Chapter 4

SECTION 4.8 Hormonal Regulation of Digestion

- Gastrin stimulates release of HCl and pepsinogen, and stimulates stomach and small intestinal motility.

Major Pancreatic and Intestinal Enzymes

- Pancreatic amylase Pancreas Digests starch - Pancreatic lipase Pancreas Digests fat into smaller components - Trypsin, chymotrypsin Pancreas Partially digest proteins - Carboxypeptidase Pancreas Breaks down certain partially digested proteins - Nuclease Pancreas Digests DNA and RNA into nucleotides - Peptidase Small intestine Breaks down certain partially digested proteins - Sucrase, maltase, lactase Small intestine Break down certain sugars into simpler sugars - Intestinal lipase Small intestine Breaks down fats into smaller components

SECTION 4.1 Overview of the Digestive System

- The primary roles of the digestive system are the breakdown of large food molecules into nutrients and the absorption of nutrients into the bloodstream. - The GI tract includes the mouth, esophagus, stomach, and small and large intestines. The other component of the digestive system, the accessory organs, assists the functioning of the GI tract.

SECTION 4.5 The Small Intestine

- The three sections of the small intestine are the duodenum, jejunum, and ileum. - In the small intestine, enzymes of the pancreas and those in the brush border of microvilli complete the process of digestion. Bile is made in the liver, stored in the gallbladder, and secreted into the duodenum, especially when fat enters the small intestine. - Nutrients are absorbed into the bloodstream or lymphatic system, depending on the nutrient. - It takes about 3 to 10 hours for chyme to move from the duodenum to the end of the ileum.

SECTION 4.6 The Large Intestine

- a minor amount of additional absorption, primarily some water and minerals, takes place in the large intestine, because its mucosa has no villi - any remaining undigested material, some water, and intestinal bacteria eventually exit the body as feces - unabsorbed water and undigested remains of chyme enter the cecum -- > ascending colon in the right side of the abdomen --> transverse colon --> descending colon --> sigmoid colon --> rectum --> anus

pancreas

- accessory organ of the digestive tract that produces and secretes many of the enzymes that break down carbohydrates, protein, and fat in the GI tract - secretes bicarbonate ions (HCO3-) - the pH of chyme is approximately 2.0 as it enters the duodenum. Pancreatic juice has an alkaline pH of approximately 8.5, so the juice neutralizes the highly acidic chyme when it enters the duodenum. This is a critical step in the digestion process, because the enzymes that function in the small intestine are not active in acidic conditions. - following bicarbonate secretion, the contents in the small intestine have a pH that is neutral or slightly alkaline—the pH at which the enzymes function best.

Crohn's disease

- autoimmune disorder - chronic condition in which the body's immune system does not function properly and begins to attack normal cells

SECTION 4.9 Common Digestive Tract Disorders

- chronic constipation may contribute to the development of diverticula and hemorrhoids. Diarrhea and vomiting can lead to dehydration - frequent chronic heartburn can be a symptom of GERD. People with GERD are at risk of esophageal cancer - irritable bowel syndrome does not result in inflammation of the large intestine. Inflammatory bowel disease includes Crohn's disease and ulcerative colitis - infection with H. pylori, smoking cigarettes, heavy consumption of alcohol, and regular use of non-steroidal anti-inflammatory drugs increase the risk of developing a peptic ulcer.

facilitated diffusion

- enterocytes absorb some nutrients by facilitated diffusion - does not require energy - nutrient moves down its concentration gradient, it still needs to be carried by a special transport protein within the membrane of the enterocyte - transport proteins are specific for the type of nutrient they are responsible for carrying. - absorption of the simple sugar fructose occurs by facilitated diffusion

duodenum

- first segment of the small intestine - most digestion and nutrient absorption occurs in the upper part of the small intestine, primarily in the duodenum and upper jejunum. - about 10 inches long - acidic stomach contents mix with alkaline fluids secreted by the pancreas and gallbladder

prebiotics

- food components, such as dietary fiber, that are not broken down by human digestive enzymes but are used as fuels by beneficial bacteria in the large intestine - in a wide variety of foods, particularly fruits, vegetables, and whole grains.

endocytosis

- in a few instances, a segment of an absorptive cell's plasma membrane surrounds and "swallows" relatively large substances, such as entire protein molecules. - this process enables an infant's intestinal tract to absorb whole proteins in human milk that provide immune benefits - not a common way for nutrients to enter enterocytes

ileum

- last segment of the small intestine - is about 5 to 9 feet long - some nutrient absorption takes place in the ileum

jejunum

- middle segment of the small intestine - most digestion and nutrient absorption occurs in the upper part of the small intestine, primarily in the duodenum and upper jejunum. - about 3 to 5.5 feet long

simple diffusion

- occurs when the concentration of a particular nutrient is higher in the lumen of the small intestine than in an enterocyte -the nutrient moves down its concentration gradient; that is, it moves from a location where it is more highly concentrated, such as in the lumen, to a place where it is less concentrated, such as inside the absorptive cells - does not require the input of energy - the digestive tract absorbs many water-soluble vitamins, lipids, and some minerals by simple diffusion

liver

- processes and stores many nutrients - synthesizes cholesterol and uses this type of lipid to make bile (greenish-yellow, bitter-tasting fluid that contains cholesterol, minerals, bile salts, and bilirubin) - bilirubin is a waste product that results from the breakdown of hemoglobin - makes about 2 to 4 cups (480 to 960 mL) of bile daily - bile flows from the liver into the gallbladder via ducts

segmentation

- regular contractions of circular intestinal muscles followed by muscular relaxations that mix chyme within a short portion of the small intestine - helps mix chyme

active transport

- some nutrients move from the lumen of the intestine and into an enterocyte against the concentration gradient; that is, the concentration of the nutrient is higher inside the absorptive cell than in the lumen - absorption of these nutrients requires both a unique transport protein and energy - enterocytes rely on active transport to absorb glucose and amino acids

Helicobacter pylori (H. pylori)

- type of bacteria that can live in parts of the stomach, was responsible for the development of most stomach ulcers - H. pylori infection makes the lining of the stomach more susceptible to being damaged by stomach acid

SECTION 4.7 Microbes in the Digestive Tract

- under normal conditions, the bacteria of the large intestine maintain a balance with each other that is beneficial for their human host

osmosis

- water from ingested foods and beverages, as well as the watery secretions of intestinal cells, enter the lumen of the GI tract - water moves freely across the cell membranes of enterocytes, because a concentration gradient is created by the absorption of water-soluble nutrients, such as sugars and minerals. - a healthy body absorbs most of the water that enters the GI tract

SECTION 4.2 The Mouth

Mechanical and chemical digestion begin in the mouth. Taste buds distinguish sweet, sour, salty, bitter, and umami tastes.

SECTION 4.3 The Esophagus

The esophagus conveys a bolus from the mouth to the stomach. The lower esophageal sphincter prevents reflux of stomach contents into the esophagus.

SECTION 4.4 The Stomach

The stomach is a muscular sac that stores and mixes food. Gastric juice contains mucus, HCl, intrinsic factor, and enzymes, such as pepsinogen. The muscular action of the stomach mixes food with gastric juice to form chyme.

peptic ulcer

a sore that occurs in the lining of the stomach or the upper small intestine.

enterocytes

absorptive cells that form the outer layer of a villus

digestive tract or gastrointestinal tract (GI tract)

alimentary canal or gut

digestive system

body system that breaks down food into its components, absorbs nutrients, and eliminates the waste

mucous cells

cells that secrete mucus

hormones

chemical messengers secreted by organs of the endocrine system that convey information to target cells

gastroesophageal reflux disease (GERD)

chronic condition characterized by frequent heartburn that can damage the esophagus

gastric juice

collection of stomach secretions that includes mucus, hydrochloric acid, intrinsic factor, and digestive enzymes

anosmia

complete inability to detect odors

gallbladder

concentrates and stores bile until it is needed for fat digestion and absorption.

diarrhea

condition characterized by frequent, watery bowel movements

diverticulitis

condition characterized by inflamed diverticula

irritable bowel syndrome (IBS)

condition characterized by intestinal cramps and abnormal bowel function, particularly diarrhea, constipation, or alternating episodes of both

diverticulosis

condition characterized by the presence of diverticula

inflammatory bowel disease (IBD)

condition that is characterized by chronic inflammation of the GI tract

hypogeusia

diminished ability to taste substances

dumping syndrome

disorder that occurs when chyme flows too rapidly into the small intestine

salivary amylase

enzyme in saliva that begins starch digestion

lysozyme

enzyme in saliva that can destroy some bacteria that are in food or the mouth

lingual lipase

enzyme secreted into saliva that begins fat digestion

epiglottis

flap of tough tissue that prevents the food from entering the larynx and trachea

bile

fluid that is produced in the liver and stored in the gallbladder until it is needed for fat digestion and absorption

lumen

hollow space through which food and fluids can pass (digestive tract)

gastrin

hormone that stimulates stomach motility and gastric gland secretions

constipation

infrequent bowel movements and feces that are difficult to eliminate

mucosa

innermost layer of the digestive tract wall

goblet cells

intestinal cells that secrete mucus

probiotics

live microorganisms that promote good health for human hosts

bolus

mass of food that has been chewed, moistened, mixed with saliva, and swallowed

oral cavity

mouth

stomach

muscular sac that stores and mixes food

esophagus

muscular tube that extends about 10 inches from the pharynx to the upper portion of the stomach

heartburn

pain generally felt in the upper chest that results from the passage of acidic contents from the stomach into the esophagus

microflora

population of several kinds of bacteria

digestion

process of breaking down large food molecules into nutrients that the body can use

absorption

process of removing nutrients from the intestinal tract and enabling them to enter the circulatory or lymphatic systems

enzyme

protein that speeds up the rate of a chemical reaction without being altered in the process

mechanical digestion

refers to physical treatments that food undergoes while it is in the intestinal tract

chemical digestion

refers to the breakdown of large nutrient molecules in food into smaller components, primarily by the action of enzymes

ileocecal sphincter

region of ileum that controls the rate of emptying undigested material into the large intestine

lower esophageal sphincter (LES)

region of the lower part of the esophagus that controls flow of material into the upper part of the stomach; also known as the gastroesophageal

pyloric sphincter

region of the stomach that regulates the flow of chyme into the small intestine

upper esophageal sphincter (UES)

region of the upper part of the esophagus that opens to allow a mass of food to enter the esophagus

pharynx

section of the alimentary canal that connects the nasal cavity with the top of the esophagus

chyme

semiliquid mass that forms when food mixes with gastric juice

cholecystokinin

slows stomach motility and stimulates the release of bile from the gallbladder and release of pancreatic enzymes

alkaline

solutions with pH values higher than 7

acidic

solutions with pH values lower than 7

external anal sphincter

sphincter that allows feces to be expelled from the anus and is under voluntary control

secretin

stimulates the release of bicarbonate-rich pancreatic juice

G cells

stomach cells that secrete gastrin

parietal cells

stomach cells that secrete intrinsic factor and the components of hydrochloric acid into the lumen of the stomach

chief cells

stomach cells that secrete some chemically inactive digestive enzymes

salivary glands

structures that produce saliva and secrete the fluid into the oral cavity

intrinsic factor

substance necessary for absorbing vitamin B-12

hemorrhoids

swollen veins in the anal canal

sphincters

thickened regions of circular muscle that control the flow of contents at various points in the GI tract

microvilli

tiny hairlike projections that form the brush border of an enterocyte

diverticula

tiny pouches that form in the wall of the colon

villi

tiny, fingerlike projections of the small intestinal mucosa that are involved in digestion and nutrient absorption

ageusia

total loss of the ability to taste substances

ulcerative colitis (UC)

type of IBD that causes ulcers to form in the mucosa of the colon and rectum

hepatic portal vein

vein that transports absorbed nutrients to the liver

lacteal

vessel of the lymphatic system

saliva

watery fluid that contains mucus and a few enzymes

mucus

watery slippery fluid secreted by special cells

peristalsis

waves of muscular contractions that help move material through most of the digestive tract