Digestive System: Structure & Function

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Questions and Answers

What type of epithelium lines the oral cavity?

Stratified squamous epithelium (correct)

Simple columnar epithelium

Cuboidal epithelium

Pseudostratified columnar epithelium

Which of the following peritoneal folds suspends the small intestines?

Greater omentum

Mesentery (correct)

Lesser omentum

Mesocolon

What is the primary function of salivary amylase in the mouth?

Breakdown of proteins

Initiation of starch digestion (correct)

Neutralization of stomach acids

Breakdown of fats

What triggers salivation according to the nervous system?

Parasympathetic nervous system Signup and view all the answers

Which part of a tooth is responsible for cutting food?

Incisors Signup and view all the answers

What is the role of the greater omentum in the digestive system?

Folding and protecting abdominal organs Signup and view all the answers

Which layer of the gastrointestinal tract is responsible for absorption and secretion?

Mucosa Signup and view all the answers

What type of digestion occurs when food is mixed with saliva and broken down into smaller pieces?

Mechanical digestion Signup and view all the answers

What is the primary function of the villi in the small intestine?

Increasing surface area for absorption Signup and view all the answers

What happens to salivary amylase in the stomach, and why?

It denatures due to stomach acidity. Signup and view all the answers

Which organ is NOT part of the gastrointestinal tract?

Liver Signup and view all the answers

What is the primary function of the serosa in the gastrointestinal tract?

To protect and anchor the GI tract Signup and view all the answers

Which section of the small intestine follows the duodenum?

Jejunum Signup and view all the answers

What type of tissue primarily makes up the muscularis layer of the GI tract?

Smooth muscle tissue Signup and view all the answers

Which part of the GI tract is responsible for the absorption of water and electrolytes?

Large intestine Signup and view all the answers

Which structure connects the small intestine to the large intestine?

Ileocaecal valve Signup and view all the answers

What is a primary secretory function of the pancreas in digestion?

Production of digestive enzymes Signup and view all the answers

Which of the following is NOT a component of the small intestine?

Colon Signup and view all the answers

Which of the following accurately describes the unique features of the large intestine?

It contains teniae coli which help form haustrae. Signup and view all the answers

Which part of the large intestine is anchored via mesocolons?

Transverse colon and sigmoid colon Signup and view all the answers

What type of cells primarily make up the mucosa of the large intestine?

Goblet cells only Signup and view all the answers

Which process primarily occurs in the large intestine involving the fermentation of substances?

Bacterial fermentation of undigested carbohydrates Signup and view all the answers

What is the role of the teniae coli in the large intestine?

To create segmentation and facilitate movement Signup and view all the answers

Which of the following accurately describes the rectum?

It represents the last 2-3 cm of the gastrointestinal tract. Signup and view all the answers

What anatomical feature is found in the large intestine that is also related to storage?

Epiploic appendages Signup and view all the answers

Which type of motion contributes to the mechanical digestion in the large intestine?

Slow peristaltic waves Signup and view all the answers

Study Notes

LECTURE 6: The Digestive System 1: Structure & Function of the Digestive System

Learning activities include viewing lecture, completing course content (Topic 3.1 – Structure & function of the Digestive System), reviewing with tutorial workbook 4, completing readings, and attending/participating in the tutorial & lab.
Lecture outcomes include identifying all anatomical and histological structures of the GI Tract, describing physiological processes in digestion, absorption, and excretion, describing factors involved in GI motility, discussing digestive secretions and their function, describing digestion, absorption, and functional importance of major nutrients and vitamins.
Terminology includes mucosa, serosa, mesentery, duodenum, jejunum, ileum, caecum, peritoneum, villi, and colon.

General Introduction

Digestive system and gastrointestinal system are used interchangeably.
The GI tract includes mouth, pharynx, esophagus, stomach, small intestine, and large intestine.
Accessory digestive organs are teeth, tongue, salivary glands, liver, gallbladder, and pancreas.
Functions include ingestion, secretion, mixing, propulsion, digestion, absorption, and defecation.

General Histology of GI Tract

The mucosa lines the lumen. It comprises epithelium (mostly simple columnar), lamina propria (loose areolar CT with capillaries), muscularis mucosae (smooth muscle), and mucosa-associated lymphoid tissue (MALT).
The submucosa is dense connective tissue with glands, blood/lymph vessels, and a submucosal plexus (regulates secretion).
The muscularis externa contains smooth muscle for segmentation and peristalsis, plus a myenteric plexus (regulates motility).
The serosa (visceral peritoneum) is the outermost layer.

General Histology of GI Tract (2)

The mucosa comprises epithelium, lamina propria and muscularis mucosae. The submucosa is dense CT with glands and blood vessels (MALT), plus a submucosal nerve plexus (controls secretion) . The muscularis externa (smooth muscle) contains a myenteric nerve plexus (controls motility). The outermost layer is the serosa.

Mouth: Ingestion & Digestion

The oral cavity (mouth) is lined with stratified squamous epithelium kept moist by saliva.
It's a site for both mechanical and chemical digestion.

Salivary Glands: Chemical Digestion

Saliva (1-1.5L) is produced by salivary glands (parotid, submandibular, and sublingual).
Saliva contains water, salivary amylase (starts starch breakdown), IgA, lysozymes, etc.
Salivation is stimulated by taste, smell, sight, or thought of food.

Teeth: Mechanical Digestion

Teeth have three parts: crown, neck, and root(s).
Gingivae (gums) cover upper and lower jaws.
Various tooth types include incisors, canines, premolars, and molars, used for cutting, tearing, crushing, and grinding.

Digestion in the mouth

Mechanical digestion involves mastication (chewing) to break food into smaller pieces and mix it with saliva.
Chemical digestion involves enzyme activity, primarily amylase that breaks down carbohydrates (starch), but this digestion stops in the stomach due to acidic environment.

Oesophagus

Muscular tube (25cm) connecting pharynx to stomach.
Upper and lower esophageal sphincters control food passage.
Peristalsis (waves of muscle contractions) transports food into the stomach.

Anatomy of stomach

Regions include cardia, fundus, body, and pyloric region.
Stomach wall layers are mucosa, submucosa, smooth muscle (3 layers), and serosa.
Gastric mucosa has folds (rugae) and glands with surface mucous cells, chief cells, parietal cells, and G cells.

Digestion in the stomach

Mechanical digestion occurs via peristaltic mixing waves to convert food into chyme.
Chemical digestion involves HCl from parietal cells to kill microbes, denature proteins, and activate pepsinogen.
Pepsin enzyme breaks down proteins into smaller peptides.

Gastric motility and emptying

Peristaltic waves propel chyme toward the pylorus.
The pyloric valve regulates chyme release into the duodenum.
Cephalic phase, gastric phase, and intestinal phase regulate gastric secretion/motility.

Anatomy of the small intestine

Small intestine (6-7m long, 2-4cm diameter): Duodenum, jejunum, and ileum.
Increases surface area for absorption through circular folds, villi, and microvilli.
Mechanoreceptors and chemoreceptors stimulate liver and pancreas to release digestive juices into the duodenum.

Histology of Small Intestine

Small intestine lining has simple columnar epithelium specialised for absorption, including circular folds, villi, and microvilli.

Absorption in the small intestine

Absorption involves nutrients, monosaccharides, amino acids, monoglycerides, fatty acids, electrolytes, vitamins, and water.

Lipoproteins

Fat globules are emulsified by bile in the duodenum, and digested into fatty acids and monoglycerides by lipase.
These combine with bile salts to form micelles.
Micelles diffuse into epithelial cells to form chylomicrons which are exocytosed into lacteals of lymphatic system.

Anatomy of large intestine

Parts include cecum, ascending colon, transverse colon, descending colon, and sigmoid colon, rectum, anus.
Unique features: teniae coli, haustrae, intestinal glands, and epiploic appendages.
Transverse colon does not border retroperitoneally; the ascending colon and descending colon do.

Processes in Large Intestine

Mucosa lacks villi, but contains goblet cells.
Mechanical digestion involves slow peristalsis mixing.
Bacterial fermentation occurs.
Water and some electrolytes are absorbed. Undigested materials, and bacteria form feces.

Absorption of water

Small intestine absorbs 8 litres of fluid each day.
Large intestine absorbs 0.9 litres.

Defecation

Colonic peristaltic contractions move feces toward the rectum.
Stretch receptors signal PNS centers (in sacral spinal cord).
Internal anal sphincter relaxes, external anal sphincter relaxes voluntarily.

Anatomy of Liver & Gallbladder

Liver (1.5 kg) is located below diaphragm in right hypochondrium, with 4 primary lobes.
Gallbladder is a pear-shaped sac that stores bile.
Cystic duct connects gallbladder to common hepatic duct, which is formed from left and right hepatic ducts.

Liver Histology

Liver cells (hepatocytes) are arranged in hexagonal lobules.
Central vein is located in the centre.
Kupffer cells (hepatic macrophages) phagocytose microorganisms.
Bile canaliculi drain produced bile into bile ducts.

Blood supply to the liver

Liver receives blood from hepatic artery and hepatic portal vein.
Blood mixes in sinusoids and flows to central vein, then to hepatic vein, and to inferior vena cava.

Main Functions of the Liver

Carbohydrate metabolism
Lipid metabolism
Protein metabolism
Processing of drugs and hormones,
Bilirubin processing and excretion
Synthesis of bile acids,
Nutrient storage (glycogen, vitamins)
Phagocytosis, and
Vitamin D activation.

Bile & Bile Secretion

Bile production (800-1000 mL/day) has water, ions, bile acids, cholesterol, lecithin, and bile pigments.
Bile emulsifies fats.
Bile acids are reabsorbed in terminal ileum; excess is excreted in feces.

Pancreas

Pancreas is retroperitoneal, in the body's upper abdomen.
Has both exocrine (1.2-1.5L digestive pancreatic juice) and endocrine (pancreatic islets producing hormones like glucagon and insulin).
Pancreatic duct joins common bile duct.

Pancreatic Juice

Composition: water, salts, bicarbonate ions. This neutralises acidic chyme, stopping pepsin action, and creating ideal pH for enzymes.
Contains digestive enzymes such as amylase, trypsin, chymotrypsin, carboxypeptidase, pancreatic lipase, deoxyribonuclease, and ribonuclease.

Control of Pancreatic Secretion

Chyme entering duodenum triggers CCK and secretin release.
CCK causes enzyme-rich pancreatic juice secretion.
Secretin stimulates HCO3- rich pancreatic juice secretion.
CCK is the major stimulus for pancreatic enzyme/bile delivery into small intestine

Digestion of Nutrients

Chemical digestion breaks carbohydrates (starch) through salivary and pancreatic amylase and ending in the small intestine with brush-border enzymes.
Chemical digestion breaks proteins in the stomach with HCI and then completed in the small intestine with pancreatic and small intestinal proteases.
Chemical digestion breaks down lipids in the duodenum with emulsification by bile and pancreatic lipase, ending in the small intestine with brush-border enzymes.
Nucleic acids are broken down to nucleotides through pancreatic ribonuclease and deoxyribonuclease, ending in the small intestine.

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Description

Explore the intricate details of the digestive system in this comprehensive quiz based on Lecture 6. You'll learn about anatomical and histological structures of the GI tract, physiological processes in digestion, and factors involved in GI motility. Participate actively in discussions and boost your understanding of major nutrients and digestive secretions.