Small Intestinal Function / Nutrient Absorption / Large Intestine Recovery and Residue

What type of gland is the pancreas?

Heterocrine (Mixed gland) - 2 distinct functions

• endocrine and exocrine roles

What are the endocrine and exocrine functions of the pancreas?

How do each areas look histologically?

1.) Endocrine - mainly functions to regulate blood sugar (e.g. producing glucagon)

• Histologically: pale stained areas are islets of langerhans (Containing the endocrine cells)

2.) Exocrine - (90-98% by mass) produces pancreatic juice, key for digestion

• Histologically: darker stained - makes up most of pancreas

What are exocrine glands?

Secrete substances onto epithelial surface by way of a duct.

• Microscopic structure of pancreas is similar to a saliva gland

How does the pancreatic juice travel to the duodenum?

Pancreatic juice travels from acini into small ducts which merge into a common duct called the pancreatic duct, carries juice to the duodenum

• Or it may join the common bile duct

What does pancreatic juice contain?

• Bicarbonate

  • Neutralizes all of the acid entering duodenum from stomach

• Digestive enzymes

  • Breaking down fat, protein, etc.

What two key cell types contribute to the composition of pancreatic juice?

1.) Cells of the intercalated duct

• Produce secretion rich in HCO3- (alkaline bicarbonate)

• To neutralize acidic contents coming from the stomach and prevents injury

• Also provides optimal pH for pancreatic enzymes to function

2.) Acinar cells

• Produce enzymes essential for breakdown of macromolecules

• e.g. trypsinogen (Activated by enteropeptidase)

What is the enzymatic composition of pancreatic juice?

• Proteases / Endopeptidases (Digest proteins/polypeptides into amino acids)

• Amylase (Carbohydrates into maltose)

• Nuclease (DNA / RNA into nucleosides)

• Lipase (Digest fats)

• Bile Salts (Emulsify fats)

What are the three phases of secretion of pancreatic juice?

1.) Cephalic Phase

• Anticipation of food, increased vagal activity to pancreas and gastrin release from stomach, increased secretion of pancreatic juice

2.) Gastric Phase

• Distention of the stomach in response to food causes a reflex, stimulating further secretion

3.) Intestinal Phase

• Acidic chyme enters the duodenum, and copious alkaline pancreatic secretion occurs in response to vagal and hormonal signals, depending on the amount and compsition of food

  • Two peptide hormones are produced by the duodenum walls in response to distention of duodenum and the presence of fatty acids

    • Secretin

    • Cholecystokinin (CCK)

Where is secretin produced and what is its function?

• Secreted by S cells of duodenum

• Stored as prosecretin, activated by gastric acid

• Function:

  • To help regulate duodenum pH by:

  • Stimulating bicarbonate production from intercalated duct

  • Inhibiting HCl secretion from parietal cells

  • Also stimulates liver to produce bile, bile helps emulsify lipids in duodenum so lipase can act upon them

Minimizes acid damage to small intestine

• As pH increases, stimulus to produce secretin is reduced

• NEGATIVE FEEDBACK

Where is cholecystokinin produced and what is its function?

Secreted by enteroendocrine cells called I-cells in mucosal lining of duodenum.

• Function:

  • Stimulates enzyme-producing cells from acini cells of pancreas and bile from the gallbladder

    • Catalyze digestion of fat, proteins and carbs

  • Released in response to increased fatty acids and peptides entering duodenum in chyme

  • As levels of nutrients fall, concentration of hormones also drops, NEGATIVE FEEDBACK

How do different species become adapted to different diets considering their pattern of pancreatic juice secretion?

Species are adapted to particular amount of acid and nutrients entering intestine from stomach.

Ruminants

• Continuous and constant rate of pancreatic secretion

• Due to continuous feeding pattern

Horses

• Continuous, but have great increase in secretion several minutes after feeding

• Enzymes can be produced rapidly so have low enzyme concentration

Cats/Dogs

• Produce little pancreatic juice when intestine is empty but production increases greatly after feeding

What are the main roles of the liver?

Main roles relate to digestion

• Production of bile by hepatocytes

• Processes nutrients absorbed from the intestine, regulates their release into blood

What is bile?

Where is it stored?

• an alkaline fluid containing cholesterol and bile acids which emulsify lipids to help break them down

• Gall Bladder

Where does the liver receive blood from?

1.) Hepatic Artery (Branch of mesenteric artery)

2.) Portal Vein (From stomach, intestines, pancreas)

What are the liver sinusoids?

• A type of capillary that serves as location for mixing of the oxygen rich blood from the hepatic artery and the nutrient rich-blood from the portal vein.

• Direct contact between hepatocytes and blood plasma

What are the Kupffer cells?

• Star shaped macrophages

  • The Kupffer cells line the sinusoids of the liver, they can take up and destroy foreign materials such as bacteria.

• Constitute more than half of the total number of macrophages in the body

Where is bile secreted from?

• Secreted into small canals between the hepatocytes, known as bile canaliculi

Where is bile salt transported from and what is it necessary for?

Where is it stored?

• Bile salts are actively transported from hepatocytes

  • Tight junctions prevent leakage into interstitial fluid

• Bile salts are necessary for degradation and absorption of fat in the small intestine

  • Into small globules with greater surface area, important for lipase to digest into FA and glycerol

• Bile is stored in gallbladder when digestion is occuring

How is bile secretion regulated?

• An increase in the concentration of bile salts in the portal blood triggers an increase in bile salts secretion, leading to an increase in bile volume.

  • Bile production and concentration are greatest when bile salts recirculate between liver and small intestine after a meal

  • Acidic chyme entrance into the duodenum stimulates release of secretin which increases production of bile (And increases HCO3- secretion)

**No specific neural stimulation of bile production

What are some species difference between species, considering bile storage?

• Dogs, cats and humans - no need for bile to be present in intestine at all times

  • Periodic storage of bile in gallbladder is thus beneficial

• In species with more continuous digestion like grazers have a more constant flow of bile into the small intestine

• Some species lack a gallbladder (Horses)

• Or have a gallbladder with a short retention time/concentrating mechanism (ruminants)

What key processes occur in the intestines?

• Most of the chemical degradation and absorption occurs in the intestines

• Fats degraded exclusively in this part of the digestive tract

• Pancreatic enzymes degrade CHOs and proteins in small intestine, digestive products then actively absorbed

• Enzymes also located on apical membrane of epithelial cells

• Any CHO and protein now broken down in SI are microbially degraded in the large intestine

What are the three key functions of muscle contractions in the small intestine?

• Mix the content

• Ensure liminal contents come in contact with the apical membranes of epithelial cells (enterocytes) to allow further chemical catalysis and absorption

• Transport chyme slowly along the digestive tract

What are the two types of movement in the small intestine?

1.) Mixing and segmentation

2.) Propulsive movements (peristalsis)

What are the key features of segmentation movements of the SI?

• Most common: divides intestinal contents into small segments, moving content back and forth

What are the key features of peristaltic movements of the SI?

• Waves on contractions moving from one section to the next, relaxation of smooth muscle ahead, contraction of circular smooth muscle behind to keep it from moving backward

• Longitudinal contraction pushes it forward

After nutrients have been absorbed, what waves or type of movement clear the rest of the contents?

• Segmentation stops and migrating motility complex (MMC) waves propagate to clear rest of contents

• Waves of electrical activity to move indigestible substances like fiber and bone through SI to the colon

• Responsible for rumbling experience

How is motility of the small intestine regulated?

• Interstitial Cells of Cajal - Pacemaker cells of GI tract

  • Found in muscular walls of the intestine

  • They produce slow waves or rhythmic depolarization, which are transferred to the smooth muscle cells.

  • When chyme enters intestines after a meal, short reflexes are triggered, stimulating cells of Cajal, more chyme = stronger reflex

• With neural, hormonal or mechanical input, slow waves may reach threshold value for generation of action potentials

• Action potentials propagate from muscle cell to muscle cell via gap junctions, causing contractions

What do epithelial cells in the crypts secrete?

• Intestinal juice

What are the main cells found in the intestinal epithelial crypts and what do they do?

• Enterocytes

  • Responsible for final degradation and absorption

• Paneth Cells

  • Secrete antimicrobial peptides

• Goblet Cells

  • Secrete mucus for lubrication/protection

How are nutrients absorbed in the small intestine?

• Nutrients broken down by enzymes

• Move from epithelial lumen into interstitial fluid via the large surface area (folds, villi, microvilli)

• Intestinal secretions are mainly H2O, ions and mucus (Most important ions are Na+ and HCO3-)

What are the main mechanisms of absorption?

1.) Diffusion - water, very small water-soluble substances, lipid-soluble substances

2.) Transcellular transport by carrier proteins - for ions, large-water soluble substances

3.) Exocytosis and endocytosis - active transport of macromolecules (peptides/proteins/antibodies from colostrum)

How are carbohydrates absorbed into the interstitial fluid?

• Apical membrane contains secondary active transport cotransporters

• Against the concentration gradient

• Bile salts and B-vitamins are transported in the same way

How are proteins absorbed into the interstitial fluid?

• They are degraded into peptides in the lumen

• Enzymes in epithelial cells degrade peptides to amino acids and di-, tri- peptides

• Active transport into the cell

Where is trypsin produced?

• Pancreas produces trypsinogen, activated to trypsin by entropeptidase or by itself (autocatalysis)

How is fat digested and degraded?

• Large proportion of carnivore diet

• Degradation of fat mainly takes place in the small intestine via the action of lipase, colipase, and bile salts

• Requires lipases from pancreas and bile salts from liver

  • Lipase breaks down triglycerides to monoglyceride and fatty acids

  • Large fat droplets are emulsified by bile salts to form smaller fat droplets

  • Degraded products absorbed or packaged by micelles

  • Micelles carry them and the products diffuse into the epithelium, bile salts return to liver

• The fats form into chylomicrons, and pass into the lymph system through exocytosis

What is a micelle?

A package of fat surrounded by bile salts

• being packaged like this allows them to move across the epithelium to be absorbed.

What do bile salts do?

Increase the rate of fat degradation, enabling monoglycerides and free fatty acids to diffuse into the epithelial cell as micelles.

• The bile salts returned to the liver rather than being absorbed.

• They simply deliver the free fatty acids and monoglycerides to the epithelium.

Where is most water absorbed in the SI?

• Derived from glands outside GI tract (Salivary, pancreas, liver)

• Significant amount is secreted from glands in stomach and crypts of small intestine

• Other water comes from eating and drinking

• Intestinal Mucosa is freely permeable to water (Small intestine absorbs 80-90% of water received)

What is the ileocaecal sphincter?

A muscular valve at the transition between the small and large intestine, its function is to limit the reflux of colonic contents back into the ilium of the small intestine

• Usually contracted but expansion of last part of small intestine causes transient relaxation

• Ensuing expansion of the large intestine thus causes the sphincter to close

  • This is controlled by sensory cells on the walls of the large and small intestine, which detect changes in pressure. chemical irritation, and fluidity of the content.

• Emptying is a highly controlled process

What are the main functions of the large intestine?

1.) Absorption of water and ions from indigestible residue.

2.) Microbial digestion and absorption of any remaining carbohydrates and proteins.

What lines the walls of the large intestine? (Or rather does not line the walls).

• The wall of the large intestine is lined with simple columnar epithelium, with invaginations.

• The invaginations are called the intestinal glands or colonic crypts.

  • The walls of the large intestine lack villi and epithelial cells do not have microvilli

What is an important process across all species that occurs in the large intestine?

• Absorption of Na+ and water

How much water supplied to the GIT is absorbed?

97% of water

What does transportation in the large intestine look like?

• Motility and mixing, allows retention and propulsion of food

• Transport is slow, especially in hindgut fermentors

What types of contractions occur in the large intestine?

1.) Segmentation

2.) Peristalsis

• Not primary propulsion (Like small intestine)

3.) Retroperistalsis (Assisting defecation)

• is the reverse of the involuntary smooth muscle contractions of peristalis

4.) Mass movements (Contractions propelling material along the large intestine)

• Majority of movement

• Triggered by chyme in stomach

In what circumstances might retroperistalsis occur?

• It can occur at the first part of the duodenum as protection from the acidity of food.

• At the terminal Ilium, where water and electrolytes are absorbed to assist defecation.

• It also occurs as a precursor to vomiting.

What regulates the contractions of the large intestines?

• Interstitial cells of cajal (pacemaker cells)

How is the rectum emptied?

• Rectum in carnivores and omnivores is empty most of the time

• When full, distention of rectum stimulates stretch receptors in the wall, causing short and long reflexes

• Causes the rectum and terminal portion of colon to contract propulsively to produce mass movement

• Animal perceives need to empty rectum (Some animals like cats have voluntary control)

What do feces from carnivores contain?

• Large proportion of shed epithelial cells and bacteria from the large intestine

What do feces from herbivores contain?

• Faecal solids are mostly undigested plant remnants

• Colon in horses have higher density of microorganisms and nitrogen-containing compounds than colon of ruminants or pigs - horse manure is a good fertilizer