L5 - Accessory Organs

Most absorption is done where in the GI tract

Small intestine

Chyme moves into small intestine how long after entering stomach

Chyme moves into small intestine 0-3 hr after entering stomach

How long does it take for chyme to move through the small intestine

Takes 1-6 hr for chyme to move through small intestine

Length of small intestine

6m

Function of small intestine

Digestion of carbohydrate, fat and protein

Role of each part of the small intestine

Duodenum: Digestion & Absorption

Jejunum: Digestion & Absorption

Ileum: Absorption

Conduction rate in small intestine

1 cm/ min

What is conduction through the small intestine influenced by

Nervous system

Hormone

What nervous system influences conduction through the small intestines

ANS

What hormones influence conduction through the small intestines

Gastrin

Cholecystokinin

Secretin

How does the surface area of the intestines change with the addition of circular folds, villi & microvilli

Surface area of intestines as a cylinder:

Surface area of intestines with circular folds:

Surface area of intestines with villi:

Surface area of intestines with microvilli:

Surface area of intestines as a cylinder: 0.33 m²

Surface area of intestines with circular folds: 1 m²

Surface area of intestines with villi: 10 m²

Surface area of intestines with microvilli: 200 m²

Explain the structure of the gut wall

Role of microvilli

The form the brush border on villi to further increase surface area

Villi & microvilli increase the surface area of the intestines by what fold

x600

What is the intestinal epithelial mucosa made up of

Absorptive cells & secretory cells

What do the secretory cells of the intestinal epithelial mucosa secrete

Enzymes, hormones, fluid, mucus

Is digestion in the small intestine affected b the pancreas

Yes - Digestion in the small intestine is aided by enzymes and fluid secreted by the pancreas

There are 3 different secretions that come from 3 different types of cells in the pancreas. What are the cells & their secretions

Endocrine cells secrete hormones (e.g. Insulin, glucagon, somatostatin)

Exocrine cells secrete enzymes

Duct cells secrete bicarbonate

Digestive secretions from the liver and the pancreas are delivered into the duodenum of the small intestine through what sphincter

sphincter of Oddi

In the pancreas, there are 2 main exocrine cells. Name them

Acini

Ducts

Role of acini

Secrete enzymes

What stimulates acini

CCK (Cholecystokinin) and ACh (Acetylcholine)

Role of ducts in the pancreas

Secrete bicarbonate

What stimulates ducts in the pancreas

Secretin stimulates ducts

What is pancreatitis

Inflammation of the pancreas

Pancreatitis can lead to what

Autodigestion of the pancreas

How does the pancreas protect itself from autodigestion

It secretes kazal to protect itself

(Kazal is an inhibitor)

Explain how bicarbonate is formed and secreted into the lumen of the pancreas/intestines

CO₂ from the interstitial fluid diffuses into the epithelial cell.

Carbonic anhydrase catalyzes the reaction combining H₂O & CO₂ to make HCO₃ & H+

HCO₃⁻ is transported into the lumen via a Cl⁻/HCO₃⁻ exchanger, where Cl⁻ moves in the opposite direction.

This allows bicarbonate-rich pancreatic secretions to neutralize stomach acid in the duodenum.

H⁺ is removed via the Na⁺/H⁺ exchanger (antiporter), ensuring pH balance.

Na⁺ moves paracellularly (between cells) to maintain electroneutrality. The Na⁺/K⁺ ATPase pump maintains low intracellular Na⁺, enabling Na⁺-dependent transport processes.

What is the main component secreted by the pancreas into intestinal fluid?

The pancreas secretes a NaHCO₃ (sodium bicarbonate) solution.

What is the role of the Cl⁻/HCO₃⁻ antiport co-transporter in pancreatic fluid secretion?

The Cl⁻/HCO₃⁻ antiport co-transporter on the apical membrane of pancreatic cells exchanges chloride ions for bicarbonate ions, allowing bicarbonate (HCO₃⁻) to enter the pancreatic duct.

How does sodium (Na⁺) get secreted into the intestinal fluid from pancreatic cells?

Sodium (Na⁺) is secreted through the cell junctions of pancreatic cells

Which organ, besides the pancreas, also secretes NaHCO₃?

Liver (in bile)

How is bicarbonate production of the pancreas regulated by the intestines

Secretin stimulates ducts in the pancreas to secrete bicarbonate

Name some pancreatic enzymes

Trypsin

Carboxypeptidase

Lipase

Amylase

Ribonuclease

The whole body typically uses ___g of carbohydrates per day

The body typically uses 250g of carbohydrates per day

The brain typically uses ___g of carbohydrates per day

The brain typically uses 125g of carbohydrates per day

Average daily carb intake

250g/day

What are the two main sources of plasma glucose

Glucose absorption from meals and hepatic glucose production

What process allows glucose to move between plasma and interstitial compartments

Transcapillary diffusion

Is glucose uptake dependent on insulin

It can be

There are 2 types of glucose uptake - insulin dependent & insulin independent

What tissue types predominantly rely on insulin-dependent glucose uptake

Muscle and adipose (fat) tissue

Which organ is responsible for producing glucose in the absence of dietary intake

The liver (hepatic glucose production)

What are the two major glucose compartments in the body

Plasma glucose and interstitial glucose

Starch is broken down to what in the mouth (& by what)

Name a monosaccharide with 3 carbons (triose)

Glyceraldehyde

Name a monosaccharide with 4 carbons (tetrose)

Erythrose

Name a monosaccharide with 5 carbons (pentose)

Ribose

Name a monosaccharide with 6 carbons (hexose)

fructose

Glucose can exist in 2 forms - what are they

Straight chain & ring form

Maltose & isomaltose are both formed by 2 glucose molecules linked around an O in a glycosidic bond. How do they differ

Maltose: 2 glucose molecules linked by an α(1→4) glycosidic bond

Isomaltose: 2 glucose molecules but linked by an α(1→6) glycosidic bond

Where does carbohydrate absorption take place in the body

In the small intestine, specifically through intestinal absorptive cells (enterocytes)

Main mechanism for glucose absorption

Na⁺ moves down its concentration gradient, driving glucose (or galactose) into the cell.

Uses energy indirectly from the Na⁺/K⁺ ATPase pump, which maintains low intracellular Na⁺ levels.

Is the fact that glucose, galactose & fructose are all polar molecules a help or a challenge in carb absorption

A challenge

what is SGLT1

SGLT1 (Sodium-Glucose Linked Transporter 1) is a protein found primarily in the small intestine and kidney. In the small intestine, SGLT1 actively transports glucose and sodium ions across the intestinal epithelial cells

The function of SGLT1 is dependent on what

dependent on the gradient of sodium

At low glucose concentrations, glucose absorption is primarily driven by

At high glucose concentrations, glucose absorption is primarily driven by

At low glucose concentrations, glucose absorption is primarily driven by SGLT1

At high glucose concentrations, glucose absorption is primarily driven by GLUT2

(GLUT2 is found in the liver - lots of glucose there!)

what does this show

GLUT2 expression increases with higher glucose concentrations, indicating that its role in glucose absorption becomes more significant at high glucose levels.

SGLT1 expression appears relatively constant across different glucose concentrations, suggesting it is always present at a basal level.

Where is glucose concentration highest in the digestive tract after meals?

The stomach and proximal small intestine have the highest glucose concentrations after meals.

GLUT2 is not always present at the brush-border membrane. When is it at the brush border membrane

when glucose levels are high

Most carbohydrates in the diet are consumed as mono/di/polysaccharides

disaccharides or polysaccharides:

– Sucrose

– Lactose

– Maltose

– Starch

– Glycogen

– Cellulose

Can mono/di/polysaccharides all be absorbed by the intestinal cells for use in the body

No. Only monosaccharides are absorbed by the intestinal cells for use in the body.

(Disaccharides and polysaccharides must be digested to monosaccharides before they can be absorbed)

Summary of carb digestion & absorption

Effect of GLP-1 on gastric emptying

GLP-1 (Glucagon-Like Peptide-1) delays gastric emptying.

is maltose or isomaltose more common

Maltose