Updated: 03/11/08 02:32 PM
HOME HEAL EDUCATE RESEARCH DIRECTORY OUTREACH


Authors: D.R. Saunders, C.E. Rubin, and J.D. Ostrow
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“A good reliable set of bowels is worth more to a man than any quantity of brains.”
Henry Wheeler Shaw

A. Objectives
  1. Know the concept of isotonicity, regional differences in absorptive function.

  2. Understand the principles of water and electrolyte absorption so that you can make sense of the causes and treatment of diarrhea.

  3. Understand the metabolism of bile salts and the enterohepatic circulation.

  4. Learn about intestinal motility, the surface area of the small intestine, the transit time of a meal.

  5. Be able to describe the polarization of an absorptive cell, paracellular absorption, active, and passive transport mechanisms.

  6. Learn about the absorption of vitamin and minerals, carbohydrate, protein, and fat so that you can understand and treat nutrient malabsorption.
B. Introduction

The small intestine absorbs all nutrients necessary to maintain body function. Its lumen receives food, vitamins, saline and other electrolytes from the external environment and enzymes, water, electrolytes and bile salts from its accessory glands (liver, pancreas). The luminal contents are prepared for absorption by digestion, and transported to the internal environment by various active and passive processes within the absorptive epithelium of the mucosa.

The small intestinal mucosa has a variety of other functions: it degrades certain substances, synthesizes others, secretes hormones and manufactures an important immunoglobulin (IgA).

Essential organs like the small bowel have a functional capacity in excess of normal needs; thus, partial loss of small intestine need not impair function. Normally, most nutrients are absorbed in the proximal half of the jejunum; when proximal absorption is impaired, the distal jejunum and ileum function as a backup system to minimize malabsorption.

In this Chapter we advise you to concentrate on understanding the normal and abnormal structure and function of the small intestine. We will especially emphasize fat absorption because it is one of the major sources of calories in the diet of an industrialized country such as ours. Furthermore, steatorrhea (impaired absorption of fat) is usually the most prominent feature of clinically significant malabsorption.

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