Chapter 15
Quantitative and Thought Questions
15.1 If the salivary glands fail to secrete amylase, the undigested starch that reaches the small intestine
will still be digested by the amylase the pancreas secretes. Thus, starch digestion is not significantly
affected by the absence of salivary amylase.
15.2 Alcohol can be absorbed across the stomach wall, but absorption is much more rapid from the
small intestine with its larger surface area. Ingestion of foods containing fat releases enterogastrones
from the small intestine, and these hormones inhibit gastric emptying and thereby prolong the time
alcohol spends in the stomach before reaching the small intestine. Milk, contrary to popular belief, does
not “protect” the lining of the stomach from alcohol by coating it with a fatty layer. Rather, the fat
content of milk decreases the rate of absorption of alcohol by decreasing the rate of gastric emptying.
15.3 Vomiting results in the loss of fluid and acid from the body. The fluid comes from the luminal
contents of the stomach and duodenum, most of which was secreted by the gastric glands, pancreas,
and liver and thus is derived from the blood. The cardiovascular symptoms of this patient are the result
of the decrease in blood volume that accompanies vomiting. The secretion of acid by the stomach
produces an equal number of bicarbonate ions, which are released into the blood. Normally, these
bicarbonate ions are neutralized by hydrogen ions released into the blood by the pancreas when this
organ secretes bicarbonate ions. Because gastric acid is lost during vomiting, the pancreas is not
stimulated to secrete HCO3- by the usual high-acidity signal from the duodenum, and no corresponding
hydrogen ions are formed to neutralize the HCO3- released into the blood by the stomach. As a result,
the acidity of the blood decreases. Loss of K+ from the loss of stomach contents can also lead to
hypokalemia.
15.4 Fat can be digested and absorbed in the absence of bile salts, but in greatly decreased amounts.
Without adequate emulsification of fat by bile salts and phospholipids, only the fat at the surface of
large lipid droplets is available to pancreatic lipase, and the rate of fat digestion is very slow. Without
the formation of micelles with the aid of bile salts, the products of fat digestion become dissolved in the
large lipid droplets, where they are not readily available for diffusion into the epithelial cells. In the
absence of bile salts, only about 50% of the ingested fat is digested and absorbed. The undigested fat is
passed on to the large intestine, where bacteria produce compounds that increase colonic motility and
promote the secretion of fluid into the lumen of the large intestine, leading to diarrhea.
15.5 Damage to the lower portion of the spinal cord produces a loss of voluntary control over defecation
due to disruption of the somatic nerves to the skeletal muscle of the external anal sphincter. Damage to
the somatic nerves leaves the external sphincter in a continuously relaxed state. Under these conditions,
defecation occurs whenever the rectum becomes distended and the defecation reflex is initiated.
15.6 Vagotomy decreases the secretion of acid by the stomach. Impulses in the parasympathetic nerves
directly stimulate acid secretion by the parietal cells and also cause the release of gastrin, which in turn
stimulates acid secretion. Impulses in the vagus nerves are increased during both the cephalic and
gastric phases of digestion. Vagotomy, by decreasing the amount of acid secreted, decreases irritation of
existing ulcers, which promotes healing and decreases the probability of acid contributing to the
production of new ulcers.
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