Chapter 22 objectives

Define alimentary canal and accessory organs and list organs that belong to each

alimentary canal- continuous tube through which food passes directly

consists of oral cavity, pharynx, esophagus, stomach, small intestine, and large intestine

accessory organs- not part of the alimentary canal but assist in digestion in some way; located around alimentary canal and include teeth, tongue, salivary glands, liver, gallbladder, and pancreas

Describe the major functions of the digestive system

1. Playing a critical role in fluid, electrolyte, and acid-base homeostasis by taking in water and electrolytes via diet and delivering them to blood
2. Ingesting vitamins and minerals
3. Producing hormones
4. Excreting metabolic wastes

Describe the six basic processes of the digestive system

1. Ingestion: bring food and water into digestive system
2. Secretion: digestive organs secrete a variety of substances—such as mucus, enzymes, acid, and hormones—to aid other digestive processes
3. Propulsion: process that allows ingested food and liquids pass from one digestive organ to next (movement)
4. Digestion: food breakdown
5. Absorption: movement of digested food particles through wall of alimentary canal into blood or lymphatic vessels
6. Defecation: exit of certain ingested materials are not digestible or usable by body

Briefly explain regulation of motility by the endocrine and nervous systems.

Nervous System Regulation

1. Enteric Nervous System (ENS): Often referred to as the "second brain," the ENS consists of a network of neurons embedded in the walls of the gastrointestinal tract. It regulates motility through:

   • Reflexes: Stretching of the gut wall triggers reflexes that stimulate contractions or relaxations in the smooth muscle.

   • Neurotransmitters: Various neurotransmitters (like acetylcholine and serotonin) are released to modulate motility.

2. Central Nervous System (CNS): The CNS influences motility through:

   • Autonomic Nervous System: The sympathetic nervous system generally inhibits motility, while the parasympathetic nervous system (via the vagus nerve) stimulates it.

Endocrine System Regulation

1. Hormones: Hormones released by the gastrointestinal tract and other organs play a crucial role in regulating motility:

   • Gastrin: Stimulates gastric motility and secretion of gastric acid.

   • Cholecystokinin (CCK): Released in response to fats and proteins, it slows gastric emptying and stimulates gallbladder contraction.

   • Secretin: Released when acidic chyme enters the small intestine, it inhibits gastric motility and stimulates bicarbonate secretion to neutralize acid

Describe the peritoneal membranes and the peritoneal cavity. Define intraperitoneal, retroperitoneal and mesenteries.

Peritoneal membranes - largest serous membrane in body
Two layer:
a. Outer parietal peritoneum lines inner surface of body wall
b. Visceral peritoneum, or serosa- where parietal peritoneum meets abdominal organs, it folds in on itself; forms outer tissue layer of such organs

Peritoneal cavity - between those two layers ^^ contains serous fluid

Intraperitoneal - organs located entirely within peritoneal cavity
Retroperitoneal - abdominal organs partly or completely outside peritoneal cavity
Mesenteries - areas where visceral peritoneum folds over on itself around certain organs, particularly small intestine
1. Support and bind these organs together and keep small intestine in a particular shape that fits within abdominopelvic cavity
2. Mesenteries also house blood vessels, nerves, and lymphatic vessels (anchor them)

Discuss the basic functions of the oral cavity, the teeth, and the tongue. Include definitions of bolus, deglutition and mastication.

Oral cavity:
a. four different digestive processes occur: ingestion, secretion, chemical and mechanical digestion, and propulsion
b. houses two accessory organs: teeth and tongue
c. salivary glands, are located in and around oral cavity; turn ingested food into a moist, chewed mass called a bolus
d. posterior oral cavity, pharynx, and esophagus then deliver bolus to stomach through process of swallowing or deglutition
e. teeth masticate, or chew, ingested food, grinding it into smaller pieces
f. tongue is involved during ingestion phase of digestion
1. movement of tongue turns food into bolus
2. pushes food against hard plate

List the two types of cells in a salivary gland and explain what product each produces and when. List the three pairs of salivary glands

Two main types of acinar cells in salivary glands:
1. Serous cells secrete a water-based fluid with enzymes and other solutes (released before/during eating)
2. Mucous cells secrete mucus; primarily involved in keeping oral mucosa moist (release continually)

Salivary Glands:

• Parotid Glands: Near the ears; produce mainly enzyme-rich serous saliva.

• Submandibular Glands: Under the jaw; produce both serous and mucous secretions.

• Sublingual Glands: Under the tongue; produce mostly mucous saliva.

Together, these glands create saliva, which aids digestion, cleanses the mouth, and helps prevent harmful bacteria growth.

Describe the composition and functions of saliva

Composition and Functions of Saliva

• Composition: Water, electrolytes, mucus, enzymes (amylase, lysozyme), IgA, and bicarbonate.

• Functions: Moistens food, begins digestion, neutralizes acid, kills bacteria, and aids in taste perception.

Describe the role of the pharynx in regards to digestion.

The pharynx's role in digestion is primarily propulsion, specifically swallowing. During deglutition, the bolus passes through the pharynx and into the esophagus. Also, the pharynx houses tonsils, which perform defensive functions.

• Propulsion: Swallowing, or deglutition, is the primary digestive function of the pharynx.

• Defense: The pharynx also houses the palatine and lingual tonsils which protect the alimentary canal from pathogens.

Describe the structure and functions of the esophagus including the roles of the upper esophageal sphincter and the gastroesophageal sphincter.

Esophagus:
1. Muscular tube about 25 cm (10 in.) long
2. Transports a bolus from pharynx to stomach
3. esophageal mucosa contains esophageal glands which secrete mucus to lubricate bolus
4. at junction of pharynx and esophagus, muscularis externa is modified into a sphincter called upper esophageal sphincter...
a. controls passage of bolus into esophagus
5. at esophagus' inferior end is another sphincter, gastroesophageal sphincter...
a. regulates passage of bolus into stomach (prevents regurge)

Explain the process of deglutition, including the three phases.

Deglutition = Swallowing

1. Voluntary phase: tongue pushes bolus toward oropharynx
2. Pharyngeal phase: bolus enters oropharynx; soft palate and epiglottis seals off nasopharynx and larynx
3. Esophageal phase: peristaltic waves move bolus down esophagus

Describe the histology of the stomach and the cells of the gastric glands. Include the functions of the types of cells the gastric glands contain. Define chyme

The histology of the stomach includes four tissue layers, similar to the rest of the alimentary canal, but with an additional oblique layer of smooth muscle that allows for churning food into a liquid called chyme. Chyme is the semi-liquid mixture of food and gastric juices produced during digestion.

Cells of the Gastric Glands

The gastric glands, located at the base of gastric pits, contain several types of cells, each with specific functions:

1. Mucous Neck Cells: Secrete acidic mucus to protect the stomach lining from its own acidic environment.

2. Parietal Cells: Produce hydrochloric acid (HCl), which activates pepsinogen and creates an acidic environment necessary for digestion.

3. Chief Cells: Secrete pepsinogen, an inactive precursor that is converted to pepsin in the acidic environment, initiating protein digestion.

4. Enteroendocrine Cells: Release hormones like gastrin, which stimulates acid secretion and influences digestion

Describe the acid secretion function of the stomach including the three regulatory phases. Define enterogastric reflex.

• Acid Secretion: Parietal cells secrete HCl.

• Phases:

  • Cephalic: Sight/smell/taste of food stimulates acid.

  • Gastric: Food in stomach and partially digested proteins stimulate acid.

  • Intestinal: Chyme in duodenum triggers the enterogastric reflex.

  The enterogastric reflex is triggered by chyme in the duodenum. It decreases vagal activity and acid secretion.

Explain the motility of the stomach in terms of its receptive function, churning function and emptying function

Motility enables stomach to receive food from esophagus, churn incoming bolus into chyme, and control rate at which chyme empties into small intestine

Receptive Function: The stomach can expand significantly (from about 50 ml to 1500 ml) to accommodate food and liquid. This is receptive relaxation and is when the gastroesophageal sphincter and the stomach's smooth muscle relax to allow the stomach to fill.

• Churning Function: After a meal, the stomach's smooth muscle layers begin peristalsis. This churns and mixes the food into chyme. A gastric pacemaker controls the rate of these peristaltic waves.

• Emptying Function: The stomach controls how quickly chyme enters the small intestine via the pyloric sphincter. Liquids pass quickly, while solids must become nearly liquid. The rate of emptying is determined by the amount and composition of chyme in the duodenum. The duodenum receives small amounts of chyme at a time.

Describe the functions of the small intestine and discuss the histology and functions of the small intestine’s circular folds, villi, and microvilli.

Main processes occur in small intestine: secretion, digestion, absorption, and propulsion
It receives partially digested food from the stomach and mixes it with digestive enzymes and bile, facilitating the breakdown of nutrients for absorption into the bloodstream or lymphatic system.

Histology and Functions of Structures in the Small Intestine

1. Circular Folds (Plicae Circulares):

   • These are large, visible folds in the intestinal lining that increase the surface area for absorption by 400 to 600 times. They also slow down the transit of chyme, allowing more time for nutrient absorption.

2. Villi:

   • Villi are small, finger-like projections that extend from the mucosal surface. Each villus contains enterocytes (absorptive cells) surrounding a core of blood capillaries and a lymphatic vessel (lacteal). This structure enhances nutrient absorption significantly.

3. Microvilli:

   • Microvilli are even smaller projections on the surface of enterocytes, creating a "brush border" appearance. They contain enzymes that further digest nutrients and increase the surface area for absorption. Each enterocyte can have up to 3,000 microvilli, maximizing the absorptive capacity of the small intestine

Describe motility in the small intestine during fasting and eating.

Motility in the Small Intestine

During Fasting:

• Migrating Motor Complex (MMC):

  • Slow, rhythmic contractions occur.

  • Clears remaining material, including leftover food and secretions.

  • Takes about 2 hours to push digesting food from the duodenum to the ileocecal valve.

During Eating:

• Peristalsis:

  • Alternating contractions of longitudinal and circular smooth muscle propel chyme toward the ileum.

• Segmentation (Churning):

  • Contractions of circular smooth muscle create a squeezing motion.

  • Aids in mechanical digestion and mixing chyme with digestive enzymes and bile.

Describe the functions of the large intestine and discuss the histology of the large intestines.

Functions:
1. Absorption - absorb water/electrolytes
2. Secretion - propulsion and defecation
3. Houses numerous bacteria that synthesize vitamins

Histology:
1. No villi or microvilli (reflects absorption is not primary function)
2. Mucosa rich with goblet cells for protection/lubrication
3. 10x more bacteria than cells in body (60% of dry mass of feces)
4. Bacteria, normal flora (gut flora) consist of about 500 different bacterial species that coexist with humans in a symbiotic (mutually beneficial) relationship

Describe the functions of the bacteria within the large intestines.

Functions:
1. Produce vitamins - such as vitamin K (necessary for blood clotting)
2. Metabolize undigested material
3. Deter growth of harmful bacteria
4. Stimulate immune system - during infancy, normal flora induce immune tolerance to their own antigens; stimulate development of MALT

Describe roles of the proximal and distal portions of the large intestines and explain the motility of the large intestine.

Proximal large intestine: ascending and transverse colon
1. primary site of water and electrolyte absorption and bacterial activity

Motility:
1. Type of segmentation (churning)
a. Circular muscle of each haustrum contracts repeatedly
b. Controlled by local neurons of ENS
2. Propulsive motion known as a mass movement
a. Mass movements occur three to four times per day, and appear to be triggered by food consumption
b. multiple haustra undergo peristalsis; propels their contents toward distal large intestine

Distal large intestine: descending and sigmoid colon, rectum, and anal canal
1. mainly storage and minimal absorption of water

Describe the pancreas and its digestive functions.

Main pancreatic duct - both pancreatic juice and enzymes
Alimentary canal - only enzymes

Functions:
1. secretes enzymes that are released into ducts of pancreas that empty into alimentary canal; makes pancreatic juice

2. Duct cells also secrete bicarbonate ions, a base, which make pancreatic juice alkaline (helps neutralize acidic chyme)

3. digestive enzymes, secreted by acinar cells, are crucial in chemical digestion

. Describe liver functions that pertain to digestion. Explain the role of bile. Define emulsification.

Main digestive function of liver is to produce bile

Two critical functions of Bile:
a. Required for digestion and absorption of lipids
b. Mechanism by which liver excretes wastes and other substances that kidneys cannot excrete

1. main organic compound of bile is bile salts which are amphiphilic (both polor and nonpolar parts). can interact with both lipids and watery envirement
2. Emulsification - bile coats lipids to physically break them down into smaller pieces

Describe the other functions of the liver.

The liver has diverse functions including: nutrient metabolism (processing nutrients)

detoxification (converting harmful substances into non-toxic forms)

excretion (bilirubin, modifying substances for kidney excretion). It also excretes bilirubin, a waste product of hemoglobin breakdown.

Describe the function of the gallbladder and trace the pathway of bile to the small intestine.

Gallbladder stores bile, concentrates it (removes water), and releases when stimulated

Pathway from gallbladder to small intestine:
1. Gallbladder to release bile into cystic duct
2. Cystic duct joins common hepatic duct to form common bile duct
3. Common bile duct joins main pancreatic duct near duodenum to form hepatopancreatic ampulla
4. Ampulla is surrounded by a ring of smooth muscle called hepatopancreatic sphincter (controls emptying of bile into duodenum)
5. Contents of hepatopancreatic ampulla empty into duodenum at major duodenal papilla

Explain the two types of digestion. Explain the process of enzymatic hydrolysis reactions of nutrient molecules

• Two types of digestion:
  1. Mechanical digestion, in which food is physically broken into smaller parts (all nutrients go through same process)
  2. Chemical digestion, in which chemical bonds between food molecules are broken (varies for each nutrient bc diff enzymes)
  

• Enzymatic Hydrolysis :Enzymatic hydrolysis is a chemical digestion process where digestive enzymes use a water molecule to break the bonds between food molecules. It's how the body breaks down food.

Enzymatic hydrolysis is a chemical digestion process where digestive enzymes use a water molecule to break the chemical bonds between food molecules. This is how your body breaks down nutrients like carbohydrates. Digestive enzymes speed up these reactions

Describe the digestion and absorption of carbohydrates, proteins, lipids and nuclei acids. Include the names of all enzymes involved. Define micelle

Carbohydrates

1. Digestion: Begins in the mouth with salivary amylase, which breaks down polysaccharides into oligosaccharides. However, this process is limited due to the short time food spends in the mouth. In the stomach, salivary amylase is inactivated by gastric acid. The main digestion occurs in the small intestine with pancreatic amylase breaking down polysaccharides into oligosaccharides and disaccharides. Brush border enzymes like lactase, maltase, and sucrase further digest these into monosaccharides (glucose, galactose, fructose) 12.

2. Absorption: Monosaccharides are absorbed in the small intestine. Glucose and galactose are actively transported via the Na+/glucose cotransport system, while fructose is absorbed through facilitated diffusion. Once in the bloodstream, they are delivered to the liver via the hepatic portal vein for further processing 2.

Proteins

1. Digestion: Protein digestion begins in the stomach with the enzyme pepsin, which is activated from its precursor pepsinogen in an acidic environment. Pepsin breaks proteins into smaller polypeptides and oligopeptides. The majority of protein digestion occurs in the small intestine with the help of pancreatic enzymes, including trypsin (activated from trypsinogen) and other brush border enzymes 3.

2. Absorption: Amino acids are absorbed in the small intestine and transported to the liver via the hepatic portal vein for processing 3.

Lipids

1. Digestion: The majority of dietary lipids are triglycerides. Digestion begins with mechanical processes (mastication, churning, segmentation) and is aided by bile salts in the small intestine, which emulsify lipids, increasing the surface area for digestive enzymes. Lipid digestion is facilitated by enzymes like lipase 3.

2. Absorption: Lipids are absorbed as fatty acids and monoglycerides, forming structures called micelles. These micelles transport lipids to the intestinal epithelial cells, where they are absorbed and reassembled into triglycerides. Lipids then enter the lymphatic system as chylomicrons before eventually reaching the bloodstream 3.

Nucleic Acids

1. Digestion: Nucleic acids begin digestion in the small intestine with the help of nucleases, which break them down into nucleotides. Further digestion occurs via brush border enzymes that remove phosphate groups and sugars, resulting in the absorption of nitrogenous bases, ribose, or deoxyribose 4.

2. Absorption: These components are absorbed into the bloodstream through active transport mechanisms 4.

Definition of Micelle

A micelle is a structure formed by the aggregation of lipid molecules in an aqueous environment, where the hydrophobic (water-repelling) tails of the lipids face inward, and the hydrophilic (water-attracting) heads face outward. This arrangement allows for the solubilization and transport of lipids in the digestive system, facilitating their absorption in the intestinal cells

Describe the absorption of water, electrolytes and both types of vitamins

Water/Electrolytes:
1. 8 liters or water are absorbed into enterocytes of small intestine
2. Most of remaining water is absorbed into enterocytes of large intestine

2. Vitamin Absorption

• Water-Soluble Vitamins (B Complex, C):

  • Absorbed in the small intestine through diffusion across enterocyte membranes.

• Fat-Soluble Vitamins (A, D, E, K):

  • Packaged into micelles with fats and absorbed along with lipids.

monomer and polymers

Here’s a simple breakdown:

• Monomers are the small building blocks.

• Polymers are long chains made by linking many monomers together.

Carbohydrates:

• Monomers: Simple sugars (monosaccharides like glucose).

• Polymers: Many sugars linked together (disaccharides like sucrose and polysaccharides like starch and glycogen).

Proteins:

• Monomers: Amino acids.

• Polymers: Chains of amino acids (polypeptides, which make up proteins).

Nucleic Acids:

• Monomers: Nucleotides.

• Polymers: Long chains (DNA and RNA).

Lipids:

• Lipids aren’t classic polymers, but they’re built from smaller units like fatty acids and glycerol (for example, in triglycerides).