and large intestine. The portion of the GIT from the stomach to the
anus -> gut.
- Chemical and mechanical breakdown of food takes place in the lumen of the gut.
- Secretions are added to ingested food by secretory epithelial cells and by
accessory organs (salivary glands, liver, gall bladder and pancreas).
- The mixture of food with secretions -> chyme
- The GIT is a long tube with muscular walls (food needs to be moved constantly)
lined by secretory (need saliva, digestive enzymes, electrolytes, tec.) and
transporting (digested food needs to be absorbed) epithelium.
- Sphincters in the GIT separate the tube into segments with distinct functions.
- Food moves through the tract propelled by waves of muscle contraction.
- Digested nutrients -> absorbed across the intestinal epithelium and pass into the interstitial fluid.
- From there, absorbed nutrients travel into blood or lymph -> then distributed throughout the body.
- Any undigested and unabsorbed food (waste) remaining in the lumen at the end of the GIT is expelled through the anus by
defecation.
- The GIT microbiome: commensal bacteria in the GIT (opens to the external environment on both ends).
,Histology
- The basic structure of the GI wall is
similar in the stomach and
intestines - variations exist from one
section to another.
- The gut wall has folds to increase its
surface area. Rugae – folds in the
stomach. Plicae – folds in the small
intestine.
- The intestinal mucosa also has tiny
finger-like projections called villi.
- Tubular invaginations of the
surface that extend down into the
supporting connective tissue –
increases surface area.
- Gastric glands – invaginations in
the stomach. Crypts – invaginations
in the intestine. Deepest invaginations form secretory submucosal glands that open into the lumen
through ducts.
- The gut wall consists of four layers: (1) an inner mucosa facing the lumen, (2) a layer known as
the submucosa, (3) layers of smooth muscle known collectively as the muscularis externa, and (4)
a covering of connective tissue called the serosa.
- The mucosa, the inner lining of the gastrointestinal tract, has three layers: a single layer of
mucosal epithelium facing the lumen; the lamina propria, subepithelial connective tissue that holds
the epithelium in place; and the muscularis mucosae, a thin layer of smooth muscle.
- Several structural modifications increase the amount of mucosal surface area to enhance
absorption.
1. The mucosal epithelium
- is the most variable feature of the GI tract, changing from section to section.
- The cells of the mucosa include transporting epithelial cells (called enterocytes in the small intestine),
endocrine and exocrine secretory cells, and stem cells. At the mucosal (apical) surface of the
epithelium, cells secrete ions, enzymes, mucus, and paracrine molecules into the lumen. On the
serosal surface of the epithelium, substances being absorbed from the lumen and molecules secreted
by epithelial cells enter the extracellular fluid.
- The cell-to-cell junctions that tie GI epithelial cells together vary. In the stomach and colon, the
junctions form a tight barrier so that little can pass between the cells. In the small intestine, junctions
are not as tight. This intestinal epithelium is considered “leaky” because some water and solutes can
be absorbed between the cells (the paracellular pathway) instead of through them. GI stem cells are
rapidly dividing, undifferentiated cells that continuously produce new epithelium in the crypts and
gastric glands.
2. The lamina propria
- subepithelial connective tissue. It contains nerve fibers and small blood and lymph vessels.
- Absorbed nutrients pass into the blood and lymph here.
- This layer also contains wandering immune cells, such as macrophages and lymphocytes, patrolling
for invaders that enter through breaks in the epithelium.
- In the intestine, collections of lymphoid tissue adjoining the epithelium form small nodules and larger
Peyer’s patches that create visible bumps in the mucosa.
- These lymphoid aggregations are a major part of the mucosa/gut-associated lymphoid tissue (MALT /
GALT).
, 3. The muscularis mucosae
- a thin layer of smooth muscle, separates the lamina propria from the submucosa. Contraction of muscles in this layer alters
the effective surface area for absorption.
Submucosa
- The submucosa is the middle layer of the gut wall. It is composed of connective tissue with larger blood and lymph vessels
running through it. The submucosa also contains the submucosal plexus, one of the two major nerve networks of the enteric
nervous system.
- The submucosal plexus (also called Meissner’s plexus) innervates cells in the epithelial layer as well as smooth muscle of
the muscularis mucosae.
Muscularis Externa
- The outer wall of the gastrointestinal tract, the muscularis externa, consists primarily of two layers of smooth muscle: an
inner circular layer and an outer longitudinal layer.
- Contraction of the circular layer decreases the diameter of the lumen.
- Contraction of the longitudinal layer shortens the tube. The stomach has an incomplete third layer of oblique muscle
between the circular muscles and the submucosa.
- The second nerve network of the enteric nervous system, the myenteric plexus, lies between the longitudinal and circular
muscle layers.
- The myenteric plexus controls and coordinates the motor activity of the muscularis externa.
4. Serosa
- The outer covering of the entire digestive tract, the serosa, is a connective tissue membrane.
- It is a continuation of the peritoneal membrane (peritoneum) lining the abdominal cavity.
- The peritoneum also forms sheets of mesentery -> keeps the intestines in place (prevent tangling as
they move).
Digestion & Absorption
, SECRETION
- In a typical day, 9 liters of fluid pass through the lumen of an adult’s gastrointestinal tract.
- Only about 2 liters of that volume enter the GI system through the mouth (consumption).
- The remaining 7 liters of fluid come from body water via osmosis along with ions and secretions with
enzymes, and mucus.
- The ions are transported from the ECF into the lumen.
- Water then follows the osmotic gradient created by this transfer of solutes from one side of the
epithelium to the other.
DIGESTIVE ENZYMES
- Digestive enzymes are secreted either by exocrine glands (salivary glands and the pancreas) or by epithelial cells in the
stomach and small intestine.
- Enzymes are proteins, which means that they are synthesized on the rough endoplasmic reticulum, packaged by the Golgi
complex into secretory vesicles, and then stored in the cell until needed.
- On demand, they are released by exocytosis.
- Many intestinal enzymes remain bound to the apical membranes of intestinal cells, anchored by transmembrane protein
“stalks” or lipid anchors.
- Some digestive enzymes are secreted in an inactive proenzyme form known collectively as zymogens.
- Zymogens must be activated in the GI lumen before they can carry out digestion – prevent degradation from proteins in
plasma/ducts
- Synthesizing the enzymes in a non-functional form allows them to be available in the cells that make them without
damaging those cells. Zymogen names often have the suffix –ogen added to the enzyme name, such as pepsinogen.
- Mucus is a viscous secretion composed primarily of glycoproteins collectively called mucins.
- The primary functions of mucus are to form a protective coating over the GI mucosa and to lubricate the contents of the gut.
- Mucus is made in specialized exocrine cell called mucous cells in the stomach and salivary glands, and goblet cells in the
intestine.
- Goblet cells make up between 10% and 24% of the intestinal cell population. The signals for mucus release include
parasympathetic innervation, a variety of neuropeptides found in the enteric nervous system, and cytokines from
immunocytes. Parasitic infections and inflammatory processes in the gut also cause substantial increases in mucus
secretion as the body attempts to fortify its protective barrier.
