GastroIntestinal Track (GI Track)

Gastrointestinal Track (GI Track)

• Also called “Alimentary Canal”

• One way tube from the mouth to the anus

  • Order: Mouth, pharynx, oesophagus, stomach, small and large intestines

  • ~7m long

• Nutrient absorption (NOT digestion, absorption) is maintained through positive feedback sans some exceptions

  • Calcium & Iron absorption is regulated based on body’s needs

GI Track: Accessory Organs/structures

Step 0: Mouth, teeth, salivary glands

Begins digestive process.

• Teeth aid in mechanical digestion

GI Track

Step 1: Mouth (Oral Cavity)

• Chews and mixes food with saliva

  • Amylase enzyme breaks down polysaccharides in sugar/carbohydrates into glucose/simpler sugar molecules

GI Track

Step 2: Pharynx (Back of the mouth, past the Uvula)

• Epiglottis shuts the trachea, allowing food to pass down the oesophagus

  • Food passes from the mouth into the oesophagus through swallowing (deglutition)

• Skeletal muscles and mucus-producing glands are present in the pharynx

• Upper oesophageal sphincter is at the lower end of the pharynx

GI Track

Step 2.5: Deglutition in Pharynx (Swallowing)

1. Voluntary phase

   Body controls how long the food remains in mouth

2. Pharyngeal phase

   Food becomes ‘bolus’. Presence of a bolus in the pharynx stimulates pharyngeal receptors, triggering a contraction from the pharynx to the oesophagus. Mucus is secreted, coating the bolus enabling smooth movement.

3. Oesophageal phase

   Presence of a bolus in the oesophagus stimulates receptors in oesophageal lining, triggering peristalsis. Mucus is secreted, coating the bolus enabling smooth movement.

GI Track

Step 3: Oesophagus

• Peristalsis propels food towards the stomach

• Each end of the oesophagus has an upper and lower oesophageal sphincter.

  • Both sphincters prevent reflux from occuring

GI Track

Step 4.1: Stomach

• Structure

• 3 Layers of muscle

  • Longitudinal layer

  • Circular layer

  • Oblique layer

    • Oblique layer is unique to stomach, allowing mechanical digestion

• Stomach-to-small intestine passage called pyloric sphincter. Located at the distal end of the stomach

GI Track

Step 4.2: Stomach

• Function

2 functions:

• Mechanical digestion:

  • Unique form of peristalsis occurs due to contraction of oblique muscle layer in stomach

• Chemical Digestion:

  • Release of gastric juices that aid the digestion of protein (e.g hydrochloric acid released by chief cells & parietal cells) ~pH = 1

‘Bolus’ enters the stomach, but leaves as ‘chyme’

GI Track

Step 5.1: Small Intestine

• Overall Structure

3 Sections:

1. Duodenum - shortest region

2. Jejunum

3. Ileum

GI Track

Step 5.2: Small Intestine

• Cell Structure

1. Inner lining of small intestine (also called musosa) is - Circular folds

   Found primarily in duodenum and jujenum

2. Cellular projections - Intestinal villi

   Projections on each intestinal wall cell (also called enterocyte) into the inside of the intestine (also called lumen)

3. Intestinal glands - Crypt cells

   Secrete digestive enzymes

4. Finer projections on intestinal villi - Microvilli

   Crucially increase the surface area of every enterocyte. Where brush border enzymes are produced - enzymes that help digest carbohydrates & proteins.

GI Track: Accessory Organs/vestigial structures

Step 5.5.1: Pancreas

Releases pancreatic juices into the small intestine (duodenum), helping break down chyme.

• Aids Nutrient processing

• Helps neutralise stomach acid

GI Track: Accessory Organs/vestigial structures

Step 5.5.2: Liver

Releases bile into the small intestine (duodenum), helping break down fats and eliminate waste products.

• Aids Nutrient processing

GI Track: Accessory Organs/vestigial structures

Step 5.5.3: Gallbladder

Stores excess bile from the liver as a concentrate.

• Bile can be stored or released

GI Track

Step 6.1: Large Intestine

• Overall Structure

Sections:

1. Cecum

2. Colon

   a. Ascending colon

   b. Transverse colon

   c. Descending colon

   d. Sigmoid colon

3. Rectum

4. Anus

GI Track

Step 6.2: Large Intestine

• Cellular Structure

• Much simpler than small intestine

• Less enzyme-secreting cells

  • Many more Enterocytes line the walls of the small and large intestine

    • Have microvilli that absorb water and salt, and remaining nutrients missed

  • Goblet cells

    • Secrete mucus to make movement of stool easier to move and protects cells

GI Track

Step 6.3: Large Intestine

• Unique Structure

1. Teniae coli

   Bands of smooth muscle that line the large intestine. Contractions aid the movement haustrum that move the stool.

2. Haustra (singular: haustrum)

   Give wrinkled appearance of large intestine

3. Epiploic appendages

   Functon unknown

GI Track Layers