ANATOMY AND PHYSIOLOGY OF

THE GASTROINTESTINAL TRACT

The GI tract is a 23- to 26-foot-long pathway that extends from the mouth through
the esophagus, stomach, and intestines to the anus The esophagus is located in the
mediastinum in the thoracic cavity, anterior to the spine and posterior to the
trachea and heart. This collapsible tube, which is about 25 cm (10 inches) in
length, becomes distended when food passes through it. It passes through the
diaphragm at an opening called the diaphragmatic hiatus. The remaining portion of
the GI tract is located within the peritoneal cavity. The stomach is situated in the
upper portion of the abdomen to the left of the midline, just under the left
diaphragm. It is a distensible pouch with a capacity of approximately1500 mL. The
inlet to the stomach is called the esophagogastric junction; it is surrounded by a
ring of smooth muscle called the lower esophageal sphincter (or cardiac sphincter),
which, on con-traction, closes off the stomach from the esophagus. The stomach
can be divided into four anatomic regions: the cardia (entrance), fundus, body, and
pylorus (outlet). Circular smooth muscle in the wall of the pylorus forms the
pyloric sphincter and controls the opening between the stomach and the small
intestine.
The small intestine is the longest segment of the GI tract, accounting for
about two thirds of the total length. It folds back and forth on itself, providing
approximately 7000 cm of surface area for secretion and absorption, the process by
which nutrients enter the bloodstream through the intestinal walls. The small
intestine is divided into three anatomic parts: the upper part, called the duodenum;
the middle part, called the jejunum and the lower part, called the ileum. The
common bile duct, which allows for the passage of both bile and pancreatic
secretions, empties into the duodenum at the ampulla of Vater. The junction
between the small and large intestine, the cecum, is located in the right lower
portion of the abdomen. The ileocecal valve is located at this junction. It controls
the passage of intestinal contents into the large intestine and prevents reflux of
bacteria into the small intestine. The vermiform appendix is located near this
junction.
The large intestine consists of an ascending segment on the right side of the
abdomen, a transverse segment that extends from right to left in the upper
abdomen, and a descending segment on the left side of the abdomen. The terminal
portion of the large intestine consists of two parts: the sigmoid colon and the
rectum. The rectum is continuous with the anus. A network of striated muscle that
forms both the internal and the external anal sphincters regulates the anal outlet.
The GI tract receives blood from arteries that originate along the entire length of
the thoracic and abdominal aorta. Of particular importance are the gastric artery
and the superior and inferior mesenteric arteries. Oxygen and nutrients are supplied
to the stomach by the gastric artery and to the intestine by the mesenteric arteries.
Blood is drained from these organs by veins that merge with others in the abdomen
to form a large vessel called the portal vein. Nutrient-rich blood is then carried to
the liver. The blood flow to the GI tract is about 20% of the total cardiac output
and increases significantly after eating. Both the sympathetic and parasympathetic
portions of the autonomic nervous system innervate the GI tract. In general,
sympathetic nerves exert an inhibitory effect on the GI tract, decreasing gastric
secretion and motility and causing the sphincters and blood vessels to constrict.
Parasympathetic nerve stimulation causes peristalsis and increases secretory
activities. The sphincters relax under the influence of parasympathetic stimulation.
The only portions of the tract that are under voluntary control are the upper
esophagus and the external anal sphincter.
FUNCTION OF THE DIGESTIVE SYSTEM
All cells of the body require nutrients. These nutrients are derived from the intake
of food that contains proteins, fats, carbohydrates, vitamins and minerals, and
cellulose fibers and other vegetable matter of no nutritional value. The primary
digestive functions of the GI tract are the following:
• To break down food particles into the molecular form for Digestion
• To absorb into the bloodstream the small molecules produced by digestion
• To eliminate undigested and unabsorbed foodstuffs and other waste products
from the body After food is ingested, it is propelled through the GI tract, coming
into contact with a wide variety of secretions that aid in its digestion, absorption, or
elimination from the GI tract.
Chewing and Swallowing
The process of digestion begins with the act of chewing, in which food is broken
down into small particles that can be swallowed and mixed with digestive
enzymes. Eating—or even the sight, smell, or taste of food—can cause reflex
salivation. Saliva is secreted from three pairs of glands: the parotid, the
submaxillary, and the sublingual glands. Approximately 1.5 L of saliva is secreted
daily. Saliva is the first secretion that comes in contact with food. Saliva contains
the enzyme ptyalin, or salivary amylase, which begins the digestion of starches.
Saliva also contains mucus and water, which help to lubricate the food as it is
chewed, thereby facilitating swallowing.
Swallowing begins as a voluntary act that is regulated by a swallowing center in
the medulla oblongata of the central nervous system. As food is swallowed, the
epiglottis moves to cover the tracheal opening and prevent aspiration of food into
the lungs. Swallowing, which propels the bolus of food into the upper esophagus,
thus ends as a reflex action. The smooth muscle in the wall of the esophagus
contracts in a rhythmic sequence from the upper esophagus toward the stomach to
propel the bolus of food along the tract. During this process of esophageal
peristalsis, the lower esophageal sphincter relaxes and permits the bolus of food to
enter the stomach. Subsequently, the lower esophageal sphincter closes tightly to
prevent reflux of stomach contents into the esophagus.

Gastric Function
The stomach stores and mixes the food with secretions. It secretes a highly acidic
fluid in response to the presence or anticipated ingestion of food. This fluid, which
may have a pH as low as 1, derives its acidity from the hydrochloric acid (HCl)
secreted by the glands of the stomach. The function of this gastric secretion is two-
fold: to break down food into more absorbable components and to aid in the
destruction of most ingested bacteria. The stomach can produce about 2.4 L per
day of these gastric secretions. Gastric secretions also contain the enzyme pepsin,
which is important for initiating protein digestion. Intrinsic factor is also secreted
by the gastric mucosa. This compound combines with dietary vitamin B12 so that
the vitamin can be absorbed in the ileum. In the absence of intrinsic factor, vitamin
B12 cannot be absorbed and pernicious anemia results.
Peristaltic contractions in the stomach propel its contents toward the pylorus.
Because large food particles cannot pass through the pyloric sphincter, they are
churned back into the body of the stomach. In this way, food in the stomach is
agitated mechanically and broken down into smaller particles. Food remains in the
stomach for a variable length of time, from a half-hour to several hours, depending
on the size of food particles, the composition of the meal, and other factors.
Peristalsis in the stomach and contractions of the pyloric sphincter allow the
partially digested food to enter the small intestine at a rate that permits efficient
absorption of nutrients. This food mixed with gastric secretions is called chyme.
Hormones, neuroregulatory, and local regulators found in the gastric secretions
control the rate of gastric secretions and influence gastric motility

Small Intestine Function

The digestive process continues in the duodenum. Secretions in the duodenum
come from the accessory digestive organs—the pancreas, liver, and gallbladder—
and the glands in the wall of the intestine itself. These secretions contain digestive
enzymes and bile. Pancreatic secretions have an alkaline pH because of high
concentrations of bicarbonate. This neutralizes the acid entering the duodenum
from the stomach. The pancreas also secretes digestive enzymes, including trypsin,
which aids in digesting protein; amylase, which aids in digesting starch; and lipase,
which aids in digesting fats. Bile (secreted by the liver and stored in the
gallbladder) aids in emulsifying ingested fats, making them easier to digest and
absorb.
The intestinal glands secrete mucus, hormones, electrolytes, and enzymes. The
mucus coats the cells and protects the mucosa from injury by HCl. Hormones,
neuroregulators, and local regulators found in these intestinal secretions control the
rate of intestinal secretions and also influence GI motility. Intestinal secretions
total approximately 1 L/day of pancreatic juice, 0.5 L/day of bile, and 3 L/day of
secretions from the glands of the small intestine and GI regulatory substances.
Two types of contractions occur regularly in the small intestine: segmentation
contractions and intestinal peristalsis. Segmentation contractions produce mixing
waves that move the intestinal con-tents back and forth in a churning motion.
Intestinal peristalsis propels the contents of the small intestine toward the colon.
Both movements are stimulated by the presence of chyme. Food, initially ingested
in the form of fats, proteins, and carbohydrates, is broken down into absorbable
particles (constituent nutrients) by the process of digestion. Carbohydrates are
broken down into disaccharides (e.g., sucrose, maltose, galactose) and mono-
saccharides (e.g., glucose, fructose). Glucose is the major carbohydrate that the
tissue cells use as fuel. Proteins are broken down into amino acids and peptides.
Ingested fats are emulsified into monoglycerides and fatty acids. These smaller
molecules are then ready to be absorbed. Chyme stays in the small intestine for 3
to 6 hours, allowing for continued breakdown and absorption of nutrients. Small,
finger-like projections called villi are present through-
out the entire intestine and function to produce digestive enzymes as well as to
absorb nutrients. Absorption is the primary function of the small intestine.
Vitamins and minerals are not digested but rather absorbed essentially unchanged.
Absorption begins in the jejunum and is accomplished by both active transport and
diffusion across the intestinal wall into the circulation. Absorption of different
nutrients takes place at different locations in the small intestine. Iron and calcium
absorption takes place in the duodenum. Fats, proteins, carbohydrates, sodium, and
chloride are absorbed in the jejunum. Vitamin B12 and bile salts are absorbed in
the ileum. Magnesium, phosphate, and potassium are absorbed throughout the
small intestine (Society of Gastroenterology Nursing and Associates, 1998).

Colonic Function
Within 4 hours after eating, residual waste material passes into the terminal ileum
and passes slowly into the proximal portion of the colon through the ileocecal
valve. This valve, which is normally closed, helps prevent colonic contents from
refluxing into the small intestine. With each peristaltic wave of the small intestine,
the valve opens briefly and permits some of the contents to pass into the colon.
Bacteria make up a major component of the contents of the large intestine. They
assist in completing the breakdown of waste material, especially of undigested or
unabsorbed proteins and bile salts. Two types of colonic secretions are added to the
residual material: an electrolyte solution and mucus. The electrolyte solution is
chiefly a bicarbonate solution that acts to neutralize the end products formed by the
colonic bacterial action. The mucus protects the colonic mucosa from the
interluminal contents and also provides adherence for the fecal mass. Slow, weak
peristaltic activity moves the colonic contents slowly along the tract. This slow
transport allows efficient reabsorption of water and electrolytes, which is the
primary purpose of the colon.
Intermittent strong peristaltic waves propel the contents for considerable distances.
This generally occurs after another meal is eaten, when intestine-stimulating
hormones are released. The waste materials from a meal eventually reach and
distend the rectum, usually in about 12 hours. As much as one fourth of the waste
materials from a meal may still be in the rectum 3 days after the meal was ingested.

Waste Products of Digestion

Feces consist of undigested foodstuffs, inorganic materials, water, and bacteria.
Fecal matter is about 75% fluid and 25% solid material. The composition is
relatively unaffected by alterations in diet, because a large portion of the fecal
mass is of nondietary origin, derived from the secretions of the GI tract. The brown
color of the feces results from the breakdown of bile by the intestinal bacteria.
Chemicals formed by intestinal bacteria (especially indole and skatole) are
responsible in large part for the fecal odor. Gases formed contain methane,
hydrogen sulfide, and ammonia, among others. The GI tract normally contains
approximately 150 mL of these gases, which are either absorbed into the portal
circulation and detoxified by the liver or expelled from the rectum as flatus.
Elimination of stool begins with distention of the rectum, which reflexively
initiates contractions of the rectal musculature and relaxes the normally closed
internal anal sphincter. The internal sphincter is controlled by the autonomic
nervous system; the external sphincter is under the conscious control of the
cerebral cortex. During defecation, the external anal sphincter voluntarily relaxes
to allow colonic contents to be expelled. Normally, the external anal sphincter is
maintained in a state of tonic contraction. Thus, defecation is seen to be a spinal
reflex (involving the parasympathetic nerve fibers) that can be inhibited voluntarily
by keeping the external anal sphincter closed. Contracting the abdominal muscles
(straining) facilitates emptying of the colon. The average frequency of defecation
in humans is once daily, but the frequency varies among individuals.