Upper GI & Monogastric Stomach Notes

Definition of Digestive System

The digestive system is defined as the collection of organs that take in, digest, absorb food for use by the body to maintain health and normal function. It encompasses all structures of the digestive tract, extending from the mouth to the anus, as well as accessory digestive organs that provide secretions aiding digestion.

Main Components

  1. Mouth/Oral Cavity (Buccal Cavity)
       - Includes:
         - Lips
         - Cheeks
         - Teeth
         - Tongue
         - Hard Palate
         - Soft Palate
         - Pharynx
         - Esophagus
         - Stomach
         - Jejunum
         - Ileum
         - Large intestines (Cecum, Colon, Rectum)
         - Anus

Accessory Components

  • Salivary Glands

  • Liver

  • Gallbladder

  • Pancreas

Digestive System: Process

The digestive system functions through five basic processes:

  1. Ingestion

  2. Mechanical Digestion

  3. Chemical Digestion

  4. Peristalsis

  5. Absorption of Nutrients

  6. Defecation / Elimination

Types of Digestion

1. Mechanical Digestion

Mechanical digestion involves breaking down food into smaller particles through physical grinding, which increases the surface area for chemical digestion.

2. Chemical Digestion

Chemical digestion is the process where various components secrete enzymes to break down chemical bonds in different components of food, including sugars/carbohydrates, fats, and proteins.

Peristalsis vs Segmentation

Peristalsis

  • Defined as wave-like smooth muscle contractions that propel food in one direction through the digestive tract.

  • Mainly involves longitudinal muscles and increases the speed of food passage.

  • Primary Action Areas: Entire GIT, particularly in the esophagus.

Segmentation

  • Characterized by squeezing smooth muscle contractions that move food back and forth, further mixing and breaking down pieces of food.

  • Involves circular muscles and decreases the speed of food passage.

  • Primary Action Areas: Small and large intestines.

Species Variation

Anatomical variations of the digestive system depend on the diet and are classified among different species:

  • Herbivores: Cattle, horses, sheep, rabbits

  • Carnivores: House cats, tigers, hyenas, orcas

  • Omnivores: Dogs, pigs, raccoons, chipmunks, bears, chimpanzees

Tissue Layers of the Digestive Tract

The walls of the gastrointestinal tract (GIT) can be divided into four layers:

  1. Mucosa
       - Closest to lumen of the tube
       - Contains sublayers:
         - Epithelium: Stratified squamous or simple columnar
         - Lamina Propria: Connective tissue
         - Muscularis Mucosae: Smooth muscle

  2. Submucosa
       - Composed of loose connective tissue

  3. Muscularis Externa
       - Contains 2-3 layers of smooth muscle, which may include oblique, circular, and longitudinal muscle layers depending on the location within the GIT.

  4. Serosa
       - Composed of loose connective tissue; the stomach has all three layers.

Mouth/Oral Cavity

Monogastric Stomach

  • The monogastric stomach receives and mixes food with saliva during mastication. It is characterized as a simple stomach with several key functions:
       - Prehension: Grasping
       - Mastication: Chewing
       - Salivation
       - Bolus Formation
       - Taste: Receptors are enclosed in gustatory papillae on the tongue.
       

Hard Palate

  • Separates the respiratory (nasal) and digestive tracts. It is a ridged bony roof of the anterior oral cavity.

  • Anomaly: Cleft palate can occur in this area.

Soft Palate

  • Separates the oral cavity from the nasopharynx. It is a fleshy caudal end of the hard palate.

  • Anomaly: An elongated soft palate results in noisy breathing, especially in brachycephalic breeds.

Gustatory Papillae

  • Raised projections on the dorsal surface of the tongue, with varied shapes.
       - Two other types of papillae serve a mechanical function:
         - Filiform: Thorn-shaped, assists in directing food toward the pharynx during lapping or grooming.
         - Conical: Cone-shaped and larger than filiform papillae; prominent in ruminants.

  • Lingual Frenulum: A fold of connective tissue located ventral to the surface of the tongue that holds the tongue to the floor of the mouth.

Salivary Glands

Salivary glands consist of two types of glandular cells:

  1. Serous Cells
       - Secrete a clear, enzyme-containing, watery fluid that initiates digestion.

  2. Mucous Cells
       - Secrete a thick, protective layer of mucous on mucous membranes, assisting in the formation of food boluses for swallowing.
       

Saliva Production

Saliva production is under autonomic (parasympathetic) control, which has been shown to increase in the presence of food or simply by thinking about food.
   

Major Salivary Gland Pairs in Most Animals:

  1. Parotid Gland
       - Located ventral to the ear; duct enters mouth near upper premolars.

  2. Mandibular Gland
       - Located ventral to the parotid salivary gland and caudal to the mandible; duct enters ventral to the tongue.

  3. Sublingual Gland
       - Situated ventro-lateral to the tongue; characterized by multiple ducts.

  4. Zygomatic Gland
       - Lies in the floor of the orbit of the eye and opens via a small duct into the roof of the mouth; challenging to locate without removing an eye.

Pharynx

The pharynx serves as a common passageway for the digestive and respiratory systems, transporting food, water, and inspired air.

Epiglottis

  • A cartilaginous, triangular-shaped flap located at the cranial entrance to the larynx within the pharynx.

  • Function: Directs food, fluids, and air correctly to prevent aspiration into the trachea during swallowing, moving caudo-dorsally to cover the laryngeal opening to the airway.

Esophagus

  • A collapsed, muscular tube located between the pharynx and the cardia (opening) of the stomach. It is distensible (able to stretch) but can also obstruct.

  • Function: Contents move through the esophagus via peristalsis.

Hiatus Esophagus

  • This area in the diaphragm allows the esophagus to pass from the thorax to the abdomen and is susceptible to tearing, resulting in a condition known as hiatus hernia.

Events in Swallowing

  1. Partially broken-down food is mixed with saliva.

  2. The tongue rolls a bolus of food against the hard palate.

  3. Food is moved caudally in the mouth to the base of the tongue.

  4. The bolus passes through the pharynx via muscle contractions.

  5. The bolus is presented at the entrance to the esophagus.

  6. The bolus enters the esophagus.

  7. Peristalsis moves the bolus down to the stomach, aided by gravity.

Monogastric Stomach Structure

  • The monogastric stomach consists of one main compartment, which is an expanded tube with an entrance and exit. The interior features include:
       - Rugae: Inner folds
       - Acidic pH signaling a strongly acidic environment.
       - The stomach is subdivided internally based upon the primary means of digestion.
       - Mechanical Digestion: Miking and breakdown of food.
       - Chemical Digestion: Involves hydrochloric acid (HCl) and enzymes.

  • Monogastric Mammals Include: Cats, dogs, horses, rabbits, rats, and mice.

Internal Areas of the Stomach:

  1. Cardia (Esophageal)
       - Entrance to stomach with a cardiac sphincter that helps to prevent reflux and produces mucus.

  2. Fundus
       - Glandular and distensible due to muscle plasticity.

  3. Body
       - The middle of the stomach, glandular, and distensible.

  4. Pyloric Antrum
       - Regions that grind food and are glandular.

  5. Pylorus
       - Thickened wall forming a muscular sphincter producing mucus; regulates unidirectional movement of chyme into the small intestine.

Chyme

  • Defined as semi-liquid, homogeneous material released from the stomach into the duodenum of the small intestine.

External Features of the Stomach

  1. Greater Curvature
       - Longest continuous uninterrupted border.

  2. Lesser Curvature
       - Inner border between the cardia and pylorus.

Glandular Cells of the Monogastric Stomach

  1. Mucous Neck Cells
       - Secrete protective mucus that coats stomach walls (rugae) to prevent autodigestion; damage could result in ulcers.

  2. Chief Cells
       - Produce enzyme precursor pepsinogen, which transforms into pepsin in the presence of HCl.

  3. Parietal Cells
       - Produce hydrochloric acid (HCl), breaking down chemical bonds in food, and produce intrinsic factor in most species.

  4. Endocrine 'G Cells'
       - In the presence of food, these cells secrete the hormone gastrin into the blood, stimulating parietal cells.