The human gastrointestinal system is composed of the organs through which the nutrients (i.e., the nutritious substances: carbohydrates, peptides and proteins, lipids, nucleic acids, and vitamins and minerals), other food components (e.g., the phy-tochemicals), and water, as well as also toxic chemicals and microorganisms (both potentially beneficial or potentially harmful), enter the body.1 Complex molecules such as proteins, lipids, and some carbohydrates are broken down (digested) into absorbable units (essentially, amino acids and some small peptides, monoglycerides, and fatty acids) and, eventually, into glycerol, simple carbohydrates, or monosac-charides (glucose, fructose, and galactose), respectively. The products of digestion plus vitamins, minerals, and water, as well as other food components, are absorbed, and enter the blood and lymph circulation. Some complex carbohydrates and some peptides or proteins, however, resist the digestive process, but they are fermented essentially in the colon because of the presence of commensal microflora. Most of the exogenous microorganisms are destroyed by various defense mechanisms, but some survive and become part of the microbial ecosystem that permanently colonizes the gastrointestinal system with either beneficial (as is the case for probiotic bacteria) or harmful (as is the case for pathogens) consequences.
The digestive, absorptive, and fermentative functions of the gastrointestinal system depend upon a variety of mechanisms that soften and dissolve the food, propel the chyme (the semifluid mass of partly digested food), and mix it with different exocrine secretions. These mechanisms depend upon intrinsic properties of the smooth muscle, involve the operation of visceral reflexes, or are under the regulation of hormones.
The human gastrointestinal system is composed of the gastrointestinal tract and the attached exocrine glands, i.e., the salivary glands, the liver plus the gall bladder that stores and secretes the bile, and the pancreas (Figure 2.1).
The gastrointestinal tract is the alimentary, tubular, food-carrying passage extending from the mouth to the anus. The different parts of the alimentary canal are: the oral cavity, the esophagus, the stomach, the small intestine composed of the duodenum, the jejunum and the ileum, the cecum plus the colon or large bowel that contains the colonic microflora, and the rectum. The large bowel (or large gut, or
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