Secretion

Question 1

What is secretion and absorption?

Answer 1

Secretion: Movement of solutes and water from the body (inside the blood and interstitial fluid) around the epithelial cells, across epithelia and into the lumen
Absorption: Movement of solutes and water from the lumen, across epithelia, around epithelial cells and into the body

Question 2

What are exocrine secretions?

Answer 2

Exocrine secretions are produced by epithelia moved into lumen. Secretions include: mucus - aids in protection, lubrication and digestion - electrolyte solutions (serous fluids) - watery NaCl and HCO3- that alter pH, dilute food, essential for chemical digestion - and digestive enzymes - enzymes that digestion and chemically breakdown food

Question 3

pH and volume of secretions from organs

Answer 3

pH all around neutral except for stomach which is very acidic, related to levels of bicarbonate (more bicarbonate = higher pH, no bicarbonate in stomach)
Volume of secretion of electrolyte solution: plasma 3L/day, salvia 1.5L/day, stomach 3L/day, pancreas 1.5L/day, liver 0.5L/day and small intestine 1.5L/day

Question 4

What is Salivary secretion?

Answer 4

3 pairs of salivary glands (sublingual 5%, submandibular 70% and parotid 25%) that produce 1.5L fluid per day - resting secretion 0.3ml/min, stimulated secretion 1.5ml/min
Composed of mucus (lubrication, protection), dilute solution of sodium bicarbonate and sodium chloride (dilution of food, optimal pH for digestive enzymes) and digestive enzymes (lingual lipase, alpha-amylase)

Question 5

Functions of salivary secretions

Answer 5

Not essential, aids in talking, chewing and swallowing (dissolves and lubricates food)
Hygiene - irrigation, xerostomia (dry mouth from reduced or absent salvia)
Digestion - dissolves food allows tasting, lingual lipase (fats), alpha amylase (starch)

Question 6

How are salivary secretions regulated?

Answer 6

Nervous - thought, smell, sight of food, presence of food in mouth
Autonomic nervous system - parasympathetic (stimulates secretion of copious quantities of fluid), sympathetic (stimulates secretion of small volumes of viscous fluid - augments parasympathetic response)

Question 7

What are gastric (stomach) secretions?

Answer 7

Stomach secretes 2-3L per day, between meals slow rate (15-30ml/h) of secretion, when eating superimposed on basal rate
Between meals - surface epithelium cells: layer of mucous and bicarbonate cells, work together as a protective barrier
When eating - surface epithelium cells, parietal cells: start secreting 150mls of HCl, pH=1 and intrinsic factor which helps stabilise B12; chief cells: pepsinogen (precursor to pepsin)

Question 8

Functions of gastric sections:

Answer 8

Mucus: protection from abrasion, contains bicarbonate which buffers acid at epithelial surface
Intrinsic factor: stabilises vitamin B12 and facilitates its absorption in the small intestine
Pepsinogen: inactive form of pepsin, converted to active form by acid, starts digestion of proteins
Gastric acid: dilutes food, denatures protein, activates pepsinogen to pepsin, creates optimum pH for pepsin action, protection

Question 9

How does the HCl get secreted?

Answer 9

Formation of carbonic anhydrase which takes CO2 and water and converts it to carbonic acid which dissociates to give hydrogen and bicarbonate
H-K ATPase, uses ATP to drive hydrogen out and pull potassium in, potassium recycled via potassium cycle. Bicarbonate is reabsorbed into bloodstream and is exchanged for chloride
Chloride enters lumen via chloride channel in apical membrane and joins hydrogen to make HCl

Question 10

How is the stomach phasically regulated?

Answer 10

As the food passes down our tract, different areas have different functions
Cephalic phase - head controls secretion
Gastric phase - stomach controls secretion
Intestinal phase - intestine controls secretion
In each different phase hormones and neural signals are regulating what is happening

Question 11

What is Cephalic phase?

Answer 11

~20% of secretion associated with meal. Preparing for meal. Stimuli - higher centres CNS for thought, smell and sight of food
Extrinsic parasympathetic nervous system response sends signals from CNS to stomach via enteric nervous system to stimulate parietal cells, chief cell and goblet cells and secretion of hormone gastrin is released into blood which stimulates parietal cells and chief cells

Question 12

What is the Gastric stage?

Answer 12

~70% of secretion associated with a meal
Stretch and distension occurs, products of digestion enter stomach lumen, elevated pH
Nervous and hormonal regulation include: local nervous reflex (ENS) and external nervous reflex to brain and back again (PNS) which both stimulate secretion from gastric glands, motility and gastrin secretion

Question 13

What is the Intestinal phase?

Answer 13

~ 10% of secretion associated with meal, controls delivery to SI
Stimuli: distension of duodenum, arrival of acidic chyme, lipids and carbohydrates
Nervous and hormonal regulation: GIP, CCK and secretin- all hormones that have an effect on the stomach, have an effect on pancreas and gall bladder. Nerves - enterogastric reflex. Both inhibit secretion and motility

Question 14

What is Pancreatic secretion?

Answer 14

Pancreas is an endocrine and an exocrine organ
Secretion 1-1.5L per day, secretes enzymes and alkaline fluid
Enzyme: acinar cells (secrete amylase, lipase, protease), chemical digestion of food
Alkaline fluid: duct cells, neutralise acid, optimum pH for enzyme

Question 15

How are enzymes are secreted by acinar cells?

Answer 15

Secretion of digestive enzymes by acinar cell stimulated by hormone cholecystokinin
Arrival of lipids and carbohydrates in duodenum stimulated by hormone cholecystokinin which in turn stimulates acinar cells to secrete digestive cells
Functions for the luminal chemical digestion of food

Question 16

Activation of proteolytic enzymes

Answer 16

Proteolytic enzymes needs to be activated in the duodenum (secreted as inactive precursors)
Trypsin is secreted as trypsinogen, chymotrypsin is secreted as chymotrysinogen and carboxypeptidase is secreted as procarboxypeptidase
In small intestine activation involves enterokinase (enteropeptdiase) which are bound to the duodenal membrane and converts trypsinogen to trypsin. Trypsin converts other enzymes to active form

Question 17

How is bicarbonate rich fluid secreted?

Answer 17

Produced by duct cells in pancreas. Secretion of alkaline fluid stimulated by hormone secretin (stimulated by arrival of acid chyme in duodenum). Alkaline fluid neutralises acid chyme delivered from stomach and creates optimum pH for pancreatic and intestinal digestive enzyme

Question 18

What is Biliary secretion?

Answer 18

0.5L/day - bile salts are vital for fat digestion and are associated with digestion. Bicarbonate is a rich fluid which neutralises acid. Excretory products include bile pigments and cholesterol

Question 19

How is biliary secretion regulated?

Answer 19

Bile stimulates its own secretion - enterohepatic circulation - necessary for fat digestion - lipids in duodenum triggers need for bile salts
Initial delivery of bile under hormonal control - CCK (cholecytsokinin), squeezes gall bladder, opens hepatopancreatic ampulla and releases bile into small intestine

Question 20

What is enterohepatic circulation?

Answer 20

Bile is metabolically expensive to produce, 95% of it is reabsorbed in ileum. Transported back to liver in enterohepatic circulation - release bile and once fats absorbed in ileum it is reabsorbed into bloodstream, travels along hepatic portal vein and secreted from liver, stored in gall bladder and secreted when CCK signals

Question 21

What is intestinal secretion?

Answer 21

Intestines can secrete about 1.5L per day from crypts/glands
SI: Consists of mucus, isomeric fluid (alkaline, mixture of NaCl and NaHCO3), helps to neutralise acid and dilute food to aid in digestion, and digestive enzymes - shed cells
LI: consist of mucus for lubrication