Lecture 2

• GI tract is msucular tube lined by epithelium, extends from oral cavvity to anus ->

  passes through:

  pharynx, oesophagus, stomach, small intest, colon, rectum and anus
• Functions of the GI tract

  • Ingestion
  • Mechanical processing
  • Digestion
  • Secretion
  • Absorption
  • Excretion
• The functions of the gastrointestinal (GI) tract include:
• Peritoneum:

  1. lines abdominal cavity (parietal peritoneum)
  2. covers organs (visceral peritoneum)
  3. forms mesenteries which suspend organs and support them and keep them from tangling
  4. secretes perotpneal fluid which provides lubrication and permits organs to move against each other w/o friction
• mesenteries:

  1. suspend organs and stop loops of gut getting tangled around each other
  2. carry blood vessels, nerves and lymphatics
• blood supply to GI tract in abdomen comes from 3 branches of aorta

  1. foregut coeliac trunk (T12)
  2. midgut superior mesentric artery (L1)
  3. hindgut inferior mesentric artery (L3)
• boundaries between supplied areas reflecs embryological development, has pathological and surgical importance
• blood largerly drains into hepatic portal vein carries absorbed nutrients to liver for processing

  liver also has direct supply via hepatic artery which is its main oxygen supply
• flow diagram of GI tract blood supply

  
• general structure of the tube, from innermost to outermost:

  1. mucosa
  2. submucosa
  3. muscularis externa
  4. serosa
• Epithelia = layers cell covering internal/external surfaces

  
• glands = structures produce secretions

  
• Epithelia have different types of intercellular junctions, such as tight junctions, adhering junctions and gap junctions
• The epithelial organisation:

  A) capillary

  B) interstitial fluid

  C) epithelial cell

  D) basolateral/serosal membrane

  E) tight junction

  F) apical (luminal/mucosal) membrane

  G) luminal/mucosal fluid

  
• The tight junctions within the epithelial prevents diffusion of plasma membrane proteins between apical and basolateral membrane
• absorption = from external compartment (gut lumen) to internal compartment (blood and lymph)

  secretion = from internal compartment to external compartment
• Example of absorption include glucose absorption in the gut and renal tubule

  example of secretion include proton and potassium in the renal tubule and hydrogen carbonate which is secreted by pancreas
• Flux = movement
• Uni-direction fluxes of a solute, such as water, can occur in both the absorptive and secretory direction which is dictated by prevailing electrochemical gradients
• Flux in both directions

  
• J(net) = J(abs) - J(sec)
• Transepithelial pathways

  A) transcellular absorption

  B) transcellular secretion

  C) paracellular secretion or absorption

  D) lumen

  E) capillary

  F) epithelial cell

  
• Absorption takes place from lumen to blood

    It can proceed:

     (i) via the cells (transcellular route) in which case, two barriers (apical and basolateral membranes) must be traversed

     (ii) in between the cells (paracellular route)
• Secretion takes place from blood to lumen via either transcellular or paracellular pathways
• Passive transport:

  • Non coupled - solute moves down its electrochemical gradient.
  • Can occur via pores or channels e.g. diffusion of ions, H2O
  • Transporters – facilitate diffusion of small non electrolytes

  can either be transcellular or paracellular
• Active Transport:

  • Primary = energy to drive transport comes directly from ATP, substrate moves against an electrochemical gradient
  • Secondary = energy to drive active transport comes from electrochemical gradient for one substrate (set up by primary active transport), second substrate moves against its electrochemical gradient
  • only transcelluar route
• water follows solutes (usually): low osmotic pressure to high osmotic pressure through paracellular or transcellular crosses plasma membranes either by diffusing through lipid or via aquaporins
• net solute movement will tend to drive net fluid movement
• The enteric nervous system:

  parasympthetic and sympathetic divisions of the ANS regulare digestive tract.

  autonomic fibres contact neurons in GI tract called ENS which is considered 3rd division of the ANS
• parasympathetic = digestive processes

  sympathetic = depress digestive activity
• ENS structure

  Myenteric & submucosal plexi

  Communication within & between plexi

  Processes extend to target cells

  Modulated by input from ANS

  
• Extrinsic nerve supply:

  •Afferent & Efferent information

  •Sympathetic = Thoraco-lumbar & inhibitory

  •Parasympathetic = Cranio-sacral & stimulatory

  
• Interstitial cells of Cajal are mediators of slow waves:

  Connections with each other & SM cells

  3-12 min-1