Week 8 - Intestines and absorption

Question 1

How is the small intestine specialised for absorption?

Answer 1

• Large SA -> the mucosa is folded into villi which have microvilli
• Plicae Circulares Slow movement of contents

Question 2

What are plicae circulares?

Answer 2

-Permanent folds in the jejunum which slow down gut contents to increase time for absorption

Question 3

What is the name for intestinal epithelia?

Answer 3

-Enterocytes

Question 4

Why does the intestine contain goblet cells?

Answer 4

-To produce mucus for protection

Question 5

What is the benefit of epithelial shedding and renewal? How does it do this?

Answer 5

• Shedding mucosa ensures that the mucosal barrier is always intact with healthy cells for protection
• Intestinal glands contain stem cells at the base which migrate to the surface whilst maturing
• When they reach the apex of the villi they undergo anoikis

Question 6

What is anoikis?

Answer 6

-Programmed cell death as the intestinal cell detaches from surroundings and is sloughed into lumen

Question 7

What cells can stem cells of the intestinal crypts mature into?

Answer 7

-Paneth cells, goblet cells, enteroendocrine cells, enterocytes

Question 8

What are paneth cells?

Answer 8

-Cells involved in innate defence via production of antimicrobials

Question 9

What do intestinal epithelia secrete?

Answer 9

-Enzymes into brush border to breakdown carbs and proteins

Question 10

Which type of carbohydrate are absorbed by the gut?

Answer 10

-Monosaccharides

Question 11

Name some monosaccharides

Answer 11

-Glucose, galactose, fructose

Question 12

Where does final enzyme digestion of carbs take place?

Answer 12

-Brush border

Question 13

Describe starch and its digestion

Answer 13

-Contains amylose (linear chains of glucose) and amylopectin (branched chains of glucose) which are broken down by amylase (salivary and pancreatic) and isomaltase (brush border) respectively

Question 14

When amylose is broken down by salivary and pancreatic amylase, what is produced?

Answer 14

-glucose and maltose

Question 15

Where is maltose broken down and by what?

Answer 15

-Brushborder by maltase

Question 16

When amylopectin is digested by amylose what is produced? Where are these further digested?

Answer 16

• a-dextrans

- Brushborder by isomaltase

Question 17

What is sucrase?

Answer 17

-Glucose and fructose

Question 18

What is lactase?

Answer 18

-Glucose and galactose

Question 19

Describe monosaccharide absorption into enterocytes

Answer 19

• NaKATPase on basolat provides the driving energy for SGLUT1 transporter to allow Na to diffuse down its conc gradient bringing glucose with it into the cell (secondary active transport)
• Uses Glut2 to be transported from the enterocyte into blood

Question 20

Which monosaccharide uses GLUT5 as facillitated diffusion?

Answer 20

-fructose

Question 21

Explain the principle of oral rehydration

Answer 21

• Uptake of Na generates an osmotic gradient and allows water to follow
• Glucose uptake stimulates Na uptake
• Therefore a mixture of glucose and salt will stimulate maximal water uptake

Question 22

What types of proteins are absorbed?

Answer 22

-a’a, dipeptides and tripeptides

Question 23

How is protein digestion started in the stomach?

Answer 23

• Pepsinogen is secreted by chief cells and converted to pepsin by HCL
• Pepsin acts on proteins and digests them to a’a and oligopeptides which are released into the small intestine

Question 24

How is protein digestion continued in the small intestine?

Answer 24

• Pancrease secretes proteases as zymogens into the small intestine
• Trypsinogen is converted to trypsin by enteropeptidase
• Trypsin then activates all the other zymogen proteases to allow protein digestion to continue

Question 25

What are the major proteases released from the pancreas?

Answer 25

- Endopeptodases -> trypsin, chymotrypsin, elastase | - Exopeptidases -> Carboxypeptidase A and B

Question 26

How are amino acids absorbed into enterocytes?

Answer 26

-Via active and passive processes using Na:amino acid transporters

Question 27

How are dipeptides and tripeptides absorbed into enterocytes?

Answer 27

-H+co-transporter and then converted to amino acids by cytosolic peptidases

Question 28

How is water absorbed into enterocytes?

Answer 28

- NaKATPase on basolat - Creates Na gradient - Na and other nutrients diffuse into epithelia - Water follows by osmosis

Question 29

What is the differnce between water uptake in small and large intestine?

Answer 29

- Small intestint absorbs more volume of water but large intestine's function is water absorption as it absorbs residual water - Small intestine Na is cotransported - Large intestine Na channels which are induced by aldosterone

Question 30

Describe calcium uptake in the intestines

Answer 30

1) Active transcellular uptake where Ca enters via facillitated diffusion -> CaATPase pumps Ca out on basolateral membrane (requires vit D)creating low intracellular Ca 2) Passive paracellular

Question 31

What is calbindin?

Answer 31

-Molecule in enterocytes acivated by vit D allowinf Ca transfer through cell

Question 32

Describe the uptake of iron in the intestines

Answer 32

- Gastric acid important in uptake - Iron absorbed across apical membrane and then binds to appoferritin which moves it across basolateral membrane and enters circulation bound to transferrin

Question 33

How are water soluble vitamins taken up in the intestines?

Answer 33

-Using Na:cotransport

Question 34

Describe how vit B12 is absorbed and its role in pernicious anaemia

Answer 34

- Absorbed in terminal ileum bound to intrinsic factor which is secreted by parietal cells - Pernicious anaemia is autoimmune destruction of parietal cells causing b12 deficiency and aneamia as b12 is necessary for erythropiesis

Question 35

Name the types of intestinal mobility in the small intestines?

Answer 35

- Intestinal gradient - Segmentation - Peristalsis

Question 36

Explain Intestinal gradient

Answer 36

-Occurs between meals-> intestinal pacemakers in the smooth muscle have a higher frequency proximally which drives slow caudal progression of contents

Question 37

Explain segmentation

Answer 37

- Occurs following meals - Different sections of gut contract at different times causing food to move back and forth to promote mixing and slowing of progression

Question 38

What is the function of peristalsis in small intestine?

Answer 38

-To move contents along

Question 39

Name the types of intestinal mobility in the large intestine

Answer 39

- Segmentation (haustra shuttling) | - Mass movement

Question 40

What is segmentation in the large intestine?

Answer 40

-Occurs in proximal colon to agitata food and allow remaining water to be absorbed forming faeces

Question 41

Explain mass movement

Answer 41

- Co-ordinated contraction which occurs 1-3 times per day - Contents move rapidly from transverse colon into rectum - Often triggered by eating (gastro-colic reflex)

Question 42

Explain defaecation

Answer 42

- Rectum normally empty but when fills to 25% there is an urge to defaecate and the internal sphincter relaxes due to pressure via parasympathetic control and external sphincter relaxes under voluntary control - Intra-abdominal pressure rises and faeces expelled

Question 43

What is the difference between internal and external sphincter?

Answer 43

-Internal is smooth muscle external is striated

Question 44

What is the 'unstirred' layer?

Answer 44

-Formed by the brush border

Question 45

What are tinea coli?

Answer 45

-Longitudinal muscle of large intestine

Question 46

What is the classical triad of pernicious anaemia?

Answer 46

-Weakness, glossitis and parasthesia

Question 47

What is the role of the intestines?

Answer 47

-To absorb nutrient, water and electrolytes from the gut lumen into the blood