GI 1 + 2 Flashcards
(24 cards)
What are the key regions of the GI tract and their functions?
Stomach: acid secretion and chemical digestion;
Small intestine: nutrient absorption;
Large intestine: water absorption and feces formation.
What are the main layers in a cross section of the GI tract?
Mucosa (epithelium, lamina propria, muscularis mucosae), Submucosa (contains submucosal plexus), Muscularis (circular and longitudinal muscle), Serosa.
What is the function of the mucosa
Mucosa - mucous epithelium, absorption - lamina propria full of blood vessels and lymph system (nutrients go here) muscularis mucosae - glands which secrete mucus lubricates bowel
What is the function of the submucosal plexus
regulates fluid secretion and absorption
What cells are responsible for acid secretion in the stomach?
Parietal cells, stimulated by histamine (H2) released by Enterochromaffin like (ECL) cells
G cells release gastrin when food is detected in the lumen. Gastrin binds to CCK2 receptors on the enteroendocrine cells which stimulates ECL to release H2
Parasympathetic system releases ACH to M3 which drives acid release
Why wouldn’t muscarinic receptor antagonists be used as an antacid
Would cause systemic effects
What is the function of enteroendocrine cells?
They release histamine to stimulate parietal cells for acid secretion. They can also be stimulated by gastrin from G cells and ACh via M1.
Why is the small intestine efficient in absorption?
It has a large surface area due to villi and microvilli, enzyme-secreting glands, and a rich blood and lymph supply.
What are goblet cells and their function in the colon?
Goblet cells secrete mucus to lubricate the bowel and protect against bacterial infection.
What are the functions of each part of the GI system
Mouth - chews + saliva
Salivary glands - saliva (amylase)
Pharynx- swallows chewed food mixed with saliva (bolus)
Esophagus - moves bolus to stomach
stomach - mixes food with gastric acid and adds pepsin
Liver - makes bile
Pancreas - release bicarbonate to neutralise acid produces enzymes
Gallbladder - stores bile and releases it into small intestine
Small intestine - Digests food and absorbs nutrients
Large intestine - absorbs water and some vitamins and minerals
Anus - lets waste leave body
List the mechanisms of nutrient absorption and the molecules involved.
Fatty acids: simple diffusion; Water: osmosis; Fructose: facilitated diffusion; Amino acids: active transport.
How does the absorption mechanism vary in different parts of the GI tract
Stomach: Acidic due to HCl secretion from parietal cells
Duodenum: Still acidic due to gastric emptying
Small intestine: Neutral pH due to HCO3 secretion from the pancreas (where most drug is absorbed)
Caecum: Acidic due to short-chain fatty acids from microbiota.
How is motility generated and controlled in the bowel?
By interstitial cells of Cajal generating slow waves; ACh stimulates contraction, NO causes relaxation; controlled by enteric nervous system. Bowel moves from proximal to distal end via peristalsis.
Describe the smooth muscle contraction mechanism in the GI tract.
Faecal pellet stimulates EC cells mechanically. They release serotonin which signals motor neurons to release ACH. ACh binds to M3 receptors (Gq) → PLC → IP3 and DAG → Ca2+ release → calmodulin activation → MLCK activation → myosin phosphorylation → contraction.
What can prevent GI contraction
Hycoscine butylbromide (M3 receptor antagonist)
How does nitric oxide cause muscle relaxation in the GI tract?
Faecal pellet triggers downstream activation and on demand synthesis and release of NO → guanylate cyclase, which converts GTP→ cGMP → MLCP inhibits Ca2+ release → myosin dephosphorylation → relaxation.
What maintains the barrier function of the GI tract?
Mucus - acts as a barrier
Occludins - tight junction between epithelial cells allowing movement of only water and ions
Immune system cells (paneth cells) release defensins targetting bacteria viruses and fungi
Plasma cells release IgA
What stimulates mucus secretion in the GI tract?
Acetylcholine via IP3-mediated Ca2+ release; Ach acts on goblet cells to increase mucus production.
Explain sodium and water absorption in the GI tract.
Jejenum and ileum -Na+/glucose or Na+/amino acid co-transporters (SGLT1); oral rehydration therapy
Duodenum and jejunum -Na+/H+ exchanger (NHE3) regulates pH and absorbs sodium
Ileum and colon - parallel Na+/H+ and Cl-/HCO3- exchangers;
Distal colon - ENaC (can be blocked by amiloride).
What is the role of oral rehydration solutions?
They enhance glucose and chloride absorption, which drives sodium and water absorption, critical in diarrhea treatment.
How do drugs like ACh antagonists or drugs that stimulate Cl- release affect the GI tract?
ACh antagonists reduce motility and fluid secretion allowing more time for fluid reabsorption, useful in treating diarrhea.
Cl- stimulators cause release of chlorine and release of water to soften stools
Where is sodium primarily absorbed in the GI tract
Jejenum
Where is chlorine primarily absorbed in the GI tract
Jejenum
Proximal colon
Distal colon
Where is potassium primarily absorbed
Distal colon
