The digestive system converts food to nutrients which the body uses for growth, energy, and cell repair.

SPECIAL CELLS/TISSUES:Intestinal cells: in small intestine secrete, hydrolytic enzymes (break down macromolecules like carbohydrates); Villi/microvilli: increase the surface area of the small intestine (absorbs more nutrients); Mucosa: surrounds the lumen, single layer of epithelial cells (secrete mucus and enzymes—protect organs and break down food); Submucosa tissue layer: blood and lymph vessels, carry absorbed nutrients to the rest of the body; Circular muscle layer: surrounds the submucosa, constricts the gut; Longitudinal muscle layer: surrounds the submucosa, shortens the gut; Sphincter muscles: help the passage of contents through the gut (in the stomach, small and large intestines, and anus); Peritoneum: surrounds the gut, secretes a fluid (lubricates organs so they easily slide against each other); Epiglottis tissues: moves down to cover the entrance to the larynx when food is swallowed; Chief cells (secrete pepsinogen/inactive pepsin) and parietal cells (secrete hydrochloric acid which kills microorganisms, helps decompose food, and activates the pepsinogen to pepsin) are in the gastric pit.

HOW IT WORKS:A mouth takes in food; saliva begins breaking down the food. The food moves through the esophagus to a storage sac, usually called a stomach. The stomach physically (and sometimes enzymatically) breaks down the food, and produces a slurry of small particles to the midgut, ready for digestion. Food travels through the midgut, often called a small intestine (the primary site of digestion and absorption). Here, hydrolytic enzymes are secreted by intestinal cells and associated glands, and macromolecules are broken down into monomers that are absorbed into the blood. The hindgut (large intestine), recovers water and ions and stores undigested wastes, or feces, until they can be expelled through what is usually called the anus.

REGULATION AND FEEDBACK:The enteric nervous system of the digestive system communicates some information to and from the CNS, and coordinates the movement of food through the gut. When food arrives in the stomach, the wall of the stomach secretes the hormone gastrin which circulates in the blood and returns to the stomach, causing an increase in motility and the secretion of digestive juices. When acidic chyme arrives in the duodenum, cells start to release secretin which stimulates the pancreas to produce and secrete bicarbonate ions that neutralize the acidic chyme. In the postabsorptive state, a lack of insulin protects glucose supplies

ORGAN FUNCTIONMouth Chewing; Breaks down starches with saliva;

Moves food to esophagus (through pharynx)Esophagus Swallowing; Transports food to stomachStomach Upper muscle relaxes and lets food enter;

lower muscles mix food with digestive juices; breaks down protein with stomach acid;

Absorbs some fat-soluble substances; releases food into duodenum (beginning of

small intestine) as chymeSmall

intestinePropels food at a slow enough rate for

digestion and absorption; absorbs breakdown products of carbohydrates,

proteins, lipids, and nucleic acids, along with vitamins, minerals, and water

Liver Breaks down fats with bile acids Gallbladder Stores, concentrates, and releases bile

Pancreas Produces digestive enzymes and bicarbonate; breaks down starches, fats, and proteins with

pancreatic juiceLarge

intestineFurther breaks down food residues; absorbs water; remaining waste material is stored as

fecesColon Part of large intestine; reabsorbs fluids;

prepares waste for its elimination

Anus Opening at lower end of digestive tract; controls expulsion of feces

for the nervous system. During the absorptive state, the liver stores fuel as glycogen and fats and synthesizes blood plasma proteins from circulating amino acids. When fuel molecule levels in the blood decline, the liver delivers the stored nutrients into the blood. High-density lipoproteins remove cholesterol from tissues and carry it to the liver, where it is used to synthesize bile. Low-density lipoproteins transport cholesterol around the body for use in biosynthesis and for storage. Very low-density lipoproteins contain mostly triglyceride fats, which they transport to fat cells. Hunger and satiety centers in the hypothalamus are powerfully affected by several digestive hormones, including insulin, CCK, and leptin. Gastrin release begins to be inhibited when the pH of the stomach contents falls below 3—negative feedback. Both secretin and CCK are negative feedback signals to the stomach to slow the delivery of chyme into the duodenum.

INTERACTIONS:Integumentary system: Vitamin D activated in the skin plays a role in absorption of calcium from the digestive tract; Skeletal System: Bones are important in mastication (chewing); Muscular System: Muscles are important in mastication, swallowing, and the mixing and moving of digestion products through the gastrointestinal tract; Nervous System: Regulates the speed at

which food moves through the digestive tract; Endocrine System: Regulates digestive function by secreting hormones (chemical messengers); Cardiovascular System: The bloodstream carries absorbed nutrients to all body cells; Lymphatic System: The lymphatic system plays a major role in the absorption of fats; Respiratory System: The digestive system and the respiratory system share common anatomical

structures; Urinary System: The kidneys and liver work together to activate vitamin D; Reproductive System: Adequate availability of nutrients, including fats, is essential for conception and normal development.

Human Amphibians Fish Reptiles BirdsMajority of nutrient absorption in jejunum

Shorter small intestine than humans, most nutrient absorption in ileum

Most nutrient absorption occurs in the intestine

Digestion is slower than in mammals; most nutrient absorption occurs in the intestine

High metabolic rates; nutrient absorption is mainly in the lower small intestine

Use teeth for all things including chewing; one set of teeth

Teeth used to hold things in place; Swallow prey whole; maxillary and vomerine teeth

Fish without teeth swallow food whole; fish with pharyngeal teeth crush food; fish with sharp pharyngeal teeth shred food

Teeth are attached by one side of the inner surface of the jaw bone; used to tear, bite, crack, and/or puncture food

No teeth; Use beaks; Swallow stones to grind food; also have gizzard to grind food

Rectum and urethra are present

Only have cloacae Food is excreted through the vent/anus

Food is excreted through the cloacae

Food is excreted through the cloacae

Tongues attached to backs of mouths; gustatory and taste receptors

Tongues connected to start of mouth; sticky end to assist prey capture

Tongue is a fold in the floor of the mouth; conducts water through mouth

Many have a forked tongue; use to smell and sense

Many have harder tongues than humans; tongue function varies based on food source