Digestive System Flashcards
Digestive functions
- ingested food yields energy molecules (ATP) that can be used by body cells for: transportation, contraction, synthesis and secretion
- food serves a source of building supplies for the body tissues
Enzyme that converts carbohydrates (CHO) into glucose
amylase
Enzyme that converts proteins into amino acids
trypsin
Enzyme that converts lipids into fatty acids
lipase
How do you liberate energy from available food
food undergoes digestion, biochemical breakdown and absorption in GI tract
4 digestive processes
- Motility
- Secretion
- Digestion
- Absorption
2 types of motility
- propulsive motility
- mixing movements
Propulsive motility
- muscular contractions propel the food contents forward in the digestive tract
- allows an appropriate velocity to a segment of the GI tract to perform its function in digestion
- ex: protein-rich food slows down in the stomach more than the mouth and esophagus (takes longer to breakdown)
Mixing movements
- mixes the food with digestive juices
- exposes all portions of the food contents to the absorbing surface of the digestive tract
Secretion
- the digestive juices from the exocrine glands into the digestive tract lumen
- comprise of water, electrolytes, and specific organic constituents such as enzymes, bile salts and mucus
- is under appropriate neural and/or hormonal stimulation
Absorbable units of CHO
monosaccharides
Absorbable units of proteins
amino acids
Absorbable units of lipids/fats
fatty acids
Carbohydrates (CHO)
are in the form of polysaccharides which consists of chains of interconnected glucose molecules
Types of CHO and their sources (polysaccharide)
- starch= plants
- cellulose= plant cell wall
- glycogen= body muscles (meat)
- dietary CHO are in the form of sucrose, lactose (milk sugar)
Digestion of carbohydrates
- Mouth-salivary enzyme amylase begins the breakdown of food starches into maltose, a disaccharide
- As the bolus of food travels through the esophagus to the stomach, no significant digestion of carbohydrates takes place. The esophagus produces no digestive enzymes but does produce mucous for lubrication. The acidic environment in the stomach stops the action of the amylase enzyme.
- Duodenum- chyme from the stomach enters the duodenum and mixes with the digestive secretion from the pancreas, liver, and gallbladder. Pancreatic juices also contain amylase, which continues the breakdown of starch and glycogen into maltose, a disaccharide. The disaccharides are broken down into monosaccharides by enzymes called maltases, sucrases, and lactases, which are also present in the brush border of the small intestinal wall. Maltase breaks down maltose into glucose. Other disaccharides, such as sucrose and lactose are broken down by sucrase and lactase, respectively. Sucrase breaks down sucrose (or “table sugar”) into glucose and fructose, and lactase breaks down lactose (or “milk sugar”) into glucose and galactose
- the monosaccharides glucose and galactose are absorbed into the interior of the cell and eventually enter the blood by means of Na+ and energy-dependent secondary active transport
- the monosaccharide fructose is absorbed into the blood by passive facilitated diffusion
Absorption
- complete digestion and absorption take place in the small intestine
- small absorbable units along with water, vitamins and electrolytes from the digestive tract –> digestive tract lumen –> blood/lymph
Digestion of proteins
- Stomach- enzyme pepsin plays an important role in the digestion of proteins by breaking down the intact protein to peptides
- Duodenum- other enzymes— trypsin, elastase, and chymotrypsin—act on the peptides reducing them to smaller peptides
- Duodenum- further breakdown of peptides to single amino acids is aided by enzymes called peptidases (those that break down peptides)
- The amino acids are absorbed into the bloodstream through the small intestines
Digestion of lipids
- Lipid digestion begins in the stomach with the aid of lingual lipase and gastric lipase
- However, the bulk of lipid digestion occurs in the small intestine due to pancreatic lipase
- When chyme enters the duodenum, the hormonal responses trigger the release of bile, which is produced in the liver and stored in the gallbladder. Bile aids in the digestion of lipids, primarily triglycerides by emulsification
- pancreatic lipases can then act on the lipids more efficiently and digest them. Lipases break down the lipids into fatty acids and glycerides
- These molecules can pass through the plasma membrane of the cell and enter the epithelial cells of the intestinal lining
4 major tissue layers of the digestive tract
- mucosa- innermost layer
- submucosa
- muscularis externa
- serosa- outer layer
Mucosa
- lines luminal surface of the digestive tract
- highly folded surface greatly increases the absorptive area
- 3 layers: mucous membrane, lamina propria and muscularis mucosa
Mucous membrane
- protective surface
- modified for secretion and absorption
- contains: exocrine gland cells (digestive juices), endocrine gland cells (blood-borne gastrointestinal hormones), epithelial cells (absorbing digestive nutrients)
Lamina propria
houses gut-associated lymphoid tissue (GALT): important in defence against disease-causing intestinal bacteria
Muscularis mucosa
a thin layer of smooth muscle
Submucosa
- provides the distensibility and elasticity to GI tract
- blood vessels and lymph vessels
- a local neural network known as as submucosal plexus
Muscularis externa
- forms smooth muscle coat of the GI tract
- consists of two layers: inner circular layer (decreases the diameter of the lumen) and outer longitudinal layer (contraction shortens the GI)
- myenteric plexus: lies between the two muscle layers
Serosa
- secretes serous fluid that lubricates and prevents friction between digestive organs and surrounding viscera
- continuous with mesentery (a fold of membrane that attaches the intestine to the wall around the stomach area and holds it in place) throughout much of the tract
Purpose for the serosa being continuous with the mesentery
- the attachment provides relative fixation
- secure the digestive organs in proper place
- allow the freedom for mixing and propulsive movements
Saliva
produced largely by 3 pairs of salivary glands
Composition of saliva
- 99.5% water
- 0.5% electrolytes and protein (amylase, mucus, lysozyme)
Functions of saliva
- salivary amylase: carbohydrate digesting enzyme
- facilitates swallowing by moistening food
- mucus provides lubrication
- antibacterial action: lysozyme destroys bacteria, saliva rinses away material that could serve as a food source for bacteria
- the solvent for molecules that stimulate taste buds
- aids speech by movements of lips and tongue
- help keep mouth and teeth clean
- rich in bicarbonate buffers
Autonomic influence on salivary secretion
- both sympathetic and parasympathetic NS act synergistically to increase salivary secretion
- parasympathetic: stimulates prompt and abundant saliva is rich in enzymes
- sympathetic: elicits smaller saliva volume; feeling mouth drier than usual (e.g. during stress)
Stomach structure
- j-shaped sac-like chamber lying between esophagus and small intestine
- divided into 3 sections: (1)fundus, (2) body and (3) antrum
Stomach functions
- food storage
- secretion of hydrochloric acid (denatures proteins) (HCl) and enzymes for protein digestion
- pulverizes semi-solid food into chyme
Pyloric sphincter
serves as a barrier between the stomach and upper part of small intestine
4 aspects of gastric motility
- gastric filling
- gastric storage
- gastric mixing
- gastric emptying
Gastric filling
Involves receptive relaxation:
- enhances the stomach’s ability to accommodate the extra volume of food
- triggered by an act of eating
- mediated by the vagus nerve
Gastric storage
the body of the stomach