1. GIT secretions & hormones
This include
Digestive enzymes (mouth-ileum): based on location and type of food
Mucus (mouth - anus)

2. Glands
They are the structures on the surface of the epithelium or invaginations of the epithelium that produce alimentary tract secretions
Examples are: mucous glands, crypts of lieberkuhn, tubular glands and complex glands (lying outside the wall of the tract) like salivary glands, pancreas, liver…..

3. Structure of a glandular cell & gland

4. Functions of mucous secretions
They have adherent qualities
Prevent direct contact of food with the mucosa
Provides a low resistance for slippage
Causes fecal particles to adhere to one another
Its resistant to digestion by GIT enzymes
The glycoproteins of mucus have amphoteric or buffering properties

5. Stimulation of the glands
The glands are stimulated by:
Mechanical presence of food in a part of the GIT
Local epithelial stimulation that activates the ENS eg tactile stimulation, chemical irritation, gut distension…..
Parasympathetic stimulation
Sympathetic stimulation has a dual effect
Regulation: majorly hormonal

6. Saliva secretion
Glands

7. Salivary secretions Glands parotid, submandibular, sublingual glands;
small buccal glands.
Secretion rate: mls/day (ave=1000)
Ph = 6.0 – 7.0

8. Composition
contains two major types of protein secretion
(1) a serous secretion that contains ptyalin (an a-amylase), which is an enzyme for digesting starches,and
(2) mucus secretion that contains mucin for lubricating and for surface protective purposes.
Parotid: majorly serous
Submandibular/sublingual: serous and mucus
Buccal: mucus

9. Mechanism for secretion of saliva

10. Mechanism Salivary secretion is in two stages:
the first stage involves the acini, and
The second, the salivary ducts.
The acini secrete a primary secretion that contains ptyalin and/or mucin in a solution of ions in concentrations not greatly different from those of typical extracellular fluid.
As the primary secretion flows through the ducts, two major active transport processes take place that markedly
modify the ionic composition of the fluid in the saliva.

11. First, sodium ions are actively reabsorbed from all the salivary ducts and potassium ions are actively secreted in exchange for the sodium.  
Therefore, the sodium ion concentration of the saliva becomes greatly reduced, whereas the potassium ion concentration becomes increased.
However, there is excess sodium reabsorption over potassium secretion, and this creates electrical negativity of about -70 millivolts in the salivary ducts; this in turn causes chloride ions to be reabsorbed passively.
Therefore, the chloride ion concentration in the salivary fluid falls.

12. Second, bicarbonate ions are secreted by the ductal epithelium into the lumen of the duct.
partly caused by passive exchange of bicarbonate for chloride ions, and
active secretory process.
Sodium conc (mEq/L)
Potassium conc (mEq/L)
Bicarbonate ions (mEq/L) ???
Increased rate of production: saliva rich in sodium is produced (copious)
Decreased rate of production: saliva rich in potassium is produced (sticky)

13. Functions of saliva For oral hygiene
Wash away pathogenic bacteria and food particles
Destroy bacteria: proteolytic enzyme and thiocyanate

14. Regulation of salivary secretion
The salivary glands are controlled mainly by parasympathetic nervous signals all the way from the superior and inferior salivatory nuclei in the brain stem.

15. Secretion is stimulated by
Taste: acid
Tactile: smooth objects
Higher centres: appetite area close to anterior hypothalamus, amygdala and cerebral cortex
Gastric and small intestinal reflexes
Sympathetic stimulation: dual
Blood vasodilators: kallikrein and bradykinin

16. Gastric secretion Glands
1. oxyntic / gastric glands: HCl, pepsinogen, intrinsic factor and mucus
location: inside surfaces of the body and fundus of the stomach
2. pyloric glands: mucus and gastrin
Location: antral portion and distal part of stomach

18. Oxyntic/ Gastric glands
Is composed of three types of cells
Mucus neck cells: mucus
Peptic or chief cells: pepsinogen
Parietal/oxyntic cells: HCl and intrisic factor

19. Mechanism of acid secretion by the oxyntic cells

20. Mechanism of acid secretion
1. Chloride ion is actively transported from the cytoplasm of the parietal cell into the lumen of the canaliculus
2. sodium ions are actively transported out of the canaliculus into the cytoplasm of the parietal cell
3. a negative potential of -40 to -70 millivolts is created in the canaliculus
4. the negative potential causes diffusion of small quantities of sodium and potassium ions back into the lumen

21. Mechanism (cont’d)
5. water inside the oxyntic cell dissociates into hydrogen and hydroxyl ions
6. hydrogen ions are actively secreted into the lumen in exchange for potasium ions using H+-K+ ATPase
7. sodium ions are reabsorbed back into the ECF using Na+-K+ATPase.
8. water moves downhill from the ECF through the cell to the lumen
9. CO2 in the cell combines with OH- in the presence of CA to form HCO3- which is reabsorbed back into the ECF
Final secretion is thus: HCl and water, conc= 150 to 160 mEq/L, KCl= 15 mEq/L

22. Secretion of pepsinogen by peptic cells
pepsinogen is secreted (no digestive activity) ,
comes in contact with HCl
it is activated to form active pepsin.
functions
an active proteolytic enzyme
necessary for protein digestion in the stomach

23. Regulation of pepsinogen secretion
(1) stimulation of the peptic cells by acetylcholine released from the VN or from the gastric ENS
(2) stimulation of peptic cell secretion in response to acid in the stomach.
Applied Physiology 
In people who have lost the ability to secrete normal amounts of acid, secretion of pepsinogen isalso decreased, even though the peptic cells may otherwise appear to be normal.

24. Secretion of intrinsic factor by parietal cells
Intrinsic factor is essential for absorption of vitamin B12 in the ileum,
it is secreted by the parietal cells along with the secretion of HCl.
Applied physio
Destruction of parietal cells caused by acute or chronic gastritis can cause:
achlorhydria (lack of stomach acid secretion)
2. pernicious anemia because of failure of maturation of the RBCs in the absence of vitamin B12 stimulation of the bone marrow

25. Secretion of mucus and gastrin
The pyloric gland secretes mucus and gastrin
Mucus
Its produced by surface mucous cells
Coats the stomach mucosa
Protects the stomach wall
Lubricates food for easy transport
Alkaline in nature
Gastrin to be discussed later

26. Stimulation of Gastric Acid Secretion
the parietal cells operate in close association with enterochromaffin-like cells (ECL cells), the primary function of which is to secrete histamine.
The rate of secretion of HCl by the parietal cells is directly related to the amount of histamine secreted by the ECL cells.
ECL is stimulated/regulated by
Gastrin
Acetylcholine
Hormones from the ENS

27. Stimulation by gastrin
Gastrin is produced by Gastrin (G) cells in response to presence of meat or other protein-containing foods in the stomach
Gastrin stimulates the ECL cells to produce histamine
Histamine stimulates the parietal cells to produce HCl

28. Phases of Gastric Secretion
a cephalic phase,
a gastric phase, and
an intestinal phase.

29. Cephalic phase
The cephalic phase occurs even before food enters the stomach
It results from the sight, smell, thought, or taste of food, and the greater the appetite, the more intense is the stimulation.
Neurogenic signals that cause the cephalic phase of gastric secretion originate in the cerebral cortex and in the appetite centers of the amygdala and hypothalamus.
They are transmitted through the dorsal motor nuclei of the vagi and thence through the vagus nerves to the stomach.
This phase of secretion normally accounts for about 20% of the gastric secretion associated with eating a meal.

30. Gastric phase Once food enters the stomach, it excites
(1) long vagovagal reflexes from the stomach to the brain and back to the stomach,
(2) local enteric reflexes,and
(3) the gastrin mechanism
The gastric phase accounts for about 70% of the total gastric secretion associated with eating a meal

31. Intestinal phase
The presence of food in the upper portion of the small intestine, particularly in the duodenum inhibits gastric secretion by
Initiation of a reverse enterogastric reflex
Release of inhibitory hormones eg secretin, GIP, VIP and somatostatin in the presence of acid, fat, protein breakdown products, hyperosmotic or hypo-osmotic fluids, or any irritating factor in the upper small intestine
Reduction of gastric motility
purpose: to slow passage of chyme from the stomach when the small intestine is already filled or already overactive.

32. Pancreatic secretion

33. Phases of pancreatic secretion
Cephalic: vagal signals, 20% of total secretion
Gastric: vagal signals fire on, 5-10%, chyme in stomach
Intestinal: chyme in the small intestine: 70-75% caused by secretin

34. Pancreatic secretion The pancreas is a large compound gland
Its pancreatic duct joins the hepatic duct immediately before it empties into the duodenum through the papilla of Vater, surrounded by the sphincter of Oddi.
Its acini produce digestive enzymes
The epithelial cells of the ductules secrete bicarbonate and water

35. Pancreatic secretion Stimulated by: Presence of chyme in the duodenum
Its islets of Langerhans secrete insulin directly into the blood
Total amount is 1 liter/day

36. Pancreatic digestive enzymes
These include:
Proteins: trypsin & chymotrypsin (proteins to peptides) , carboxypolypeptidase (peptides to amino acids)
CHO: pancreatic amylase (CHO, glycogen to tri n di saccharides)
Fat: pancreatic lipase (neutral fat to f.a. & monoglycerrides), cholesterol esterase, phospholipase

37. Activation of the enzymes
The enzymes become activated only in the intestine.
Trypsinogen is converted to trypsin in the presence of enterokinase
Chymotrypsinogen is activated to form chymotrypsin in the presence of trypsin
Note: trypsin inhibitor is formed in the cytoplasm of the glandular cells, and it prevents activation of trypsin both inside the secretory cells and in the acini and ducts of the pancreas.
trypsin inhibitor prevents activation of the others as well.

38. Applied physiology
Acute pancreatitis: occurs when the effect of trypsin inhibitor is overwhelmed eg when a duct is blocked
Thus the pancreatic secretions rapidly become activated and can literally digest the entire pancreas within a few hours,

39. Bicarbonate ion secretion
Are secreted by epithelial cells of the ductules and ducts of the pancreas along with water
Normal conc is 29mEq/L (can rise to 145 mEq/L, to neutralize acidic chyme)
Steps involved: 3 2 1 4

40. Regulation of pancreatic
ACh: more enzymes, less water & electrolytes
CCK: secreted by I cells of the mucosa of duodenum and jejunum. more enzymes, less water & electrolytes
Secretin: secreted by S cells of the mucosa of duodenum and jejunum. Causes production of large water & bicarbonate, less enzyme all in response to acidic chyme.
An important protective mechanism to prevent the development of duodenal ulcer.
HCl +NaHCO NaCl +H2CO3
H2O
CO2

41. CCK Secreted by the I cells of the mucosal of the duodenum and jejunum
Release is potentiated by presence of proteoses, peptones and long chain f.a in the chyme

42. Regulation

43. Bile secretion by the liver
The liver produces ml of bile per day.
Functions of bile
1. fat digestion and absorption
2. means of excretion of waste products like bilirubin
Secretion of bile: secreted in two stages
1. hepatocytes secrete large amounts of bile acids, cholesterol, other organic constituents into bile canaliculi
2. bile flows in the system of ducts through the cystic duct into the gall bladder or empties directly into the duodenum: these ducts add Na+ & HCO3- to the secretion (secretin effect)

44. Gallbladder: for storage and concentrating bile
water, sodium, chloride, and most other small electrolytes are continually absorbed through the gallbladder mucosa, concentrating the remaining bile constituents that contain the bile salts, cholesterol, lecithin, and bilirubin.
Most of this gallbladder absorption is caused by active transport of sodium through the gallbladder epithelium, and this is followed by secondary absorption of chloride ions, water, and most other diffusibleconstituents.
Bile is normally concentrated in this way about 5-fold, but it can be concentrated up to a maximum of 20-fold.

45. Composition of bile

46. CCK stimulates gallbladder emptying
Stimulus: food digestion in the stomach and presence of fatty foods in the duodenum, Ach & ENS
Rhythmical contraction of the bladder wall begins under the influence of CCK
Sphincter of Oddi relaxes
gallbladder empties its store of concentrated bile into the duodenum

47. Regulation

48. Role of bile salts in fat digestion and absorption
Emulsifying or detergent function: to decrease the surface tension of the particles
Help in the absorption of fatty acids, monoglycerrides, cholesterol and other lipids from the tract
They form physical complexes called micelles, through which the lipids are ferried into the blood

49. Applied physio: Gallstone
Bile salts are formed in the hepatic cells from cholesterol in the blood plasma.
Under abnormal conditions, the cholesterol may precipitate in the gallbladder causing gallstone formation
Other causes include
Too much absorption of water from bile
Too much absorption of bile acids from bile
Too much cholesterol in bile
Inflammation of the epithelium

50. Formation of gallstone

51. Small intestinal secretions
A. Brunner’s glands secrete alkaline mucus in response to:
(1) tactile or irritating stimuli on the duodenal mucosa;
(2) vagal stimulation: increased; and SNS inhibits
(3) gastrointestinal hormones, especially secretin.
Functions
to protect the duodenal wall from digestion by the highly acid gastric juice emptying from the stomach
Neutralizes the HCl entering the duodenum from the stomach.

52. Intestinal secretions contd
B. Crypts of Lieberkühn with villi:
contains goblet cells that secrete mucus and enterocytes which secrete water and electrolytes
Mainly to aid digestion and absorption

53. Large intestinal secretions
Crypts of Lieberkühn without villi:
The epithelial cells secrete only mucus
Regulation
Tactile stimulation
Functions
protects the intestinal wall against excoriation
provides an adherent medium for holding fecal matter together
protects the intestinal wall from the great amount of bacterial activity that takes place inside the feces
provides a barrier to keep acids formed in the feces from attacking the intestinal wall.
Local reflexes: vagal stimulation through pelvic nerves increases mucus production