Guyton and Hall Physiology Chapter 65 - Secretory Functions of the Alimentary Tract

• Glands in the gastrointestinal tract secrete digestive enzymes and mucus for lubrication and protection.
• Secretions are formed in response to food presence, with the quantity varying in each tract segment.
• Enzyme types and secretion constituents change based on food types.
• The distal end of the ileum and the mouth to the anus are sites of digestive enzymes and mucous glands, respectively

Types of Alimentary Tract Glands

• Mucous cells (goblet cells) extrude mucus for lubrication and protection, responding to local irritation.
• Crypts of Lieberkühn invaginate into the submucosa and contain specialized secretory cells.
• Tubular glands, in the stomach and upper duodenum, secrete acid and pepsinogen.
• Complex glands; salivary glands, pancreas, and liver secrete for digestion or emulsification, comprised of acini lined with glandular cells that empty into ducts.
• Salivary glands and the pancreas lie outside the walls of the alimentary tract.

Stimulation of Alimentary Tract Glands

• Food stimulates glands to secrete juices in the region and adjacent areas.
• Mucous cell secretion results from direct contact stimulation.
• The enteric nervous system is activated by tactile stimulation, chemical irritation, and gut wall distention.
• This stimulates mucous cells and deep glands to increase secretion.
• Gut epithelium activates the enteric nervous system.

Autonomic Stimulation

• Parasympathetic stimulation increases glandular secretion rates in the upper tract, like salivary glands and the duodenum, and in the distal large intestine.
• Sympathetic stimulation causes a slight secretion increase and vasoconstriction, which reduces secretion if parasympathetic or hormonal stimulation is already causing copious secretion.
• Secretion in the small intestines occur mainly in response to local neutral and hormonal stimuli

Hormonal Regulation

• Gastrointestinal hormones regulate secretion volume and composition, liberated from the mucosa in response to food.
• These hormones, polypeptides or derivatives, stimulate secretion after absorption into the blood.
• Hormones are valuable to increase the amount of gastric and pancreatic juice when food enters the duodenum or stomach

Basic Mechanism of Secretion by Glandular Cells

• Nutrient material is diffused or actively transported into the glandular cell via capillaries.
• Mitochondria form adenosine triphosphate (ATP) with oxidative energy.
• Energy from ATP synthesizes organic secretory substances in the endoplasmic reticulum and Golgi complex.
• Ribosomes are responsible for secreted protein formation.
• Secretory materials pass through the endoplasmic reticulum tubules to Golgi complex vesicles in about 20 minutes.
• In the Golgi complex, materials are modified, concentrated, and discharged into vesicles stored in secretory cells.
• Hormonal or nervous control signals extrude vesicular contents, releasing calcium into the cell to fuse vesicles with the cell membrane, which breaks open in exocytosis.

Water and Electrolyte Secretion

• Hormones that act on the cell's membrane cause secretory effects, similar to nervous stimulation.
• Secretion necessitates sufficient water and electrolytes for organic substances to pass through.

Lubricating and Protective Properties of Mucus

• Mucus contains water, electrolytes, and glycoproteins for lubrication and protection.
• Mucus adheres to food and spreads as a film over surfaces.
• It coats the gut wall, preventing food contact with the mucosa.
• Mucus allows easy slippage with low resistance and causes fecal particles to adhere with resistance to digestion.
• Glycoproteins buffer acids or bases, often containing HCO3- to neutralize acids.

Secretion of Saliva

• Saliva contains serous secretion (ptyalin for digesting starches) and mucus secretion (mucin for lubrication and protection) from parotid, submandibular, sublingual, and buccal glands.
• Daily secretion ranges from 800 to 1500 ml with a pH between 6.0 and 7.0.
• Serous and mucous secretions differ, with parotid glands secreting serous secretion and submandibular/sublingual glands secreting both, while buccal glands secrete mucus.

Secretion of Ions in Saliva

• Saliva secretion occurs in two stages: acini and salivary ducts.
• The acini secrete ptyalin or mucin in an ionic solution.
• The salivary ducts reabsorb Na+ actively and secrete K+ actively, reducing Na+ concentration and increasing K+ concentration.
• Passive reabsorption of Cl- and secretion of HCO3- by the ductal epithelium occurs.
• Under resting conditions, saliva contains low Na+ and Cl- and high K+ and HCO3-.
• During maximal salivation, the acini secretion rate increases, reducing ductal reconditioning.
• Electrical negativity is created at -70mv due to the exchange of sodium and potassium

Function of Saliva for Oral Hygiene

• Saliva plays a key role in maintaining healthy oral tissues and preventing deterioration.
• Its flow washes away pathogenic bacteria and food particles.
• Saliva contains thiocyanate ions and proteolytic enzymes that destroy bacteria.
• It also contains antibodies that can destroy oral bacteria and prevent dental caries.

Nervous Regulation of Salivary Secretion

• The salivary glands are controlled by parasympathetic nervous signals from the brain stem’s salivatory nuclei.
• Taste and tactile stimuli from the mouth and pharynx excite the salivatory nuclei.
• Especially the sour taste elicits copious saliva secretion.
• Salivation can be stimulated or inhibited by signals from higher brain centers, like the appetite area in the hypothalamus.
• Reflexes originating in the stomach and upper small intestine also cause salivation.
• Sympathetic stimulation can cause slight salivation, forming thicker saliva than parasympathetic activity.

Secondary Factors & Blood Supply

• Adequate nutrients for the glands are required because secretion requires blood supply.
• Parasympathetic nerve signals and salivation dilate blood vessels and kallikrein stimulates bradykinin for vasodilation.

Esophageal Secretion

• Esophageal secretions consist entirely of mucus, preventing excoriation and protecting from acidic gastric juices.

Gastric Secretion

• The stomach mucosa has oxyntic glands, which secrete hydrochloric acid, pepsinogen, intrinsic factor, and mucus, and pyloric glands.
• Pyloric glands secrete mucus and gastrin.
• Oxyntic glands are located in the body and fundus, while pyloric glands are located in the antral stomach.

Secretions From Gastric (Oxyntic) Glands

• An oxyntic gland is composed of mucous neck cells, peptic cells, and parietal cells.
• Parietal cells: secrete hydrochloric acid and intrinsic factor.
• Peptic cells: secrete pepsinogen.
• Mucous neck cells: secrete mainly mucus.
• Some oxyntic glands also contain enterochromaffin-like cells (ECL) that secrete histamine.
• Hydrochloric acid secretion by parietal cells involves special mechanisms.

Mechanism of Hydrochloric Acid Secretion

• Parietal cells secrete an acid solution containing 160 mmol/L of hydrochloric acid when stimulated, with a pH of about 0.8.
• High H+ concentration requires energy.
• Bicarbonate diffuses so that gastric blood has a higher pH than that of arterial blood.
• The intracellular canaliculi is where hydrochloric acid forms.

Hydrochloric Acid Secretion

• The canaliculus consists of water, hydrochloric acid (150-160 mEq/L), potassium chloride (15 mEq/L), and sodium chloride.
• Gastric barrier prevents acid leakage to prevent damage to the mucosa
• Alkaline mucus, tight junctions, and toxic substances may damage that barrier
• Production of H+ requires minimal backleak into the mucosa of the secreted acid

Basic Factors That Stimulate Gastric Secretion

• Basic factors which stimulate gastric secretion include acetylcholine, gastrin, and histamine
• Acetylcholine excites secretion by peptic, parietal, and mucous cells.
• Gastrin and histamine strongly stimulate parietal cells’ acid secretion.

Pepsinogen Secretion and Activation

• Activation of pepsinogen depends on multiple types of pepsinogen that perform a specific functions
• Pepsinogen becomes activated when contacting hydrochloric acid, forming active pepsin.
• Pepsin functions as a proteolytic enzyme in an acidic environment.
• Hydrochloric acid is necessary for protein digestion in the stomach, similar to pepsin.

Intrinsic Factor

• The secretion of intrinsic factor is produced by parietal cells of the stomach that is essential for B12 absorption in the ileum
• Destruction of parietal cells may lead to achlorhydria or pernicious anemia.

Pyloric Glands Secretion

• Pyloric glands secrete alkaline and viscid mucus
• They are structurally similar to the oxyntic glands but contain mostly mucous cells and secrete gastrin, unlike other cells

Surface Mucous Cells

• Secrete the thick mucus coat on the stomachs outer surface
• Another characteristics is that they are alkaline that aren't directly exposed to any acidic or proteolytic stomachs

Stimulation of Gastric Acid Secretion

• The acidity of the fluid can be great, where the fluid in the glands can be great
• Parietal cells of the oxyntic glands are the only cells which secrete hydrochloric acid

Acid secretion in the stomach

• Acid secretion is highly regulated with both the endocrine and the nervous signals
• They operate in close association with the enterochromaffin which is to secrete histamine for hydrochloric acid formation
• Hydrochloric acid is only to be secreted if histamine has been released by the ECL cells

Regulation of Small Intestine Secretion

• The secretion of gastric acid stimulates gastrin
• The blood is also transported to the ECL cells of the stomach which transports to the blood
• histamine is what releases into the peptic acid which stimulates more acid secretions

Pepsinogen Secretion

• Pepsinogen is stimulated to be secreted in the stomachs
• They have the acetylcholine that us directly connected to the CNS
• They also rely acid in the stomach to further aid with the acid secretions

Phases of small intestine secretion

• They can occur in three forms, the cephalic, gastric and intestinal phases
• The cephalic phases is occurs when food enters the stomach
• It uses both the sense of smell and tasting for food
• Neurogenic signals originate in parts of the cerebral cortex, hypothalamus, and amygdala.
• The phase accounts for 30 percent of all stomach meal

The gastric phases

• Occurs when the food enter in the stomach to increase secretion
• It increases the vasovagal reflexes from the brain to the gut to have for the gastrin mechanism
• Accounts for 60 percent of the small intestine secretes
• It accounts for most of the small intestine secretion

Inhibition of gastric secretin intestinal factors

• The intestinal chyme stimulates smaller amount s for the gastric juices
• It has two influences with at least two
• The duodenum causes the stimulation with the prescence of acids, proteins, foods and hormone releases

1. the reverse enterogastric reflex with the myenteric nervouss system for what is located in small

Gastric secretion

• During this time the stomach only secretes nonaxyntix with mucus
• The body of the small intestine secretes over 50 ml each hour
• Emotional stimulate the small intestine and cause it to be highly active for an inter digestion

Composition of gasstrin for the CNS

• Consists of large polypepetides that has molecaular weights as high as 2000 to 4200 by weight
• Function is resides in the terminal 4 of the of the acid groups
• Synthetic gstrin can lead all with the physiological for what is considered natural gastrins

The PancreaticSecretions

• The pancreas, secretes both digestive enzymes and bicarontaes to for digestion
• They flow through the pancreas which is the vatyer to the duodenum
• To break down chyme and or for food to travel in the small instetsines
• The pancreas also secretes insulin into the pancreastic juicer
• However the glands secretes this structure that can be located throughout pancreas

Pancerati enzymes

• Consists of many enzymes that are proteins, carbs and for fats
• Some consits of large amounts of CO3 which help neturazie
• Enterkinsea helps activate enzymes for the intersnetial tratc
• Trypsin is activated by the enzymes cause they secrete more of they

Inibitor for the tryspin for the digsetion

• The digstion has to become actiavted in the interned tracts
• Becuase if it does not then it will digsete into the small isntiens
• Trypsin inhibitor in pancreatic cells prevents trypsin activation inside secretory cells

Secrection of Bicarbonte Ions

• Although has 2 functions whcih will sectret from the cells adn for whater
• The HCO3 and water is the cell from to whihc it will then lead to the acini's
• Copious quantities of pancreatic juice allow the concentration of bicarbonate to rise to 145mEq

Regulation of Pancreatic secretions

• Relegation of the 3 secretions with are importatn during during pancrteic flow

1. Acytlcholine has relaesed more and from other choingerl
2. Choecytokinin which is located for the duodendeal

Multiplicative effects of secreteions

• when all the stimulus occure with the pancratix
• and if they do not at seperate its does not cause as much secreation
• therfore it must has to be the mutiple for all the stimulas for only that it can affect it

Phases of Pancreataic action

• the phases of with are the 3 which are the cephalci, gastric and intersetnal phases

1. During that the cns signals increase secretions of in both the brain and stomachs 2 .During the interstinal phases after the chyme will become coppios which causes secretion within that specific hormones

Intestinal phases during secretion

• Secretins stimulates for Bicarbonata and neutalize acid contents
• With those 3 hormones whcih has around has 34 amino acids and the weight

Cholercistokinin controls eznymes

• Foods in the u[[er smal also causes hormones to relaease dfrom ther small inteatins
• This relaease cause with the fats and amino aids for what enters in to hte stetomachs

Bile to secrete hrugh teh

• One that can many of the fuctions and secretion form the licesr is
• And also play an imputabt role to for fat the desotion nad pasoribtions

Physiogolcal that has secretion

• Bile is serctret in that the two has that form

1. is how the cell that consits fo rhte acid bile chlostrol and other stuff this relaeses in the form or in te small intestine
2. . then will the bile flow in the small intestiens from the ducts which will then divert hte acid and wht it has int eh small iesnts is how the bile consset

Gaaldder of bile

• Galbradders can secrete form the cell from that the bile has secretred
• However teh volume fo that the gallbadder will has in around 30 o 60 mln for teh can haold
• Becuase ofr watr and electrolytes for what it relaes from the intestens

compsoiiton of biles

• bile is comrpised of the folwoing in the tabele that it displays when and then teh it becomes contested byhte gallbaldddert
• bile acid is the that the bile are sereted that the contednts are half teh solutes htat teh biLe Has

Cholcoykinim that stimules gallbladrd emptyying

• when foods begin to travel adn secrete throughout the body the small instiens will begins to mpty teh fodoss
• but if only emtpty with a small btu if it contains it mpty it quickler
• Gall Bladder is to secrete with the cholecstyimin with relaes a fat into the duodenum

Functions of Bile and fats.

• Liver makes bile salts 6G BY GIVING THE BIELS
• Cholesterol converts by both choeric fats for te cells with gives enrgey the acid
• Bile Salts has many action that cause

Bile salts action

• bile salts actisnos:

1. has many actions that has the fats it contedns 2 It Helps with hte absortbions: fats, mono and and lipids 3.. detergent with the fats is for how the intestine tract to also take down the small fats

• the "ferried " will go to intestines ad what hte liver wil lmake bile to secrette
• Without of hte prescence of bile over 40 percents will lost and the metabilloci deficits will occur
• enterohepait circulation is how bile salt with recorculoated back in the bile

secretetion intestine fromt hhe muces

• An array for comopons which it is called brunner glands it lies i nthe wall of the small intstiens between the pyloris and the small testines and where that pancreatice secretinos for also from them
• Brunner glands secrete alkaline mucus to protect the duodenal wall from acidic gastric juice

bruners for secreetion stimulation

• This for which stimulated
• It hleps relaes form stimulas from small intestions in the walls to help reduce or reduce acid compaierds tp others

1. Tactile or irtats stimulas from ht emucosa
2. Vagal htat cuase increases and the stomach will start to
3. Gastro with the secrtin

Secretion for digesetions for te juciers

• located ofr what is called juciers htat the cover the intestiens and the covers
• cells htat will secreted and elrctyes in the intersnes and and will
• Crypts of Lieberkühn are small that the pit calls the is in located throughout the intensines
• Vili's whcih also be the part for hte the surface for what the crypt and to absorb

Brunner’s Glands Stimulation and Inhibition

• Brunner's glands are stimulated by tactile or irritating stimuli, vagal stimulation, and gastrointestinal hormones.
• The glands are inhibited by sympathetic stimulation and protects the duodenal from the HCL acid.

Regulation of Small Intestine Secretion-Local Stimuli

• The most important regulation is through local enteric nervous reflexes that are caused by tactile or other irritating stimuli