5 - Digestive system I Flashcards
Digestive system
Four basic processes:
- GI tract Approx. 4.5 meters (in living), 9m (at autopsy)
digestion, absorption, motility
- Accessory glands (salivary glands, pancreas, liver, gall bladder)
secretion
digestion
physical and chemical breakdown of food
absorption
transport of digestive end-products in to bloodstream
motility
peristaltic activity of muscle, propelling food along GI tract
secretion
transport of digestive fluids into the GI tract
General structure of the GI tract
Lumen
Mucosa (3 layers: epithelium, lamina propria, muscularis mucosa)
Submucosa (connective tissue, blood + lymphatic vessels)
Muscularis (circular + longitudinal layer)
Serosa (connective tissue)
Nerve plexus = enteric nervous system
Mucosa
• Consists of mucous membrane, lamina propria (connective tissue,
capillaries, nerves, lymphoid tissue) and muscularis mucosae
• Mucous membrane is composed of epithelial cells or enterocytes which include absorptive cells, exocrine cells, goblet cells (secrete mucus) and endocrine cells
Exocrine = secretion of enzymes into a duct directed at target
Endocrine = secretion of hormones into the bloodstream
• Mucus membrane (or Epithelial lining) is replaced approx. every 5 days in a healthy adult
Saliva
• Secreted by salivary glands at 0.75 – 1.5 litres/day (near zero at night)
• Stimulated by autonomic nervous system
• Contains salivary α-amylase (digests glycogen and starch) and lingual lipase
Other functions:-
• Lubrication (mucus)
• Buffering and diluting noxious substances
• Antibiotic action (lysozyme, lactoferrin)
• Taste
• Cleans teeth
• Fluoride, calcium uptake into teeth
Oesophagus
- Muscular tube (~ 25 cm long) connecting pharynx to the stomach
- Upper and lower oesophagal sphincters usually closed, but backflow can occur causing heartburn
- Upper 1/3 has skeletal muscle
- Lower 2/3 has smooth muscle
Stomach
- Can expand from 50 ml to 1-2 litres
- Gastric glands (mucosa) contain parietal cells* (secrete HCl) and chief cells (secrete pepsinogen)
- Can secrete 2 litres HCl/day (10 mM, pH 2!!)
- Pepsinogen (inactive zymogen), becomes pepsin
- Mucus (neck cells) and gastric lipase also secreted
- *Intrinsic factor = glycoprotein required for absorption of vitamin B12 in the ileum
- Rennin coagulates milk
Stomach + nerves
The hormone gastrin and the vagus nerve, trigger the release of pepsinogen and HCl
The vagus nerve is part of the parasympathetic nervous system, involved in rest-and-digest
Mechanism of HCl secretion
H+ ions, made from CO2 and water by carbonic anhydrase(CA), are actively transported into the lumen in exchange for K+ ions
• Bicarbonate ions are exchanged for chloride ions, which diffuse into the lumen
• Nett result is the accumulation of HCl in the lumen
Activation of pepsinogen
- Pepsinogen is not activated until it encounters HCl in the stomach
- The first 44 amino acids of pepsinogen are removed to generate pepsin
- Pepsin can then activate more pepsinogen
- Pepsin exhibits maximal activity at pH 2.0 and is inactive at ≥ pH 6.5
- Pepsin is an endopeptidase that starts to break internal peptide bonds of proteins to generate smaller fragments
- Exopeptidases remove one amino acid at a time from either end of a polypeptide
Activation of chymotrypsinogen
Produced in pancreas and secreted into the duodenum.
trypsin causes cleavage into 2 fragments
chymotrypsin allows release of 2 dipeptides
Small intestine
- Approx. 2.5 - 3 metres long, big SA is optimal for absorption
- First 30 cm = duodenum, then jejenum and ileum
- Duodenum receives chyme from stomach, enzymes from pancreas, and bile from liver & gallbladder
- Responsible for digestion (duodenum) and absorption (all 3 regions) of all nutrients, water, vitamins, minerals
- Crypts of Lieberkuhn secrete copious amounts of bicarbonate-rich fluid to neutralise chyme from the stomach
