Small Bowel

Question 1

What is the function of the small bowel?

Answer 1

To absorb nutrients, salt and water

Question 2

Describe the structure of the small bowel

Answer 2

The small bowel is approx 6m long and consists of the duodenum which leads on from the stomach (25cm), the jejunum (2.5m) and ileum (3.75m). There is no suddentransition between them and all have the same basic histological structure. The transition from duodenum to jejunum occurs around the duodenojejunal flexure.

Question 3

What is the mesentery and what is its function?

Answer 3

The mesentery is a fold of peritoneal tissue. It has 2 main functions:

1. Suspends small & large bowel from posterior abdominal wall, anchoring them in place while also allowing some movement.
2. Provides a conduitfor blood vessels, nerves & lymphatic vessels - main vessels are superior mesenteric artery, jejunal/ileal arteries (arise from SMA), Ileocolic artery which supplies terminal ileum and ascending colon + caecum, right colic artery which supplies ascending colon and middle colic artery which supplies hepatic flexure, splenic flexure and part of descending colon.

Question 4

Describe the digestive epithelium

Answer 4

Outer covering of small bowel known as serosa underneath which is the longitudinal muscle, circular muscle (important for motility) and plicae circulares.
The plicae circulares contains villi.

Question 5

Describe the villi

Answer 5

1. Only occur in small intestine and are motile.
2. Rich blood supply & lymph drainage for absorption of digested nutrients
3. Have good innervation from the submucosal plexus
4. Simple epithelium - 1 cell thick and dominated by enterocytes.

Question 6

Describe structure of villi

Answer 6

Consist of epithelium and rich network of blood vessels + lymph vessels inside villi. Between villi are crypts within layer known as lamina propria. These are a layer of the mucous membrane which also contains a muscularis mucosae layer. Underneath this is the submucosa and then muscularis externa (circular and longitudinal).

Question 7

Describe structure of villi epithelium

Answer 7

Contains numerous cell types. Projecting part of villi has absorptive cells such as enterocytes, secreting cells such as goblet cells, enteroendocrine cells and tuft cells. Lying between bottom of crypt and projecting villi are progenitor cells known as transit-amplifying cells. At base of crypt are stem cells and Paneth cells.

Question 8

What are the features and role of enterocytes?

Answer 8

Most abundant cells in small bowel. Are tall columnar cells with microvilli & a basal nucleus, specialised for absorption & transport of substances. Short lifespan of 1-6 days. Main function is to increase the surface area of small bowel for absorption, which it does 500 fold time.

Question 9

What are the features and role of microvilli?

Answer 9

Microvilli (~0.5-1.5m high) make up the “brush border”. Several thousand microvilli per cell and surface covered with glycocalyx.

Question 10

What is glycocalyx?

Answer 10

Rich carbohydrate layer on apical membrane which serves as protection from digestional lumen yet allows for absorption. Traps a layer of water & mucous known as “unstirred layer” and regulates rate of absorption from intestinal lumen.

Question 11

What are the features and role of goblet cells?

Answer 11

2nd most abundant epithelial cell type. Mucous containing granules accumulate at apical end of cell, causing ‘goblet’ shape. Mucous is a large glycoprotein that facilitates passage of material through bowel. High abundance of goblet cells along entire length of bowel, low in duodenum but high in colon.

Question 12

What are the features and role of enteroendocrine cells?

Answer 12

Columnar epithelial cells, scattered among enterocytes and most often found in lower part of crypts. Are hormone secreting so can influence gut motility. Also referred to as chromaffin cells.

Question 13

What are the features and role of Paneth cells?

Answer 13

Found only in the bases of crypts and contain large, acidophilic granules. Granules contain: antibacterial enzyme lysozyme which protects stem cells and glycoproteins + zinc which is an essential trace metal for a no. of enzymes. Also engulf some bacteria & protozoa so may have a role in regulating intestinal flora.

Question 14

What are the features and role of stem cells?

Answer 14

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 older cells that die by apoptosis and are digested + reabsorbed. Differentiate into various cell types (pluripotent).

Question 15

Why do enterocytes & goblet cells have a short life span

Answer 15

Enterocytes are first line of defense against GI pathogens & may be directly affected by toxic substances in diet. Effects of agents which interfere with cell function, metabolic rate etc will be diminished. Any lesions will be short-lived. If escalator-like transit of enterocytes is interrupted through impaired production of new cells (e.g. radiation) severe intestinal dysfunction will occur.

Question 16

What are some general differences between duodenum, jejunum and ileum?

Answer 16

Duodenum is distinguished by presence of Brunner’s glands. Jejunum is wider and redder than the duodenum + thicker – are thicker as plicae circulares are bigger, more numerous and more tightly set. Jejunum mesentry is also attached above and to the left of the aorta whereas ileum is below+right. Jejunul arterial arcades are only 1-2 and have long terminal arterioles in contrast to the ileum. Peyer’s patches are aggregations of lymphoid tissue, only found in the lower ileum on the antimesenteric border – to do with gut immunity.

Question 17

What are Brunner’s glands?

Answer 17

Submucosal coiled tubular mucous glands secreting alkaline fluid. These open into the base of the crypts. The alkaline secretions of Brunner’s glands:

1. Neutralizes acidic chyme from stomach, protecting proximal small bowel
2. Help optimise pH for action of pancreatic digestive enzymes

Question 18

What are the functions of small intestine motility?

Answer 18

1. To mix ingested food with digestive secretions & enzymes
2. To facilitate contact between contents of intestine & the intestinal mucosa
3. To propel intestinal contents along alimentary tract

Question 19

What is segmentation?

Answer 19

Type of motility which mixes contents of lumen. Occurs by stationary contraction of circular muscles at intervals.More frequent contractions in duodenum cf. ileum. This allows pancreatic enzymes & bile to mix with chyme. Although chyme moves in both directions, net effect is movement towards colon.

Question 20

What is peristalsis?

Answer 20

Involves sequential contraction of adjacent rings of smooth muscle. Propels chyme towards colon. Most waves of peristalsis only travel about 10cm. Segmentation & peristalsis result in chyme being segmented, mixed & propelled towards colon.

Question 21

What is the migrating motor complex?

Answer 21

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

Question 22

Describe digestion in duodenum

Answer 22

Digestion in small bowel occurs in an alkaline environment. Pancreatic digestive enzymes & bile enter duodenum from MPD & CBD. Duodenal epithelium also produces its own digestive enzymes. Digestion occurs in lumen & in contact with the membrane.

Question 23

What are the types of carbohydrates and where are they digested?

Answer 23

Simple carbohydrates include monosaccharides like glucose and fructose plus disaccharides like sucrose or maltose. Complex carbohydrates include sugars bonded together in a chain such as cellulose, pectins and starch. Digestion begins in mouth by salivary alpha-amylase which is destroyed in the stomach due to acidic pH. Most of digestion of carbohydrates occurs in small intestine.

Question 24

Describe activity of pancreatic alpha amylase

Answer 24

Secreted into duodenum in response to a meal. Continues digestion of starch & glycogen in small bowel (started by salivary amylase). Needs Cl- for optimum activity & neutral/slightly alkaline pH. Acts mainly in lumen (some also adsorbs to brush border). Digestion of amylase products & simple carbohydrates occurs at the brush border.

Question 25

How are different carbohydrates digested?

Answer 25

Absorption of glucose & galactose is by secondary active transport through carrier protein SGLT1 in apical membrane of brush border. Absorption of fructose is by facilitated diffusion through GLUT-5. GLUT-2 facilitates exit at basolateral membrane for glucose and fructose.

Question 26

Describe digestion of starch in lumen

Answer 26

Alpha-amylase breaks third bond on amylose forming maltotriose and maltose as the terminal bond cannot be broken. It also digests amylopectin, forming alpha-limit dextrins as adjacent 1,4 linkages, branching linkages (1,6) and terminal linkages can’t be broken by alpha-amylase. This occurs in the lumen of the small intestine.

Question 27

Describe oligosaccharide digestion at brush border

Answer 27

Lactase splits lactose and both monomers are transported via SGLT-1. Maltase breaks maltotriose and maltose down to glucose monomers for transport through SGLT-1. Sucrase-isomaltose are actually two enzymes linked together, where sucrase moiety splits sucrose, maltose and maltotriose. The isomerase moiety splits alpha-limit dextrins, maltose and maltotriose.

Question 28

Describe digestion of proteins

Answer 28

Protein digestion begins in lumen of stomach by pepsin. Pepsin then inactivated in alkaline duodenum. 5 pancreatic proteases are secreted as precursors into lumen of small bowel and trypsinogen converted to trypsin by enterokinase - an enzyme located on duodenal brush border. Trypsin then activates other proteases which hydrolyse proteins into amino acids and oligopeptides.

Question 29

Describe action of luminal, brush-border & cytosolic peptidases

Answer 29

Variety of peptidases at brush borders of enterocytes progressively hydrolyse oligopeptides into amino acids. Enterocytes directly absorb some of small oligopeptides via action of H+/oligopeptide cotransporter PepT1. These small peptides are digested to AAs by peptidases in cytoplasm of enterocytes.

Question 30

Describe digestion of lipids

Answer 30

Lipids are poorly soluble in water and more complicated to digest. 4 stage process in small bowel:

1. Secretion of bile salts & pancreatic lipases
2. Emulsification (↑s surface area for digestion)
3. Enzymatic hydrolysis of ester linkages - Colipase complexes with lipase prevents bile salts displacing lipase from fat droplet
4. Solubilisation of lipolytic products in bile salt micelles

Question 31

Describe absorption of lipids at enterocyte

Answer 31

Fatty acids (FAs) & monoglycerides (MG) leave micelles and enter enterocytes. FAs & MG resynthesized into tri-glycerides (TGS) by 2x pathways: Monoglyceride acylation (major) and Phosphatidic acid pathway (minor). Chylomicrons are lipoprotein particles synthesised as an emulsion (80-90% TGs, 8-9% phospholipids, 2% cholesterol, 2% protein, trace carbohydrate) in Golgi apparatus. These are secreted across basement membrane by exocytosis. Chylomicrons enter a lacteal (lymph capillary) and lymph transports them away from bowel.

Question 32

What is the role of the ileocaecal valve?

Answer 32

Ileum is separated from the colon by the ileocaecal valve. Also prevents back flow of bacteria into ileum.