Metabolism

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Digestive Physiology Lecture 1

• Objectives
• Identify digestive system organs their
  functions.
• Outline the structure of the GIT.
• Describe the functional anatomy of the digestive
  tract organs.

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Functional Anatomy of the Digestive System

• Digestive System consists of
• Gastrointestinal Tract (GIT) A series of
  organs extending from mouth to anus, forming a
  passageway for food digestion products
• Accessory Glands Secrete fluids enzymes into
  the lumen of GIT

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Overview of Digestive System FunctionFour Basic
processes

• Digestion
• Mechanical Digestion Food broken into smaller
  pieces to become dissolved and mixed with
  digestive enzymes.
• Chemical Digestion Food broken into smaller
  molecules by enzymes smaller pieces enhance
  digestion.

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Overview of Digestive System Function

• 2 Secretion
• Fluids containing enzymes other fluids
  transported into the lumen.
• 3 Absorption
• Products of digestion absorbed through wall of
  GIT, then into bloodstream
• 4 Motility
• Muscular wall of digestive tract propels its
  contents through the lumen.

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Structure of the Gastrointestinal Tract

• Mucosa Lines the lumen
• Submucosa Underlying connective tissue
• Muscularis Externa Smooth muscle fibres
• Serosa Connective tissue

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Mucosa (3 sublayers)

• Mucous Membrane consists of epithelial cells
  called enterocytes.
• Some cells are absorptive transport nutrients
  etc from lumen to bloodstream.
• Some cells are exocrine, secreting fluids
  enzymes into the lumen eg goblet cells secrete
  mucous.
• Some other cells are endocrine, secreting
  hormones into blood

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Mucosa (3 sublayers)

• Lamina Propria Connective tissue containing
  small blood vessels, nerves lymphatic vessels
  that communicate with larger nerves vessels in
  deeper layers.
• Also contains lymphoid tissue eg lymph nodes
  which protect against bacteria (GIT contains many
  bacteria).

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Mucosa (3 sublayers)

• Muscularis Mucosae
• Thin layer of longitudinal circular smooth
  muscle.
• Contracts mucosa into folds, stirring contents
  of lumen promoting contact between contents
  mucosa.

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Submucosa

• Thick layer of connective tissue - Elastic
• Tolerates stretch.
• Contains many larger blood lymphatic vessels.
• Contains the submucosal plexus (the enteric
  nervous system ENS).
• ENS influences muscle, exocrine endocrine cell
  function in the GIT.
• ENS can operate independently, but may be
  influenced by input from autonomic nerves
  sensory neurons located in GIT wall.

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Muscularis Externa

• Inner layer of circular smooth muscle outer
  layer of longitudinal smooth muscle ? GIT
  motility.
• Motility mixes food with digestive enzymes
  promotes contact between products of digestion
  mucosa (necessary for absorption).

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Serosa

• Inner layer of fibrous connective tissue
  (structural support) outer layer of epithelial
  tissue (mesothelium), which secretes a
  lubricating fluid, allowing organs to slide past
  each other.
• The mesothelium is continuous with mesenteries,
  which are continuous with the peritoneum, which
  lines the abdominal cavity.

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The Stomach

• 4 regions cardia, fundus, body, pylorus
• Pyloric sphincter regulates stomach emptying,
  prevents digestive absorptive capacity of
  duodenum being overwhelmed by chyme.

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Digestion in the Stomach

• The stomach
• Holds ingested food
• Degrades this food both physically and chemically
  (e.g., pepsin digests proteins)
• Delivers chyme to the small intestine
• Secretes intrinsic factor required for absorption
  of vitamin B12

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The Stomach

• Gastric glands at base of gastric pits secrete
  gastric juice.
• Chief Cells secrete pepsinogen pre-curser of
  pepsin.
• Parietal Cells secrete H (acidifies stomach)
  intrinsic factor (essential for absorption of
  Vitamin B12 by ileum).
• G Cells secrete Gastrin (hormone - regulates acid
  secretion)
• Neck Cells secrete mucous - protects gastric
  epithelium from HCl. (Gastric Mucosal Barrier)
• Epithelial Cells secrete bicarbonate which
  neutralises acidity near stomach lining

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Small Intestine

• 3 Regions
• Duodenum 0.3 m
• Jejunum 1 m
• Ileum 1.5 m

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Small Intestine

• Duodenum Receives digestive enzymes
  bicarbonate ions (pancreas) bile (liver)
• Absorbs most nutrients
• Villi - folds in mucosal surface Brush border
  Microvilli on apical surface (3000-6000 per
  cell) ? SA
• Villi microvilli less abundant towards ileum.

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Small Intestine

• Circulation
• Hepatic portal vein conveys nutrients from
  intestinal capillaries to liver for reprocessing.

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Motility of Small Intestine

• Chyme in small intestine stimulates brief
  peristaltic waves, which alternate with longer
  periods of segmentation
• Influenced by distention extrinsic nerves
  hormones
• Moderate distention increases contractile force
  ?Propels chyme ? reduces distention
• Gastrin stimulates motility in ileum relaxes
  ileocecal sphincter

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Motility of Small Intestine
Peristalsis
Segmentation
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Motility of the Small Intestine

• Intestino-Intestinal Reflex
• Severe distention or injury to small intestine ?
  small intestine contractions ? protects injury
  from stretching
• Ileogastric Reflex
• Distention of ileum ? gastric motility
• Gastroileal Reflex
• Chyme in stomach ? motility of ileum

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Large intestine
Cecum ? ascending colon ? transverse colon ?
descending colon ? sigmoid colon ?
rectum Absorption of water ions, Storage
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Large Intestine

• Ileocecal Sphincter Regulates flow of material
  from ileum to colon.
• Cecum Blind-ended pouch below junction of small
  intestine colon
• Vermiform (wormlike) Appendix An appendage with
  no known function

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Colon

• Teniae coli Longitudinal muscle of muscularis
  externa compressed into 3 narrow bands and form
  the colon into a series of pouches (haustra)

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• Rectum Anus
• Rectum stores faeces until both the internal
  anal sphincter (smooth muscle) and the external
  anal sphincter (Skeletal muscle) relax,
  permitting defecation.

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Motility of large intestine Haustral churning
haustra relaxed distended ? wall contract ?
squeeze content to next haustra. Peristalsis
slower rate (3-12 / min) than proximal
portion. Mass Peristalsis a strong peristaltic
wave from middle transverse colon ? quickly drive
content towards rectum (3-4 times a day during /
after meal).