digestive system

substances are not considered IN the body until…

they cross a cell membrane

why is the mouth to anus considered an “open tube” ?

continuous with exterior

what are the 4 main functions of the GI tract ?

motility, secretion, digestion and absorption (into blood or lymph)

what is the automatic wave-like movement of the muscles that line your gastrointestinal tract called ?

peristalsis

role of mouth

mastication

initial stages of carb digestion

role of stomach

protein digestion, churns food with acid & enzymes

role of small intestine

main site of digestion and absorption (proteins, carbs, fats, nucleic acids)

role of large intestine

final digestion and absorption

water absorption

waste concentration

what are the 4 layers of the GI wall ?

muscosa

submucosa

muscularis externa

serosa

serosa

outer-layer of GI wall

lubricates and prevents friction between organs

muscularis externa

smooth muscle for peristalsis

contains myenteric plexus - nerve network between 2 layers

mucosa

secretes digestive juices and blood-borne hormones

highly folded

submucosa

connective tissue for elasticity

submucosal plexus

muscle tone

amount of tension (or resistance to movement) in muscles

role of the myenteric plexus nerve

peristaltic movement of the bowels

in what layer of the GI tract is the myenteric plexus nerve found ?

muscularis externa

role of submucosa plexus

sensory and secretory, controls endocrine and exocrine secretion

why do different parts of the GI need to communicate ?

sufficient secretion when food present and avoid overabundance in absence

enteric nervous system

operates independently of CNS to control digestive system

swallowing and saliva production is stimulated by…

chemoreceptors and mechanoreceptors - respond to food presence

afferent impulses to salivary centre in brainstem

PNS stimulates saliva secretion

saliva contains what enzymes ? what is their role ?

salivary amylase and bicarbonate

moistens food, antibacterial effects

the stomach secretes…

• intrinsic factors

• mucus

• pepsinogen

• acid

pepsinogen is activated by

Hcl

intrinsic factor

vitamin B12 absorption in ileum

why does peristalsis start and end ?

pacemaker region

antrum and pyloric sphincter

contraction of pyloric sphincter causes…

gastric mixing

gastrin

hormone secreted by stomach in 3 phases

in response to GRP (gastric-releasing-peptide)

what are the 3 phases of gastric secretion ?

cephalic, gastric, intestinal

function of intestinal phase of gastric secretion

triggers chyme release in duodenum

inhibits further gastric secretions

carb state leaving stomach

partial disaccharides (amylase)

protein state leaving stomach

small peptides (pepsin and HCl)

fat state leaving stomach

triglycerides (not digested yet)

why are bile salts and enzymes inactivated once reaching duodenum ?

acidic environment (neutralised by bicarb)

size and capacity of gall bladder

9cm long, 50ml

the gall bladder releases bile into the

duodenum

the liver produces approx. how much bile per day ?

1000ml

bile contains

water, bile salts, bilirubin, cholesterol, phospholipids

role of secretin

stimulates bile generation

gall bladder contraction

bile secretion into duodenum

CCK release is stimulated by

fats and proteins in duodenum

role of CKK (cholecystokinin)

stimulates bile & pancreatic enzyme release

regulates gastric emptying

role of hydrochloric acid (HCl)

does not digest any nutrients

• antimicrobial activity - kills microorganisms ingested with food

• aids breakdown of connective tissue and muscle fibres

• denatures proteins

• activates pepsinogen - forms pepsin

pepsin functions optimally in what pH

acidic

where is intrinsic factor synthesised ?

stomach

intrinsic factor is required for

absorption of VitB12 at terminal ileum - required for RBC synthesis

a deficiency in intrinsic factor can lead to

pernicious anaemia - not enough RBCs due to deficiency in vitamin B12

weakness and fatigue

pernicious anaemia is treated with

intramuscular injections

why does the stomach absorb alcohol and drugs but not nutrients ?

alcohol and aspirin - lipid soluble, diffuses across plasma mem and enters blood

the stomach produces…

HCl, peptidase and mucosa

what is meant by the pancreas has a “dual function”

endo and exocrine

the release of pancreatic juice is stimulated by

secretin and CCK

2 components of pancreatic juice

bicarbonate - neutralises stomach acid (allows pancreatic enzymes to work)

pancreatic enzymes - amylase, lipase, proteolytic

why are proteolytic enzymes secreted as inactive precursors ?

prevent digestion of cells and tissues that produce them

where does protein digestion begin ?

stomach

amino acids in the small intestine are broken down by

peptidases on microvilli

carbohydrates are digested by what enzyme ?

amylase

lactose is composed of what monosaccharides ?

Glucose + Galactose

sucrose is composed of what monosaccharides ?

Glucose + Fructose

maltose is composed of what monosaccharides ?

Glucose + Glucose

role of bile salts

breakdown fat droplets into lipid emulsions so lipase can access triglycerides beneath surface

prevent re-aggregation of fat droplets that have been broken down by mechanical digestion

pancreatic lipase

breaks down TGs into FFAs and monoglycerides