chemical digestion

Gastric secretions include:

• Mucus → To protect the stomach lining and assist with further lubrication

• Hydrochloric acid (HCl) → Provides proper pH for enzymatic reactions to occur

• Pepsinogen → When acted on by HCl, turns into pepsin and breaks down protein into polypeptides

• (pancreatic amylase in dogs)

gastric secretions, Also produces: 

• Intrinsic factor (glycoprotein) → Necessary for cobalamin absorption in the small intestine
  
  

• Gastrin → Helps regulate acid production
  
  

• Creates an acidic environment to kill most bacteria

Topic 8: Stomach to Small Intestine

• The stomach has a pacemaker that produces five slow waves per minute, which initiate muscular contractions

• These contractions mix food with the gastric secretions to form a product called chyme, which must be passed into the small intestine for further digestion

• Emptying rate of chyme to the small intestine can vary depending on the meal:

  • Larger meals → slower emptying rate

  • Meals high in fat and soluble fiber → slower emptying rate

  • Meals higher in liquid or insoluble fiber → faster emptying rate

Up to this point, most of digestion has been mechanical. This changes once we reach the small intestine.

Topic 9: The Small Intestine

Three parts with no clear division:

• Duodenum → First and shortest portion; where pancreatic and bile ducts enter
  
  

• Jejunum → Makes up part of the bulk of the small intestine
  
  

• Ileum → Makes up part of the bulk of the small intestine
  
  

• Lined with villi and microvilli; cells lining each of these are called enterocytes
  
  

• The surface of the microvilli is called the brush border
  
  

• This structure increases the absorption surface area

Topic 10: Enzymatic Breakdown in the Small Intestine

• There is still some mechanical digestion occurring here through muscular contractions

• The pancreas and duodenal glands secrete crucial enzymes

Fat digestion:

• Intestinal lipase → breaks fats into monoglycerides, diglycerides, glycerol, and fatty acids

• Pancreatic lipase → also helps break down fats

Protein digestion:

• Aminopeptidase → cuts amino acids off the end of protein chains

• Dipeptidase → splits dipeptides into single amino acids

• Enterokinase → activates trypsinogen into trypsin (a key protease)

• Trypsin → activates itself and other protein-digesting enzymes like chymotrypsin and carboxypeptidase

Nucleic acid digestion (DNA/RNA from food):

• Nucleotidase & nucleosidase → break down nucleoproteins into bases and sugars

• Pancreatic nuclease → also breaks apart nucleic acids

Carbohydrate digestion (brush border enzymes):

• Maltase → maltose → glucose
  

• Lactase → lactose → glucose + galactose
  

• Sucrase → sucrose → glucose + fructose
  

• Pancreatic amylase → starch → smaller sugars
  

• Brush border enzymes complete final digestion
  

• The acidic chyme is neutralized by bicarbonate salts so that digestive enzymes can work more effectively