Digestion

Learning objectives

  • Why do we need a digestive system

  • Describe the four digestive processes

  • Identify the major organs of the gastrointestinal (GI) tract


Why do we need a digestive system

  • We consume nutrients that are too large to enter the bloodstream or cells directly

  • Large molecules need to be broken down- digested to allow absorption

  • Digestive system uses mechanical , chemical and enzymatic processes


carbohydrates= polysaccharides —> disaccharide —> monosaccharide

Proteins= protein—> peptide—> amino acids

Fats= triglycerides—>monoglycerides—> glycerol and fatty acids

Nutrients in cellular metabolism

  • Catabolism- break down of molecule = ATP

  • Anabolism- synthesis of new molecule = new molecule

Other functions

  • Ingestion- intake of food and water

  • Mechanical processing- mechanical breakdown of food to increase surface area

  • Excretion- removal of undigested molecules

  • Defence- GI is essential in removing pathogens found in food



4 digestive processes

Digestion- mechanical and chemical breakdown of complex macromolecules into absorbable units

Absorption- active and passive transfer of digested products from GI tract anf to the extracellular fluid

Secretion- release of digestive fluid into the GI tract to aid digestion

Motility- Muscle contraction to mix and move contents of the GI tract


Deglutination

  • swallowing

  • controlled by the autonomic nervous system

  • oral phase- voluntary stage of swallowing (food is chewed)- soft palate closes to prevent nasal aspiration of food

  • pharyngeal phase- bolus moved by peristalsis (involuntary) epiglottis blocks trachea

  • oesophageal phase-peristalsis waves move bolus of food. gastroesophageal sphincter relaxes allowing bolus into stomach


The digestive system

Upper GI tract

Mouth

Main function- mastication

  • mechanical digestion of food

  • Teeth and tongue aid in breakdown of food

Saliva- from salivary glands

  • lubricates food to help with swallowing

  • enzymatic digestion of starch and glycogen (amylase)

Food-saliva mix aka bolus

  • propelled into pharynx by the tongue


Pharynx

  • funnel shaped opening surrounded by a mucous membrane and a skeletal muscle.

  • Air and food pass through the pharynx (common pathway)

  • air enters the larynx via the epiglottis

  • food enters the oesophagus via the upper oesophageal sphincter

Oesophagus

  • muscular tube descending from the pharynx to the stomach

  • thin walled to allow for distension

  • upper third is skeletal muscle'

  • lower third is smooth muscle

  • junction between oesophagus and stomach is the lower oesophageal sphincter.


upper oesophageal sphincter

  • controlled by skeletal muscle

lower oesophageal sphincter

  • controlled by smooth muscle

  • prevents the stomach acid from entering the oesophagus

Stomach

  • An elastic enlargement of the GI tract- accommodate large volumes of food

  • acts as a reservoir - mix food with gastric juices known as chyme

  • controls release into small intestine

structure

fundus

  • extends to increas volume

Body

  • mucosa folds into ruggae- flatten to increase volume

Antrum

  • muscular lower region

  • mixing and emptying stomach


Gastric pits

exocrine secretory celss

  • neck cells- secrete mucus at the top of gastric glands

  • chief cells- secrete pepsinogen (inactive pepsin)

  • parietal cells- secrete hydrochloric acid

enteroendocrine cells

  • G cells- secrete hormone gastrin into bloodstream


gastric juice

  • mucus- protects mucosa from low pH

  • hydrochloric acid- reduces pH killing bacteria and helps digestion

  • Pepsin (secreted as pepsinogen)- activated by low pH

  • Intrinsic factor- necessary for b12 absorption

  • gastric lipase- breaks down triglycerides


Functions

  • antrum mixes food and saliva with gastric juices- producing a chyme

  • contractions force small amount of chyme through pyloric sphincter into pylorus

  • secretions of gastrin into the bloodstram stimulating motiliy

  • starts to digest protein an lipids

  • pyloric sphincter regulates passage of chyme into small intestine.


Lower GI tract

Small intestine

coiled, hollow tube 3.5 m long

primary site of digestion and absorption

hugh surface area 200m2 - deep folds called plicae circulares- folds covered in villi


Villi

  • each villi contains blood vessels and a lacteal

  • the epithelial cells are further specialised with microvilli

  • the microvilli form the brush border and contain several digestive enzymes


secretory cells

  • located in the pits known as crypts of lieberkuhn - secrete bicarbonate rich intestinal juices


functions

  • secretion- pancreatic juice and bile enters duodenum

  • intestinal juice is secreted by intestinal glands pH 7.6 cotaing bicarbonate and mucus

  • intestinal enzymes synthesised by absorptive cells that line the villi

  • also contain endocrine cells secreting hormones

  • k cells-gastric inhibitory peptide GIP

  • s cells -secretin

  • CCK cells- secrer cholecystokinin CCK

  • chemical digestion - chyme mixed with pancreatic juice and bile

  • absorb nutrients

  • peristalsis

duodenum

  • 0.5 m starts from pylorus

Jejunum

  • 1m middle portion

Ileum

  • 2m final portion joining colon

Large intestine

can be divided into three sections caecum, colon and rectum

teniae coli- 3 bands of longitudinal smooth muscle in muscularis externa

haustra- pocket like sacs caused by teniae coli

epithelium is adapted to absorb ions and water

lacks folds that the small intestine has

colon

  • ascending

  • transverse

  • descending

  • sigmoid

  • no enzyme secretion

  • concentrate waste into faeces

  • absorb water

  • store faeces until defecation

  • absorb vitamins produced by bacteria


no digestion happens here

Rectum and anus

sigmoid colon emptied into rectum for defecation through the anus

internal and external anal sphincter relaxation of both required for defecation



accessory glands

Specialised structures that secrete products, via ducts, into another structure or organ

in the digestive system it secretes digestive juices into the lumen of the GI tract


structure

  • 2 parts

  • body of the gland and main duct.

  • main duct has many branches that lead to acini (ball like shape)- produce secretions into the main duct

Salivary glands

Secrete saliva helps to moisten the food.

Parotid gland- present on both sides of head by the ear level

sublingual gland- located beneath the tongue

submandibular gland- located beneath the lower jaw

secrete saliva- 99.5% water and 0.5% solutes- sodium, potassium, chloride, phosphates, bicarbonate, mucus, enzymes (salivary amylase and lysozymes)


Liver

Largest organ in the abdominal cavity.

  • secretes bile- fatty acids

  • processing nutrients

  • removal of old red blood cells- macrophages

  • elimination of waste products

  • Bilirubin is secreted as waste product.

Anatomy

  • two main lobes

  • hepatocytes- produce bile carried by hepatic duct

  • hepatic duct form a common hepatic duct= common bile duct= pancreatic duct (ampulla of vater)

  • sphincter of Oddi- controls release of bile into small intestine. only opens after meals

  • Hepatocytes- absorb materials for bile in sinusoids

  • Bile is then secreted into bile canaliculi- small canals- located opposite side of cells

  • canaliculi flow into bile duct and onto the common hepatic duct


Gallbladder

Pancreas

Pancreatic juice- rich in bicarbonate (neutralise stomach acid), rich in pro-enzymes (zymogens) inactive forms of pancreatic amylasem lipase, proteases (trypsin, chymotrypsin, carboxypeptidase. and nucleases.

zymogens stored as granules in acini cells.- safe storage of digestive enzyme. Inactive from when not needed.

become activated by endopeptidases on the surface of enterocytes.

Zymogen activation- Trypsinogen activates trypsin- this can then activate the other inactive zymogens.

divided into an exocrine and endocrine portions

99% cells (exocrine) arranged into acini associated with the pancreatic duct. - exocrine portion, secrete pancreatic juice

1% (endocrine) arranged in islets of Langerhans- endocrine portion, secretes hormones such as insulin and glucagon


The biliary system

liver- synthesis bile

Gallbladder- stores and concentrates bile

associated ducts- transport bile to the duodenum

common bile duct terminates at the ampulla of vater- junction between common bile duct and pancreatic duct.

flow controlled by sphincter of Oddi


GI tract wall

Made up of 4 distinct layers

  • Mucosa- lines the lumen- provides protection

  • Submucosa- connective tissue support the mucosa- provide elasticity

  • Muscularis externa- smooth muscle layer- helps with motility of GI tract

  • Serosa-strong layer of connective tissue- Provide the support to the GI tract

  • Mesentery- connect the different organs in the abdomen, contains nerves and tissues


Mucosa

Innermost layer of the GI tract lines the lumen.- separates the contents of GI tract from the internal environment

Secrets mucus

Absorption of the digestive products

Protects against infectious disease

3 layers

mucous membrane-enterocytes.

  • absorptive cells- absorb nutrients

  • exocrine cells- secrete directly into lumen of GI tract eg goblet cells (mucus)

  • endocrine cells- produce secretions into the bloodstream eg G cells gastrin (produce acid)

lamina propria-connective tissues, small blood vessels, lymphatic vessels and nerves


muscularis mucosae- thin layer of smooth muscles, primary function is to mix content of the lumen

Submucosa

  • thick layer of connective tissues

  • allows GI tract to be elastic (contract) and distensible (stretchy)- prevents damage

  • Large blood vessels

  • Lymphatic vessels

  • Submucosal plexus- nervous cells

Muscularis externa

  • Layers of smooth muscle for motility

  • inner layer- circular muscle

  • outer layer- longitudinal muscle

  • coordinated contraction propel the lumen contents in a process called peristalsis

  • Coordinated by the myenteric plexus- nervous system

Serosa

inner layer - connective tissues, provides strength

outer layer- epithelial tissue aka mesothelium

mesothelium attaches to the mesentery maintaining position of each organ but flexible enough to allow some movement

The enteric nervous system

plays a big role in motility - automatic process.

  • system of nerves throughout GI tract

  • Composed of 2 major intrinsic nerve plexuses

  • submucosa plexus- regulates glands in the epithelium and smooth muscle in the mucosa

  • myenteric nerve plexus- major nerve supply controlling GI motility