Previous Page | Right click this page to print.

Anatomy and Physiology of the Gastrointestinal Tract

Audio transcript; narrated by Annette Buyserie, Oregon State University

The anatomy and physiology of domestic species’ GI tracts share many similar characteristics. However, each species has specific variations in the anatomy and physiology of their digestive tract that influence the feedstuff consumed and utilization efficiency. The anatomical and physiological variations between the species are the rationale for differential feeding between species.

The initial component in the GI tract of all domestic species is the mouth and its associated structures such as the tongue and teeth. The primary functions of the mouth are prehension and mastication. As stated, species' variations exist that influence the feedstuff consumed and utilization efficiency. For example, prehension and therefore selectivity differ between species such as cattle, sheep, and horses. During the mastication process, saliva is added to the feed. The functions of saliva are to moisten the feed, aid in bolus formation, and coat the bolus. A bolus is a mass of moistened and chewed feed. In some species, the saliva contains enzymes and therefore initiates enzymatic digestion of the feed. Finally, the saliva contains sodium bicarbonate that acts as a buffer. In ruminants, the buffering capacity of saliva is essential to maintain rumen pH.

From the mouth, the consumed feed travels to the esophagus. The esophagus is essentially a tube from the mouth to the stomach or, in ruminants, to the reticulorumen. As the first two compartments of the rumen system, the reticulum and rumen, are not completely separate, the initial portion of the system may be referred to as the reticulorumen. The primary function of the esophagus is to transport the bolus from the mouth to the stomach or reticulorumen via muscular contractions. In nonruminants, the esophagus only moves the bolus downward. In contrast, in ruminants, the esophagus has the ability to move the bolus both downward and upward. The upward movement of a bolus from the reticulorumen to the mouth is part of the process of rumination. Rumination is the process of regurgitation and remastication to reduce the particle size of feed and aid in digestion of fibrous components.

The glandular stomach is the next component of the GI tract for nonruminants and nonpoultry. The variations in the GI tract of ruminants and poultry will be discussed later in the lecture. The functions of the glandular stomach include temporary storage and mixing of the feed. In addition, the stomach also functions as the initial site of protein digestion. Finally, the high concentration of hydrochloric acid (HCl) in the stomach is bactericidal. The cumulative activities of the stomach produce chyme. Chyme is the partially digested, acidic mixture of feed that travels to the small intestine.

The component subsequent to the glandular stomach is the small intestine. The small intestine is the primary site in the GI tract for enzymatic digestion. The small intestine is divided into three regions. The first region of the small intestine is the duodenum. Bile salts are released into the duodenum. Bile salts are produced by the liver and stored in the gall bladder until their release into the duodenum. Bile salts function to neutralize the chyme entering the small intestine and to emulsify fats preparing them for further digestion. Relatively large number and quantities of pancreatic enzymes are also secreted into the duodenum. The enzymes are produced in the pancreas and secreted into the duodenum via a duct. The pancreatic enzymes aid in the digestion of proteins, carbohydrates (CHO), and lipids. Additional duodenal enzymes aid in the digestion of proteins and CHO. The second and third portions are the jejunum and the ileum, respectively. Enzymatic digestions continue in the jejunum and ileum.

The small intestine is a primary site for nutrient absorption. The interior of the small intestine is lined with both villi and microvilli. Villi and microvilli are present in each section of the small intestine. The primary function of the villi and microvilli are to increase the surface area and therefore absorption efficiency of the small intestine. Figure 1.5 illustrates the structure and orientation of the villi and microvilli in the small intestine.

The villi are finger-like projections that extend into the lumen of the small intestine. As illustrated, the villi are a composite of many cells and contain extensive circulatory and lymph system components to transport the nutrients following absorption. Also illustrated in Figure 1.5 are the microvilli. Microvilli are the microscopic projections from each cell that comprise the villi.

The final component of the GI tract is the large intestine. The large intestine is divided into three sections. The three sections are the cecum, colon, and rectum. The functions of the cecum and colon are absorption of some organic compounds, absorption and resorption of water, and some microbial digestion. The primary type of digestion that occurs in the cecum and colon is microbial digestion. However, as the cecum and colon are located subsequent to the primary absorptive site, the absorption efficiency of microbial fermentation is lower than pre-absorption efficiency. The function of the rectum is excretion of fecal material.

The GI tract is also the site for a few additional functions. The GI tract functions as a route for excretion of processed toxicants and excess minerals. Finally, the GI tract is the site of synthesis of nutrients by microorganisms. Examples of the nutrients synthesized by microorganisms are water-soluble vitamins, amino acids, proteins, CHO, and lipids. The location and extent of microbial fermentation vary between species. Further, the location in the GI tract relative to the digestion and absorptive sites determine utilization efficiency.

Previous Page | Right click this page to print.