Human Digestive System: Glands, Stages, and Nutrient Absorption

Accessory Digestive Glands

Salivary Glands

These glands secrete saliva into the mouth:

• Parotid: Located in front of and below the ears.
• Submaxillary (Submandibular): Located against the inner face of the lower jaw.
• Sublingual: Located on the floor of the mouth.

Pancreas: A Mixed Gland

The pancreas is an elongated, spongy gland located underneath and behind the stomach. It performs both exocrine and endocrine functions:

• Exocrine Function (Digestion): It secretes pancreatic juice into the small intestine (duodenum) through the Ampulla of Vater.
• Endocrine Function (Hormone Secretion): Secretes hormones into the blood. Example: Insulin regulates the transport of glucose inside the cells.

Liver: The Body's Largest Gland

Location: Right side of the stomach within the abdominal cavity.

Bile Secretion and Function

The liver secretes bile, which is stored in the gallbladder (a small sac). Bile is poured into the duodenum through the Ampulla of Vater. Bile lacks digestive enzymes but contains bile salts that facilitate the emulsion (breakdown) of fats, making them easier for digestive juices to process.

Key Functions of the Liver

• Production of bile (aids in lipid digestion).
• Glucose storage (forms glycogen).
• Reservoir of blood.
• Elimination of toxic substances from the blood (e.g., alcohol).
• Recycling of red blood cells; non-useful parts are excreted as bile pigments.

Stages of Digestion

Mechanical Digestion

This involves physical transformations (cutting, crushing, mixing, kneading) to reduce food size, making subsequent chemical digestion easier.

Chewing: Crushing of food and mixing it with saliva, leading to the formation of the bolus.

Swallowing and Digestive Transit

1. The tongue pushes the bolus against the palate and raises the soft palate.
2. The bolus passes the pharynx.
3. The bolus passes the esophagus via contraction waves (peristalsis).

Transit continues through the digestive tract, mixing the food mass with digestive juices. The resulting semi-liquid mass (pap) is named differently depending on location:

• Chyme (Quimo): Formed in the stomach.
• Chyle (Quilo): Formed in the intestine.

Chyme and Chyle Formation

Chyme: The acidic, semi-liquid result of gastric digestion. It is prepared to pass into the small intestine.

Chyle: Formed in the small intestine by the action of digestive juices. This whitish liquid contains:

• Products resulting from digestion (nutrients).
• Various non-digestible substances.
• Water and mineral salts.

Absorption of Nutrients

Intestinal Villi

Location: Primarily in the jejunum and ileum.

Function: To absorb nutrients that have been transformed into small molecules.

Absorbed nutrients pass into the blood through the Hepatic Portal Vein, which transports them directly to the liver. Note that many lipids pass from the intestine into the lymphatic vessels instead of the portal vein.