Ganong's Review of Medical Physiology, 23rd Edition

CHAPTER 26

Overview of Gastrointestinal Function & Regulation 431

protects the cells to some extent from the effects of digestive
enzymes. Some digestive enzymes are also actually part of the
brush border, being membrane-bound proteins. These so-
called “brush border hydrolases” perform the final steps of
digestion for specific nutrients.

GASTROINTESTINAL SECRETIONS

SALIVARY SECRETION

The first secretion encountered when food is ingested is saliva.
Saliva is produced by three pairs of salivary glands that drain into
the oral cavity. It has a number of organic constituents that serve
to initiate digestion (particularly of starch, mediated by amylase)
and which also protect the oral cavity from bacteria (such as
immunoglobulin A and lysozyme). Saliva also serves to lubricate
the food bolus (aided by mucins). Saliva is also hypotonic com-
pared with plasma and alkaline; the latter feature is important to
neutralize any gastric secretions that reflux into the esophagus.
The salivary glands consist of blind end pieces (acini) that
produce the primary secretion containing the organic constit-
uents dissolved in a fluid that is essentially identical in its

composition to plasma. The salivary glands are actually

extremely active when maximally stimulated, secreting their

own weight in saliva every minute. To accomplish this, they

are richly endowed with surrounding blood vessels that dilate

when salivary secretion is initiated. The composition of the

saliva is then modified as it flows from the acini out into ducts

that eventually coalesce and deliver the saliva into the mouth.

Na

+

and Cl

• are extracted and K
  
  
  • and bicarbonate are added.
    Because the ducts are relatively impermeable to water, the loss
    of NaCl renders the saliva hypotonic, particularly at low
    secretion rates. As the rate of secretion increases, there is less
    time for NaCl to be extracted and the tonicity of the saliva
    rises, but it always stays somewhat hypotonic with respect to
    plasma. Overall, the three pairs of salivary glands that drain
    into the mouth supply 1000 to 1500 mL of saliva per day.
    Salivary secretion is almost entirely controlled by neural
    influences, with the parasympathetic branch of the autonomic
    nervous system playing the most prominent role (Figure 26–3).
    Sympathetic input slightly modifies the composition of saliva
    (particularly by increasing proteinaceous content), but has little
    influence on volume. Secretion is triggered by reflexes that are
    stimulated by the physical act of chewing, but is actually initi-
    ated even before the meal is taken into the mouth as a result of
    central triggers that are prompted by thinking about, seeing, or
    smelling food. Indeed, salivary secretion can readily be condi-
    tioned, as in the classical experiments of Pavlov where dogs
    were conditioned to salivate in response to a ringing bell by
    associating this stimulus with a meal. Salivary secretion is also
    prompted by nausea, but inhibited by fear or during sleep.
    Saliva performs a number of important functions: it facili-
    tates swallowing, keeps the mouth moist, serves as a solvent
    for the molecules that stimulate the taste buds, aids speech by
    facilitating movements of the lips and tongue, and keeps the
    mouth and teeth clean. The saliva also has some antibacterial
    action, and patients with deficient salivation
    (xerostomia)
    have a higher than normal incidence of dental caries. The
    buffers in saliva help maintain the oral pH at about 7.0. They
    also help neutralize gastric acid and relieve heartburn when
    gastric juice is regurgitated into the esophagus.
  
  
  
  

GASTRIC SECRETION

Food is stored in the stomach; mixed with acid, mucus, and

pepsin; and released at a controlled, steady rate into the

duodenum (see Clinical Box 26–1).

ANATOMIC CONSIDERATIONS

The gross anatomy of the stomach is shown in Figure 26–4.

The gastric mucosa contains many deep glands. In the cardia

and the pyloric region, the glands secrete mucus. In the body

of the stomach, including the fundus, the glands also contain

parietal (oxyntic) cells,

which secrete hydrochloric acid and

intrinsic factor, and

chief (zymogen, peptic) cells,

which se-

crete pepsinogens (Figure 26–5) These secretions mix with

FIGURE 26–2
The structure of intestinal villi and crypts.
(Reproduced with permission, from Fox SI:
Human Physiology
, 10th ed. McGraw-Hill,
2008.)

Simple columnar

epithelium

Lacteal

Capillary network

Goblet cells

Intestinal crypt

Lymph vessel

Arteriole

Venule

Villus