Ingestion and Digestion Notes

Digestion begins in the oral cavity, continues in the stomach, and finishes in the duodenum (the first part of the small intestine).

Breaks down chemical bonds in macromolecules using enzymes from saliva.
Saliva moistens and lubricates food, aiding mechanical digestion.
Salivary glands are innervated by the parasympathetic nervous system.
The presence or even the sight/smell of food triggers increased parasympathetic stimulation and salivation.
Saliva contains:
- Salivary amylase (ptyalin): Hydrolyzes starch into smaller sugars like maltose and dextrins.
- Lipase: Catalyzes the hydrolysis of lipids.
The amount of chemical digestion in the mouth is minimal due to the short duration food stays there.
The tongue forms the food into a bolus, which is then swallowed into the pharynx.

The cavity connecting the mouth and posterior nasal cavity to the esophagus.
Connects to both the esophagus and the larynx (part of the respiratory tract).
Divided into three parts:
- Nasopharynx: Behind the nasal cavity.
- Oropharynx: At the back of the mouth.
- Laryngopharynx: Above the vocal cords.
The epiglottis prevents food from entering the larynx during swallowing.
Failure of this mechanism can lead to aspiration of food and choking.

A muscular tube connecting the pharynx to the stomach.
Composed of:
- Top third: Skeletal muscle (under somatic/voluntary control).
- Bottom third: Smooth muscle (under autonomic/involuntary control).
- Middle third: Mix of both.
Peristalsis: Rhythmic contraction of smooth muscle that propels food toward the stomach.
Emesis (vomiting): Reversal of peristalsis due to factors like chemicals, infections, physical stimulation, or even cognitive factors.
Swallowing is initiated in the oropharynx muscles at the upper esophageal sphincter.
The lower esophageal sphincter (cardiac sphincter) relaxes to allow the bolus to enter the stomach.

The three main energy sources are carbohydrates, fats, and proteins.
Chemical digestion of carbohydrates and fats begins in the mouth.
No mechanical or chemical digestion occurs in the esophagus, other than the enzymatic activity started in the mouth.
Highly muscular organ with a capacity of about 2 liters in humans.
Located in the upper left quadrant of the abdominal cavity, under the diaphragm.
Uses hydrochloric acid (HCL) and enzymes to digest food, creating a harsh environment.
The mucosa is thick to prevent autodigestion.
Four main anatomical divisions:
- Fundus and body: Contain mostly gastric glands.
- Antrum and pylorus: Contain mostly pyloric glands.
Internal curvature: Lesser curvature.
External curvature: Greater curvature.
Lining of the stomach is thrown into folds called rugae.

Gastric glands respond to the vagus nerve (parasympathetic nervous system), which is activated by the sight, taste, and smell of food.
Three cell types:
- Mucus cells: Produce bicarbonate-rich mucus that protects the stomach wall from the acidic environment. The pH within the stomach is around $pH = 2$ .
- Chief cells: Secrete pepsinogen (inactive zymogen form of pepsin).
- Parietal cells: Secrete hydrochloric acid (HCL) and intrinsic factor.
Gastric juice: Combination of secretions from chief and parietal cells.
Pepsinogen is cleaved to pepsin by hydrogen ions and digests proteins by cleaving peptide bonds near aromatic amino acids.
Pepsin is most active at low pH, unlike most human enzymes.
Stomach acid kills most harmful bacteria, except Helicobacter pylori.
The acidic environment also denatures proteins and breaks down some intramolecular bonds.
Parietal cells secrete intrinsic factor that involved in vitamin B12 absorption.
Pyloric glands contain G cells that secrete gastrin.
Gastrin induces parietal cells to secrete more HCL and signals the stomach to contract.
Chyme: Acidic semi-fluid mixture resulting from the digestion of solid food.
The combined activities of the stomach significantly increase the surface area of food particles.
Few substances like alcohol and aspirin are absorbed directly from the stomach
The stomach is primarily an organ of digestion.

The small intestine consists of the duodenum, jejunum, and ileum.
The small intestine is up to 7 meters long.
The duodenum is responsible for most chemical digestion and some absorption.
Most absorption occurs in the jejunum and ileum.
Food leaves the stomach through the pyloric sphincter and enters the duodenum.
The presence of chyme in the duodenum causes the release of:
- Brush border enzymes: disaccharidases (maltase, isomaltase, lactase, and sucrase) and peptidases (dipeptidase) present on the luminal surface of cells lining the duodenum, break down dimers and trimers of biomolecules into absorbable monomers.
- Enteropeptidase: Involved in the activation of other digestive enzymes from accessory organs.
- Hormones: Secretin and cholecystokinin (CCK).

Maltase: Digests maltose.
Isomaltase: Digests isomaltose.
Lactase: Digests lactose.
Sucrase: Digests sucrose.
Lack of a particular disaccharidase causes an inability to break down the corresponding disaccharide.
Undigested disaccharides lead to bacterial hydrolysis, producing methane gas and causing osmotic effects, leading to bloating, flatulence, and diarrhea (e.g., lactose intolerance).

Break down proteins or peptides.
Aminopeptidase: Removes the N-terminal amino acid from a peptide, secreted by glands in the duodenum.
Dipeptidases: Cleave the peptide bonds of dipeptides to release free amino acids.
Proteins can be absorbed into di-and tripeptides.

A peptide hormone.
Causes pancreatic enzymes to be released into the duodenum.
Regulates the pH of the digestive tract by reducing HCL secretion and increasing bicarbonate secretion from the pancreas.
An enteroagastrone: Slows motility through the digestive tract, allowing more time for digestive enzymes to act on chyme.

A complex fluid composed of bile salts, pigments, and cholesterol.
Bile salts are derived from cholesterol and have both hydrophobic and hydrophilic regions, acting like detergents.
Emulsify fats and cholesterol into micelles in the small intestine which increases the surface area of fts.
Without bile, fats would separate out of the aqueous mixture and wouldn't be accessible to pancreatic lipase.
Proper fat digestion depends on both bile, mechanical digestion, and lipase, chemical digestion.

A complex mixture of several enzymes in a bicarbonate-rich alkaline solution.
Bicarbonate neutralizes acidic chyme and provides an ideal working environment for digestive enzymes (pH 8.5).
Contain enzymes that can digest all three types of nutrients: carbohydrates, fats, and proteins.