Anatomy and Function of the Small and Large Intestine

The Intestines: Structure and Function

The Small Intestine

The main function of the small intestine is the absorption of necessary nutrients for the human body. It is the part of the digestive tract that starts just after the stomach and ends at the cecum of the colon.

Divisions of the Small Intestine

The small intestine is divided into three parts:

• Duodenum
• Jejunum
• Ileum

It measures approximately 3 meters long in a living person, but extends up to about 6.5 meters post-mortem due to the loss of muscle tone. It is located between two sphincters: the pyloric sphincter (connecting to the stomach) and the ileocecal sphincter (communicating with the large intestine).

The Large Intestine (Colon)

The large intestine is the penultimate portion of the digestive tract, formed by the cecum, the colon, the rectum, and the anal canal. The small intestine joins the large intestine in the lower right abdomen through the ileocecal valve.

The large intestine is a muscular tube about 1.5 meters long. The first part of the large intestine is called the cecum.

Functions of the Large Intestine

The large intestine continues to absorb water and mineral nutrients from the remaining food matter and serves as a storage area for feces. After about 32 hours after ingestion, food reaches the large intestine, where it is not processed further in this final stage of digestion.

The large intestine's primary roles are limited to:

• Absorbing vitamins that are released by bacteria in the colon.
• Absorbing water.
• Compacting and storing the stool in the rectum until it is passed through the anus.

Accessory Digestive Glands

Accessory glands are organs that secrete digestive fluids necessary to transform complex food into simple substances for easy digestion. These fluids contain substances called enzymes, which are responsible for simplifying food molecules.

Major Accessory Glands of Digestion

• Salivary Glands: Secrete saliva.
• Liver: Secretes bile.
• Pancreas: Secretes pancreatic juice.

Salivary Glands and Saliva Characteristics

The major salivary glands are the parotid, submandibular, and sublingual glands. Additionally, there are many small buccal glands.

Daily saliva secretion ranges between 800 and 1500 ml, with an average of 1000 ml.

Components and Function of Saliva

Saliva contains two main types of protein secretion:

1. A serous secretion containing the enzyme ptyalin (salivary amylase), which begins the digestion of starch.
2. A mucous secretion containing mucin, which acts primarily for lubrication and surface protection.

The parotid glands secrete only serous saliva, while the submandibular and sublingual glands secrete both serous and mucous fluids. Buccal glands secrete mucus only.

The pH of saliva is typically 6.0 to 7.0, a range favorable to the digestive action of ptyalin.

Control of Salivary Secretion

Salivary glands are controlled mainly by parasympathetic nerve signals. They are excited by both tactile stimuli from the tongue and other areas of the mouth and pharynx.