Digestive System Organization

Study Notes

The digestive system consists of the gastrointestinal tract (GIT) and accessory organs.
The GIT includes the mouth, oropharynx, esophagus, stomach, small intestine, and large intestine.
Accessory organs include salivary glands, liver, gallbladder, and pancreas.

Ingestion: food and liquid enter the oral cavity.
Mechanical digestion: crushing of food in the oral cavity and mixing in the stomach.
Chemical digestion: breakdown of food into small organic molecules.
Secretion: enzymes and digestive fluids secreted by the digestive tract and its accessory organs facilitate chemical digestion.
Absorption: passage of nutrients from the digestive tract into blood or lymph for distribution to tissue cells.
Elimination: undigested material is released through the rectum and anus by defecation.

Mucosa (inner lining) secretes mucus that lubricates and protects the GIT.
Submucosa (layer of dense irregular connective tissue) contains blood vessels and lymphatic vessels.
Muscularis propria (smooth muscle layer) has two layers: inner circular layer and outer longitudinal layer.
Serosa (outermost layer) is a layer of loose connective tissue that secretes fluid to lubricate the outside of the GIT.

Longitudinal muscle layer shortens the segment of the intestine and expands the lumen.
Circular muscle layer reduces the diameter of the lumen and increases its length.
Oblique muscle layer (found in the stomach only) helps to mix food with digestive enzymes.

Each muscle layer functions as a syncytium.
Actin filaments are attached to dense bodies, which are bonded together by intracellular protein bridges called gap junctions.
The slow waves of electrical activity occur at different frequencies in different parts of the GIT.

Autonomic nervous system (ANS) regulates GI tract activities:
- Sympathetic nerves inhibit GI tract activities.
- Parasympathetic nerves stimulate GI tract activities.
Enteric nervous system (ENS) regulates GI tract activities:
- Myenteric plexus controls movements and contraction.
- Submucosal plexus controls secretions.
Hormonal control:
- Hormones from the GI tract and endocrine glands regulate GI tract activities.
- Enteroendocrine cells release peptide hormones and biogenic amines.

Mechanical digestion through mastication.
Chemical digestion through amylase enzyme in saliva that breaks down polysaccharides into disaccharides.
Food particles are mixed with saliva during mastication, resulting in a moist lump called a bolus.

Three pairs of salivary glands: sublingual, parotid, and submandibular glands.
Saliva consists of 99.5% water and 0.5% dissolved substances, including HCO3-, electrolytes, and amylase enzyme.
Functions of saliva:
- Moistens food and the mouth during mastication.
- Cleans the teeth and inhibits bacterial growth.
- Digests a small amount of starch via amylase.

Actively moves food from the throat to the stomach.
Control of movement:
- Upper esophageal sphincter prevents air from entering the esophagus.
- Lower esophageal sphincter (cardiac sphincter) prevents backflow of stomach contents.

Muscular, expandable, J-shaped organ that can expand to contain 1-1.5 L of material (chyme).
Regions:
- Fundus
- Cardia
- Body
- Antrum
- Pylorus
Functions:
- Temporary storage of ingested food.
- Mechanical digestion of ingested food.
- Chemical digestion of food through the action of acid and enzymes.
- Production of intrinsic factor.

Glands in the fundus and body:
- Secrete acid and enzymes.
- Parietal cells secrete intrinsic factor and HCl.
- Chief cells secrete pepsinogen.
Glands in the pylorus:
- Secrete mucus and hormones that coordinate and control digestive activity.

Parietal cells do not create HCl in their cytoplasm.
H+ and Cl- are transported and secreted separately.
H+ is generated as carbonic anhydrase converts CO2 and H2O to carbonic acid.
Bicarbonate ions are ejected into the interstitial fluid in exchange for a chloride ion.

Stomach accommodates a 20-fold increase in volume with little change in tension in its wall.
Receptive relaxation of the stomach is mediated by the vagus nerve.
Gastric mixing:
- Each time a strong antral contraction occurs, it pushes food forward toward the pyloric sphincter, which is closed, so food returns back.
- Such movements mix food with digestive enzymes, converting food into chyme.