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Digestive system summary
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Physiology of the digestive system
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Summary of the Digestive System lectureWe need to digest food into small molecules that can be used as fuel to give us energy or as building
blocks to build and repair our tissues. We ingest food into our gastro-intestinal tract (GIT), a tube
lined with specialised epithelium, covered by a layer of mucous and supplied with digestive juices
produced by glands. The GIT moves food from the mouth to the anus by peristalsis, a concerted
contraction of circular and longitudinal muscles in the GIT wall. Peristalsis is controlled by the
autonomic nervous system. In addition to mechanical breakdown (chewing, churning), food is
broken down chemically by enzymes, into molecules that can be absorbed by the GIT epithelium, to
be distributed throughout the body by the blood. When taken up by the cells new biomolecules are
produced by assimilation, to keep a homeostasis of plasma nutrients and to store for later use.
What cannot be digested will be eliminated as faeces.
A good diet has a balance of carbohydrates, lipids, proteins, vitamins and minerals: nutrients mixed
with good amounts of water and fibre (keeps stools soft by holding on to water). Carbohydrates:
sugars and starch provide glucose the fuel used for aerobic respiration. Starch (chains of glucose
molecules) needs to be broken down into individual glucose molecules by enzymes like amylase
(produced in saliva glands and pancreas). Lipids provide fuel, padding and insulation, but also form
the basic substance of cell membranes (phospholipids), vitamin D and steroid hormones
(cholesterol). Lipids are absorbed into the blood stream after they are broken down into individual
fatty acids by the pancreatic lipase enzymes. Proteins are broken down to shorter peptides by
pepsin in the stomach and then into individual amino acids by pancreatic protease enzymes. The
amino acids can then be absorbed. Vitamins are organic compounds needed for vital cell functions
and absorbed directly (e.g. vitamin C), or synthetized in our body (e.g. vitamin D). Minerals
(inorganic elements) are needed for many proteins and cell function and are absorbed directly.
From the mouth where food is chopped up and chewed, wetted and mixed with amylase in saliva, it
moves by peristalsis through the oesophagus to the stomach, where food is churned and gastric
juice (containing hydrochloric acid and pepsin) is added to form chyme. The gastric epithelium is
constantly renewed and produces an alkaline mucus to prevent gastric juice to dissolve the stomach.
The pyloric valve allows the passage of small quantities of acidic chyme (dependent on its content
e.g. fats slow down) into duodenum (the first 12 inches of the small intestine). In the duodenum
chyme is made alkaline by bicarbonate, secreted by the pancreas, mixed with the digestive enzymes
amylase, lipase and protease from the pancreas and bile from the liver, to digest food to molecules
that can be absorbed. Absorption takes mainly place in the jejunum and ileum that have their
surface area for absorption increased by (micro)villi. In the large intestine or colon most of the
water is extracted and the chyme is turned into faeces, which is stored in the rectum and expelled
by the anus.
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