HAPIB Module 2 - Digestive and Urinary Systems

What are the major organs of the GIT?

• A series of hollow organs

• Mouth/oral cavity

• Oesophagus

• Stomach

• Small intestine (duodenum, jejunum, ileum)

• Large intestine (cecum, ascending colon, transverse colon, sigmoid colon, and descending colon)

• Rectum

• A*nus

What are the accessory organs of teh GIT?

• Produce secretions to assist digestion

• Salivary glands

• Liver

• Gallbladder

• Pancreas

what are teh functions of the digestive system?

• The digestive system converts food into its simplest components

  • Absorbed into the bloodstream or excreted as waste

• Ingestion

• Mechanical digestion

• Chemical digestion

• absorption

• elimination of waste

What is ingestion?

The entry of food/liquids into the digestive tract via the mouth

what is mechanical digestion?

Crushing/shearing of ingested food

• the physical/mechanical breakdown of food into smaller components

what is chemical digestion?

(extraction of nutrients)

• Enzymatic breakdown of food into substances that can be absorbed

what is absorption?

Movement of nutrients into the bloodstream -> sent to the liver for processing

what is elimination of waste?

Indigestible food is compacted into faeces

what are the organs of the upper GIT?

• mouth/oral cavity

• oesophagus

• stomach

• small intestine (duodenum only)

what is the function of the oral cavity?

• first organ in the upper GIT

• Mechanical digestion (mastication = chewing) and mixing of food with saliva

• Enzymes in saliva begin the process of chemical digestion

what is the function of the oesophagus?

• second organ of the upper GIT

• A fibromuscular tube that transfers food (bolus) into the stomach

what is the function of the stomach?

• fourth organ of the upper GIT

• Chemical digestion - bolus is mixed with gastric juices (HCI and digestive enzymes)

• Mechanical churning - facilitate digestion

• Bolus is converted into chyme (liquified form of the food ingested mixed with acid and digestive enzymes)

what is the function of the small intestine (duodenum)?

• last organ of the upper GIT

• Duodenum = first 20-25cm

• Chemical digestion -> aided by digestive juices from the pancreas and gallbladder

what are the organs of the lower GIT?

• small intestine (jejunum and ileum)

• large intestine

• rectum/anus

what is the function of the small intestine (jejunum and ileum)?

• first organ in the lower GIT

• Jejunum and ileum = remaining ~3-6m of the small intestine

• Where nutrient absorption occurs

what is teh function of the large intestine?

• second organ in hte lower GIT

• ~1.5m long

• Cecum, ascending colon, transverse colon, descending colon, sigmoid colon

• Reabsorption of water

• Compacts waste (faeces) for elimination

what is the function of the rectum/anus?

• final organ of the lower GIT

• Store and eliminate waste (faeces)

what are the functions of teh accessory organs of the GIT?

• Secretion (to facilitate chemical digestion)

  • Salivary glands - salivary amylase

  • Pancreas - pancreatic enzymes, buffers (breaks down proteins, fats, carbohydrates)

  • Liver - bile (emulsify fats)

• Detoxification

  • Liver - inactivates toxins

• Storage

  • Gallbladder - bile

  • Liver - iron, glucose, fat soluble vitamins

what are teh four layers of the GIT wall?

• The structure of the digestive tract varies slightly from region to region, but generally consists four major layers:

  1. Mucosa (innermost layer)

     • Epithelial mucous membrane and connective tissue

     • In contact with the bolus/chyme moving through (in the lumen)

  2. Submucosa

     • Dense irregular connective tissue

     • Contains blood vessels, nerves, glands

  3. Muscularis externa

     • 2-3 layers of smooth muscle

     • Facilitates movement of materials and mechanical digestion

  4. Serosa/adventitia (outermost layer)

what is the mucosa of the GIT wall?

(innermost layer)

• Epithelial mucous membrane and connective tissue

• In contact with the bolus/chyme moving through (in the lumen)

• three layers

  • epithelium

  • loose ct

  • muscularis

what is the epithelium layer of the GIT mucosa?

in direct contact with the contents of the lumen of the GIT

• Epithelial composition depends on its function

  • Mouth + oesophagus -> stratified squamous (protection)

  • Stomach, small intestine + large intestine -> simple columnar (absorption)

  • Rectum + anus -> stratified squamous (protection)

• Contains specialised goblet cells that secrete mucus to keep the epithelium moist

• In the small intestine, contains microvilli -> increase surface area for nutrient absorption

what is the loose ct layer of the GIT mucosa?

(lamina propria)

• Nutrients can travel through easily

• Contains:

  • Blood vessels

  • Lymphatic vessels

  • Sensory nerve endings

  • Gut associated lymphoid tissue (GALT)

  • Some mucous glands

  • Fibroblasts

  • Macrophages

what is the muscularis layer of teh GIT mucosa?

(muscularis mucosa)

• Thin layer of smooth muscle cells

• Its function is still under debate

  • Believed to alter the shape of the lumen and move mucosal folds/villi

what is the submucosa of the GIT wall?

• A layer of dense irregular CT (but still relatively loose)

• Provides physical support to the mucosa

• Connects the mucosa to the underlying muscularis

• Contains:

  • Large blood vessels (receive absorbed nutrients)

  • Large lymphatic vessels

  • Submucosal glands (exocrine)

    • Secrete enzymes and buffers into the lumen via ducts

  • Submucosal plexus (Meissner's plexus) - sensory neurons and autonomic nerve fibres that innervate the glands

    • Controls glandular secretions, alters electrolyte and water transport, regulates local blood flow (i.e. controls fluid secretion and absorption)

what is the muscularis layer of teh GIT wall?

• Multiple muscle layers

  • Organised in circular and longitudinal layers

  • Composition depends on location within the GIT

    • Oesophagus - contains 2 layers of muscle -> mixture of skeletal muscle (voluntary swallowing) and smooth muscle

    • Stomach - contains 3 layers of smooth muscle -> inner oblique, middle circular, outer longitudinal

    • Small intestine, large intestine, rectum - contains 2 layers of smooth muscle -> inner circular and outer longitudinal

  • Contraction of these muscle layers is essential for mechanical processing and  moving materials along the GIT

  • The muscularis externa also contains the myenteric plexus (Auerbach plexus)

    • A network of sensory neurons + autonomic nerve fibres that runs between the two layers of muscle

    • Located between the circular and longitudinal muscle layers

    • Coordinates digestive muscle activity (i.e. controls motility)

      • Parasympathetic stimulation - increased muscle tone

      • Sympathetic stimulation - decreased muscle tone

what is teh serosa/adventitia layer of teh GIT wall?

• Depending on the region of the GIT, the outermost layer of the GIT is either called adventitia or serosa

• Serosa

  • Found in the intraperitoneal regions of the GIT (i.e. most of the stomach, parts of the small + large intestine and rectum)

  • A thin layer of loose CT covered with mesothelium -> lubricates the outer surface of the GIT -> decreased friction

  • Serosa = lubricates structures

• Adventitia

  • Found in the retroperitoneal regions of the GIT (i.e. oral cavity, pharynx, oesophagus, parts of the stomach, small and large intestine, and the anus)

  • Contains a dense network of collagen fibres

  • Adventitia = binds/attaches structures

how does the GIT wall change?

Mucosa

• Epithelial type

  • Stratified squamous/simple columnar

• Physical appearance

  • Protrusions (villi)

  • Invaginations (glands)

Muscularis

• Number of layers

  • 2 layers/3 layers

  • Complete/incomplete layers

• Muscle type

  • Smooth muscle/skeletal muscle

 

 

what are the structures of the oral cavity?

• Lined by oral mucosa (stratified squamous epithelium) -> needs to be very protective

• Hard palate - bony support structure at the top of the oral cavity

• Soft palate - soft muscular structure, posterior to the hard palate

• Tongue - skeletal muscle

• Uvula - dangly piece of tissue that hands at the back of the oral cavity -> prevents food from entering pharynx prematurely

• Teeth

• Gingiva (gums)

• Tonsils - lymphoid tissue

what are teh functions of teh oral cavity?

• Mechanical digestion

  • Mastication (chewing)

  • Saliva + food = bolus

• Chemical digestion

  • Digestion of carbohydrates and lipids begins here

    • Salivary amylase - carbohydrate digestion

    • Lingual lipase - lipid (triglyceride) digestion

what are teh salivary glands?

• three major pairs of salivary glands

• sublingual

• submandibular

• parotid

what are teh sublingual salivary glands?

• Lie on either side of the tongue

• Produce a mucous secretion that acts as a bufffer/lubricant

• Produces ~5% of saliva entering the oral cavity

what are the submandibular salivary glands?

• Lie on the inner surface of the mandible (jaw)

• Produces a mixed serous/mucous secretion containing buffers, mucin, and salivary amylase

• Secretions released under the tongue

• Produces ~75% of the saliva entering the oral cavity

what are teh parotid salivary glands?

• Large glands located just in front of the ears

• Produce a serous secretion containing lots of salivary amylase

• Produces ~20% of saliva entering the oral cavity

• Mumps = viral infection of the parotid gland

what is saliva?

• The salivary glands produce 1-1.5 litres of saliva every day

• Saliva contains ~99.4% water

• The remaining ~0.6% contains:

  • Electrolytes (mainly Na+, CI- and HCO3-)

  • Buffers (keeps mouth at pH 7.0 preventing the build up of acids produced by bacteria)

  • Antibodies (IgA)

  • Enzymes - salivary amylase

    • Lysozyme (antibacterial enzyme)

    • Lingual lipase

    • Salivary mucin (a slimy visco-elastic material)

what are teh functions of saliva?

• Lubrication

• Chemical digestion - enzymes

• Protection - antibacterial lysozymes, buffers, IgA

• Dissolves food - taste

what is teh structure of the oesophagus?

• The oesophagus is a collapsed tube that runs posteriorly to the trachea

• Structure of the oesophageal wall

  • ~25cm long, ~2cm in diameter

• Mucosa

  • Stratified squamous epithelium (protection; abrasion/bacteria)

  • Contains large folds - keep the lumen closed unless swallowing is occurring

  • Allows for expansion when swallowing and food is moving through

• Submucosa

  • Contains large mucus glands (lubrication)

• Muscularis

  • Upper 1/3 = skeletal muscle (swallowing is voluntary)

  • Middle 1/3 = mixture of skeletal and smooth muscle

  • Lower 1/3 = smooth muscle

• Adventitia

  • Not serosa

  • Connective tissue layer lies outside the muscularis layer, anchoring the oesophagus to neighbouring structures

• The oesophagus contains two sphincters

1. Upper oesophageal sphincter

2. Lower oesophageal sphincter

what is the upper oesophageal sphincter?

Upper oesophageal sphincter - prevents air entering digestive tract and the reflux of food into airways

what is teh lower oesophageal sphincter?

Lower oesophageal sphincter - prevents reflux of stomach contents

what is teh function of the oesophagus?

• Convey food and liquids from the mouth into the stomach

• No absorption!

what are the three phases of swallowing?

1. buccal phase (voluntary)

2. pharyngeal phase (mainly involuntary)

3. oesophageal phase (mainly involuntary)

what is the buccal phase of swallowing?

(voluntary) - first phase of swallowing

• Contraction of the tongue to push bolus up against the soft palate and into the oropharynx

what is the pharyngeal phase of swallowing?

(mainly involuntary) - second phase of swallowing

• Tongue blocks off the oral cavity

• Epiglottis blocks the larynx/trachea

• Upper oesophageal sphincter (UES) relaxes and opens

• Food enters the oesophagus

what is the oesophageal phase of swallowing?

(mainly involuntary) - last phase of swallowing

• UES contracts and closes

• Bolus is forced through the oesophagus by peristalsis

• Food enters the stomach

• Once food has reached the stomach, LES closes (to prevent regurgitation

what is the stomach?

• An expandable muscular organ that churns food into chyme

• four main regions;

  • fundus

  • cardia

  • body

  • pylorus

what is the fundus of the stomach?

• superior to the esophageal junction

• top part of teh stomach

what is the cardia of the stomach?

• within ~3cm of gastroesophageal junction

  • Entry point for food/liquids into the stomach

  • Oesophageal sphincter keeps stomach contents from reentering the oesophagus

  • Abundant mucus glands - protection from acid

what is the body of the stomach?

• the largest region

  • Lies between the fundus and the curve of the "J"

  • The "mixing bowl" for ingested food

what is the pylorus of the stomach?

•  the curve of the "J" leading to the small intestine

  • Can be further subdivided into the pyloric antrum and the pyloric canal

  • Pyloric sphincter regulates chyme -> small intestine

what is the structure of the stomach wall?

• Mucosa - lined with simple columnar epithelium

  • Produces a carpet of alkaline mucus (protection)

  • Cells are replenished every 3-7 days

  • Contains deep folds that form gastric glands

• Submucosa - fibrous connective tissue containing blood vessels, lymphatic vessels and nerve cells

• Muscularis - made up of three layers

  • Inner oblique - responsible for creating the churning motion to aid mechanical breakdown of food

    • Not present in any other part of the digestive system

  • Middle circular

  • Outer longitudinal - responsible for moving the bolus towards the pylorus (through muscular shortening)

• Serosa - a thin ct layer that secretes a lubricating fluid to reduce friction

  • Continuous with the visceral peritoneum

what are teh rugae of the stomach?

• When empty, the mucosa and submucosa layers of the stomach wall form prominent folds/ridges called rugae

• As the stomach fills with food these rugae flatten out to allow the stomach to expand

• The mucosal layer of the stomach forms a series of gastric pits, each housing a number of gastric glands

what are the gastric glands of the stomach?

• Mostly exocrine

• Opens into the stomach through gastric pits in the mucosa

• Secrete most of the acid and enzymes required for chemical digestion in the stomach (~1.5L per day)

• Gastric glands contain specialised cells - the composition of these cells depends on the gland's location within the stomach

what are goblet cells?

• Cells of the gastric glands:

• secrete mucus - protects the mucosal wall from acid

what are parietal cells?

• cells of teh gastric glands

• secrete intrinsic factor - necessary for vitamin B12 absorption (small intestine)

• Produce HCI - kill microbes, denatures proteins

what are G cells?

• cells of the gastric glands

• enteroendocrine cells - produces hormones

  • Produce gastrin - increases stomach motility

    • Stimulates HCI/enzyme production

    • Relaxes pyloric sphincter

what are chief cells?

• cells of the gastric glands

• secrete pepsinogen (inactive) which is converted into pepsin (active) by HCI -> degrades proteins

  • Secrete gastric lipase -> breaks down lipids

what are the functions of the stomach?

• Temporary storage of ingested food

  • Stomach contents (up to 2-4L of food) are held for approximately 2 hours

• Mechanical digestion of ingested food

  • Achieved by contraction/relaxation of the three muscle layers

• Chemical digestion of ingested food

  • Stomach mucosa produces HCI and digestive enzymes (~1.5L of gastric juice produced each day)

  • Food is broken down into a thick, acidic, soupy chyme

• Production of intrinsic factor

  • Help absorb Vitamin B12

• The stomach doesn’t play a big role in the absorption of food, but it can absorb water (especially if the body is dehydrated), alcohol (~10-20% of ingested ethanol), some drugs (e.g. aspirin)

what is the small intestine?

• Averages 3-5m in length and is divided into three segments

  • duodeunum

  • jejunum

  • ileum

what is the duodenum?

• First 20-25cm of the small intestine, proximal to the stomach

• Receives chyme from the stomach and digestive secretions from the pancreas and liver

• Considered the "mixing bowl" of the small intestine

• Characteristic features:

  • Contains few circular folds

  • The villi are small

  • The submucosa contains lots of duodenal glands -> mucus

• Why?

  • Structure = function!

  • The duodenum receives chyme from the stomach and must neutralise the acid before it damages the absorptive surfaces of the small intestin

what is the jejunum?

• Approx. 1-2m long

• Marked by a sharp bend at the beginning

• Where the majority of chemical digestion and nutrient absorption occurs

• Characteristic features:

  • Contains lots of circular folds

  • The villi are abundant and very long

• Why?

  • To maximise nutrient absorption

what is the ileum?

• The longest segment - approx 2-3m long

• Where absorption of vitamin B12, fats (eps. Fatty acids and glycerol) and bile salts occurs

• Characteristic features:

  • Contains fewer circular folds than jejunum

  • The villi are stumpy

  • The submucosa contains lymphoid nodules -> protect the small intestine from bacteria colonising the large intestine

what is teh structure of the small intestine wall?

• Mucosa

  • Lined with simple columnar epithelium

  • Produced a carpet of alkaline mucus (protection)

  • Contains a series of finger like projections called villi (increase surface area for absorption)

• Submucosa

  • Fibrous connective tissue containing blood vessels, lymphatic vessels and nerve cells

• Muscularis

  • Made up of two layers of smooth muscle;

  1. Inner circular

  2. Outer longitudinal

• Serosa

  • Thin connective tissue layer that secretes a lubricating fluid (to reduce friction)

what are the structural characteristics of the small intestine?

• The small intestine contains many structures that increase the surface area for absorption

  • Plicae circulares

  • Villi

  • Microvilli

• Together, these structural adaption increase the surface area of the intestinal wall by 600x

what are the plicae circularis of the small intestine?

• the intestinal lining contains a series of permanent structural folds called plicae circulares -> greatly increase the surface area of the lumen within the small intestine

• Deep ridges in the mucosa and submucosa

• Begin near the proximal part of the duodenum and extends to the middle of the ileum

what are the villi of the small intestine?

• the mucosal wall of the plicae circulares projects into a series of finger like projections called villi -> further increase surface area available for nutrient absorption

• The surface of the plicae circulares contains small vascularised projections of the mucosa called villi

• There are approx. 20-40 villi per mm2 of small intestine

• Each villus contains a capillary bed which houses:

  1. 1 x arteriole (carry absorbed nutrients to the liver)

  2. 1 x venule (carry absorbed nutrients to liver)

  3. 1 x lacteal (lymphatic capillary) -> transports materials that cant enter the blood capillaries (e.g. fatty acids)

what are the microvilli of the small intestine?

• the surface of each villus is covered by epithelial cells which contain microvilli on their apical surface (brush border)

• The simple columnar epithelial cells lining the intestinal villi contain cell membrane extensions (microvilli)

• There are approx. 200 million microvilli per mm2 of small intestine

  • Significantly increases surface area of the plasma membrane for absorption

• Because they like the bristles on a brush, said to have a brush border

what is the function of the plicae circularis of the small intestine?

• facilitate absorption (esp. jejunum)

  • Their shape causes the chyme to move in a spiral motion through the small intestine -> slows the movement of chyme, increasing time available for absorption

what are hormones of the digestive system?

• The digestive system utilises a variety of hormones to regulate digestion and nutrient absorption

• Tehse hormones act on various parts of the GIT to coordinate processes such as

  • The release of stomach acid

  • The release of pancreatic enzymes

  • The release of bile

  • Gut motility (mixing and propulsion)

  • Metabolism

  • Hunger and satiety

  • Etc

• In particular, the duodenum plays an important role in regulating digestive function by stimulating the release of specific hormones, depending on the characteristics of the arriving chyme

what are the four main endocrine hormones of the digestive system?

1. Gastrin

2. Secretin

3. Gastric inhibitory peptide (GIP)

4. Cholecystokinin (CCK)

what is gastrin?

• endocrine hormone of the digestive tract

• Source: enteroendocrine G cells in the stomach (gastric glands) and the duodenum

• Released: when food arrives in stomach; when large quantities of partially digested proteins are present in the duodenum

what is the function of gastrin?

• Stomach: stimulates acid production by parietal cells; stimulates muscle contractions (increased gastric motility)

• Duodenum: stimulates gallbladder to empty bile into the duodenum; stimulates the pancreas to release pancreatic enzymes into the duodenum

what is secretin?

• endocrine hormone of the digestive tract

• Source: enteroendocrine S cells in the duodenum

• Released: when acidic chyme arrives in the duodenum (pH 2-4)

what is the function of secretin?

• stimulates the pancreas to secrete bicarbonate/buffers (increases chyme pH/neutralise acidic pH

• allows pancreatic enzymes to function optimally)

• stimulates bile production by the liver

• inhibits acid secretion from parietal cells in the stomach

• inhibits/slows gastric emptying

what is gastric inhibitory peptide (GIP)?

• endocrine hormone of the digestive tract

• Source: enteroendocrine K cells in the duodenum

• Released: when fats and carbohydrates (esp. glucose) enter the duodenum

what are the functions of GIP?

• Stimulates pancreas to release insulin

• Stimulates lipid synthesis (fat accumulation)

• Inhibits lipid breakdown

• Stimulates glucose uptake by skeletal muscle

• Regulates appetite

what is cholecystokinin (CCK)?

• endocrine hormone of the digestive tract

• Source: enteroendocrine I cells in the duodenum

• Released: when fats and undigested proteins enter the duodenum

what are the functions of CCK?

• Promotes digestion of fat and protein

• Stimulates the pancreas to secrete pancreatic enzymes into the duodenum

• Stimulates gallbladder to empty bile into the duodenum

• Inhibits gastric activity

• Suppresses hunger

what is peristalsis and segmentation?

• The muscularis layer;

  • Made up of two layers of smooth muscle

    1. Inner circular

    2. Outer longitudinal

• Coordinated contraction of these smooth muscle layers plays a vital role in peristalsis and segmentation of the bolus/chyme as it moves through tehb digestive tract

• Peristalsis = moving material along the digestive tract (propulsion)

• Segmentation = mechanical mixing

what is peristalsis?

• Is what pushes bolus/chyme forwards through the GIT

• Occurs throughout the GIT

• Peristalsis is so powerful that if you swallow food while standing on your head it will enter your stomach

1. Bolus of food arrives

2. Circular muscles contract behind bolus -> narrows internal lumen diameter

3. Longitudinal muscles contract ahead of the bolus

4. Waves of circular muscle contractions behind bolus force it forwards

what is segmentation?

• Occurs mainly in the small intestine (but also in the large intestine)

• Not involved with pushing chyme forwards through the GIT - it occurs in a stationary way in localised regions of the intestine

• Localised contractions of circular muscle isolate chyme in small sections of the intestine

• Chyme is continually pushed back and forth between adjacent sections (like repeatedly squeezing a tube of toothpaste back and forth)

  • Thoroughly mixes chyme with digestive juices

  • Facilitates mechanical breakdown

  • Maximises contact between chyme and mucosa

• When most of the chyme has been absorbed, segmentation stops

what is the large intestine?

• Averages approx. 1.5m in length and 3 inches in diameter, and is divided into 3 regions

  • cecum

  • colon

  • rectum

what is teh cecum?

• first region of teh large intestine

• An expanded pouch (6cm) where material from the large intestine is stored and faecal compaction begins

• The flow of chyme entering from the small intestine is controlled by the ileocecal valve

• Has an attached appendix

what is the colon?

• second region of the large intestine

• Divided into four regions (1. ascending colon, 2. transverse colon, 3. descending colon, 4. sigmoid colon)

what is the rectum?

• last region of the large intestine

• The last 15cm of the digestive tract

• Where faeces are stored prior to defecation

what is teh structure of the large intestine wall?

• Mucosa

  • Simple columnar epithelium (cecum and colon)

  • Stratified squamous (rectum and anus)

  • No circular folds or villi

    • Mucosa drops down into intestinal glands

  • Contains lots of goblet cells (lubrication)

• Muscularis - two layers of smooth muscle

  1. Inner circular

  2. Outer longitudinal (incomplete) - "teniae coli"

what are teniae coli of the large intestine?

• The outer longitudinal layer is incomplete - it is present in three separate longitudinal ribbons running down one side

• The teniae coli contract lengthwise to bunch up the colon into a series of pouches called haustra

what are the haustra of the large intestine?

• Series of sac like pouches within the colon ; as the chyme passes through the large intestine, it progressively moves from one pouch to the next

• The presence of chyme stimulates slow moving haustral contractions (->segmentation, primarily in the transverse and descending colon)

• Contractions occur every 25min and last 1min

• These contractions push the intestinal contents from one pouch to the next, facilitating mixing and H2O reabsorption

what are the functions of the large intestine?

1. Final absorption of nutrients and water

   • Only about 10% of nutrient absorption occurs in the large intestine

2. Chemical digestion

   • Facilitated by bacteria - no digestive enzymes secreted by the large intestine

   • Remaining indigestible carbohydrates serve as a nutrient source for bacteria in the colon

     • Bacterial activity; produces B vitamins, vitamin K, releases CO2 nad methane

3. Formation and elimination of faeces

   • Faeces = undigested food (cellulose), dead epithelial cells, mucus and bacteria

   • Frequency of defecation can be affected by diet, health, stress

   • The number of bowel movements varies greatly - ranges from 2-3 per day to 3-4 per week

what is absorption in the large intestine?

• Less than 10% of nutrient absorption occurs in the large intestine - the chyme that enters the large intestine contains few nutrients except water

• On average, 1.5L of material enters the large intestine every day, however only 200mL of faeces is eliminated

• If faecal elimination (defecation) is delayed, additional water is absorbed -> constipation

• If waste moves too quickly through the large intestine, not enough water is absorbed -> diarrhoea

what is the human microbiome?

• Humans are colonised by an estimated 100 trillion microorganisms

• These microorganisms account for approx. 1-2kg of a persons weight (roughly size of the brain)

• Include bacteria, fungi, viruses

• Distinct microbial populations reside al over our body

  • The bulk live in the GIT (esp. the cecum)

• Most of the 1000s of species of bacteria in the GIT are symbiotic (benefits both host and microbiota)

• The gut microbiota derives its nutrients from the hosts diet

  • Diet has a dramatic impact on the composition and metabolic activity of your microbiome

what is the gut microbiome?

• The gut microbiome kicks in from the moment you are born, and is fully established by age 3

• Microbiome development is affected by;

  • Gestational age (full term or prem)

  • Mode of delivery (vaginal or c section)

  • Type of feeding (breast milk or formula)

  • Maternal nutritional status (overweight or undernourished)

• As you grow your gut microbiome begins to diversify (i.e. contain different types of microbial species)

• A higher microbiome diversity = indicator of good health

what does the gut microbiome do?

1. Facilitates chemical digestion and absorption in the large intestine

2. Syntehsises specific vitamins (K and B), enzymes and amino acids

3. Ferments dietary fibre - produces a short chain of fatty acids (SCFAs)

   • Source of nutrients

   • Provides energy for epithelial cells; increased barrier integrity

   • Produces proteins to maintain desmosomes; increased barrier integrity

   • Used in cholesterol metabolism

   • Regulate appetite

   • Decreases pH of colon -> decreased growth of pathogenic bacteria

4. Affects drug metabolism - microbial metabolites can regulate liver function

5. Aids the immune system - activates intestinal immune cells to secrete immunoglobulins (IgA)

how does the gut microbiome impact overall health?

• Mental health: hormones/neurotransmitters/immunological factors released from the gut send signals to the brain -> regulates mood

• Immune health: signals derived from gut microbiota are critical for immune system development and resistance against infection (innate and adaptive immunity)

• Metabolism: gut microbiota stimulate the release of hormones (e.g. CCK, PYY, serotonin) that regulate metabolic processes (e.g. glucose metabolism, fat storage, appetite)

• Cardiovascular health: metabolites from gut microbiota affect cholesterol transportation and platelet aggregation

• Inflammation: altered immune responses to gut microbiota can trigger inflammation -> disease

how does disease impact the gut microbiome?

• Disturbance in the balance of gut microorganisms ("dysbiosis", due to infectious illness, prolonged use of antibiotics) -> decreased microbiome diversity -> increased susceptibility to disease

• The challenge is to identify whether alterations to the gut microbiome are cause or effect

• Changes in the gut microbiota have been associated with:

  1. Inflammatory bowel disease - IBD patietns have decreased bacterial diversity (fecal transplants)

  2. Diabetes - specific bacteria appear to be protective, toehrs are associated with diabetes

  3. Obesity - gut micobiomes can influence appetite and metabolism

  4. Eczema/allergies - lack of early life exposure to antigens (i.e. hygienic environments, C-section) alters the gut microbiome  -> disrupts immune development

• Manipulation of the gut microbiome (antibiotics/probiotics, microbiota transplant) may be useful to treat disease

what is the liver?

• The second largest organ in the body (~1.5kg)

• Sits just under the rib cage on the right side of the abdomen

• The only organ that can regenerate - it can regenerate itself back to full size from as little as 51% of the original liver mass

• Not just an accessory digestive organ - its the largest gland in the body - plays important roles in regulating metabolism

what is the anterior view liver anatomy?

• The falciform ligament divides the liver into two primary lobes (left and right) and attaches the liver to the abdominal wall

• The coronary ligament attaches the superior surface of the liver to the diaphragm

• The round ligament is a thickening in poster end of the falciform ligament which connects the liver to the umbilicus

what is teh posterior view liver anatomy?

• The caudate lobe lies between the left lobe and the inferior vena cava

• The quadrate lobe is sandwiched between the left lobe and the gallbladder

• Major vessels:

  • The hepatic artery brings oxygenated blood from the heart to the liver

  • The hepatic portal vein brings nutrient rich blood from the spleen and GIT to the liver

  • The inferior vena cava takes deoxygenated blood away from the liver, back to the heart

  • The bile duct carries bile away from the liver to the gallbladder and duodenum

• Hepato = related to the liver

what are liver lobules?

the structural units of the liver

• The liver is divided into approx. 100,000 lobules, each ~1mm in diameter

• Each lobule is hexagonal in shape, are packed with liver cells (hepatocytes) and are separated by interlobular septa (connective tissue)

• At each corner of these lobules is a portal triad consisting of:

  • An interlobular vein

  • An interlobular artery

  • An interlobular bile duct

• Blood from the interlobular veins and arteries feed into the liver sinusoids (large, fenestrated capillaries)

• At the centre of each lobule is a central vein which takes blood from the sinusoids to the inferior vena cava

what are hepatocytes?

• part of the liver lobule

• Make up 80% of the liver's mass

• Have secretory, metabolic and endocrine functions

• Within each lobule, hepatocytes are arranged in a series of irregular plates (like spokes of a wheel) -> plates are only one cell thick

• Hepatocytes are tightly packed around fenestrated vessels (sinusoids) -> easy access to blood -> able to process the nutrients, toxins and wastes in the blood

what are sinusoids?

• part of teh liver lobule

• Liver sinusoids are open, fenestrated capillaries which lack a basement membrane

• Also contain Kupffer cells (phagocytes) that remove dead blood cells, bacteria and foreign materials

how does blood flow in teh liver?

1. Oxygen rich blood arrives from the heart via the hepatic artery (1/3 blood supply)

2. Nutrient rich blood arrives from the spleen and GIT via the hepatic portal vein (2/3 blood supply)

3. Blood combines in liver capillaries (sinusoids)

   • Hepatocytes filter and process the blood

     • Regulate solute and nutrient levels

     • Absorb or secrete molecules such as proteins

4. Sinusoids all drain into a central vein

5. All central veins merge to form hepatic veins, which empty into the vena cava