Small bowel

Question 1

What is the function of the small bowel?

Answer 1

Absorb nutrients, salt and water

Question 2

What is the structure of the small bowel?

Answer 2

Approx 6m long and 3.5cm in diameter

Duodenum: 25cm

Jejunum: 2.5m

Ileum: 3.75m

No sudden transition between each part, all have the same basic histiological organisation

Question 3

What is the mesentery?

Answer 3

Suspends small and large bowel from posterior abdominal wall

Anchors them in place whilst still allowing some movement

Provides a conduit for blood vessels, lymphatics and nerves

Blood vessels include superior messnteric artery which supplies: jejunal and Ilial arteries, ileocolic artery, right colic artery, middle colic artery

Question 4

What is the digestive epithelium of the small bowel?

Answer 4

What Outer covering is the serosa

Underneath is the longitudinal muscle

Underneath that is the circular muscle (important for motility)

On this is the plica circularis, which has villi on it

Structure of villus: apex at top. Lots of microvilli Arteries veins and lymphatics at the centre. Crypt of lieberkuhn at the bottom of each villi

Question 5

What are some facts about villi?

Answer 5

Only occur in small intestine

Motile

Have rich blood supply and lymph drainage for absorption of digested nutrients

Have good innervation from the submucosal plexus

Have simple epithelium: 1 cell thick, dominated by enetetocytes (columnar absorbative cells)

Question 6

What cell types are found in the small bowel?

Answer 6

Villi lined with:

Mostly enterocytes (absorbative)

Scattered goblet cells

Enteroendocrine cells

Crypts of lieberkuhn:

Paneth cells

Stem cells

Question 7

What are enterocytes?

Answer 7

Most abundant cells in small bowel

Tall columnar cells with microvilli and a basal nucleus

Specialise for absorption and transport of substances

Short lifespan of 1 to 6 days

Surface area:

Cylindrical internal surface area of small-bowel is 0.4 m²

Folds, villi and microvilli increase surface area to around 200 m² (at least a 500 fold increase) (size of a tennis court)

Question 8

What are microvilli?

Answer 8

They make up the brush border

Several thousand microvilli Per cell

Surface of microvilli covered with glycocalyx

Glycocalyx:

Rich carbohydrate layer on apical membrane

Serves as protection from digestional lumen
Yet allows for absorption

Traps a layer of water and mucus known as “unstirred layer”

This regulates the rate of absorption from intestinal lumen

Question 9

What are goblet cells?

Answer 9

Second most abundant epithelial cell type

Mucus containing granules accumulate at apical end of cell, causing goblet shape

Mucous -> large glycoprotein that facilitates passage of material through bowel

Increasing abundance of goblet cells along entire length of bowel (lower in duodenum , higher in colon)

Question 10

What are the enteroendocrine cells of the small bowel?

Answer 10

Columnar epithelial cells

Scattered among enterocytes

Most often found in lower part of crypts

Hormone secreting (eg. To influence gut motility)

(May be referred to at chromaffin cells)

Question 11

What are paneth cells?

Answer 11

Found only in the bases of crypts

Contain large, acidophilic granules

Granules contain: antibacterial enzyme lysozyme (pretexts stem cells). Glycoproteins and zinc (essential for a number of enzymes)

Also engulf some bacteria and Protozoa

May have a role in regulating intestinal flora

Question 12

What are stem cells?

Answer 12

Undifferentiated cells which remain capable of cell division to replace cells which die

Epithelial stem cells are essential in the GI tract to continually replenish the surface epithelium

Continually divide by mitosis

Migrate up to tip of villus, replacing all the cells that die by apoptosis (these are digested and reabsorbed)

Differentiate into various cell types (pluripotent)

Question 13

What is the duodenum?

Answer 13

Distinguished by presence of brunners glands

Submucosal coiled tubular mucous glands secrete img alkaline fluid

Open into the base of the crypts

Alkaline secretion: neutralises acidic chyme from stomach, protecting proximal small bowel. Helps optimise pH for action of pancreatic digestive enzymes

(It goes duodenum, jejunum, ileum, large bowel)

Question 14

What are the differences between the jejunum and ileum?

Answer 14

Jejunum is wider and thicker walled, and redder than the ileum

As they have larger, and more numerous plica circularis

The jejunal mesentery is above and to the left of the aorta, the ileal mesentery is attached below and to the right of the aorta

Ileum also had peyers patches (lymphoid tissue) on anterior mesenteric border of lower ileum. Involved in gut immunity

Question 15

What are the functions of the small bowel motility?

Answer 15

To mix ingested food with digestive secretions and enzymes

To facilitate contact between contents of intestine and the intestinal mucosa

To propel intestinal contents along alimentary tract

Question 16

What are the three different types of movement in small bowel motility?

Answer 16

1. Segmentation (mixing)

Mixes contents of lumen

Occurs by stationary contraction of circular muscles at intervals (more frequent in duodenum)

Allows pancreatic enzymes and bile to mix with chyme (net movement of chyme is to the colon)

2. Peristalsis (propelling)

Sequential contraction of adjacent rings of smooth muscle (most waves only travel about 10cm)

Propels chyme towards colon

3. Migrating motor complex

cycles of smooth muscle contractions sweeping through gut

Begin in stomach -> small intestine -> colon -> next wave starts in duodenum

Prevents migration of colonic bacteria to ileum

Question 17

What are the digestive and absorbative mechanisms in the duodenum?

Answer 17

Digestion:

Occurs in an alkaline environment

Pancreatic digestive enzymes and bile enter duodenum from major pancreatic duct and common bile duct

Duodenal epithelium also produces its own digestive enzymes

Digestion occurs in lumen and in contact with the membrane

Question 18

What happens in the digestion of carbohydrates?

Answer 18

These contain around 50% of ingested calories in western diet

Digestion begins in the mouth but salivary alpha amylase (destroyed in the stomach by acid pH)

Most carbohydrate digestion occurs in the small intestine

Monosaccharides - (glucose and fructose). disaccharides (sucrose and maltose). Complex carbohydrates (starch, cellulose, pectins)

Pancreatic a amylase : secreted into duodenum in response to a meal

Continues digestion of starch and glycogen in small bowel

Needs Cl- and neutral/alkaline pH for optimum activity

Acts mainly in lumen

Digestion of amylase products and simple carbohydrates occurs at the brush border (enzymes such as sucrase, isomaltase, lactase)

Question 19

How does the absorption of digested carbs work?

Answer 19

Absorption of glucose and galactose by secondary active transport - carrier protein SGLT-1 on apical membrane

Absorption of fructose is by facilitated diffusion - carrier protein GLUT-5 on apical membrane

GLUT-2 facilitates exit at basolateral membrane

Human small intestine can absorb 10kg of simple sugars a day

Question 20

How does the digestion of proteins work?

Answer 20

Begins in lumen of stomach by pepsin (which is deactivated in the alkaline duodenum)

5 pancreatic proteases are secreted as precursors (eg. Trypsinogen). These travel to the lumen of the small bowel

Trypsin is activated by enterokinase (which is located on the brush border)

Trypsin -> activates other proteases

This hydrolysed proteins into single amino acids and oligopeptides

Question 21

How are digested proteins absorbed?

Answer 21

Action of luminal, brush border and cytosolic peptidases

A variety of peptidases at brush borders of enterocytes progressively hydrolyse oligopeptides to amino acids

Enterocytes directly absorb some small oligopeptides via the action of H+/oligopeptide cotransporter PepT1

These small oligopeptides are digested to amino acids by peotidases in the cytoplasm of enterocytes

Question 22

How are lipids digested?

Answer 22

Lipids are poorly soluble in water so more difficult to digest

4 stage process in small bowel:

• secretion of bile salts and pancreatic lipases
• emulsification increases surface are
• enzymatic hydrolysis of ester linkages (colipase complexes with lipase - prevents bile salts displacing lipase from fat droplet)
• solubilisation of lypolytic products in bile salt micelles

Question 23

How are lipids absorbed?

Answer 23

Fatty acids and monoycerides leave micelles and enter enterocytes

Fatty acids and monoglycerides resynthesised into triglycerides by 2 pathways: monoglyceride acylation (major) and phosphatidic acid pathway (minor)

Chylomicrons - lipoprotein particles synthesised as an emulsion in Golgi apparatus

Chylomicrons secreted across basement membrane by exocytosis

Chylomicrons enter lacteal (lymph) -> lymph transports them away from bowel

Question 24

What is the ileocaecal valve?

Answer 24

Separates the ileum from the colon

Relaxation and contraction controls passage of material into colon

Also prevents back flow of bacteria into colon