Digestive System

Digestive System – Fig. 15-1, Table 15-1

Digestion – process of changing food substances into forms that can be absorbed; digestive system consists of an alimentary canal & several accessory organs

Digestive structures are adapted for mechanical and chemical digestion; enzyme secretion, and absorption

http://highered.mcgraw-hill.com/sites/0072495855/student_view0/chapter26/animation__organs_of_digestion.html (overview of digestive system)

Digestive System – Fig. 15-1, Table 15-1

General Characteristics of the Alimentary Canal – various regions are specialized to perform specific functions

Structure of the wall – 4 layers – Fig. 15-2 Mucous membrane – inner most layer; function

to protect, absorption, secretion Submucosa – functions to nourish surrounding

tissues, tranports absorbed materials Muscular layer (muscularis) – functions in the

movement of the tube and its contents; smooth muscle fibers are arranged in circular & longitudinal groups

Serous (serosa) – composed of visceral peritoneum; functions to protect and lubricate

Wall of Digestive Tract

Digestive System Movements of the tube – Fig. 15-3

Motor functions include mixing and propelling movements

Mixing – small segments of smooth muscle contraction

Movement by peristalsis – tube undergoes receptive relaxation just ahead of a peristaltic wave

Peristalsis http://www.yout

ube.com/watch?v=o18UycWRsaA

Digestive System Innervation of the tube

Innervated by branches of sympathetic and parasympathetic divisions of the autonomic nervous system

Parasympathetic impulses generally cause an increase in digestive activities; sympathetic impulses generally inhibit digestive activities

Mouth – adapted to receive food and begin preparing it for digestion. It also serves as an organ of speech & sensory reception. Fig. 15-4

Mouth Cheeks & lips

Cheeks form the lateral walls of the mouth

Lips – highly mobile & possess a variety of sensory receptors useful in judging the characteristics of food

Tongue Thick, muscular organ that aids in the

mastication of food with saliva and moving it toward the pharynx

Rough surface (papillae) aids in handling food & contains taste buds

Lingual tonsils (lymphatic tissue) are located on the root of the tongue

Mouth Palate

Comprises the roof the mouth and includes hard & soft palates

Soft palate closes the opening to the nasal cavity during swallowing

Palatine tonsils (lymphatic tissue) are located on either side of the tongue in the back of the mouth

Pharyngeal tonsils (adenoids) are located on the posterior wall of the pharynx above the border of the soft palate

Mouth Teeth

Two sets of teeth develop in sockets of the mandibular & maxillary bones

20 primary (deciduous) & 32 secondary teeth - Fig. 15-6

Break food into smaller pieces to increase surface area for digestive enzymes to act upon

Teeth are adapted to handle food in different ways Incisors – biting Canines (cuspids) – grasping Molars - grinding

Mouth Teeth

Anatomy – Fig. 15-6 – tooth is divided into crown, neck, root, and is attached/anchored by periodontal membrane (ligament) Enamel – protects Dentin – makes up the bulk of the tooth

Salivary Glands Salivary glands – secrete saliva,

moistens food, binds food particles together, begins digestion of carbohydrates, makes taste possible, helps cleanse the mouth, & regulates pH in the mouth (pH 6.5 – 7.5) – Fig. 15-7

Salivary secretions – glands include Serous cells that secrete digestive

enzymes (amylase) that are more liquid/watery

Mucous cells that secrete mucus that is more viscous

Major salivary glands Parotid – largest; secretion rich in amylase Submandibular – secretes combinations of

enzymes and mucus Sublingual – secretes mucus

Pharynx & Esophagus – serve as passageways

Pharynx – throat, passage for air & food

Swallowing mechanism – 3 stages food is mixed with saliva & forces into

pharynx Involuntary reflex actions move the food

into esophagus Esophagus to stomach by peristalsis

Pharynx & Esophagus – serve as passageways

Esophagus Passes through mediastinum &

penetrates diaphragm Cardiac sphincter – circular ring of

muscle that prevents regurgitation Hiatal hernia – weak diaphragm, some part

of stomach, large intestine or other abdominal organ protrudes into thoracic cavity

Heart burn – gastric juice enters esophagus & causes mucosa to become inflamed

Stomach Mixes food with gastric juices carries

on a limited amount of absorption; & moves food to small intestine – Fig. 15-8

Parts of stomach Cardiac, fundic body & pyloric regions Pyloric sphincter – serves as a valve

between stomach & small intestine; regulates how much “chyme enters

Gastric glands secrete pepsin (breaks down proteins), HCl (kills bacteria, and keeps stomach at pH of about 2 which is optimal for action of pepsin), and lipase (breaks down lipids)

Stomach Stomach not well adapted for

absorption, however it can absorb some water, lipid soluble drugs, and alcohol

Mixing & Emptying Actions as stomach fills the walls stretch –

internal pressure remains unchanged Mixing movements (3 directions of

smooth muscle) aid in producing chyme; peristaltic waves move chyme to pyloric region

Muscular wall of pyloric region pumps chyme into the small intestine

Rate of emptying depends on fluidity of chyme & the type of food present

Pancreas (accessory organ) – Fig. 15-10

Pancreatic juice digests all food types

Has a high HCO3- (bicarbonate)

content that helps to neutralize chyme & causes intestinal contents to be alkaline

Pancreatic duct merges with bile duct & enters small intestine at duodenum

Accessory Organs of Digestive Tract

Liver/Gall Bladder – Accessory Organs

Functions of the liver Metabolism of fats, sugars, & proteins Filters blood Storage of substances – glycogen, iron,

vitamins Detoxifies – converts ammonia to urea Secretes bile – emulsifies lipids

Structure Located in upper R. & central portion of

abdominal cavity Highly vascular & divided into lobes Functional unit – hepatile lobule

Liver/Gall Bladder – Accessory Organs

Function of the Gall Bladder – Fig. 15-10

Store bile between meals Release of bile form the common bile duct is

controlled by sphincter muscle Gallstones – cholesterol that has crystallized

(pg. 405) Release of bile is stimulated by

cholescystokinin from the small intestine which causes the gall bladder to contract

Small Intestine – extends from pyloric sphincter to the large intestine Fig. 15-9

Functions Complete digestion of all nutrients Absorbs nutrients which then enter

blood stream Transports residues to the large intestine Secrete digestive enzymes

Parts Duodenum, jejunum, & ileum Suspended from the posterior abdominal

wall by mesentery (extension of peritoneum)

Small Intestine Structure (circular folds – plica{e})

Lined with villi that increase surface area & aid in mixing & absorbing

Microvilli on the free ends of epithelial cells increase surface area even more

Regulation/absorption/movements Secretion of intestinal enzymes are

enhanced by the presence of gastric juice & chime and distension of small intestinal wall

Villi absorb water & nutrients Movements include mixing by

segmentation & peristalsis Ileocecal valve controls movements of

contents from small to large intestine

Small Intestine

Villi of Small

Intestine

Large Intestine Reabsorbs water & electrolytes and forms

7 stores feces – Fig. 15-12, 15-13 Parts divided into cecum, ascending colon,

transverse colon, descending colon, sigmoid colon, rectum, and anal canal

Functions Little or no digestive function; secretes mucus  Absorption of H2O, electrolytes, & vitamins

(produced by bacteria that inhabit large intestine) 

 Movements – similar to those of small intestine

Feces – stored in rectum and eliminated through anus

Large Intestine

Large Intestine

http://www.youtube.com/watch?v=6kg5wZQfADQ (colonoscopy)

http://www.youtube.com/watch?v=nGyzZyxMxkk&NR=1 (colonoscopy showing rectal cancer)