Lecture 1 Digestion 1 Flashcards
Salivary glands
3 pairs:
Parotid: One Infront of each ear either side of face
Sublingual: below tongue
Submandibular: under jaw
Pharynx
Membrane lined cavity behind nose and mouth connecting to oesophagus
GI tract
Same general structure from oesoph to anus w/ 4 major tissue layers. (Innermost) Mucosa, submucosa, Muscularis externa and serosa (outermost)
Mucosa - 1st layer: mucous membrane
Serves as protective surface. Modified for secretion/ absorption contains exocrine/ endocrine gland cells & epithelial cells
Mucosa 2nd layer: Lamina Propia
Houses gut-associated lymphoid tissue (GALT) important defence against disease causing intestinal bacteria.
Mucosa: 3rd layer: Muscularis mucosa
Sparse layer of smooth muscle
Submucosa
Thick layer of connective tissue provides digestive tract w/ distensibility and elasticity. Contains larger blood and lymph vessels. Contains nerve network known as submucosal or Meissner’s plexus
Muscularis externa
Contraction activity produces propulsion and mixing movement. Major smooth muscle coat of digestive tube usually 2 layers:
Circular: inner, contraction decreases diameter of lumen
Longitudinal: outer, contraction shortens tube. Myenteric (Auerbach’s) plexus Lise between the 2 layers
Serosa
Secretes serous fluid, lubricates and prevents friction between digestive organs and surrounding viscera. Continuous w/mesentery attachment provides relative fixation, supports digestive organs in place but w/enough freedom for propulsion and mixing
Nervous innervation
Enteric: intrinsic - myenteric plexus between the two layers of muscularis externa and in the submucosa the submucous/Meissner’s plexus. Motor, secretory and sensory neurones
Extrinsic: autonomic - controls parasympathetic (cholinergic - relaxed) and sympathetic (adrenergic-alert) activity.
Vili
Total SA of small intestines 300m2 epithelial layer shed and replaced every 5 days. Absorption via capillaries, fatty acids and glycerol absorbed into lacteals (inside villi).
Single layer epithelium (stomach onwards)
Luminal tight junctions, exo&endocrine cells invaginations form exocrine glands
Secretions
Salivary: salt water, mucous, amylase - lubricates, digests polysacc
Tongue: lingual lipase, IgA and lysozyme.
Oesoph & pharynx: mucous - lubricates
Stomach: HCl, pepsin(ogen) mucous HCO3- - solubilise food, protein digestion and protection
Pancreas: enzymes, HCO3- - digest fat, CHO, protein, neutralise chyme
Liver: Bile salts, HCO3- & waste products - solubilise fat pH adj, removal of toxic substances.
Gallbladder: store and conc. bile - used in fat digestion
Small intestine: enzymes, salt, H2O & mucous - digest + maintain fluidity
Large intestine: mucous - lubrication
Rectum - defecation
Movement of food/ fluid throughout the day
1500ml saliva, 2000ml gastric secretion, 500ml bile, 1500ml pancreatic secretion, 1500ml intestinal secretion + 1200ml fluid intake of which 8.1l is reabsorbed and 100ml excreted as feces
Food type/ absorbed as
Carb/polysaccharide
Protein/amino acids (peptides)
Fat/ glycerol and fatty acids
Water/vits/ions directly absorbed
Carb absorption
Starch > polysaccharides > simple sugars. Glucose and galactose transferred w/ Na co transportation, secondary active transport. Fructose by diffusion.
Protein digestion
Stomach: pepsin hydrolyses bonds between amino acids to polypeps of various sizes
Small intestine:
endopeptidase- (pancreatic trypsin and chymotrypsin) break peptide bonds within protein molecules
exopeptidase - cleaves peptide bonds at terminals of protein molecules (pancreatic carboxy/carboxy/brush border aminopeptidases)
Dipeptidase- secreted on brush border of villi cleave dipeptides
> Many proteolytic enzymes, excreted in inactive form to prevent self digestion and activated at point of function
Digestion of fats (revisit needed)
Mouth: lingual lipase (also in stomach) digests 30% of lipid
Stomach: lingual lipase still active, gastric lipase (not v. Important)
duodenum: pancreatic lipase most important triglycerides > monoglyceride+ 2 fatty acids.
Bile salts act as detergents to emulsify fats globules and w/phospholipids form emulsion droplets increasing SA so that lipases can break them down to micelles. Monoglyceride and fatty acids from micelles diffuse into villi. Triglyceride reformed in smooth ER and chylomicron sent to lacteal
Fat absorption summary
Monoglycerides and fatty acids diffuse into enterocyte ( a cell of the intestinal lining.) Triglycerides resynth in smooth ER . Droplets coated in emulsifying agent secreted by exocytosis as cholomicrons into interstitial water diffuse into lacteals of lymph cannot enter capillaries as too large
Sodium & water absorption
Water: passive, follows solute to maintain osmotic equilibrium Na- Cl- & HCO3- follow Na+ absorption absorbed w/glucose (coabsorption)
Water soluble vits absorb by diffusion or mediated transport except B12
Fat soluble vits (ADEK) follow pathway for fat absorption
B12 absorption
vitamin B12 is absorbed from food in a two-step process. First, hydrochloric acid in the stomach separates vitamin B12 from the protein that it’s attached to. Second, the freed vitamin B12 then combines with a protein made by the stomach, called intrinsic factor, and the body absorbs them together.
Dysbiosis
Imbalance of microbial communities in the gut linked with disease when imbalance disturbs functions needed for health or introduces processes that promote disease e.v. loss of complex community of anaerobes in adult gut microbiome results in lower overall microbial diversity and increased facultative anaerobes. Low gut diversity can lead to Eubiosis c. Dificile infection IBS and liver disease
how emulsification speeds up fat absorption
Fats are hydrophobic and thus form large globules that globular enzymes can only act on the surface of - so digestion would be v. slow
Bile is amphipathic and contains phospholipids and bile salts. It causes fat droplets to break down into smaller emulsion droplets
this process increases the SA which enzymes can act on increasing the speed of fat digestion
effect of gallstones
Bile is made by the liver and stored in the gall bladder. If gall stones block the bile duct preventing bile from entering the GI tract then fat digestion will not take place efficiently
Can get bile duct inflammation, pain, infection and pancreatitis
Respiratory and conduction zone
