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Wales College of Medicine, Biology, Life and
Health Sciences
School of Nursing and Midwifery Studies
DIGESTION
Most foods are complex compounds that cannot be utilised by the body
in their original form. After ingestion, the process of digestion enables
these complex structures to be broken down into their more easily
metabolised molecules. While this session mainly deals with the fate of
carbohydrates, proteins and fats, do not forget the dietary importance of
fluids, vitamins and minerals.
Mechanical processes assisting in digestion.
Following ingestion, food is ground down by the teeth (mastication) and mixed with
saliva by the tongue. The food is formed into a ball (bolus) before being pushed
across the roof of the mouth to the pharynx to be swallowed (deglutition). Muscular
waves (peristalsis) within the walls of the oesophagus move the bolus on to the
stomach, entering via a muscle ‘ring’ (cardiac sphincter).
The longitudinal, circular and oblique muscles within the stomach wall churn the
food, mixing in the gastric juices, until liquid chyme is released via a second muscle
‘ring’ (pyloric sphincter).
FUNDUS
Relatively thin muscle wall providing gentle contractions
BODY
ANTRUM
30ml capacity. Each relaxation of the pyloric sphincter ejects just 3 ml
chyme. The remaining 27 ml is ‘thrown back’ for further mixing. With
approximately 3 contractions per minute, the stomach will usually
empty in 4 hours (6 hours with a high fat content).
The small intestine continues to mix chyme via localised contraction and relaxation of
adjacent segments (segmentation). Digestion and absorption are maximised by overall
gut length and the presence of structural modifications that increase surface area:
 circular folds creating a slow ‘spiral’ movement of chyme along the lumen,
 villi,
 Microvilli.
The gastro-intestinal tract is prepared for its role in digestion by both nervous and
hormonal control. The thought, sight or smell of food triggers parasympathetic
stimulation of salivary glands and gastric gland pits, resulting in early production of
both saliva (up to 1.5 litres daily) and gastric juices (2-3 litres daily). This is further
enhanced by the presence and/or taste of food in the mouth, the latter triggering
production of the hormone gastrin from endocrine tissue within the stomach lining.
Digestion and the mouth
Parotid (25%) – secretion mostly
water
Sublingual (5%) – secretion mostly
mucous
Submandibular – (70%) – contains
water and mucous.
Consisting of up to 99.5% water,
saliva keeps the mouth moist. This
helps
prevent
infection
and
ulceration, aided by a protective
bacterial flora, bacteriostatic enzymes and antibodies. It contains the electrolytes
sodium, potassium and chloride and, importantly, a high concentration of bicarbonate
to neutralise the acids produced by the flora. This neutralisation also minimises tooth
decay while calcium salts prevent passive dissolution of tooth calcium.
Mucous, containing mucin (a water-soluble protein), aids in the swallowing of food.
Sympathetic induced vasoconstriction to salivary glands may reduce the water content
of saliva, increasing the mucin concentration and causing breath odour (halitosis).
Digestion is limited in the mouth with salivary amylase (pH 6.35-6.85) commencing
the breakdown of starch (polysaccharide). Recent suggestions are that a salivary
lipase is also present.
Digestion and the stomach
Apart from water, the major component of gastric
juice is hydrochloric acid (HCl). Secreted by the
parietal cells, along with the polypeptide intrinsic
factor, it creates a stomach pH <2. Acid production
is also influenced by levels of histamine released by
ECL cells. To protect itself against the low pH,
gastric juice also produces mucous (from goblet
cells) and bicarbonate (from surface epithelial cells).
Stomach activity is influenced by other hormonal secretions. Serotonin (also from
ECL cells) increases stomach contraction while somatostatin inhibits stomach
emptying. Once in the stomach, chyme is subjected to the many influences of gastric
juice. HCl has a range of functions including:
 ‘denatures’ proteins – weak bonds creating tertiary structures are weakened;
 activates pepsinogen, an inactive (….ogen) proteolytic enzyme stored in, and
secreted by, the chief cells;
 deactivates salivary amylase remaining in the chyme.
Active pepsin breaks down the now redundant salivary amylase and also commences
the breakdown of ingested proteins into smaller polypeptides. The intrinsic factor is
needed to ‘mark’ vitamin B12 for absorption in the terminal ileum.
Digestion and the gut
The microvilli, projections of the mucosal cells often referred to as the brush border,
contain many digestive enzymes […ase]. Goblet cells, secreting mucous, are found
throughout its length though they too are more specialised in the duodenum. Here they
are referred to as Brunner’s glands and secrete a thick, alkaline mucous to combat the
acid chyme from the stomach. In addition enteroendocrine cells secrete controlling
hormones while Peyer’s patches (lymphoid tissue) add a defence against ingested
organisms. Between the villi are deep gland pits (crypts of Lieberkuhn) secreting a
watery intestinal juice. The base of the crypts is the site for the formation of the goblet,
endocrine and lining epithelial cells. These gradually move to the tips of the villi
before being worn away. Thus, the lining is replaced every 5-6 days. The intestinal
juice therefore contains small amounts of mucous, hormones and enzymes from the
cell remnants.
In the duodenum, the presence of fats and/or proteins in the chyme triggers the release
of the intestinal hormones cholycystokinin/cck (increases gall bladder contraction and
pancreatic enzyme production) and secretin (bile and pancreatic juice). Bile and
pancreatic juices (up to 1.5 Litres each daily) are released into the duodenum via the
ampulla of Vater, ‘controlled’ by the sphincter of Oddi. Bile and pancreatic juice is
mixed with the chyme by segmentation.
Most fats (triglycerides) are too complex to allow easy digestion. Bile, released from
the gall bladder, acts as an emulsifier (saponification) – a similar action to detergent to split the fats into smaller droplets. This increases the surface area of the fats,
allowing the fat enzymes to work more effectively. Pancreatic lipase breaks these
down into fatty acids and monoglycerides.
Pancreatic juice is highly alkaline (pH 8), further ‘buffering’ the acid chyme and
inactivating pepsin. As well as triggering the release of cck and secretin, the presence
of food in the duodenum also stimulates the intestinal wall to secrete the enzyme
enterokinase. As previously seen in the stomach, all protein enzymes are stored and
secreted in an inactive form to prevent autodigestion of the body’s own organs.
Enterokinase is the activator of the pancreatic protein enzymes. Initially trypsinogen
is activated to trypsin which in turn activates chymotrysinogen to chymotrypsin.
Together, they break down the peptide bonds (endopeptidases), within the protein
chains, to form smaller peptides. Additionally, carboxypeptidase strips amino acids
from the end of the chains (aminopeptidase). Similarly, aminopeptidases within the
intestinal wall complete the breakdown into smaller amino acids, dipeptides and
tripeptides. Ingested DNA and RNA are broken down by nucleases
Carbohydrate digestion continues with more amylase from the pancreas. This
continues the conversion (see below) of polysaccharides to smaller disaccharides
(twin sugars).
Polysaccharides
(i)
Disaccharides
(ii)
Monosaccharides
(Pancreatic Amylase)
maltose
fermentation sugar
(maltase)
glucose
fruit/honey sugar
sucrose
cane/beet sugar
(sucrase)
fructose
fruit sugar
+
glucose
lactose
milk sugar
(lactase)
galactose
+
glucose
As nutrients are absorbed across the
intestinal wall they come into contact
with brush border enzymes. Lipase
continues the digestion of monoglycerides
during their absorption into the central
lacteals of the villi. The enzymes maltase,
sucrase and lactase act on their respective
disaccharide (double sugars) to complete
(see above) the final digestion of
carbohydrates
to
monosaccharides
(single/simple sugar).
A.Matthews/07/05