body odor
Smell that is produced by the breakdown of sweat by bacteria that live on the skin.
dermis
The layer of skin directly under the epidermis; made of a tough connective tissue that contains the protein collagen.
epidermis
The outermost layer of the skin; forms the waterproof, protective wrap over the body's surface; made up of many layers of epithelial cells.
integumentary system
The outer covering of the body; made up of the skin, hair, and nails.
keratin
Tough, waterproof protein that is found in epidermal skin cells, nail, and hair.
melanin
The brownish pigment that gives skin and hair their color.
melanocyte
Melanin-producing cells; found in the bottom layer of the epidermis.
melanoma
Cancer of melanin-containing cells (melanocytes); mostly linked to long-term exposure to UV radiation.
oil gland
Skin organ that secretes an oily substance, called sebum, into the hair follicle.
subcutaneous tissue
Fatty layer of tissue that lies under the dermis, but is not part of the skin.
sunburn
A burn to the skin that is caused by overexposure to UV radiation from the sun's rays or tanning beds.
sweat gland
Gland that opens to the skin surface through skin pores; found all over the body; secretes sweat.
Points to Consider
How might what you eat affect your bones?
What do you think is the most important function of your skeletal system?
Lesson 16.3: Skeletal System
Lesson Objectives
Identify the main tissues and organs of the skeletal system.
List four functions of the skeletal system.
Describe three movable joints.
Identify two nutrients that are important for a healthy skeletal system.
Describe two skeletal system injuries.
Check Your Understanding
What is an organ system?
What is connective tissue?
Introduction
How important is your skeleton? Can you imagine your body without it? You would be a wobbly pile of muscle and internal organs, and you would not be able to move around much. You will learn about these functions in this lesson. Your skeleton is important for many different things. Bones are the main organs of the skeletal system. They are made up of living tissue. If you think you have broken a bone it’s important to visit a health-care professional. A broken bone may not heal properly by itself. A sprain can be bandaged properly to reduce swelling and discomfort. A doctor or other health-care professional can also give you advice on how to manage such an injury at home.
Your Skeleton
Humans are vertebrates, which are animals that have a backbone. The sturdy scaffolding of bones and cartilage that is found inside vertebrates is called a skeleton. The adult human skeleton has about 206 bones, some of which are named in Figure below. The skeletons of babies and children have many more bones and more cartilage than adults have. As a child grows, these “extra” bones grow into each other, and cartilage gradually hardens to become bone tissue.
You may think that bones are dry and lifeless, but they are very much alive. The white, hard bones that you might see in a museum or science book, are only the hard mineral remains of the bone tissue. Living bones are full of life. They contain many different types of tissues.
Cartilage is found at the end of bones and is made of tough protein fibers called collagen. Cartilage creates smooth surfaces for the movement of bones that are next to each other, like the bones of the knee. Ligaments are made of tough protein fibers and connect bones to each other. Your bones, cartilage, and ligaments make up your skeletal system.
Figure 16.12
The skeletal system is made up of bones, cartilage, and ligaments. The skeletal system has many important functions in your body.
Functions of Bones
Your skeletal system gives shape and form to your body, but it is also important in other homeostatic functions. The main functions of the skeletal system are:
Support The skeleton supports the body against the pull of gravity. The large bones of the lower limbs support the trunk when standing.
Protection The skeleton provides a framework that supports and protects the soft organs of the body. For example, the skull surrounds the brain to protect it from injury. The bones of the rib cage help protect the heart and lungs.
Movement Bones work together with muscles as simple mechanical lever systems to move the body.
Making Blood Cells Blood cells are made mostly inside certain types of bones.
Storage Bones store calcium. They contain more calcium than does any other organ. Calcium is released by the bones when blood levels of calcium drop too low. The mineral phosphorus is also stored in bones.
Structure of Bones
Bones are organs. Recall that organs are made up of two or more types of tissues. Bones come in many different shapes and sizes, but they are all made of the same materials. The two main types of bone tissue are compact bone and spongy bone. Compact bone makes up the dense outer layer of bones. Spongy bone is lighter and less dense than compact bone, and is found toward the center of the bone. The tough, shiny, white membrane that covers all surfaces of bones is called the periosteum.
Many bones also contain a soft connective tissue called bone marrow. There are two types of bone marrow: red marrow and yellow marrow. Red marrow makes red blood cells, platelets, and most of the white blood cells for the body (discussed in the Diseases and the Body’s Defenses chapter). Yellow marrow makes white blood cells. The bones of newborn babies contain only red marrow. As children get older, their red marrow is replaced by yellow marrow. In adults, red marrow is found mostly in the bones of the skull, the ribs, and pelvic bones. Bone come in four main shapes. They can be long, short, flat, or irregular. Identifying a bone as long, short, flat, or irregular is based on the shape of the bone not the size of the bone. For example, both small and large bones can be classified as long bones. The small bones in your fingers and the largest bone in your body, the femur, are long bones. The structure of a long bone is shown in Figure below.
Figure 16.13
Bones are made up of different types of tissues.
How Bones Develop and Grow
Your skeleton began growing very early in your development. After only eight weeks of growth from a fertilized egg, your skeleton was formed by cartilage and other connective tissues. At this point your skeleton was quite bendy and flexible. After a few more weeks of growth, the cells that form hard bone began growing in the cartilage, and your skeleton began to harden. However, not all of the cartilage is replaced by bone. Cartilage remains in many places in your body including your joints, your rib cage, your ears, and the tip of your nose.
A baby is born with zones of cartilage in its bones that allow growth of the bones. These areas, called growth plates, allow the bones to grow longer as the child grows. By the time the child reaches an age of about 18 to 25 years, all of the cartilage in the growth plate has been replaced by bone. This stops the bone from growing any longer.
Even though bones stop growing in length in early adulthood, they can continue to increase in thickness throughout life. This thickening occurs in response to strain from increased muscle activity and to weight-bearing exercise.
Joints and How They Move
A joint is a point at which two or more bones meet. There are three types of joints in the body: fixed, partly movable, and movable. Fixed joints do not allow any bone movement. Many of the joints in your skull are fixed (Figure below). Partly movable joints allow only a little movement. Your backbone has partly movable joints between the vertebrae (Figure below). Movable joints allow movement and provide mechanical support for the body. Joints are a type of lever, which is a rigid object that
is used to increase the mechanical force that can be applied to another object. Can openers and scissors are examples of levers. Joints reduce the amount of energy that is spent moving the body around. Just imagine how difficult it would be to walk about if you did not have knees!
Figure 16.14
The skull has fused joints. Fused joints do not allow any movement of the bones, which protects the brain from injury.
Figure 16.15
The joints between your vertebrae (b) are partially movable.
Movable Joints
Movable joints are the most mobile joints of all. They are also the most common type of joint in your body. Your fingers, toes, hips, elbows, and knees all provide examples movable joints. The surfaces of bones at movable joints are covered with a smooth layer of cartilage. The space between the bones in a movable joint is filled with a liquid called synovial fluid. Synovial fluid is a thick, stringy fluid that looks a lot like egg white. The fluid lubricates and cushions the bones when they move at the joint. There are many different types of movable joints, and many different examples. Four types of movable joints are shown in Figures below, below and below.
In a ball and socket joint the ball-shaped surface of one bone fits into the cup-like shape of another. Examples of a ball and socket joint include the hip, shown in Figure below, and the shoulder.
Figure 16.16
Your hip joint is a ball and socket joint. The ball end of one bone fits into the socket of another bone. These joints can move in many different directions.
In a hinge joint, the ends of the bones are shaped in a way that allows motion only in two directions, forward and backward. Examples of hinge joints are the knees and elbows. A knee joint is shown in Figure below.
Figure 16.17
The knee joint is a hinge joint. Like a door hinge, a hinge joint allows backward and forward movement.
The pivot joint is formed by a process that rotates within a ring, the ring being formed partly of bone, and partly of ligament. An example of a pivot joint is the joint between the radius and ulna that allows you to turn the palm of your hand up and down. A pivot joint is shown in Figure below.
A gliding joint is a joint which allows only gliding movement. The gliding joint allows one bone to slide over the other. The gliding joint in your wrist allows you to flex your wrist. It also allows you to make very small side-to-side motions. There are also gliding joints in your ankles.
Figure 16.18
Pivot Joint The joint at which the radius and ulna meet is a pivot joint. Movement at this joint allows you to flip your palm over without moving your elbow joint.
Keeping Bones and Joints Healthy
Just like a houseplant depends on your taking good care of it by watering it and giving it plant food, so too does your body depend on you! You can help keep your bones and skeletal system healthy by eating well and getting enough exercise. Weight-bearing exercises help keep bones strong. Weight-bearing exercises work against gravity. Such activities include basketball, tennis, gymnastics, karate, running, and walking. When the body is exercised regularly by performing weight-bearing activity, bones respond by adding more bone cells to increase the bones' density.
Eating Well
Did you know that what you eat as a teenager can affect how healthy your skeletal system will be in 30, 40, and even 50 years? Calcium and vitamin D are two of the most important nutrients for a healthy skeletal system. Your bones need calcium to grow properly. If you do not get enough calcium in your diet as a teenager, your bones may become weak and break easily later in life. Osteoporosis is a disease in which bones become lighter and more porous than they should be. Light and porous bones are more likely to break, which can cause pain and prevent a person from walking. Being immobile can cause more bone loss, making the disease even worse.
Older women are most likely to develop osteoporosis because it is linked to the decrease in production of sex hormones. However, poor nutrition, especially diets that are low in calcium and vitamin D, increase the risk of osteoporosis in later life. Not doing regular weight-bearing exercises is also linked to having thinner, weaker bones. Two of the easiest ways to prevent osteoporosis is to eat a healthful diet that has the right amount of calcium and vitamin D, and to do some sort of weight-bearing exercise every day.
Foods that are a good source of calcium include: milk, yogurt, and cheese. Non-dairy sources of calcium include Chinese cabbage, kale, and broccoli. Many fruit juices, fruit drinks, tofu, and cereals have calcium added into them. These foods are also an important source of calcium.
It is recommended that teeagers get 1300 mg of calcium every day. For example, one cup of milk provides about 300 mg of calcium, or about 30% of the daily requirement. Other sources of calcium are shown in Figure below.
Figure 16.19
There are many different sources of calcium. Getting enough calcium in your daily diet is important for good bone health. How many ounces of cheddar cheese would provide your recommended daily intake of calcium?
Your skin makes vitamin D when exposed to sunlight. The pigment melanin in the skin acts like a filter that can prevent the skin from making vitamin D. As a result, people with darker skin need more time in the sun than people with lighter skin to make the same amount of vitamin D.
Fish is naturally rich in vitamin D. Vitamin D is added to other foods including milk, soy milk and breakfast cereals. Teenagers are recommended to get 5 micrograms (200 IU/day) of vitamin D every day. A 3 ½ ounce portion of cooked salmon provides 360 IU of vitamin D.
Lack of vitamin D, or deficiency, can be caused by two different things: not enough sunlight exposure, and lack of vitamin D in the diet. Vitamin D deficiency results in problems with bone growth and hardening. This leads to bone softening diseases such as rickets in children and osteomalacia in adults. Osteomalacia is a bone disease in which the bones do not harden properly and they can break easily. Rickets is a type of osteomalacia. An X-ray of a child that has rickets is shown in Figure below. Lack of vitamin D may also be related to osteoporosis.
Figure 16.20
Rickets is a softening of the bones in children that may cause fractures and bending of the bones, especially of the legs. The bones have not hardened properly because of lack of vitamin D, and bend under the weight of the body.
Bone Fractures
Even though they are very strong, bones can fracture, or break. Fractures can happen at different places on a bone. They are usually due to excess bending stress on the bone. Bending stress is what causes a pencil to break if you were to bend it too far. Soon after a fracture, the body begins to repair the break. The area becomes swollen and sore. Within a few days bone cells travel to the break site and begin to rebuild the bone. It takes about 2 to 3 months before compact and spongy bone form at the break site.
Sometimes the body needs extra help in repairing a broken bone. In such a case a surgeon will piece a broken bone together with metal pins. Moving the broken pieces together will help keep the bone from moving, and give the body a chance to repair the break. A broken ulna has been repaired with pins in Figure below.
Figure 16.21
The upper part of the ulna, just above the elbow joint is broken, as you can see in the X-ray at left. The x-ray at right was taken after a surgeon inserted a system of pins and wires across the fracture to bring the two pieces of the ulna into close proximity. Such a system is called a tension band, and is very stable. The line of staples were used to close the skin incision.
Cartilage Injuries
Osteoarthritis is a condition in which the cartilage at the ends of the bones breaks down. The break down of the cartilage leads to pain and stiffness in the joint. Decreased movement of the joint because of the pain may lead to weakening of the muscles that normally move the joint, and the ligaments surrounding the joint may become looser. Osteoarthritis is the most common form of arthritis. It has many causes, including aging, sport injuries, fractures involving a joint that are not stabilized, and obesity. Total
hip replacement is a common treatment for osteoarthritis of that joint.
Ligament Injuries
Recall that a ligament is a short band of tough connective tissue that connects bones together to form a joint. Ligaments can get injured when a joint gets twisted or bends too far. The protein fibers that make up a ligament can get strained or torn, causing swelling and pain. Injuries to ligaments are called sprains. Ankle sprains are a common type of sprain. A small ligament in the knee, called the anterior cruciform ligament (ACL), is a common site of injury in athletes. Ligament injuries can take a long time to heal. Treatment of the injury includes rest and special exercises that are developed by a physical therapist.
Preventing Injuries
Preventing injuries to your bones and ligaments is easier and much less painful than treating an injury. Wearing the correct safety equipment when performing activities that require such equipment can help prevent many common injuries. For example, wearing a bicycle helmet can help prevent a skull injury if you fall. Warming up and cooling down properly can help prevent ligament and muscle injuries. Torn ligaments and fractured bones are common sport injuries. Such injuries need to be treated by a doctor. Overuse injuries such as ligament strains and tears are common injuries for teenage athletes. Correct conditioning and enough rest can help prevent overuse injuries.
CK-12 Life Science Page 40