Motility

• Contraction of smooth muscle in the digestive tract
• Mixing and Moving the contents in the GIT

Secretion

• Both exocrine and endocrine secretions
• Mucus – along entire digestive tract
• Saliva, acid, enzymes, bile, bicarbonate, hormones et

Absorption
Digested material transferred from GIT lumen into blood or lymph. Occurs mainly in small intestine

Submucosal plexus

Myenteric Plexus

Functions

• Breaks food into smaller pieces
  
  • Facilitate swallowing
  • Increases surface area of food particles
• Mixes food with saliva
• Expose food to taste buds
  
  • Increases secretions to prepare for the arrival of food

Functions

• Initiates digestion of dietary starch by salivary amylase
• Initiates digestion of some lipids by lingual lipase
• Some antibacterial effect (lysozyme and salivary IgA)
• Dissolving molecules (taste buds)
• Helps in swallowing, speech and oral health

Oropharyngeal stage

• Initiated by tongue voluntarily pushing bolus of food to pharynx
• Note functions of uvula, tongue, glottis and epiglottis

Storage

• Body of stomach
• Receptive relaxation
  
  • Expansion of stomach cavity with little change in intragastric pressure

• When chyme that was being propelled forward hits the closed sphincter, it is tossed back into the antrum. Mixing of chyme is accomplished as chyme is propelled forward and tossed back into the antrum with each peristaltic contraction, a process called retropulsion.

These factors in the duodenum inhibit further gastric motility and emptying until the duodenum has coped with factors already present

Vomitting

• Not a function of retroperistalsis in the stomach
• Contraction of respiratory muscles and abdominal muscles
• Stomach, esophagus and sphincters relaxed
• Controlled by vomiting center (area postrema – medulla of brainstem)
• Causes
  
  • Throat stimulation, irritation of stomach, elevated intracranial pressure etc

Activates pepsinogen, breaks down connective tissue, denatures proteins, kills microorganisms

Protects mucosa against mechanical, pepsin, and acid injury

Functions of HCL

• Formation of pepsin
  
  • Pepsinogen to Pepsin
• Breakdown of connective tissue and muscle fibers
  
  • So that it's easier for pepsin to break down proteins
• Denaturation of protein
• Killing of microorganisms

Directed by CNS via vagus nerve and submucosal plexus

Increased production of gastric juice (mucous, Chief and parietal cells) and release of gastrin (G cells)

Sustained increased production of gastric juice and increased motility (mixing)

Distension of duodenum trigger enterogastric reflex (inhibits gastric motility)

• ↓ gastric contraction
• ↓ gastrin production

Feedback inhibition of pepsinogen and HCl production and gastric motility

Secretion of bile salts

Metabolic processing of nutrients

Detoxifying or degrading

Synthesizing plasma proteins

Storing glycogen, fats, vitamins etc

Activating vitamin D

Hormone secretion

Excrete cholesterol and bilirubin

Remove bacteria and worn-out RBCs

Arterial blood, which provides the liver’s O2 supply and carries blood-borne metabolites for hepatic processing, is delivered by the hepatic artery.

Venous blood draining the digestive tract is carried by the hepatic portal vein to the liver for processing and storage of newly absorbed nutrients.

Bile salts travel from

• Liver
• Common bile duct
• Duodenum, via sphincter of Oddi
• Terminal ileum, where 95% of bile salts are reabsorbed
  
  • 5% lost in faeces
• Hepatic portal vein
• Liver
  This is the enterohepatic circulation

Functions

• Aid fat digestion and absorption
• Detergent action of bile salts:
  
  • Bile salts’ ability to convert large fat globules into a lipid emulsion consisting of many small fat droplets suspended in the aqueous chyme
  • Formation of micelles: bile salts and lecithin aggregate in small clusters with their fat- soluble parts huddled together in the middle to form a hydrophobic (“water-fearing”) core, hence allowing fats to be absorbed
• Excretion of water insoluble substances
  
  • Cholesterol and the waste product bilirubin for example
  • Bilirubin has no role in digestion
• Bile salts stimulate bile secretion
• CCK promotes gallbladder emptying (contraction of gallbladder) and relaxation of Sphincter of Oddi /Hepatopancreatic Sphincter (HPS)
  
  • ↑ CCK secretion
  • ↑ Plasma CCK
  • Gallbladder contraction and HPS Relaxation
  • ↑ bile flow into duodenum
  • Aids fat digestion
• Hepatitis and cirrhosis are the most common liver disorders

Pancreatic Amylase

• Secreted in active form
• Breaks down complex carbohydrates like starch and glycogen

Pancreatic Lipase

• Main enzyme for fat digestion in GIT
• Hydrolyses dietary triglycerides into monoglycerides and free fatty acids

Pancreatic Proteolytic Enzymes (Proteases)

• Trypsin, chymotrypsin and carboxypeptidase
• Secreted as inactive forms (trypsinogen, chymotrypsinogen and procarboxypeptidase)
• Trypsinogen activated by enteropeptidase (secreted by cells of small intestines)
• Trypsin activates other enzymes and is also autocatalytic (self activation)

Pancreatic insufficiency

• Deficient of pancreatic enzymes
• Dietary fat digestion is seriously impaired

Gastrin

• Stimulated by protein in stomach
• ↑ secretion of HCl and pepsinogen
• Enhances gastric motility

Secretin

• Stimulated by acid in duodenum
• Inhibits gastric emptying and gastric secretion
• Stimulates pancreas to produce HCO3-

CCK

• Stimulated by chyme in duodenum
• Inhibits gastric motility and secretion
• Stimulates pancreatic enzyme secretion
• Stimulates contraction of gallbladder and relaxation of hepatopancreatic sphincter

Vasoactive intestinal peptide
Stimulates the secretion of intestinal glands, dilates regional capillaries, and inhibits acid production in the stomach. By dilating capillaries in active areas of the intestinal tract, VIP provides an efficient mechanism for removing absorbed nutrients.

Converts pepsinogen to pepsin to break down proteins