The digestive system & metabolism

Chpt 14

Important terms

Ingestion – taking in of food Digestion – breaking down of food, both physically

& chemically Absorption – movement of nutrients throughout the

bloodstream Defecation – ridding the body of indigestible wastes Alimentary canal – true “digestive tract” – the path

food takes once it enters the mouth and exits the anus

Accessory organs – help with the process of digestion, but food does not actually travel there Teeth, salivary glands, pancreas, liver, gallbladder

Organs of the Alimentary Canal: Mouth

Lips (labia)—protect the anterior opening Cheeks—form the lateral walls Hard palate—forms the anterior roof Soft palate—forms the posterior roof Uvula—fleshy projection of the soft palate Vestibule—space between lips externally

and teeth and gums internally Oral cavity proper—area contained by the

teeth Tongue—attached at hyoid bone and styloid

processes of the skull, and by the lingual frenulum to the floor of the mouth

Physiology of the Mouth

Mastication = chewingSaliva is released once food is masticated

Initiation of swallowing by the tongue Allows for sense of taste

Esophagus

About 10 inches long (3 lengthwise fists) Runs from the pharynx to the stomach,

through the diaphragm Moves food to the stomach through

rhythmic contractions called peristalsis Passageway for food only

Respiratory system branches off after the pharnyx (hence the larnyx only serves a respiratory function)

Ogars have layers, onions have layers

All alimentary canal organs have many layers Mucosa

Innermost , moist membrane▪ Surface epithelium▪ Small amount of connective tissue & smooth muscle

Submucosa Just beneath the mucosa layer▪ Soft connective tissue containing▪ Blood vessels, nerve endings & lymphatic tissue

Muscularis externa Two layers of smooth muscle

Serosa Outermost wall containing fluid producing cells▪ Parietal peritoneum: lines the abdominopelvic cavity▪ Visceral peritoneum: outermost layer that is connected to parietal

peritoneum

Stomach

Located on the left side of the abdominal cavity

Food enters at the cardioesophageal sphincter

Food empties into the small intestine at the pyloric sphincter (valve)

Regions of the stomach Cardiac region—near the heart Fundus—expanded portion

lateral to the cardiac region Body—midportion Pylorus—funnel-shaped

terminal end Rugae—internal folds of the

mucosa

Stomach Physiology & Mucosa Structure

Temporary storage tank for food Site of food breakdown Chemical breakdown of protein begins Delivers chyme (processed food) to the small

intestine Mucosa is simple columnar epithelium

Mucous neck cells—produce a sticky alkaline mucus Gastric glands—situated in gastric pits and secrete

gastric juice Chief cells—produce protein-digesting enzymes

(pepsinogens) Parietal cells—produce hydrochloric acid Enteroendocrine cells—produce gastrin

Hormones that act in digestion

All the hormones below are made by the stomach mucosa & stimulated by food entering the stomach.

Gastrin: Stimulates the emptying of the stomach, stimulates contraction of

intestinal muscles Relaxes ileocecal valve Stimulates mass movement of the contents of the large intestine

Serotonin: Causes contractions of the stomach muscle

Histamine: Activates the parietal cells to release HCl (starting the digestion

process) Somatostatin

Inhibits secretion of all gastric products (important for negative feedback)

Slows down blood flow to intestines & decreases intestinal absorption Inhibits the release of bile from the gallbladder

Small Intestine & its subdivisions

The body’s major digestive organ Site of nutrient absorption into the blood Muscular tube extending from the pyloric sphincter

to the ileocecal valve Suspended from the posterior abdominal wall by

the mesentery Duodenum

Attached to the stomach Curves around the head of the pancreas

Jejunum Attaches anteriorly to the duodenum

Ileum Extends from jejunum to large intestine

Chemical Digestion in the Small Intestine Chemical digestion begins in the small

intestine Enzymes are produced by▪ Intestinal cells▪ Pancreas

Pancreatic ducts carry enzymes to the small intestine

Bile, formed by the liver, enters via the bile duct

Small Intestine Anatomy

Three structural modifications that increase surface area Microvilli—tiny projections

of the plasma membrane (create a brush border appearance)

Villi—fingerlike structures formed by the mucosa

Circular folds (plicae circulares)—deep folds of mucosa and submucosa

Hormones that act in digestion

All hormones listed below are produced by the duodenal mucosa

Intestinal gastrin: stimulated by acidic & partially digested foods entering the duodenum Stimulates secretion of gastric products & intestinal motility

Secretin: stimulated by acidic chyme Inhibits secretion of gastric products Increases output of pancreatic juices to breakdown carbohydrates and

proteins Increases bile output

Cholecystokinin (CCK): stimulated by fatty chyme Aids in the effectiveness of bile Increases output of pancreatic juice Stimulates the gallbladder to expel bile Relaxes the hepatopancreatic sphincter to allow entry of bile into

duodenum

Hormones that act in digestion

Gastric inhibitory peptide (GIP): stimulated by fatty or glucose-containing chyme Inhibits gastric gland secretion & motility

Vasoactive inhibitory peptide (VIP): chyme containing all kinds of partially digested foods Stimulates buffer secretion Dilates intestinal capillaries to prepare for

absorption of nutrients Inhibits HCl production (stops further digestion) Relaxes the intestinal smooth muscle to increase

surface area for absorption

Large Intestine

Larger in diameter, but shorter in length, than the small intestine

Frames the internal abdomen

Cecum—saclike first part of the large intestine

Appendix Accumulation of lymphatic

tissue that sometimes becomes inflamed (appendicitis)

Hangs from the cecum

Large Intestine Colon

Ascending—travels up right side of abdomen Transverse—travels across the abdominal cavity Descending—travels down the left side Sigmoid—enters the pelvis

Rectum and anal canal—also in pelvis Anus—opening of the large intestine

External anal sphincter—formed by skeletal muscle and under voluntary control

Internal involuntary sphincter—formed by smooth muscle

These sphincters are normally closed except during defecation

Large Intestine No villi present Goblet cells produce alkaline mucus

which lubricates the passage of feces Muscularis externa layer is reduced to

three bands of muscle called teniae coli These bands cause the wall to pucker

into haustra (pocketlike sacs)

Liver & gallbladder Work together to create & store bile The liver ejects bile directly into the duodenum to

emulsify fats When fats are broken down into tiny particles, they are

more easily digested Liver – appx. 3 lbs

Extends from the right hypochondriac to epigastric regions

Divided into 4 lobes with its own specific circulation (hepatic portal)

Entire liver is covered by serosa (visceral peritoneum), which attaches the liver to the diaphragm

Gallbladder is chiefly responsible for storage of bile

Gallbladder Thin walled, green, muscular sac

About the size of a kiwi (4 inches) Gland that extends from the interior margin of the

liver Stores bile that is not needed immediately for

digestion Since it is not needed immediately, the gallbladder

concentrates that bile by absorbing some of its water content

When empty or only containing a small amount of bile, it can fold up like the rugae of the stomach to allow for quick expansion and expulsion of bile when needed

Pancreas Extends across the abdomen

Lies deep to the greater curvature of the stomach Produces a broad spectrum of enzymes to breakdown

foodstuffs Pancreas delivers pancreatic juice directly to the duodenum

Usually fuses with the bile duct from the liver/gallbladder Pancreatic juice mainly consists of water, electrolytes, and

enzymes Has a high pH – enables neutralization of acidic chyme

Pancreatic enzymes include: Amylase, lipases, nucleases Carboxypeptidase: broad range of functions: mainly protein digestion Chymotrypsin: breaks down proteins: specifically: tyrosine (cheese

protein), tryptophan & phenylalanine Enterokinase – a serine protease – breaks bonds between peptides