Small Intestine BASICS Flashcards
What are the three portions of the small intestine?
Duodenum
Jejunum
Ileum
How long are the three portion of the small intestine and what are the main roles of each of the sections
The duodenum: (0.25 m) - key roles in digestion and gut regulation The jejunum (2.5 m) - key roles for absorption The ileum (3.75 m) - key roles for absorption
What is a noteworthy feature of the epithelial layer of the small intestine?
Finger like projections called villi that increase internal SA and are motile with rich blood supply and lymph drainage
How is the mucosae arranged?
In folds like a concertina
What are the invaginations in the small intestine called
cyrpts of Lieberkuhn
What are enterocytes?
Cells most abundant in the gut and are responsible for absorption
What do enterocytes look like?
Tall columnar epithelia with microvili on apical surface and nucleus that sits toward basolateral membrane along with mitochondria
What populates the apical region of the enterocyte?
Rough endoplasmic reticulum
How are enterocytes connected to each other?
Tight junctions
What is the lifespan of enterocytes?
between 1 and 6 days
What is the enterocyte apical surface also referred to as and why?
Brush border
Each cell has approx 2000 microvilli on apical surface
What are the carbs found on the apical membran eof the enterocytes called?
Rich network of carbohydrates called glycocalyx
What does the glycocalyx do on enterocytes?
Help trap a layer of water, mucus and enzymes on the surface of call to protect it from the luminal contents eg enzyme that may damage the cell/membrane and regulate digestion and absorption
What is the unstirred layer?
The layer of water, mucus and enzymes that has the rich network of carbs called glycocalyx
What is found in the apical part of the goblet cytosol?
Mucin granules that distort the shape of the cell
What does mucous consist of?
Water and glycoproteins
What does mucous do?
Serve as lubricant to facillitate gut passage, especially useful bc water constantly being absorbed so steadily dehydrates contents
How does the number of goblet cells change along entire length of the intestine?
Increases along the length due to water slowly absorbed so food bolus slowly drier
What are enteroendocrine cells?
Hormone secreting epithelial cells that secrete into the local blood supply
What are examples of enteroendocrine cells?
G-cells secrete gastrin
I-cells secrete cholecystokinin
S-cells secrete secretin
D-cells secrete somatostatin
What do enteroendocrine cells look like?
Roughly columnar in shape and sensory apparatus is the apical portion with collections of manufactured hormones found near the basolateral membrane
What does a closed enteroendocrine cell mean?
Not exposed to the gut lumen at all
What are paneth cells?
Immunological cells that help protect stem cells and engulf bacteria and regulate intestinal flora
What do paneth cells contain?
Lots of acidophillic granules
Lysozyme: antibacterial enzyme
Glycoproteins: protect local cells from enzymes
Zinc: common cofactor for lysozymes
Where are paneth cells located?
Near stem cells at the bottom of crypts
Where are stem cells in the small intestine found?
Pluripotent stem cells proliferate in the crypts and move up the sides of the villus towards the tip where they become senescent and naturally slough off, get broken down and reabsorbed
What do stem cells allow in the small intestine?
Effects of gut borne toxins/drugs to eventually run their course - it is easier to continualy replace these cells than protect the huge no. of cells at risk
Lesions will be short lived and repaired quickly
What happens to stem cell activity following radiation therapy/exposure?
Stem cell activity impaired so severe GI dysfunction
What does the duodenum have that distinguishes it from the rest of the intestine?
Coiled mucous-secreting submucosal gland that secrete bicarbonate alkaline solution which open up into the base of the Crypts. Helps to neutralise acid chyme, protect the lining of the proximal small intestine, and provide an optimum pH for enzymes.
What does the Jejunum have that distinguishes it from the rest of the intestine?
Large submucosal folds called plicae circularis. These are similar to the folds in the rest of the small intestine, but they are considerably larger and closer together. They look much more like frills than folds.
What does the ileum have that is different to the rest of the intestine
Peyer’s patches, which are aggregates of specialised lymphoid tissue; key to the immune response to gut-borne pathogens.They can initiate leukocyte and immunoglobulin responses to pathogens and contain M-cells which do not have microvilli like the neighbouring enterocytes
What are the three main functions of intestinal motility?
Mix ingested food with digestve secretions and enzymes
Facilliate the contact between contents and mucosal surface
Propel the contents along the small intestine
What are the three mechanisms that allow for motility to be achieved?
Segmentation
Peristalsis
Migrating motor complex
What is Segmentation?
Alternate contraction and relaxation of non adjacent segments of circular muscle in the gut wall, pushing food forwards and then backwards
What is the major effect of segmentation? Minor effect?
Mixing and mechanical breakdown
minor: propulsion
What is peristalsis?
Sequential contraction of adjacent segments of circular muscle in the gut wall combined with contractions of logitudinal muscle
What is the major and minor effect of peristalsis?
Major: propulsion along GI tract
Minor: some mixing
What is the migrating motor complex?
Periodic contractions from stomach to distal ileum during fasted state that restart once complete. It is present in the fed state too but less ordered?
What are the effects of the migrating motor complex?
Prevent colonic flora travelling backwards and to help cleanse small intestine of residual food
What are the two sites that the digestion of macronutrients by enzymes tends to be effective?
Cleave big nutrient into small nutrients in gut lumen - usually released from glandular organs
Cleave dimeric nutrients into monomers for absorption in the brush border of the enterocytes
Where are secretions that facillitate digestion in the small intestine principally derived from?
Liver, gall bladder, pancreas and small intestine wall
What is the first stage of carb break down and digestion? Where does it occur?
First stage: salivary amylase in the mouth is a minor effect
Where does the digestion of carbohydrates resume after the mouth? What enzymes does it use
In the duodenum via pancreatic amylase and brush border enzymes
What is the goal of digestion of carbohydrates?
Break long polymers into simple monomers that can be absorbed by the gut wall
What does pancreatic amylase break polysaccharides into?
Sucrose, maltose, lactose
What does lactose break down into
Glucose and galactose
What does sucrose break down into
Fructose and glucose
How are glucose and galactose absorbed through apical surface?
Through apical surface by secondary acitve transport through SGLT 1 transporter - sodium glucose linked transporter
How is fructose absorbed through apical surface?
GLUT-5 facillitated diffusion
How are the monosaccharides moved through the basolateral membrane of the enterocytes?
GLUT2 channels, diffuse down conc gradient
How do SGLT 1 transporters work?
They are supporters transporting one carbohydrate and one glucose into the cell at a time. Sodium pumped out of the basolateral surface by a sodium potassium ATPase to ensure the sodium gradient
What kind of environment does pepsin need to function?
Acidic
Where does protein digestion happen?
Duodenal lumen
What happens in the duodenal lumen to proteins?
Protease rich pancreatic juice break longer peptides into tripeptides and dipeptides.
What are examples of pancreatic enzymes?
Trypsin, chymotrypsin, carboxypeptidase
Where does the final stage of protein digestion occur?
In the brush border by enzymes secreted from the epithelial cells
What enzymes are involved in the final stage of protein digestion?
Tripeptidases and dipeptidases, specifically endopeptidase, dipeptidase, aminopolypeptidase, and carboxypeptidase
How are single amino acids absorbed by enterocytes
Mainly secondary active transport using transporters eg. AA/Na+ symporter on apical membrane
How are short dipeptides and tripeptidases absorbed by enterocytes
Apical transporter : AA/H+ symporter
Basolateral - n/a
How are fats broken down in the mouth?
Linguinal lipase hydrolysis triglycerides in mouth
How does fat digestion continue in stomach?
Ingested linguinal lipase and secreted gastric lipase cleaves single fatty acid chains from free triglycerides
What does the mechanical churning of the stomach do to fats
Slightly emulsifies the fat to increase SA
What does bile do to fats in the duodenum?
Provide chemical emulsification with a hude increase of SA by creating small fat droplets
What does pancreatic juice do to fats in duodenum?
Acts on the emulsified fats to cleave two fatty acid chains from triglycerides to form monoglycerides and free fatty acids via pancreatic lipase and colipase - luminal digestion
What happens to digestion products of fats as they reach the brush border?
They are combined with bile salts to form micelles that are soluble enough to cross the aqueous unstirred layer
How are lipids absorbed by enterocytes?
In the brush border, lipolytic products diffuse through the apical membrane and bile salts maintain in lumen, and the monoglycerides and free FA resynthesized into triglycerides via two separate pathways.
What are the pathways that monoglycerides and free FA are resynthesized into triglycerides in digestive epithelial cells?
The primary mechanism is the monoglyceride acylation pathway, and the secondary mechanism is the phosphatidic pathway (the details of these catabolic pathways are not essential information).
What happens to triglycerides once resynthesized in the epithelia?
Packaged with proteins, phospholipids, cholesterol into a lipoprotein called chylomicrons which are exocytosed and are too large to enter capillaries so enter lymphatic system via villi lacteal
