CK-12 Life Science
Page 42
Figure 16.27
Anaerobic exercises involve the muscles working against resistance. In this case the resistance is the persons own body weight.
Aerobic Exercise and Muscular Endurance
Aerobic exercises are exercises that cause your heart to beat faster and allow your muscles to use oxygen to contract. Aerobic exercise causes many different changes in skeletal muscle. Muscle energy stores are increased, the ability to use oxygen improves, and more capillaries surround the muscle fibers. These changes result in the ability of the muscle to avoid getting tired, and to use oxygen and food more efficiently. Aerobic exercise also helps improve cardiac muscle. It results in the heart being able to pump a larger volume of blood with each beat due to an increase in the size of the heart’s ventricles. Examples of an aerobic exercise are shown in Figure below.
Figure 16.28
When done regularly, aerobic activities such as cycling, make the heart stronger.
Both aerobic and anaerobic exercises also improve the ability of the heart to pump blood around the body. Aerobic exercise causes the heart to get bigger, and anaerobic exercise causes the walls of the heart to get thicker. These changes allow the heart to push more blood throughout the body with every heartbeat.
Keeping Muscles Healthy
Being physically active for 60 minutes a day for at least five days a week improves your physical fitness. Being physically active can also help you to reduce your risk of developing diseases such as cardiovascular disease, Type 2 diabetes, obesity, and certain forms of cancer. Being physically active does not mean you have to do boring workouts. You do not have to join a gym or be in a sports team to be physically active. Physical activities can include everyday things such as walking your dog, vacuuming or sweeping, cycling to school, skating, or climbing a flight of stairs (Figure below).
Figure 16.29
Adding more physical activity in your daily life does not mean boring or expensive activities, it can be fun! Local and community pools often run swim classes that do not cost a lot, and are designed for beginners.
Muscle Injuries
Sometimes muscles and tendons get injured when a person starts doing an activity before they have warmed up properly. A warm up is a slow increase in the intensity of a physical activity that prepares muscles for an activity. Warming up increases the blood flow to the muscles and increases the heart rate. Warmed-up muscles and tendons are less likely to get injured. For example, before running or playing soccer, a person might jog slowly to warm muscles and increase their heart rate. It is important that warm ups prepare the muscles that are to be used in the activity. Even elite athletes need to warm up, as shown in Figure below. Some injuries are caused by overuse. An overuse injury happens if the muscle or joint is not rested enough between activities.
A strain is an injury to a muscle in which the muscle fibers tear because the muscle contracts too much or contracts before the muscle is warmed up. Strains are also known as pulled muscles. Overuse injuries often involve tendons. Overuse of tendons can cause tiny tears within the protein fibers of the tendon, which gradually weakens the tissue. These tiny tears lead to swelling, pain, and stiffness; a condition called tendinitis. Tendinitis can affect any tendon that is overused. Strains and tendinitis are usually treated with rest, cold compresses, and stretching exercises that a physical therapist designs for each patient.
Figure 16.30
Warming up before the game helps the players avoid injuries. Some warm-ups may include stretching exercises. Some researchers believe stretching before activities may help prevent injury.
Proper rest and recovery are also as important to health as exercise is. If you do not get enough rest, your body will become injured and will not improve or react well to exercise. It is important to remember to allow enough recovery time for muscles and tendons to rest between exercise sessions. You can rest muscles by doing a different activity to what you normally do. For example, if you run, you can rest your running muscles and joints by swimming. This type of rest is called active rest.
Anabolic steroids are hormones that cause the body to build up more protein in its cells. Muscle cells, which contain a lot of protein, get bigger when exposed to anabolic steroids. Your body naturally makes small amounts of anabolic steroids. They help your body repair from injury, and help to build bones and muscles. Anabolic steroids are used as medicines to treat people that have illnesses that affect muscle and bone growth. However, some people who do not need anabolic steroid as medicine try to increase their muscle size by taking these steroids. When taken in this way, anabolic steroids can have long-term affects other body systems. They can damage the person’s kidneys, heart, liver, and reproductive system. If taken by adolescents, anabolic steroids can cause bones to stop growing, resulting in stunted growth.
Lesson Summary
The body has three types of muscle tissue: skeletal, cardiac, and smooth. Muscles move the body by contracting against the skeleton. Muscles are controlled by the nervous system.
Nerves send messages to the muscular system from the brain. Nerves also send messages to the brain from the muscles. Regular exercise improves the health of the muscular system. Muscles that are exercised are bigger and stronger than muscles that are not exercised.
Exercise improves both muscular strength and muscular endurance. Muscular strength is the ability of a muscle to exert force during a contraction. Muscular endurance is the ability of a muscle to continue to contract over a long time without getting tired. Identify two types of injuries to the muscular system.
A strain is an injury to a muscle in which the muscle fibers tear because the muscle contracts too much or contracts before the muscle is warmed up. Tiny tears and swelling in a tendon results in tendinitis.
Review Questions
Name the three types of muscle tissue in the body.
Which of the three types of muscles in the body are voluntary?
What is another name for muscle cells?
Describe how skeletal muscles and bones work together to move the body.
What is a tendon?
How does aerobic exercise affect the heart?
How does aerobic exercise affect skeletal muscle?
How does anaerobic exercise affect skeletal muscle?
What is a muscle strain?
Why is warming up before exercise a good idea?
Why are taking anabolic steroids a dangerous way to try to build up muscles?
Further Reading / Supplemental Links
http://www.hmc.psu.edu/healthinfo/m/musclestrain.htm
http://www.cdc.gov/nccdphp/dnpa/physical/everyone/index.htm
http://en.wiki.org
Vocabulary
aerobic exercises
Types of exercises that cause the heart to beat faster and allow the muscles to obtain energy to contract by using oxygen.
anabolic steroids
Hormones that cause the body to build up more protein in its cells.
anaerobic exercise
Types of exercises that involve short bursts of high-intensity activity; forces the muscles to obtain energy to contract without using oxygen.
cardiac muscle
An involuntary and specialized kind of muscle found only in the heart.
contraction
Shortening of muscle fibers.
extensor
The muscle that contracts to cause a joint to straighten.
flexibility exercises
Stretching exercises that improve the range of motion of muscles and joints.
flexor
The muscle that contracts to cause a joint to bend.
involuntary muscle
A muscle that a person cannot consciously control; cardiac muscle and smooth muscle are involuntary.
muscle cells
Long, thin cells that can contract; also called muscle fibers.
muscle fibers
Long, thin cells that can contract; also called muscle cells.
musc
ular endurance
The ability of a muscle to continue to contract over a long time without getting tired.
muscular strength
The ability of a muscle to exert force during a contraction.
muscular system
The body system that allows movement.
physical exercise
Any activity that maintains or improves physical fitness and overall health.
physical fitness
The ability of your body to carry out your daily activities without getting out of breath, sore, or overly tired.
skeletal muscle
The muscle that is usually attached to the skeleton.
smooth muscle
Involuntary muscle found within the walls of organs and structures such as the esophagus, stomach, intestines, and blood vessels.
strain
An injury to a muscle in which the muscle fibers tear because the muscle contracts too much or contracts before the muscle is warmed up.
tendinitis
A condition in which tiny tears form in the protein fibers of the tendon and gradually weaken the tissue.
tendon
A tough band of connective tissue that connects a muscle to a bone.
voluntary muscle
A muscle that a person can consciously control; skeletal muscle is voluntary.
warm-up
A slow increase in the intensity of a physical activity that prepares muscles for an activity.
Points to Consider
How does your muscular system depend on your digestive system?
How does what you choose to eat affect your muscular system and your skeletal system?
Chapter 17: Food and the Digestive System
Lesson 17.1: Food and Nutrients
Lesson Objectives
Explain why the body needs food.
Identify the roles of carbohydrates, proteins, and lipids.
Give examples of vitamins and minerals, and state their functions.
Explain why water is a nutrient.
Check Your Understanding
What are the four types of organic compounds?
What do all cells need in order to function?
What are muscles made of?
Introduction
Did you ever hear the old saying “An apple a day keeps the doctor away”? Do apples really prevent you from getting sick? Probably not, but eating apples and other fresh fruits can help keep you healthy. The girl shown in Figure below is eating fresh vegetables as part of a healthy meal. Why do you need foods like these for good health? What role does food play in the body?
Figure 17.1
This girl is eating a salad of tomatoes and leafy green vegetables. Fresh vegetables such as these are excellent food choices for good health.
Why We Need Food
Your body needs food for three reasons:
Food gives your body energy. You need energy for everything you do.
Food provides building materials for your body. Your body needs building materials so it can grow and repair itself.
Food contains substances that help control body processes. Your body processes must be kept in balance for good health. For example, your body needs the right balance of water and salts.
For all these reasons, you must have a steady supply of nutrients. Nutrients are chemicals in food that your body needs. There are six types of nutrients: carbohydrates, proteins, lipids, vitamins, minerals, and water. Carbohydrates, proteins, and lipids give your body energy. Proteins provide building materials. Proteins, vitamins, and minerals help control body processes.
Nutrients that Provide Energy
Molecules of carbohydrates, proteins, and lipids contain energy. When your body digests food, it breaks down the molecules of these nutrients. This releases the energy so your body can use it. The energy in food is measured in units called Calories.
Carbohydrates
Carbohydrates are nutrients that include sugars, starches, and fiber. How many grams of carbohydrates you need each day are shown in Figure below. It also shows some foods that are good sources of carbohydrates.
Figure 17.2
Up to the age of 13 years, you need about 130 grams of carbohydrates a day. Most of the carbohydrates should be complex. They are broken down by the body more slowly than simple carbohydrates. Therefore, they provide energy longer and more steadily. What other foods do you think are good sources of complex carbohydrates?
Sugars are small, simple carbohydrates that are found in foods such as fruits and milk. The sugar found in fruits is called fructose. The sugar found in milk is called lactose. These sugars are broken down by the body to form glucose, the simplest sugar of all. Glucose is used by cells for energy. Remember the discussion of cellular respiration in the Cell Functions chapter? Cellular respiration turns glucose into the usable form of chemical energy, ATP. One gram of sugar provides your body with four Calories of energy.
Some people cannot digest lactose, the sugar in milk. This condition is called lactose intolerance. If people with this condition drink milk, they may have cramping, bloating, and gas. To avoid these symptoms, they should not drink milk, or else they should drink special, lactose-free milk.
Starches are large, complex carbohydrates. They are found in foods such as vegetables and grains. Starches are broken down by the body into sugars that provide energy. Like sugar, one gram of starch provides your body with four Calories of energy.
Fiber is another type of large, complex carbohydrate. Unlike sugars and starches, fiber does not provide energy. However, it has other important roles in the body. There are two types of fiber found in food: soluble fiber and insoluble fiber. Each type has a different role.
Soluble fiber dissolves in water. It helps keep sugar and fat at normal levels in the blood.
Insoluble fiber does not dissolve in water. As it moves through the large intestine, it absorbs water. This helps keep food waste moist so it can pass easily out of the body.
Eating foods high in fiber helps fill you up without providing too many Calories. Most fruits and vegetables are high in fiber. Some examples are shown in Figure below.
Figure 17.3
Between the ages of 9 and 13 years, girls need about 26 grams of fiber a day, and boys need about 31 grams of fiber a day. Do you know other foods that are high in fiber?
Proteins
Proteins are nutrients made up of smaller molecules called amino acids. As discussed in the Introduction to Living Things chapter, the amino acids are arranged like "beads on a string." These amino acid chains then fold up into a three-dimensional molecule. Proteins have several important roles in the body. For example, proteins:
Make up muscles.
Help control body processes.
Help the body fight off bacteria and other “foreign invaders.”
Carry substances in the blood.
If you eat more proteins than you need for these purposes, the extra proteins are used for energy. One gram of protein provides four Calories of energy. This is the same amount as one gram of sugar or starch. How many grams of proteins you need each day are shown in Figure below. It also shows some foods that are good sources of proteins.
Figure 17.4
Between the ages of 9 and 13 years, you need about 34 grams of proteins a day. What other foods do you think are good sources of proteins?
There are many different amino acids, the building blocks of proteins, but your body needs only 20 of them. Your body can make ten of these amino acids from simpler substances. The other ten amino acids must come from the proteins in foods. These ten are called essential amino acids. Only animal foods, such as milk and meat, contain all ten essential amino acids in a single food. Plant foods are missing one or more essential amino acids. However, by eating a combination of plant foods, such as beans and rice, you can get all ten essential amino acids.
Lipids
Lipids are nutrients such as fats that store energy. The heart and skeletal muscles rely main
ly on lipids for energy. One gram of lipids provides nine Calories of energy. This is more than twice the amount provided by carbohydrates or proteins. Lipids have several other roles in the body. For example, lipids:
Protect nerves.
Help control blood pressure.
Help blood to clot.
Make up the membranes that surround cells.
Fats are one type of lipid. Fat is the main form in which the body stores energy. Stored fat gives your body an energy reserve. It’s like having money in a savings account. It’s there in case you need it. Stored fat also cushions and protects internal organs. In addition, it insulates the body. It helps keep you warm in cold weather.
Fats and other lipids are necessary for life. However, they can be harmful if you eat too much of them, or the wrong type of fats. Fats can build up in the blood and damage blood vessels. This increases the risk of heart disease. There are two types of lipids: saturated lipids and unsaturated lipids.
Saturated lipids are harmful even in very small amounts. They should be avoided as much as possible. Saturated fats are found mainly in animal foods, such as meats, whole milk, and eggs. Saturated fats increase cholesterol levels in the blood. Cholesterol is a fatty substance that is found naturally in the body. Too much cholesterol in the blood can lead to heart disease. It is best to limit the amount of saturated fats in your diet.