Lecture 3: Digestion, Absorption, and Transport Flashcards
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
circulatory system
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
Main Organs
Mouth
Esophagus
Stomach
Small intestine
Large intestine
Accessory Organs
Salivary glands
Liver
Pancreas
Gallbladder
They are outside the GI tract but aid in digestion
Sphincters
allows food to pass from one organ to the next
Where does digestion begin?
Mouth
Mechanical Digestion
Breaking down food through
chewing, grinding, squeezing, and moving food
through the GI tract by peristalsis and segmentation
Chemical Digestion
Breaking down food through
enzymatic reactions
Saliva
- Dissolves small food particles
- Contains the enzyme amylase, which begins to break
down carbohydrates à chemical digestion - In adults, no other chemical digestion takes place in
the mouth
Bolus
Once food has been
adequately chewed and
moistened, the tongue rolls
it into a bolus and it enters
the pharynx to be swallowed
Epiglottis
Closes off the trachea and prevent choking
Esophagus
transports food and fluids to the
stomach
Bolus enters esophagus
Upper esophageal sphincter
Allows the bolus of food to enter the esophagus
Lower esophageal sphincter (LES)
Allows the bolus of food to enter the stomach
The LES relaxes after swallowing to allow the bolus to enter the stomach
The LES contracts to prevent stomach contents from returning to the esophagus
Stomach
Stores, mizes and prepares food for digestion
- Chemical digestion à mixes food with gastric juices
to break it down into smaller pieces - Mechanical digestion à muscles of the stomach mix,
churn, and push the contents with the gastric juices - Empty: holds 1 cup
- Fully expanded: up to 1 gallon
Chyme
- the semiliquid, partially digested food mass
that leaves the stomach and enters the small intestine - Approximately 1 tsp of chyme leaves the stomach
and enters the small intestine every 30 seconds
Pyloric Sphincter
the gateway for chyme to enter the small intestine from the stomach
Three segments of small intestine
Duodenum: 10 inches long
Jejunum: 8 feet long
Ileum: 12 feet long
Digestion in small intestine
- Mechanical digestion à muscular contractions push
chyme forward - Chemical digestion à digestive secretions break down
nutrients - The contact time in the small intestine is 3 to 10 hours,
depending on the food eaten
Structure of the small intestinal wall
surface contains circular folds, villi, and microvilli
All help increase surface area to help maximize absorption
Three segments of Large Intestine
Cecum: beginning of large intestine
Colon: largest part of the large intestine
Rectum: Final 8-inch portion of the large intestine
Large Intestine
Absorbs water and some nutrients
Chyme enters large intestine through the ileocecal valve
is 5 feet long and 2.5 inches in diameter
Purpose of Large Intestine
- Site of water, sodium, potassium, and chloride
absorption - Bacteria produce vitamin K, thiamin, riboflavin, biotin,
and vitamin B12 - Only biotin and vitamin K can be absorbed
- Bacteria in the colon ferment some undigested and
unabsorbed carbohydrates into simpler compounds,
methane gas, carbon dioxide, and hydrogen. - Fermented fiber produces short-chain fatty acids
- 1 liter of fluid material is gradually reduced to 200
grams of brown fecal material
Purpose of the end of the Large Intestine
- The intestinal matter passes through the large
intestine in 12 to 70 hours, depending on the
person’s health, age, diet, and fiber intake - Stool is propelled through the large intestine until it
reaches the rectum, the 8-inch portion of the large
intestine - Anus is the opening of the rectum, or end of the GI tract
- The final stage of defecation is under voluntary control
~Influenced by age, diet, prescription medications,
health, and abdominal muscle tone
Salivary Glands
- Dissolve small food particles to ease the
process of swallowing food - The body produces 1 quart of saliva per day
- Saliva contains water, mucus, electrolytes,
and a few enzymes
Liver
- Largest internal organ in the body à 3
pounds - Major player in the digestion, absorption,
and transport of nutrients - Essential in carbohydrate metabolism
- Makes proteins
- Manufactures bile salts used to digest fats
- Site of alcohol metabolism
- Removes and degrades toxins and excess
hormones
Gallbladder
- Receives bile from the liver via the common
hepatic duct - Concentrates bile
- Releases bile into the small intestine via the
common bile duct
Pancreas
- Endocrine function: Releases hormones to
maintain blood glucose levels - Exocrine function: Secretes digestive
enzymes into the small intestine
How is food propelled through GI tract
Peristalsis
Segmentation
Peristalsis
Squeezes food forward through the GI tract
* Mechanical digestion
Segmentation
Shifts food back and forth along the GI tract in the intestines
* Allows contact with surface of small and large intestines and increases absorption
How is food chemically digested?
- Digestive enzymes and other substances
- Regulated by hormones
- Completed by the time the food reaches the
large intestine
What aids food in chemical digestion?
Enzymes
Enzymes
Proteins that drive the process of digestion by catalyzing (speeding up) hydrolysis
Hydrolysis
- Chemical reaction that uses water to split chemical
bonds of digestible nutrients - The hydroxyl group (OH) from water joins one
molecule while the hydrogen ion (H) joins the other
molecule
The three conditions required for enzymes to work
- Presence of compatible enzyme and nutrient
- pH of surrounding environment must fall within a certain range
- Must have optimal environment temperature
The three conditions required for enzymes to work
- Presence of compatible enzyme and nutrient
~Enzymes are compatible only with a specific
compound or nutrient (substrate) - pH of surrounding environment must fall within a certain range
~Most active and efficient within a certain range of
acidity and alkalinity - Must have optimal environment temperature
~Enzyme activity is slowed with too low a temperature and stopped when the temperature is too high.
Where does majority of nutrient absorption take place?
Small intestine
Methods of Absorption
Passive Diffusion
Facilitated diffusion
Active Transport
Endocytosis
Passive Diffusion
Nutrients move from high concentration to low concentration
– no energy required
– e.g. water, small lipids
Facilitated Diffusion
Nutrients move from high concentration to low concentration with the help of a carrier protein
– no energy required
– E.g. fructose
Active Transport
Nutrients move from low concentration to high concentration with the help of a carrier protein
– energy required
– E.g. glucose, amino acids
Endocytosis
Cell forms a vesicle to surround and engulf a nutrient
– E.g. whole proteins such as immunoglobulins in breast milk
Where does most of fluid absorption occur?
Large Intestine
Fluid Absorption
- The majority of nutrients have been absorbed when chyme enters the large intestine
- Water and sodium are absorbed in the large intestine
- The same mechanisms used in the small intestine are also used in the large intestine
~Water: Passive diffusion
~Sodium: Active transport
What can influence digestion and absorption?
Psychological influences
Chemical influences
Bacterial influences
Psychological influences
Taste, smell, presentation of food, expectations that food tastes good, environment
Chemical influences
Preparation of food, digestibility, temperature
Bacterial influences
Amount of stomach acid, microbes in large intestine
What can increase satiety?
Fat
Protein
Fiber and whole grains
Water
Regulation of Digestion
Endocrine system and Nervous system work together to coordinate digestion, absorption, and excretion of waste
products
What is digestion controlled by?
Enteric nervous system
Enteric Nervous System
- a meshwork of nerve fibers that innervate the GI
tract, pancreas, and gallbladder
~Monitor stomach contractions after eating
~Monitor secretions of the cells in the GI tract
How does the nervous systems communicate with GI tract?
Extrinsic nerves
Intrinsic nerves
Extrinsic Nerves
- Communicate changes in the GI tract and
stimulate motility - Originate in the brain or spinal cord
Intrinsic Nerves
- Receive the message from the extrinsic
nerves and respond by stimulating the
release of digestive juices - Interwoven in the linings of the esophagus,
stomach, and small and large intestines
Hormones
Regulates digestion by controlling:
* The release of gastric and pancreatic secretions
* Peristalsis
* Enzyme activity
Enterogastrones
produced and secreted by cells lining the stomach and small intestine
~Influence GI motility, stomach emptying, gallbladder
contraction, intestinal absorption, and hunger
~Release is stimulated by the types of food passing
through the digestive tract
Key Hormones
Gastrin
Secretin
Cholecystokinin
Gastric inhibitory peptide (GIP)
Ghrelin
PYY
Gastrin
Stimulates HCl production and the release
of gastric enzymes
* Lowers pH
Secretin
Stimulates the pancreas to release
bicarbonate in the small intestine
* Raises pH
Cholecystokinin (CCK)
Stimulates the pancreas to release lipase and the gallbladder to release bile
* Slows gastric motility
Gastric inhibitory peptide (GIP)
Inhibits gastric motility and stomach secretions
Ghrelin
Increases hunger
PYY
Decreases hunger
How are nutrients transported throughout the body?
- Nutrients are absorbed into the bloodstream or lymphatic
system. - The waste products that remain after nutrient
absorption are removed by the excretory
system - The kidneys filter the blood, allowing waste
products to be concentrated in the urine and
excreted
Water soluble nutrients
~Absorbed into the cardiovascular system through the hepatic portal vein to the liver
-Carbohydrates, amino acids, and water-soluble vitamins
Fat-soluble Nutrients
Absorbed into the lymphatic system
~Fat-soluble vitamins, long-chain fatty acids, and
proteins
-Too large to be transported via the capillaries
Common Digestive Disorders
Range from minor to serious in nature
- Heartburn
- Esophageal cancer
- Belching
- Gastroenteritis
- Ulcers
- Gallbladder disease
- Flatulence
- Diarrhea/constipation
- Hemorrhoids
- Irritable bowel syndrome
- Ulcerative colitis
- Crohn’s disease
- Colon cancer
- Celiac disease
Gastroesophageal Reflux Disease (GERD)
Esophageal problem
Most commonly known as indigestion or acid reflux
- The lower esophageal sphincter doesn’t close
properly, allowing HCl from the stomach to flow into
the esophagus. - Leads to chronic heartburn and stomach acid reflux
- Factors generally associated with GERD include:
~Chocolate, fatty foods, coffee, soda, onions, garlic
~Smoking, being overweight or obese, drinking alcohol,
wearing tight-fitting clothing, eating large evening meals,
reclining after eating - Dietary changes, behavior modification, antacids,
prescription drugs, or surgery may help
Ulcers
Disorder of the Stomach
- A sore or erosion in the lining of the lower region of
the stomach or upper part of the duodenum - 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
changes, and/or surgery - An untreated ulcer can result in peritonitis (a scar
tissue that can obstruct food and cause vomiting and
weight loss, and greater risk for stomach cancer)
Gallstones
Gallbladder Disease
- Diagnosed most frequently in women and older Americans
- Obesity and rapid weight loss are contributing factors
- An unhealthy gallbladder can create gallstones
~Stones are formed from cholesterol in the gallbladder
or bile duct
~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
Celiac Disease
Intestinal Disorder
- A genetic autoimmune disorder that causes damage to the small intestine when foods containing gluten are consumed
- Causes the small intestinal villi to flatten out, causing nutrient malabsorption
- Caused by an abnormal reaction to the protein
gluten, found in rye, wheat, and barley - Symptoms
~Reoccurring abdominal bloating
~Cramping and/or gas
~Diarrhea
~Foul-smelling stools
~Weight loss
~Anemia
~Bone or joint pain - Increases risk for osteoporosis, diminished
growth, seizures - Diagnosis is made with a blood test and tissue
biopsy of the small intestine - Treatment
- Strict gluten-free diet
Irritable bowel syndrome (IBS)
Intestinal Disorders
Changes in colon rhythm
* Those with IBS experience an over-response to colon stimuli,
resulting in alternating patterns of diarrhea, constipation, and
abdominal pain
* Exact cause unknown
* Treatment may include increased dietary fiber, stress management, medications, FODMAP diet, probiotics
Diarrhea
Intestinal Disorder
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 - Treated with fluid and electrolyte replacement
Constipatio
Intestinal Disorders
Infrequent passage of dry, hardened stools
Often due to insufficient fiber or water intake
* Other causes include stress, inactivity, smoking
cessation and various illnesses
* Treatment
~Exercise, normal eating patterns, and proper rest
~Laxatives should be used sparingly, as they can
cause dehydration, salt imbalances, and laxative
dependency
~Avoid enemas
Diverticulosis
Intestinal Disorders
- Small pouches develop along the large intestine
- May become infected (diverticulitis)
- Often caused by chronic constipation
- May be a chronic condition
- Worsened by low-fiber diets
Hemorrhoids
Intestinal Disorders
Swelling of the veins of the rectum and anus
- Can lead to bleeding, itching, and/or pain
- Cause unknown
~Straining to pass dry stools, pregnancy, constant
constipation or diarrhea, and aging contribute - Treatment: increased dietary fiber and fluid
intake
~Itching and pain can be relieved through use of
creams, ice packs, and soaking in a warm bath
~Severe cases may require surgery
Ulcerative Colitis and Crohn’s Disease
Type of Inflammatory Bowel Disease (IBD):
- Chronic inflammation of the GI tract, resulting in ulcers
- Tends to run in families
- No known cause or cure
- Treatment includes drug therapy and surgery
Colon Cancer
Intestinal Disorders
The third leading cause of cancer death, but one of the most curable cancers if detected early
- 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, and stage of cancer diagnosis
