25!! Flashcards
Secretion definition
Movement of solutes and water from the body to the lumen
Absorption definition
Movement of solutes and water from the lumen into the body
Endocrine vs exocine ‘secretion?’
- Endocrine “secretions” move into the body (into the blood stream)– thus they are not secreted as such.
- Exocrine secretions produced by epithelia – move into the lumen
3 main components of exocrine secretin’s and their functions
Mucus
- Protection and lubrication
- Aids mechanical digestion
Electrolyte solution (serous fluids)
- Dilutes food & provides optimal pH
- Essential for chemical digestion of food
Digestive enzymes
- Essential for chemical digestion of food
- Aids absorption
LEARN PH AND VOLUME IN BELOW TABKE - key ideas
- everything is around neutral apart from the stomach - relate to levels of biocrbonate - lots of it = not much hydrogen
≈ 8 L/day secreted from 3 L of plasma – therefore _________ is important
Reabsortion is important
How many pairs of salivary glands
3
How many L of fluid do the salivary glands secrete a day? - what is the rate at rest and when stimulated?
Produce 1.5 L fluid per day
- Basal (Resting) secretion - 0.3 ml/min
- Stimulated secretion - 1.5 ml/min
Where does most of the salivia content come from
Submandibular
What is the composition of salvia
Mucus
- Lubrication (bit of protection)
Dilute solution of NaHCO3/NaCl
- Dilution of food (important for ability to translate things)
- Optimal pH for digestive enzymes (sodium bicarbonate makes it neutral)
Digestive Enzymes
- Lingual lipase
- α-amylase
Function of salivary secretions
Not essential?
Aids with:
- Talking
- Chewing and swallowing
- Dissolves food
- Lubricates
Hygiene
- Irrigation
- Xerostomia
- Dry mouth from reduced or absent saliva
Digestion
- Dissolves food allows tasting
- Lingual lipase
- Fats
- α amylase
- Starch
Regulation of salivary secretions - NERVOUS
- Thought, smell, sight of food
- Presence of food in mouth
- autonomic
Regulation of salivary secretion - Autonomic Nervous system - para vs symp
Parasympathetic
- Stimulates secretion of copious quantities of fluid
(Main stimulator)
Sympathetic
- Stimulates secretion of small volumes of viscous fluid – augments parasympathetic response
Both produce effects
Gastric secretion (stomach) - how much is secreted per day
2-3 L per da
Rate of gastric secretion between meals
Slow rate (15-30 ml/h) of secretion
- Surface epithelium cells – mucous and bicarbonate
(Generates this layer of mucus with bicarbonate in it - protective barrier )
When eating - gastric secretion is superimposed on…
basal rate
Different cell types (volume and composition) when eating - GASTRIC SECRETION
Surface Epithelial cells
- Goblet (Mucous) cells - Mucus
- Bicarbonate
Down in Gastric glands…
Parietal cells
- 150 mls per h HCl acid, pH = 1.0
- Intrinsic factor (helps stabilise b-12??)
Chief cells
- Pepsinogen
Function of the mucus (gastric secretions)
Protection
- abrasion
- acid - mucus contains bicarbonate which buffers acid at the epithelial surface
Function of the intrinsic factor (gastric secretions)
- stabilises Vitamin B12 and facilitates it’s absorbtion in small intestine
Function of Pepsinogen
(Gastric secretions)
- Inactive form of pepsin
- Gastric proteolytic (protein digesting) enzyme
- Converted to active form pepsin by acid
- Starts digestion of proteins
Secretion of HCL acid by parietal cells - what is the source of acid?
Source of acid (H+)
- Formation of H2CO3 by carbonic anhydrase
- CO2 + H2O —-> H2CO3
- Dissociation of H2CO3 to give H+ and HCO3-
- H2CO3 ——-> HCO3- + H+
Secretion of HCL acid by parietal cells - 4 main components
(1) Carbonic anhydrase
- H+ and HCO3- from CO2 and H2O
(2) Secretion of acid (H+)
- H+-K+ ATPase
- Apical membrane of parietal cells Pumps H+ ions into lumen in
exchange of potassium ions (K+)
- K+ return to lumen through membrane channel
(3) Source of chloride (Cl-)
- Anion counter transporter
- Serosal membrane of parietal cells
- Ejects HCO3- into interstitial fluid
- Imports Cl- ions into cell
(4) Secretion of chloride (Cl-)
- Cl- diffuse across cell
- Enters lumen via Cl- channel in apical membrane
Yappage
1.carbonic anhydrase produces H2CO3 and then HCO3 and H+ which then needs to be sent to the lumen
2. Transporter called H+K+ ATPase, using ATP as energy to drive hydrogen out and pull potassium in. Potassium will then be recycled using postassium channel.
3. Biocarbonate is reabsorbed into the blood stream - alkaline tide ? - anion cotransporteer brings in Cl- at the same time
4. Chloride enters the lumen via a chloride channel and joins up with H+ to make HCL
What happens during the cephalic phase ?
Preparation for arrival of food
- stimuli
- higher centres
- thought, smell, sight of food
- chewing action, taste
Extrinsic Parasympathetic nervous system response
- via enteric nervous system
- stimulates parietal cells, chief cells and goblet cells
- stimulate secretion of hormone gastrin (from G cells)
- release it into blood
- stimulates parietal cells and chief cells
What happens during the gastric phase?
Ensures sufficient secretion to handle ingested food
- Stimuli in stomach
- Stretch/distension stomach wall
- Products of digestion stomach lumen
- Elevated pH
Nervous and hormonal regulation includes:
- Local nervous reflex
- enteric nervous system
- external (long) nervous reflex (to Brian and back again) - receptors
- parasympathetic nervous system
- both reflexes stimulate:
- secretion from gastric glands
- motility
- gastrin secretion (g-cells detect partly digested peptides)
- further stimulation of secretion and motility
Intestinal phase – small intestine regulates secretion - what happens?
Controls delivery to small intestine
Stimuli
- Distension of duodenum
- Arrival in duodenum of:
- Acid chyme
- Lipids and carbohydrates
(Nutrients picked up by enteroendocrine cells that release hormones)
Nervous and hormonal regulation
- Hormones
- GIP, CCK, Secretin (ASWELL as affect on stomach they also have
an effect on the pancreas and gallbladder)
- Nerves
- Enterogastric reflex (long reflex duodenum to brain to
stomach) - mainly down regulated motility - a bit of secretion
- Both (hormones and nerves) inhibit secretion and motility
- secretin tells stomach to slow down acid secretion
- CCK and GIP produced by nutrients tells stomach to slow down motility rate and slow down enzyme production
2 components that make up the composition of pancreatic secretion
- Enzymes
- secreted by Acinar cells
- Chemical digestion of food
- Alkaline fluid
- secreted by Duct cells
- Neutralise acid (bicarbonate)
- Optimum pH for enzymes
When stimulated secretion increases - due to secretin for bicarbonate and due to CCK for the enzymes as they enter the GI tract
Activation of proteolytic enzymes - what are they secreted as? What are some examples and where are they activated ?
Proteolytic enzymes secreted as inactive precursors & activated in duodenum
- Trypsin – trypsinogen
- Chymotrypsin – chymotrypsinogen
- Carboxypeptidase – procarboxypeptidase
In small intestine activation involves
- Enterokinase (=enteropeptidase)
- Bound to duodenal membrane
- Converts trypsinogen to trypsin
- Trypsin converts other enzymes to active form
Regulation of biliary secretion - initial delivery of bile under HORMONAL control (all one process with the last cue card)
Hormone CCK
- produced in response to produces of digestion (nutrients) in duodenum
- contraction of gallbladder
- relaxation of hepatopancreatic ampulla
- contraction and relaxation together relases the bile into the SI
Hormone secretin
- milf stimulation of bile by liver
Enterohepatic circulation
- bile metabolically expensive to produce
- 95% reabsorbed in ileum
- transported back to liver in enterohepatic circulation
- reabsorbed and secreted
- stimulation of bile secretion
How much secretions are released from small intestine and where from? - what secretions ? Composition?
-1.5 L per day - from crypts / glands
Mucus
- Lubrication
Isosmotic fluid
- Alkaline - mixture of NaCl and NaHCO3
- Helps neutralize acid
- Dilutes food to aid digestion
Digestive enzymes
- Shed cells
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