Absorption

Question 1

structure of large intestines

Answer 1

• caecum
• ascending colon
• transverse colon
• descending colon
• rectum (a layer of smooth and another of skeletal muscle = voluntary control of defecation)

colon : absorbs water, electrolytes. Bacteria

1. food enters the large intestinve through the ileocecal valve (region of muscle and separates ileum with caecum)
2. no villi (only needed during digestion and absorption to increase SA and thats all occured and completed in SI)

Question 2

Absorption of carbohydrates

Answer 2

1. monosaccharides absorbed from lume of SI and move across intestinal mucosa
2. via faciliated diffusion : SGLT (sodium glucose linked transporter = dependent on Na+) for glucose and galactose and GLUT-5 for fructose
3. exit via GLUT-2 and enter blood stream
   => fructose no Na+ dependent

Question 3

Absorption of proteins

Answer 3

1. proteins digested by different proteases (pepsin, trypsin, chymotrypsin, carboxypeptidase)
2. Di- and tripeptides absorbed together with H+ via PepT1 across membrane
3. Amino acids co transport with Na+ across membrane
4. Di- and tripeptides can be further
   digested into amino acids in the cytoplasm during absorption
5. In some instances, peptides/proteins can also be absorbed by transcytosis
6. transporters also help them to exit the cell and enter the blood circulation to the liver

Question 4

function of Large intestine

Answer 4

The large intestine concentrates waste
* Longitudinal muscle contracts, forms these sacs called haustra
* Chyme enters through ileocecal valve
o relaxes/valves opens when a peristaltic wave reaches it
o also relaxes due to gastroileal reflex

• Segmental contractions (haustral contractions) for mixing and absorption
  o Triggered by presence of chyme
  o Water, ions
• Bacteria break down undigested matter and produce and secrete vitamins (K) => bacteria produce vitamin K and abosrbed in LI
• motility = Mass movement/mass peristalsis (large wave of contraction) sends material forward (SI absorbs most of H2O so left over mostly solid waste)
• Occurs 3-4 times daily- gastrocolic reflex (eating and distension in stomach)

Question 5

Absorption of micronutrients

Answer 5

Monosaccharides and amino acids move into capillaries directly under the enterocytes in the villi –> into blood
=> then enter the hepatic portal vien to the liver

• liver removes remove potentially harmful products from the venous blood thats passing through via the hepatic portal veinbefore entering systemic circulation

Nutrients go to heart and then redistributed
systemically via the aorta

Question 6

Absorption of lipids

Answer 6

1. micelles absorbed by brush border
2. monoglycerides etc are hydrophobic so enter via diffusion
3. cholesterol travels via transporter due to its polar group through an energy dependent process
4. enzymes in cells of the smooth ER convert FFAs and monoglycerides back to triglycerides
5. cholesterol + triglycerides + apoprotein = chylomicrons (Apo B48) and move to the golgi for exocytosis
6. Chylomicrons too large to move into capillaries- move into lacteals to enter the lymphatic system and will end up going to vena cava

Question 7

Absorption of ions and water

Answer 7

Iron can be absorbed as heme iron (found in meat) or Fe2+ (spinach)
* Heme iron is converted to Fe2+, and then both pools (Fe2+ and new Fe2+ from heme) leave via ferroportin transporter from lumen into ECF
=> Fe used to make Hb as part of RBC synthesis

• Absorption of ions (Na+ and Cl- travel via facilitated diffusion) from lumen to ECF creates an osmotic gradient
  o pulls water to move to ECF via paracellular pathway (between cells)
• Most water absorption occurs in the
  small intestines (~7.5 L). The large
  intestine also absorbs water (~1.4 L)

Question 8

Briefly describe two main differences between amino acid and fatty acid
absorption and/or distribution (2 mark

Answer 8

AA: required to use transporters to enter/exit cells/ FA: are hydrophobic and can diffuse across

AA: distribtion = capillaries/ FA: distribtion = lacteals

Question 9

gastroileal reflex

Answer 9

pushes stuff in ileum into LI
=> clear the small intestine and make room

when chyme enters from stomach into SI (duodenum) causes distension
= also cuases long reflex arc via CNS /vagus nerve
=> ileocecal valve relaxes

Question 10

gastrocolic reflex
(eating and distension in stomach)

Answer 10

similar gastroileal reflex
–> CNS via vagus nerve when food enters stomach
=> promotes mass movement in LI
–> push waste towards rectrum

Question 11

Defecation reflex

Answer 11

function : removes waste
1. distension sensed via stretch receptors of the rectal wall = activates defecation reflex
2. internal anal sphincter relaxed (smooth = involuntary)
3. external anal sphincter is skeletal = voluntary : still have control

= promoted by abdominal contractions and the Valsalva maneuver

Question 12

Valsalva maneuver

Answer 12

when you hold your breath, epiglottis folds down which increases pressure in body in respiratory system
==> push to poo

Question 13

Migrating motor complex

Answer 13

FUNCTION : “housekeeping” to remove undigested material, bacteria and dead cells to large intestine

Between meals (fasting = empty GIT), series of contractions begin at stomach and through the GIT
1. peristalsis starts in stomach, SI and LI
* Increases in motilin hormone secretion associated with themigrating motor complex
* Takes ~ 90mins to reach large intestine (quite slow from stomach –> LI)

MAIN DIFFERENCE between other types of peristalsis:
stimulated by hormone, motilin during fasting

Question 14

Mass balance

Answer 14

• Vomiting (reverse perstalsis = moving things back up from SI-> stomach-> oral cavity) and diarrhea
  => result in more GI secretions and water lost instead of being absorbed
  ==> trying to flush out whatever it is, out your system
• Digestive system needs to match fluid input with fluid removal
• If secreted fluid not reabsorbed, not enough water and rapidly dehydrated, difficult for defecation
• Oral rehydration therapies- contain ions + glucose
  –> ions (Na+) and glucose can be transported via enterocyes
  == osmotic gradient and allow H2O to flow back

Question 15

Describe the different types of utility observed in the gastrointestinal
system and the stimulus for this movement. (5 marks)

Answer 15

1. peristalsis in esophagus; propulsion
2. peristalsis in stomach; churning
3. segmental contractions + little bit of peristalsis in SI
4. segmental contractions + little bit of peristalsis in LI -> mass movement
5. migrating motor complex = stimulated by motilin during fasting phase