Chapter 3 - Digestion, Absorption and Transport

Question 1

Digestion

Answer 1

The process by which food is broken down into absorbable units.
• digest = take apart

Question 2

Gastrointestinal (GI) tract

Answer 2

A flexible muscular tube that extends from the mouth, through the esophagus, stomach, small intestine, large intestine, and rectum to the anus.

Question 3

Lumen

Answer 3

The inner space within the GI tract is continuous from one end to the other.

Question 4

Mouth

Answer 4

The process of digestion begins in the mouth. During chewing, teeth crush large pieces of food into smaller ones, and fluids from foods, beverages, and salivary glands blend with these pieces to ease swallowing.

Fluids also help dissolve the food so that the tongue can taste it; only particles in the solution can react with taste buds.

Question 5

Absorption

Answer 5

The uptake of nutrients by the cells of the small intestine for transport into either the blood or the lymph.

• absorb = suck in

Question 6

Pharynx

Answer 6

When a mouthful of food is swallowed, it passes through the pharynx, a short tube that is shared by both the digestive system and the respiratory system.

To bypass the entrance to the lungs, the epiglottis closes off the airway so that choking doesn’t occur when swallowing, thus resolving the first challenge.

Question 7

Bolus

Answer 7

a portion; with respect to food, the amount swallowed at one time.

• bolos = lump

Question 8

Stomach

Answer 8

Churns, mixes, and grinds food to a liquid mass, add acids, enzymes, and flud.

Retains the bolus for a while in its upper portion. Little by little, the stomach transfers the food to its lower portion, adds juices to it, and grinds it to a semiliquid mass called chyme. Then, bit by bit, the stomach releases the chyme through the pyloric sphincter, which opens into the small intestine

Question 9

Chyme

Answer 9

The semiliquid mass of partly digested food is expelled by the stomach into the duodenum.

• chymos = juice

Question 10

Small Intestine

Answer 10

Secretes enzymes that digest all energy-yielding nutrients to smaller nutrient particles

Cells of wall absorb nutrients into blood and lymph

Intestine At the beginning of the small intestine, the chyme bypasses the opening from the common bile duct, which is dripping fluids into the small intestine from two organs outside the GI tract—the gallbladder and the pancreas.

The chyme travels on down the small intestine through its three segments—the duodenum, the jejunum, and the ileum—almost 10 feet of tubing coiled within the abdomen.*

Question 11

Large intestine (colon)

Answer 11

Absorbs water and minerals; passes waste (fiber, bacteria and unabsorbed nutrients) along with water to the rectum.

From the small intestine, the remaining content arrives at another sphincter - the ileocecal valve.

Upon entering the colon, the contents pass another opening. Should any intestinal contents slip into this opening, it would end up in the appendix, a blind sac about the size of your little finger. Normally, the contents bypass this opening, however, and travel along the large intestine up the right side of the abdomen, across the front to the left side, down to the lower left side, and finally below the other folds of the intestines to the back of the body, above the rectum

Question 12

Peristalsis

Answer 12

The entire GI tract is ringed with circular muscles. Surrounding these rings of muscle are longitudinal muscles. When the rings tighten and the long muscles relax, the tube is constricted. When the rings relax and the long muscles tighten the tube bulges.

occurs continuously and pushes the intestinal contents along

wavelike muscular contractions of the GI tract that push its contents along.

• peri = around
• stellein = wrap

Question 13

Stomach action

Answer 13

The stomach has the thickest walls and strongest muscles of all the GI tract organs. In addition to the circular and longitudinal muscles, it has a third layer of diagonal muscles that also alternately contract and relax (see Figure 3-3). These three sets of muscles work to force the chyme downward, but the pyloric sphincter usually remains tightly closed, preventing the chyme from passing into the duodenum of the small intestine. As a result, the chyme is churned and forced down, hits the pyloric sphincter, and remains in the stomach. Meanwhile, the stomach wall releases gastric juices. When the chyme is completely liquefied with gastric juices, the pyloric sphincter opens briefly, about three times a minute, to allow small portions of chyme to pass through.

Question 14

Esophagus

Answer 14

Passed food from the mouth to the stomach.

Has a sphincter muscle at each end. During a swallow, the upper esophageal sphincter opens. The bolus then slides down the esophagus, which passes through a hole in the diaphragm to the stomach.

The lower esophageal sphincter at the entrance to the stomach closes behind the bolus so that it proceeds forward and doesn’t slip back into the esophagus

Question 15

Sphincter contractions

Answer 15

Contractions Sphincter muscles periodically open and close, allowing the contents of the GI tract to move along at a controlled pace

1. Upper esophageal - between mouth and esophagus
2. Lower esophageal - between esophagus and stomach.
3. Pyloric - between stomach and duodenum
4. Ileocecal - between small intestine and caecum
5. Anal - at end of rectum

Question 16

Reflux

Answer 16

A backward flow

Question 17

Secretions of digestion

Answer 17

The breakdown of food into nutrients requires secretions from five different organs: the salivary glands, the stomach, the pancreas, the liver (via the gallbladder), and the small intestine.

These secretions enter the GI tract at various points along the way, bringing an abundance of water and a variety of enzymes.

Question 18

Segmentation

Answer 18

The circular muscles of the intestines rhythmically contract and squeeze their contents. These contractions, called segmentation, mix the chyme and promote close contact with the digestive juices and the absorbing cells of the intestinal walls before letting the contents move slowly along.

Question 19

Hydrolysis

Answer 19

Hydrolysis—the addition of water (hydro) to break (lysis) a molecule into smaller pieces.

Question 20

Enzyme

Answer 20

A protein that facilitates a chemical reaction—making a molecule, breaking a molecule apart, changing the arrangement of a molecule, or exchanging parts of molecules.

As a catalyst, the enzyme itself remains unchanged. The enzymes involved in digestion facilitate a chemical reaction known as hydrolysis

Question 21

Gastric Juice

Answer 21

In the stomach, gastric glands secrete gastric juice, a mixture of water, enzymes, and hydrochloric acid, which acts primarily in protein digestion.

The acid is so strong that it causes the sensation of heartburn if it happens to reflux into the esophagus.

The strong acidity of the stomach prevents bacterial growth and kills most bacteria that enter the body with food. It would destroy the cells of the stomach as well, but for their natural defenses. They secrete mucus that protects them.

Question 22

Pancreatic Juice and Intestinal Enzymes

Answer 22

By the time food leaves the stomach, digestion of all three energy nutrients (carbohydrates, fats, and proteins) has begun, and the action gains momentum in the small intestine.

There the pancreas contributes digestive juices by way of ducts leading into the duodenum. The pancreatic juice contains enzymes that act on all three energy nutrients.

Pancreatic juice contains sodium bicarbonate, which is basic or alkaline—the opposite of the stomach’s acid.

The pancreatic juice thus neutralizes the acidic chyme arriving in the small intestine from the stomach. From this point on, the chyme remains at a neutral or slightly alkaline pH. The enzymes of both the intestine and the pancreas work best in this environment.

Question 23

Saliva

Answer 23

The salivary glands squirt just enough saliva to moisten each mouthful of food so that it can pass easily down the esophagus.

The saliva contains water, salts, mucus, and enzymes that initiate the digestion of carbohydrates.

Saliva also protects the teeth and the linings of the mouth, esophagus, and stomach from substances that might cause damage.

Question 24

The final stage

Answer 24

The three energy-yielding nutrients— carbohydrate, fat, and protein—have been digested and are ready to be absorbed.

Some vitamins and minerals are altered slightly during digestion, but most are absorbed as they are.

Undigested residues, such as some fibers, are not absorbed. Instead, they continue through the digestive tract, carrying some minerals, bile acids, additives, and contaminants out of the body.

Intestinal bacteria ferment some fibers, producing water, gas, and small fragments of fat that provide energy for the cells of the colon.

This semisolid mass helps exercise the GI muscles and keeps them strong enough to perform peristalsis efficiently. Fiber also retains water, accounting for the consistency of stools.

The waste that is finally excreted has little or nothing of value left in it. The body has extracted all that it can use from the food.

Question 25

Bile

Answer 25

Flows into the duodenum. The liver continuously produces bile, which is then concentrated and stored in the gallbladder.

The gallbladder squirts bile into the duodenum of the small intestine when fat arrives there.

Bile is not an enzyme; it is an emulsifier that brings fats into suspension in water so that enzymes can break them down into their component parts.

Question 26

Anatomy of the absorption system

Answer 26

The inner surface of the small intestine under a microscope has wrinkled, folds and finderlike projections or villi

Question 27

Villi

Answer 27

Fingerlike projections as numerous as the hairs on fabric that is composed of hundreds of cells each covered in microvilli and between each lie the crypts.

In constant motion. Each is lines by a thin sheet of muscle so that it can wave, sqirm and wriggle.

Nutrient molecules are trapped among the microvilli and then drawn unto the cells. The cells of the villi select nutrients the body needs and regulates their absorption.

Question 28

Crypts

Answer 28

Tubular glands that secrete the intestinal juices into the small intestine

Question 29

Goblet cells

Answer 29

secrete mucus

Question 30

Microvilli

Answer 30

Each villi is coated with thousands that project from the cell’s membrane.

Enzymes and “pumps” within recognize and act on different nutrients

Question 31

Specialized cells

Answer 31

There are specialized cells of the intestinal tract that absorb different nutrients.

Question 32

Prepareing nutrients for transport

Answer 32

When a nutrient molecule has crossed the cell of a villus, it enters either the bloodstream or the lymphatic system.

Both transport systems supply vessels to each villus.

The water-soluble nutrients and the smaller products of fat digestion are released directly into the bloodstream and guided directly to the liver where their fate and destination will be determined.

The larger fats and the fat-soluble vitamins are insoluble in water, however, and blood is mostly water. The intestinal cells assemble many of the products of fat digestion into larger molecules. These larger molecules cluster together with special proteins, forming chylomicrons.

Question 33

Chylomicrons

Answer 33

Carry fats and they are released into the lymphatic system. They move through the lymph until they can enter the bloodstream at a point near the heart. Consequently, chylomicrons bypass the liver at first.

Question 34

The circulatory system

Answer 34

Once a nutrient enters the bloodstream it may be transported to any of the cells in the body.

Deliveres nutrients whever they are needed.

Question 35

The vascular system

Answer 35

Blood circulatory system

Closed system of vessels through which blood flows continuously with the heart serving as the pump

All the body tissues derive nutrients and oxygen from the blood and deposit carbon dioxide and other wastes back into the blood.

The digestive system supplies the nutrients. The lungs exchange oxygen and carbon dioxide. The kidneys filter wastes other than carbon dioxide out of the blood to be excreted in the urine.

Question 36

Hepatic portal vein

Answer 36

Directs blood not back to the heart but to another organ, the liver.

the vein that collects blood from the GI tract and conducts it to the liver

This vein branches into a network of large capillaries so that every cell of the liver has access to the blood.

• portal = gateway hepatic vein

Question 37

hepatic vein

Answer 37

Collects blood from the liver and returns it to the heart.

• hepatic = liver

Heart to arteries to capillaries (in intestines) to hepatic portal vein to capillaries (in liver) to hepatic vein to heart

Question 38

The liver

Answer 38

The liver’s placement ensures that it will be first to receive the nutrients absorbed from the GI tract. In fact, the liver has many jobs to do in preparing the absorbed nutrients for use by the body. Of all the body’s organs, the liver is the most metabolically active.

The liver defends the body by detoxifying substances that might cause harm and preparing waste products for excretion.

Question 39

The lymphatic system

Answer 39

System provides a one-way route for fluid from the tissue spaces to enter the blood. Unlike the vascular system, the lymphatic system has no pump; instead, lymph circulates between the cells of the body and collects into tiny vessels.

The fluid moves from one portion of the body to another as muscles contract and create pressure here and there.

Ultimately, much of the lymph collects in the thoracic duct behind the heart. The thoracic duct opens into the subclavian vein, where the lymph enters the bloodstream.

Thus nutrients from the GI tract that enter lymphatic vessels (large fats and fatsoluble vitamins) ultimately enter the bloodstream, circulating through arteries, capillaries, and veins like the other nutrients, with a notable exception—they bypass the liver at first.

Once inside the vascular system, the nutrients can travel all over the body where they can be taken into cells and used as needed.

Question 40

Gastrointestinal Bacteria

Answer 40

Bacteria An estimated 10 trillion bacteria representing some 400 or more different species and subspecies live in a healthy GI tract.

The prevalence of different bacteria in various parts of the GI tract depends on such factors as pH, peristalsis, diet, and other microorganisms.

Relatively few microorganisms can live in the low pH of the stomach with its relatively rapid peristalsis, whereas the neutral pH and slow peristalsis of the lower small intestine and the large intestine permit the growth of a diverse and abundant bacterial population.

Most bacteria in the GI tract are not harmful; in fact, they are actually beneficial. Provided that the normal intestinal flora are thriving, infectious bacteria have a hard time establishing themselves to launch an attack on the system.

Diet is one of several factors that influence the body’s bacterial population and environment.

Question 41

Probiotics

Answer 41

The living microorganisms found in foods and dietary supplements that, when consumed in sufficient quantities, are beneficial to health.

• pro = for
• bios = life

Change the conditions and native bacterial colonies in the GI tract in ways that seem to benefit health.5

Helping to alleviate diarrhea, constipation, inflammatory bowel disease, ulcers, allergies, lactose intolerance, and infant colic; enhance immune function; and protect against colon cancer.

Question 42

Prebiotics

Answer 42

Food components (such as fibers) that are not digested by the human body but are used as food by the GI bacteria to promote their growth and activity.

Digest fibers and complex proteins.

Question 43

Gastrointestinal hormones and nevre pathways

Answer 43

Two intricate and sensitive systems coordinate all the digestive and absorptive processes: the hormonal (or endocrine) system and the nervous system.

Even before the first bite of food is taken, the mere thought, sight, or smell of food can trigger a response from these systems.

As food travels through the GI tract, it either stimulates or inhibits digestive secretions by way of messages that are carried from one section of the GI tract to another by both hormones and nerve pathways.

Question 44

Homeostasis.

Answer 44

Survival depends on body conditions staying about the same; if they deviate too far from the norm, the body must “do something” to bring them back to normal.

Question 45

Feedback mechanism

Answer 45

A certain condition demands a response. The response changes that condition, and the change then cuts off the response.