What are the basic processes of the digestive system?
digestion (biochemical breakdown of complex food)
motility (muscular contractions that mix and move food forward in digestive tract)
secretion (digestive juices)
absorption (small monomers and water)
Trace the path of the digestive system.
oral cavity, esophagus, stomach, small intestine, large intestine, rectum
Describe the phyiosological features of the stomach and the small intestine.
stomach: fundus, body, antrum -> have pyloric sphincter: pinches opening to small intestine so chime is released bit by bit
rugae: folds disappear when there is stomach distension
small intestine: duodenum, jejunum, ileum
plica: folds that remain there even when there is distension and have villi and microvilli on it to increase the surface area for max. absorption
each villus has its own artery and vein to absorb nutrients
What is mesentery?
connective tissue that holds organs in place an blood vessels in their places so not everything clumps together when you stand up straight
Describe the layers of tissue in the digestive tract from outside to inside.
serosa: connective tissue + peritoneum
submucosa: gland in submucosa + duct from gland
mucosa: mucous epithelium, basal lamina, muscularis mucosae
muscularis: circular layer and longitudinal layer
What is the mucosa? What are the 3 layers?
lines luminal surface
highly folded surface increasing absorptive areas
- mucous membrane: epithelium
secretion and absorption
exocrine: digestive juices and endocrine: into blood or tissues - lamina propria: thin layer of CT
houses GALT lymphoid tissue for defense of intestinal bacteria - muscularis mucosa: sparse layer of smooth muscle
Describe the epithelial lining of stomach.
highly folded called rugae marked by depressions = gatric pits [des creux]
coiled glands are found under mostly in the fundus and body of stomach
Describe the secretion and mesentery in the serosa.
secretes serous fluid: lubricates and prevents friction between digestive organs and surrounding viscera
mesentery is continuous throughout much of the tract and provides:
attachment for fixation
support to digestive organs in the proper place while having freedome of propulsive movements and mixing
What kind of secretions can be found in the mouth/oral cavity?
-saliva: produced by salivary glands {parotid, submandibular, sublingual} composed mostly of water and electrolytes
moistens food to swallow
rich in bicarbonate buffers since amylase works best at slightly alkaline pH
-amylase (begins CHO digestion by serous cell), mucus (lubrication), lysozyme(antibacterial action)
Describe the esophagus.
flattened in the resting state
lined with stratified squamous epithelium
between pharynx and stomach
has sphincters at both ends:
- pharyngoesophageal sphincter keeps entrance closed so not huge amounts of air enter esophagus
- gastroesophageal sphincter prevents reflux of gastric contents
What are the GI tract contractions?
peristalsis: promotes forward movement
segmental: contractions promote mixing
What is gastric motility?
mixing action in the stomach of both repulsion (back ward) and propulsion (forward)
as peristaltic contractions become stronger: liquid chyme squirts out of pyloric sphincter into duodenum
stomach continues to mix chime while it gradually releases the chyme
Describe the steps in gastric emptying.
- direction of movement of peristaltic contraction (from fundus to antrum)
- peristaltic contraction
- movement of chime squirts through the pyloric sphincter slowly
How does the duodenum control gastric emptying?
- fat: digested and absorbed more slowly than other nutrients
- acid: HCL secretion+highly acidic chime neutralized in duodenum by bicarb secreted by pancreas
* unneutralized acid in duodenum inhibits further gastric emptying - hypertonicity: amino acid and glucose molecules slows gastric emptying
if increase osmolarity of duodenal contents, stomach closes - distension: too much chime in the duodenum inhibits emptying of more gastric contents
What are the three responses of the duodenum to gastric emptying?
fat, hypertonicity, acid, distension trigger:
-neural response enterogastric reflec acts to slow peristaltic activity [smooth muscle contraction]
-hormonal response
release of GIP [gastric inhibitory peptide] act on gastric muscle to decrease peristalsis and slow passage of food into the duodenum
-additional factors
emotions: sadness + fear = decrease motility
anger+ aggression = increase motility
intense pain= inhibit motility
Where do gastric secretions come form? What are the gastric sections?
gastric juices secretions
oxyntic mucosa
lines the body and fundus
pyloric gland area [PGA]
lines the antrum
Describe the phases in gatric empyting.
cephalic
- sensation or thought relayed to brainstem
- response: parasympathetic signals to gastric mucosa
- stimulates gastric juice secretion
gastric
- stomach distension of stomach caused by presence of food
- triggers parasympathetic reflexes that increase secretion of gastric juice
- Gastrin (by endocrine G cells) is a digestive hormone secreted by mucosa in the presence of food to cause increased gastric juice
intestinal phase
-once food enters the small intestine
-stomach activity is inhibited by less gastric juice secretion
-via GIP, CCK and secretin
=oppose gastric juice secretion and gastrin action
=stimulate bile ejection from pancreas to neutralize HCL in duodenum
Where are the gastric pit located?
in the middle portion of the mucosa of the stomach
In the stomach, in the oxynctic mucosa, what are the endocine cells?
mucous cell: secrete mucus to protect against the pepsin and acid (lines)
chief cells: pepsinogen when activated to pepsin and beings protein digestion
parietal cells: HCl
- activates pepsinogen
- denatures proteins
- breaks down connective tissue
- kills microrganisms
- intrinsic factor important for vit B12 absorption
ECL: secretes histamine stimulates parietal cells
In the pyloric gland area, what is the function of the endocrine cells?
G cells: secrete gastrin which stimulates exocrine cells to secrete more gastric juice [high in HCl and pepsinogen]
has mechanism to ensure that when food is in stomach, there is enough juice to digest it = promotes digestion
D cells: produce inhibitory secretions ONLY = somatostatin
act to inhibit parietal cells [HCl], G cells [gastrin that increases the digestive secretion] and ECL cells
What kind of cell types and secretions are there during the gastric phase?
- mucous cells secrete
-mucous - tonic with mucosa irritation to create a physical barrier between lumen and epithelium
-bicarbonate - secreted with mucous to buffer gastric acid to prevent damage to epithelium - parietal cells secrete
-gastric acid [HCl] to activate pepsin and kill bacteria
-intrinsic factor to complexe with vitamin B12 for absorption
as stimulus to release acetylcholine, gastrin, histamine - enterochromaffin like cell (ECL cells) secrete histamine to stimulate acetylcholine and gastrin to stimulate gastric acid secretion
- Chief cells secrete
-pepsinogen
-gastric lipase
to stimulate release of acetylcholine , acid and secretin to digest proteins and fats - D cells secrete somatostatins [inhibitory] to stimulate acid in stomach which inhibits gastric acid secretion
- G cells secrete gastrin to stimulate acetylcholine, peptides and amino acids to stimulate gastric acid secretion
What is the purpose HCl secretions and from where do they originate?
from parietal cells
activates pepsinogen to pepsin by providing acid medium for optimal pepsin activity
breaks down connective tissue and muscle fibres
denatures protein
kills most microorganism ingested with salivary lysozyme as a team
How is HCl formed?
carbon dioxide + water = carbonic acid - dissociates to form H+
bicarbonate - exchange for Cl into parietal cells by open channels
combine H+ (H/K pump) and Cl- (diffuses) move into ducts
What are the three ways HCl cannot enter into the cell?
- luminal membrane gastric mucosal cells are impermeable to H+ so HCl cannot penetrate into cells
- cells are joined by tight junctions so HCl cannot penetrate between them
- mucous coating over the gastric mucosa protects even more.
