Human Digestive System: Anatomy and Functions
Small Intestine Anatomy and Function
Intestinal Wall Structure
The thin wall of the small intestine is composed of several layers: the intestinal mucosa, muscular layer, and serous layer. The intestinal mucosa throughout the entire intestine has a simple columnar epithelium. It also contains defensive lymphoid tissue, called Peyer’s patches, which are abundant in the jejunum and ileum.
Mucosal Features for Absorption
To increase the absorption surface, the mucosa features:
- Valvulae conniventes (or plicae circulares): Transverse folds of the intestinal mucosa.
- Villi: Finger-like extensions of the mucosa, present even on the valvulae conniventes.
- Microvilli: Tiny extensions on the surface of columnar cells, forming a brush border.
Intestinal Glands
The mucosa presents a series of glands:
- Brunner’s Glands: Located only in the duodenum, they secrete mucus to protect against gastric juice.
- Crypts of Lieberkühn (or Intestinal Glands): Found throughout the intestinal mucosa, they produce enzyme-rich intestinal juice.
Muscular Layer and Motility
The muscle layer consists of longitudinal and circular smooth muscle, performing two types of motion:
- Segmentation: Mixing of chyme with digestive juices in the gut.
- Peristalsis: Rhythmic contractions that advance food through the digestive tract.
Large Intestine: Structure and Role
Primary Functions of the Large Intestine
The large intestine primarily functions in water absorption, transforming indigestible liquid material into a semi-solid consistency. It is also responsible for the transportation and evacuation of fecal material. Additionally, it performs biological functions such as participating in the synthesis of Vitamin K.
Cecum and Appendix
The cecum, a blind pouch located in the right iliac fossa, is the beginning of the large intestine. It features a prolongation called the vermiform appendix. The cecum is separated from the small intestine by the ileocecal valve, which allows the passage of substances and prevents reflux.
Segments of the Colon
The colon extends from the cecum to the rectum, forming what is called the ‘colic frame’. It is divided into several parts:
- Ascending Colon: Extends upwards from the cecum on the right side, reaching the liver.
- Transverse Colon: Stretches from the liver to the spleen.
- Descending Colon: Extends downwards from the spleen into the pelvis.
- Sigmoid Colon: An S-shaped segment that crosses the pelvis to the left side.
Rectum and Anal Canal
The rectum is situated in the hypogastrium. It presents a portion or dilated rectal ampulla and another portion or narrowed conduit, the anal canal, which opens to the outside through the anus in the perineal region.
Anal Sphincters
The anus has two sphincters:
- The internal anal sphincter: Composed of involuntary smooth muscle.
- The external anal sphincter: Composed of voluntary striated muscle.
Salivary Glands and Saliva Production
Composition and Role of Saliva
The salivary glands are positioned around the oral cavity and secrete saliva, a transparent, whitish, and sparkling liquid discharged into the mouth. Saliva is composed of water, electrolytes, mucus, and enzymes like salivary amylase (ptyalin), which begins the digestion of carbohydrates. The primary functions of saliva are to aid in grinding food and initiate digestion.
Major Salivary Glands
- Parotid Glands (two): Located behind the ascending ramus of the maxilla, next to the ear canal. Their excretory duct, Stensen’s duct, opens near the second upper molar.
- Submandibular Glands: Located under the lower maxilla. Their excretory duct, Wharton’s duct, flows along the lingual frenulum.
- Sublingual Glands: Several small glands located in the floor of the mouth, with multiple excretory ducts.
The Digestion Process: From Mouth to Stomach
Overview of Digestion
The digestion process involves physical and chemical alterations of food, transforming complex substances into simpler, easily absorbable nutrients that cells can utilize. Unabsorbed substances are eliminated through defecation.
Oral Phase of Digestion
The initial phases of digestion occur in the mouth, involving three key processes:
Ingestion, Mastication, and Insalivation
- Ingestion: Taking food into the mouth.
- Mastication (Chewing): Mechanical breakdown of food.
- Insalivation: Mixing food with saliva.
During insalivation, salivary amylase (ptyalin) begins the degradation of carbohydrates. The processed food forms a mass called a bolus.
Bolus Formation and Swallowing
Swallowing (Deglutition): The bolus passes from the mouth through the oropharynx and esophagus to the stomach. This is a voluntary act initially, then becomes involuntary, propelling the bolus towards the stomach, where it will be stored in the gastric cavity.