The Digestive System

Chapter 16

Overview

• The digestive system functions to break down food into a usable form for the body

• Uses include energy for cellular metabolism and growth (you are what you eat)

• Nutrients can be extracted from nearly any food and the molecular building blocks are then rearranged according to the body’s needs.

Divisions of the Digestive System

• The digestive system is divided into two basic parts; the alimentary canal and accessory organs

• The alimentary canal (also the gastrointestinal tract or GI tract) is a hollow tube extending from the opening of the mouth to the anus

• Accessory organs are located within or outside the alimentary canal and assist with functions of the organs of the alimentary canal

Parts of the Digestive System

• Alimentary Canal – mouth, pharynx, esophagus, stomach, small intestine, and large intestine

• Accessory Organs – teeth, tongue, salivary glands, pancreas, liver, and gallbladder

Digestive Processes

• Ingestion – bringing food in the digestive system

• Mechanical digestion – breaking food into smaller pieces and mixing it up

• Propulsion – movement of food through alimentary canal; includes swallowing and peristalsis

Digestive Processes

• Chemical digestion – breakdown of large molecules of food into their basic chemical building blocks which are small enough to be absorbed

• Absorption – the diffusion of food into the bloodstream from the alimentary canal

• Defecation – the elimination of indigestible material from the body in the form of feces

Wall Structure of Alimentary Canal

• Each part of the alimentary canal has the same 4 layers, though the cellular structure of each layer in different parts may differ slightly

• Mucosa – deepest layer, mucous membrane; protects from microorganisms, absorbs digested food, and secrete mucous and digestive enzymes

Wall Structure of Alimentary Canal

• Submucosa – rich in blood vessels, lymphatic vessels, nerve endings, and small glands; blood vessels carry away absorbed materials

• Muscularis – consists of smooth muscle in two layers (longitudinal and circular) forming sphincters at several points; mixes and propels food

Wall Structure of Alimentary Canal

• Serosa – covers the alimentary canal and secretes serous fluid to lubricate movements between its parts

The Mouth

• The mouth receives food and prepares it for swallowing by mechanically breaking it down and mixing it with saliva

• The lips are rich in blood vessels and nerve endings and help keep food in the mouth while chewing

• The tongue helps to mix food with saliva while chewing and moves it to the back of the throat while for swallowing; covered with taste buds which provide information about food to the brain

Dentition

• The teeth of the mouth are of two different types

• Deciduous teeth (baby teeth) develop first and fall out as permanent teeth grow beneath them; all 20 are usually present by about 24 months of age

• Permanent teeth (adult teeth) emerge throughout late childhood and adolescence except for the wisdom teeth which may not erupt until the mid-twenties

Types of Teeth

• Incisors – chisel shape helps to cut food

• Canines – conical shape helps to pierce and tear food

• Premolars and molars – help to grind and crush food

Salivary Glands

• Secrete saliva, which is mostly water (99.5%) with some solutes, mostly mucous and enzymes

• Necessary for gustation (tasting)

• Binds and lubricates food for swallowing forming a bolus

• Begins the breakdown of carbohydrates and kills many bacteria

Salivary Glands

• Largest is the parotid gland located in front of and below each ear

• Submandibular gland is located along the inner surface of the jaw

• Sublingual gland is located in front of the submandibular gland and below the tongue

Pharynx

• Transports food from the oral cavity to the esophagus

• The epiglottis closes to keep food out of the trachea

Esophagus

• Extends approx. 10 inches to the stomach

• The esophageal sphincter controls the flow of food into the stomach and keeps it in there

• If this sphincter is weak it can permit the backflow of chyme from the stomach to produce heartburn

Stomach

• Highly extensible folded bag with an extra layer of oblique muscle fibers in the muscularis

• Mixes bolus with HCl, pepsinogin, pepsin (breaks down most proteins), and intrinsic factor which enables the absorption of vitamin B 12

• Some mechanical and major chemical digestion take place here

• A limited amount of absorption also takes place• Entrance to the small intestine is controlled by

the pyloric sphincter

Pancreas

• Produces about a 1.5 liters of pancreatic juice daily which enters the duodenum via the common bile duct

• Pancreatic juice contains powerful enzymes which can break down carbohydrates, proteins, and fats

Liver

• The liver, among its other functions, produces bile for the emulsification of fats

• This is stored in the gall bladder then released into the duodenum by way of the common bile duct

• Sheep’s Liver

Small intestine

• Extends almost 20 feet, divided into three sections – the duodenum, jejunum, and ileum

• The mucosa is highly folded and has fingerlike projections called villi which intrude approx. 1 mm into the lumen

• The villi are also studded with microvilli which dramatically increase surface area for absorption

Large Intestine

• Extends for about 5 feet

• Extracts water from indigestible food, eventually forming feces

• Feces is made up of indigestible food (fiber), mucous, old cells from the alimentary canal lining, and bacteria that reside within

How Many Structures Can You Identify?