The Digestive System

Digestion• Digestion = the

mechanical and chemical breakdown of foods into nutrients that cell membranes can absorb

• 2 Components of the digestive system:– Alimentary canal – mouth,

pharynx, esophagus, stomach, small intestine, large intestine, anal canal

– Accessory organs – secrete products into the canal; salivary glands, liver, pancreas, gallbladder

Components of the Digestive System

Alimentary Canal

• Around 9 meters long• Muscular tube that

passes through the ventral cavity

• Lumen

Alimentary Wall Structure

• Mucosa• Submucosa• Muscular layer

– Circular fibers

– Longitudinal fibers

– Oblique fibers

• Serosa

Movements of the Alimentary Canal

• Mixing movements• Propelling movements

Mouth Structure

• Surrounded by lips, cheeks, tongue, and palate

• Oral cavity• Vestibule• Cheeks• Lips

Tongue

• Frenulum• Papillae• Hyoid bone• Lingual tonsils

Palate

• Hard palate• Soft palate• Uvula• Palatine tonsils• Pharyngeal tonsils

(adenoids)

Primary and Secondary Teeth

• Primary teeth– Deciduous teeth

– Erupt between 6 months and 2-4 years

– 20 teeth

• Secondary teeth– Appear around 6 years

– 32 teeth

Tooth Types

• Incisors – Chisel-shaped– Bite off large pieces of

food

• Cuspids– Cone-shaped– Grasp and tear food

• Bicuspids and Molars– Flattened surfaces– Grind food

General Tooth Structure• Crown • Root• Neck• Enamel• Dentin• Pulp• Root canals• Cementum• Periodontal ligament

Salivary Glands

• Secrete saliva:– Moistens food particles– Helps bind food

particles– Begins chemical

digestion of carbohydrates

– Dissolves food for tasting

– Helps cleanse mouth and teeth

Salivary Cells

• Serous cells– Produce watery fluid

that contains amylase

– Amylase splits starch and glycogen into disaccharides

• Mucous cells– Secrete mucus to bind

food particles and lubricate during swallowing

Major Salivary Glands• Parotid glands

– Largest– Mostly serous

secretions

• Submandibular glands– Mostly serous

secretions

• Sublingual glands– Smallest– Mostly mucous

secretions

Regions of the Pharynx• Nasopharynx

– Open to nasal cavity– Passage for air during

breathing

• Oropharynx – Behind soft palate– Passage for air and

food

• Laryngopharynx– Passage for food to the

esophagus

Swallowing Reflex

• Food is chewed and mixed with saliva to form a mass called a bolus.

• Bolus is forced into the pharynx.

• Swallowing reflex is stimulated by sensory receptors around the pharyngeal opening.

Swallowing Reflex

• Soft palate rises to prevent food from entering the nasal cavity.

• Hyoid bone and larynx are elevated, and the epiglottis of the larynx closes off the top of the trachea.

• Breathing is briefly inhibited.

Swallowing Reflex

• Tongue presses against the soft palate, sealing the oral cavity off from the pharynx.

• Longitudinal muscles in the pharyngeal wall contract, moving the pharynx up toward the bolus.

• Muscles in the lower pharynx relax, and the esophagus opens.

• Peristalsis moves the bolus through the esophagus.

Esophagus

• Straight, collapsible tube

• Approximately 25 cm long

• Passageway from pharynx to stomach

• Cardiac sphincter• Mucous glands for

lubrication

Movement through Esophagus - Peristalsis

Stomach

• J-shaped, pouchlike organ

• Hangs under the diaphragm

• 1 liter capacity• Rugae

Stomach Functions

• Receives food from the esophagus

• Mixes food with gastric juices

• Initiates protein digestion

• Performs limited absorption of water, salts, alcohol, and lipid-soluble drugs

• Moves food into the small intestine

Stomach Regions

• Cardiac• Fundic• Body • Pyloric

Gastric Secretions

• Gastric pits

• Gastric glands– Goblet cells

– Chief cells – pepsinogen

– Parietal cells – HCl and intrinsic factor

• Gastric juice

• Regulated by ACh, gastrin, and cholecystokinin

Mixing and Emptying Actions of the Stomach

Pancreas

• Secretes pancreatic juice from acinar cells

• Mixed gland• Pancreatic duct• Hepatopancreatic

sphincter

Pancreatic Secretions

• Pancreatic juice contains several enzymes:– Pancreatic amylase– Pancreatic lipase– Nucleases– Trypsin, chymotrypsin,

and carboxypeptidase– Bicarbonate ions

• Release regulated by secretin

Liver• Located in upper right

quadrant below the diaphragm

• Color from rich supply of blood vessels

• Divided into left and right lobes by fibrous capsule

• Each lobe separated into hepatic lobules functional units of liver

Hepatic Lobule Structure

• Consists of many hepatic cells radiating out from a central vein

• Hepatic sinusoids• Portal vein• Central veins• Kupffer cells• Bile canals• Common hepatic duct

Liver Functions• Cells respond to insulin and glucagon to maintain normal

glucose levels• Carbohydrate, lipid, and protein metabolism

– Glucose Glycogen– Noncarbs Glucose– Makes cholesterol and fats– Amino acids Urea– Makes plasma proteins

• Storage of glycogen, iron, vitamins A, D, and B12• Blood filtering• Detoxification• Secretion of bile

Bile• Yellowish-green

liquid that contains:– Bile salts – for

emulsification and absorption of fatty acids, cholesterol, vitamins A, D, E, and K

– Bile pigments – bilirubin and biliverdin

– Cholesterol– Electrolytes

Gallbladder• Pear-shaped sac on the

inferior liver surface• Connects to the cystic duct

which feeds into the common hepatic duct

• Stores bile between meals• Reabsorbs water to

concentrate bile• Releases bile into the small

intestine• Common bile duct• Stimulated by cholecystokinin

Gallbladder and Liver Problems

• Jaundice• Hepatitis• Gallstones

Small Intestine• Extends from pyloric

sphincter to the large intestine

• Receives secretions from the pancreas and liver

• Completes digestion of nutrients in chyme and absorbs products of digestion

• Mixing movements and peristalsis – chyme moves through in 3-10 hours

• Transports digestive residue to the large intestine

Regions of the Small Intestine

• Duodenum– 25 cm long

– Most fixed portion of the small intestine

• Jejunum• Ileum

– Jejunum and ileum are not distinctly separate

– Both are mobile

Mesentery• Double-layered fold of

peritoneal membrane• Suspends the jejunum

and ileum from the posterior abdominal wall

• Supports the blood vessels, nerves, and lymphatic vessels that supply the intestinal wall

Greater Omentum

• Filmy, double-layered fold of the peritoneal membrane

• Drapes like an apron from the stomach over the transverse colon and the folds of the small intestine

• May adhere to infected areas of the alimentary canal to wall it off

Intestinal Villi• Tiny projections on the

inner wall off the small intestine

• Densest in the duodenum• Increase surface area for

absorption• Lacteals – absorb fatty

acids and glycerol• Goblet cells• Intestinal glands• Microvilli

– Secrete peptidases, sucrase, maltase, lactase, intestinal lipase

• Capillaries absorb simple sugars, amino acids, electrolytes, and water

Large Intestine

• Ileocecal valve• 1.5 meters long• Extends up right side,

crosses obliquely to the left side, and descends into the pelvis

• Opens to the outside of the body as the anus

Regions of Large Intestine• Cecum

– Vermiform appendix

• Colon– Ascending colon– Transverse colon– Descending colon– Sigmoid colon

• Rectum• Anal canal

Anal Canal Structure• Anal columns• Anus

– Internal anal sphincter

– External anal sphincter

• Hemorrhoids

Large Intestine Anatomy

• Lack villi• Teniae coli• Many goblet cells

– Protect intestinal wall

– Bind particles of fecal matter

– Help control pH

Large Intestine Functions• Proximal end functions primarily in water

and electrolyte absorption

• Distal end functions primarily to store feces

• Little to no digestive function

• More sluggish movements – peristaltic waves 2-3 times per day (mass movements)

Defecation Reflex

• Can be initiated by person (deep breath and abdominal contraction)

• Forces feces into rectum• Reflex involves relaxation of the internal

anal sphincter and peristaltic waves through the descending colon

• Can be prevented by contraction of the external anal sphincter

Feces

• Made of materials not digested or absorbed – Water– Electrolytes– Mucus– Bacteria

• 75% water• Color from bile pigments

altered by bacterial action• Odor from compounds

produced by bacteria