Ch 26: Digestion, Absorption, and Nutrition

What converts pepsinogen into pepsin?

acidity and pepsin (autocatalytic)

What signals stimulate the secretion of pepsinogen?

• vagal stimulation via acetylcholine

• direct response to gastric acid

What is the other name for Rennin (not renin!)

chymosin

What is the function of Rennin?

This is a proteolytic enzyme that causes milk to curdle in the stomach.

Milk is retained in the stomach and released more slowly.

How does rennin secretion change after birth?

Rennin secretion (specifically prochymosin) is maximally secreted the first few days after birth.

Over time it is replaced by pepsin as the primary gastric protease

What activates prochymosin?

stomach acid converts prochymosin into Chymosin/Rennin

What is the role of gastric intrinsic factor?

This protein binds to vitamin B12 in the duodenum

What pathway does vitamin B12 take through the digestive system?

B12 is bound to proteins in the stomach (R-factor). This complex passes to the duodenum where pancreatic proteases digest the R factor and release B12. 

Intrinsic factor binds to B12 and this B12-Intrinsic Factor complex is absorbed in the ileum.

What can result from insufficient intrinsic factor?

• ileal disease

• resection

• atrophic gastritis

• Pernicious anemia due to a lack of vitamin B12

What is the function of the Brunner’s glands?

These glands, present in the duodenum, release alkaline mucus to protect the mucosa from acidic chyme. 

What stimulates release of mucus by Brunner’s glands?

• local irritation, vagus (ACH)

What inhibits release of mucus by Brunner’s glands

sympathetic nervous system (NE)

What is the function of Crypts of Lieberkühn?

secretion of water-like fluid (1800mL/day)

What do the large intestines secrete?

The large intestines lack villi or enzymes for digestion. Primary secretion is alkaline mucus from the Crypts of Lieberkühn 

Which of the following statements concerning HCL in the stomach is correct?

a. It inactivates pancreatic enzymes

b. it increases the pH in the stomach

c. it activates pepsinogen

d. it is secreted by ECL cells

e. all of the above

The correct answer is C.

Stomach acid doesn’t inactivate pancreatic enzymes because it doesn’t come in contact with  them.

Stomach acid decreases pH in the stomach

Stomach acid components are secreted by parietal cells. 

What digestive enzymes are released by the salivary glands?

• alpha amylase

• lingual lipase

What is mostly absorbed in the jejunum? (6)

• ethanol

• nutrients

• vitamins

• various ions

• water

• electrolytes

What is absorbed in the ileum?

• vitamin B12

• bile salts

What is absorbed in the colon

• water

• electrolytes

What is absorbed in the rectum?

some drugs like steroids and salicylates

How many barriers must a nutrient cross to be absorbed? Name them, superficial to deep. 

8 barriers 

• Unstirred layer

• glycocalyx (membranous carbohydrates)

• apical cell membrane

• cytoplasm of enterocyte

• basolateral cell membrane

• intercellular space

• basement membrane

• wall of capillary or lymph vessel

Where are lactose and sucrose digested?

The enzymes for digestion of lactose and sucrose are only present at the brush border

How does disaccharide digestion work?

enzymes are present on the brush border membrane. Disaccharides are split into monosaccharide components and then absorbed into the cytosol. 

How is glucose absorbed into the cytosol of the intestines?

Secondary active transport via a Sodium/Glucose symporter. 

How is sucrose absorbed into the cytosol?

facilitated diffusion via GLUT5 transporter

What is the clinical significance of malabsorption of sugars?

Sugars that are unabsorbed will increase the osmolarity of the lumen. This increase in solutes will draw fluid out of the cells and increase the fluid volume in the lumen.

this may cause diarrhea

What composes sucrose?

glucose and fructose

What are two examples of endopeptidases in the small intestine?

trypsin and chymotrypsin

What is an example of an exopeptidase in the small intestine?

carboxypeptidase

What is the molecular pathway for protein digestion?

proteins → polypeptides → polypeptides and AA → amino acids

What enzyme activates trypsinogen?

enterokinase (although trypsin is autocatalytic for trypsinogen activation)

What is the primary means of absorption for amino acids?

secondary active transport with sodium ions and facilitated diffusion

What is emulsification?

the process of aggregating dietary triglycerides via bile salts. 

What is the purpose of enzymatic digestion of lipids?

this process yields monoglyceride and fatty acids. both of these can diffuse into the enterocyte. 

What is the result of pancreatic lipase action upon triglycerides?

Triglycerides will produce 2-monoglyceride and free fatty acids

How does inadequate fat absorption impact vitamin absorption?

Insufficient fat absorption also reduces the absorption of fat soluble vitamins (A,D,E,K).

What effect does GLP-1 have on Orexin-A?

GLP-1 inhibits Orexin-A and thus inhibits hunger responses and promotes satiety. 

What are two key peripheral factors that regulate hunger/satiety? What do they do?

• Leptin is a hormone secreted by adipocytes in response to fat intake, usually after a meal. Leptin is anorexigenic and inhibits hunger signaling.

• Ghrelin is a hormone secreted by the stomach and other cells when the stomach is empty. Ghrelin is orexigenic and stimulates hunger signaling.