Digestion and absorption

Digestion refers to the chemical and physical modifi cations
which render ingested food constituents absorbable
by the small intestine. Absorption is the process by
which the products of digestion move from the lumen
of the small intestine into the enterocytes and thence
into the bloodstream or lymphatic system. A complex
meal may be fully digested and absorbed in 3 hours.
After the digested food has passed through the wall
of the small intestine, nutrients other than fats or fatsoluble
substances enter the venules within the lamina
propria. The venules eventually lead to the portal vein,
which conveys nonlipid products of digestion from
the intestine to the liver. Most of the absorbed nonlipid
material passes through the liver, whose cells
extract nutrients for storage or metabolic processing.
The only exception to this portal route are the internal
iliac veins which drain directly back into the systemic
circulation (Granger et al., 1987). The nutrient-enriched
blood that has passed through the liver is then
sent to the heart and thence to the lungs. The oxygenated
blood returns to the heart for redistribution to all
parts of the body. The nutrients pass from the blood
capillaries into the extracellular fl uid from which the
cells take up the nutrients they need.
Bile produced by the liver is secreted continuously
into the gall bladder, which discharges it intermittently
into the duodenum. Bile is necessary for the emulsifi
cation of ingested fats. Many vital micronutrients,
including vitamin D, folate and vitamin B12, are conserved
through their excretion in bile and subsequent
reabsorption by the small intestine (enterohepatic
circulation). The liver also produces copious amounts
of lymph to sustain the lymphatic system. Absorbed
lipids and lipid-soluble substances in the form of chylomicrons
are transported via the lymphatic system
and thoracic ducts to the subclavian veins and into the
systemic circulation, bypassing the liver.
Digestion commences as soon as the food enters the
mouth. Chewing helps to break up large particles of
food, whilst also mixing food with saliva, which acts as
a lubricant and contains the enzymes salivary amylase
and lingual lipase. Lingual lipase has only a minor role
in digestion of dietary triglycerides; however, salivary
amylase plays a major role in digestion of dietary
starch. Most of the enzymatic activity of salivary
amylase occurs in the stomach, where there is a much
longer time for it to interact with the starch.
In the stomach, the churning action and the presence
of hydrochloric acid and pepsin convert the food
bolus into a liquid chyme. The stomach has three
regions with regard to its glandular secretions: the
Fig. 3.22 Longitudinal section of an epithelial
cell of the proximal convoluted tubule.
capillary basal lamina lumen of
tubule
basolateral
membrane
epithelial cell
brush-border membrane
tight junction
intercellular
space
basal
channels
Background physiology and functional anatomy 37
fundus (in the proximal stomach), the body and the
antrum (in the distal stomach). The folds of the stomach
lining contain microscopic gastric pits into each
of which drain four or fi ve gastric glands. The mucosa
of the body and fundus contains oxyntic glands
whereas the mucosa of the antrum contains pyloric
glands. Oxyntic glands are lined by parietal cells that
secrete hydrochloric acid and intrinsic factor and by
chief cells that secrete pepsinogen (the precursor of
pepsin) and gastric lipase. The pyloric glands contain
almost no parietal cells or chief cells but, rather, contain
mucus-secreting cells and G cells, which produce
the hormone gastrin.
Most absorption takes place and is completed in
the small intestine. On arrival at the duodenum, the
acidic chyme is buffered by the bicarbonate in pancreatic
juice and bile. Brunner’s glands in the duodenum
produce an alkaline secretion containing mucus. The
proteases, amylase, lipase and other enzymes of pancreatic
origin are also secreted into the duodenum.
The fi nal stages of digestion take place on the luminal
surface or within the epithelial cells lining the small
intestine. When absorption has been accomplished,
the jejunum and ileum are actively involved in the
regulation of electrolyte and fl uid balance. The various
stages of digestion are co-ordinated by the action
of the nervous system, endocrine system and circulatory
system.
The principal functions of the colon are (1) absorption
of water and inorganic salts (mainly sodium)
from the chyme to form solid faeces and (2) storage of
faecal matter until it can be expelled.
3.4.2 The luminal environment within the
small intestine
The bulk luminal phase of the upper gastrointestinal
tract is characterized by a wide range of pH values.
Postprandial pH values in humans are within the following
ranges: 1.8–3.4 in the stomach, 6.8–7.8 in the
lower small intestine, and 3.5–7 in the duodenum and
proximal jejunum.
Bulk contents of the intestinal lumen are mixed by
segmentation and peristalsis, and water and solutes
are brought to the surface of the mucosa by convection.
However, the luminal environment immediately
adjacent to the brush-border membrane is stationary
and unaffected by gut motility. The lack of convective
mixing in this region creates a series of thin layers,
each progressively more stirred, extending from the
surface of the enterocyte to the bulk phase of the
lumen. Together, these are the so-called ‘unstirred
layer’, whose effective thickness in the human jejunum
has been calculated to be 35 μm based on the
rate of disaccharide hydrolysis at the brush border,
rather than the 600-μm value derived from osmotic
transient measurements (Levitt et al., 1992).
Solute movement within an unstirred layer takes
place by diffusion, which is slow compared to the
convective movement in the bulk luminal phase. The
pH at the luminal surface is approximately two units
lower than that of the bulk phase and varies less than
±0.5 units despite large pH variations in the intestinal
chyme. It has been suggested that the formation of the
low-pH microclimate is due to the presence of mucin
which covers the entire surface of the epithelium
(Nimmerfall & Rosenthaler, 1980; Shiau et al., 1985).
Mucopolysaccharides possess a wide range of ionizable
groups and hence mucin is an ampholyte. If the
luminal chyme is of low pH, the ampholyte is positively
charged and so it repels additional hydrogen ions
entering the microclimate. If, on the other hand, the
chyme is alkaline, the ampholyte becomes negatively
charged and retains hydrogen ions within the microclimate.
In this manner, the mucin layer functions as
a restrictive barrier for hydrogen ions diffusing in and
out of the microclimate.
3.4.3 Adaptive regulation of intestinal
nutrient transport
Many patterns of adaptation fall into one or the other
of two categories: (1) a non-specifi cally increased absorption
of all nutrients, arising ultimately from an
increase in the animal’s overall nutrient requirements,
and with an increase in absorptive surface area as the
primary mechanism, and (2) phenomena involving
the induction or repression of a specifi c transport
mechanism, depending on the dietary availability or
body store of the transported substrate.
Non-specifi c anatomical adaptation to changing
metabolic requirements and food deprivation
Increases in metabolic requirements such as arise
during pregnancy, lactation, growth, exercise and cold
stress are met by an increased absorption of all available
nutrients, mediated at least in part by an induced
increase in food intake (hyperphagia). The increased
38 Vitamins: their role in the human body
absorption is due to an increase in mucosal mass per
unit length of intestine and a consequent increase in
absorptive surface area. Not only is there an increase
in the total number of cells (hyperplasia) but the villi
become taller.
The mammalian intestine adapts to prolonged
food deprivation by dramatically slowing the rate
of epithelial cell production in the crypts in order to
conserve proteins and biosynthetic energy. This effect
on mitosis and enterocyte renewal leads to markedly
shortened villi. Because cell migration along the
crypt/villus axis is also slowed, more cells lining the
villi are functionally mature. Therefore, food deprivation,
by reducing mucosal mass and increasing
the ratio of transporting to nontransporting cells,
effectively increases solute transport per unit mass
of intestine.