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Chapter 38
# Title
Digestive and Excretory Systems
Summary
38–1 Food and Nutrition
One calorie is equal to the amount of energy needed to raise the
temperature of one gram of water by one degree Celcius. The
energy in food is measured in dietary Calories (with a capital C).
One Calorie is equal to 1000 calories. The number of Calories you
need each day depends on your age, sex, and activity level.
Nutrients are substances in food that supply the body with
energy and raw materials needed for growth, repair, and maintenance. The nutrients that the body needs are water, carbohydrates,
fats, proteins, vitamins, and minerals.
Every cell in the human body needs water because many
of the body’s processes take place in water. Water makes up
a large part of blood and other body fluids. Sweating removes
water in order to cool the body by evaporation. Water must be
replaced regularly.
Simple and complex carbohydrates are the body’s main source
of energy. Simple carbohydrates do not have to be digested or
broken down. They provide quick energy for the body. Complex
carbohydrates must be broken down into simple sugars to be used
for energy.
Fats are formed from fatty acids and glycerol. Saturated fats
are usually solid at room temperature. Unsaturated fats are usually
liquid at room temperature. Fats protect organs and joints. They help
make up cell membranes, and insulate the body.
Proteins supply raw materials for growth and repair of the
body. Amino acids form proteins. The human body can produce
twelve of the twenty amino acids. The other eight must be obtained
from food. These amino acids are called essential amino acids.
Vitamins are organic molecules that help regulate body
processes. Fat-soluble vitamins can be stored in fatty tissues.
Water-soluble vitamins cannot be stored and should be in the
foods that a person eats every day. Some diseases result when
the body does not receive a sufficient supply of vitamins. Vitamins
can have serious effects on a person’s health.
Minerals are inorganic nutrients. Your body usually needs
minerals in small amounts. Examples of minerals include calcium
and iron. The body loses minerals in sweat, urine, and other waste
products, so they must be replaced by eating foods.
The Food Guide Pyramid can help you choose a balanced diet.
Foods at the base of the pyramid should make up most of your diet.
Foods at the top of the pyramid should be used in small amounts.
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38–2 The Process of Digestion
The digestive system breaks down food into simpler molecules
that can be absorbed and used by cells. The human digestive
system is a one-way tube. It includes the mouth, pharynx,
esophagus, stomach, small intestine, and large intestine. Other
structures—salivary glands, pancreas, and liver—add secretions
to the digestive system.
1. Digestion starts in the mouth. Teeth tear and crush food
to begin mechanical digestion. Mechanical digestion is the
physical breakdown of large chunks of food into smaller
pieces. Salivary glands in the mouth secrete saliva, which
contains enzymes that break down starches into sugars. This
begins the process of chemical digestion. Chemical digestion
breaks down large food molecules into smaller food molecules.
2. The swallowed clump of food passes through the pharynx and
into the esophagus. A flap of skin, the epiglottis, keeps the food
from entering the trachea. Muscle contractions, called peristalsis,
squeeze food through the esophagus to the stomach.
3. Chemical and mechanical digestion take place in the stomach.
Glands in the stomach lining make hydrochloric acid and the
enzyme pepsin. The hydrochloric acid and pepsin start the
chemical digestion of protein. Stomach muscles contract to
churn and mix the stomach contents. This mechanical digestion
forms a liquid mixture.
4. Most chemical digestion and absorption of food occurs in the
small intestine. Enzymes from the pancreas help digest starch,
protein, and fat. A liquid called bile from the liver dissolves
and breaks up fat droplets. Several enzymes help break down
carbohydrates and proteins. Tiny fingerlike projections called
villi (singular: villus) increase the surface area of the small
intestine. Cells at the small intestine’s surface absorb nutrients.
5. The large intestine removes water from the undigested
material. The remaining waste passes out of the body.
Peptic ulcers, diarrhea, and constipation are digestive system
disorders. Bacteria cause most peptic ulcers. Diarrhea occurs when
too little water is removed from waste in the large intestine.
Constipation occurs when too much water is removed.
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Class __________________
Date __________
38–3 The Excretory System
During metabolism, cells make wastes such as carbon dioxide and
urea. Excretion is the process in which the body eliminates wastes.
The main organs of excretion are the kidneys. The kidneys play
an important role in maintaining homeostasis. They remove
waste products from blood, maintain blood pH, and control the
water content of blood.
Two kidneys are located in the lower back. Blood containing
wastes enters the kidneys. The kidneys remove urea, excess water,
and other substances from the blood. This cleaned blood returns to
circulation. The wastes are excreted. The wastes are removed and
passed to the ureter.
The basic unit of function in a kidney is the nephron. Each
nephron is a small self-sufficient processing unit. As blood enters
a nephron, impurities are filtered out and emptied into the
collecting duct. The purified blood exits the nephron through the
venule. The processes of filtration and reabsorption take place in
the nephrons.
• Filtration removes wastes from the blood. It occurs in a part
of the nephron called the glomerulus. The glomerulus is
enclosed within a structure called Bowman’s capsule.
• Reabsorption returns some of the filtered materials back to
the blood. These materials include nutrients and water.
Fluid that remains in the kidneys is called urine. Urine contains
urea, excess salts, and other substances. After some water is
removed the urine leaves each kidney through a tube called the
ureter. The ureters carry urine to the urinary bladder, where urine
is stored. Urine leaves the body through a tube called the urethra.
The kidney’s activity is controlled by hormones and by the
composition of blood. Drinking excess water increases the amount
of water in the blood. This causes the kidneys to decrease the
amount of water they reabsorb and return to the blood. Similarly,
salty foods result in excess salt in the blood. To keep the composition of blood the same, the kidneys excrete the excess salt in urine.
A person can live with only one kidney. If both kidneys
fail, the person must receive a kidney transplant or undergo
dialysis. Dialysis purifies the blood by passing it through a
filtering machine.
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