Introductory Mammalian Physiology (PHOL0002) Notes - The Gastrointestinal System
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Course
Introductory Mammalian Physiology (PHOL0002)
Institution
University College London (UCL)
Explore Introductory Mammalian Physiology with these specialized notes tailored for Year 1 students at University College London. Immerse yourself in the complexities of the gastrointestinal system, where discussions delve into the intricacies of digestion, absorption, gastric secretions, and the p...
Introduction to Gastrointestinal Physiology
Structure and Function
• Structure of the GI tract and its associated digestive organs
o Hollow tube starting at the oral cavity and ending at the anal canal
o Each part of the GI tract has a specialised function
▪ Exocrine glands – secretions crucial for digestion and absorption
• Salivary glands
• Liver
• Pancreas
▪ Stomach – storage and breakdown of food
▪ Small intestine – digestion and absorption of food
▪ Large intestine – storage and absorption
• General structure of the digestive canal
o
▪ Mucosa
• Made up of a layer of epithelial cells – attached to lamina propria – attached to
muscularis mucosae
▪ Submucosa
• Innervated with submucosal plexus – part of enteric nervous system
• Surrounded by muscularis – made up of circular and longitudinal muscles
o Activation of muscles by myenteric plexus = allows activation and
stimulation of muscles – to allow you to mix and propel food down digestive
tract
▪ Serosa
• Connective tissue that holds together muscularis
o Mucosal differences along the GI tract – mucosal layer changes the most as you move across the GI
tract
▪
• Oesophagus
o Layer of tough epithelial cells
o Multiple layer of cells protects oesophagus from potentially damaging food
that is swallowed that moves down from the mouth to the stomach
,Introduction to Gastrointestinal Physiology
• Stomach
o Single layer of columnar epithelial cells dotted with gastric pits
• Small intestine
o Finger like projections (villi) – epithelial surface
o Enable the efficient absorbance of the different components of our diet
• Large intestine
o Flat in nature
o Made up of a single layer of epithelial cells
▪ Involved in absorption of water and electrolytes
o Dotted with goblet cells = mucus secreting cells
▪ Provide mucus to aid in the movement of solid faecal content
o Cell types in the intestinal epithelium
▪ Continuously renewed every three to six days – particularly prone to damage and hypoxia
▪ Originate within the crypts of villi from stem cells
• Stem cells as they move up the crypt become transit amplifying cells = differentiate
into four different cell types → move up through the villi until ejected into the
intestinal lumen
o Absorptive cell
▪ Have microvilli present on the apical surface
o Secretory cells
▪ Goblet cell
• At the apical region of the cytoplasm – full of mucus
secreting granules – when activated = secrete mucus into
intestinal lumen
▪ Enteroendocrine cell
• Packed full of secretory granules on the basal surface of the
cell
o Contain endocrine hormones that are released upon
activation of the cell
▪ Paneth cell
• Packed with secretory granules – lysozyme and antibacterial
agents
o Play a role in defence within the small and large
intestine
• Functions of the GI tract
o Digestion and absorption of nutrients
o Absorption and retention of water and electrolytes
o Elimination of toxins
o Maintenance of barrier function
o Immunological barrier
• Overview of GI function
,Introduction to Gastrointestinal Physiology
o
▪ Oral cavity – secretions
• Salivary glands produce a fluid that is rich in mucus – used in lubrication and
protection of the upper GI tract
• Enzymes required for the initiation of starch digestion
▪ Stomach
• When food is present – stimulates secretions
o HCl = generates low pH in lumen – breaks down contents
o Pepsin enzyme = required for protein digestion
o Intrinsic factor = needed for protecting vitamin B12 for absorption down in
later parts of the small intestine
o Alkaline mucus secretion – for protection of the stomach lumen
• Mixes and breaks up food into the small intestine
▪ Small intestine
• Efficient nutrient digestion and absorption
o Vitamin absorption
o Ion absorption
o Water absorption
▪ Distal small intestine – ileum
• Vitamin B12 absorption
• Bile salt absorption
• Ion and water absorption
▪ Large intestine
• Fine tuning of electrolyte and water absorption
• Daily fluid movements across the GI tract
o
, Introduction to Gastrointestinal Physiology
• Intestinal electrolyte handling: a balance sheet
o
• Splanchnic circulation
o
▪ Represents 20-25% cardiac output
▪ 1.5L/min in fasting state
▪ 3L/min during digestion
▪ Large reservoir – holds 30% blood volume under normal conditions
Control of GI function
• Control of GI function
o 3 different ways
▪ Endocrine control
• Release of hormones from the cells in which they are produced into the circulation
▪ Neurocrine control
• Neurotransmitter release from neurons to act on non-excitatory cells
▪ Paracrine control
• Hormone is released and acts on a local target cell
o Responses
▪ Secretion
▪ Motility
▪ Transport
▪ Release of other hormones
• Gastric acid secretion
o Endocrine, Neurocrine and paracrine mechanisms control gastric acid secretion – control secretion
of HCl
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