The Digestive System

THE DIGESTIVE SYSTEM

Divided into two parts:

* Alimentary tract

o Or Gastrointestinal tract

o Hollow tube from mouth to anus

o Food material inside the tract is considered to be outside the body b/c the canal is open to the external environment at both ends

o It digests food and absorbs the digested fragments

o Organs include

§ Mouth

§ Pharynx

§ Esophagus

§ Stomach

§ Small intestine

§ Large intestine

* Accessory digestive organs

o Organs include

§ Teeth

§ Tongue

§ Gall bladder

§ Salivary glands

§ Liver

§ Pancreas

o The teeth and tongue are in the oral cavity while digestive glands and gallbladder lie outside the GI tract and connect to it by ducts

o Most of the accessory digestive organs are in the abdomino-pelvic cavity

§ Covered by a serous membrane called the visceral peritoneum

§ Parietal peritoneum covering the abdominal walls of the abdomino-pelvic cavity

§ B/w these two layers is a thin layer of fluid produced by the serous membranes

* Lubricates b/c these organs are moving

§ Organs are held in place, and anchored to the abdominal wall (b/c they are moving) by mesentery

* A double layer of peritoneum

o Sheet of two serous membranes fused back to back that extends to the digestive organs from the body wall

* Also a fat storage area

* Provide routes for blood vessels, lymphatics and nerves to reach the digestive viscera

GI tract

* Hollow tube w/ 4 distinct layers

o Central is the hollow portion called the lumen

o Mucosa

§ Layer closest to the lumen

§ Wet epithelial membrane (simple columnar epithelium)

§ Often w/ a lot of goblet cells to secret mucous

* Mucous lubricates food to help it pass through the canal

* Mucous lining protects the wall of the GI tract from digestive enzymes (in the stomach and small intestine) and HCl from the stomach--PROTECTIVE

§ Epithelial layer rests on a thin layer of loose connective tissue called lamina propria

* Has a lot of blood vessels to get broken down nutrients into the blood so they can be distributed to tissues

* Blood vessels also nourish the epithelium

§ Beyond the lamina propria is a thin layer of smooth muscle called the muscularis mucosae

* Contraction of this layer moves the epithelium around so food trapped in the mucosa is dislodged and proceeds through the tract

* In the small intestine this layer throws the mucosa into a series of small folds that increases the surface area greatly

o Submucosa

§ Layer of dense connective tissue just external to the mucosa

§ Contains lots of blood vessels, lymphatic vessels, lymphoid follicles and nerve fibers

§ Has a rich supply of elastic fibers that enables the stomach to regain its normal shape after temporarily storing a large meal

§ The vascular network supplies the surrounding tissues of the GI tract wall

o Muscularis externa

§ Muscular layer

§ Responsible for segmentation and peristalsis

§ In most of the GI tract it is 2 layers of smooth muscle

* Inner circular layer

* Outer longitudinal layer

* In several places along the tract, the circular layer thickens, forming sphincters that act as valves to prevent the backflow and control food passage from one organ to the next

o Visceral peritoneum or Serosa

§ Protective outermost layer

§ Formed by areolar connective tissue covered w/ mesothelium (a single layer of squamous epithelial cells)

** FUNCTIONAL ANATOMY OF THE GI TRACT**

* Oral cavity (mouth)

o Extends form the lips anteriorly to the oropharynx

o Lined w/ stratified squamous epithelium

§ Protective

§ On gums, hard palate and dorsum of the tongue the epithelium is keratinized to give extra protection against abrasion when eating

o Lips and cheeks

§ Lips extend from the bottom of the nose to indentation on the chin

§ What we call lips (where lipstick is applied) is called the red margin

* This area is red b/c it is poorly keratinized

* No sweat glands, no sebaceous glands

o Causes dryness and cracking due to the elements

§ Lips and cheeks help to keep food b/w the teeth when we chew and play a small role in speech

* Lips formed by the obicularis oris muscle

* Cheeks are formed largely by the buccinators