Flashcards in Week 8 - Intestines and absorption Deck (47):
How is the small intestine specialised for absorption?
-Large SA -> the mucosa is folded into villi which have microvilli
-Plicae Circulares Slow movement of contents
What are plicae circulares?
-Permanent folds in the jejunum which slow down gut contents to increase time for absorption
What is the name for intestinal epithelia?
Why does the intestine contain goblet cells?
-To produce mucus for protection
What is the benefit of epithelial shedding and renewal? How does it do this?
-Shedding mucosa ensures that the mucosal barrier is always intact with healthy cells for protection
-Intestinal glands contain stem cells at the base which migrate to the surface whilst maturing
-When they reach the apex of the villi they undergo anoikis
What is anoikis?
-Programmed cell death as the intestinal cell detaches from surroundings and is sloughed into lumen
What cells can stem cells of the intestinal crypts mature into?
-Paneth cells, goblet cells, enteroendocrine cells, enterocytes
What are paneth cells?
-Cells involved in innate defence via production of antimicrobials
What do intestinal epithelia secrete?
-Enzymes into brush border to breakdown carbs and proteins
Which type of carbohydrate are absorbed by the gut?
Name some monosaccharides
-Glucose, galactose, fructose
Where does final enzyme digestion of carbs take place?
Describe starch and its digestion
-Contains amylose (linear chains of glucose) and amylopectin (branched chains of glucose) which are broken down by amylase (salivary and pancreatic) and isomaltase (brush border) respectively
When amylose is broken down by salivary and pancreatic amylase, what is produced?
-glucose and maltose
Where is maltose broken down and by what?
-Brushborder by maltase
When amylopectin is digested by amylose what is produced? Where are these further digested?
-Brushborder by isomaltase
What is sucrase?
-Glucose and fructose
What is lactase?
-Glucose and galactose
Describe monosaccharide absorption into enterocytes
-NaKATPase on basolat provides the driving energy for SGLUT1 transporter to allow Na to diffuse down its conc gradient bringing glucose with it into the cell (secondary active transport)
-Uses Glut2 to be transported from the enterocyte into blood
Which monosaccharide uses GLUT5 as facillitated diffusion?
Explain the principle of oral rehydration
-Uptake of Na generates an osmotic gradient and allows water to follow
-Glucose uptake stimulates Na uptake
-Therefore a mixture of glucose and salt will stimulate maximal water uptake
What types of proteins are absorbed?
-a'a, dipeptides and tripeptides
How is protein digestion started in the stomach?
-Pepsinogen is secreted by chief cells and converted to pepsin by HCL
-Pepsin acts on proteins and digests them to a'a and oligopeptides which are released into the small intestine
How is protein digestion continued in the small intestine?
-Pancrease secretes proteases as zymogens into the small intestine
-Trypsinogen is converted to trypsin by enteropeptidase
-Trypsin then activates all the other zymogen proteases to allow protein digestion to continue
What are the major proteases released from the pancreas?
-Endopeptodases -> trypsin, chymotrypsin, elastase
-Exopeptidases -> Carboxypeptidase A and B
How are amino acids absorbed into enterocytes?
-Via active and passive processes using Na:amino acid transporters
How are dipeptides and tripeptides absorbed into enterocytes?
-H+co-transporter and then converted to amino acids by cytosolic peptidases
How is water absorbed into enterocytes?
-NaKATPase on basolat
-Creates Na gradient
-Na and other nutrients diffuse into epithelia
-Water follows by osmosis
What is the differnce between water uptake in small and large intestine?
-Small intestint absorbs more volume of water but large intestine's function is water absorption as it absorbs residual water
-Small intestine Na is cotransported
-Large intestine Na channels which are induced by aldosterone
Describe calcium uptake in the intestines
1)Active transcellular uptake where Ca enters via facillitated diffusion -> CaATPase pumps Ca out on basolateral membrane (requires vit D)creating low intracellular Ca
What is calbindin?
-Molecule in enterocytes acivated by vit D allowinf Ca transfer through cell
Describe the uptake of iron in the intestines
-Gastric acid important in uptake
-Iron absorbed across apical membrane and then binds to appoferritin which moves it across basolateral membrane and enters circulation bound to transferrin
How are water soluble vitamins taken up in the intestines?
Describe how vit B12 is absorbed and its role in pernicious anaemia
-Absorbed in terminal ileum bound to intrinsic factor which is secreted by parietal cells
-Pernicious anaemia is autoimmune destruction of parietal cells causing b12 deficiency and aneamia as b12 is necessary for erythropiesis
Name the types of intestinal mobility in the small intestines?
Explain Intestinal gradient
-Occurs between meals-> intestinal pacemakers in the smooth muscle have a higher frequency proximally which drives slow caudal progression of contents
-Occurs following meals
-Different sections of gut contract at different times causing food to move back and forth to promote mixing and slowing of progression
What is the function of peristalsis in small intestine?
-To move contents along
Name the types of intestinal mobility in the large intestine
-Segmentation (haustra shuttling)
What is segmentation in the large intestine?
-Occurs in proximal colon to agitata food and allow remaining water to be absorbed forming faeces
Explain mass movement
-Co-ordinated contraction which occurs 1-3 times per day
-Contents move rapidly from transverse colon into rectum
-Often triggered by eating (gastro-colic reflex)
-Rectum normally empty but when fills to 25% there is an urge to defaecate and the internal sphincter relaxes due to pressure via parasympathetic control and external sphincter relaxes under voluntary control
-Intra-abdominal pressure rises and faeces expelled
What is the difference between internal and external sphincter?
-Internal is smooth muscle external is striated
What is the 'unstirred' layer?
-Formed by the brush border
What are tinea coli?
-Longitudinal muscle of large intestine
What is the classical triad of pernicious anaemia?
-Weakness, glossitis and parasthesia