Human Nutrition Ch. 3

Mucosa

innermost lining of the GI tract; made up of epithelial cells

What is the primary role of the mucosa?

production of GI secretions

How long do the cells of the mucosal lining live?

2-5 days, they need a constant supply of nutrients 

submucosa

beneath the mucosa that contains blood vessels that deliver nutrients and oxygen to outer layers 

What are the functions of the submucosa?

lymphatic vessels and fluid for circulation and a network of nerves for regulating digestion 

muscularis

muscular layer of the GI tract

What are the functions of the muscularis?

controls speed and rate of gastrointestinal motility, contractions promote mixing of food with digestive secretions and keeps food moving throughout GI tract

Serosa

protective barrier that encloses GI tract, connective tissue

What is the function of the serosa?

secretes fluid that lubricated digestive organs, preventing them from adhering to one another 

transit time

amount of time between consumption of food and its elimination from body (typically 24-72 hours)

GI motility

mixing and propulsive movements of the GI tract caused by contraction and relaxation of the muscularis 

Segmentation

muscular movement in the GI tract that moves the contents back and forth within a small region

Peristalsis

waves of muscular contractions that move materials in the GI tract in a forward direction

What is the difference between segmentation and peristalsis?

segmentation - mixing with digestive juices 

peristalsis - pushing chyme forward in GI tract 

chyme

liquid, semi-solid mass

Exocrine cells

produce and release their secretions into ducts that empty directly into the lumen of the GI tract 

Goblet cells

cells that synthesize and secrete mucus that forms a protective coating that lubricates

Acinar cells

found in the pancreas, produce and secrete digestive enzymes into the small intestine 

Digestive enzymes

biological catalysts that facilitate chemical reactions that break chemical bonds by the addition of water (hydrolysis), resulting in the breakdown of large molecules into smaller components 

How does the ENS connect to the CNS?

the vagus nerve

Enteric nervous system (ENS)

responsible for controlling smooth muscles and release of GI secretions and receives information from sensory receptors throughout GI tract 

Where are ENS neurons located?

within the submucosa and muscularis layers

Sensory receptors (cells)

receptors that monitor conditions and changes in the GI tract

Chemoreceptors

sensory receptor that responds to a chemical stimulus (ex: pH)

Mechanoreceptors

sensory receptor that responds to pressure, stretching, or mechanical stimulus

What sensory receptors are abundant in the stomach?

mechanoreceptors 

How are stimulus responses carried out in the GI tract?

information from sensory receptors goes to the ENS which communicates with muscles, endocrine glands, and secretes appropriate hormones in response 

Endocrine glands/cells

glands that produce and release hormones into the bloodstream

GI hormones

hormones secreted by the mucosal lining of the GI tract that regulate GI motility and secretion 

Gastrin

hormone produced in the stomach that stimulates GI motility, secretion of gastric juice, and increases gastric emptying 

What stimulates the production of gastrin?

food enters the stomach, stomach walls stretch, alcohol, caffeine, smell/taste of food

Secretin

hormone produced in the SI, inhibits GI motility, inhibits secretion of gastric juice and stimulates release of pancreatic juice

What stimulates secretin production?

arrival of chyme into the duodenum

Cholecystokinin (CCK)

hormone produced in the SI, stimulates gallbladder to contract and release bile, stimulates release of pancreatic enzymes into the SI

Ghrelin

hormone produced in the stomach, stimulates hunger in the brain

Gastric inhibitory peptide (GIP)

hormone produced in the SI, inhibits gastric motility

What are the phases of digestion?

cephalic phase (brain signals before eating), gastric phase (stomach responds to food) , and intestinal phase (SI manages remaining food & absorption)

What initiates the cephalic phase of digestion?

the central nervous system

Cephalic phase

response of CNS to sensory stimuli that occurs BEFORE food enters GI tract 

What is the cephalic phase characterized by?

increased GI motility and release of GI secretions

Gastric phase

food arrives in the stomach; increases GI motility and release of GI secretions 

Intestinal phase

chyme enters the SI, decrease in gastric motility and decrease of gastric juice secretion 

When does digestion begin?

when food enters the mouth

Mastication

chewing and grinding of food by teeth to prepare for swallowing

Saliva

secretion released into mouth by salivary glands to moisten food and start digestion 

Bolus

soft, moist mass of chewed food

Pharynx

region toward back of the mouth that is the shared space between oral and naval cavities 

Dysphagia

difficulty swallowing

Esophagus

passageway from pharynx to end of the stomach

Gastroesophageal sphincter

circular muscle that regulates the flow of food between the esophagus and the stomach (lower or cardiac sphincter) 

Gastroesophageal Reflex Disease (GERD)

condition caused by weaking of GE sphincter, enables gastric juices to reflux into the esophagus, causing irritation to the mucosal lining 

Why is the gastroesophageal sphincter important?

it must stay closed to prevent the stomach acid from going back up into the esophagus, if not it causes heartburn

What are the sections of the stomach?

fundus, body, and antrum

Fundus

top part of the stomach

Body

middle part of the stomach

Antrum

bottom part of the stomach

What are the important functions of the stomach?

temporary storage of food, production of gastric secretions for digestion, mixing of food with gastric secretions

How much can the stomach stretch?

empty - ¼ c to full - 16 cups

Gastric pits

infoldings of the mucosal lining of the stomach that contain specialized endocrine and exocrine cells 

Gastric juice

digestive secretions produced by exocrine cells that make up gastric pits

What is gastric juice made of?

water, HCl, digestive enzymes, mucus, and intrinsic factor

Parietal cells

exocrine cells within the gastric mucosa that secrete HCl and intrinsic factor

Chief cells

exocrine cells in the gastric mucosa that produce digestive enzymes

Pyloric sphincter

a circular muscle that regulates the flow of food between the stomach and the duodenum

Gastric emptying

food leaves the stomach and enters SI

What influences the rate of gastric emptying?

volume, consistency of food, and chemical composition of food

Gastroparesis

condition that affects gastric motility and gastric emptying that slows movement of food from the stomach and into the SI

What is the primary site of chemical digestion and nutrient absorption?

small intestine

How long is the small intestine?

about 20 feet long

Duodenum

the top of the SI, receives chyme from the stomach and secretions from gallbladder and pancreas 

Jejunum

middle of the SI

Ileum

bottom of the SI

Enterocytes

epithelial cells that make up the luminal surface of each vilus (SI)

What is the absorptive surface of the SI?

the brush border

What does the pancreas secrete for digestion?

pancreatic juice that contains bicarbonate and enzymes (neutralizes chyme) 

What does the gallbladder release?

bile, made by the liver (breaks down fat) 

Nutrient absorption

transfer of nutrients from the lumen of the GI tract to the circulatory system (enterocytes) 

Once water-soluble nutrients cross basolateral membrane they are

circulated in blood

Once fat-soluble nutrients cross basolateral membrane they are

circulated in lymph

Bioavailability

extent to which nutrients are absorbed into the blood/lymphatic system

What is the LI role in digestion? (aka colon)

absorption and reabsorption of fluids and electrolytes, microbial action, and waste elimination 

Colonic microbiota

bacteria that reside in the LI (over 1,000 types)

Functions of colonic microbiota

break down fiber, produce small amounts of some nutrients, protect us from infection by competing with pathogenic bacteria 

Probiotic foods

foods or supplements that contain beneficial live bacteria (yogurt, fermented foods) 

Prebiotic foods

foods that feed the bacteria of the LI

Dysbiosis

an imbalance in the body’s microbial community in terms of numbers and diversity 

Inflammatory Bowel Disease

chronic conditions such as ulcerative colitis and Crohn’s disease that cause inflammation of the lower GI tract, autoimmune disease

Leaky gut

increased permeability of the intestinal lining

Irritable Bowel Syndrome (IBS)

condition that affect lower GI tract, causing abdominal pain, muscle spasms, diarrhea, constipation, considered a functional GI disoder 

What system circulates nutrients, oxygen, and other substances?

cardiovascular system

What organs release digestive secretions?

salivary glands, stomach, pancreas, liver, gallbladder, SI and LI

Water-soluble nutrients absorbed from the digestive tract are circulated in the blood directly where?

to the liver via the hepatic portal vein 

The process of metabolism generates what cellular waste products?

carbon dioxide, urea, and water

How are metabolism waste products removed from the body?

by the lungs, kidneys, and skin