Module 6 Section 5

Question 1

small intestine

Answer 1

• the small intestine is the part of the gastrointestinal tract between the stomach and the large intestine, and is where the majority of digestion and absorption occurs
• it is divided into 3 segments: duodenum, jejunum, and ileum

Question 2

small intestine motility

Answer 2

• the term for motility in the small intestine is segmentation, which both mixes and slowly propels the chyme
• this movement is not peristaltic

Question 3

steps of small intestine motility

Answer 3

1. occurs by localized concentric contractions, which occur every few centimeteres, trapping the chyme in the relaxed areas
2. after a brief period of time, the contracted areas relax and ring-like contractions appear in the previosuoly relaxed areas
3. in this manner, the chyme is moved back ad forth for thorough mixing and is slowly moved forward

Question 4

segmentation of the small intestine

Answer 4

• segmentation in the empty ileum is caused by gastrin, which is secreted due to the presence of chyme in the stomach
• this is the gastroileal reflex
• segmentation is initiated by pacemaker cells that generate a basic electrical rhythm (BER) similar to that seen in stomach
• if the BER brings the circular smooth muscle to threshold, then contraction will occur
• within the duodenum and jejunum the BER is about 12/minute while only about 9/minuye in the terminal ileum
• the excitability of the smooth muscle is enhanced by distention of the small intestine, the hromone gastrin, and extrinsic nerve acitvity
• segmentation is absent between meals

Question 5

migrating motility complex

Answer 5

• when most of the meal has been absorbed, the segmentation contractions end and are replaced by the migrating motility complex
• this motility consists of weak peristaltic waves that are sustained for only short distances before dying out
• they start at the stomach and slowly move their way down the intestine
• its purpose is to move any remnants of the previous meal, any mucosal debris, and any bacteria forward toward the colon
• when the persistaltic waves reach the end of the ileum, they start again at the beginning of the duodenum

Question 6

small intestine secretions and digestion

Answer 6

• as with other parts of the digestive system, the small intestine mucosa has exocrine glands that secrete mucus
• about 1.5L/day is secreted and its purpose is to provide lubrication and protection
• it also provides lots of water for the hydrolytic reactions carried out by the digestive enzymes
• no intestinal enzymes are secreted inot the small intestine lumen, digestion is the result of pancreatic enzymes
• there are some intestinal enzymes that are membrane bound and not secreted into the lumen

Question 7

brush border

Answer 7

• the luminal surface of the small intestine has small, hair-like projections called microvilli that form the brush border which air in the process of absorption
• the brush border also contains membrane-bound enzymes that are categorized into 3 classes

Question 8

3 classes of membrane-bound enzymes in brush border

Answer 8

• enterokinases: can convert pancreatic trypsinogen to trypsin
• disaccharidases: complete carb digestion. by hydrolyzing disaccharides (maltose, sucrose, lactose) into monosaccharides
• aminopeptidases: hydrolyze small peptide fragments into individual aa

Question 9

absorption in the small intestine

Answer 9

• absorption is the primary function of the small intestine
• all carbs, proteins, fats, electrolytes, vitamins, as well as majority of water is absorbed in this part
• almost all absorption occurs in the duodenum and jejunum with little in the ileum
• the ileum is vital for absorption of somethings such as vitamin B12 and bile salts
• on a daily basis about 200g+ or more of carb, 100g+ of fat, 50g+ of aa, and 7-8L of water are absorbed
• the absorptive capacity of the small intestine is much greater, meaning that pretty much anything ingested will be absorbed
• absorption of some compounds, such as calcium and iron, are regulated with excess being excreted in the feces
• mucosa lining of the small intestine is highly adapted for absorption as it has a very large surface area and has a variety of specialized transport mechanisms

Question 10

surface area of the small intestine

Answer 10

• the structure of the small intestine is such that it increases the surface area 600 fold more than a simple tube of similar length and diameter
• in order the maximize surface area of epithelial lining, the inside surface is not smooth
• the inner surface of small intestine has many circular folds that increase the surface area 3-fold
• projecting from the folded structures are microscopic, finger-like projections called villi that increase th esurface area a further 10-fold
• the epithelial cells of villi also have smaller hair-like projections called microvilli of the brush border, whcih increase the surface area another 20-fold

Question 11

absorption of major organic nutrients in the small intestine

Answer 11

chemical digestion and nutrient absorption are essentially finished by the time food residue leaves the small intestine and enters the cecum

Question 12

water and sodium

Answer 12

• sodium can be absorbed both passively and actively
• chloride follows sodium due to the electrical gradient and water follows sodium due to the osmotic gradient

Question 13

passive absorption of sodium

Answer 13

in the presence of an appropriate electrochemical gradient, sodium can passively move down this gradient into the interstitial fluid by moving between intestinal epithelial cells through leaky tight junctions

Question 14

active absorption of sodium

Answer 14

• movement of sodium through cells requires active transport
• it can passively cross the luminal membrane through sodium channels or cotransported with either glucose or aa (sodium reabsorption in kidneys)
• once inside an epithelial cell, sodium is actively pumped out across the basolateral membrane into the interstitial fluid where it diffuses into the capillaries

Question 15

3 sources of exogenous proteins

Answer 15

along with ingested proteins, there are 3 sources of exogenous proteins that need to be digested and absorbed

1. digestive enzymes are all proteins and need to be absorbed
2. proteins from epithelial mucosal cells that have been sloughed off
3. plasma proteins that normally leak from the capillaries into the digestive tract lumen

Question 16

proteins

Answer 16

• proteins from the 3 sources alone can account for up to 40g
• regardless the source of proteins, they are all broken down into individual aa and small peptides for absorption
• these are absorbed by secondary active transport, similar to glucose and galactose
• small peptides have a different carrier and are broken down both by the brush border aminopeptidasess as well as intracellular peptidases
• aa move out across the basolateral membrane and into the blood

Question 17

carbohydrates

Answer 17

• all carbs are absorbed as monosaccharides through active transport
• they are transported either by sodium-monosaccharide cotransport, or sodium-independent facilitated diffusion
• there are preferences for monosaccharide absorption

Question 18

galactose absorption

Answer 18

• galactose is transported first and can be absorbed by secondary acitve transport
• basolateral Na+-K+ pumps require energy to move Na+ into epithelial cells
• this Na+ is then used by cotransport carries to bring cluose and galactose into epithelial cell
• once concentrated inside, galactose moves down its concnetration gradient across the basolateral membrane and into blood

Question 19

glucose absorption

Answer 19

• glucose is transported second and, very similar to galactose, can be absorbed by secondary active transport
• just in the case of galactose transport, basolateral Na+-K+ pumps require energy and bring Na+ inot epithelial cells
• this Na+ is then used by cotransport carries to bring glucose and galactose inot the epithelial cell
• once concetrated inside, glucose moves down its concetration gradient across the basolateral membrane and into blood

Question 20

fructose absorption

Answer 20

fructose is transported last and is transported by faciliated diffusion

Question 21

steps of fat aborption in small intestine - step 1

Answer 21

bile salts emulsify fats and that pancreatic lipases digest the triglycerides into monoglyceride and fatty acids within dat droplets

Question 22

steps of fat aborption in small intestine - step 2

Answer 22

• when the micelles reach the luminal membrane of the epithelial cells, monoglycerides and fatty acids can diffuse from the micelles inot the epithelial cells
• the micelles then retun to the chyme to pick up more triglycerides
• this process repeats along the length of the small intestine until all fat is absorbed

Question 23

steps of fat aborption in small intestine - step 3

Answer 23

• once inside epithelial cells, the monoglycerides and free fatty acids are ocnverted back into triglycerides
• the triglycerides are attracted to each other and form lipid droplets and get coated with a layer of lipoprotein to make the fat droplets water soluble
• in this form they are called chlyomicrons

Question 24

steps of fat aborption in small intestine - step 4

Answer 24

• chlyomicrons leave the endothelial cells by exocytosis
• because capillaries have basement membranes, chlyomicrons cannot driectly enter them
• instead, the chylomicrons are taken up into the lymphatic system before being delivered to the blood

Question 25

minerals - iron

Answer 25

- iron is essential for the production of haemoglobin - the typical dietary intake of iron is 15-20 mg/day with men absorbing 0.5-1mg/day and women absorbing 1-1.5mg/day - women need more iron due to blood loss in menstural flow

Question 26

steps of iron absorption within small intestine - step 1

Answer 26

- the amount of iron absorbed depends on its state - ferrous iron (Fe2+) is more readily absorbed than ferric iron (Fe3+) - only a portion of ingested iron is in the right form to be absorbed

Question 27

steps of iron absorption within small intestine - step 2

Answer 27

- dietary iron that is absorbed into the smal intestine epithelial cels and is immediately needed forRBC production, is trasnferred into the blood - once inside the epithelial cell, iron has 2 fates

Question 28

steps of iron absorption within small intestine - step 3

Answer 28

if it is immediately needed for RBC production it is delivered to the blood where it is bound to transferrin for delivery to the bone marrow

Question 29

steps of iron absorption within small intestine - step 4

Answer 29

if it is not immediately used it is stored within the epithelial cell in grandular form called ferritin

Question 30

steps of iron absorption within small intestine - step 5

Answer 30

if iron levels within the epithelial cells become too high- iron is secreted into the intestinal lumen

Question 31

steps of iron absorption within small intestine - step 6

Answer 31

- any ferritin is also dumped into the lumen when the epithelial cell is sloughed off within 3 days - all excess iron is excreted int he feces

Question 32

diarrhoea

Answer 32

- diarrhoea is the second major way to lsoe water and disrupt body acid-base balance after vomiting - diarrhoea is beneficial when the rapid emptying of the intestine aids in the removal of potentially harmful materials from the body - it is detremental though when it is excessive and prolonged by causing dehydratio, loss of nutrients, and metbaolic acidosis from loss of HCO3- - there are 3 main causes of diarrhoea

Question 33

causes of diarrhoea - excessive small intestine motility

Answer 33

- this usually arises from a local irritation (Bacterial, viral) or emotional stress - the increased motility does not allow sufficient time for water absorption

Question 34

causes of diarrhoea - excess of osmotically active particles

Answer 34

ex. lactose intolerance where the undigested lactose exerted an osmotic effect to draw water into the intestines

Question 35

causes of diarrhoea - toxins from v. cholerae and other microorganisms

Answer 35

these toxins promote excessive fluid secretion by the intestinal mucosa