Digestive System Flashcards
Ingestion
Act of eating
Digestion
Breaking down food into small enough molecules for the body to absorb
Starts in the mouth
Chemical and mechanical breakdown of food into smaller units that can be taken across the intestinal epithelium into the body
Mostly happens in the lumen of the gut
Absorption
Molecules are taken by cells lining the digestive tract
Movement of substances from the GI tract lumen to ECF
Elimination
Passage of undigested material out of the digestive tract
Organic compounds
Carbs, fats, proteins, vitamins
Inorganic compounds
Minerals (calcium, iron, magnesium, sodium) and water
Are nutrients organic compounds, inorganic compounds, both, or neither
Both
Chemical digestion
Breaking down macromolecules with acids and enzymes
Mechanical digestion
Muscular contractions, physical force
Gastrointestinal tract
Most animals have it, 2 openings with specialized regions so digestion and absorption can happen simultaneously
Esophagus, stomach, small intestine, large intestine
Mouth
Mechanical digestion: chewing
Chemical digestion: enzymes (salivary amylase, begins hydrolysis for starch) and acids secreted by salivary glands
Tongue forms food into a bolus and pushes it into the pharynx
Esophagus
Food is pushed down through peristalsis to the stomach
Narrow tube through the thorax to the abdomen with walls made of skeletal muscle that transitions to smooth muscle 2/3 of the way down
Stomach
Mechanical digestion: churning and squeezing of muscles
Chemical digestion: stomach acids and pepsin (protein to polypeptide enzyme)
3 M HCl maintains pH 2 for pepsin
Stores and breaks down food
Mucosa protects stomach walls from gastric acid (pepsin and HCl) and is completely replaced every 3 days
Baglike organ at the end of the esophagus that can contain up to 2L of food/fluid when fully expanded
Creates chyme by mixing food, acid, and enzymes
Intermediary between ingestion and digestion/absorption
Pancreas
Releases pancreatic juices into the duodenum
Pancreatic amylase breaks starch into maltose
Trypsin breaks polypeptides into amino acids
Lipase breaks lipids into fatty acids and glycerol
Small intestine
Most chemical digestion happens here because of enzymes secreted by the pancreas, liver (makes bile that breaks down fat), gallbladder (stores bile from liver), and brush border cells of the duodenum
Brush border cells of the duodenum
Beginning of the small intestine below the stomach
Maltase: breaks down maltose
Sucrase: breaks down sucrose
Lactase: breaks down lactose
Peptidase: breaks down polypeptides
Microvilli
Small fingerlike protrusions in the small intestine that increase surface area
What absorbs monomers in the small intestine?
Villi and microvilli
Where is most food broken down into monomers?
Duodenum
Fistula
Opening into the lumen
Gastrointestinal system
Digestive system
Digestive system
Mouth, stomach, small intestine, large intestine, rectum
Gut
The part of the GI tract from the stomach to the anus
Accessory glandular organs
Salivary glands, liver, gallbladder, pancreas
Add secretions to ingested food
Chyme
A soupy mix of secretions and food
What kinds of epithelium line the GI tract?
Secretory, transportational
What do rings of muscle in the GI tract do?
Function as sphincters, divide it into parts with distinct functions, move food through the tract with waves of muscle contraction
What absorbs products of digestion and where do they go?
Intestinal epithelium, pass through interstitial fluid and move to blood or lymph for distribution
What happens to waste still in the lumen at the end of the GI tract?
It exits through the anus
What part of the digestive system is part of the external environment?
Tract lumen, allows bacteria to live in the lumen, particularly the large intestine
Salivary glands
Secrete saliva in oral cavity, first stage of digestion
What are the three kinds of salivary glands and where are they?
Sublingual glands under the tongue
Submandibular glands under the mandible/jawbone
Parotid glands near the hinge of the jaw
What are the three parts of the stomach?
Upper part is the fundus
Central part is the body
Lower part is the antrum
Pylorus
Opening between the stomach and small intestine, guarded by pyloric valve
Pyloric valve
Thickened band of muscle that limits the amount of chyme entering the small intestine at once
What prevents the small intestine from getting overwhelmed?
Integrated signals and feedback loops between the stomach and small intestine that regulate the rate of chyme entering the duodenum
What are the 3 parts of the small intestine?
Duodenum (25cm)
Jejunum and ileum (260 cm collectively)
What carries out digestion in the small intestine?
Intestinal enzymes and exocrine secretions (from pancreas and liver)
How do secretions from accessory glandular organs enter the small intestine?
Ducts in the duodenum with continuously contracted sphincters that prevent pancreatic fluid and bile from entering outside of meals
Where is chyme mostly digested and absorbed?
Small intestine, ~1.5 L of it passes through to the large intestine
Colon
Proximal section of the large intestine, watery chyme becomes semi solid feces as water and electrolytes are absorbed by extracellular fluid (ECF)
Rectum
Terminal section of the large intestine, triggers defecation reflex when feces enter
Anus
Where feces leave the GI tract, has an external anal sphincter of skeletal muscles with voluntary control
How long is the digestive system?
~450 cm (~15 ft) in a live person, roughly doubles after death due to longitudinal muscle relaxation
How much of the digestive system is intestines?
395 cm (~13 ft)
What are the similarities and differences between stomach and intestine structure?
Gut wall is crumpled into folds to increase surface area (rugae in stomach, plicae in small intestine)
Small intestine projects into the lumen with villi
Surface area is added by tubular invaginations that extend into supporting connective tissues (gastric glands in stomach, crypts in small intestine), some of the deepest ones form secretory submucosal glands that open into the lumen through ducts
What are the 4 layers of the gut?
inner mucosa facing the lumen, submucosa, layers of muscle collectively called the muscularis externa, serosa
Mucosa
Inner lining of the GI tract, has 3 layers, structural modifications increase mucosal surface area to enhance absorption
What are the three layers of the mucosa?
Mucosal epithelium, lamina propria (subepithelial tissue that holds the epithelium in place), muscularis mucosae
Mucosal epithelium
Most variable feature of the GI tract, cells of mucosa include transporting epithelial cells (enterocytes in the small intestine), endocrine and exocrine secretory cells and stem cells, cells at the surface secrete ions, enzymes, mucus, and paracrine molecules into the lumen, substances from the lumen and molecules from epithelial cells absorbed in the outer level enter the ECF
Are stomach and colon junctions or small intestine junctions tighter?
Stomach and colon, small intestine junctions are considered “leaky” since some water and solutes can be absorbed between cells (paracellular pathway) instead of through them
GI stem cells
Rapidly dividing undifferentiated cells that continuously produce new epithelium in crypts and gastric glands, as they divide new ones are pushed to the luminal surface of the epithelium
What is the average lifespan of GI stem cells?
A few days
Lamina propria
Subepithelial connective tissue, contains nerve fibers, small blood/lymph vessels here, contains wandering immune cells patrolling for invaders entering through breaks in the epithelium, in the small intestine collections of lymphoid tissue form smaller nodules and larger Peyer’s patches that create visible bumps in the mucosa and are a major part of the gut associated lymphoid tissue (GALT)
What wandering immune cells are in the lamina propria?
Macrophages and lymphocytes
Muscularis mucosae
Thin layer of smooth muscle, separates lamina propria and submucosa, contraction changes effective surface area for absorption by moving villi
Submucosa
Middle layer of gut wall, made of connective tissue with large blood and lymph vessels running through it, contains submucosal plexus (1 of 2 major nerve networks of enteric nervous system, aka Meissner’s plexus, gives cells in the epithelial layer and smooth muscle in the muscularis mucosae nerves)
Muscularis externa
Outer wall of the GI tract, primarily 2 layers of smooth muscle (inner layer is circular, outer layer is longitudinal), circular contraction decreases lumen diameter, longitudinal contraction shortens the tube, stomach has an incomplete third layer of oblique muscles between circular muscles and submucosa
Myenteric plexus
2nd nerve network of enteric nervous system, between circular and longitudinal muscle layers, aka Auerbach’s plexus, controls and coordinates muscularis externa motor activity
Serosa
Outer covering of digestive tract, connective tissue membrane (continuation of peritoneal membrane lining abdominal cavity, aka peritoneum, forms sheets of mesentery (holds intestines in place to not tangle when moving))
What is the primary function of the digestive system?
Moving nutrients, water, and electrolytes from outside the body to the body’s environment, has 4 basic processes
What are the 4 basic processes of the digestive system?
Digestion, absorption, secretion, motility
Secretion
Movement of water and ions from ECF to digestive tract lumen or release of GI epithelial cells synthesized substances to lumen or ECF
Movement of material from cells into lumen or ECF
What two processes of the digestive system are opposites?
Secretion and absorption
Motility
Movement of material in the GI tract because of muscle contraction
How are substances input into the digestive system?
2L from ingestion (food and drink through the mouth)
7L from secretions (1.5L saliva from salivary glands, 0.5L bile from liver, 2L gastric secretions from stomach, 1.5L pancreatic secretions from pancreas, 1.5L intestinal secretions from both intestines)
9L total
How are substances removed from the digestive system?
8.9L from absorption (7.5L by the small intestine, 1.4L by the large intestine)
0.1L from excretion (feces)
9L total
What are the three main challenges the digestive system faces?
Avoiding autodigestion: food is mostly macromolecules so require strong enzymes but also must avoid digesting GI tract cells, if can’t raw patches (peptic ulcers) develop on GI tract walls
Mass balance: matching fluid input and output, when ingest 2L of fluid body secretes 7L of water, electrolytes, mucus, and enzymes (1/6 of total body water, 2x plasma volume), must reabsorb secreted fluids to prevent dehydration, normal intestinal reabsorption only loses 100ml in feces, vomiting and diarrhea can be emergencies when GI secretions are lost to environment instead of being absorbed, when severe can deplete enough extracellular fluid that circulatory system is unable to maintain blood pressure
Defense: largest area of contact between in and outside of body is digestive system lumen, GI tract with ~1 tennis court of surface area must balance keeping bacteria, viruses, and pathogens out and absorbing water and nutrients, GI tract transporting epithelium is assisted by physiological defense mechanisms, meets challenges by coordinating motility and secretion to maximize digestion and absorption
What physiological defense mechanisms does the GI tract use?
Mucus, digestive enzymes, acids, and GALT
What is most fluid that passes through the lumen secreted with?
Ions, enzymes, and mucus
Digestive enzymes
Secreted by exocrine glands (salivary glands and pancreas) or epithelial cells in stomach or small intestine, enzymes are synthesized on the rough endoplasmic reticulum, packaged by Golgi complex into secretory vesicles, and stored in cell until needed, on demand released by exocytosis, many intestinal enzymes stay bound to apex membranes by transmembrane protein stalks or lipid anchors, some digestive enzymes are secreted as inactive proenzymes known collectively as zymogens
Zymogens
Proenzymes, must be activated in GI lumen to carry out digestion, synthesizing enzymes in nonfunctional form stockpiling in cells without damage
Often end in -ogen after enzyme name, ex. pepsinogen
Mucus
Viscous secretion mostly made of glycoproteins collectively called mucins, primary function is forming protective coating over GI mucosa and lubricating hut contents, made in specialized exocrine cells called mucus cells in stomach and salivary glands or goblet cells in intestines, goblet cells are 10-24% of intestinal cell population, signals for mucus release include parasympathetic innervation, variety of neuropeptides in enteric nervous system, cytokines from immunocytes, parasitic infections, inflammatory processes in the gut (last 2 are body attempts to fortify protective barrier)
What does motility in the GI tract do?
Moves food from mouth to anus and mechanically mixes and breaks food into uniform small particles, mixing maximizes exposure to digestive enzymes by increasing surface area, GI motility is determined by properties of GI smooth muscles and modified by chemical input from nerves, hormones, and paracrine signals
Tonic contractions
Sustained for minutes or hours, occur in smooth muscle sphincters and anterior portion of stomach
Phasic contractions
Contraction relaxation cycles last a few seconds, in posterior region of stomach and small intestine
Slow wave potentials
Cycles of depolarization and repolarization associated with cycles of muscle contraction and relaxation
When do action potentials fire?
When slow wave potentials exceed threshold
Slow waves
Spontaneous depolarizations in GI smooth muscle
Migrating motor complex (MMC)
A series of contractions that begin in the empty stomach and end in the large intestine
When mostly empty between meals tract goes through contractions from stomach to large intestine over 90 minutes, housekeeping function that sweeps remaining food/bacteria from upper GI tract to large intestine
Interstitial cells of Cajal
Origin of slow waves, network of cells, aka ICCs, modified smooth muscle cells between smooth muscle layers and intrinsic nerve plexuses that may act intermediary between neurons and smooth muscle
Peristaltic contractions
Responsible for forward movement
Segmental contractions
Responsible for mixing
Are slow wave potentials or myocardial potentials slower?
Slow wave, 3-12 waves/min vs 60-90 waves/min
Peristalsis
Progressive waves of contraction that move through sections of GI tract, circular muscle contracts just behind bolus and pushes it forward into receiving segment with relaxed circular muscles, receiving segment contracts continuing forward movement, moves bolus forward at 2-25 cm/sec, in esophagus moves bolus from pharynx to stomach, contributes to food mixing in stomach but normally intestinal peristaltic waves are limited to short distances
Segmental contractions
1-5cm segments of intestine alternately contract and relax, in contracting segment circular muscles contract and longitudinal relax, may be random or at regular intervals, alternating segmental contractions mix contents and keep them in contact with absorptive epithelium, when segments contract sequentially from mouth direction to away intestinal contents are propelled short distances
Large intestine
after leaves small intestine, enters large intestine and water is absorbed from undigested material, once all absorbed waste passes through rectum and is stored until excretion through anus
