35.1 - The Digestive System

Digestion 

Digestion: is the process of breaking down food into molecules the body can use, the absorption of nutrients, & the elimination of waste. Carbohydrates are broken down to

monosaccharides (example: glucose) Proteins are broken down to amino acids Lipids are broken down to fatty acids and glycerol 

2 types of digestion Mechanical digestion: breaking chunks of food

into smaller pieces Chemical digestion: changing food into the subunits listed above by the action of enzymes

The Gastrointestinal Tract 

The gastrointestinal tract, or digestive tract, is a long, winding tube which begins at the mouth and winds through the body to the anus.

The Mouth  Mechanical

mouth:

digestion begins in the

Incisors—sharp front teeth—cut the food Molars—broad, flat back teeth—grind up

food The tongue manipulates the food against the hard palate  Chemical

digestion

Salivary glands produce saliva, a mixture of

water, mucus, and a digestive enzyme called salivary amylase. ○ Salivary amylase begins the chemical

digestion of some starch into disaccharides.

The Esophagus 

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Bolus (moistened ball of chewed up food) is forced into the pharynx. The epiglottis prevents food from entering the trachea during swallowing, and passes bolus into the esophagus The esophagus is a muscular tube that connect pharynx to the stomach. The series of rhythmic contractions & relaxations is called peristalsis. Peristalsis pushes the bolus through the esophagus into the stomach.

The stomach 

Mechanical digestion The walls of the stomach have several

muscle layers. These muscles work together to churn the food, breaking up food particles and mixing them with gastric fluid. This results in a mixture called chyme. Chyme moves from the stomach to the small intestine. Gastric glands release gastric juice, a mixture of mucus, digestive enzymes, and hydrochloric acid.

Chemical digestion in the stomach 

Gastric fluid carries out chemical digestion in the stomach Hydrochloric acid ensures a low pH in the

stomach, dissolves minerals and kills bacteria An enzyme called pepsin begins the break down of proteins into peptide chains (chains of amino acids). Pepsin functions in a pH of about 2. Mucus coats the stomach and protects it from HCl & digestive enzymes ○ When the mucus coating breaks down allowing

digestive enzymes to eat through part of the stomach lining, the result is called an ulcer.

The liver, gallbladder, and pancreas 

The liver, gallbladder, and pancreas are involved in digestion but food does not pass through them.

Liver  The

liver performs several functions

Stores glucose as glycogen Makes proteins Breaks down toxic substances such as

alcohol Produces bile

○ Bile breaks fat globules into small droplets

(mechanical digestion), forming a milky fluid in which fats are suspended. This exposes a greater surface area of fats to the action of digestive enzymes

Gallbladder Bile is concentrated and stored in the gallbladder  When chyme is present in the small intestine, the gallbladder releases bile into the small intestine  If bile becomes too concentrated due to high levels of cholesterol in the diet, or if the gallbladder becomes inflamed, gallstones can form 

Pancreas The pancreas produces pancreatic fluidand releases it into the small intestine. Pancreatic fluid contains: 2. sodium bicarbonate, which neutralizes stomach acids (pancreatic fluid raises the pH of chyme from an acid to a base)  Neutralizing stomach acids helps protect the

lining of the small intestine & ensures that enzymes secreted by the pancreas can function

3.

Enzymes that break down  disaccharides into monosaccharides  Fats into fatty acids and glycerol  Proteins into amino acids

Small Intestine The small intestine is a long, narrow tube that extends about 7 m. The first section (25 cm) of the small intestine is called the duodenum  Mucus protects the intestinal wall from protein digesting enzymes and acidic chyme 

Secretions from the liver and pancreas enter

the duodenum where chemical digestion of chyme is completed: ○ Peptides into amino acids ○ Disaccharides into monosaccharides ○ Fats into fatty acids and glycerol

Absorption  Absorption

is the process of transferring the end products of digestion (amino acids, monosaccharides, and fatty acids) into the circulatory system and lymph vessels in the lining of the small intestine  The highly folded lining of the small intestine is covered with million of fingerlike projections called villi,

Absorption through villi 

Nutrients are absorbed through this surface by means of diffusion and active transport  Fatty acids and

glycerol enter the lymph vessels & are eventually transferred to the bloodstream  Amino acids and monosaccharides enter the capillaries in the villi and are carried to the liver

Large intestine 

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After absorption is complete in the small intestine, peristalsis move the remaining material on to the large intestine. The large intestine, or colon, is the final organ of digestion  Water and salts are absorbed leaving behind a

more solid material  Large intestine secretes mucus to lubricate the intestinal wall. This lubrication makes the passing of feces less abrasive. Mucus also binds together the fecal matter.  Anaerobic bacteria in the large intestine synthesizes some B vitamins and vitamin K  These bacteria stop harmful bacteria from colonizing

The Rectum  The

rectum is the last part of the digestive system  Feces are eliminated from the rectum through the anus.  The meal’s entire journey through the digestive tract takes between 24-33 hours.