The GI tracts can be divided into compartments that are separated from neighbours by sphincters.
Starting at the entrance these are the oral cavity (mouth and pharynx), the esophagus, the stomach, the small
intestine, and the large intestine.
Once ingested, food moves in sequence through these compartments. Specific details of digestion,
absorption, secretion and motility vary with compartment. What remains of the meal at the end of the large
intestine is excreted from the tract. In this unit you will follow a meal starting when it is ingested in the mouth.
You will discover what happens in each compartment and the control mechanisms involved.
Most of the control mechanisms that we have encountered so far regulate variables in the "internal
environment". The control mechanisms of the GI system are different; they regulate the conditions in the lumen
of the gastrointestinal tract (external environment), that is, the volume and composition of the luminal contents.
Both neural and hormonal control mechanisms are involved in this regulation. Regulation phase is identified by
the site of the initiating stimulus cephalic, gastric or intestinal.
One major goal of this unit is to know the specific details concerning the 4 processes digestion,
absorption, motility and secretion and each compartment of the GI tract. A second goal is to know the
cephalic, gastric and intestinal phases of control.
3 PHASES OF CONTROL:
The neural and hormonal regulation of the GI system is divided into three phases: cephalic, gastric and
intestinal. These phases are named for the site at which various stimuli initiate the reflex and NOT for the sites
of effector activity. Each of these phases can affect virtually all organs in the gastrointestinal tract. In addition,
all of these phases may be occurring simultaneously.
Cephalic Phase Gastric Phase Intestinal Phase
Digestive reflexes triggered by When food enters stomach, stimuli Triggers a series of reflexes that
stimuli received in the brain, such in the gastric lumen initiate a seriesfeed back to regulate the delivery
as the smell or sight of food of short reflexes that constitute rate of chyme from the stomach and
digestion feed forward to promote digestion,
motility and utilization of nutrients
THE CEPHALIC PHASE:
• In the cephalic phase of digestion, the sight, smell, or taste of food initiates GI reflexes
• Mechanical digestion begins with chewing (mastication)
o Saliva moistens & lubricates food
o Salivary amylase digests carbohydrates
• Swallowing, or deglutition, is a reflex integrated by a medullary center
The mouth, pharynx and esophagus serve as the starting point for this section.
The secretion of saliva by the salivary glands is controlled by both the parasympathetic and sympathetic
nervous systems. The parasympathetic nervous system produces the greatest secretion of saliva, although both
systems enhance salivary secretion.
1 Module XIII
Swallowing (deglutition) is a complex reflex. Figure 2124, page 710, illustrates the movement of food
through the pharynx and upper esophagus during swallowing. You should know how the swallowing reflex is
initiated and controlled. Note the function of the upper and lower esophageal sphincters and the peristaltic
waves. You should understand how the anatomical position of the lower esophageal sphincter can change
during pregnancy, and why this change in position can give rise to heartburn. You should also understand why
heartburn can occur after a large meal.
Chemical & Mechanical Digestion Begins in the Mouth:
• Salivary secretion is under autonomic control
o Stimuli include sight, smell, touch, thought of food
• Salivary amylase begins chemical digestion
• 3 functions of saliva
o Soften & lubricate food, making it easier to swallow
o Dissolves food so that we can taste it
• Begins in the oral cavity with chewing
• Lips, tongue and teeth all contribute to the mastication of food
o Creates a softened, hardened mass (bolus) that can be swallowed
Function of the Esophagus:
• Connects the mouth and stomach
• Food is passed through the esophagus by the process of peristalsis (rhythmic, wavelike muscle
o Forces food from throat into the stomach
The Swallowing Reflex:
• Deglutition is a reflex action that pushes a bolus of food or liquid into the esophagus
• Steps of the reflex
1. Tongue pushes bolus against soft palate & back of the mouth ▯Triggers swallowing reflex
2. Upper esophageal sphincter relaxes while epiglottis closes to keep swallowed material out of
3. Food moves downward into the esophagus, propelled by peristaltic waves & aided by graviety
THE GASTRIC PHASE:
• The stomach stores food, begins protein and fat digestion and protects the body from swallowed
• The stomach secretes (secretory cells of the gastric mucosa):
o Mucus and bicarbonate from mucus cells
o Gastric acid from parietal cells
o Pepsinogen from chief cells
o Somatostatin from D cells
o Histamine from ECL cells
o Gastrin from G cells
2 Module XIII
• Gastric function is integrated with the cephalic and intestinal phases of digestion
• Initiated by the arrival of food in the stomach
The body and fundus of the stomach have cells that secrete mucus, hydrochloric acid (HCl) and
pepsinogen. The antrum of the stomach secretes little acid but does secrete mucus and pepsinogen. It also has
endocrine cells that secrete gastrin. To be anatomically accurate, there is a fourth region in the stomach. It is
called the "cardia". The cardia is the region of the stomach where the esophagus enters the stomach.
Substances secreted by cells of the stomach, along with the stimuli for their release and their functions
are listed in Figure 2125. Figure 2126 illustrates the integration of secretion.
• A painful burning sensation in the chest caused by gastroesophageal reflux
o Backflow from the stomach irritating the esophagus
o Lower esophageal sphincter doesn’t stay contracted ▯Gastric acid & pepsin irritates lining of the
o Occurs after a large meal
Expansion creates pressure in esophageal lumen that can suck acidic contents out of
stomach is sphincter is relaxed
o Churning action of stomach when filled with food can also squirt acid back into stomach if
sphincter isn’t fully contracted
Regions & Functions of the Stomach:
• 3 regions – central body, fundus & antrum
1. Storage – Stores food & regulates its passage into the small intestine where most digestion and
absorption take place
2. Digestion – Chemically & mechanically digests food into chyme
3. Protection – Destroys many bacteria/pathogens that are swallowed with food; also protects itself from
its own secretions
Central Body Fundus Antrum
Contain cells that secrete mucus, Contain cells that secrete mucus, Secretes mucus, pepsinogen and
HCL & pepsinogen HCL & pepsinogen some acid
HCl secretion is controlled by a number of factors. Control is divided into the three phases discussed
earlier. You must know the stimuli that initiate each phase, the neural or hormonal pathways involved, and the
result. Zantac and Tagamet are drugs that block histamine receptors on parietal cells. How might this affect HCl
secretion? The intestinal phase of gastric function is illustrated in figure 2128. Review the properties of the
hormones gastrin, CCK and secretin and note how these hormones affect hydrochloric acid secretion.
• Acetylcholine, gastrin & histamine stimulate HCl release
• Parietal cells in gastric glands secrete gastric acid or HCl
o Kills bacteria & denatures proteins (activates pepsin)
3 Module XIII
o Continues carbohydrate digestion
You should understand pepsin's role in protein digestion. You should also know the endproducts when
protein is digested by pepsin in the stomach.
• Acetylcholine & acid secretion stimulate pepsinogen release
• Chief cells in the gastric glands secrete the inactive enzyme pepsinogen
o Cleaved to pepsin in lumen of the stomach by H
o Pepsin is an endopeptidase that carries out initial digestion of proteins
Gastric motility in the stomach is described on page 711 in the section The Stomach stores food. The
intestinal phase control of gastric motility is illustrated in figure 2128. As you read through this section, try to
understand the factors that control stomach movement and how these stomach movements mix and deliver the
food to the small intestine.
Before you leave this section make sure that you understand how the contents of the stomach and small
intestine can affect gastric emptying (motility). Stomach contents tend to stimulate gastric emptying whereas
duodenal contents tend to be inhibitory. In addition, factors that affect gastrin release also affect gastric motility
since gastrin increases the force of antral contractions.
• When food arrives, the stomach relaxes & expands to hold the increased volume
o Receptive relaxation
• However, when we ingest more than we need, stomach must regulate the rate at which food enters the
small intestine enhance gastric motility during a meal
o Upper stomach holds food, while lower stomach digests
o Peristaltic waves pushes food down toward pylorus, mixing food with acid & digestive enzymes
o As it digests into chyme, each contractile wave pushes food into small intestine and stomach