digestive system

what makes up the digestive system

digestive tract and accessory organs

digestive tract

oral cavity, pharynx, oesophagus, stomach, small intestine, large intestine, anus

accessory organs

salivary gland, pancreas, liver & gallbladder, spleen

digestive system functions

ingestion, mechanical processing (chewing), digestion, absorption of nutrients, minerals & water, excretion, endocrine (e.g. insulin, pancreas), immune (e.g. tonsils), detoxification, storage of nutrients and regulation of their blood levels

layers of the GI tract

mucosa, submucosa, muscularis, serosa

mucosa of the GI tract

moist mucous epithelial membrane, contains lymphoid tissue

submucosa of the GI tract

contains nerves, blood vessels & small glands

muscularis of the GI tract

lies in two planes: inner circular smooth muscle layer and outer longitudinal layer, myenteric plexis between layers = nerves regulate movement and secretions

serosa of the GI tract

outer membrane, inferior to diaphragm = visceral peritoneum

peritoneum

serous membrane lining abdominal cavity & organ, largest serous membrane, not smooth - lots of folds to make mesenteries

visceral peritoneum

covers organs (serosa tunic)

parietal peritoneum

covers interior surface of body wall

peritoneal cavity

between layers, filled with serous fluid

mesenteries

two layers of peritoneum with thin layer of loose connective tissue

function of mesenteries

holds organs to abdominal cavity, routes by which vessels & nerves pass from body wall to organs

mesentery associated with small intestine

mesentery proper

mesenteries of colon

transverse & sigmoid mesocolon

mesentery of appendix

mesoappendix

greater omentum

connecting greater curvature of stomach to transverse colon, lots of adipose & lymph

lesser omentum

connects lesser curvature of stomach & proximal duodenum to liver & diaphragm

peritonitis

acute inflammation of peritoneal membranes - rubbing together of inflamed peritoneal surfaces causes abdominal pain. irritation of peritoneum due to escaped bile, bacteria from external due to wound in abdominal wall, ruptured appendix so bacteria enters peritoneum

abdominal wall and its layers

boundaries of abdominal cavity, made up of common layers - deep (extraperitoneal fat, parietal peritoneum & fascia), more superficial on anterior wall (layers of muscle), superficial (superficial fascia & skin)

hernias

protrusion of an organ/fascia through wall of the cavity that normally contains it, often arise when pressure in residing cavity is increased

hiatus hernia

when stomach protrudes into mediastinum through oesophageal opening in diaphragm, mostly no symptoms but can cause pain from reflux gastric acid

oral cavity

formed by cheeks, hard & soft palates and tongue

the tongue

forms floor of oral cavity, strong skeletal muscle: attached post. to hyoid bone, temporal bone & mandible, free anteriorly, attached to floor via lingual frenulum

muscles of tongue

intrinsic muscles: no attachment to bones, lie within tongue; extrinsic muscles: anchor tongue to surrounding bones, move tongue

composition of the tongue

covered in stratified squamous epithelium, dorsum (upper surface) covered in papilla = projections of connective tissue (some contain taste buds), lymphoid tissue embedded in posterior surface = lingual tonsil

mastication (chewing)

breaks food into smaller parts to increase surface area for digestion - incisors & canines bite food, molar teeth grind food

mastication reflex

chewing stimulated by sensory receptors, signals from brain provide conscious control to initiate or stop chewing or change rate

salivary glands

3 main pairs located outside mouth & empty saliva into it: parotid, submandibular, sublingual; small glands also found in mucous membrane of oral cavity itself

parotid salivary gland

largest, anterior to ear, serous, produce watery saliva

submandibular salivary gland

below mandible, serous & mucus

sublingual salivary gland

below tongue, smallest, mostly mucus glands

role of the salivary glands

secretion of lubricating fluid containing enzymes that break down carbohydrates

saliva functions

prevents bacterial infection, lubrication, contains salivary amylase to break down starch, helps to form bolus for swallowing

mumps

infections of salivary glands: inflamed & large glands, fever, malaise, painful throat especially when swallowing

swallowing

involves distinct phases of food moving from mouth > pharynx > oesophagus

what prevents food from entering the larynx

epiglottis

oesophagus

transports food from pharynx to stomach, collapsible, mucosa is moist stratified squamous epithelium

three phases of swallowing

voluntary, pharyngeal, oesophageal

swallowing: voluntary phase

bolus of food moved by tongue from oral cavity to pharynx

swallowing: how is the pharyngeal phase controlled

reflex/involuntary, controlled by swallowing centre in medulla oblongata

swallowing: describe the pharyngeal phase

soft palate elevates, epiglottis tipped post prevents food passing into larynx, pharyngeal constrictor muscles contract in a sequence to push food downward, upper oesophageal sphincter relaxes, elevated pharynx opens oesophagus, food pushed into oesophagus

swallowing: how is the oesophageal phase controlled

reflex, moves food into stomach

swallowing: describe the oesophageal phase

peristaltic contractions of the oesophagus, lower oesophageal sphincter relaxes, food enters stomach

heartburn

lower oesophageal sphincter fails to close after food enters stomach so contents can go back up to the oesophagus, acid from stomach can irritate oesophagus & cause burning sensation

heartburn treatment

treat with antacids to neutralise acid, less problematic when eating smaller amounts & not lying down after

vomiting

forcible expulsion of stomach contents through mouth; stimulated by irritation & distension of stomach; stomach, diaphragm & abdominal walls contract & oesophageal sphincters open; can lead to disturbed fluid balances if prolonged

stomach

stores & mixes food, moves food into small intestine at small intervals after eaten, very little absorption of nutrients (glucose and some drugs can be absorbed)

stomach openings

gastroesophageal (from oesophagus to cardiac sphincter) and pyloric (duodenum to pyloric sphincter)

label the parts of the stomach

A - cardia, B - fundus, C - body, D - pylorus

layers of the stomach

serosa (visceral peritoneum), muscularis, submucosa & mucosa

submucosa and mucosa of stomach

folded into rugae when stomach is empty, allowing stomach lining to stretch; contains gastric pits

what are the three sections of the small intestine

duodenum, jejunum, ileum

what are the main functions of the small instestine

digestion of food, absorption of nutrients and vitamins, removal of water from ingested fluids

which part of the small intestine is the major site of nutrient absorption

the jejunum

what is the function of Peyer’s patches in the ileum

they defend against ingested microorganisms by providing immune protection

what is the function of the mucosa in the small intestine

it secretes water, electrolytes, and mucus to lubricate the intestine and protect the intestinal wall from acidic chyme and digestive enzymes

what is the role of folds and projections in the small intestine

they increase the surface area for digestion and absorption by approximately 600-fold

ileocecal junction

the junction between the small and large intestine containing a sphincter and valve that ensure one-way flow

what is the secretory function of the stomach

secretes acid and digestive enzymes that begin chemical digestion of food

why must mucus be secreted into the small intestine

to protect it from acidic chyme leaving the stomach

what is bile and what is its function

produced by the liver to neutralise stomach acid and emulsify fats for digestion

how much bile does the liver produce each day

approximately 600-1000mL per day

what are the major functions of the liver

produce bile, store glycogen, vitamins, iron and copper, detoxify drugs and toxins, remove worn-out blood cells, interconvert nutrients

what is the hepatic porta

an area on the inferior surface of the liver where the hepatic portal vein and hepatic artery enter and hepatic ducts exit

what is the function of the hepatic portal vein

to carry nutrient-rich, deoxygenated blood from the intestines to the liver for processing

what is the ‘first-pass effect’

drugs absorbed from the GI tract are metabolised by the liver before reaching general circulation

which organs are considered accessory digestive organs

salivary glands, liver, gallbladder, pancreas, teeth and tongue

what is the function of the gallbladder

it stores and concentrates bile by absorbing water from it

what happens to the gallbladder after a meal

it contracts and releases bile into the duodenum

what are gallstones

precipitated cholesterol or bile pigments that can block the cystic duct and cause pain, nausea, and jaundice

what are the endocrine and exocrine functions of the pancreas

endocrine: produces insulin to regulate blood glucose, exocrine: produces digestive enzymes released into the duodenum

why is the pancreas described as retroperitoneal

because it lies behind the peritoneum

what is pancreatitis

inflammation of the pancreas caused by pancreatic enzymes digesting pancreatic tissue itself

what are the main parts of the large intestine

cecum, ascending colon, transverse colon, descending colon, sigmoid colon, rectum

what are the main functions of the large intestine

absorption of water and salts, formation of faeces, mucus secretion, action of gut microorganisms

how long does material typically remain in the large intestine

around 18-24 hours

what is unique about the mucosa of the large intestine

it contains no villi or folds, but has numerous straight tubular glands called crypts

intraperitoneal segments of the large intestine

cecum/appendix, transverse colon, sigmoid

retroperitoneal segments of the large intestine

ascending colon, descending colon, rectum

what is the function of the appendix

it contains lymph nodules and contributes to immune function

what forms the internal and external anal sphincters of the anal canal

internal = smooth muscle, external = skeletal muscle

what is hepatitis

inflammation of the liver caused by alcohol or viral infection

what is cirrhosis

replacement of hepatocytes with fibrous scar tissue causing reduced liver function

what causes jaundice in liver disease

build-up of bile pigments due to impaired liver function

what is appendicitis

inflammation of the appendix, usually caused by obstruction leading to swelling and infections

what is peritonitis

infection of the peritoneal cavity that can occur if the appendix bursts

what is a peptic ulcer

a lesion in the lining of the stomach or small intestine, commonly caused by bacterial infection

what is IBS (irritable bowel syndrome)

a condition of unknown cause involving alternating constipation and diarrhoea

what causes constipation

slow movement of faeces through the large intestine causing excessive water absorption and hard, dry stool