Human Physiology, 14th edition (2016)

The Digestive System 635

Fluid and Electrolyte

Absorption in the Intestine

The GI tract receives about 1.5 L per day of water from food and

drink; additionally, the GI tract secretes 8–10 L/day of fluid into the

lumen. This includes contributions from the salivary glands, stom-

ach, intestine, pancreas, liver, and gallbladder. The small intestine

both secretes and absorbs water accompanying different transport

processes, but these are not in balance. The small intestine secretes

about 1 L per day but absorbs most of the fluid in the chyme. As a

result, only about 2 L per day of fluid pass into the large intestine.

The large intestine absorbs about 90% of this remaining volume,

leaving less than 200 ml of fluid to be excreted in the feces.

Absorption of water in the intestine occurs passively as a result

of the osmotic gradient created by the active transport of ions. The

epithelial cells of the intestinal mucosa are joined together much

like those of the kidney tubules and, like the kidney tubules, con-

tain Na^1 /K^1 pumps in the basolateral membrane. The analogy with

kidney tubules is emphasized by the observation that aldosterone,

which stimulates salt and water reabsorption in the renal tubules,

also appears to stimulate salt and water absorption in the ileum.

The handling of salt and water transport in the large intes-

tine is made more complex by the ability of the large intestine to

secrete, as well as absorb, water. The secretion of water by the

mucosa of the large intestine occurs by osmosis as a result of

that the commensal bacteria of the intestine are needed for the
development of regulatory T lymphocytes (T reg cells), which
may help prevent intestinal inflammation.
Intestinal dendritic cells are antigen-presenting cells needed
for the activation of T lymphocytes. They reside in the lamina
propria, but send thin processes between columnar epithelial
cells that can engulf live bacteria that reach the epithelium from
the lumen. These activated dendritic cells, able to present bacte-
rial antigens to T cells, can travel through lymphatic vessels as far
as the mesenteric lymph nodes. They can then activate regulatory
T cells to suppress inflammation in response to commensal bac-
teria while stimulating killer T cells to combat pathogenic bacte-
ria (chapter 15, section 15.3).
Infection with Clostridium difficile is the most common
cause of nosocomial (hospital acquired) infectious diarrhea.
While this is usually well treated with antibiotics, it can some-
times recur and may become life threatening. The frequent use
of antibiotics may actually increase a patient’s susceptibility to
C. difficile by killing commensal bacteria, which provide a resis-
tance to colonization by pathogenic bacteria. A procedure called
fecal microbiota transplantation, which is the administration of
processed feces from healthy donors to the intestine of people
with recurrent C. difficile infection (through colonoscopy, an
enema, or a nasal tube), was recently found to be more effective
than antibiotic therapy alone in treating the infection. Fecal trans-
plantation helps to reestablish a healthy intestinal microbiota, and
its apparent effectiveness helps illustrate the importance of the
intestinal microbiota to immunity.

CLINICAL APPLICATION

Inflammatory bowel disease ( IBD ) includes Crohn’s dis-

ease and ulcerative colitis. Many genes are associated with

increased susceptibility to IBD, which involve altered immune

responses to intestinal bacteria. The ability of immune cells

to eliminate bacteria, and the functions of Paneth cells, may

be particularly compromised. In addition, there is decreased

mucus secretion by goblet cells and changes in tight junctions

between epithelial cells that increase intestinal permeability.

These can lead to erosions and ulcerations of the intestinal

mucosa. Crohn’s disease is characterized by inflammation

that can cause fibrosis, prolonged diarrhea with abdominal

pain, fatigue, and weight loss. IBD may be promoted by a

poor diet and the overuse of antibiotics that disturb the nor-

mal intestinal microbiota, which are believed to contribute to

the rising incidence of IBD in the United States.

Irritable bowel syndrome ( IBS ) differs from inflamma-

tory bowel disease in that IBS is not characterized by tissue

or biochemical changes, but is caused by complex factors

that affect the neural regulation of the GI tract. This can pro-

duce abdominal pain and discomfort with diarrhea, consti-

pation, or alternating periods of each. Defective interactions

of the intestinal microbiota with the enteric nervous system

are believed to contribute to IBS.

CLINICAL APPLICATION

Diarrhea is the excretion of excessive fluid in the feces.

There are many different causes, the most common of which

is viral gastroenteritis (stomach flu), where inflammation of

the intestine causes excessive fluid secretion. Some bacte-

ria (including the ones responsible for cholera) cause diar-

rhea by releasing enterotoxin, which is believed to stimulate

the intestinal secretion of Cl^2. This attracts Na^1 , and water

follows by osmosis. The osmolality of the lumen may also

be increased by undigested lactose in people with lactose

intolerance, thereby causing more water to remain in the

colon. Celiac disease, in which gluten—a protein found in

wheat and other grains—causes damage to the intestinal

mucosa in susceptible people, results in the malabsorption

of nutrients. These undigested nutrients produce diarrhea

by increasing the osmolality of the chyme to promote water

retention in the colon.

Clinical Investigation CLUES

George had abdominal pain with alternating diarrhea

and constipation, but no abnormal findings from his

colonoscopy.

• What is IBD, and might George have had it?


• What is IBS, and might George have had it?