ANIMAL DIGESTIVE SYSTEM

Nutrition

The process by which an organism takes in, digests, absorbs, and utilizes food substances to obtain chemical energy and organic building blocks.

Herbivores

Animals that mainly consume autotrophs, such as plants and algae, to obtain their nutrition.

Omnivores

Animals that regularly consume both plant matter and other animals as part of their diet.

Carnivores

Animals that mainly consume other animals for their nutritional needs.

Essential Nutrients

Chemical substances that an animal requires but cannot synthesize on its own, meaning they must be obtained preformed from the diet.

Essential Amino Acids

The subset of amino acids that an animal cannot synthesize from other raw materials and must absorb directly from food (8 essential for adult humans).

Essential Fatty Acids

Specific unsaturated fatty acids, such as linoleic acid, that animals cannot synthesize themselves and must obtain from their diet to maintain cell membrane and signaling functions.

Vitamins

Organic molecules required in the diet in very small amounts, acting primarily as coenzymes or regulators in metabolic pathways.

Vitamin B1 (Thiamine)

Function: Coenzyme used in removing CO² from organic compounds (carbohydrate metabolism).

Sources: Pork, legumes, whole grains.

Deficiency: Beriberi (nervous system disorders, muscle weakness, heart failure).

Vitamin B2 (Riboflavin)

Function: Component of coenzymes FAD and FMN used in cellular respiration.

Sources: Dairy products, meats, enriched grains.

Deficiency: Skin lesions such as cracks at the corners of the mouth (cheilosis).

Vitamin B3 (Niacin)

Function: Component of coenzymes NAD^+ and NADP^+ used in redox reactions.

Sources: Nuts, meats, grains.

Deficiency: Pellagra (characterized by the 4 Ds: Dermatitis, Diarrhea, Dementia, Death).

Vitamin B5 (Pantothenic Acid)

Function: Component of Coenzyme A (CoA) critical for cellular respiration and lipid synthesis.

Sources: Meats, dairy, whole grains.

Deficiency: Fatigue, numbness, tingling of hands and feet ("burning feet syndrome").

Vitamin B6 (Pyridoxine)

Function: Coenzyme used in amino acid metabolism.

Sources: Meats, vegetables, whole grains.

Deficiency: Irritability, convulsions, muscular twitching, anemia.

Vitamin B7 (Biotin)

Function: Coenzyme in synthesis of fat, glycogen, and amino acids.

Sources: Legumes, vegetables, meat.

Deficiency: Scaly skin inflammation, neuromuscular disorders.

Vitamin B9 (Folic Acid/Folate)

Function: Coenzyme in nucleic acid and amino acid metabolism; critical for fetal neural tube development.

Sources: Green vegetables, oranges, legumes.

Deficiency: Anemia, birth defects (Neural Tube Defects like Spina Bifida).

Vitamin B12 (Cobalamin)

Function: Coenzyme in nucleic acid metabolism; necessary for maturation of red blood cells.

Sources: Meats, eggs, dairy (absent in plant foods).

Deficiency: Pernicious anemia, neurological disorders (degeneration of myelin sheets).

Vitamin C (Ascorbic Acid)

Function: Antioxidant; essential for collagen synthesis (bone, cartilage, teeth matrix).

Sources: Citrus fruits, broccoli, tomatoes.

Deficiency: Scurvy (bleeding gums, weak blood vessels, delayed wound healing).

Vitamin A (Retinol)

Function: Component of visual pigments (rhodopsin); maintains epithelial tissues.

Sources: Dark green and orange vegetables (as beta-carotene), dairy.

Deficiency: Blindness and night blindness (nyctalopia), skin disorders.

Vitamin D (Calciferol)

Function: Aids in absorption and use of calcium and phosphorus for bone growth.

Sources: Dairy, egg yolk (also synthesized in skin via UV light).

Deficiency: Rickets in children (soft bones/bowed legs), osteomalacia in adults.

Vitamin E (Tocopherol)

Function: Antioxidant; prevents damage to cell membranes by neutralizing free radicals.

Sources: Vegetable oils, nuts, seeds.

Deficiency: Nervous system degeneration, hemolytic anemia (red blood cell rupture).

Vitamin K (Phylloquinone)

Function: Essential for blood clotting (required for prothrombin synthesis in liver).

Sources: Green vegetables, tea (also synthesized by colon bacteria).

Deficiency: Defective blood clotting, prolonged bleeding.

Calcium (Ca)

Function: Bone and teeth formation, blood clotting, nerve function, muscle contraction.

Sources: Dairy, dark green vegetables.

Deficiency: Impaired growth, loss of bone mass (osteoporosis).

Phosphorus (P)

Function: Bone and teeth formation, acid-base balance, component of ATP and nucleic acids.

Sources: Dairy, meats, grains.

Deficiency: Weakness, demineralization of bone.

Sulfur (S)

Function: Component of certain amino acids (methionine, cysteine), essential for protein structural folding.

Sources: Proteins from meats, eggs, legumes.

Deficiency: Fatigue, impaired growth (rare, occurs only during severe protein malnutrition).

Potassium (K)

Function: Main intracellular cation; acid-base balance, water regulation, nerve function.

Sources: Meats, dairy, fruits and vegetables (bananas).

Deficiency: Muscular weakness, paralysis, nausea, heart failure.

Chlorine (CI)

Function: Main extracellular anion; acid-base balance, gastric juice formation (HCl).

Sources: Table salt.

Deficiency: Muscle cramps, reduced appetite.

Sodium (Na)

Function: Main extracellular cation; acid-base balance, water regulation, nerve impulse conduction.

Sources: Table salt.

Deficiency: Muscle cramps, reduced appetite.

Magnesium (Mg)

Function: Enzyme cofactor; structural component of bone.

Sources: Whole grains, green leafy vegetables.

Deficiency: Nervous system disturbances.

Iron (Fe)

Function: Component of hemoglobin and electron carriers (cytochromes) in cellular respiration.

Sources: Meats, eggs, legumes, green leafy vegetables.

Deficiency: Iron-deficiency anemia, weakness, impaired immunity.

Fluorine (F)

Function: Maintenance of tooth structure and bone density.

Sources: Fluoridated drinking water, seafood.

Deficiency: Increased frequency of tooth decay.

Iodine (I)

Function: Component of thyroid hormones (thyroxine and triiodothyronine).

Sources: Seafood, iodized salt.

Deficiency: Goiter (enlargement of the thyroid gland).

Malnutrition

A failure to obtain adequate nutrition, resulting from a diet that lacks one or more essential nutrients or supplies too much energy.

Undernourishment

A condition resulting from a diet that fails to provide sufficient chemical energy, forcing the body to break down its own molecules (glycogen, fat, then proteins) for fuel.

Epidemiology

The study of the distribution and determinants of health and disease states in human populations.

Ingestion

The first stage of food processing, which involves the act of eating or feeding.

Digestion

The second stage of food processing, involving the breakdown of food into molecules small enough for the body to absorb.

Absorption

The third stage of food processing, during which the animal's cells take up (absorb) small molecules such as amino acids and simple sugars.

Elimination

The fourth and final stage of food processing, which involves the passing of undigested material out of the digestive system.

Filter Feeding

A feeding mechanism where an animal strains small organisms or food particles from the surrounding aquatic medium.

Bulk Feeding

A feeding mechanism where an animal eats relatively large pieces of food all at once.

Substrate Feeding

A feeding mechanism where an animal lives in or on its food source and eats its way through it.

Fluid Feeding

A feeding mechanism where an animal sucks nutrient-rich fluid from a living host.

Intracellular Digestion

The hydrolysis of food inside vacuoles after a cell brings in food via phagocytosis or pinocytosis.

Extracellular Digestion

The breakdown of food in compartments that are continuous with the outside of the animal's body.

Gastrovascular Cavity

A digestive pouch with a single opening that functions in both ingestion and elimination (found in simpler animals like hydras and flatworms).

Complete Digestive Tract (Alimentary Canal)

A digestive tube extending between two separate openings, a mouth and an anus, moving food in a single direction.

Salivary Glands

Accessory glands that deliver saliva into the oral cavity to lubricate food and initiate chemical digestion.

The pancreas

An accessory gland that produces an alkaline secretion containing digestive enzymes and bicarbonate ions, delivered into the small intestine.

The liver

A multifunctional organ that produces bile, handles nutrient conversion, and processes toxins absorbed from the digestive tract.

The gallbladder

An organ that stores and concentrates bile produced by the liver until it is needed in the small intestine.

Oral Cavity

The mouth, where mechanical digestion begins via teeth and chemical digestion of carbohydrates starts via saliva.

Esophagus

A muscular tube that conducts food from the pharynx down to the stomach using peristaltic movements.

Stomach

A muscular, stretchable organ that stores food and performs major mechanical churning and preliminary protein digestion.

Small Intestine

The longest section of the alimentary canal, where the vast majority of enzymatic hydrolysis and nutrient absorption occurs.

Pharynx

The throat region that opens into both the esophagus (leads to stomach) and the trachea (leads to lungs).

Large Intestine

The terminal portion of the alimentary canal (including the colon, cecum, and rectum) primarily functioning in water reabsorption and feces formation.

Rectum

The terminal portion of the large intestine where feces are stored prior to elimination.

Anus

The opening at the end of the alimentary canal through which solid waste matter is eliminated from the body.

Trachea

The windpipe; the respiratory tube that conducts air to the lungs.

Contracted Sphincter

A ring-like valve of muscle that stays tightly shut to regulate the passage of material between different compartments of the alimentary canal.

Bolus

A lubricated, ball-like mass of chewed food mixed with saliva, shaped by the tongue before swallowing.

Larynx

The upper part of the respiratory tract containing the vocal cords, which moves upward during swallowing to allow the epiglottis to tip over the glottis.

Esophageal Sphincter

A muscular ring at the top of the esophagus that relaxes during swallowing to let a bolus pass from the pharynx.

Peristaltic Contractions

Rhythmic waves of contraction and relaxation by smooth muscles in the wall of the alimentary canal that push food forward.

Glottis

The physical opening or vocal apparatus of the larynx leading into the trachea.

Mucus

A viscous, slippery mixture of glycoproteins, cells, salts, and water that protects lining cells and lubricates paths.

Amylase

An enzyme that breaks down starch and glycogen into smaller polysaccharides or maltose.

Gastric Juice

A highly acidic digestive fluid secreted by the stomach glands, composed of hydrochloric acid (HCl) and pepsin.

Chyme

The nutrient-rich, semi-liquid mixture of partially digested food and gastric juice formed in the stomach.

Protease

A general term for any enzyme that hydrolyzes peptide bonds, breaking down proteins into smaller peptides or amino acids.

Parietal Cells

Cells in the gastric glands that use ATP pumps to secrete hydrogen and chloride ions separately into the stomach lumen to form HCl.

Chief Cells

Cells in the gastric glands that secrete inactive pepsinogen into the stomach lumen.

Mucous Cells

Cells in the gastric glands that secrete mucus to protect the stomach lining from self-digestion by acid and pepsin.

Gastric Ulcers

Damaged lesions or open sores that develop in the protective lining of the stomach wall.

Helicobacter Pylori

An acid-tolerant bacterium that causes inflammation of the stomach lining, leading to the formation of most gastric ulcers.

Churning

The mechanical mixing action of the stomach brought about by coordinated contractions of its smooth muscle layers every 20 seconds.

Heartburn

A painful, burning sensation felt in the chest caused by the backflow (reflux) of acidic chyme past the lower esophageal sphincter into the esophagus.

Reverse Peristalsis

The backward movement of smooth muscle contractions in the alimentary canal, leading to vomiting (emesis).

Digestion of Carbohydrates

Oral Cavity: Polysaccharides are broken down into smaller polysaccharides by Salivary Amylase (Salivary Glands).

Small Intestine (Lumen): Remaining polysaccharides are broken down into maltose and disaccharides by Pancreatic Amylase (Pancreas).

Small Intestine (Epithelium/Brush Border): Disaccharides are cleaved into monosaccharides by Maltase, Lactase, Sucrase, and Dextrinase (Small Intestine Epithelium).

Digestion of Proteins

Stomach: Proteins are broken down into small polypeptides by Pepsin (Chief Cells/Stomach as Pepsinogen) activated by HCl.

Small Intestine (Lumen):

Inactive Trypsinogen and Chymotrypsinogen (Pancreas) enter the lumen. Enterokinase (Small Intestine Epithelium) activates trypsinogen into active Trypsin, which then activates Chymotrypsin. Trypsin and chymotrypsin cleave polypeptides into even smaller peptides.

Pancreatic Carboxypeptidase (Pancreas) cleaves amino acids one by one from the carboxyl (COOH) end.

Small Intestine (Epithelium/Brush Border): Aminopeptidase (Small Intestine Epithelium) cleaves amino acids from the amino (NH_2) end; Dipeptidase (Small Intestine Epitestine Epithelium) splits dipeptides into single amino acids.

Digestion of Nucleic Acids

Small Intestine (Lumen): DNA and RNA are broken down into nucleotides by Pancreatic Nucleases (Pancreas).

Small Intestine (Epithelium/Brush Border):

Nucleotides are broken down into nucleosides by Nucleotidases (Small Intestine Epithelium).

Nucleosides are broken down into nitrogenous bases, sugars, and phosphates by Nucleosidases and Phosphatases (Small Intestine Epithelium).

Digestion of Fats

Small Intestine (Lumen): Large fat globules are physically emulsified into small droplets by Bile Salts (Liver, stored in Gallbladder). These emulsified droplets are chemically broken down into fatty acids and monoglycerides by Pancreatic Lipase (Pancreas).

Common Bile Duct (Choledochal Duct)

The anatomical duct formed by the union of the common hepatic duct and the cystic duct, responsible for carrying bile from the liver and gallbladder into the duodenum.

Main Pancreatic Duct (Wirsung Duct)

The primary duct that traverses the pancreas from tail to head, carrying enzyme-rich pancreatic juices to join the common bile duct at the duodenum.

Ampulla of Vater (Hepatopancreatic Ampulla)

The dilation or chamber formed by the confluence of the common bile duct and the main pancreatic duct just before they enter the duodenum.

Gastrin

Site of Secretion: G cells in the stomach lining (gastric mucosa) in response to the arrival of food.

Target Tissue: Parietal and chief cells of the stomach wall.

Action: Stimulates the secretion of gastric juice (hydrochloric acid and pepsinogen) and promotes gastric motility.

Cholecystokinin (CCK)

Site of Secretion: Duodenum (lining of the small intestine) in response to the presence of amino acids and fatty acids in chyme.

Target Tissue: Pancreas and Gallbladder.

Action: Stimulates the gallbladder to contract and release bile, triggers the pancreas to release digestive enzymes, and slows down stomach churning to optimize digestion.

Secretin

Site of Secretion: Duodenum (lining of the small intestine) in response to low pH (acidic chyme entering from the stomach).

Target Tissue: Pancreas and Stomach.

Action: Stimulates the pancreas to release bicarbonate (HCO_3^-) to neutralize the acidic chyme, and inhibits gastric juice secretion and motility.

Villi, Microvilli, and Brush Border

Villi are large finger-like projections of the small intestine wall, and microvilli are microscopic cellular projections on individual epithelial cells. Together, they form the brush border, which massively increases the surface area for nutrient absorption.

Hepatic Portal Vein

A major blood vessel that carries nutrient-rich blood directly from the capillaries of the villi to the liver for nutrient processing and detoxification before blood enters general circulation.

Chylomicrons

Small, water-soluble globules formed inside intestinal epithelial cells by combining triglycerides with phospholipids, cholesterol, and proteins, allowing fats to be transported out of the cell.

Lacteal

A specialized microscopic lymphatic capillary located at the center of each intestinal villus, responsible for absorbing chylomicrons.

Colon

The longest section of the large intestine, primarily responsible for reabsorbing water and ions from indigested material and consolidating waster.

Cecum

A blind pouch at the junction of the small and large intestines, structurally important for fermenting ingested plant matter (much larger in herbivores).

Rectum

The terminal segment of the large intestine where compressed feces are stored before being expelled through the anus.

Appendix

A small, finger-like extension of the cecum that serves as a reservoir for symbiotic gut bacteria and contributes to structural immunity.

Feces

The semi-solid waste material produced in the large intestine, consisting of undigested food matter, cellulose, bacteria, and bile pigments.

Incisors

The chisel-shaped front teeth adapted for cutting, nipping, or biting off pieces of food.

Canines

The pointed, cone-shaped teeth situated next to the incisors, structurally specialized for tearing and piercing flesh.

Premolars

Transitional teeth located between the canines and molars, featuring flat surfaces with cusps for crushing and grinding food.