429

CHAPTER

SECTION V GASTROINTESTINAL PHYSIOLOGY

26

Overview of

Gastrointestinal

Function & Regulation

OBJECTIVES

After studying this chapter, you should be able to:

■

Understand the functional significance of the gastrointestinal system, and in par-

ticular, its roles in nutrient assimilation, excretion, and immunity.

■

Describe the structure of the gastrointestinal tract, the glands that drain into it, and

its subdivision into functional segments.

■

List the major gastrointestinal secretions, their components, and the stimuli that

regulate their production.

■

Describe water balance in the gastrointestinal tract and explain how the level of

luminal fluidity is adjusted to allow for digestion and absorption.

■

Identify the major hormones, other peptides, and key neurotransmitters of the

gastrointestinal system.

■

Describe the special features of the enteric nervous system and the splanchnic

circulation.

INTRODUCTION

The gastrointestinal tract is a continuous tube that stretches

from the mouth to the anus. Its primary function is to serve as

a portal whereby nutrients and water can be absorbed into the

body. In fulfilling this function, the meal is mixed with a vari-

ety of secretions that arise from both the gastrointestinal tract

itself and organs that drain into it, such as the pancreas, gall-

bladder, and salivary glands. Likewise, the intestine displays a

variety of motility patterns that serve to mix the meal with

digestive secretions and move it along the length of the gas-

trointestinal tract. Ultimately, residues of the meal that cannot

be absorbed, along with cellular debris and lipid-soluble met-

abolic end products that are excreted in the bile rather than

the urine, are expelled from the body. All of these functions

are tightly regulated in concert with the ingestion of meals.

Thus, the gastrointestinal system has evolved a large number

of regulatory mechanisms that act both locally and to coordi-

nate the function of the gut, and the organs that drain into it,

over long distances.

The lumen of the gastrointestinal tract is functionally contig-

uous with the outside of the body. The intestine also has a very

substantial surface area, which is important for its absorptive

function. Finally, the gut is an unusual organ in that it becomes

colonized, almost from birth, with a large number of commen-

sal bacteria (particularly in the colon, or large intestine). This