Chap 24.1 Functions of the Digestive System

Describe each of the functions involved in the normal functions of the digestive system.

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Large intestine

Absorption. The proximal half of the colon absorbs salts (e.g., sodium chloride), water, and vitamins (e.g., K) produced by bacteria. Storage. The distal half of the colon holds feces until they are eliminated. Mixing and propulsion. Slight segmental mixing occurs. Mass movements propel feces toward the anus, and defecation eliminates the feces. Protection. Mucus provides lubrication; mucus and bicarbonate ions protect against acids produced by bacteria.

Secretion.

As food moves through the digestive tract, secretions are added to lubricate, liquefy, buffer, and digest the food. Mucus, secreted along the entire digestive tract, lubricates the food and the lining of the tract. The mucus coats and protects the epithelial cells of the digestive tract from mechanical abrasion, stomach acid, and digestive enzymes. The secretions also contain large amounts of water, which liquefies the food, making it easier to digest and absorb. Water also moves into the intestine by osmosis. Liver secretions break large lipid droplets into much smaller droplets, which makes the digestion and absorption of lipids possible. Enzymes secreted by the oral cavity, stomach, small intestine, and pancreas break down large food molecules into smaller molecules that can be absorbed by the intestinal wall.

Oral cavity

Ingestion. Solid food and fluids are taken into the digestive tract through the oral cavity. Taste. Tastants dissolved in saliva stimulate taste buds in the tongue. Mastication. Movement of the mandible by the muscles of mastication causes the teeth to break food into smaller pieces. The tongue and cheeks help place the food between the teeth. Digestion. Amylase in saliva begins carbohydrate (starch) digestion. Swallowing. The tongue forms food into a bolus and pushes the bolus into the pharynx. Communication. The lips, cheeks, teeth, and tongue are involved in speech. The lips change shape as part of facial expressions. Protection. Mucin and water in saliva provide lubrication, and lysozyme (an enzyme that lyses cells) kills microorganisms. Nonkeratinized stratified squamous epithelium prevents abrasion.

Small intestine

Neutralization. Bicarbonate ions from the pancreas and bile from the liver neutralize stomach acid to form a pH environment suitable for pancreatic and intestinal enzymes. Digestion. Enzymes from the pancreas and the lining of the small intestine complete the breakdown of food molecules. Bile salts from the liver emulsify fats. Absorption. The circular folds, villi, and microvilli increase surface area. Most nutrients are actively or passively absorbed. Most of the ingested water or the water in digestive tract secretions is absorbed. Mixing and propulsion. Segmental contractions mix the chyme, and peristaltic contractions move the chyme into the large intestine. Excretion. Bile from the liver contains bilirubin and excess cholesterol. Protection. Mucus provides lubrication, prevents digestion of the intestinal wall, and protects the small intestine from stomach acid. Peyer patches protect against microorganisms.

Esophagus

Propulsion. Peristaltic contractions move the bolus from the pharynx to the stomach. The lower esophageal sphincter limits reflux of the stomach contents into the esophagus. Protection. Glands produce mucus, which provides lubrication and protects the inferior esophagus from stomach acid.

Mixing

Some contractions do not propel food from one end of the digestive tract to the other but, rather, move it back and forth within the digestive tract to mix it with digestive secretions and help break it into smaller pieces. Segmental contractions are mixing contractions that occur in the small intestine

Stomach

Storage. Rugae allow the stomach to expand and hold food until it can be digested. Digestion. Protein digestion begins as a result of the actions of hydrochloric acid and pepsin. Absorption. Absorption of a few substances (e.g., water, alcohol, aspirin) takes place in the stomach. Mixing and propulsion. Mixing waves churn ingested materials and stomach secretions into chyme. Peristaltic waves move the chyme into the small intestine. Protection. Mucus provides lubrication and prevents digestion of the stomach wall. Stomach acid kills most microorganisms.

Pharynx

Swallowing. The involuntary phase of swallowing moves the bolus from the oral cavity to the esophagus. Materials are prevented from entering the nasal cavity by the soft palate and kept out of the lower respiratory tract by the epiglottis and vestibular folds. Breathing. Air passes from the nasal or oral cavity through the pharynx to the lower respiratory tract. Protection. Mucus provides lubrication. Nonkeratinized stratified squamous epithelium prevents abrasion.

List the regions of the digestive tract, from beginning to end.

The regions of the digestive tract include the following: (1)Oral cavity, or mouth, with the salivary glands and (2)tonsils as accessory organs (3)Pharynx, or throat (4)Esophagus (5)Stomach (6)Small intestine, consisting of the duodenum, jejunum, and ileum, with the liver, gallbladder, and pancreas as major accessory organs 7)Large intestine, including the cecum, colon, rectum, and anal canal (8)Anus

(Propulsion) Mass movements

are contractions that move material in some parts of the large intestine. Mass movements extend over much larger parts of the digestive tract than peristaltic movements.

mechanical digestion

involves the mastication and mixing of food

chemical digestion

is accomplished by digestive enzymes secreted along the digestive tract. Large organic molecules must be digested into their component parts before they can be absorbed by the digestive tract. Minerals and water are not broken down before being absorbed. Vitamins are also absorbed without digestion; in fact, they lose their function if their structure is altered by digestion.

Digestion

is the breakdown of large organic molecules into their component parts: carbohydrates into monosaccharides, proteins into amino acids, and triglycerides into fatty acids and glycerol.

Ingestion

is the intake of solid or liquid food into the stomach. The normal route of ingestion is through the oral cavity.

Propulsion

is the movement of food from one end of the digestive tract to the other. The total time it takes food to travel the length of the digestive tract is usually about 24-36 hours. Each segment of the digestive tract is specialized to assist in moving its contents from the oral end to the anal end:

Absorption

is the movement of molecules out of the digestive tract and into the circulation or into the lymphatic system. The mechanism by which absorption occurs depends on the type of molecule involved. Molecules pass out of the digestive tract by diffusion, facilitated diffusion, active transport, symport, or endocytosis

Mastication

is the process by which the teeth chew food in the mouth. Digestive enzymes cannot easily penetrate solid food particles and can work effectively only on particle surfaces. It is vital, therefore, that solid foods be mechanically broken down by mastication into smaller particles to increase the total surface area of food for digestion.

Elimination

is the process by which the waste products of digestion are removed from the body. During this process, which occurs primarily in the large intestine, water and salts are absorbed, changing the material in the digestive tract from liquefied to semi solid. These semi solid waste products, called feces, are then eliminated from the digestive tract by the process of defecation.

(Propulsion) Swallowing,

or deglutition (dē′gloo-tish′ŭn), moves a mass of food or a liquid, called a bolus (bō′lŭs), from the oral cavity into the esophagus.

(Propulsion) Peristalsis

propels material through most of the digestive tract. Peristaltic (per-i-stal′tik) waves are muscular contractions consisting of a wave of relaxation of the circular muscles in front of the bolus, followed by a wave of strong contraction of the circular muscles behind the bolus, which force the bolus along the digestive tube. Each peristaltic wave travels the length of the esophagus in about 10 seconds. Peristaltic waves in the small intestine usually travel only short distances.