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Digestive System Study Notes, Study notes of Anatomy

Central Philippine University (CPU)Anatomy

Subject: Biology Year: 2025 Course: Anatomy and Physiology Subtopics: Part I: Functions of the Digestive System Part II: Anatomy of the Digestive System Part III: Organs of the Alimentary Canal Part IV: Layers of the Alimentary Canal Part V: Salivary Glands Part VI: Gastrointestinal Processes and Controls Part VII: Digestive Enzymes and Absorption Part VIII: Activities Occurring in the Mouth, Pharynx, and Esophagus Part IX: Metabolism and Nutrition Part X: Body Energy Balance Part XI: Developmental Aspects of the Digestive System and Metabolism

Typology: Study notes

2024/2025

Available from 01/11/2025

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C.A.B.M. | 2024

1

Digestive System and Body Metabolism

Functions of the Digestive System

1. Ingestion: Taking in food.

2. Digestion: Breaking food into nutrient

molecules.

3. Absorption: Movement of nutrients into the

bloodstream.

4. Defecation: Elimination of indigestible waste.

Anatomy of the Digestive System

Two Main Groups of Organs

1. Alimentary Canal (GI Tract):

o A continuous, coiled, hollow tube from

mouth to anus.

o These organs ingest, digest, absorb,

defecate

o Includes:

▪ Mouth

▪ Pharynx

▪ Esophagus

▪ Stomach

▪ Small intestine

▪ Large intestine

▪ Anus

2. Accessory Digestive Organs:

o Include teeth, tongue, salivary glands,

pancreas, liver, and gallbladder.

o Assist digestion by breaking down food

or secreting digestive fluids.

Organs of the Alimentary Canal

Mouth (Oral Cavity)

• Lined with a mucous membrane.

• Anatomy:

o Mouth (oral cavity): mucous

membrane–lined cavity

o Lips (labia): Protect the anterior

opening.

o Cheeks: Form lateral walls.

o Hard Palate: Anterior roof.

o Soft Palate: Posterior roof.

o Uvula: Fleshy projection of the soft

palate.

o Vestibule: Space between lips/cheeks

and teeth/gums.

o Oral Cavity Proper: Area inside teeth.

o Tongue: Attached by hyoid bone,

styloid processes, and lingual frenulum.

o Tonsils:

▪ Palatine: Located at the

posterior end of the oral cavity.

▪ Lingual: Found at the base of

the tongue.

• Functions:

o Mastication (chewing).

o Mixing food with saliva.

o Swallowing initiation.

o Taste via taste buds.

Pharynx

• Food is propelled by two skeletal muscle layer:

longitudinal outer layer; circular inner layer

• Passageway for food, fluids, and air.

• Regions:

1. Oropharynx: Posterior to the oral

cavity.

2. Laryngopharynx: Continuous with the

esophagus.

• Propels food via peristalsis (muscle

contractions).

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Digestive System and Body Metabolism Functions of the Digestive System

Ingestion : Taking in food.
Digestion : Breaking food into nutrient molecules.
Absorption : Movement of nutrients into the bloodstream.
Defecation : Elimination of indigestible waste. Anatomy of the Digestive System Two Main Groups of Organs
Alimentary Canal (GI Tract) : o A continuous, coiled, hollow tube from mouth to anus. o These organs ingest, digest, absorb, defecate o Includes: ▪ Mouth ▪ Pharynx ▪ Esophagus ▪ Stomach ▪ Small intestine ▪ Large intestine ▪ Anus
Accessory Digestive Organs : o Include teeth, tongue, salivary glands, pancreas, liver, and gallbladder. o Assist digestion by breaking down food or secreting digestive fluids. Organs of the Alimentary Canal Mouth (Oral Cavity)

Lined with a mucous membrane.
Anatomy : o Mouth (oral cavity) : mucous membrane–lined cavity o Lips (labia) : Protect the anterior opening. o Cheeks : Form lateral walls. o Hard Palate : Anterior roof. o Soft Palate : Posterior roof. o Uvula : Fleshy projection of the soft palate. o Vestibule : Space between lips/cheeks and teeth/gums. o Oral Cavity Proper : Area inside teeth. o Tongue : Attached by hyoid bone, styloid processes, and lingual frenulum. o Tonsils : ▪ Palatine : Located at the posterior end of the oral cavity. ▪ Lingual : Found at the base of the tongue.
Functions : o Mastication (chewing). o Mixing food with saliva. o Swallowing initiation. o Taste via taste buds. Pharynx
Food is propelled by two skeletal muscle layer: longitudinal outer layer; circular inner layer
Passageway for food, fluids, and air.
Regions:

Oropharynx : Posterior to the oral cavity.
Laryngopharynx : Continuous with the esophagus.

Propels food via peristalsis (muscle contractions).

Esophagus

~10 inches long.
Pharynx → stomach through diaphragm
Conducts food to the stomach via peristalsis.
Passageway for food only (respiratory system branches off). Layers of the Alimentary Canal

Mucosa : o Innermost, moist membrane lining the lumen. o Mostly simple columnar epithelium (stratified squamous in the esophagus).
Submucosa : o Beneath the mucosa o Connective tissue with blood vessels, nerves, lymphatic tissue, and lymph vessels.
Muscularis Externa : o Smooth muscle (inner circular and outer longitudinal layers).
Serosa : o Outermost layer with fluid-producing cells. o Divided into: ▪ Visceral Peritoneum : Covers organs. ▪ Parietal Peritoneum : Lines the abdominal cavity. Alimentary Canal Nerve Plexuses

Two intrinsic nerve plexuses regulate GI tract mobility and secretions (part of the autonomic nervous system):

Submucosal Plexus
Myenteric Plexus The Stomach

C-shaped organ on the left side of the abdominal cavity.
Food enters at the cardio esophageal sphincter from the esophagus
Food empties into the small intestine at the pyloric sphincter (valve)
Regions :

Cardial Region (Cardia) : Near the heart.
Fundus : Expanded portion lateral to the cardia.
Body : Midportion. ▪ Greater curvature is the convex lateral surface ▪ Lesser curvature is the concave medial surface
Pylorus : Funnel-shaped terminal end leading to the small intestine via the pyloric sphincter.

Stomach can stretch and hold 4 L (1 gallon) of food when full o Rugae —internal folds of the mucosa present when the stomach is empty
Lesser omentum o Double layer of the peritoneum o Extends from liver to the lesser curvature of stomach
Greater omentum o Another extension of the peritoneum o Covers the abdominal organs o Fat insulates, cushions, and protects abdominal organs

Subdivisions : o Cecum- saclike first part of the large intestine ▪ Appendix ➢ Hangs from the cecum ➢ Accumulation of lymphoid tissue that sometimes becomes inflamed (appendicitis) o Colon : travels up right side of abdomen and makes a turn at the right colic (hepatic) flexure o Ascending : travels across the abdominal cavity and turns at the left colic (splenic) flexure o Transverse : travels down the left side o Descending : enters the pelvis o Sigmoid : S-shaped region; enters the pelvis o Rectum o Anal Canal (ends at the anus)- opening of the large intestine ▪ External Anal Sphincter : Voluntary skeletal muscle. ▪ Internal Anal Sphincter : Involuntary smooth muscle. ▪ These sphincters are normally closed except during defecation
Functions : o Absorption of water, vitamins, and ions. o Elimination of feces.
Goblet cells produce alkaline mucus to lubricate the passage of feces
Muscularis externa layer is reduced to three bands of muscle, called teniae coli
These bands of muscle cause the wall to pucker into haustra (pocketlike sacs) Accessory Digestive Organs Teeth- masticate (chew) food into smaller fragments - Deciduous (Baby) Teeth : 20 teeth by age 2; lower central incisors first to appear - Permanent Teeth : 32 teeth, including wisdom teeth. - Classification according to shape and function : o Incisors - cutting. o Canines (eyeteeth)- tearing. o Premolars (biscupid) and Molars - grinding. - Two major regions of a tooth o Crown - exposed part of tooth above the gingiva (gum) ▪ Enamel —covers the crown ▪ Dentin —found deep to the enamel and forms the bulk of the tooth, surrounds the pulp cavity ▪ Pulp cavity —contains connective tissue, blood vessels, and nerve fibers (pulp) ▪ Root canal —where the pulp cavity extends into the root o Root ▪ Cement—covers outer surface and attaches the tooth to the periodontal membrane (ligament)

▪ Periodontal membrane holds tooth in place in the bony jaw

Note: The neck is a connector between the crown and root o Region in contact with the gum Salivary Glands
Three pairs: o Parotid Glands: Anterior to the ears; mumps affect these salivary glands. o Submandibular Glands: Floor of the mouth. o Sublingual Glands: Below the tongue. ▪ Both submandibular and sublingual glands empty saliva into the floor of the mouth through small ducts
Saliva o Mixture of mucus and serous fluids o Helps to moisten and bind food together into a mass called a bolus ▪ Salivary amylase : moistens food and begins starch digestion. ▪ Lysozymes and antibodies : inhibit bacteria ▪ Dissolves chemicals so they can be tasted Pancreas
Produces digestive enzymes and alkaline fluid.
Secretes enzymes into the duodenum
Alkaline fluid introduced with enzymes neutralizes acidic chyme coming from stomach
Releases insulin and glucagon. Liver
Largest gland in the body.
Digestive role is to produces bile , which emulsifies fats. o Bile leaves the liver through the common hepatic duct and enters duodenum through the bile duct o Bile is yellow-green , watery solution containing: ▪ Bile salts and bile pigments (mostly bilirubin from the breakdown of hemoglobin) ▪ Cholesterol, phospholipids, and electrolytes
Consists of four lobes suspended from the diaphragm and abdominal wall by the falciform ligament Gallbladder
Green sac found in a shallow fossa in the inferior surface of the liver
Stores and concentrates bile.
When no digestion is occurring, bile backs up the cystic duct for storage in the gallbladder
While in the gallbladder, bile is concentrated by the removal of water
When fatty food enters the duodenum, the gallbladder spurts out stored bile Gastrointestinal Processes and Controls

Ingestion : Placing food into the mouth.
Propulsion : movement of foods from one region of the digestive system to another o Peristalsis —alternating waves of contraction and relaxation that squeeze food along the GI tract o Segmentation —movement of materials back and forth to foster mixing in the small intestine
Food Breakdown : Mechanical Breakdown o Chewing, stomach churning, and segmentation in the intestines.
Digestion : o Enzymes break down macromolecules into building blocks. o Each major food group uses different enzymes

Carbohydrates are broken down to monosaccharides (simple sugars)
Proteins are broken down to amino acids
Fats are broken down to fatty acids and glycerol

Food propulsion

Peristalsis: waves of peristalsis occur from the fundus to the pylorus, forcing food past the pyloric sphincter
Grinding: the pylorus meters out chyme into the small intestine (3 ml at a time)
Retropulsion: peristaltic waves close the pyloric sphincter, forcing contents back into the stomach; the stomach empties in 4–6 hours Activities of the Small Intestine

Chyme breakdown and absorption o Intestinal enzymes from the brush border function to: ▪ Break double sugars into simple sugars ▪ Complete some protein digestion o Intestinal enzymes and pancreatic enzymes help to complete digestion of all food groups o Pancreatic enzymes play the major role in the digestion of fats, proteins, and carbohydrates o Alkaline content neutralizes acidic chyme and provides the proper environment for the pancreatic enzymes to operate o Release of pancreatic juice from the pancreas into the duodenum is stimulated by: ▪ Vagus nerves ▪ Local hormones that travel via the blood to influence the release of pancreatic juice (and bile) - Secretin - Cholecystokinin (CCK)
Hormones (secretin and CCK) also target the liver and gallbladder to release bile o Bile ▪ Acts as a fat emulsifier ▪ Needed for fat absorption and absorption of fat-soluble vitamins (K, D, E, and A)
Water is absorbed along the length of the small intestine
End products of digestion o Most substances are absorbed by active transport through cell membranes o Lipids are absorbed by diffusion
Substances are transported to the liver by the hepatic portal vein or lymph
Chyme propulsion o Peristalsis is the major means of moving food o Segmental movements ▪ Mix chyme with digestive juices ▪ Aid in propelling food Activities of the Large Intestine
Nutrient breakdown and absorption o No digestive enzymes are produced o Resident bacteria digest remaining nutrients ▪ Produce some vitamin K and some B vitamins ▪ Release gases
Water, vitamins, ions, and remaining water are absorbed
Remaining materials are eliminated via feces
Feces contains: Undigested food residues, mucus, bacteria, water
Propulsion of food residue and defecation o Sluggish peristalsis begins when food residue arrives o Haustral contractions are the movements occurring most frequently in the large intestine o Mass movements are slow, powerful movements that occur three to four times per day
Presence of feces in the rectum causes a defecation reflex
Internal anal sphincter is relaxed
Defecation occurs with relaxation of the voluntary (external) anal sphincter

Metabolism and Nutrition

Most foods are used as metabolic fuel o Foods are oxidized and transformed into adenosine triphosphate (ATP) o ATP is chemical energy that drives cellular activities
Energy value of food is measured in kilocalories (kcal) or Calories (C) Nutrition
Nutrient —substance used by the body for growth, maintenance, and repair
Major nutrients Carbohydrates, lipids, proteins, water
Minor nutrients : Vitamins, minerals
A diet consisting of foods from the five food groups normally guarantees adequate amounts of all the needed nutrients Dietary Recommendations
Healthy Eating Pyramid o Issued in 1992 o Six major food groups arranged horizontally
MyPlate o Issued in 2011 by the USDA o Five food groups are arranged by a round plate Dietary Sources of the Major Nutrients
Carbohydrates o Dietary carbohydrates are sugars and starches o Most are derived from plants such as fruits and vegetables o Exceptions: lactose from milk and small amounts of glycogens from meats
Lipids o Saturated fats from animal products (meats) o Unsaturated fats from nuts, seeds, and vegetable oils o Cholesterol from egg yolk, meats, and milk products (dairy products)
Proteins o Complete proteins—contain all essential amino acids ▪ Most are from animal products (eggs, milk, meat, poultry, and fish) ▪ Essential amino acids: those that the body cannot make and must be obtained through diet ( PVTMATHILL ) o Legumes and beans also have proteins, but the proteins are incomplete
Vitamins o Most vitamins function as coenzymes o Found mainly in fruits and vegetables
Minerals o Mainly important for enzyme activity

Excess dietary fat is stored in subcutaneous tissue and other fat depots
When carbohydrates are in limited supply, more fats are oxidized to produce ATP o Excessive fat breakdown causes blood to become acidic (acidosis or ketoacidosis) ▪ Breath has a fruity odor ▪ Common with: ➢ No carbohydrate‖ diets ➢ Uncontrolled diabetes mellitus ➢ Starvation Protein Metabolism
Proteins form the bulk of cell structure and most functional molecules
Proteins are carefully conserved by body cells
Amino acids are actively taken up from blood by body cells
Amino acids are oxidized to form ATP mainly when other fuel sources are not available
Ammonia, released as amino acids are catabolized, is detoxified by liver cells that combine it with carbon dioxide to form urea The Central Role of the Liver in Metabolism
- Liver is the body’s key metabolic organ
- Roles in digestion: o Manufactures bile o Detoxifies drugs and alcohol o Degrades hormones o Produces cholesterol, blood proteins (albumin and clotting proteins) o Plays a central role in metabolism
Liver can regenerate if part of it is damaged or removed
To maintain homeostasis of blood glucose levels, the liver performs: o Glycogenesis ―glycogen formation‖ ▪ Glucose molecules are converted to glycogen and stored in the liver o Glycogenolysis ―glycogen splitting ▪ Glucose is released from the liver after conversion from glycogen o Gluconeogenesis ―formation of new sugar ▪ Glucose is produced from fats and proteins
Fats and fatty acids are picked up by the liver o Some are oxidized to provide energy for liver cells o The rest are either stored or broken down into simpler compounds and released into the blood
Blood proteins made by the liver are assembled from amino acids o Albumin is the most abundant protein in blood o Clotting proteins
Liver cells detoxify ammonia o Ammonia is combined with carbon dioxide to form urea, which is flushed from the body in urine

Cholesterol metabolism and transport o Cholesterol is not used to make ATP o Functions of cholesterol: ▪ Structural basis of steroid hormones and vitamin D ▪ Building block of plasma membranes o Most cholesterol (85%) is produced in the liver; only 15% is from the diet
Cholesterol and fatty acids cannot freely circulate in the bloodstream
They are transported by lipoproteins (lipid- protein complexes) known as LDLs and HDLs
Low-density lipoproteins (LDLs) transport cholesterol to body cells o Rated ―bad lipoproteins‖ since they can lead to atherosclerosis
High-density lipoproteins (HDLs) transport cholesterol from body cells to the liver o Rated ―good lipoproteins‖ since cholesterol is destined for breakdown and elimination Body Energy Balance
Energy intake = Total energy output (heat + work + energy storage) o Energy intake is the energy liberated during food oxidation ▪ Energy produced during glycolysis, citric acid cycle, and the electron transport chain o Energy output ▪ Energy we lose as heat (60%) ▪ Energy stored as fat or glycogen
Interference with the body’s energy balance leads to: o Obesity o Malnutrition (leading to body wasting)
Regulation of food intake o Body weight is usually relatively stable ▪ Energy intake and output remain about equal o Mechanisms that may regulate food intake ▪ Levels of nutrients in the blood ▪ Hormones ▪ Body temperature ▪ Psychological factors
Metabolic rate and body heat production o Nutrients yield different amounts of energy o Energy value is measured in kilocalories (kcal) ▪ Carbohydrates and proteins yield 4 kcal/gram ▪ Fats yield 9 kcal/gram
Basic metabolic rate (BMR) —amount of heat produced by the body per unit of time at rest
Average BMR is about 60 to 72 kcal/hour for an average 70-kg (154-lb) adult
Factors that influence BMR o Surface area —a small body usually has a higher BMR o Gender —males tend to have higher BMRs o Age —children and adolescents have higher BMRs o The amount of thyroxine produced is the most important control factor ▪ More thyroxine means a higher metabolic rate
Total metabolic rate (TMR) —total amount of kilocalories the body must consume to fuel ongoing activities
TMR increases dramatically with an increase in muscle activity
TMR must equal calories consumed to maintain homeostasis and maintain a constant weight
Body temperature regulation o When foods are oxidized, more than 60% of energy escapes as heat, warming the body o The body has a narrow range of homeostatic temperature ▪ Must remain between 35.6ºC and 37.8ºC ▪ (96ºF and 100ºF)
The body’s thermostat is in the hypothalamus
Hypothalamus initiates mechanisms to maintain body temperature o Heat loss mechanisms involve radiation of heat from skin and evaporation of sweat o Heat-promoting mechanisms involve vasoconstriction of skin blood vessels and shivering

Digestive System Study Notes, Study notes of Anatomy

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