Physiology lecture 27

Gut function, saliva and the stomach

processing of food

• Motility - mechanical breakdown and controlled movement along gut

• Secretion - addition of enzymes, electrolytes etc.

• Digestion - chemical breakdown

• Absorption - transport proteins

• Excretion - secretion of metabolic waste products and xenobiotics, and elimination of faeces

Compartments of GI tract

Hollow tube

Oral cavity and easophagus (only non mucosal part)

Lower esophageal sphincter not as effective as other spincters

Stomach - around 25% of fat digestion (gastric lipase)

Small intestine

Large intestine

All have roles in MDSA (motility, digestion, secretion and adsorption)

structure of gut tube

has mucosa and submucosa secreted from glands

layers of muscle to aid movement and motility

Large surface area due to plica, villi and microvilli

Regulation of gut function

• Neural (neuralcrine) - autonomic NS, vagus nerves, enteric NS

• Hormonal (endocrine) - dispersed entero-endocrine cells

• Local (paracrine)​

• E.g. histamine

• Three key stages of regulation​

• Cephalic​

• Gastric​

• Intestinal

saliva

1. Function

2. composition

3. control of saliva

Saliva functions

• Lubrication - fluid and mucus

• Digestion - amylase and R proteins

• Solution

• Moistness

• Protection - fluid and bicarbonate, lysozyme, immunoglobulins

salivary glands

• 3 pairs account for 90% of saliva

• Parotid glands - serous (watery), amylase

• Sublingual and submandibular glands - mixed secretion (serous and mucus)

• 10% from minor glands - mucus, mucins

• Total secretion around 1.5 litres a day

salivary gland structure

salivary fluid secretion

• primary fluid from acinar epithelial cells - isotonic, plasma like

• Secondary ductal modification - Na+Cl- reabsorption, K+ HCO3 secretion, ducts impermeable to water therefore final saliva is hypotonic and bicarb rich

control of salivation

• cephalic phase (unconditioned and conditioned) - sight, smell, taste and thought of food

• Parasympathetic (cholinergic) - cranial nerves VII and IX (NOT VAGUS), large volume

• Sympathetic (noradrenergic) - smaller volume rich in enzymes and mucus

stomach to know

gastric functions, structure, gastric juice comp, control

gastric functions

• Reservoir

• Mixing food with gastric secretions

• Digestion - mechanical, chemical

• Controlled gastric emptying - moves food (chyme) to duodenum

• Protection - bactericidal, mucus and bicarbonate

Structure of stomach

Fundus, body, then antrum

Movement controlled by pyloric sphincter (more effective than esophageal sphincter)

Microanatomy of stomach

Muscles, lymphs, arteries and veins

cells in gastric mucosa

Composition of gastric juice

• HCl - parietal cells

• Pepsinogen to pepsin - chief cells

• Mucus and bicarbonate - mucus neck and surface epithelial cells

• intrinsic factor - parietal cells

Around 2 litres produced a day

parietal cells

also known as oxyntic

present in antrum and body

Secrete gastric acid and intrinsic factor

Stimulated

parietal cells in action

carbonic anhydrase converts water and carbon dioxide into H+ and bicarbonate ions

Proton pumps allow H+ to enter stomach lumen - also contain transporters for potassium to facilitate efflux from parietal cell

Bicarbonate secreted out through a chloride transporter into interstitial fluid

Proton pump inhibitors like omeprazole work to attempt to prevent acid reflux from causing issue

what do parietal cells secrete?

HCl

• Carbonic anhydrase - catalyses formation of H+

• Apical membrane

• H+,K+-ATPase​ - Primary proton pump​

• K+ channels​ - Supplies K+ for recycling through the proton pump​

• Cl- channels​ - Supplies anion for HCl

• Basal membrane​

• Na+/K+-ATPase​

• Cl-/HCO3- exchange

Chief cells

Or peptic cells, in body and antrum

Secretes pepsin and gastric lipase for digestion of proteins and fats

Ach, acid, secretin cause stimulation of chief cells

pepsinogen converted to pepsin by acid which then digests proteins to peptides and AAs

pepsin mostly inactivated at pH above 3.5 and denatured at 7.2 (irreversibly)

Mucus neck cells and surface cells

Secrete mucus and bicarbonate

Stimulated for mucus secretion by tonic secretion or irritation of mucosal layer

Bicarb secreted with mucus

Mucus provides physical barrier between lumen and epithelium

Bicarb buffers gastric acid to prevent epithelial damage

how stomach prevents autodigestion

mucus-bicarbonate barrier

if this doesn’t happen peptic ulcers can form

control of perietal cell function

• Direct and indirect chemical regulators:

• Histamine from ECL cells​

• Somatostatin from D cells ​

• Gastrin from G cells​

• Acetylcholine from vagus and enteric neurons

ECL and D cells​

G cells

H2 receptor inhibitor

Cimetidine

control of parietal cells

Overview of control mechanisms​

• Cephalic phase​

• Sight, smell, taste & thought of food​

• Vagal efferent nerves​

• direct effect​

• histamine release​

• gastrin release​

• Gastric phase​

• Food in the stomach (stretch & chemical nature)​

• Gastrin release ​

• Negative feedback inhibition by acid​

• somatostatin release​

• Intestinal phase​

• Inhibitory

LOs

• List the functions of saliva​

• Describe the mechanisms of saliva secretion​

• Explain the control of saliva secretion​

• List the functions of the stomach​

• Explain the mechanism of gastric acid secretion​

• Give an account of the control of gastric acid secretion