Chapter 9 The Digestive System

Intracellular Digestion

involves oxidation of glucose and fatty acids for energy

Alimentary Canal

runs from mouth to anus and is sectioned off by sphincters

Sphincters

smooth muscles around canal that can contract to allow compartmentalization of function

Digestion

breakdown of food into constituent organic molecules

What do starches break down into?

monosaccharides

What do lipids break down into?

free fatty acids and glycerol

What do proteins break down into?

amino acids

Mechanical Digestion

physical breakdown of large food particles into smaller ones

Chemical Digestion

enzymatic cleavage of chemical bonds

Absorption

transport of products of digestion from the digestive tract into circulatory system for distribution to body’s tissues and cells

Digestive Tract Contents

oral cavity, pharynx, esophagus, stomach, SI, LI, rectum

Oral Cavity

mouth

Pharynx

shared pathway for food entering digestive system and air entering respiratory system

Esophagus

transports food to stomach

What help provide enzymes and lubrication to aid digestion?

salivary glands, pancreas, liver, gallbladder

Enteric Nervous System

collection of 100M neurons that govern function of GI system

Peristalsis

rhythmic contractions of gut tube

What substances trigger thirst?

ADH and aldosterone

What substances stimulate hunger?

glucagon and ghrelin

What substances stimulate satiety?

leptin and cholecystokinin

Mastication

chewing; creates more surface area for enzymatic digestion and decreases risk of obstruction

Saliva Function

moistens and lubricates food

Salivary Amylase

hydrolyze starch into smaller sugars

Lipase

catalyzes hydrolysis of lipids

Bolus

form of food by tongue

Pharynx Description

cavity form mouth to posterior nasal cavity to esophagus and larynx

Nasopharynx Location

behind nasal cavity

Oropharynx Location

back of mouth

Laryngopharynx Location

above vocal chords

Epilgottis

cartilaginous structure that folds down to cover laryngeal inlet when swallowing

Esophagus Description

muscular tube connecting pharynx to stomach

How much of the esophagus is skeletal muscle?

top third

How much of the esophagus is smooth muscle?

bottom third

Emesis

vomiting

Upper Esophageal Sphincter Function

initiates swallowing

Lower Esophageal Sphincter

relaxes and opens to allow passage of food

Stomach Description

highly muscular organ with capacity of approximately 2 L

Three Main Energy Sources of Stomach

carbohydrates, fats, proteins

What does the stomach use to digest food?

HCL and enzymes

Four Parts of the Stomach

fundus, body, antrum, pylorus

Fundus and Body Contents

mostly gastric glands

Gastric Glands Function

respond to signals from vagus nerve of PNS in response to sight, taste, smell of food

Mucous Cells Function

produce bicarbonate rich mucus protecting cell wall from harshly acidic and proteolytic environment of stomach

Gastric Juice

combination of chief cells and parietal cell secretions

Chief Cells

secrete pepsinogen, forming pepsin that digests proteins by cleaving peptide bonds near aromatic amino acids

Parietal Cells

secrete H+, intrinsic factor (glycoprotein involved in proper absorption of vitamin B12

Antrum and Pylorus Contents

mostly pyloric glands

Pyloric Glands

contain G-cells secreting gastrin

Gastrin Function

secrete more HCl and signals stomach contraction

Lesser Curvature of Stomach

internal curvature

Greater Curvature of Stomach

external curvature

Rugae of Stomach

lining of stomach with folds

Chyme

digested food

SI Segements

duodenum, jejunum, ileum

Duodenum Function

digestion

Jejunum Function

absoprtion

Ileum Function

absoprtion

Pyloric Sphincter

food leaves stomach and enters duodenum

Brush-Border Enzymes

present on luminal surface of cells lining duodenum and break down dimers and trimers of biomolecules into absorbable monomers

Disaccharides Function

digest disaccharides

What do undigested disaccharides cause?

diarrhea

Maltase Function

digests maltose

Isomaltase Function

digests isomaltose

Lactase Function

digests lactose

Sucrase Function

digests sucrose

Peptides Function

breaks down protein/peptides

Aminopeptidase

secreted by glands in duodenum that removes N-terminal amino acid from peptide

Dipeptidase Function

cleave peptide bonds of dipeptides to release free amino acids

Enteropeptidase

enzyme critical for activation of trypsinogen to trypsin

Trypsin Function

initiates activation cascade

Secretin

peptide hormone causing pancreatic enzymes to be released into duodenum

Secretin Functions

regulates pH by decreasing HCl secretion from parietal cells and increase bicarbonate secretion from pancreas

Enterogastrone

hormone allowing motility through digestive tract

Cholecystokinin (CKK)

secreted in response to entry of chyme into duodenum

CKK Function

stimulates release of bile and pancreatic juices, and satiety form brain

Bile

complex fluid of bile salts, pigments, cholesterol

Bile Salts

derived from cholesterol; not enzymes

Bile Salts Function

emulsify fats and cholesterol into micelles

Pancreatic Juices

complex mixture of several enzymes in bicarbonate-rich alkaline solution that helps to neutralize chyme

Pancreas Endocrine Functions

release insulin, glucagon, somatostatin

Pancreas Exocrine Function

made of acinar cells that produce pancreatic juices

Pancreatic Amylase Function

breaks down large polysaccharides into small disaccharides, responsible for carbohydrate digestion

Pancreatic Lipase Function

break down fats into fatty free acids and glycerol

Liver Bile Ducts

connect liver with gallbladder and SI

Liver Function

bile production, process nutrients, detoxification and drug metabolism

Bile Function

emulsify fat in digestive system

Bilirubin

major pigment in bile; byproduct of hemoglobin breakdown

Jaundice

yellowing of skin if cannot excrete bilirubin

Albumin

protein maintaining plasma oncotic pressure and carrier for drugs/hormones and clotting factors

Gallbladder Functions

stores and concentrates bile, excrete bile, site of stone formation

Microvilli

increase surface area for absorption

Do small fatty acids need transporters?

no, they are nonpolar

Vitamins Absorbed in SI

A, B, C, D, E, K

Fat Soluble Vitamins

A, D, E, K

What does failure to absorb fat-soluble vitamins cause?

pathologies in liver, gallbladder, pancreas, or SI, leading to deficiencies

Water-Soluble Vitamins

B and C

Transcellularly

water across cell membrane

Paracellularly

water squeezing between cells

Large Intestine Function

water absoprtion

Sections of LI

cecum, colon, rectum