The GI tract’s purpose is to digest food (breaks it down into smaller fragments) and absorb the digested food through its lining into the bloodstream.  Accessory organs such as the teeth, tongue, and digestive glands assist with this process in various ways. Food within the tract is technically outside the body because it has contact only with cells lining the tract and the tract is open to the environment at both ends. 

              In order to accomplish its purpose, once in the GI tract, food must be propelled from one area (organ) of the GI tract to the next.  Peristalsis, involuntary wave-like movements that alternately contract and relax, occurs throughout the entire length of the tract and propels the food distally.  The net effect is to squeeze the food along the tract.  While peristalsis can occur in either direction from any point along the tract, it normally is more active or forceful in moving material caudally (analward).  

            Food enters the GI tract through the mouth and digestion is initiated here.  Once in the mouth, food is chewed and mixed with saliva beginning the breakdown of food before it even leaves the mouth.  Consider the premature infant receiving orogastric (og) feedings—the initiation of digestion may be delayed.  No data were available to support or dispute whether this plays any role in the subsequent GI problems the infant experiences.  

        The act of swallowing propels the food past the oropharynx, down the esophagus and into the stomach.  Vagal reflexes that are part of the overall swallowing mechanism initiate peristaltic waves in the esophagus.  At the opening of the stomach is a muscular ring, the Lower Esophageal Sphincter (LES) that remains tonically constricted until a peristaltic wave stimulates it to relax and allow food to enter the stomach.  The LESs primary function is to remain tonically closed preventing reflux of stomach contents into the esophagus [Gastroesophageal Reflux (GER)] except during times or retching and/or vomiting.  

            The C-shaped stomach is located on the left side of the abdominal cavity, partially hidden by the liver and diaphragm.  The stomach consists of 3 portions:

a.  Fundus:  rounded upper portion 

b.  Body:  as its name implies, the mid-portion 

c. Pylorus:  funnel shaped terminal portion, connects upper and lower portions of the GI tract.  The pylorus is continuous with the small intestine through the pyloric sphincter.

The stomach acts as a temporary “storage tank” for food as well as a site for food breakdown.  Muscles in the stomach wall not only move food along the tract but also churn, mix, and beat the food, physically breaking it down into smaller fragments.  The chemical breakdown of proteins also begins in the stomach.  Most digestive activity occurs in the pyloric region of the stomach.  After food has been processed in the stomach, it forms a milky, semi-fluid mixture resembling heavy cream called chyme.  Chyme enters the small intestine through the pyloric sphincter.

            The ileum or small intestine is the body’s major digestive organ, preparing food for its absorption into the cells of the body.  The small intestine is a convoluted tube extending from the pyloric sphincter to the ileocecal valve, making it the longest section of the GI tract.  The small intestine hangs in coils in the abdominal cavity suspended from the posterior abdominal wall by the fan-shaped mesentery.  The large intestine surrounds and frames it within the cavity.  The small intestine is divided into 3 sections:

a. Duodenum:  first portion, curves around the head of the pancreas

b. Jejunum:  mid-portion, extends from duodenum to the ileum

c.  Ileum:  terminal portion of small intestine, it connects the small and large intestines at the ileocecal valve.

        Chemical digestion of foods is completed in the small intestine by digestive enzymes, bile, and pancreatic secretions entering its lumen.  Since the small intestine is able to process only a small amount of food at one time, the pyloric sphincter controls the movement of food from the stomach into the small intestine to minimize overwhelming it.  Nearly all food absorption occurs in the small intestine via hundreds of finger-like projections called villi that cover its surface.  Each villi is rich in capillary vessels and lymph ducts that pick up the nutrients;  the veins picking up the glucose and amino acids, while the fatty acids and glycerol are picked up by the lymph ducts.  On the average, it takes 3-5 hours for the chyme to pass the length of the small intestine and arrive at the ileocecal valve.  The ileocecal valve’s primary function is to prevent backflow of fecal contents from the large intestine into the small intestine.  A portion of the valve actually protrudes into the first portion of the large intestine and is forced closed as the large intestine fills----accomplishing its function.  On reaching the ileocecal valve, actual entry into the large intestine may be delayed until another meal is eaten, stimulating or intensifying peristaltic activity in the small intestine to force the remaining chyme through the valve.  

The large intestine or colon extends from the ileocecal valve to the anus and is divided into 4 sections:

a.  Cecum:  Sac-like first portion of the large intestine

b.  Appendix:  worm-like appearance, hangs from the cecum

c.  Colon:  sub-divided into the ascending, transverse, descending, and sigmoid colons

The Ascending colon travels up the right side of the abdominal cavity, makes a left turn, travels across the abdominal cavity as the Transverse colon, turns right, and continues down the left side of the abdominal cavity as the Descending colon, entering the pelvis as the S-shaped Sigmoid colon.  

d. Rectum & Anal canal:  along with the sigmoid colon, lie in the pelvis.  Anal canal ends at the anus which opens to the external environment.

The large intestine’s principal function is to dry out the indigestible food residue by absorbing water and to eliminate this residue from the body as feces.  Since most nutrient absorption occurs before the large intestine is reached, no villi are seen in this portion of the GI tract, but there are vast numbers of goblet cells that produce mucus.  This mucus acts as a lubricant to ease the passage of feces. The proximal half of the colon is concerned with the former (absorption) and the distal half of the colon with the latter (elimination).

            The human gastrointestinal tract is fully developed anatomically by 20 weeks' gestation but many of its physiologic functions develop much later.  Thus, the prematurely born infant will have certain gastrointestinal limitations.  While some functions appear to start-up at birth regardless of gestational age (e.g., a decrease in intestinal permeability) other functions appear to start-up only when a certain post-conceptual age has been reached (e.g., onset of peristalsis at 28-30 weeks, coordination of suck/swallow at 33-36 weeks.)  Despite these functional shortcomings, initiation of slow, careful feedings, known as Trophic Feedings is recommended.  Trophic feedings (minimal in amount and/or calorie) have been shown to have very positive effects on the developing GI tract of the premature infant.  As positive as these trophic feedings may be, they are not without associated concerns.  One in particular is the development of Necrotizing Enterocolitis (NEC).