Lecture 22 - GI III (The Digestion & Absorption of Food)

Digestive function of the liver

produce bile for export to the duodenum

What is bile?

a fat emulsifier: it breaks up fat into tiny particles so that the TGs are more accessible to digestive enzymes

Gallbladder function

stores and concentrates bile

From which duct does the gallbladder receive bile?

cystic duct

The liver receives blood from two sources

1. Arterial blood (from hepatic artery)

2. Venous blood (carried by hepatic portal vein)

Arterial blood provides the liver with what?

- the liver's O2 supply

- metabolites for hepatic processing

Venous blood provides the liver with what?

- newly absorbed nutrients for processing

From which vein does blood leave the liver?

hepatic vein

portal triad

hepatic artery, portal vein, bile duct

Lobule of the liver

functional unit of the liver

Where are portal triads located?

at each of the 6 corners of the lobule

Bile and blood flow in the liver

bile and blood will flow in opposite directions through each lobule

Composition of bile

A yellow-green, alkaline solution containing

- bile salts

- bile pigments

- cholesterol

- neutral fats

- phospholipids

- electrolytes

Main bile salts

- Cholic acid

- Chenodeoxycholic acid

Hepatocytes produce how much bile daily? and what does it depend on?

- 500-1000ml

- Depends on composition + amount of fatty acids

The more fat ingested....

the more bile produced

Pancreatic colipase function

links pancreatic lipase enzyme with bile salts to anchor lipase close to site of action

How are bile salts conserved?

by recycling through the enterohepatic circulation

Main bile pigment

bilirubin

most bilirubin is converted to...?

urobilinogen by intestinal bacteria

Key stimulus of bile production/secretion

increased rate of return of recycled bile salts to liver via the enterohepatic circulation

Rate of bile salt recycling is greatest when?

at the time of food intake

Secreted bile salts consist of what?

- 95% old recycled bile salts

- 5% newly synthesized bile salts

95% of bile salts are reabsorbed by the...?

terminal ileum

How does the gallbladder concentrate bile?

by absorbing water and ions

What pushes bile into the cystic duct?

muscular contractions

How are gallstones formed?

supersaturation of bile salts with cholesterol (+ stasis of bile fluid) causes cholesterol to crystallize forming them

Other name for hepatopancreatic sphincter

Sphincter of Oddi

If there is no digestion, the hepatopancreatic sphincter...?

is closed and bile backs up into cystic duct into gallbladder

Gallbladder contraction is stimulated by..?

CCK (signalled by fatty acid, chyme)

Three processes that are CCK's responsibility

- stimulates gallbladder contraction

- stimulates secretion of pancreatic juice

- relaxes hepatopancreatic sphincter

Other than CCK, what hormone stimulates bile secretion?

secretin

Secretin mainly stimulates...?

bicarbonate-rich fluid secretion

focusing on epithelial cells lining bile ducts

Where is digestion completed and most of absorption occuring?

small intestine

once food reaches the small intestine is fat digested?

no

three regions of the small intestine

duodenum, jejunum, ileum

How long does it take for chyme to traverse the small intestine?

3-6hrs

What do intestinal glands secrete and how much per day?

1-2L of intestinal juice daily

What is the major stimulus of intestinal juice secretion?

distension/irritation of intestinal mucosa by hypertonic or acidic chyme

Composition of intestinal juice

- mainly water & mucus

- No enzymes

If there are no enzymes in intestinal juice, where do the enzymes for digestion in the small intestine come from?

from the epithelial microvilli (brush border enzymes)

Motility of small intestine

segmentation (mixing)

What initiates segmentation in the small intestine?

intrinsic pacemaker cells in the longitudinal layer

Frequency of contraction and region in the small intestine

Higher frequency in duodenum (12x/min) than ileum (9x/min)

What is the effect of having more contractions in the duodenum compared to the ileum?

keep things moving to provide room for new material

Once absorption is complete, what happens to motility?

segmentation replaced by peristalsis

Migrating motility complex

form of peristalsis in the small intestine acting as a "housekeeping" function to clear the GI tract of undigested material

Motilin

peptide hormone, secreted by the duodenum endothelial cells, that stimulates peristaltic waves

How long does it take for an MMC to go from the duodenum to the ileum

2hrs

why is the ileococal sphincter usually closed?

to prevent bacteria entering the small intestine

Two ways the ileocecal sphincter opens

1. pushed open by ileal contents

2. Gastroileal reflex

Gastroileal reflex

triggers relaxation of ileocecal valve due to enhanced secretory & contractile activity of stomach

Gastrin

increases motility of ileum & relaxes ileocecal sphincter

How is regurgitation back into the ileum prevented?

once chyme has passed through, backward pressure from cecum closes valve flaps

In order to absorbed, food products must be...?

hydrolysed into monomers

Monosaccharides in diet

glucose, fructose, galactose

more complex carbohydrates

- sucrose

- lactose

- maltose

- starch

- glycogen

Why is cellulose a source of fiber?

it cannot be digested due to its different glucose linkages

Digestion of carbs begins where with what?

in the mouth with salivary amylase

Salivary amylase purpose

break carbs down into smaller chains

What inactivates salivary amylase?

gastric acid, and then it's digested by pepsin

Pancreatic amylase

breaks down carbs in the small intestine

intestinal brush border enzymes

- complete digestion to monosaccharides

dextrinase, glucoamylase, maltase, sucrase, lactase

Latose intolerance

Inability to digest lactose due to lacking the enzyme lactase

GLUT5 transporter substrate

fructose

GLUT5 transporter is

Na+ independent

does not require sodium ions (Na⁺) to facilitate the transport of fructose across the cell membrane. GLUT5 operates through facilitated diffusion

Glucose and galactose absorption

Secondary active transport coupled with sodium

Digestion of proteins begins where and with what?

in the stomach with pepsinogen (converted to pepsin)

Optimal pH for pepsin

1.5-3.5

Primary activity of pepsin

cleaves bonds with tyrosine & phenylalanine hydrolyses 10-15% of ingested protein

What inactivates pepsin?

high pH of small intestine

Pepsin breaks proteins down into...?

smaller polypeptides + some amino acids

Digestion of proteins continues with

many proteolytic enzymes in small intestine

proteolytic enzymes of the small intestine

- pancreatic trypsin & chymotrypsin

- carboxypeptidase (bb/pancreatic)

- aminopeptidase (bb)

- dipeptidase (bb)

bb - brush border

Carboxypeptidase action

breaks off amino acids, 1 by 1 from carboxyl end of chain

Aminopeptidase action

breaks off amino acids, 1 by 1 from amino end of chain

Dipeptidase action

splits dipeptides into amino acids

Protein digestion in the small intestine relies on...?

both pancreatic & brush border enzymes

What protein can the small intestine epithelium absorb?

mono-, di- and tripeptides

absorption of individual amino acids

secondary active, carrier-mediated transport linked to Na+; carriers recognize groups of amino acids (eg: acidic, basic, neutral)

absorption of dipeptides/tripeptides

transported into epithelial cells with H+; digested to single amino acids within cytoplasm

(makes use of a hydrogen gradient maintained by an apical sodium/hydrogen exchanger)

Intestinal cells are joined at luminal surfaces by

tight junctions

Why does having tight junctions make it necessary to have carrier/transporters in intestinal cells?

Therefore we need the transporters/carriers we have to be able to absorb nutrients, otherwise the tight junctions are too tight for absorption

Key site of fat digestion

small intestine with pancreatic lipases

emulsion

droplets of fat each ~1 μm - suspended in aqueous environment

Fats must be ___ with ___ before absorption.

emulsified, bile salts

lipase action

cleave TGs to form fatty acids + glycerol/2-monoglycerides

where is fat absorption completed?

ileum

What fat digestion products can diffuse through the plasma membrane?

FFA, monoglycerides, cholesterol & fat-soluble vitamins

and micelles

Inside epithelial cells, fat digestion products are...?

FFA & monoglycerides resynthesized into TGs

Chylomicron

TGs + phospholipids + cholesterol + protein coating

Lacteal

the lymphatic vessels of the small intestine that absorb digested fats.

chylomicrons are packaged by the golgi body and sent to the ___ vessels

lacteal vessels

fat in the lymphatic system (chylomicrons) empty where?

into venous blood via thoracic duct

What happens to fats once they reach the bloodstream?

fats hydrolyzed lipoprotein lipase to FFA & glycerol ⇒ can pass through capillary walls

Lipoprotein lipase

capillary endothelial cell derived enzymes that hydrolyze TGs