The Intestines

Question 1

The small intestine requires a large surface area for which its luminal contents are agitated for several hours. How is this achieved?

Answer 1

1. Very long
2. Millions of villi project into lumen
3. Micro-villi on eneterocytes

Question 2

How is the brush border formed?

Answer 2

Enterocytes (epithelial cells) undergo rapid cell division in crypts in between the villi. They then migrate towards to the tip of the crypt from which they are shed, and as they migrate, they mature and their luminal surface is covered with millions of micro-villi. This is the brush border.

Question 3

What is the function of the brush border?

Answer 3

To form an unstirred layer where nutrients meet and react with enzymes secreted by enterocytes, allowing completion of digestion before absorption.

Question 4

Where is chyme produced and secreted into?

Answer 4

Produced in stomach and secreted into the duodenum where it is conditioned.

Question 5

What is the process of conditioning of chyme?

Answer 5

Correction of acidicity, hypertonicity, and digestion.

Acidicity - HCO3- is added from the pancreas, liver and duodenal mucosa. HCO3- is produced during the production of gastric acid.

Hypertonicity - osmotic movement of water across duodenal wall

Digestion - addition of enzymes from pancreas and duodenal mucosa, with bile acids from the liver.

Question 6

What happens to chyme in the large intestines?

Answer 6

Water is absorbed from any indigestible residues of chyme - converting it into semi-solid stool or faeces that is temporarily stored and accumulated until defecation.

Question 7

What is the musculature of the large intestines?

Answer 7

Large intestine have teniae coli - 3 longitudinal bands of muscle that begins at the appendix and merges at the rectosigmoid junction to form a continuous layer of muscle around the rectum. Their tonic contraction shortens the part of the wall in which they are present causing the colon to be sacculated (baggy) between the teniae coli, forming Haustra.

Question 8

What is haustra in the large intestine?

Answer 8

The parts of the intestinal wall not occupied by teniae coli and not controlled by their tonic contraction -> sacculation.

Question 9

What does the duodenum absorb and secrete?

Answer 9

Absorbs: Iron

Secretes:
HCO3- from mucosa 
Water via osmosis 
Bile and pancreatic secretions 
Protease/Carbohydrase/Secretin/Gastrin/CCK

Question 10

What does the jejunum absorb and secrete?

Answer 10

Absorb: 
Carbs 
AAs
FAs 
Vitamins and Minerals 
Electrolytes 
Water 

Secretes: Protease/Carbohydrase/secretin/gastrin/CCK

Question 11

What does the ileum absorb and secrete?

Answer 11

Absorb:
Vitamin B12
Bile
Anything else not absorbed by jejunum

Secretes:
(same as jejunum)
Proteases/Carbohydrases/Secretin/Gastrin/CCK

Question 12

What does the large intestine absorb and secrete?

Answer 12

Absorb:
Water
Any remaining nutrients
Colonic bacterial vitamins - vit K, vit b12, thiamine, riboflavin

Question 13

How are carbs broken down? where does this occur?

Answer 13

Carbs are ingested in the form of amylose (straight chain alpha-1,4 bonds), amylopectin (branched with alpha-1,6 bonds at branches) or disaccharides.

Alpha-amylases act on alpha-1,4 bonds and are secreted in the saliva and by pancreas. They yield glucose and maltose from amyloses and alpha-dextrins from amylopectins.

In the small intestine, brush border enzymes complete breakdown to glucose via isomaltase, maltase, sucrase and lactase.

Question 14

What do the following enzymes do:

1. isomaltase
2. maltase
3. sucrase
4. lactase

Where are they found?

Answer 14

Isomaltase breaks down branched molecules at alpha-1,6 bonds

Maltase breaks down one maltose => 2 glucose

Sucrase breaks down sucrose => glucose and fructose

Lactase breaks down lactose into glucose and galactose

Question 15

How is glucose absorbed? Where does this occur?

Answer 15

Glucose is actively absorbed using the energy from a Na+ gradient set up by NA/K/ATPase pump in the basolateral membrane.

Glucose enters enterocytes via the apical membrane through SGLT1, a Na+/glucose symporter.

Glucose then leaves the epithelia cell into the bloodstream via the BLM via facilitated diffusion through GLUT2 transporter.

Question 16

How are galactose and fructose absorbed?

Answer 16

Fructose - facilitated diffusion

Galactose - SGLT1 also supports galactose transport

Question 17

How are proteins broken down and where?

Answer 17

Proteins -> oligopeptides (10-20 AAs long) by pepsin from chief cells in the stomach. Digestion of peptides also occur by peptidases secreted from the pancreas into the duodenum - trypsin, chymotrypsin, carboxypeptidase

Question 18

Where is pepsin secreted from?

Answer 18

Chief cells in the stomach

Question 19

What does pepsin do?

Answer 19

Peptide digestion - breaks bonds near aromatic AA side chains

Question 20

Trypsin

Answer 20

Protein digestion - breaks bonds near basic AA side chains

Question 21

Carboxypeptidase

Answer 21

Breaks C-terminal AAs with basic side chains

Question 22

What enzymes are involved in the breakdown of peptides- where are they found?

Answer 22

Pepsin - stomach, chief cells

Trypsin, Chymotrypsin, Carboxypeptidase - pancreatic peptidases -> dudodenum

Question 23

How does the absorption of proteins in babies differ to adults? What are the implications of this for passive immunity?

Answer 23

Neonates have open guts - they are able to pick up whole proteins so passive immunity can occur through breast feeding with the absorption of IgA immunoglobulins.

Question 24

How are AAs absorbed? Where does this occur?

Answer 24

AAs are actively taken up in the small intestine - mostly jejunum via Na+amino acid co-transporters which use the Na+ gradient set up by the Na/K/ATPase pump. Some also occur by facilitated passive diffusion.

Question 25

5 main Na/AA co-transporters exist. What are they?

Answer 25

Small, neutral AAs 
Neutral AAs, Basic AAs, Cystine 
Acidic AAs 
Imuno-AAs 
beta AAs - mainly taurine

Question 26

How are dipeptides and tripeptides absorbed?

Answer 26

by active mechanism associated with pumping H+ into the lumen - which then returns by co-transport with the peptide

Question 27

What are the implications of fats being relatively insoluble and their digestion.

Answer 27

Fats are relatively insoluble in water so they tend to aggregate into large globules -> prevents effective action of digestive enzymes which is exacerbated by stomach acid

Question 28

How are fats digested? Where does this happen? What happens to the digested fats?

Answer 28

In duodenum - bile acids enable fats to be incorporated into small micelles with fats in the middle and polar components of bile acids in the outside. The micelles generate a large surface area for action of lipases which cleave the fatty acids from glycerol. Micelles also carry FAs and glycerol into the unstirred layer, next to the mucosa where they can be released for diffusion -> epithelial cells. Once inside epithelial cells, they re-form triglycerols and re-expelled as chylomicrons - lipids covered by phospholipids, which facilitate the transport of fat in the lymphatic system from the gut -> systemic veins.

Question 29

How are salts and water absorbed? Where?

Answer 29

Sodium diffuses into the cell and actively transported across BLM via NA/K/ATPase - provides driving force for most of the absorptive processes

Cl- follows Na+ —–> osmotic movement of water -> reabsorption of water too

Question 30

What is oral rehydration therapy?

Answer 30

Oral rehydration therapy consists of giving fluid with glucose and NaCl as Na uptakes generates an osmotic gradient -> water reuptake and glucose stimulates na+ uptake, plus generates its own osmotic gradient.

Question 31

What patterns of motility occur in the small intestine?

Answer 31

Segmenting - allows very slow movement of intestinal contents whilst being gently agitated for effective absorption. Small intestine is divided into sections - each with a pacemaker - frequency of which decreases from duodenum -> ileum (12/min -> 8/min) - known as the intestinal gradient. Each pacemaker drives a small section of intestine -> smooth muscle contracts - these contractions separate the intestine into segments and the contents of the section of muscle that is not contracted is mixed by movement from the portions that do contract. Segmenting DOES NOT PROPEL JUST MIXES CONTENTS! but intestinal gradient means that there is a gradual net movement caudally.

Question 32

What patterns of motility occur in the large intestine?

Answer 32

haustral shuffling - the large intestine is divided into segments knwon as haustra as the circular muscles are more complete thatn the longitudinal which have been reduced to teniae coli (3 thin bands of longitudinal muscle vs. thick circular).

Contractionof the smooth muscle in the walls of the haustra shuffles the contents back and forth allowing for slow reabsorption of water and salts and the formation of feaces, with overall progression of contents towards the sigmoid colon.

Mass movement, also.

Question 33

What patterns of motility from the transverse colon to the rectum?

Answer 33

Mass Movement - Once or twice a day, there is a peristaltic propelling pattern form the transverse through to the descending colon which rapidly forces feaces into the rectum which is normally empty, thus inducing the urge to defecate due to pressure receptors.

Waves of contraction the rectal muscle then forces faeces towards the anus ->
parasympathetic control to smooth muscle of internal anal sphincter causes the muscle to relax and somatic control to voluntary striated muscle that constitutes the external anal sphincter also causs it to relax => faeces expulsion.

Question 34

How is mass movement triggered?

Answer 34

1. eating - the gastro-colic reflex

2. time of day