Nutrition and Metabolism
nutrient - substance in food used by the body to promote normal growth, maintenance, and repair
some nutrients are used to build cellular molecules and replace worn-out parts
most nutrients are transformed into ATP
nutrients divided into six categories
carbohydrates, lipids, proteins
***these 3 are the major bulk of the diet
vitamins, minerals
***these 2 required in minute amounts
water
***about 60% of the volume of the food eaten
***considered to be a major nutrient
most foods offer a combination of nutrients
Dietary sources of the Major Nutrients
Carbohydrates
lactose from milk/dairy products
small amounts of glycogen in meats
all others (sugars and starches) are derived from plants
sugars come mainly from fruits, sugar cane
polysaccharide starch is found in grains, legumes, and root vegetables
polysaccharide cellulose in vegetables, is not digestible but provides roughage
Lipids
ingest cholesterol and phospholipids in the diet
most dietary lipids are neutral fats
saturated fats in meat, dairy, and in a few plant products like coconut
unsaturated fats in seeds, nuts, and most vegetable oils
major sources of cholesterol are egg yolks, meats, and milk products
Proteins
animal products contain the highest-quality proteins
complete proteins meet all the body's amino acid requirements for tissue maintenance/growth
eggs, milk, and most meat
incomplete proteins are low in one or more of the essential amino acids proteins
legumes (beans and peas), nuts, and cereals
Vitamins
organic nutrients of various forms
body requires in very small amounts
found in all major food groups but no one food contains all the required vitamins
most function as coenzymes (acts with an enzyme to accomplish a particular catalysis)
Minerals
needs an adequate supply of Ca, P, K, S, Na, Cl, Mg and trace amounts of about a dozen others
fats and sugars have practically no minerals
cereals and grains are poor sources
mineral-rich foods are vegetables, legumes, milk, and some meats
Metabolism
carbohydrates (particularly glucose)
preferred fuel to produce cellular energy (ATP)
excesses are converted to fat
empty-calorie foods (candy/sweets) causes a rapid deposit of fat in adipose tissues
lipids
handled by the liver
used to make myelin sheaths around neurons
insulate the body/organs with a fatty cushion
body cells convert blood fat products/cholesterol into membranes/steroid hormones
also the main energy molecule for making ATP when there is not enough carbs in the diet
proteins
proteins make up the bulk of cellular structures
broken down to amino acids
amino acids tend to be carefully conserved and even hoarded by the body cells
cells remove amino acids from the blood and use them to build proteins
*own use (enzymes, membranes, mitotic spindle proteins muscle proteins)
*export (mucus, hormones, and others)
released ammonia is toxic to body cells (especially nerve cells)
liver combines the ammonia with CO2 to form urea
Liver and Metabolism
major metabolic organ with an important role in digestion
phagocytic cells remove/destroy bacteria that moved from the digestive tract into the blood
brings nutrient-rich blood from digestive viscera to the liver (liver's needs will be met first)
liver cells remove amino acids, fatty acids, and glucose from blood
- all may be stored for later use or processed in various ways
products of liver metabolism released into blood
- nutrients not needed by liver cells become available to other body cells
Overview of Gastrointestinal Processes and Controls
essential activities of the GI tract include six processes:
Ingestion
food must be placed into the mouth to be acted on
active, voluntary process called ingestion
Propulsion
food must be moved from one organ to the next
swallowing is an active process involving the tongue and inferior oral cavity
mostly involves peristalsis
involuntary squeezing of food along the tract
involves alternating waves of contraction and relaxation of the muscles in organ wall
Food breakdown by mechanical digestion
physical processes
prepares food for further degradation by enzymes
mixing of food in mouth (tongue) and stomach
Food breakdown by chemical digestion
sequence of steps in which food molecules are broken down to their building blocks by enzymes
carbohydrates
building blocks are monosaccharides
3 are common in our diet:
glucose
- most important (blood sugar)
fructose
- most abundant sugar in fruits
galactose
- found in milk
the only carbohydrates our digestive system can digest:
sucrose (table sugar)
lactose (milk sugar)
maltose (malt sugar)
starch (a polysaccharide formed of hundreds of glucose units)
carbohydrates our digestive system cannot digest:
cellulose (a plant polysaccharide)
no enzymes capable of breaking them down
do not provide any nutrients
helps move foodstuffs in GI tract by providing bulk/fiber in the diet
proteins
digested to their building blocks (amino acids)
intermediate products are polypeptides and peptides
lipids (fats)
yield two different types of building blocks (fatty acids and glycerol)
Absorption
transport of digested end products
moves from the Iumen of the GI tract to the blood/lymph
digested foods must enter cells by active/passive transport
small intestine is the major absorptive site
Defecation
elimination of indigestible substances from the body
via the anus in the form of feces
some processes are the job of a single organ
only the mouth ingests
only the large intestine defecates
most digestive system activities occur bit by bit as food is moved along the tract
can be viewed as a "disassembly line"
food is carried from one processing stage to the next
nutrients are made available to the cells in the body en route
the alimentary canal is a continuous, coiled, hollow, muscular tube
winds through the ventral body cavity
open at both ends
Mouth
food enters the digestive tract
mucous membrane-lined cavity
lips (labia) protect its anterior opening, cheeks form its lateral walls, hard palate forms its anterior roof,
soft palate forms its posterior roof, and uvula is a fleshy fingerlike projection of the soft palate
area contained by the teeth is the oral cavity proper
muscular tongue occupies the floor of the mouth
has several bony attachments
two of these are to the hyoid bone and the styloid processes of the skull
as food enters the mouth, it is mixed with saliva and masticated (chewed)
cheeks and closed lips hold the food between the teeth during chewing
tongue continually mixes food with saliva during chewing and initiates swallowing
the breakdown of food begins before the food has even left the mouth
papillae containing taste buds (taste receptors) are found on the tongue surface
food manipulating function and enjoyment/appreciation of food
Pharynx
common passageways for food, fluids, and air
walls of the pharynx contain two skeletal muscle layers
cells of the inner layer run longitudinally
cells of the outer layer (the constrictor muscles) run around the wall in a circular fashion
alternating contractions of these two muscle layers propel food through the pharynx into the esophagus
propelling mechanism is called peristalsis
Esophagus
from the pharynx through the diaphragm to the stomach
10 inches long
passageway that conducts food to the stomach by peristalsis
walls of the alimentary canal organs from the esophagus to the large intestine are made up of the same
four basic tissue layers (tunics)
1. mucosa is the innermost layer
moist membrane that lines the cavity
2. submucosa is just beneath the mucosa
contains blood vessels, nerve endings, lymph nodules, and lymphatic vessels
3. muscularis externa muscle layer
made of an inner circular layer and an outer longitudinal layer of smooth muscle
4. serosa is the outermost layer of the wall
lines the abdominopelvic cavity
Activities Occurring In the Mouth, Pharynx, and Esophagus
Food Ingestion and Breakdown
mechanical and chemical digestion begins once food is in the mouth
food is physically broken down into smaller particles and mixed with saliva by chewing
(when food enters the mouth more saliva is produced)
salivary amylase begins chemical digestion of starch into maltose
essentially no food absorption occurs in the mouth
pharynx and esophagus have no digestive function
provide passageways to carry food to the stomach
Food Propulsion-Swallowing and Peristalsis
deglutition (swallowing) is a complicated process
involves coordinated activity of tongue, soft palate, pharynx, and esophagus
two major phases:
1. bolus is forced into the pharynx by the tongue
2. food moved through the pharynx and esophagus by peristaltic contractions
movement of food through the pharynx/esophagus is automatic
(gravity plays no part in the transport of food)
at the end of the esophagus the bolus enters the stomach
Stomach
C-shaped
on the left side of the abdominal cavity nearly hidden by the liver and diaphragm
food enters the stomach from the esophagus
continuous with the small intestine
approximately 10 inches long
diameter depends on how much food it contains
full - it can hold about 1 gallon of food
empty - it collapses inward on itself and mucosa is thrown into large folds called rugae
acts as a temporary "storage tank” for food as well as a site for food breakdown
usual longitudinal and circular layers, but the wall contains a third obliquely arranged layer of muscle
muscularis externa –
allows the stomach to move food along the tract
churn, mix, and pummel the food, physically breaking it down to smaller fragments
chemical breakdown of proteins begins in the stomach
mucosa produces large amounts of mucus
dotted with millions of deep gastric pits that lead into gastric glands that secrete gastric juice
produces:
protein-digesting enzymes (mostly pepsinogens)
hydrochloric acid - makes the stomach contents acidic and activates the enzymes
sticky mucus - protects the stomach from being damaging and digesting itself
hormones such as gastrin that are important to the digestive activities
after food has been processed it resembles heavy cream and is called chyme
Activities of the Stomach
Food Breakdown
secretion of gastric juice is regulated by neural and hormonal factors including sight, smell, and taste
secretion of gastric juice by the stomach glands increases
presence of food / falling pH stimulates stomach cells to produce pepsin, mucus, and HCl
(2 to 3 liters of gastric juice are produced every day)
HCl makes stomach contents very acidic
pepsin is a protein-digesting enzyme
rennin (another protein-digesting enzyme) works primarily on milk protein
(not believed to be produced in adults)
little chemical digestion occurs in the stomach
virtually no absorption occurs through stomach walls
all the following occurs at about the same time:
food enters the stomach
stomach begins filling
secretion of gastric juices stimulated
stomach wall begins to stretch
muscle layers of the stomach wall become active
- compress and pummel the food
- physically breaks it apart
- continually mixing food with the enzyme-containing gastric juice
- forms semifluid chyme
Food Propulsion
once food has been well mixed peristalsis begins in the lower half of the stomach
only liquids and very small particles to pass through pyloric sphincter
each contraction of the stomach muscle releases 3 ml or less of chyme into the small intestine
contraction closes the valve so remaining chyme is propelled backward into stomach for more mixing
takes about 4 hours for the stomach to empty completely after eating a well-balanced meal
(6 hours or more if the meal has a high fat content)
Small Intestine
body's major digestive organ
twisted passageways
usable food is prepared for its journey into the cells of the body
longest section of the alimentary tube with an average length of 6 feet
hangs in sausage-like coils in the abdominal cavity
encircled and framed by the large intestine in the abdominal cavity
three subdivisions
duodenum
curves around the head of the pancreas
about 10 inches long
jejunum
about 8 feet long
extends from the duodenum to the ileum
ileum
about 12 feet long
joins the large intestine
chemical digestion of foods begins in earnest
able to process only a small amount of food at one time
muscles control food movement into the small intestine from the stomach
prevents the small intestine from being overwhelmed
enzymes from pancreas ducted into the duodenum through the pancreatic duct
completes the chemical breakdown of foods in the small intestine
bile also enters the duodenum through the bile duct (in the same area)
nearly all food absorption occurs in the small intestine
wall has 2 structures that increase the absorptive surface
decrease in number toward the end of the small intestine
1. microvilli
- tiny projections of the plasma membrane
- give the cell surface a fuzzy appearance, sometimes referred to as the brush border
2. villi
- fingerlike projections of the mucosa that give it a velvety appearance and feel
- has a rich capillary bed
Activities of the Small Intestine Food Breakdown and Absorption
food reaching the small intestine is partially digested
carbohydrate / protein digestion has been started
virtually no fats digested up to this point
chemical digestion is accelerated in small intestine
chyme takes 3-6 hours to move thru loops/coils/twists of small intestine
at the end digestion is complete and nearly all absorption has occurred
chyme entering duodenum is exposed to enzyme-rich pancreatic juice
pancreatic amylase - completes digestion of starch
trypsin - protein digestion
lipase - totally responsible for fat digestion
nuclease - digest nucleic acids
contains bicarbonate (pH 8) to neutralizes chyme coming from the stomach
the gallbladder to contract and release stored bile into the bile duct
- bile and pancreatic juice enter the small intestine together
- bile is not an enzyme
- bile emulsifies/mechanically breaks down large fat globules
- bile is necessary for absorption of fats and fat-soluble vitamins (K, D, and A)
absorption of water and end-products of digestion occurs along the length of the small intestine
most absorbed through intestinal cell plasma membranes (active transport)
enter the capillaries and transported to the liver for processing
chyme in distal ileum contains water, indigestible food materials (including cellulose), and bacteria
debris enters the large intestine
Food Propulsion
peristalsis (waves of contraction followed by waves of relaxation)
Large Intestine
about 5 feet long
extends from the small intestine to the anus
functions to dry out indigestible food residue by absorbing water and to eliminate residues as feces
frames the small intestine on three sides
has the following subdivisions
cecum
- saclike
- first part
appendix
- wormlike and usually twisted
- potential trouble spot (an ideal location for bacteria to accumulate and muItiply)
colon
- divided into several distinct regions
ascending colon travels up the right side of the abdominal cavity
travels across the abdominal cavity as the transverse colon
continues down the left side as the descending colon
becomes the S-shaped sigmoid colon
rectum
- lies in the pelvis
anal canaI
- lies in the pelvis
- ends at the anus which opens to the exterior
- has both internal involuntary and external voluntary muscles
open and close the anus
usually closed except during defecation
most nutrient absorption has occurred before the large intestine is reached so no villi are seen
lots of goblet cells in mucosa produce mucus (lubrication to ease the passage of feces)
Activities of the Large Intestine Food Breakdown and Absorption
matter delivered to the large intestine contains few nutrients
residue spends 12 to 24 hours in large intestine
colon produces no digestive enzymes
"resident" bacteria metabolize some remaining nutrients and release gases
(methane and hydrogen sulfide)
- about 500 ml of gas (flatus) is produced each day (more with carbohydrate-rich foods)
- contribute to the odor of feces
bacteria make some vitamins (vitamin K and some B vitamins)
absorption is limited to the absorption of these vitamins, some ions, and most of the remaining water
feces contain undigested food residues, mucus, millions of bacteria, enough water to allow transport
delivered to the rectum
Propulsion of the Residue and Defecation
peristalsis and mass movements are two major types of propulsive movements in the large intestine
peristalsis is sluggish and contributes very little to propulsion
mass movements are long, slow-moving, powerful contractile waves
move over large areas of the colon 3-4 times daily to force contents toward the rectum
occurs during or just after eating as food begins to fill the stomach and small intestine
bulk / fiber increases the strength of colon contractions and softens the stool
rectum is generally empty but its wall is stretched when feces are forced into it by mass movements
defecation reflex is initiated by the spine
feces are forced through the anal canal
conscious decision to open or constrict external muscles
defecation can be delayed temporarily
within a few seconds, reflex contractions end, and the rectal walls relax
the next mass movement initiates another defecation reflex
Accessory Digestive Organs
Pancreas
soft, pink, triangular gland
extends across the abdomen (spleen to duodenum)
produces enzymes that break down all categories of digestible foods
secreted into the duodenum in an alkaline fluid
neutralizes acidic chyme coming from stomach
also has an endocrine function to produce insulin and glucagon
Liver and Gallbladder
largest gland in the body
located inferior to the diaphragm, more to the right side of the body
overlies and almost completely covers the stomach
four lobes
digestive function is to produce bile
bile leaves liver through the common hepatic duct / enters the duodenum through the bile duct
a yellow-to-green, watery solution
contains bile salts, bile pigments (chiefly bilirubin - a breakdown product of hemoglobin),
cholesterol, phospholipids, and a variety of electrolytes
only bile salts (derived from cholesterol) and phospholipids aid the digestive process
bile salts emulsify fats by physically breaking large fat globules into smaller ones
(more surface area for the fat-digesting enzymes to work on)
gall bladder is a small, thin-walled green sac that stores bile
found on the inferior liver surface
while being stored the bile is concentrated by the removal of water
when fatty food enters the duodenum, a hormonal stimulus prompts the gallbladder to contract and
spurt out stored bile into the duodenum
Salivary Glands
three pairs that empty their secretions into the mouth
product of the salivary glands is saliva –
moistens and binds food together into a mass called a bolus
makes chewing and swallowing easier
contains the enzyme salivary amylase - begins the process of starch digestion in the mouth
also contains Iysozyme and antibodies that inhibit bacteria
saliva dissolves food chemicals so they can be tasted
Teeth
used for mastication (chewing)
opening, closing, moving jaws from side to side while continually using the tongue to move food
teeth tear and grind food to break it down into smaller fragments
classified according to shape and function
incisors - chisel-shaped - adapted for cutting
canines - fang-like - tearing or piercing
premolars and molars - broad crowns with rounded cusps (tips) - grinding