Lecture 1 Digestion 1

Question 1

Salivary glands

Answer 1

3 pairs:
Parotid: One Infront of each ear either side of face
Sublingual: below tongue
Submandibular: under jaw

Question 2

Pharynx

Answer 2

Membrane lined cavity behind nose and mouth connecting to oesophagus

Question 3

GI tract

Answer 3

Same general structure from oesoph to anus w/ 4 major tissue layers. (Innermost) Mucosa, submucosa, Muscularis externa and serosa (outermost)

Question 4

Mucosa - 1st layer: mucous membrane

Answer 4

Serves as protective surface. Modified for secretion/ absorption contains exocrine/ endocrine gland cells & epithelial cells

Question 5

Mucosa 2nd layer: Lamina Propia

Answer 5

Houses gut-associated lymphoid tissue (GALT) important defence against disease causing intestinal bacteria.

Question 6

Mucosa: 3rd layer: Muscularis mucosa

Answer 6

Sparse layer of smooth muscle

Question 7

Submucosa

Answer 7

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

Question 8

Muscularis externa

Answer 8

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

Question 9

Serosa

Answer 9

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

Question 10

Nervous innervation

Answer 10

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.

Question 11

Vili

Answer 11

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).

Question 12

Single layer epithelium (stomach onwards)

Answer 12

Luminal tight junctions, exo&endocrine cells invaginations form exocrine glands

Question 13

Secretions

Answer 13

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

Question 14

Movement of food/ fluid throughout the day

Answer 14

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

Question 15

Food type/ absorbed as

Answer 15

Carb/polysaccharide
Protein/amino acids (peptides)
Fat/ glycerol and fatty acids
Water/vits/ions directly absorbed

Question 16

Carb absorption

Answer 16

Starch > polysaccharides > simple sugars. Glucose and galactose transferred w/ Na co transportation, secondary active transport. Fructose by diffusion.

Question 17

Protein digestion

Answer 17

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

Question 18

Digestion of fats (revisit needed)

Answer 18

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

Question 19

Fat absorption summary

Answer 19

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

Question 20

Sodium & water absorption

Answer 20

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

Question 21

B12 absorption

Answer 21

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.

Question 22

Dysbiosis

Answer 22

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

Question 23

how emulsification speeds up fat absorption

Answer 23

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

Question 24

effect of gallstones

Answer 24

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

Question 25

Respiratory and conduction zone