Case 2 - The stomach Flashcards
Explain how CO2 and HCO3- act as intraceullar buffers
- CO2 is kept constant by respiration
- the cell is highly permeable to CO2 which enters the cell via diffusion and reacts with water
- H2O + CO2 = H+ + HCO3-
- Carbonic anhydrase speeds up dissociation
- if the hydrogen ions are in excess this will shift the equilibrium to form CO2 and H2O
- Therefore more CO2 leaves the cell into the extracellular fluid
What is absorbed in the ileum?
- Proteins
- lipids
- sodium
- water
- carbohydrates
- bile acids
- cobalamin (Vitamin B12)
What is the exocrine function of the stomach?
- Releases pepsin for protein digestion,
- HCL for stomach acidity
What is the paracrine function of the stomach?
Use of histamine to stimulate gastric acid secretion
What do the mucuous neck cells of gastric glands secrete?
- Mucous secretion
What is intrinsic factor and what is its importance?
- It is secreted by parietal cells
- it binds to vitamin b12 (x2) and forms a complex that is resistant to digestion
- Allows b12 to be absorbed in the terminal ileum
- without intrinsic factor b12 is absobed inefficiently by passive diffusion - leads to pernicious anemia
Describe the basolateral secretion of parietal cells.
- bircabonate leaves the cell passively by a 1:1 exchange for Cl-
- basolateral Na+, K+ - ATPase maintains concentratins of [Na+<strong> </strong>] [K+]
What is the function of the fundus and corpus of the stomach
- What types of cells are present
- Fundus and corpus - main secretory and for storage, mainly gastric glands
- Chief - pepsinogen
- Parietal - HCl and intrinsic factor
What is absorbed in the jejunum?
- Carbohydrates
- proteins
- lipids
- sodium
- water
What is the range of intracellular ph
6.8 - 7.2
In gastric glands what do the surface epithelial cells secrete?
- Secrete mucous (HCO3-) to protect the stomach from secretions from acidity and proteases from gastric juice, neutralises acid
Explain how macromolecules control intracellular buffering
- Macromolecules present in the cell have weak acid (COO-) and base groups (NH2)
- These groups can pick up a proton
- They help to minimsise pH fluctuations
- They provide temporary relief as they have a limited capacity
What is absorbed in the duodenum?
- Iron
- carbohydrtaes
- lipids
- proteins
- sodium
- water
What are secretory canaliculi and what are their function?
- An extension of the lumen
- Full of microvilli, not just around the lumen
- when the cell is stimulated there is a structural change and the canaliculi are released
- Apical membrane is withdrawn into close vesicles during resting phase
- Massive decrease in surface area
- Stoping secretion pulls back the villi into the cell, due to PP they damage the cell therefore cell is restored during rest
What is the function of the stomach?
- Acts as a reservoir
- Mix and churns food with gastric secretions to form chyme
What is the MoA of PPI?
- They are lipid soluble weak bases they enter the cell and accumulate in acid spaces i.e. canaliculi and tubulovesicles of parietal cells
- They are activated in acid - chemically altered by H+ to an active sulphonamide form
- Sulphonamides are cationic which are trapped in canaliculi
- Forms irreversible disulphide bond with H+ - K+ - ATPase and blocks H+ secretion until new pumps are synthesized
- They are given in encapsulated formulation to avoid breakdown by stomach acid, it is absorbed in the SI and travels to parietal cells via the blood (onset 1 - 2 hours)
How is glucose absorbed in the SI
- There is a Na+ - K+- ATPase at the basolateral membrane which extrudes Na+ out of the cell
- This creates a Na+ gradient
- Na+ and glucose come into the cell via a co-transporter (SGLT) symporter)
What transporters act as acid extruders?
sodium hydrogen exchanger
proton pump e.g. parietal cells
sodium bicarbonate co - transppoter: import of bicarbonate ions into the cell to neutralise the acid within the cell
How is fat absorbed in the SI?
- Liver produces bile which emulsifies fat, fat lobules are broken down into smalelr fat droplets coated in bile salts
- Increases surface area for an enzyme attack
- Broken down further
- THey pass through the membrane into the lacteal to venous circulation, to prevent accumulation of vascular pathology in vessels
Describe the apical secretion of parietal cells
- Active transport of H+ions by a proton pump: H+, K+ - ATPase, antiporter mechanism and conformational change is driven by phosphorylation
- K+ recycles via a K+ channel, no net gain/loss (recycled)
- Cl- ions recycle via Cl- ion channels
What is the function of the pylorus and antrum of the stomach
What types of cells are present
- Few parietal cells
- mainly endocrine - gastrin - g cells
- mixing and movements control of emptying into duodenum
How does a high flow rate of gastric secretion affect the gastric juice composition?
- Increase in
- chloride ions
- potassium ions
- hydrogen ions
- Decrease in sodium ions
- Rate of parietal cells increases
- Mucous cell secretion is passive therefore remains unaffected
- Less dilution of chyme
Why must pH be kept optimal
For the optimal functioning of proteins within the body:
- Enzymes
- Receptors
- Transporters
- Structural/contractile proteins
What do the parietal cells release
- Intrinsic factor
- HCl
