Digestive System Flashcards
Functions of the Digestive System
- Supply of nutrients dependent on food ingested - not nutritional requirements
- GI tract optimises conditions for digestion/absorption
- Ingested food is broken down into substrates available for cells
Functions of the Digestive System: Ingestion
Material enters digestive tract via mouth
Functions of the Digestive System: Mechanical Processing
- Easier to proper along tract
- Increases surface area to allow easier breakdown
Functions of the Digestive System: Digestion
- Chemical breakdown of food for absorption
- Large molecules (e.g. starch) broken down small molecules (e.g. absorbable sugars)
Functions of the Digestive System: Secretions
Release of water, acids, enzymes, buffers, salts
Functions of the Digestive System: Absorption
Movement of substrates, electrolytes, vitamins and water across epithelium into interstitial fluid
Functions of the Digestive System: Excretion
Removal of waste products from body
End result of Function of DS
Body is supplied with nutrients, electrolytes, water and waste is removed
Digestive Organs: Oral Cavity
Ingestion, mechanical processing with accessory organs (teeth and tongue), moistening, mixing with salivary secretions
Digestive Organs: Pharynx
Muscular propulsion of materials into the oesophagus
Digestive Organs: Oesophagus
Transport of materials to the stomach
Digestive Organs: Stomach
Chemical breakdown of materials by acid and enzymes and mechanical processing through muscular contraction
Digestive Organs: Small Intestines
Enzymatic digestion and absorption of water, organic substances/substrates, vitamins and ions
Digestive Organs: Large Intestines
Dehydration and compaction of indigestible materials in preparation for elimination
Accessory Organs: Teeth
Mechanical processing by chewing (mastication)
Accessory Organs: Tongue
Assists mechanical processing with teeth, sensory analysis
Accessory Organs: Salivary glands
Secretion of lubricating fluid containing enzymes that breakdown carbohydrates
Accessory Organs: Liver
Secretion of bile (important for lipid digestion), storage of nutrients, many other vital functions
Accessory Organs: Gallbladder
Storage and concentration of bile
Accessory Organs: Pancreas
Exocrine cells secrete buffers and digestive enzymes; endocrine cells secrete hormones
Accessory Organs: Serous Membrane - two sheets
Areolar tissue in between sheets
> blood supply, nerves, lymphatics
Functions:
- Stabilisation and attachment of digestive organs to the peritoneal cavity
- Stops digestive organs from entangling
Histological Organisation
4 layers
- Mucosa (secretion/absorption)
- Submucosa
- Muscularis Externa
- Serosa
Histological Organisation: Mucosa
- Inner lining
Mucous membrane - Digestive Epithelium > Simple columnar or stratified squamous > Enteroendocrine cells secrete hormones > Moistened by glandular secretions >Longitudinal folds, circular folds and villi to increase surface area
Lamina propria
- Areolar tissue containing: Blood and Lymphatic vessels, Sensory nerve endings, Lymphoid tissue and Smooth muscle cells (muscularis mucosae)
Digestive epithelium: Stratified squamous
- Oral cavity, pharynx, esophagus
- Mechanical stresses
Digestive epithelium: Simple columnar
- Stomach, small intestine, large intestine
- Absorption
- Presence of goblet (mucous) cells
Digestive epithelium provides protection against
- Digestive acids and enzymes
- Mechanical stresses, such as abrasion
- Bacteria Ingested with food and reside in digestive tract
Specialised Epithelial Cells: Stem Cells
Constant cell renewal - digestive epithelial are renewed every 3 days on average
Specialised Epithelial Cells: Enteroendocrine Cells
secrete hormones
Specialised Epithelial Cells: Goblet Cells
Secrete mucus
Specialised Epithelial Cells: Paneth’s Cells
Secrete antimicrobial peptides
Histological Organisation: Submucosa
- Dense, irregular connective tissue
- Surrounds muscularis mucosae
- Large blood vessels and lymphatic vessels
- May contain exocrine glands: secrete buffers and enzymes into digestive tract
Submucosal Plexus (Meissner’s Plexus)
- Innervates the mucosa and submucosa
- Sensory neurons, ganglionic neurons & postganglionic neurons
Histological Organisation: Muscularis Externa
Smooth muscle cells arranged in:
- inner circular layer
- outer longitudinal layer
Involved in:
- Mechanical processing
- Movement of materials along digestive tract
- Movements coordinated by enteric nervous system (ENS) which are primarily innervated by parasympathetic division of the ANS
Serosa
- Serous membrane covering muscularis externa
- Attachment to mesentery
Except in oral cavity, pharynx, esophagus, and rectum:
> Adventitia
- firmly attaches the digestive tract to adjacent structures
- dense sheath of collagen fibers
Digestive Movement
Involves contraction and relaxation of smooth muscle
Coordinated contractions vital for movement of materials along the digestive tract
Pacesetter cells
- Spontaneous depolarisation
- Tigger waves of contraction that spread throughout the entire muscular sheet
- Located in the muscularis mucosae and muscularis externa
Peristalsis
Moves food/material along tract - waves of muscular contraction - circular muscles - push forward - longitudinal muscles - shorten - propels bolus forward > moves along SI in approx. 90-120 mins
Segmentation
Mixing/churning - Cycles of contraction > churn and fragment bolus > mix with intestinal secretions (e.g. duodenum) - not directional
Regulation of Digestive Tract
Need to control rate of secretion/absorption so process is efficacious. Required to maintain homeostasis
Regulation of Digestive Tract: Receptors
- Detect a change in something
- Chemoreceptors, osmoreceptors, stretch receptors
- External e.g. sight, smell
Regulation of Digestive Tract: Processing/control centres
- Local factors
- Nerves:
enteric nervous system - part of PNS, housed within digestive tract, local regulation
ANS - Hormones - endocrine e.g. gastrin, GIP, CCK, secretin, VIP
- Effectors - smooth muscle, exocrine glands
Mouth/oral cavity
- sensory analysis before swallowing: tongue
- mechanical processing: teeth, tongue and palatal surfaces
- lubrication: mixing food with mucus and saliva
- carbohydrates and lipids: salivary amylase breaks down starch and lingual lipase breaks down lipids
- salivary glands: produce saliva
> parotid (25%) - salivary amylase
> submandibular (70%) - buffers
> sublingual (5%) - buffers, glycoproteins (mucins) and salivary amylase
Saliva functions
- Lubrication of mouth and pharynx (mucins)
- Cleanse mouth and teeth
- Moistening food
- Dissolve food and chemicals (stimulate taste buds)
- Begins digestion
Oesophagus
- Hollow muscular tube about 25cm in length
- Conveys food to stomach by peristalsis
- Pierces diaphragm through oesophageal hiatus
- Upper and lower ‘sphincters’
> prevents air from entering the oesophagus
> prevents backflow (reflux) from stomach
Oesophagus: Deglutition
Buccal Stage:
- Voluntary
- Bolus: oropharynx
Pharyngeal Stage:
- Involuntary
- Bolus: oesophagus
Oesophageal Stage:
- Involuntary
- Bolus: Stomach
Stomach
- 3 layers of smooth muscle and rugae
- Storage and mixing chamber
> starch digestion continues
> protein and triglyceride digestion begins
Stomach: Mechanical Digestion
- Mixing waves every 15-25sec
- Forms chyme (partially digested semi-fluid material)
- Forces chyme into duodenum
Stomach: Chemical Digestion
- HCl: denatures proteins
- Pepsinogen: converted to pepsin (HCl and already converted pepsin), hydrolyses peptide bonds
- Gastric lipase: hydrolyses triglycerides
Stomach Mucosa
Secretory cells form gastric glands which line gastric pits
- Mucous cells: mucus
- Parietal cells: HCl, intrinsic factor: required for absorption of vitamin B12
- Chief cells: pepsinogen, gastric lipase
- G Cells: gastrin (hormone that stimulates parietal and chief cells and increases secretions and motility)
Regulation of Gastric Activity
- Involves CNS, entericNS, hormones
- 3 overlapping phases: names according to where the food is:
> Cephalic phase
> Gastric phase
> Intestinal Phase
Cephalic Phase: CNS/brain
- Begins when on sight, smell, taste or thought of food
- Prepares stomach for arrival of food
> impulses to submucosal plexus (vagus nerve)
> increases gastric secretion: increases secretion from gastric glands and motility
Gastric Phase: Builds on cephalic phase
- Arrival of food into stomach: presence of undigested food/materials in stomach
- Triggers stretch receptors and chemoreceptors (pH): enhanced secretion (gastrin, pepsinogen, HCl), stimulates gastric motility
Intestinal Phase: Starts when chyme arrives in duodenum
- Increases intestinal secretion: secretin (reduces gastric secretion), CCK (inhibits gastric emptying), GIP (gastric inhibitory polypeptide)
- Controls rate of chyme exiting from stomach (gastric emptying)
Hormone control of food intake
- Insulin and CCK are released during food absorption: act as satiety signals to depress hunger
- Ghrelin is an appetite stimulant: acts as a ‘dinner bell’
- Leptin: secreted by adipose tissue when fat stores increase, acts by inhibiting appetite stimulant neuropeptideY (NPY)
Vomiting
Forceful expulsion of gastric contents out through the mouth
- Stomach doesn’t actively participate
- Major force for expulsion from diaphragm and abdominal muscles
Vomiting Reflex
1) Deep inspiration and closure of glottis
2) Contraction of diaphragm and abdominal muscles, compressing stomach
3) Gastric contents forced out through relaxed gastroesophageal sphincter
4) Distension of the oesophagus induces peristalsis to force contents back to stomach
5) cycle repeats - retching
6) pressure in oesophagus increases, jaw thrust out, pharyngeal sphincter opens and gastric contents forced out
Vomiting symptoms
- Preceded by profuse salivation, sweating, rapid HR and nausea
- These sensations characteristic of rapid discharge by ANS
- Co-ordinated by vomiting centre in medulla
- Excessive vomiting can lead to large losses of secreted fluids and acids that are normally reabsorbed
> decreases plasma volume
> metabolic alkalosis
Small Intestine
- Digestion and absorption of nutrients
- 90% of nutrient absorption
- Approx. 6m - large surface area
> further increased by circular folds, villi, microvilli
Total absorptive capacity is more than 200m2
Circular Folds
- Plicae Circulares
- Many present in SI and mix chyme
Small Intestine: Duodenum
- 25cm
- Receives chyme from stomach and mixes with digestive enzymes from pancreas
- Neutralises acids to avoid damage to absorptive surfaces and avoid inactivating digestive enzymes
Small Intestine: Jejunum
- 2.5m
- Chemical digestion and nutrient absorption
Small Intestine: Ileum
- 3.5m
- Nutrient absorption
> Vitamin B12, bile salts, leftover undigested products
> Controls flow of from ileum to large intestine
Intestinal Villi
Covered by simple columnar epithelium
> microvilli: brush border
Lamina propria
- nerve endings
- network of capillaries: carry absorbed nutrients to liver then around body
Lacteals
- transport material that cannot enter capillaries
- fatty acids (chylomicrons)
Small intestinal movement
- Involves contraction and relaxation of smooth muscle from duodenum to ileum
- Important in small intestine to allow for proper digestion and absorption
- Peristalsis and segmentation
> weaker peristaltic contractions slowly propels bolus forward
> moves along SI in 90-120mins
> cycles of contraction mix with intestinal secretions and make absorption more efficient by allowing contact with villi
Pancreas
3 Regions
- Head, body and tail
Connected to duodenum via pancreatic duct
Pancreas Function
- Pancreas has distinct lobules that contain acini and islets
Pancreas Function: Exocrine
- Acinar glands secrete pancreatic juice
- Digestive enzymes and buffers
Pancreas Function: Endocrine
- Pancreatic islet cells
- Secrete insulin and glucagon into the bloodstream
Pancreatic Secretions
Water, salts, bicarbonate and phosphate buffers
Enzymes to digest: Carbohydrates: pancreatic alpha-amylase Proteins: proteolytic enzymes (trypsin)/proteases Triglycerides: pancreatic lipase RNA and DNA: nucleases
Liver
- Heaviest glandular organ in body (1.5kg)
- Metabolic and synthetic function
- Right hypochondriac and epigastric regions
- 4 lobes: large right, smaller left, caudate, quadrate lobe
- Bile secreted into bile canaliculi and moves to gallbladder
Liver: in lobule
- Hepatocytes arranged around sinusoids and central vein: adjust levels of circulating nutrients
- Blood passes through sinusoids and drains into central vein
- Kupffer cells: phagocytic - engulf pathogens, cell debris and damaged RBCs
Liver: Hepatic Artery
Delivers oxygenated blood
Liver: Hepatic Portal Circulation
Delivers nutrient rich blood from intestines
Liver Function: Metabolic Regulation
- carbohydrate, lipid and protein metabolism
- waste removal
- store vitamins and minerals
- process drugs (inactivation)
Liver Function: Haematological Regulation
- Removal of bacteria and old RBC, WBC
- Make plasma proteins
- Removal of hormones and antibodies
- Removal or storage of toxins
- Activation of vitamin D
Liver Function: Bile production
- Synthesis and secretion of bile into duodenum
- Excrete bilirubin
Liver failure > build up of hormones and toxins
Bile function
- Digestion of lipids: not water soluble
- Mechanical processing in stomach creates large drops containing lipids
- Pancreatic lipase is not lipid soluble: interacts only at surface of lipid droplet
- Bile salts break droplets apart (emulsification): increases surface area exposed to enzymatic attack, creates tiny emulsion droplets coated with bile salts and this promotes absorption of the lipids by epithelium
Gallbladder
- Small pear-shaped, muscular sac
- Stores bile
- Concentrates bile: water absorbed, bile salts and solutes concentrated
- Releases bile into duodenum: under stimulation of CCK
Gallstones
- Solidified cholesterol and bile salts
- 80-90% are cholesterol gallstones: liver secretes bile abnormally saturated with cholesterol > excess crystallises > forms stones stored in gallbladder or cystic duct
Can also form due to low levels of bile acids and bile lecithin
Catabolism
Breakdown of structure of food: disassembles molecules
Hydrolysis
Break molecular bonds in large organic molecules
Digestive enzymes: Amylases
Break bonds between simple sugars (carbohydrates)
Digestive enzymes: Proteases/peptidases
Break bonds between amino acids (proteins
Digestive enzymes: Lipases
Separate fatty acids from glycerides (lipids)
Digestive enzymes: Nucleases
Break nucleotides into sugars, phosphates and nitrogenous bases
Carbohydrate digestion
Carbohydrases from salivary glands and pancreases brush border enzymes. Monosaccharides are absorbed into capillaries
Protein Digestion
- Complex and long
- Mastication allows access for enzymes
- Stomach - pepsin
- Pancreatic and brush border enzymes
- Amino acids and peptides > absorbed into capillaries
Carbohydrate and protein absorption
Monosaccharides
> 2° active transport with sodium
> Facilitated diffusion (fructose)
Amino acids, dipeptides, tripeptides
> amino acids 1° or 2° active transport
> di and tripeptides 2° active transport
All move into capillaries in villus
Lipid Digestion
- Lingual lipase
- Bile salts
> emulsification
> access for enzymes - pancreatic lipase
- fatty acids and monoglycerides
> absorbed into lacteals
Lipid Absorption
Simple diffusion
> SCFA move into capillaries in villus
> others move into lacteals
Bile combines with LCFA and monoglycerides to form micelles
- contact epithelial cell membrane
- lipids diffuse through membrane
- resynthesised into triglycerides in epithelial cells
- coated with proteins > chylomicrons
- chylomicrons are too large to move into capillaries and move into lacteals
Large intestine
- 1.5m long
- ileocaecal sphincter to anus
- tonic contraction of 3 longitudinal muscles (teniae coli) to form pouches (haustra)
> aids in mechanical digestion: mixing chambers, segmentation
-serosa: teardrop shaped sacs of fat (omental appendices)
Large intestine: Caecum
- Ileocecal valve
- Appendix
Large intestine: colon
- Ascending
- transverse
- descending
- sigmoid
Large intestine: rectum
- anal canal
> internal sphincter: smooth muscle
> external sphincter: skeletal muscle
Mucosa/ glands in LI
No villi or circular folds in mucosa
> haustra instead: expansible paches
Epithelium
> absorptive cells: water absorption
> goblet cells: mucus secretion
Organised into crypts (intestinal glands)
Large intestine functions
- Reabsorption of water
- compaction of intestinal contents into faeces
- Absorption of important vitamins produced by bacteria
- breakdown of some remaining products for removal
- storage of faecal material prior to defecation
Digestion in the Large Intestine: Mechanical Digestion
- movement of LI begins when substances pass ileocecal sphincter
- peristalsis slower than SI
- Haustral churning (segmentation): initiated by distension of haustra as chyme enters LI
Mass movement
- strong peristaltic wave beginning in transverse colon that drives contents into rectum
- occurs during or immediately after meal when food enters stomach
- causes defecation reflex
Digestion in the Large Intestine: Chemical Digestion
- Final stage of digestion occurs through activity of bacteria
- Produces gases and other by-products
> vitamin K, biotin, pantothenic acid, organic waste products, short chain fatty acids, toxins - Bacteria produce necessary enzymes that humans don’t have genetic capacity to produce: able to digest complex polysaccharides (indigestible carbohydrates)
Diarrhoea
- Passage of highly fluid faecal matter
- Increased frequency and decreased consistency of stools
- Results in rapid dehydration and metabolic acidosis: loss of nutrient material
- Less time in colon = less water reabsorbed = dehydration
Diarrhoea - Causes
- Exposure to damaging agents, e.g. radiation
- Excessive intestinal motility: substances move through faster and less water absorption occurs
- Excessive osmotically active particles: will draw water out of cells into intestinal contents
- Toxins e.g. vibrio cholera: excessive secretor by SI can lose 10L fluid/day
