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Summary Animal Nutrition and Physiology
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Animal nutrition and physiologyANU30806
,1. GIT anatomy
1.1 GIT functions
The main function of the GIT is to break down foods and absorb the broken down nutrients. This is done
mechanically to enlarge the surface area of the food, in preparation of chemical breakdown by
enzymes. This ensures that absorption is possible through the intestinal wall. The last function is to
excrete waste.
1.2 GIT wall
Starting from the lumen (inside), the GIT wall is composed of a number of structures. This is similar for
almost all structures in the GIT, but there are some differences related to function. It is an essential
barrier to the outside world, protecting from toxins, pathogens, abrasion and digestive juices. However,
they also facilitate absorption and secretion, making them selective permeable.
- The mucosa forms a selective permeable barrier between the gut lumen and body tissues. It
consists of a single layer of epithelial cells, with mucus glands in between them, covered by loose
connective tissue, the lamina propria, and a thin muscle layer called the muscularis mucosae.
Throughout the GIT, enteroendocrine cells (EECs) are present, playing roles through nutrient
sensing and signalling.
- The submucosa is a layer of connective tissue that contains blood vessels, collagen elastic and nerve
fibres, with the latter being called the submucosal plexus.
- The muscularis propria consists of an inner layer of circular muscle fibres and an outer layer of
longitudinal cells, to be able to make peristaltic movements. Between the layers is the myenteric
plexus which are nerve fibres. However, the upper third part of the oesophagus consists of striated
muscle, as well as the anal sphincter and the stomach contains three layers of muscle fibres.
- The serosa wraps the intestine and connects it to the mesentery, which holds the abdominal cavity
in place. Through the mesentery run blood and lymph vessels, as well as nerves to access the GIT
- Mucus is produced all along the GIT to protect the GIT wall against toxins, pathogens, physical
friction and chemical digestion. In addition, it also lubricates food to facilitate transport. In the
upper part of the GIT, it is rich in bicarbonate (HCO3-), to neutralize the acids from the stomach,
and in the lower part, it is rich in antimicrobial peptides to protect against ingested pathogens.
Mucus is built up of mucins, which are highly glycosylated proteins containing serine, proline and
threonine. They are resistant to digestion because of their long polypeptide chains. In the intestine,
an additional layer of glycoproteins and lipids called the glycocalyx protects and aids in digestion
and absorption.
The GIT does not digest itself due to regulation of e.g. secretion and motility, because many enzymes
are stored as inactive pro-enzymes, the GIT barrier with its non-digestible mucus coat and due to a high
cell turnover rate.
,1.3 Mouth, pharynx and oesophagus
The mouth is where food is masticated and lubricated by saliva, which is secreted by salivatory glands.
It consists of water, mucins, HCO3-, amylase (starch digestion enzyme), lipase (fat digestion enzyme) and
antimicrobial compounds. When food is swallowed, it enters the pharynx, which directs the food bolus
to the oesophagus by the epiglottis. The soft palate and the uvula prevent the bolus from entering the
nasopharynx and nasal cavity. When swallowed, it enters the oropharynx followed by the
laryngopharynx and then the oesophagus. The oesophagus transports food to the stomach. It contains
an upper and a lower (cardia) sphincter. The pharyngeal parts of the oesophagus are lined with stratified
squamous epithelium to protect against extreme temperatures.
1.4 The stomach
The food enters through the lower oesophagus into the cardia of the stomach. In this organ, food is
mixed, stored and partially digested. The stomach is divided in a non-functional cardiac region, and a
functional oxyntic (fundic) region including the fundus and the corpus (body); and the pyloric region
including the pyloric antrum. It can also be functionally divided in a proximal stomach which acts as a
reservoir and a distal stomach which is mainly involved in mixing and churning
.
, The mucosa is folded in gastric folds. The cardiac region is covered with stratified squamous epithelium
which partly also covers the fundum in horses. The rest is covered with columnar epithelium facilitating
gastric secretion. Gastric pits in this epithelium connect to the lamina propria and secrete the gastric
juices. These juices consist of water, mucins, enzymes, electrolytes and neuricriene, endocriene and
paracrine agents. Due to high concentrations of HCl, gastric juice is very acideous (pH < 2). Mucus glands
are present in all regions of the stomach and are called foveolar cells. Chief and pariental cells are found
mainly in the glands of the oxyntic region. Chief cells synthesize and secrete pepsinogen and gastric
lipase, and some other species produce chymosin which aids in the digestion of milk in young animals.
Pariental cells secrete HCl and intrinsic factor and may secrete HCO3- to prevent alkalinization of the
cells. HCl promotes the conversion of proenzyme pepsinogen to active pepsin which denatures proteins
and HCl destroys ingested bacteria. Intrinsic factor is a glycoprotein which binds to vitamin B12 which is
essential to its absorption. EECs are also hound in the glands of the stomach, where enterochromaffin
cells (ECs) secrete serotonin, adrenomedullin and antrial natriuretic peptide (ANP), with the latter
reducing the expanded extracellular volme by incresing renal sodium excretion. Enterochromaffin-like
cells (ELCs) secrete histamine. D cells in the oxyntic and pyloric region secrete somatostatine (growth
and insulin hormone) and amylin (hormone slowing gastric emptying and therefore promoting satiety).
G cells in the pyloric region secrete gastrin, which stimulates HCl secretion by pariental cells and aids in
gastric motility. The stomach contains a third muscle layer; the oblique layer, which helps to churn and
mix the food, mechanically breaking it down. Depeing on duodenal signals and the physiochemical
characteristics of the food, food is retained in the stomach for minites to 8-10 hours. Solids are retained
longer than liquids. When food is mixed with gastric juices, water etc. it is called chyme and is forced
through the pyloric sphincter to the duodenum.
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