GI Secretions and Gland Types

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Questions and Answers

What initiates gastric secretion upon food entering the stomach?

Sympathetic nervous system activation

Presence of food in the small intestine

Local enteric reflexes and gastrin-histamine mechanism (correct)

Secretion of intestinal hormones

What percentage of gastric secretion occurs when food is in the duodenum?

30%

60%

10% (correct)

20%

Which of the following is NOT a stimulus for the entero-gastric reflex?

Large bowel distension (correct)

Presence of protein breakdown products

Irritation of the mucosa

Presence of acid in the upper intestine

Which hormone is primarily responsible for inhibiting gastric secretion?

Secretin Signup and view all the answers

What is the primary function of Brunner’s glands in the proximal duodenum?

Production of alkaline mucus Signup and view all the answers

What role do mucous cells in the colon play?

Protecting intestinal walls and holding fecal matter Signup and view all the answers

How do enterocytes in the small intestine contribute to digestion and absorption?

They secrete and absorb water and electrolytes. Signup and view all the answers

Which factor is NOT a type of intestinal hormone that inhibits gastric secretion?

Gastrin Signup and view all the answers

What is the primary role of ptyalin (α-amylase) in carbohydrate digestion?

It converts starch into maltose and glucose polymers. Signup and view all the answers

Which carbohydrates are hydrolyzed into monosaccharides during digestion?

Sucrose into fructose and glucose. Signup and view all the answers

How long does hydrolysis of carbohydrates continue in the stomach?

1 hour Signup and view all the answers

What is the optimal pH for pepsin activity during protein digestion?

pH 2-3 Signup and view all the answers

What percentage of protein digestion occurs in the stomach?

10-20% Signup and view all the answers

Which enzyme cleaves individual amino acids from the carboxyl end of polypeptides?

Carboxypolypeptidase Signup and view all the answers

What happens to meats if achlorydria occurs?

They may be poorly digested. Signup and view all the answers

Which of the following is true about monosaccharides after carbohydrate digestion?

They are immediately absorbed by portal blood. Signup and view all the answers

What is the primary role of mucus in the gastrointestinal (GI) tract?

Acts as a lubricant and protects the GI wall Signup and view all the answers

Which type of glands is responsible for secreting hydrochloric acid and pepsinogen in the stomach?

Oxyntic glands Signup and view all the answers

Which component is NOT typically found in saliva?

Hydrochloric acid Signup and view all the answers

What stimulates the secretion of glandular cells in the alimentary tract?

Hormonal regulation and also physical stimulation Signup and view all the answers

What is the primary function of intrinsic factor in the GI system?

Facilitates vitamin B12 absorption Signup and view all the answers

Which secretion primarily protects the esophagus during swallowing?

Mucous secretion from simple mucous glands Signup and view all the answers

How does parasympathetic stimulation affect glandular secretion?

Increases glandular secretion Signup and view all the answers

Which of the following is a key characteristic of the secretion from pyloric glands?

Produces alkaline viscid mucus Signup and view all the answers

What triggers the gastric phase of secretion before food enters the stomach?

The sight, smell, thought, or taste of food Signup and view all the answers

What is the primary stimulant for HCl secretion by parietal cells in the stomach?

Gastrin Signup and view all the answers

Which electrolyte is typically present in high concentrations in saliva?

Potassium Signup and view all the answers

What is the role of the hydrogen-potassium pump (H+-K+ ATPase) in gastric secretion?

Creates an acidic environment in the stomach Signup and view all the answers

Which of the following glands is responsible for serous secretion?

Parotid glands Signup and view all the answers

What is the primary function of bile salts in fat digestion?

To break down fat globules into smaller particles Signup and view all the answers

What is the role of micelles in the digestion of fats?

To transport monoglycerides and fatty acids to the intestinal mucosa Signup and view all the answers

Sodium absorption in the small intestine primarily occurs through which mechanism?

Active transport using ATP Signup and view all the answers

Which structure significantly increases the absorptive surface area in the small intestine?

Microvilli Signup and view all the answers

How are carbohydrates primarily absorbed in the small intestine?

Active sodium co-transport and secondary active transport Signup and view all the answers

During fat absorption, what happens to triglycerides inside the epithelial cells?

They form chylomicrons for transport Signup and view all the answers

What triggers the absorption of chloride ions in the small intestine?

Electrical gradients established by sodium absorption Signup and view all the answers

What happens to the absorbed water in the small intestine?

It relies on the usual laws of osmosis for absorption Signup and view all the answers

What type of transport mechanism is involved in the absorption of dipeptides and tripeptides?

Active transport via sodium co-transport Signup and view all the answers

What is the result of having an abundance of bile micelles during fat digestion?

Increased fat absorption efficiency Signup and view all the answers

How are fats transported once they are absorbed by the epithelial cells of the small intestine?

They are released as chylomicrons into the lymphatic system Signup and view all the answers

Which of the following ions undergoes co-transport during absorption in the small intestine?

Sodium ions Signup and view all the answers

What is the primary method by which free fatty acids and monoglycerides are taken up by epithelial cells?

Diffusion out of micelles Signup and view all the answers

Study Notes

Main GI Secretions

Digestive enzymes and mucus are released in response to food in the GI tract.
The body secretes the precise amount of enzymes needed for proper digestion.
Mucus is a thick secretion that lubricates and protects the GI wall.

Anatomical Types of Glands

Single-cell mucous glands (goblet cells) release mucus directly onto the epithelial surface.
Pits are invaginations of the epithelium into the submucosa (e.g., crypts of Lieberkühn).
Tubular glands are located in the stomach and upper duodenum (e.g., oxyntic glands).
Complex glands (e.g., salivary glands, pancreas, and liver) secrete digestive or emulsification fluids.

Stimulation of Alimentary Tract Glands

Contact of Food with the Epithelium:
- This local effect stimulates both mucous cells and glands.
- It triggers the Enteric Nervous System (ENS) via tactile stimulation, chemical irritation, and distention.
Regulation of Glandular Secretion by Hormones:
- Hormones regulate volume and character of secretions in the stomach and intestine.
Autonomic Stimulation of Secretion:
- Parasympathetic Stimulation:
  - Increases glandular secretion (e.g., glossopharyngeal and vagus nerves).
- Sympathetic Stimulation:
  - Slightly increases secretion on its own.
  - If parasympathetic or hormonal stimulation is present, sympathetic stimulation reduces secretion by vasoconstricting blood supply.

Secretion Of Saliva

Saliva is secreted by the parotid, submandibular, sublingual, and buccal glands.
Saliva contains serous secretion (ptyalin, α-amylase) and mucous secretion (mucin).
It is rich in potassium and bicarbonate ions.
Saliva secretion occurs in two stages.
Saliva concentration: Sodium (15 mEq/L), Chloride (15 mEq/L), Potassium (30 mEq/L), and Bicarbonate (50-70 mEq/L).

Esophageal Secretion

Simple mucous glands in the esophageal body provide lubrication and protection from acidic gastric juices.
Compound mucous glands in the distal esophagus protect from acidic gastric juices.

Gastric Secretion

Oxyntic glands (body and fundus): Secrete hydrochloric acid (HCl), pepsinogen, intrinsic factor, and mucus.
Pyloric glands (antrum): Secrete gastrin and mucus.

Oxyntic Gland

Oxyntic glands contain mucous cells, chief cells that produce pepsinogen, and parietal cells that produce HCl.

Hydrochloric Acid Secretion

HCl is produced in the canaliculi of villi.
It has a pH of 0.8 and requires 1500 calories per liter of gastric juice.
The hydrogen-potassium pump (H+-K+ ATPase) is essential for HCl secretion.
Final HCl secretion consists of 150-160 mEq/L HCl, 15 mEq/L KCl, and a small amount of NaCl

Gastric Secretion Stimulation

Parasympathetic stimulation (e.g., Ach) triggers:
- Chief cells to secrete pepsinogen.
- Parietal cells to secrete HCl.
- Mucous cells to secrete mucus.
Pepsinogen is secreted in inactive form; it activates into pepsin in contact with HCl.
Pepsin is a highly proteolytic enzyme active at pH 1.8-3.5.

Intrinsic Factor (IF) Secretion

Intrinsic factor is essential for vitamin B12 absorption in the ileum.
It is secreted by parietal cells.
Chronic gastritis can reduce parietal cell volume, leading to achlorhydria (lack of stomach acid secretion) and lack of IF.
This lack of IF can lead to reduced vitamin B12 absorption, which can cause pernicious anemia due to reduced bone marrow stimulation.

Pyloric Gland - Mucus Secretion

Surface mucous cells produce alkaline viscid mucus to protect the stomach and lubricate food transport.

Pyloric Gland - Gastrin Secretion

Gastrin is produced by G cells in the pyloric gland and is the main stimulator of HCl secretion by parietal cells.
It directly stimulates parietal cells and indirectly stimulates ECL cells which produce histamine, stimulating parietal cells to secrete HCl.

Phases Of Gastric Secretion

Cephalic Phase (30%):
- Occurs before food enters the stomach from stimuli like sight, smell, thought, and taste.
- Appetite increases the intensity of secretion via neurogenic signals from the cortex, amygdala, and hypothalamus.
Gastric Phase (60%):
- Begins when food enters the stomach.
- Triggered by long vago-vagal reflexes, local enteric reflexes, and the gastrin-histamine mechanism.
Intestinal Phase (10%):
- Occurs when food enters the duodenum.
- Duodenal mucosa releases small amounts of gastrin, stimulating parietal cells to produce HCl.

Inhibition Of Gastric Secretion

Entero-gastric Reflex:
- Triggered by food in the small intestine, distension, acid, protein breakdown products, irritation of the mucosa.
- Transmitted through the Myenteric Nervous System (MNS), sympathetic and vagus nerves.
- Inhibits stomach secretion.
Intestinal Hormones:
- Secretin, Gastric Inhibitory Peptide (GIP), Vasoactive Intestinal Polypeptide (VIP), and Somatostatin.
- Triggered by acid, fat, protein, osmotic imbalance, and irritants in the duodenum.
- All four hormones inhibit gastric secretion.

Secretion Of Small Intestine

Brunner’s Glands (proximal duodenum): Secrete alkaline mucus in response to irritation, vagal stimulation, and secretin.
Crypts of Lieberkühn (entire small intestine):
- Contain goblet cells that secrete mucus, and enterocytes which secrete and absorb water and electrolytes.
- Crypts contribute 1.8 l/d of fluid to facilitate absorption from chyme.

Secretion Of The Colon

Mucous cells in the colon protect the intestinal wall from excoriation, create a medium for holding feces, protect the wall from bacterial activity, and act as a barrier to acids formed in the feces.

Digestion Of Carbohydrates

Carbohydrates are broken down into simple sugars like glucose, galactose, and fructose.
Disaccharides: Sucrose and Lactose.
Polysaccharides: Starches.

Digestion Of Carbohydrates In The Mouth And Stomach

Ptyalin (α-amylase) in saliva: Begins starch digestion in the mouth.
Stomach: Salivary amylase continues to break down starches in the body and fundus for roughly an hour.
Gastric acid: Inactivates salivary amylase.
All carbohydrates are digested to simple sugars within 15-30 minutes.

Digestion Of Proteins

proteins are broken down into amino acids.

Digestion Of Proteins In The Stomach

Pepsin: A proteolytic enzyme active at pH 2-3.
HCl: Provides the right pH for pepsin to function.
Pepsin breaks down protein into proteoses, peptones, and polypeptides.
Pepsin digests around 10-20% of total protein.
Pepsin specifically digests collagen efficiently.

Digestion Of Proteins In The Small Intestine

Pancreatic Proteolytic Enzymes (duodenum and upper jejunum): Trypsin, chymotrypsin, carboxypolypeptidase, proelastase (becomes elastase) break down proteins into smaller peptides.
Intestinal Epithelial Peptidases (brush border):
- Aminopolypeptidase, dipeptidases, and cytosol peptidases further break down peptides to individual amino acids.

Digestion Of Fats - Emulsification

Emulsification:
- Bile salts and lecithin break down large fat globules into smaller sizes to increase surface area.
- This process decreases fat tension, making the fat droplets more soluble in watery fluids.
Micelles formation: Bile salts remove monoglycerides and free fatty acids from the area of fat digestion and form micelles. Micelles are small spheres with a fat-soluble interior (sterol nucleus) and a water-soluble exterior (polar group).
- The micelle’s polar groups project outward, coating the surface and allowing it to dissolve in water. Micelles act as transport for monoglycerides and free fatty acids until absorption.

Anatomical Basis of Absorption

The small intestine absorbs a large amount of fluid:
- 1.5 l ingested liquids + 7 L secretions = 8.5 L juice to be absorbed daily.
The small intestine has adaptations to enhance its absorptive surface:
- Folds of Kerckring: Folds within the small intestine.
- Villi: Finger-like projections.
- Brush Border: Microscopic projections on each epithelial cell on each villus.
- These adaptations significantly increase the surface area of the small intestine for absorption.

Absorption In The Small Intestine

Carbs, fats, proteins, ions, and water are absorbed in this region.
Water is transported primarily by diffusion and osmosis.

Absorption Of Ions - Sodium

Sodium is actively transported into the paracellular spaces using energy from adenosine triphosphatase (ATP) enzymes.
Some sodium is absorbed along with chloride ions as negatively charged chloride ions passively follow the sodium gradient.
Sodium is co-transported through the brush border membrane with glucose, amino acids, and hydrogen ions.
This transport of sodium creates an osmotic gradient that pulls water across the epithelial cells.

Absorption Of Ions - Chloride And Bicarbonate Ions

Chloride ions are actively transported:
- Rapidly absorbed by diffusion in the upper small intestine.
- In the ileum and colon, absorbed through a chloride-bicarbonate exchanger.
Bicarbonate ions are indirectly absorbed:
- Large amounts are absorbed in the duodenum and jejunum.
- Sodium ions are absorbed, and H+ is secreted creating carbonic acid (H2CO3).
- Carbonic acid is converted to water (H2O) and carbon dioxide (CO2).
- Water is absorbed, while CO2 is absorbed to be expired or moved into the blood.

Absorption Of Nutrients - Carbohydrates

Glucose transport occurs in two stages:
- First stage (active transport): Sodium is actively transported from the GI lumen across the brush border membrane to the cell's interior.
- Second stage (active co-transport): Sodium ions bind with a transport protein, which then binds to a glucose molecule. This complex is transported across the membrane.
- Final step (diffusion): Glucose diffuses from the cell's basolateral membrane into the paracellular space and blood.

Absorption Of Nutrients - Proteins

Dipeptides, tripeptides, and free amino acids are absorbed through a sodium co-transport mechanism.
Amino acids and peptides bind with a specific sodium-dependent transport protein on the brush border membrane.
Sodium moving into the cell pulls the amino acid or peptide along with it.
Some amino acids utilize special membrane transport proteins for facilitated diffusion.

Absorption Of Nutrients - Fats

Micelles carrying monoglycerides and fatty acids migrate to the microvilli of the epithelial cell brush border.
Monoglycerides and fatty acids diffuse from the micelles into the epithelial cells.
They are then taken up by the smooth endoplasmic reticulum and reformed into triglycerides.
These triglycerides are released as chylomicrons, which enter the blood through the thoracic lymph duct.
Small amounts of short-chain and medium-chain fatty acids can be directly absorbed into the portal blood.
Bile micelles remain in the chyme to continue their transport function:
- 97% of fat is absorbed with bile micelles.
- Only 40-50% of fat is absorbed without bile micelles.

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Description

Explore the exciting world of gastrointestinal secretions and the various anatomical types of glands. This quiz delves into the digestive processes, including how enzymes and mucus facilitate digestion and protect the GI tract. Test your knowledge on the stimulation of alimentary tract glands and their regulation!