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Chapter 3 : The Remarkable Body The Body’s Cells ●Cells require nutrients -each of the body’s cells is a self-contained, living entity -however, each cell depends on the rest to supply its needs -these needs include energy, oxygen, water, building blocks, control systems, and especially the essential nutrients ●All living things including cells die -some skin cells and red blood cells must replenish themselves every 10 to 120 days -cells lining the digestive tract replace themselves every three days -ordinarily many muscle cells reproduce only once every few years -liver cells have the ability to reproduce quickly and do so whenever repairs are necessary -certain brain cells do not reproduce at all ●The workings of Genes -Each gene is a blueprint that directs the production of protein machinery, often an enzyme -a protein that speeds up a specific chemical reaction -Genes provide the instructions for the structural components cells need to survive -Nutrients affect the genes’ activities within the cells ●Cells, Tissues, Organs, Systems -Cells are organized into tissues -tissues perform specialized tasks -Tissues are grouped together to form organs -Several related organs working together comprise a body system The Body Fluids and the Circulatory System ●Body fluids supply tissues with energy, oxygen, and nutrients, including water -fluids circulate to pick up fresh supplies and deliver waste to points of disposal -every cell draws oxygen and nutrients from those fluids -every cell releases carbon dioxide and other waste products into the body fluids ●The body’s main fluids are -Blood -the fluid of the cardiovascular system -composed of water, red and white blood cells, other foreign particles, nutrients, oxygen, etc. -Blood travels within arteries, veins, and capillaries, as well as within the heart’s chambers -circulating within these vessels is the plasma of the blood -Lymph -the fluid that moves form the blood stream into tissue spaces and then travels in its own vessels -which eventually drain back into the bloodstream -Extracellular Fluid -Fluid surrounding cells -derived from blood in the capillaries -flows around the outside of cells, permitting exchange of materials -some returns to the blood by reentering capillaries -the remaining fluid forms lymph -Intracellular Fluid -Fluid inside cells -medium in which all cell reactions take place -its pressure helps the cells hold their shape -is drawn from the extracellular fluid ●Lungs -Where the blood picks up oxygen and releases carbon dioxide -The blood returns to the heart where the pumping heartbeats push this oxygenated blood from the lungs to the body tissues -As blood passes through the digestive system, blood delivers oxygen and picks up most nutrients from the intestine -other than fats and their relatives -these nutrients will be distributed to other body cells -Lymphatic vessels pick up most fats from the intestine -then transport these fats to the blood ●All blood leaving the digestive system is routed directly to the liver -The liver filters blood, removes and processes nutrients, manufactures materials for exports to other parts of the body, and destroys toxins or stores them to keep them out of circulation ●Blood passes through the kidneys -where waste is filtered from the blood, urine is made and released to the bladder for excretion from the body ●Ample fluid intake is needed to ensure efficient circulation of fluid to all your cells -this means drinking sufficient water to replace the water lost each day -cardiovascular fitness is also essential The Hormonal and Nervous Systems ●Hormones -Chemicals secreted by glands in response to conditions in the body that require regulation -serve as chemical messengers -act on organ to maintain constant conditions ●What do Hormones have to do with Nutrition? -Each gland monitors a condition -and produces one or more hormones to regulate it -each hormone acts as a messenger that stimulates various organs to take appropriate action -For example, the pancreas -an organ that plays a role in the regulation of blood sugar -as well as the secretion of digestive enzymes -When the pancreas detects a high concentration of blood glucose it releases insulin -insulin stimulates cells to remove glucose from the blood -When the pancreas detects low blood sugar levels it secrets glucagon -glucagon stimulates the liver to release stored glucose into the blood -Nutrition and exercise affect hormonal balances -people who become very thin have an altered hormonal balance that may make them unable to maintain their bones -people who eat high-fat diets have hormone levels that may make them susceptible to certain cancers ●Hormones also affect nutrition -Both the nervous and hormonal systems regulate hunger and affect appetite -They carry messages to regulate the digestive system -Telling the digestive organs the kind of foods that have been eaten and how much of each digestive juice to secrete -A hormone produced by fat tissue informs the brain about the degree of body fatness and helps to regulate appetite -Hormones also regulate the menstrual cycle in women, etc. -Hormones maybe partially responsible for the loss of appetite that sick people experience ●How does the Nervous System Interact with Nutrition? -With the brain and spinal cord as central controllers, the nervous system receives and integrates information from sensory receptors all over the body -which communicate to the brain the state of both the outer and inner worlds -the nervous system also tells the muscles and glands what to do -the nervous system’s role in hunger is coordinated by the brain -sensations of hunger and appetite are perceived by the brain’s cortex -the part of the brain where conscious thought takes place -Hunger -the physiological need for food -to signal huger the digestive tract sends messages to the hypothalamus via hormones and nerves -the signals also stimulate the stomach to intensify its contractions and secretions -causing hunger pains -When the brain’s cortex perceives hunger sensations you want to eat -the conscious mind can override such signals ●The fight-or-flight reaction -A.K.A. the stress response -when danger is detected nerves release neurotransmitters -chemicals that facilitate communication between nerve cells -when danger is detected glands release epinephrine -the major hormone that elicits the stress response -glands also release Norepinephrine -another hormone that helps to elicit the stress response -As a result of the release of these neurotransmitters and hormones metablolism speeds up -the sum of all physical and chemical changes taking place in living cells -As a result of an increase in metabolism: -the pupils of the eyes widen -muscles tense up -breathing quickens and deepens -heart rate and blood pressure increase -the liver releases glucose from its stores -fat cells release fat -the digestive system shuts down ●In ancient times stress often involved physical danger -the response was violent physical exertion ●In the modern world stress is seldom physical -but the body reacts the same way Atherosclerosis ●The accumulation of fat and other constituents in the arteries and stresses that strain the heart and often lead to heart attacks -especially when a body accustomed to underexertion experiences sudden high blood pressure -this is why dialy exercise is part of a healthy lifestyle The Immune System ●The skin presents a physical barrier to infection ●The body’s cavities are lined with membranes that resist penetration by unwanted substances and microbes -bacteria, viruses, and other organisms invisible to the naked eye ●These linings are sensitive to vitamin and other nutrient deficiencies -health-care providers inspect both the skin and the mouth to detect signs of malnutrition ●Antigen – any substance that is foreign to the body ●If an antigen penetrates the body’s barriers, the immune system rushes in to defend the body -The immune system consists of tissues and organs that defend the body against antigens ●One in every hundred human body cells is a white blood cells -Phagocytes are scavenger cells that travel throughout the body -these are the first cells to defend body tissues against invaders -these cells act by engulfing the invader and then attacking the invader with oxidative chemicals or otherwise digesting or destroying them -these cells leave a chemical trail that helps other immune cells join in the defense of the body -Lymphocytes -T-cells and B-Cells -killer T-cells recognize chemical messages from phagocytes -they read and remember the identity of invader from the messages -they then seek out and destroy all foreign particles having the same identity -these are the cells that are targeted by immunosuppressive drugs following surgery ●Helper T-cells do not attack invaders -They help other immune cells attack invaders -People with AIDs are rendered defenseless against other diseases because HIV attacks and destroys their T-cells ●B-cells respond rapidly to infection by dividing and releasing antibodies into the blood -antibodies are proteins designed to combine with and inactivate specific antigens -B-cells retain a chemical memory of each invader, therefore ensuring a swift response should a reinvasion occur ●Immunizations work by injecting a disabled or harmless form of a disease-causing organism into the body -B-cells learn to recognize it -assuring a quick response if the real infectious organism invades The Digestive System ●Taste buds guide you in judging what foods are acceptable -basic chemical tastes include: -Sweet, sour, bitter, salty, umami -the taste of MSG ●A food’s flavor is also affected boy: -Aroma -our sense of smell is thousands of times more sensitive than our sense of taste -Texture -Temperature -Other flavor elements ●Why do people like sugar, fat and salt -while most people have aversions to bitter and sour tastes in isolations ●The enjoyment of sugars and fats encourage people to consume ample energy -sugars provide energy for the brain -fats provide energy and essential nutrients needed by all body tissues -enjoyment of salt assures the consumption of sodium and chloride -the aversion to bitterness discourages consumption of foods containing bitter toxins ●The digestive system digests and absorbs the mixture of chewed and swallowed food Digest – to break molecules into smaller molecules Absorb – the movement of nutrients into intestinal cells after digestion The Digestive Tract ●The digestive tract is a flexible muscular tube extending from the mouth to the anus -the human body surrounds this digestive canal -when you swallow something it is not inside your body until you absorb it -many things pass through the digestive tract without being absorbed ●The digestive system: -absorbs nutrients -absorbs some nonnutrients -leaves behind substances, such as fiber, that are excreted The Mechanical Aspect of Digestion ●Mechanical digestion begins in the mouth -chewing shreds food into pieces small enough to swallow -adds water in the form of saliva -softens rough/sharp foods -saliva moistens and coats food making it slippery ●Chewing releases nutrients trapped inside indigestible skins -for example, corn kernels -once a food is mashed and moistened, there is no advantage to additional chewing ●At the base of the esophagus is a sphincter muscle -this muscle prevents reflux of stomach contents into the esophagus ●The stomach and intestines liquefy foods -peristalsis -wave-like muscular squeezing -begins at the esophagus and pushes food along the digestive tract ●The stomach holds food and mashes it into a fine paste -the stomach and intestines add water to the paste so that it becomes more fluid as it moves along ●The stomach stores food in its upper portion -little by little the food is squeezed into its lower portion -chyme -the fluid resulting from the action, both mechanical and chemical breakdown, of the stomach upon a meal ●Large intestine = colon -digestion and absorption are nearly complete by the time food arrives here -the colon primarly reabsorbs water and absorbs minerals -fiber and undigested materials make up the feces -fiber provides bulk against which muscles can work -the rectum stores fecal material that is later excreted ●Transit from mouth to rectum takes from 1 to 3 days ●The timing of meals can affect how a person feels -this is not due to a requirement of the digestive tract -it is due to the body’s need for nutrients -Digestion is limited only by sleep and exercise -eating late can interfere with normal sleep -exercising too soon after eating can inhibit physical work The Chemical Aspect of Digestion ●How do “Digestive Juices” work? -Digestion begins in the mouth -An enzyme in saliva breaks down starch -Another enzyme begins the digestion of fat -Saliva helps maintain the teeth -it washes away food particles that would otherwise promote tooth decay -it neutralizes acids produced by oral bacteria ●Protein digestion begins in the stomach ●The stomach releases gastric juice -A mixture of water, enzymes, HCl -the acid is needed to activate a protein-digesting enzyme -protein digestion is the stomach’s main function ●pH is a measure of acidity -the lower the pH, the more acidic ●The digestive tract is protected from acid by mucus ●Most digestion and absorption occur in the small intestine -Hormonal messengers stimulate the gall bladder to release bile into the intestine -bile is produced by the liver and is an emulsifier -a compound that attracts fats into water -Hormonal messengers stimulate the pancreas to release pancreatic juice -containing both enzymes and bicarbonate -bicarbonate neutralizes stomach acid ●As pancreatic and intestinal enzymes break down nutrients small pieces are released into intestinal fluids ●Eventually pieces are small enough such that nutrients can be absorbed ●At this point only water, fiber, and some mineral remain in the digestive tract ●Certain fibers cannot be digested by human enzymes -these fibers are often digested by bacteria living in the human digestive tract -intestinal cells can absorb small fat fragments released from fiber as a result of bacterial cell metabolism If “I Am What I Eat,” Then How Does A Sandwich Become “Me”? ●In the Mouth: food is chewed, mixed and chemically broken down by teeth, tongue and saliva -each swallow triggers peristalsis that moves food from the esophagus to the stomach ●In the Stomach: Starch digestion continues in the upper storage area until gastric juice puts a halt to this -Acid unwinds proteins which are then broken down by enzymes -The end result is chyme -Water carbohydrate- and protein-rich chyme enters the small intestine -followed by fat ●In the Small Intestine: Some sugars are absorbed almost as soon as they enter the small intestine -Bile emulsifies fat -pancreatic enzymes break down fat, protein, and starch -cells of the small intestine produce enzymes that complete chemical breakdown ●small chemical fragments are then absorbed into the blood and lymph via the cells of the small intestine’s wall -vitamins and minerals are also absorbed ●In the Large Intestine (Colon): fiber fragments, fluid, some minerals are absorbed in the large intestine -fibers are partly digested by bacteria in the colon -some of these products are absorbed -most fiber is not absorbed, and along with other components, is excreted as feces ●Food Passage time In the mouth: less than a minute In the stomach: about 1-2 hours In small intestine: about 7-8 hours In the colon: about 12-14 hours ●Absorption and Transportation of Nutrients -Absorption in the intestine is selective -for example, when calcium is in short supply it’s absorption increases -the small intestine is lined with projections called villi off of which project microvilli -this serves to increase the absorptive surface area ●After nutrients pass through the cells of the villi the blood and lymph transport the nutrients to the body’s cells -Lymphatic vessels initially transport most of the products of fat digestion and a few vitamins -these are later delivered to the blood stream -blood vessels carry the products of carbohydrate and protein digestion, most vitamins, and minerals to the liver ●The digestive system’s cells are sensitive to a lack of energy, nutrients, or dietary fiber -Severe undernutrition can cause the absorptive surface of the small intestine to shrink -without fiber the digestive tract’s muscles become weak from lack of exercise -malnutrition itself impairs digestion and thus becomes self-perpetuating ●A letter from Your Digestive Tract -Gurgling, belching, gas, hiccups -hiccups are spasms of the vocal cords and the diaphragm -can be caused by irritation of the diaphragm, indigestion, etc. ●Overeating causes heartburn -the reflux of acidic stomach contents into the esophagus -acid poses no problem to a healthy stomach -acid burns the unprotected surface of the esophagus -leaning over or lying down after a meal can open the muscular sphincter allowing acid into the esophagus -excess body fat can squeeze the stomach causing acid to back up ●Some food choices are more irritating, especially: -“hot” components of chilly peppers -chemicals in coffee -fat -chocolate -carbonated soft drinks -alcohol -smoking ●Antacids provide only temporary relief by neutralizing stomach acidity -to which the stomach responds by producing more acid to restore normal acid conditions -antacids can also interfere with the digestive tract’s ability to absorb nutrients -check with a physician before taking acid reducers ●Stomach acid fights bacterial infections -acid reducers can cause indigestion and diarrhea -acid reducers can mask the symptoms of ulcer, hernia, GERD ●An ulcer is an erosion in the layers of cells that form a lining -the bacterium that causes stomach ulcer may also cause stomach cancer -therefore, treatment involves antibiotic drugs ●A hernia is a protrusion of an organ or body part through the wall of the body chamber that normally contains the organ -A hernia can cause food to back up into the esophagus -often hernias require corrective treatment by a physician ●GERD = gastroesophageal reflux disease -severe and chronic exposure of the esophagus, throat, mouth, or airway to stomach acid and enzymes -causes inflammation and injury -untreated GERD may increase the risk of esophageal cancer -treatment may require surgery or management with medication ●Constipation and Diarrhea -Constipation is infrequent difficult bowel movements -often caused by diet, inactivity, dehydration, or medication -chronic constipation is associated with 2X the risk of colon cancer -Diarrhea is frequent watery bowel movements -often caused by diet, stress, or irritation of the colon -severe prolonged diarrhea causes dehydration and mineral imbalances -Don’t use laxatives to treat constipation -defecate when your body tells you to -the longer you wait, the more water extracted from the feces, the harder the feces -consume sufficient fiber as to create softer, bulkier stools that stimulate muscle contraction -fiber helps the digestive muscles stay fit -drink water -be physically active -If you have diarrhea: -rest and drink fluids -To avoid diarrhea, don’t change diet too drastically or quickly -One cause of diarrhea is food poisoning ●If diarrhea lasts more than a day or two, or if it alternates with constipation, it could be irritable bowel syndrome, and you should see a physician -IBS is an intermittent disturbance of bowel function -it is associated with diet, lack of physical activity, or psychological stress The Excretory System ●Waste must be eliminated -CO2 is eliminated via lungs where it is exchanged for O2 -Other wastes are pulled out of the blood by the liver -the liver processes these wastes and either: -sends them to the digestive tract with bile, to leave the body with feces -or prepares them to be sent to the kidneys for disposal in the urine ●Kidneys are waste- and water- removal specialists -Kidneys filter blood -waste, dissolved in water, is collected by the kidney’s nephrons -nephrons are the functional units of kidneys -Waste becomes concentrated in the urine which travels through tubes to the urinary bladder -which is periodically emptied -The excretion or retention of sodium by the kidneys is a vital part of the body’s blood pressurecontrolling mechanism -Kidneys have a high metabolic rate -Kidneys also regulate fluid volume and the concentrations of substances in the blood and extracellular fluids -Kidney function is regulated by hormones -which are also secreted by the kidneys -Due to their role in toxin removal, whatever is good for the kidneys is good for the body -a strong cardiovascular system -an abundant supply of water -energy -vitamins -minerals -exercise Storage Systems ●When I eat more than my body needs, what happens to the extra nutrients? -Excess energy-containing nutrients are stored in two forms -the liver makes some into glycogen -a storage form of carbohydrate -some is stored as fat -Liver glycogen meets the body’s glucose needs -this can meet the body’s need for 3 to 6 hours -Glycogen stored by the muscle is only used by muscle -The liver ships out fat in packages to be picked up by cells that need it -Excess fat is stored in the cells of andipose tissue -These glucose and fat storage systems ensure that the body cells will have a source of energy even if the body is hungry -Body stores also exist for many other nutrients ●Variations in Nutrient Stores -Some nutrients are stored without limit, even if they reach toxic levels -Other nutrients are stored in only small amounts and can readily be depleted -You don’t need to eat fat at every meal because it is stored abundantly -In contrast, you usually need to have a source of carbohydrate at intervals throughout the day because the liver has less than one day’s supply of glycogen