Download Chapter 3 : The Remarkable Body

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