Alimentary System

СРС

Тheme   :  «Alimentary System.»

                                                                         Выполнила:Сапаргалиева Ольга.

                                                            028-2 ОМ

               Alimentary System

The alimentary system (Lt. alimentum :nourishment ) includes all the body structures involved in preparing food for absorption into the body and excretion of waste products ie.

• oral cavity
• pharynx
• oesophagus
• stomach
• small intestine
• large intestine
• anus

The gastrointestinal system is (strictly speaking) composed of only the stomach and small and large intestines but is commonly deemed to include the oesophagus as well.

A knowledge of the normal microanatomy of the gastrointestinal system is vital to the understanding of:

• the normal function of the gastrointestinal system
• the pathology of gastrointestinal disease
• staging of gastrointestinal tumours

Normal function of the gastrointestinal system

The main functions of the gastrointestinal system are:

• absorption of nutrients from ingested food
• protection against ingested pathogens

Absorption of nutrients from ingested food occurs in four main phases within defined regions of the gastrointestinal tract

1. fragmentation

• starts in the oral cavity, continues in the stomach
• aided by secretion of saliva from salivary glands

2. digestion

• starts in the stomach, continues in the duodenum
• aided by pancreatic enzymes, bile salts, brush border enzymes

3. absorption

• occurs mainly in the jejunum and ileum
• aided by the large surface area produced by mucosal villi and microvilli
• absorbed nutrients are transported away from the mucosa via lacteals and capillaries

4. elimination of waste products

• liquid residue passes into the large intestine where water is progressively absorbed from it
• resultant solid faeces are expelled through the anal canal

In order for food to be properly absorbed, it must be propelled smoothly from one segment to another. This occurs via:

• voluntary muscular action
• striated muscle in the oral cavity, pharynx and upper oesophagus
• involuntary muscle contraction (peristalsis)
• smooth muscle of the muscularis propria
• modulated by autonomic nerves in the submucosal and myenteric plexuses

Protection against ingested pathogens occurs as a result of

• lymphoid aggregates distributed throughout the gastrointestinal tract (mucosa-associated lymphoid tissue or MALT)
• IgA secreted into the gut lumen

Structure of the gastrointestinal system  
The gastrointestinal system (or tract) is essentially a muscular tube which has the same basic structure throughout. Four layers are recognised, from inside out:

1. mucosa
2. submucosa
3. muscularis propria
4. serosa

The mucosa varies greatly in the different regions of the gastrointestinal tract while the other layers are essentially the same.

Mucosa

• is composed of
• epithelium
• lamina propria (loose connective tissue)
• muscularis mucosae (thin layer of smooth muscle)
• changes abruptly from one type to another at several points along the tract

Submucosa

• is composed of fairly dense connective tissue containing blood vessels, lymphatics and nerves

Muscularis propria

• is divided into two layers
• inner circular layer
• outer longitudinal layer
• has inherent rhythmicity which results in peristalsis