Assignment for lecture 7 (digestive system)
The digestive system is a long tube running from mouth to anus, consists of the following
parts: mouth, oesophagus, stomach, small intestine, large intestine, rectum, anus. In addition
two accessory organs add secretions to the digestive tract: the liver and the pancreas. There
are exchanges between blood and the digestive system contents, in two directions: secretion
of the “tools” for digestion (primarily enzymes which break down food into its simplest
components), and absorption of nutrients in the form of amino acids, simple sugars, nucleic
acids and fats. Blood leaving the digestive system does not return immediately to the heart,
but first passes through the hepatic portal vein which takes it to the liver.
Food remains for approximately 1-3 hours in the stomach, and reaches the small intestine
after 7-9 hours and the large intestine after 25-30 hours. Total transit time (mouth to anus)
varies from 30 to 120 hours. The contents are propelled along the digestive tract by a wave of
rhythmic contractions (relaxation ahead of a food bolus, contraction behind it); this is termed
peristalsis.
In the mouth the salivary glands produce saliva, which begins the process of digestion
because it contains an enzyme which is called salivary amylase; this name is derived from the
fact that it breaks down starch to form the disaccharide maltose. There are 20 deciduous teeth
(milk teeth, baby teeth) and 32 adult teeth. The stomach secretes two substances, which are
particularly important in breaking down food; these are acid and the enzyme pepsin, which is
secreted as the inactive precursor pepsinogen. In addition it secretes mucus whose main
function is to prevent the stomach lining from being digested.
The pancreas has two divisions, endocrine (secreting into the blood) and exocrine (secreting
into the small intestine). It has two important functions in digestion: it secretes bicarbonate,
which neutralises stomach acid, and enzymes. These substances are secreted from the
exocrine division. The liver is divided into hexagonal lobules, through which blood travels
from the hepatic portal vein to the central vein.
The small intestine is about 6 metres long in total and is made up of three sections, the
duodenum, the jejunum and the ileum; of these, most of the absorption takes place in the
duodenum and jejunum. The large intestine absorbs water from its contents and the contents
are then known as faeces. The contents are propelled along the large intestine by waves of
contraction known as mass movements.
Although our fluid intake may be low, large movements of water nevertheless take place
across the wall of the digestive tract. On a typical day we secrete 7000 ml of water into the
digestive tract and absorb 8100 ml, leaving about 100 ml of water in the faeces.