Lab 3 BIO104

The oral cavity is separated from the nasal cavity by the palate. Specifically, the soft palate serves as the divider between the oral cavity/oropharynx and the nasal cavity/nasopharynx.
The soft palate plays a critical role during deglutition (swallowing), where it moves in a superior and posterior direction to seal off the nasopharynx, thereby preventing food or liquid from entering the nasal cavity.
The hard palate forms the anterior portion of the roof of the mouth and is a horizontal plate comprised of two distinct bones: the maxillary and the palatine bones.
The lips serve several vital functions, including the retention of food within the mouth during the process of mastication and assisting in speech and articulation.

There are a total of $32$ permanent teeth in the adult human. These are distributed between the upper arch, located in the maxilla, and the lower arch, located in the mandible.
Specific tooth types serve different mechanical functions:
- Incisors: Essential for the initial biting and cutting of food.
- Canines (Cuspids): Characterized as the longest teeth, they are specialized for grasping and holding food.
The tongue is involved in several processes: mastication (the manipulation and mixing of food), deglutition (swallowing), the sensation of taste, and the facilitation of speech.
Saliva is produced by three primary pairs of salivary glands, each contributing a specific volume to the total output:
- Parotid Glands: Contribute approximately $\sim 25\%$ of total saliva.
- Submandibular Glands: Contribute approximately $\sim 70\%$ of total saliva.
- Sublingual Glands: Contribute approximately $\sim 5\%$ of total saliva.
Saliva serves four essential functions:
- Moistening and dissolving food to facilitate the formation of a bolus.
- Initiating the chemical digestion of starch through the enzyme salivary amylase.
- Providing lubrication to assist in the swallowing process.
- Offering antibacterial protection through the presence of lysozyme.

The term "pharynx" is derived from the Latin word for "throat." It is anatomically divided into three subdivisions:
- Nasopharynx
- Oropharynx
- Laryngopharynx
During the process of swallowing, the tongue (glossal) muscles are responsible for pushing food particles into the pharynx.
The epiglottis is the specific structure that acts as a gatekeeper, preventing food from entering the respiratory system during the act of swallowing.
Once food is masticated and mixed with saliva, the resulting mass is referred to as a bolus.

The esophagus is a muscular tube that conveys food from the pharynx to the stomach. It is divided into three sections:
- Cervical part
- Thoracic part
- Abdominal part
The esophagus passes through the diaphragm to reach the abdominal cavity via an opening known as the esophageal hiatus.
Involuntary, wavelike muscular contractions known as peristalsis propel food through the esophagus and the rest of the digestive tract.
The lower esophageal (cardiac) sphincter is the muscular structure that prevents the reflux of stomach contents back into the esophagus.
Reflux esophagitis, commonly known as heartburn or Gastroesophageal Reflux Disease (GERD), occurs when stomach acid or contents leak back into the esophagus.

The stomach is located in the upper left quadrant of the abdomen, specifically residing in the left hypochondriac and epigastric regions. It sits between the esophagus and the duodenum of the small intestine.
The stomach is divided into four main regions (listed from proximal to distal):
- Cardia
- Fundus
- Body
- Pylorus
The wall of the stomach consists of four layers, from the outermost to the innermost:
- Serosa
- Muscularis
- Submucosa
- Mucosa
The muscularis layer of the stomach is unique compared to the rest of the digestive tract. While most organs have two layers (circular and longitudinal), the stomach features three layers of smooth muscle:
- Oblique layer
- Circular layer
- Longitudinal layer
The additional oblique layer allows the stomach to perform complex churning motions for mechanical digestion.
Rugae, or gastric folds, are internal structures that allow the stomach to expand as it fills, significantly increasing its internal surface area.
The stomach serves two primary digestive processes: mechanical digestion (churning) and chemical digestion (enzymatic and acid breakdown). These processes convert the bolus into a semi-liquid state called chyme.

Gastric pits contain four specialized cell types that produce essential secretions:
- Mucous (Surface Mucous) Cells: Secrete mucus to provide lubrication and protect the stomach lining from self-digestion by $HCl$ and pepsin.
- Parietal Cells: Secrete hydrochloric acid ( $HCl$ ) and intrinsic factor.
- Chief Cells: Secrete pepsinogen, the inactive precursor to the protein-digesting enzyme pepsin.
- G (Enteroendocrine) Cells: Secrete the hormone gastrin.
The transition from the stomach to the small intestine is regulated by the pyloric sphincter, which controls the release of chyme into the duodenum and prevents backflow.

The small intestine is the primary site for the absorption of nutrients.
It consists of three parts, ordered from proximal to distal:
- Duodenum
- Jejunum
- Ileum
The major duodenal papilla is a critical opening in the duodenum where bile and pancreatic secretions enter the digestive tract.

The liver is divided into anterior lobes by the falciform ligament.
Histologically, the liver is composed of functional units known as hepatic lobules.
The liver performs two basic categories of functions:
- Bile production: Essential for the emulsification and digestion of fats.
- Metabolic functions: Including detoxification of substances, storage of glycogen, and the synthesis of plasma proteins.
Bile produced in the liver is transported through the right hepatic duct and the left hepatic duct.
Pancreatic secretions are carried to the duodenum via the pancreatic duct.

The digestive system is defined by four main functions:
- Ingestion: Taking food into the body.
- Digestion: The breakdown of food, beginning in the mouth. This occurs in two forms: mechanical (physical breakdown like mastication/chewing) and chemical (enzymatic/acidic breakdown).
- Absorption: The process of taking nutrients into the bloodstream.
- Defecation: The elimination of undigested waste from the body.