Digestion PhysiologyChapter 26

Test (Chapters 25 & 26) April 6th and 7th

(next block day)

March 28, 2011Announcements:• Turn in last week’s warm ups to the box• Pick up new warm up sheet• Practical grades in gradebook• Practical make up:

– Today after school @ 3:15– Tuesday morning 6:45

Warm Ups:1. In the large intestine longitudinal muscles are grouped

into strips called _______; circular muscles are grouped into rings are form _______.

2. What is produced in the appendix?3. The two lobes of the liver are connected by the:4. What are gallstones?5. Through what two pathways does blood enter the

hepatic lobules?

Primary Mechanisms of Digestive System (Table 26-1)

• Ingestion – take food in• Digestion – break down complex nutrients

into simpler nutrients• Motility – movement of GI tract; aids in

digestion• Secretion – enzymes are required for

digestion to take place• Absorption – movement across GI mucosa

into the internal environment• Elimination – process of eliminating

unabsorbed material

Mechanical Digestion

• All motility of the digestive tract that cause the following changes:– Change in physical state of the food for

large particles to smaller particles– Churning of the GI lumen to mix

particles with digestive juices– Propelling food forward ending with

elimination

Mastication

• Mastication = chewing• Requires tongue, cheek and lips• Reduces particle size• Mix food with saliva

Deglutition• Deglutition = swallowing1. Oral stage

• Bolus is formed voluntarily• Tongue pushes bolus against the palate and into

the oropharynx• Soft palate acts as a valve to prevent food from

entering the nasopharynx

2. Pharyngeal stage• Involuntary reflexes push bolus toward

esophagus• Epiglottis prevents food from entering trachea

3. Esophageal stage• Involuntary reflexes move bolus towards stomach

Motility

• Smooth muscle contractions take over in the lower portion of the esophagus

• Peristalsis – progressive wavelike ripple of the muscle layer of a hollow organ– Bolus stretches the GI tract wall

triggers contraction of circular smooth muscle bolus moves forward

Motility

• Segmentation – mixing movement; back and forward movement within a single region– Mechanically breakdown food particles– Mix food and digestive juices together– Facilitate absorption

• Peristalsis and segmentation can occur in alternating sequence to churn/mix and progress food

Peristalsis & Segmentation

Regulation of Motility

• Gastric Motility– Emptying the stomach takes approx 2-6

hours after a meal– Food is churned with digestive juices to

form chyme • Ejection every 20 seconds into the

duodenum• Controlled by hormonal and nervous

mechanisms

Regulation of Motility

• Hormonal Control– Fats and nutrients in duodenum cause

secretion of gastric inhibitory peptide (GIP) from the intestinal mucosa into the bloodstream• Slows peristalsis in stomach; decreasing

passage of food into the duodenum

Regulation of Motility

• Nervous Control– Nerve receptors in duodenum are

sensitive to acid and distention • Sensory and motor fibers within the vagus

nerve (CN X) cause a reflex inhibition of gastric peristalsis (enterogastric reflex)

Regulation of Motility• Intestinal Motility• Takes approx 5 hours for food to pass through

the small intestine– Segmentation

• Mixes chyme and digestive juices from liver, pancreas and intestinal mucosa

• causes contact with intestinal mucosa to increase absorption

– Peristalsis• Continues in the jejunum to move food into the large intestine • Stimulated by the hormone cholecystokinin-pancreozymin

(CCK)– Secreted by intestinal endocrine cells in the presence of chyme

Mechanical Digestion

• Summarized in Table 26-2; page 775

Chemical Digestion

• Consists of all the changes in the chemical composition of food

• Result of hydrolysis– Compounds combine with water then

split into simpler compounds– Enzymes catalyze the hydrolysis of

foods

Chemical Digestion

• Six main types of chemical substances:– Carbohydrates, proteins, fats, vitamins,

mineral salts, water– Only carbohydrates, proteins and fats

must undergo chemical digestion to be absorbed

Digestive Enzymes Properties

• Extracellular enzymes• Classified as hydrolases• Function optimally at a specific pH

– Ex: amylase vs pepsin• Continually destroyed or eliminated• Most digestive enzymes are

synthesized and secreted as proenzymes– Kinases convert proenzymes to active

enzymes

Carbohydrate Digestion

• Carbohydrates are saccharide compounds– Contain one or more saccharide groups

• Polysaccharides – starches & glycogen• Disaccharides – sucrose, lactose and

maltose• Monosaccharides – glucose, fructose &

galactose

Carbohydrate Digestion

• Polysaccharides are hydrolyzed by amylases– Present in saliva and pancreatic juice

• Sucrose, lactose and maltose are hydrolyzed by sucrase, lactase, and maltase– Located on epithelial cells lining villi in

small intestine– End result (usually glucose) is located at

site of absorption (“contact digestion”)

Protein Digestion

• Proteins are large molecules composed of twisted chains of amino acids

• Proteases catalyze the hydrolysis of proteins into smaller compounds– Proteoses peptides amino acids

• Proteins have varying peptide bonds holding amino acids together = increased need for varying proteases

Protein Digestion - Proteases

• Pepsin – gastric juice• Trypsin and chymotrypsin –

pancreatic juice• Peptidases – intestinal brush border

Proteoses

Fat Digestion

• Fats are insoluble in water – must be emulsified prior to digestion– Emulsify = dispersed as small droplets– Lecithin and bile salts emulsify oils and fats in

the small intestine by forming micelles (fig 26-8, page 778)

– Lecithin mixes with fat to form micelles

• Fats broken down by mechanical digestion are further broken down by lipase

• Action of lipase is enhanced by colipase (released from the pancreas)

Residuals of Digestion• Certain compounds cannot be

digested in humans b/c we lack the enzyme required for hydrolysis

• These compounds are excreted in the feces

• Cellulose (dietary fiber), connective tissue from meat (collagen), undigested fats combined with calcium and magnesium, bacteria, pigments, water, mucous

Chemical Digestion

• Summarized in Table 26-3, page 779

Secretion

• Release of substances from exocrine glands in the GI tract– Saliva, gastric juice, bile, pancreatic

juice, intestinal juice

Saliva• Secreted from salivary glands• Water component helps liquefies food chyme

– Allows enzymes to mix with food particles

• Mucus lubricates food to protect mucosa lining• Amylase – chemically digest starch and glycogen• Lipase (small amounts) – digest lipids

– Decreased function when fat are not emulsified

• Sodium bicarbonate (NaHCO3)– Dissociated in water– Bicarbonate ions bind with H+ to increase pH

Control of Salivary Secretion

• Controlled by reflex mechanisms:– Olfactory & visual stimuli send impulses

to centers in the brainstem efferent impulses to salivary glands

– Chemical and mechanical stimuli comes from the presence of food in the mouth

Gastric Juice

• Secreted by gastric glands surrounds by gastric pits

• Chief cells – secrete enzymes of digestive juices– Pepsin (proenzyme = pepsinogen)

• Pepsinogen is activated by hydrochloric acid (HCl)

• Parietal cells – Secrete HCl

• Decreases stomach pH; increases blood pH

– Secrete intrinsic factor • Binds to molecules of vitamin B12 to facilitate

absorption in the small intestine

Control of Gastric Secretion• Gastric secretion is controlled by 3

phases:1. Cephalic phase (“psychic phase”)

– Sight, smell, taste, thought of food activate control centers in medulla oblongata

– Parasympathetic fibers of the vagus nerve conduct impulses to gastric glands

– Vagal impulses stimulate production of gastrin• Gastrin stimulates gastric secretion

Control of Gastric Secretion

2. Gastric phase: – Gastrin secretion is further stimulated

by the presence of products of protein digestion & distention

– Gastrin continue to stimulate the secretion of gastric juices (high pepsinogen and HCl content)

Control of Gastric Secretion3. Intestinal phase:

– Gastric inhibitory peptide (GIP) in secreted in the small intestine in the presence of fats and carbohydrates

• Decrease gastric motility and secretion– Secretin secreted in the small intestine in the

presence of acid, digested proteins and fats• Inhibit gastric secretion• Simulate secretion of pancreatic enzymes• Stimulate ejection of bile into small intestine

– CCK• Secreted in the small intestine in the presence of chyme• Stimulates ejection of bile from gallbladder• Stimulates secretion of pancreatic juices• Opposes action of gastrin; raises pH of gastric juice

Pancreatic Juice

• Secreted by exocrine acinar cells of the pancreas

• Mostly water• Enzymes:

– Trypsin and chymotrypsin (proteases)– Lipases– Nucelases (RNA and DNA digesting enzymes)– Amylase (starch digesting enzyme)

• Secrete bicarbonate into the GI lumen and H+ into the blood to buffer the effects parietal cell secretion (fig 26-10 and fig 26-11)

Control of Pancreatic Secretion

• Secretin– Stimulates the secretion of pancreatic

fluid high in bicarbonate to neutralize acidity of chyme in the small intestine

– See notes under “Control of Gastric Section – Intestinal phase”

• CCK – see above

Bile

• Secreted by liver and stored in gallbladder• Lecithin and bile salts

– Emulsify fats by creating a hydrophilic “shell” around tiny fats droplets

• Sodium Bicarbonate – increase pH of chyme in small intestine

• Excretions:– Cholesterol, products of detoxification, bilirubin

(product of hemolysis)

Control of Bile Secretion

• Controlled by CCK and secretin• See Table 26-5, page 782

Intestinal Juice

• Mucus – provides lubrication• Sodium bicarbonate – increases pH

to allow intestinal enzymes to function at optimal level

• Water – carries mucus and NaHCO3

**Study These Tables**

Table 26-4: Digestive SecretionsTable 26-5: Actions of Digestive

Hormones

Both on page 782

Wednesday/Thursday 3/30-3/31

Warm Up:1. Name the 5 components of saliva.2. What is the proenzyme of pepsin? What

is needed to activate this proenzyme?3. Explain the role of bicarbonate in the GI

tract.4. The control of gastric secretion can be

broken up into 3 phases. Name these phase and briefly describe each one.

5. Name and give the function of the 5 enzymes found in pancreatic juice.

Absorption

• Passage of substances (digested foods, vitamins, salts, water) across the mucosa into the blood

• Majority of absorption takes place in small intestine where surface area is increased

Mechanisms of Absorption

• Some substances (water) are absorbed via diffusion

• Secondary Active Transport (ex: Sodium)– Na+ is actively transported from the basal

(backside) of epithelial cells lining the lumen of the small intestine into blood capillaries

– Creates a low intracellular sodium concentration

– Na+ ions diffuse passively from the lumen into epithelial cells

– Fig 26-14, page 785

Mechanisms of Absorption• Sodium cotransport

– Glucose is very large and hydrophilic – Requires carrier to cross intestinal mucosa– Carriers bind sodium and glucose together to

passively transport out of lumen

• Amino Acid absorption– Transported by passive carriers on luminal and

basal surfaces of absorptive cells– Brush border enzymes can also act as carriers– Polypeptides can diffuse into absorptive cells,

hydrolyze into amino acids, diffuse into blood

Mechanisms of Absorption

• Fatty acid/monoglyceride absorption– Bile salts and lecithin form micelles– Fat digestion takes place within these

tiny spheres– At the intestinal brush border, micelle

contents can diffuse into absorptive cells– Inside triglycerides reform within

chylomicrons– Water soluble chylomicrons allow fats to

be transported through lymph and into bloodstream

Mechanisms of Absorption

• Absorption of vitamins– Vitamins A, D, E, K are fat-soluble

• Depend on bile salts for absorption

– B Vitamins require carrier-mediated transport• Ex: Vit B12 and intrinsic factor

Summary of Absorption

• See table 26-6, page 787

Elimination

• Expulsion of digestive residuals from the digestive tract in the form of feces– Defecation– Normally rectum is empty– Massive peristalsis of feces into the

rectum stimulate receptors and relax the external anal sphincter

– Voluntary control

Elimination

• Constipation– Contents move through large intestine

at a slower rate– Increased water absorption occurs

resulting in hardened feces

• Diarrhea– Result of increased motility of the small

intestine– Water absorption does not occur

GI Tract Disorders

• Common S/S– Gastroenteritis

• Gastritis – stomach inflammation• Enteritis – intestinal inflammation

– Anorexia: chronic loss of appetite– Nausea: feeling of needing to vomit; may

progress to vomiting– Emesis: vomiting– Diarrhea: elimination of liquid feces; abdominal

cramps may also be present– Constipation: decreased motility of colon;

difficulty in defecating

GI Tract Disorders

• Ulcers– In stomach or duodenum– Cause pain and may lead to perforation

of the wall of the GI tract– Bleeding anemia– Causes:

• Hyperacidity• H. phylori bacterium

GI Tract Disorders

• Stomach cancer– Linked to excessive alcohol use,

chewing tobacco, eating heavily preserved foods

– Early signs:• Heartburn, belching, nausea

– Later signs:• Chronic indigestion, vomiting, anorexia,

stomach pain, blood in feces

GI Tract Disorders

• Diverticulosis – presence of abnormal sac-like projections on the large intestine (diverticula)– When inflamed causes diverticulitis– S/S: pain, tenderness, fever

GI Tract Disorders

• Colitis – inflammation of the large intestine– s/s: diarrhea, abdominal cramps,

constipation, bleeding, intestinal ulcers– Crohn’s Disease: autoimmune colitis– Treatment: surgical removal of affected

portions of the intestine

GI Tract Disorders

• Irritable bowel syndrome (IBS)– “spastic colon”– Noninflammatory condition usually

caused by stress– Diarrhea or constipation

GI Tract Disorders

• Colorectal cancer– Occurs after 50– Associated with low-fiber, high-fat diet– Early signs: change in bowel habits,

fecal blood, rectal bleeding, abdominal pain, unexplained anemia, weight loss, fatigue

Liver Disorders

• Hepatitis – inflammation of the liver– S/S: jaundice, liver enlargement,

anorexia, abdominal discomfort, gray-white feces, dark urine

– Causes: alcohol or drug abuse; bacterial or viral infection

• Cirrhosis – degenerative liver condition– Tissue can no longer regenerate