Intro to GI Physiology Flashcards
What is the GI tract also called?
Alimentary canal
What is the GI tract? What is it lined by?
It is a muscular tube, lined by an epithelium
Where does the GI tract extend from and to?
From the oral cavity to anus (passes through pharynx, oesophagus, stomach, and small and large intestines)
How long is the GI tract?
About 8 or 9 metres
How long is the small/large bowel?
Small - 6m Large - 1.5m
What are the 6 main functions of the GI tract?
- Ingestion 2. Mechanical processing 3. Digestion 4. Secretion 5. Absorption 6. Excretion
What is ingestion?
Materials enter digestive tract via the mouth
What is mechanical processing?
Crushing and shearing, wetting and softening
What is purpose of mechanical processing?
Makes materials easier to propel along digestive tract
What is digestion?
The chemical breakdown of food into small organic fragments for absorption by digestive epithelium
What is secretion?
Is the release of water, acids, enzymes, buffers, and salts by epithelium of digestive tract or by glandular organs
What is absorption?
Movement of organic substrates, electrolytes, vitamins, and water across digestive epithelium into interstitial fluid of digestive tract
What is excretion?
Removal of indigestible material and waste products from body fluids
What is the peritoneum?
Serous membrane forming the lining of the abdominal cavity (parietal peritoneum) Covers organ (visceral peritoneum)
What does the peritoneum form?
Mesenteries
What are mesenteries?
An organ that attaches the intestines to the posterior abdominal wall in humans and is formed by the double fold of peritoneum. Suspend the organs and keep them from tangling
What does the peritoneum secrete?
Peritoneal fluid
What is function of peritoneal fluid?
Provides lubrication and permits organs to move against each other without friction
What is function of mesenteries?
Helps in storing fat (fat protects organs) and carries blood vessels, lymphatics, and nerves to supply the intestines
Where does the blood supply to the GI tract in the abdomen come from?
3 branches of the (abdominal) aorta: - Coeliac artery - Superior mesenteric artery (SMA) - Inferior mesenteric artery (IMA)
What does the coeliac artery then supply?
Stomach Spleen Gall bladder Pancreas
What does the coeliac artery also branch off to form?
Hepatic artery which supplies the liver
What does the SMA supply?
Pancreas, small intestine, large intestine
What does the IMA supply?
Large intestine, rectum
Where does deoxygenated blood from the GI largely drain into?
Hepatic portal vein
What carries absorbed nutrients to the liver for processing?
Hepatic portal vein
What are the 4 main layers of the GI tract? Innermost to outermost?
- Mucosa (innermost) 2. Submucosa 3. Muscularis externa 4. Serosa (outermost)
What is purpose of villi?
Increases surface area for absorption
What conditions does flattening of villi cause?
Coeliac
What are epithelia?
Layers of cells covering internal or external surfaces
How can sheet of epithelial cells form an epithelial tube/gland?
Invagination of epithelial sheet by organised tightening of adhesion belts in selected regions of cell sheet Epithelial tube pinches off from overlying sheet of cells
What are glands?
Structures that produce secretions
What are the 4 types of junctions between cells (moving from apical to basal)?
- Occluding junction 2. Cell-cell anchoring junctions 3. Channel-forming junctions 4. Cell-matrix anchoring junctions
What is other name for occluding junctions? What is their function?
Tight junctions seals gap between epithelial cells –> prevent leakage of transported solutes and water
What is function of cell-cell anchoring junctions / adherens junction?
Anchoring junctions are protein complexes that mediate the adhesion of cells to other cells or to the extracellular matrix
- Stabilise the cells position, provide stability and rigidity, and support tissue integrity by holding cell sheets together.
- Connect actin filament bundle in one cell with that in the next cell
What are channel-forming junctions?
Gap junction allows the passage of small water-soluble molecules from cell to cell
What are cell-matrix anchoring junctions?
Actin linked cell-matrix adhesion anchors actin filaments in cell to extracellular matrix
What junctions are of particular significance for epithelial cells?
The tight junctions near the apical surface: it is these which define the apical and basolateral domain, and stop proteins from these domains mixing
Describe epithelial organisation in organ such as gut/kidney
Epithelia are usually closely associated with a capillary network, which supplies O2 and nutrients and removes CO2 and metabolites. The interstitial fluid surrounding the capillaries is in contact with the basolateral (serosal) membrane of epithelial cells. The domain of the plasma membane in contact with the external compartment is called the apical membrane. This segration of the cell into functionally discrete domains is crucial to the process of vectorial (directed) movement of solutes and water.
What is function of tight junctions?
Prevents diffusion of plasma membrane proteins (e.g. receptors, channels and transporters) between apical and basolateral domains
Solutes and water can travel in 2 directions across epithelia (absorption and secretion). Describe direction of absorption
From external compartment (gut lumen) to internal compartment (blood and lymph) E.g. glucose absorption in gut and renal tube
Describe direction of secretion
From internal compartment to external compartment Good examples of this are protons (H+) and potassium in the renal tubule and HCO3- which is secreted by the pancreas.
What is equation for net flux?
Net flux (Jnet) = Jabs - Jsec
Where can absorption take place?
From lumen to blood: - via the cells (transcellular route) in which case, two barriers (apical and basolateral membranes) must be traversed - in between the cells (paracellular route)
How can secretion take place?
- Transcellular secretion 2. Paracellular secretion
What route must active transport take?
Transcellular route
What routes can passive transport take?
Paracellular or transcellular
What is 1ary active transport?
- Energy to drive transport comes directly from ATP - Substrate moves against an electrochemical gradient
What is 2ary active transport?
- 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
GI tract blood supply
Sugar absorption
What is active transport of solute usually followed by?
Passive transport of water (osmosis)
Therefore, if there is an accumulation of osmotically active solutes in a compartment, water will tend to move so as to equalise the osmotic gradient between those compartments.
What is effect of parasympathetic impulses on GI?
‘Rest and digest’ –> promote digestive processes
The vagus controls foregut and midgut derivatives, while pelvic splanchnic nerves supply hindgut components.
What is effect of sympatehtic impulses on GI?
Depress digestive acitvity
What is the ‘enteric nervous system’?
- Much of the activity of the muscle in the gut wall is locally regulated and controlled, by a complex set of nerves lying between the muscle laters, and underneath the submucosa.
- Controls much of the normal gut motility essentially autonomously
- Parasympathetic and sympathetic nerves allow some degree of central control
- Communication withina and between plexi (myenteric and submucosal plexi)
Extrinsic nerve supply
What are interstitial cells of Cajal? Where are they located?
Located within the muscle layers of the alimentary tract that mediate communication between the autonomic nervous system and smooth muscle –> mediators of slow waves
What are slow waves?
Periodic shifts (depolarisations) in the resting membrane potential of the cells
What is purpose of slow waves?
This makes it easier for a small signal to initiate a contraction and/or an action potential. In practice, what this means is that neural and hormonal signals will initiate activity only during a slow wave: thus the slow waves give an underlying structure to the activity of the smooth muscle.
Are epithelial cells polarised?
Yes - Polarized cells can target proteins to discrete cell domains (allowing vectorial transport in or out)
