Chapter 3: Digestion, Absorption, and Transport

© 2010 Pearson Education, Inc.

What Are…?

Digestion: The process of breaking down food into individual molecules small enough to be absorbed through the intestinal wall

Absorption: The process of moving nutrients from the gastrointestinal (GI) tract into the bloodstream

Transport: The process of moving absorbed nutrients throughout the body through the circulatory and lymph systems

Elimination: The excretion of undigested and unabsorbed food through the feces


Gastrointestinal Tract

A 23-foot-long muscular tube comprised of the organs of the digestive tract

Extends from the mouth through the esophagus, stomach, and small and large intestines to the anus

Sphincters allow food to pass from one organ to the next

Saliva Dissolves small food particles Contains the enzyme amylase, which begins to break down

carbohydrate

Figure 3.1

Journey through the GI Tract: Ingestion

Figure 3.3

Sphincters at Work

Figure 3.1

Journey through the GI Tract: Digestion and Absorption

Figure 3.4

Anatomy of the Stomach


Small Intestine

Most digestion occurs in the small intestines Extends from the pyloric sphincter to the ileocecal valve Contains three sections

• Duodenum

• Jejunum

• Ileum Takes 3 to 10 hours for food to traverse the small intestine

Villi require proper nutrients or will deteriorate and flatten causing malabsorption

Figure 3.5

Anatomy of the Small Intestine

Figure 3.6

Anatomy of the Large Intestine


Large Intestine

Site of water, sodium, potassium, and chloride absorption Bacteria produces vitamin K, thiamin, riboflavin, biotin,

and vitamin B12

• Only biotin and vitamin K can be absorbed In the large intestine, 1 liter of fluid material is gradually

reduced to 200 grams of brown fecal material

• Brown color is due to unabsorbed iron mixed with yellowish-orange substance called bilirubin

• Greater the iron content, the darker the feces

Figure 3.1

Journey through the GI Tract: Elimination

Final stage of defecation is influenced by age, diet, prescription medications, health, and abdominal muscle tone

Figure 3.7

The Accessory Organs



Liver

• Is the first organ to receive absorbed nutrients from the portal vein

• Is essential in carbohydrate metabolism

• Produces proteins

• Manufactures bile salts that are used to digest fats

• Is the site of alcohol metabolism

• Removes and degrades toxins and excess hormones



Gallbladder

• Receives bile from the liver via common hepatic duct

• Releases bile into small intestine via common bile duct Pancreas

• Endocrine function – releases hormones to maintain blood glucose levels

• Exocrine function – secretes digestive enzymes into the small intestine


Quick Review

In the mouth, saliva mixes with and moistens food Bolus of food mixes with gastric juices in the stomach,

becoming chyme The majority of digestion and absorption occur in the small

intestine Undigested residue enters the large intestine, where water

is removed from the chyme The remnants of digestion reach the anus and exit the body

in the feces Accessory organs include the liver, which produces bile,

the gallbladder, which concentrates and stores it, and the pancreas, which produces enzymes and hormones.


Propelling Food through the GI Tract

Food is propelled through the GI tract by synchronized contractions of the diagonal, circular, and longitudinal muscles

Two primary contractions are

• Peristalsis – Squeezes food through the GI tract

• Segmentation – Shifts food back and forth along the GI tract

- Allows contact with surface of small and large intestine and increase absorption

• Dependent on coordination between muscles, nerves, and hormones

Figure 3.8

Peristalsis and Segmentation


Quick Review

Food is propelled through GI tract by strong muscular contractions

• Peristalsis squeezes food and propels it forward

• Segmentation shifts food back and forth along intestinal walls


Chemical Digestion of Food

Aided by digestive enzymes and other substances Regulated by hormones Completed by the time the food reaches the large intestine


Chemical Digestion of Food

Enzymes Proteins Catalyze hydrolysis

• Compatible with a specific compound or nutrient• Optimal pH range for enzyme function• Optimal temperature for enzyme function

Reactions do not change them Named by type of substrate plus suffix –ase

• Example: sucrase and maltase• Exception: pepsin

Secreted along the GI tract Majority produced by the pancreas

Concentration of hydrogen ions in a solution

The more hydrogen ions (H–) the more acidic

The more hydroxide ions (OH–) the more basic

pH


Quick Review

Foods are chemically digested by hydrolysis Enzymes catalyze hydrolysis when

• Correct substrate is available

• pH is optimal

• Temperature is optimal Secretions of the GI tract help optimize the environment for

digestion

• Saliva

• Gastric juices

• Bile

• Bicarbonate


Nutrient Absorption

Majority of absorption takes place in the small intestine

• Nutrients are absorbed via

- Passive diffusion – nutrients move from high concentration to low concentration; no energy is required

- Facilitated diffusion – nutrients move from high concentration to low concentration with the help of a carrier protein; no energy is required

- Active transport – nutrients move from low concentration to high concentration with the help of a carrier protein, energy is required

- Endocytosis – cell forms a vesicle to surround and engulf a nutrient

Figure 3.10

Nutrient Absorption


Nutrient Absorption

Some absorption takes place in the stomach and large intestine

• Water and salt are absorbed in the large intestine


Quick Review

Brush border of the small intestine is the major site of nutrient absorption

Water and salt not absorbed in the small intestine are absorbed in the large intestine

Nutrients are absorbed by

• Passive diffusion

• Facilitated diffusion

• Active transport

• Endocytosis


Hormones Affect Digestion

The GI tract releases hormones

• Enterogastrones

- Are produced and secreted by the cells lining the stomach and small intestine

- Gastrin, secretin, cholesystokinin (CCK), and gastric inhibitory peptide (GIP)

- Influence GI motility, stomach emptying, gallbladder contraction, intestinal absorption, and hunger

• Release is stimulated by the types of food passing through the digestive tract


Hormones Affect Digestion

Hormones

• Regulate digestion by controlling

- The release of gastric and pancreatic secretions

- Peristalsis

- Enzyme activity


How Does the Nervous System Affect Digestion?

Lets you know when you need to eat and drink Extrinsic nerves originate in the brain and spinal cord Intrinsic nerves are woven into the lining of the esophagus,

stomach, and small and large intestine Hormones work with nerves to communicate feelings of

hunger and fullness

• Ghrelin – hormone of hunger

• Peptide YY – signals that you have eaten or are full


Quick Review

Enterogastrones regulate digestion by stimulating or inhibiting the release of stomach, small intestine, pancreas, and gallbladder secretions, and influencing GI motility

• Gastrin, secretin, cholecystokinin, and gastric inhibitor peptide

Extrinsic and intrinsic nerves communicate and interpret changes in the GI tract

• Affect gastric motility, release or inhibition of digestive juices, and hunger

• Ghrelin and peptide YY communicate with the nervous system

- Help decide when to eat and when to stop eating


• Water-soluble nutrients

GI tract Capillaries

• Water-soluble nutrients

Hepatic Portal Vein • Water-

soluble nutrients

Liver

Transportation of Nutrients Throughout the Body Nutrients are absorbed into the circulatory or lymphatic

system

• Water-soluble nutrients are absorbed into the circulatory system

- Carbohydrates, amino acids, and water-soluble vitamins


• Fat-soluble nutrients

Lymph Capillaries

• Fat-soluble nutrients

Lymphatic vessels • Fat-

soluble nutrients

Thoracic Duct

Transportation of Nutrients Throughout the Body

• Fat-soluble nutrients are absorbed into the lymphatic system

- Fat-soluble vitamins, long-chain fatty acids, and proteins too large to be transported via the capillaries


Quick Review

The circulatory and lymph system transport absorbed nutrients throughout the body and deliver them to the cells

Water-soluble nutrients are transported via the circulatory system

• Carbohydrates, proteins, and water-soluble vitamins Fat-soluble nutrients are transported via the lymph system

• Fat-soluble vitamins and long-chain fatty acids


Common Digestive Disorders

Digestive disorders can range from annoying to serious in nature

The esophagus, stomach, gallbladder, and large and small intestines can all be affected by problems ranging from ulcers to cancer


Disorders Affecting the Esophagus

Heartburn, also known as indigestion or acid reflux

• The lower esophageal sphincter doesn’t close properly, allowing HCl from the stomach to flow into the esophagus

• Chronic heartburn and stomach acid reflux are typical of gastroesophageal reflux disease (GERD)

- Certain foods and lifestyle factors are generally associated with GERD

• Dietary changes, behavior modification, antacids, prescription drugs, or surgery may help


Disorders Affecting the Esophagus

Esophageal cancer

• This is one of the most common cancers of the digestive tract

• Sufferers are typically males over age 50 who smoke and drink heavily, living in urban areas

• Treatment can include surgery, radiation, and chemotherapy


Disorders Affecting the Stomach

Belching

• Generally caused by swallowing amounts of air

• It usually happens as a result of eating too fast, drinking carbonated beverages, or anxiety



Stomachache can occur for a number of reasons

• Gastroenteritis (stomach flu) is the inflammation of the stomach or intestines

- It can be caused by a variety of viruses

- Symptoms can include nausea, vomiting, diarrhea, and abdominal cramping

- Treat symptoms through rest, rehydration, and eating soft foods

• Consuming food or drink fluid contaminated with a pathogenic microbe can cause vomiting, abdominal cramps, diarrhea, and fever



Ulcers

• Helicobacter pylori, a bacterium, is often involved in the creation of ulcers

• Common symptoms are vomiting, fatigue, bleeding, general weakness, and burning pain

• Treatment can include prescription drugs, dietary recommendations, and/or surgery

An untreated ulcer can result in peritonitis, scar tissue that can obstruct food and cause vomiting and weight loss, and greater risk for stomach cancer


Disorders Affecting the Gallbladder

Gallbladder disease

• Diagnosed most frequently in women, older Americans

• Obesity and rapid weight loss are contributing factors

• An unhealthy gallbladder can create gallstones

- Treatment includes surgery for gallbladder removal, prescription medication, shock-wave therapy, or a combination of therapies

- The body eventually adapts to the removal of the gallbladder by secreting bile directly into the duodenum

An Ulcer and a Large Gallstone


Disorders Affecting the Intestines

Flatulence

• Intestinal gas, which needs to be released 10 to 20 times a day

• Caused by:

- foods high in fiber and starch

- eating quickly

- drinking carbonated beverages

- lack of exercise

- smoking



Diarrhea: the passage of watery, loose stools more than three times a day

• Generally the result of bacterial, viral, or parasitic infections that cause food and fluids to pass too quickly through the colon

- Chronic diarrhea may be the sign of a more serious problem

- Untreated diarrhea can lead to malnutrition

• Diarrhea can lead to dehydration and potentially death, particularly in children and the elderly



Constipation: infrequent passage of dry, hardened stools

• Often due to insufficient fiber or water intake.

- Exercise, normal eating patterns, and proper rest can help resolve constipation

- Laxatives should be used sparingly as they can cause dehydration, salt imbalances, and laxative dependency

• Colon cleansing (enema) is not recommended as a treatment



Hemorrhoids: swelling of the veins of the rectum and anus

• Can lead to bleeding, itching, and/or pain

• Caused by a variety of factors, including diarrhea and constipation

• Treatment includes increased dietary fiber and fluid intake

- Some symptoms (itching and pain) can be relieved through use of creams, ice packs, and soaking in a warm bath

- Severe cases may require surgery



Irritable bowel syndrome (IBS): changes in colon rhythm

• Those with IBS experience an overresponse to colon stimuli, resulting in alternating patterns of diarrhea, constipation, and abdominal pain

• The exact cause is not known

• Treatment includes increased dietary fiber, stress management, and prescription drugs



Ulcerative colitis: a chronic inflammation of the large intestine, resulting in ulcers in the lining of the colon

• Tends to run in families

• Afflicts both men and women, beginning between the ages of 15 and 30

• There is no known cause or cure

• Treatment includes drug therapy and surgery



Crohn’s disease

• Similar to ulcerative colitis, but ulcers can occur throughout the GI tract

• There is no known cause or cure

• Treatment includes drug therapy and possibly surgery Celiac disease

• Consuming foods containing gluten leads to damage of the small intestine


Sample Celiac Disease

Serum Test



Colon cancer

• The third-leading cause of cancer death

- One of the most curable cancers if detected early

• It begins with polyps on the lining of the colon that are often small, benign, and can be surgically removed

- Polyps can develop into cancerous tumors if not detected early

• Treatment includes radiation, chemotherapy, or surgery

• Survival rates vary depending on age, treatment response, stage of cancer diagnosis


Quick Review


Putting It All Together

The digestive process

Chapter 3: Digestion, Absorption, and Transport

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Transcript of Chapter 3: Digestion, Absorption, and Transport