[MOD10A - PHYSIO] GIT2: Secretion, Digestion, Absorption

It is like a long conveyor belt with different functions occurring either simultaneously or in succession

Gastrointestinal Tract

True or False. As food enters the GIT, it is chemically broken down into macromolecules (carbohydrates, proteins, lipids, water, and electrolytes)

False. It is mechanically broken down.

True or False. The macromolecules are then acted upon by enzymes from the saliva, stomach, pancreas, bile, and small intestine, resulting in small molecules that are then readily absorbed

TRUE

True or False. Molecules that are not digested or absorbed are then excreted by the stool.

TRUE

What are the parts encountered by the meal and its end products?

The mouth, esophagus, stomach, duodenum, jejunum, ileum, cecum, colon, rectum, and anus.

Through the GIT's length, glands produce secretions into the lumen. Other secretions come from the liver and pancreas through the __________.

Pancreaticobiliary System

What is the first secretion encountered by ingested food and is produced by 3 pairs of salivary glands that drain to the oral cavity?

Saliva

Where does carbohydrate digestion begin?

Mouth

Chewed food is mixed with saliva, containing the enzyme _________, mainly secreted by the parotid glands.

Ptyalin (α-amylase)

Less than 5% of dietary starch is hydrolyzed in the ______ since food stays in it for only a short time.

Mouth

30-40% of dietary starch is hydrolyzed in the ________ before the amylase is inactivated by gastric secretion

Gastric fundus and body of the stomach

Amylase works best at a pH of ___

6.7

Amylase is inactivated once pH becomes ______

pH < 4

An oral pH of ____ is maintained by salivary buffers

7

True or False. Saliva also facilitates swallowing, moisturizes the mouth (i.e., lubrication), acts as a solvent for molecules for taste bud stimulation, and keeps the mouth and teeth clean (antibacterial), but does not aid in speech

False. Saliva aids in speech

It is a condition wherein there is deficient saliva, causing the oral cavity to lose its antibacterial properties, and a higher incidence of dental caries and oral ulcerations

Xerostomia

What are the contents of saliva?

Water, bicarbonate, mucins, amylase, lysozyme (IgA), electrolytes

Content of saliva that facilitates taste, dissolution of nutrients, and aids swallowing and speech

Water

Content of saliva that buffers esophageal reflux

Bicarbonate

Content of saiva that lubricate food bolus and protect mucosal surface

Mucins

Content of saliva that initiates starch digestions

Amylase

The stage of salivary secretion that involves acini.

1st Stage

Secretes the primary secretion containing ptyalin, and/or mucus (contains mucin), and ions closely similar to extracellular fluid

Acini

What major active transport processes take place as the primary secretion flows through the ducts in the 1st stage of salivary secretion?

Na+ is actively reabsorbed (decreased) and K+ is actively secreted (increased)

In the 1st stage of salivary secretion, the excessively reabsorbed Na+ creates an electronegativity of -70mV in the ducts which causes ____ to be passively reabsorbed (decreased), resulting in its low concentration

Cl-

The stage of salivary secretion that involves salivary ducts

2nd Stage

In the salivary ducts, ______ from the ductal epithelium is secreted partly due to passive exchange for chloride, but may also be partly from an active process at resting conditions

HCO3-

What is the tonicity of saliva at resting conditions?

Hypotonic

How much saliva is produced per day under resting conditions?

1-1.5 L/day

What is the sodium (Na⁺) concentration in saliva at resting conditions?

15 mEq/L

How does sodium (Na⁺) concentration during maximal salivation compare to plasma?

It reaches half to ⅔ of plasma levels

What is the potassium (K+) concentration in saliva at resting conditions?

30 mEq/L

What is the chloride (Cl⁻) concentration in saliva at resting conditions?

15 mEq/L

How does potassium (K⁺) concentration during maximal salivation compare to plasma?

It is about 4 times that of plasma in resting conditions (30 mEq/L)

What is the bicarbonate (HCO₃⁻) concentration in saliva at resting conditions?

50-70 mEq/L

How much can acinar secretion increase during maximal salivation?

Up to 20-fold

Why does NaCl concentration increase during maximal salivation?

Ductal secretion fails to catch up with the rapid secretion, leading to NaCl levels of half to ⅔ of plasma, while K+ levels of only 4x that of plasma

True or False. Saliva contains mucus secretion only.

False. Saliva contains a serous and mucus secretion

Secretion of saliva that contains ptyalin

Serous secretion

Secretion of saliva that contains mucin, primarily for lubrication and mucosal surface protection

Mucus secretion

True or False. Parotid glands mostly secrete mucus

False. Parotid glands mostly secrete serous

True or False. Submandibular and sublingual glands secrete both mucus and serous

TRUE

True or False. Buccal glands secrete both mucus and serous

False. Buccal glands only secrete mucus

A thick secretion composed of water, electrolytes, and glycoproteins

Mucus

True or false. Mucus has a high resistance for slippage so the particles can slide easily along the epithelium

FALSE. It has a low resistance for slippage.

True or False. Lubrication is brought about mainly by mucus

TRUE

True or false. Saliva keeps the oral tissues healthy by washing away pathogenic bacteria and food particles needed by the bacteria

TRUE

True or false. Saliva contains antibodies to destroy oral bacteria

TRUE

Its importance in ____ hygiene reiterates how its absence can result in ulcerations, infections, and dental caries (xerostomia)

Oral

_____ is mainly parasympathetic and is controlled by the superior and inferior salivatory nuclei in the brain stem, located at the pontomedullary junction (medulla and pons)

Saliva Regulation

What is the secondary factor that also affects secretion, since secretion requires adequate nutrients from the blood?

The blood supply to the glands

Salivary cells can secrete ______, which acts on α2-globulin to produce bradykinin, a strong vasodilator

Kallikrein

line the main body of the esophagus

Simple mucous glands

line the gastric end, and to a lesser extent, the initial portion of the esophagus

Compound mucous glands

Located at the fundus and body of the stomach

Oxyntic/Gastric glands

Located at the antrum and pylorus of the stomach

Pyloric glands

_____ is formed at the villous-like projections inside the canaliculi and is conducted through the secretory end of the cell

HCL

True or false. Secretion is inhibited by fear or during sleep

TRUE

______ is the main driving force for HCl secretion by parietal cells

H+-K+ ATPase pump (or proton pump)

In the formation of HCL, water inside the cell dissociates into H+ and OH- in the _____

Cytoplasm

OH- and CO2 (from blood metabolism) will form HCO3-, facilitated by what enzyme?

Carbonic anhydrase

What are the two components of the gastric barrier, which are located between epithelial cells?

Alkaline mucus and tight junctions

What prevents the minimal backleak of the secreted acid into the mucosa?

Gastric barrier

True or False. The gastric barrier stimulates the production of high concentrations of H+ found in the gastric juice to aid in the mixing of food in the stomach.

False. It inhibits the production of high concentrations of H+ found in gastric juice

What are the three substances that can damage the gastric barrier, leading to mucosal injury?

Alcohol, aspirin, NSAIDs

This type of gradient allows acid to leak into the mucosa when the gastric barrier is compromised.

Electrochemical gradient

What ion is found in high concentrations in gastric juice but is inhibited from back-leaking by the gastric barrier?

Hydrogen ions (H⁺)

What are the three primary stimuli of gastric secretion?

Gastrin, histamine, acetylcholine

This hormone is released by G cells in response to gastrin-releasing peptide (GRP) and oligopeptides in the gastric lumen.

Gastrin

Gastrin travels through the bloodstream to stimulate secretion in these gastric glands.

Fundic glands

Gastrin binds to these cells to release histamine, which triggers acid secretion.

Enterochromaffin-like (ECL) cells

True or False. Gastrin and histamine have strong effects on all gastric cells, while acetylcholine has only little effects on other cells.

False. Gastrin and histamine mainly stimulate acid secretion and have little effect on other cells.

True or False. Acetylcholine is released from ECL cells.

False. It is released from enteric nerve endings in the fundus

True or False. Acetylcholine can stimulate parietal and chief cells, pepsinogen, HCl, and mucus secretion

True

The three primary stimuli of gastric secretion bind to receptors on this part of the cell.

Basolateral membrane

True or False. Gastrin and acetylcholine increase intracellular cAMP, whereas histamine promotes secretion by elevating cytosol Ca+.

False. Gastrin and acetylcholine promote secretion by elevating cytosol Ca+, whereas histamine increases intracellular cAMP.

Gastrin and acetylcholine promote secretion by increasing this intracellular ion.

Cytosolic calcium (Ca²⁺)

The mechanism where small changes in the availability of each gastric secretion stimulus can facilitate high secretion rates.

Synergistic effect

Blocking even one of the primary stimuli can significantly reduce gastric secretion.

TRUE

This type of drug is used to inhibit gastric secretion by blocking histamine receptors.

Histamine-2 antagonists (H2 blocker or H2RA)

True or False. Pepsinogen is produced by parietal cells, while lipase is produced by chief cells.

False. Both are produced by chief cells

This enzyme precursor has no digestive activity until it is activated by HCl.

Pepsinogen

The active form of pepsinogen, which functions as a proteolytic enzyme in an acidic environment.

Pepsin

True or False. HCl is essential for protein digestion because it activates pepsinogen into pepsin.

TRUE

The optimal pH range for pepsin activity.

pH 1.8-3.5

This neurotransmitter from the vagus nerve and enteric nervous plexus stimulates pepsinogen secretion.

Acetylcholine

True or False: Lipase does not require activation and starts dietary fat digestion immediately.

TRUE

A glycoprotein secreted by parietal cells that is essential for vitamin B12 absorption.

Intrinsic factor

The site of vitamin B12 absorption in the small intestine.

Ileum

Two consequences of parietal cell destruction in chronic gastritis

Pernicious anemia, Achlorhydria

True or False. Vitamin B12 deficiency prevents the proper maturation of red blood cells.

TRUE

Enumerate the three phases of gastric secretion in order of increasing percentage of their gastric secretion

Intestinal phase, Cephalic phase, Gastric phase

The phase of gastric secretion that begins before food enters the stomach.

Cephalic phase

Four triggers of the cephalic phase

Sight, smell, thought, or taste of food

True or False. The greater the appetite, the lower the stimulation of the cephalic phase.

False. Greater appetite leads to more intense stimulation

What are the two brain regions involved in appetite stimulation during the cephalic phase?

Hypothalamus and amygdala

Where in the brain does neurogenic signals originate?

Cerebral cortex

The cranial nerve responsible for transmitting neurogenic signals from the brain to the stomach during the cephalic phase.

CN X (Vagus) nerve

This phase occurs when food enters the stomach

Gastric phase

A reflex pathway between the stomach and brain that plays a role in the gastric phase of secretion.

Vagovagal reflex