Digestion and Absorption

Question 1

What is the main function of the small intestine?

Answer 1

Major site for digestion and absorption

Question 2

What is the length of the small intestine?

Answer 2

Approx. 6m long with a 3.5cm diameter

Question 3

What does the small intestine recieve?

Answer 3

Chyme from the stomach via the pyloric sphincter
Pancreatic juice from the pancreas (sphincter of oddi)
Bile from the gall bladder (sphincter of oddi)

Question 4

Generally what does the small intestine secrete?

Answer 4

Intestinal juices (succus entericus)

Question 5

What valve does the small intestine move residues thorough?

Answer 5

The ileocaecal vale which opens in response to proximal pressure and in response to gastrin

Question 6

How has the small intestine adapted to be good at absorption?

Answer 6

Circular folds
Villi
Microvilli

Question 7

What peptide hormones are secreted into the blood via endocrine cells within the mucosa of the small intestine?

Answer 7

Gastrin 
Cholecystokinin (CCK) 
Secretin
Motilin
Glucagon-like insulinotropic peptide (GIP) and incretin 
Glucagon-like peptide-1 (GLP-1)
Ghrelin

Question 8

What cell secretes gastrin?

Answer 8

G cells of the gastric antrum and duodenum

Question 9

What cell secretes CCK?

Answer 9

I cells of the duodenum and jejunum

Question 10

What cell secretes secretin?

Answer 10

S cells of the duodenum

Question 11

What cell secretes motilin?

Answer 11

M cells of the duodenum and jejunum

Question 12

What cell secretes GIP and incretin?

Answer 12

K cells of the duodenum and jejunum

Question 13

What cell secretes GLP-1 and incretin?

Answer 13

L cells of the gut

Question 14

What cell secretes ghrelin?

Answer 14

Gr cells of the gastric antrum, small intestine and elsewhere (pancreas)

Question 15

What do all the peptide hormones from the small intestine act on except from incretin?

Answer 15

G-protein coupled receptors

Question 16

What does incretin act on?

Answer 16

Beta cells of the pancreas in essentially a feed-forward manner to stimulate the release of insulin

Question 17

What are the positive control mechanisms for the release of succus entericus?

Answer 17

Distension/irritation
Gastrin
CCK
Secretin
Parasymp 
ALL enhance

Question 18

What is the negative control mechanism for the release of succus entericus?

Answer 18

Symp nerve activity

Decreases

Question 19

What does the succus entericus contain?

Answer 19

Mucus for protection (goblet cells)
Aqueous salt - enzymatic digestion (crypts of lieberkuhn)
NO digestive enezymes

Question 20

What occurs during segmentation (mixing) of chyme?

Answer 20

The chopping action moves chyme back and forth - very vigorous after a meal
Alternating contraction and relaxation of segments of circular muscle

Question 21

What initiates segmentation?

Answer 21

Small intestine pacemaker cells causing the BER which is continous. At threshold, it activates segmentation which in the duodenum is primarily due to distension by entering chyme

Question 22

How is segmentation in the empty ileum triggered?

Answer 22

By gastrin from the stomach (gastroileal reflex)

Question 23

How is the strength of segmentation altered?

Answer 23

It is enhanced and decreased by parasympathetic and sympathetic activity

Question 24

What is the migrating motor complex (MMC)?

Answer 24

Occurs between meals every 90-120 mins and is a strong peristaltic contraction passing the length of the intestine (stomach - ileocaecal valve)

Question 25

What is the function of the migrating motor complex?

Answer 25

Clears the small intestine of debris, mucus and sloughed epithelial cells between meals

Question 26

What inhibits the MMC?

Answer 26

Feeding and vagal activity

Question 27

What triggers the MMC?

Answer 27

Motilin, suppressed by gastrin and CCK

Question 28

What are the endocrine pancreatic secretions?

Answer 28

Insulin and glucagon - secreted to the blood

Question 29

What are the exocrine pancreatic secretions?

Answer 29

Digestive enzymes (acinar cells), aqueous NaHCO3- solution (duct cells) which are secreted to the duodenum as pancreatic juice

Question 30

What is the purpose of the duct cells producing alkaline fluid?

Answer 30

To neutralise the acidic chyme entering the duodenum, this provides optimum pH for the pancreatic enzyme function and protects the mucosa from erosion by acid

Question 31

What are the proteases stored within the acinar cells?

Answer 31

Trypsinogen, chymotrypsinogen and procarboxypeptidae A and B

Question 32

What are the amylases stored within the acinar cells?

Answer 32

Pancreatic amylase

Question 33

What are the lipases stored within the acinar cells?

Answer 33

Pancreatic lipase

Question 34

Where are the enzyme stored within the acinar cells?

Answer 34

In zymogen granules and released in response to elevated calcium

Question 35

Where are the proteases activated?

Answer 35

Within the duodenum

Question 36

What are the 3 phases to pancreatic secretion?

Answer 36

Cephalic Gastric Intestinal

Question 37

What occurs during the cephalic phase?

Answer 37

Mediated by the vagal stimulation of mainly the acinar cells

Question 38

What occurs during the gastric phase?

Answer 38

Gastric distension evokes a vasovagal reflex resulting in parasymp stimulation of acinar and duct cells

Question 39

What occurs during the neutralisational intestinal phase?

Answer 39

Acid in duodenual lumen, increased secretin release, secretin carried to pancreaetic duct cells resulting in increased secretion of alkaline fluid

Question 40

What occurs during the digestion part of intestinal pancreatic secretion?

Answer 40

Fat and protein in duodenal lumen Increased CCK release from I cells which is carried by the blood to the pancreatic acinar cells. This results in an increased secretion of digestive enzymes into duodenal lumen

Question 41

What are the main constituents of carbohydrates?

Answer 41

Starch (amylose and amylopectin) Cellulose Glycogen Disaccharides (sucrose, lactose)

Question 42

What are the main constituents of lipids?

Answer 42

``` Triacylglycerols Phospholipids Cholesterol and cholesterol esters Free fatty acids Lipid vitamins ```

Question 43

What are the main constituents of proteins?

Answer 43

Ingested plus some from endogenous sources such as digestive enzymes and dead cells from GI tract

Question 44

What is digestion?

Answer 44

The enzymatic conversion of complex dietary substances to a form that can be absorbed

Question 45

What do most digestive proccesses occur in the small intestine as?

Answer 45

Luminal digestion - mediated by pancreatic enzymes secreted into the duodenum Membrane digestion - mediated by enzymes situated at the brush border of epithelial cells

Question 46

What is absorption?

Answer 46

The proccesses by which the absorbable products of digestion are transferred across both the apical and basolateral membranes of enterocytes (absorptive cells of the intestinal epithelium)

Question 47

What are the different types of digestible carbohydrates?

Answer 47

Polysaccharides Oligosaccharides Monosaccharides

Question 48

What are the 2 different types of polysaccharides?

Answer 48

Starch (amylose, amylopectin), glycogen (animal)

Question 49

What are the different types of oligosaccharides?

Answer 49

Sucrose (glucose and fructose) | Lactose (glucose and galactose)

Question 50

What are the different types of monosaccharides?

Answer 50

Glucose and fructose and galactose

Question 51

What do carbohydrates need to be converted to to be digested?

Answer 51

They need to be converted to monosaccharides

Question 52

How are starch converted to oligosaccharides?

Answer 52

Alpha-amylase by salivary and pancreatic

Question 53

How are oligosaccharides converted to monosaccharides?

Answer 53

Via oligosaccharidases (lactase, maltase and sucrase-isomaltase)

Question 54

What is alpha-amylase?

Answer 54

Endoenzyme

Question 55

What does alpha-amylase break down?

Answer 55

It breaks down linear internal alpha-1,4 linkages but not terminal alpha-1,4 linkages and therefore there is no production of glucose

Question 56

What are the products when alpha amylases break down amylose or amylopectin?

Answer 56

Products are thus linear glucose oligomers (maltotriose, maltose) and alpha limit dextrins

Question 57

What are the role of oligosaccharidases?

Answer 57

They are integral membrane proteins with a catalytic domain that faces the lumen on the GI tract

Question 58

What is the substrate of lactase?

Answer 58

Has only one substrate, breaks down lactose to glucose and galactose

Question 59

What do all other oligosaccharides do (apart from lactase)?

Answer 59

Cleave the terminal alpha-1,4 linkages of maltose, maltotriose and alppha limit dextrins (to yield glucose)

Question 60

What is sucrose responsible for?

Answer 60

It is specifically responsible for hydrolysing sucrose to glucose and fructose

Question 61

How can lactose intolerance result?

Answer 61

Primary lactase deficiency Secondary lactase deficiency Congenital lactase deficiency

Question 62

Where does absoprtion of the final products of carbohydrate digestion occur?

Answer 62

In the duodenum and jejunum via the apical and basolateral membranes.

Question 63

What receptor absorbs glucose and galactose?

Answer 63

Seconadry active transport mediated by SGLT1

Question 64

How is fructose absorbed?

Answer 64

Facilitated diffusion mediated by GLUT5

Question 65

How do all monosaccharides exit the small intestine?

Answer 65

Facilitated diffusino via GLUT

Question 66

How does SGLT1 work?

Answer 66

2 Na+ bind increasing the affinity for glucose. Na+ and glucose translocate from extracellular to intracellular where the Na+ dissociate and the affinity for glucose decreases

Question 67

How must protein be digested before it can be absorpbed?

Answer 67

To oligopeptides and amino acids by pepsin and pancreatic proteases

Question 68

What do pepsidases do?

Answer 68

They further hydrolyse oligopeptides to amino acids at the brush border

Question 69

How are amino acids transported across the apical membrane?

Answer 69

Via a variety of amino acid transporters, some of which are Na+ dependent and others Na+ independent

Question 70

How are oligopeptides transported across the apical membrane?

Answer 70

Via H+/oligopeptide co-transporter PepT1

Question 71

What happens to oligopeptides once they cross the apical membrane/

Answer 71

Within the cytoplasm they are further hydrolysed to amino acids by pepsidases within the enterocyte

Question 72

How do amino acids exit the enterocyte?

Answer 72

By crossing the basolateral membrane by several Na+ independent transporters