small bowel Flashcards by Chloe Tan

What is the general function of the small bowel?

- Approximately, how long is the small bowel and what is its diameter?

To absorb nutrients, salt and water

6m long 
3.5cm in diameter

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What are the functions of the mesentery?

Suspends small and large bowel from the posterior abdominal wall, anchoring them in place whilst still allowing some movement

Provides a conduit for blood vessels, nerves and lymphatic vessels

it is also a fat store

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What are the functions of the mesentery?

Suspends small and large bowel from the posterior abdominal wall, anchoring them in place whilst still allowing some movement

Provides a conduit for blood vessels, nerves and lymphatic vessels

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What are the 3 main regions of the small intestine/bowel?

- What are the 6 layers of the small intestine from superficial to deep?

Duodenum , Jejunum , Ileum

Serosa, longitudinal muscle layer, circular muscle layer, submucosa, mucosa, lumen

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What innervates villi?

- How thick is the epithelium of villi what what cells are they dominated by?

Innervation from submucosal plexus (part of the enteric nervous system)

1 cell thick (dominated by enterocytes - simple columnar absorptive cells)

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two features of the villi?

motile

have a rich blood supply and lymphatic drainage for absorption of digested nutrients

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What are villi (mucosa) lined with?

Simple columnar epithelium consisting of:

Primarily enterocytes

Scattered goblet cells

Enteroendocrine cells (hormone secreting cells eg GLP-1, serotonin)

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What types of cells does the epithelium of the Crypts of Leiberkuhn include?
What is the most abundant type of cell in the small bowel?

Paneth cells (secretory cells- antimicrobial peptides and proteins)
Stem cells

Enterocytes (tall columnar cells with microvilli and a basal nucleus)

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What are enterocytes cells specialised for and what are their lifespans?
What is the cylindrical internal surface area of the small bowel?
What increases the surface are of the small bowel and by how much?

Absorption and transport of substances
1-6 days (relatively short)

0.4m^2

Villi and microvilli
~500 fold ^

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What structure makes up the ‘brush border’?

- What is glycocalyx?

Microvilli - several thousand microvilli per cell

Rich carbohydrate layer on apical membrane of microvilli

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What is the purpose of glycoalyx?

- What is the “unstirred layer”?

Covers surface of microvilli
Serves as protection from digestional lumen yet allows for absorption

Traps a layer of water and mucous → regulates rate of absorption from intestinal lumen

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What is the second most abundant epithelial cell type?

- What is mucous and what does it do?

Goblet cells

Large glycoprotein that facilitates passage of material through bowel

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What happens to the number of goblet cells along the entire length of bowel?

Abundance increases along entire length of bowel

Low in duodenum and high in colon

As more lubrication is needed further down the system

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What classification of epithelial cells are enteroendocrine cells and where are they most often found?
What is the function of enteroendocrine cells?

Columnar epithelial cells
Lower parts of crypts

Hormone secretion for example to influence gut motility

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What do enteroendocrine cells have a high affinity for in terms of staining?

Chromium/silver salts (hence being called chromaffin cells in older textbooks)

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Where are Paneth cells exclusively found?

Bases of crypts

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What do Paneth cells contain that are important for their functions and what are some of these functions?

Large, acidophilic granules containing anti-bacterial lysozyme (protects stem cells) and glycoproteins and zinc (essential trace metal for a number of enzymes)

Engulf some bacteria and protozoa

May have a role in regulating intestinal flora

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Why are epithelial stem cells essential in the GI tract?

- where do their daughter cells go

To continually replenish the surface epithelium as cells such as enterocytes have a very short life span

They continually divide by mitosis and migrate to the top of villus, replacing older cells that die by apoptosis → digested and reabsorbed

differentiate into various cell types (pluripotent)

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How long is the lifespan of enterocytes and goblet cells present in the small bowel?

~36 hours (relatively short)

List reasons why enterocytes and goblet cells in the small bowel have this lifespan

Enterocytes are first line of defence against GI pathogens and may be directly affected by toxic substances in diet

Effects of agents which interfere with cell function, metabolic rate will be diminished

Any lesions → short-lived

What structure distinguishes the duodenum from the jejunum and ileum?
What does this gland secrete and why?

Brunner’s glands
which are submucosal coiled tubular mucous glands

Alkaline fluid to neutralise chyme from stomach, protecting proximal small bowel

And to help optimise pH for action of pancreatic digestive enzymes

Compare the walls of the jejuum and the ileum and what is the reason for this difference
what are plicae cirulares

Jejunum is wider and thicker walled and redder than the ileum

The plicae circulares in the jejunum are larger and more numerous and so they are more closely set

circular folds in proximal duodenum to middle ileum
aka valves of kerckring
helps absorption and slows food down by spiralling chyme

Compare the jejunal and ileal mesenteries

Jejunal mesentery forms 1-2 arterial arcades whereas ileal forms 3-4, but these 3-4 are shorter

Jejunal mesentery is above and to left of aorta whereas ileal mesentery is attached below and to the right of the aorta

Where in the small intestine are Peyer’s Patches found?

- List the functions of small intestine motility?

Lower part of the ileum

Mix ingested food with digestive secretions and enzymes
Facilitate contact between contents of intestine and intestinal mucosa
Propel intestinal contents along alimentary contact

What is segmentation?

Mixes contents of lumen Occurs due to stationary contraction of circular muscles at intervals. More frequent contraction in duodenum Contractions allow pancreatic enzymes and bile to mix with chyme Although chyme moves in both directions, net effect is movement into the colon

what is peristalsis?

Peristalsis (propelling) Involves sequential contraction of adjacent rings of smooth muscle Propels chyme towards colon. Most waves of peristalsis only travel about 10cm. Segmentation and peristalsis result in chyme being segmented, mixed and propelled into the colon

what is Migrating motor complex

Cycles of smooth muscle contractions sweeping through gut. Begin in the stomach → small intestine → colon → next wave starts in duodenum. Prevents migration of colonic bacteria into ileum.

- Describe the environment that digestion in the small bowel occurs in - How do bile and pancreatic digestive enzymes enter the duodenum?

Alkaline environment Main pancreatic duct (MPD) and common bile duct (CBD)

- What does duodenal epithelium produce to aid digestion?

Its own digestive enzymes

- Where does digestion of carbohydrates begin and by what enzyme? - Where does most of the digestion of carbohydrates occur?

Mouth by salivary-alpha-amylase which is destroyed in stomach by acidic pH Small intestine

- Give examples of simple and complex carbohydrates | - What is secreted into the duodenum in response to a meal to aid digestion of carbohydrates?

Simple: monosaccharides such as glucose and fructose; disaccharides such as sucrose and maltose. Complex: starch, cellulose and pectin. Pancreatic-alpha-amylase

- What does pancreatic alpha amylase enzyme need for optimum activity? - Where does digestion of amylase products and simple carbohydrates occur?

Cl- and neutral/slightly alkaline pH (enzyme acts mainly in lumen - some also adsorbs to brush border) Brush Border

- What carrier protein is used for absorption of glucose and galactose? - What carrier protein is used for absorption of fructose?

SGLT-1 on apical membrane (absorption is by secondary active transport) GLUT-5 on apical membrane (done by facilitated diffusion)

- What carrier protein facilitates exit of digestion products of carbohydrates into the intestinal space at the basolateral membrane? - How much simple sugar can the human small intestine absorb daily?

GLUT-2 10kg

- Where does protein digestion begin and by what enzyme?

Lumen of stomach by pepsin (pepsin inactivated in alkaline duodenum)

- List the 5 pancreatic proteases secreted as precursors into the the lumen of the small bowel

Trypsinogen → Trypsin (via enterokinase) Chymotrypsinogen → Chymotrypsin Proelastase → Elastase Procarboxypeptidase A - Carboxypeptidase A Procarboxypeptidase B - Carboxypeptidase B

-What does Trypsin do?

activate other proteases to hydrolyse proteins | into single amino acids (AA) and oligopeptides (AA)n

- Where is the enzyme enterokinase located? | - What are oligopeptides?

Duodenal brush border Peptide whose molecules contain a relatively small number of amino-acid residues, but more than 2

- How do enterocytes directly absorb small amino acids and transport them into the interstitial space?

Via action of H+/oligopeptide cotransporter → PepT1 and other cotransporters, These peptides are then broken down by enzymes such as peptidases in enterocyte cytoplasm so that the individual AAs can be transported into the interstitial space Other AAs are broken down to form smaller AAs by variety of peptidases on the brush border

- Why are lipids more complicated to digest than carbohydrates and proteins?

Lipids are poorly soluble in water

- Outline the 4 stage process of lipid digestion in the small bowel

Secretion of bile salts and pancreatic lipases Emulsification of lipids to increase SA for digestion Enzymatic hydrolysis of ester linkages via colipase-lipase complex (complex prevents bile salts displacing lipase from fat droplet) Solubilisation of lipolytic products in bile salt micelles (Holding station)

- Outline the absorption of lipids

Fatty acids (FAs) and monoglycerides (MGs) leave micelles and enter enterocytes. FAs and MGs resynthesised into triglycerides (TGs) via 2 pathways: Monoglyceride acylation (major) Phosphatidic acid pathway (minor) Chylomicrons synthesised and secreted across basement membrane by exocytosis Chylomicrons enter a lacteal (lymph capillary) → lymph transports them away from bowel

- What is the purpose of colipases in lipid digestion?

Prevents bile salts from displacing lipase from fat droplet

- What are chylomicrons synthesised by and describe their composition?

Golgi apparatus 80-90% Triglycerides 2% Cholesterol 2% Protein Small traces of carbohydrate

- What separates the ileum from the colon and what is its purpose?

Ileocaecal valve Relaxation and contraction controls passage of material into colon and it prevents backflow of bacteria into ileum