CHAPTER 38 –
DIGESTIVE AND EXCRETORY
SYSTEMS
38-1 – Food and Nutrition
Food and Energy
• Food is fuel for your body. It gives our body energy. Cells convert the
chemical energy stored in the sugar glucose and other molecules into
ATP.
Energy in food can be measured by burning it. When food is burned,
the energy content of the food is converted to heat, which is measured
in terms of calories. The amount of heat needed to raise the
temperature of 1 gram of water by 1 degree Celsius is 1 calorie.
Scientists refer to the energy stored in food as dietary Calories with a
capital C. One Calorie is equal to 1000 calories, or 1 kilocalorie (kcal).
An average sized teenager needs about 2200 Calories a day for females
and 2800 Calories for males. More Calories are needed for more active
people.
Nutrition is the study of food and its effects on the body. The science
of nutrition tries to determine how food helps the body meet all of its
various needs. Nutritionists recommend balanced diets that include
many different types of food. Nutritionists also plan diets for people
with particular needs, like diabetics.
Nutrients
• Nutrients are substances in food that supply the energy and raw
materials your body uses for growth, repair, and maintenance.
• The nutrients that the body needs are water, carbohydrates, fats,
proteins, vitamins, and minerals.
• The most important nutrient is water. Every cell in the human body
needs water because many of the body’s processes, including
chemical reactions, take place in water. Water makes up most of
blood, lymph, and other body fluids. When we sweat, it is water.
• People need at least 1 liter of water per day. If not enough water is
taken in, dehydration can result.
Carbohydrates – simple and complex carbohydrates are the main
source of energy for the body. Sugars in fruits, honey, and sugar cane
are simple carbohydrates (monosaccharides and disaccharides).
Starches found in grains, potatoes, and vegetables are complex
carbohydrates (polysaccharides).
Starches are broken down by the digestive system into simple sugars.
These molecules are absorbed into the bloodstream and carried to cells
throughout the body. Sugars that are not immediately used to supply
energy are converted into the complex carbohydrate glycogen, which is
stored in the liver and in skeletal muscles.
Fats – including fats (lipids) in your diet is very important. Fats are
formed from fatty acids and glycerol. Your body needs essential fatty
acids to produce cell membranes, myelin sheaths, and certain
hormones. They also help the body absorb fat-soluble vitamins. When
a person eats more food than is needed, the body stores the extra
energy as fat. Deposits of fat protect body organs and insulate the
body.
There are serious health consequences from a diet high in fat. This
includes an increased risk of high blood pressure, heart disease,
obesity, and diabetes.
Proteins – proteins supply raw materials for growth and repair of
structures such as skin and muscle. Proteins have regulatory and
transport functions (insulin and hemoglobin are proteins)
Proteins are polymers of amino acids. The body can make 12 of the 20
amino acids. The other 8 we must get from food. Meat, fish, eggs, and
milk contain all 8 esssential amino acids.
Vitamins – organic molecules that help regulate body processes, often
working with enzymes. Most vitamins but be obtained from food. A
diet with not enough vitamins can have serious consequences.
• There are two types of vitamins: fat-soluble and water-soluble
• The fat-soluble vitamins A, D, E, and K can be stored in the fatty
tissues of the body. The body can store it for future use.
• The water-soluble vitamins C and the B vitamins dissolve in water and
can’t be stored in the body. They should be included in the foods you
eat each day. Eating a diet with a variety of foods is important to
supply the daily vitamin needs of your body.
Taking extra doses of vitamin supplements doesn’t benefit the body
and can sometimes be harmful. Too much vitamins A, D, E, and K can
be toxic.
Minerals – minerals are inorganic nutrients that the body needs in
small amounts. Some examples are calcium, iron, and magnesium.
Calcium is needed for bones and teeth, iron makes hemoglobin.
Calcium, sodium, and potassium are needed for normal functioning of
nerves.
Nutrition and a Balanced Diet
The Food Guide Pyramid classifies foods into six groups. It shows how
many servings from each group should be eaten every day.
38-2 The Process of Digestion
The digestive system includes the mouth, pharynx, esophagus,
stomach, small intestine, and large intestine. Several major accessory
structures, including the salivary glands, the pancreas, and the liver,
add secretions to the digestive system.
The Mouth
• Mechanical digestion is the physical breakdown of large pieces of
food into smaller pieces. Chewing begins the process of mechanical
digestion.
• As you chew your food, digestive enzymes begin the breakdown of
carbohydrates into smaller molecules. This is called chemical
digestion. During chemical digestion, large food molecules are broken
down into smaller food molecules.
• The function of the digestive system is to help convert foods into
simpler molecules that can be absorbed and used by the cells of the
body.
• Teeth are anchored in the bones of the jaw. The surfaces of the teeth
are protected by enamel. Teeth cut, tear, and crush food into small
fragments.
• Salivary glands secrete saliva, which helps to moisten the food and
make it easier to chew. The release of saliva can be triggered by the
scent of food, especially when you are hungry!
• Amylase – an enzyme in saliva that breaks the chemical bonds in
starches and releases sugars
The Esophagus
• The Esophagus is a food tube that leads to your stomach.
• Smooth muscle contractions move the food down the esophagus to
the stomach. These contractions are called peristalsis.
• A thick ring of muscle, called the cardiac sphincter, closes the
esophagus after food goes into the stomach to prevent the food from
coming back up into the esophagus.
• Heartburn is a painful, burning sensation just above the stomach.
This is caused by the backflow of stomach acid. This can be caused by
overeating or drinking too much caffeine.
The Stomach
• Food from the esophagus empties into a large muscular sac call the
stomach. The stomach continues the mechanical and chemical
digestion of food.
• Chemical digestion – the stomach releases substances, including
enzymes, that break down the food. Pepsin and hydrochloric acid are
enzymes that breaks down protein. Chemical digestion of
carbohydrates does not happen in the stomach. It resumes in the
small intestines.
• Mechanical digestion – stomach muscles contract to churn and mix
stomach fluids and food, which creates a mixture called chyme. After
an hour or two the chyme enters the small intestine.
The Small Intestine
• Small intestine – digestive organ where most chemical digestion takes
place
• Food enters the duodenus first. This is the first of three parts of the
small intestine and is where almost all of the digestive enzymes enter
the intestine. Most of the chemical digestion and absorption of food
happens in the small intestine. Chyme mixes with enzymes and
digestive fluids from the pancreas, the liver, and the lining of the
duodenum.
• Pancreas – a gland behind the stomach.
• The pancreas produces hormones to regulate blood sugar levels. It
also produces enzymes to break down carbohydrates, proteins, lipids,
and nucleic acids. The pancreas also produces sodium bicarbonate,
which neutralizes stomach acid so the enzymes can be effective. (acid
can made enzymes not work well)
Liver – a large organ above and to the right of the stomach that
produces bile. Bile is a fluid loaded with lipids and salts. It dissolves
and disperses fat found in fatty foods so enzymes can reach the smaller
fat molecules and break them down. Bile is stored in the gallbladder.
Absorption in the Small Intestine
• The three parts of the small intestine are the duodenum, the
jejunum, and the ileum. All together the small intestine is about 6
meters long.
• The small intestine absorbs nutrients. It is covered with finger-like
projections called villi.
• Villi provide a lot of surface area for the absorption of nutrient
molecules. Slow, wavelike contractions of smooth muscle s move the
chyme along this surface.
By the time food leaves the small intestines, there are no more
nutrients in it. Only water, cellulose, and other indigestible substances
are left.
The Large Intestine
• When chyme leaves the small intestine, it enters the large intestine,
or colon.
• The large intestine removes water from the undigested material that
is left. It then passes through the rectum and is eliminated from the
body.
Digestive System Disorders
• Acids from the stomach can sometimes damage the stomach’s lining,
producing a hole in the stomach wall. This is a peptic ulcer. Doctors
used to this that peptic ulcers were caused by too much acid in the
stomach. They now know that they are caused by an infectious
disease that ca be cured with antibiotics. The cure rate is 90%.
• Other disorders include diarrhea and constipation. If not enough
water is absorbed, diarrhea occurs. If too much water is absorbed,
constipation occurs.
38-3 The Excretory System
Functions of the Excretory System
• The role of the excretory system is to get rid of waste products.
• Excretion is the process in which metabolic wastes are eliminated.
• The skin and the lungs are two organs of excretion. The skin excretes
excess water, salts, and urea in the form of sweat. Lungs excrete
carbon dioxide.
• The liver also plays an important role in excretion. It converts amino
acids into useful compounds, leaving waste. The liver converts the
waste into urea. Urea and other wastes are removed from the
bloodstream through the kidneys.
• The kidneys play an important role in maintaining homeostasis. They
remove waste products from the blood, maintain blood pH and
regulate the water content of the blood and blood volume.
The Kidneys
• The kidneys are located on both sides of the spinal column new the
lower back.
• Ureter – a tube that leaves the kidneys and carries urine to the
urinary bladder.
• Urinary bladder – a saclike organ where urine is stored before being
excreted.
• There are two regions in the kidneys. The inner part is called the
renal medulla. The outer part is called the renal cortex. The
functional parts of the kidney are called nephrons.
• Each nephron has its own blood supply: an arteriole, a venule, and a
network of capillaries connecting them. As blood enters a nephron
through the arteriole, impurities are filtered out and emptied into the
collecting duct. The purified blood exits the nephron through the
venule.
Blood purification involves filtrations and reabsorption.
• Filtration – passing a liquid or gas through a filter to remove wastes
• The filtration of blood takes place in the glomerulus.
• Glomerulus – small network of capillaries encased in the upper end
of the nephron by a hollow, cup-shaped structure called Bowman’s
capsule
• Fluid from blood flows into Bowman’s capsule. The materials that are
filtered from the blood are called the filtrate. The filtrate contains
water, urea, glucose, salts, amino acids, and some vitamins.
• Reabsorption – the process in which liquid is taken back into a vessel.
• Most of the material removed from the blood at Bowman’s capsule
makes its way back into the blood.
• The material that remains is called urine.
• Urine is concentrated in the loop of Henle.
• Loop of Henle – a section of the nephron tubule in which water is
conserved and the volume of urine is minimized.
Urine is stored in the bladder until it is released from the body through
a tube called the urethra.
Urine Testing
• Drugs remain in the filtrate and come out in the urine. Testing the
urine is a good way to see if a person has done drugs.
Kidney Stones
• Calcium, magnesium, or uric acid salts in the urine can crystallize and
form kidney stones. The stones can block the ureter and cause a lot
of pain. Kidney stones can be treated with ultrasound waves. The
sound waves pulverize the stones into smaller fragments, and then
they can come out with urine.
Homeostasis by Machine
• If anything goes wrong with the kidneys, serious medical problems
can develop. Humans have two kidneys and can live with just one,
but if both are damaged there are two things that can keep a person
alive: a kidney transplant or dialysis.
A kidney dialysis machine removes blood from the body through a tube
inserted in the arm and pumps it through special tubing that acts like
nephrons. Tiny pores in the tubing allow salts and small molecules,
including nitrogen wastes to pass through. Wastes diffuse out of the
blood into the fluid-filled chamber which allows purified blood to be
returned to the body.
• Dialysis is expensive and time-consuming. It takes several hours a day
and about three times a week.
• Medical science is working on making an artificial kidney.