6th-Chapter 2: Lesson 2.1 Supporting and Moving the Body

Organization of the Body

The is organized into , , , and body systems (Figure 2.1). are the basic unit of life. All living things, including the human body, are made of cells.

In the body, cells are organized as tissues. A is a collection of similar cells that do a certain job for the body. For example, is made of muscle cells. Muscle tissue can contract and shorten, enabling muscles to move.

Some tissues form glands. A is a group of cells that produce and release substances into the body. For example, the glands in the mouth release saliva. This liquid breaks down food so that it can be swallowed.

Tissues work together to form organs. An is a collection of tissues that perform a specific job. For example, the stomach is an organ. Its job is to store and . The stomach is made of several kinds of tissue, which include the following:

• muscle

• tissue

• tissue

work with other organs. A collection of organs that works together is a . This chapter reviews most of the major body systems.

Each body system performs a set of important functions and makes up the human organism. Like the dimensions of health, body systems work very closely together. For example, the and systems work together so you can walk and run and move your arms. The respiratory and circulatory systems work closely together to bring air into the body and move it through the blood. Taking care of each system enables the others to work effectively.

Integumentary System

The system is one of three systems that support and move the body. The integumentary system includes the , , and . It covers and protects the entire body and may be the most familiar body system. You see the integumentary system as you look at another person and when you look in the mirror. It is the only body system completely exposed to the world outside the body.

Skin

The is the largest organ in the human body (Figure 2.2). If spread flat, the skin would cover square feet, about the size of a bedsheet. Skin protects the body and does a surprising number of important jobs. Taking care of the skin ensures it can do these jobs. Some of the tasks the skin performs include the following:

• keep out that could infect the body

• remove some waste and make D to build strong bones

• house nerve endings that allow people to , touch, and pressure

The skin houses various nerves, pores, , veins, and hair follicles. These items are housed within the three main layers of the skin: the epidermis, the dermis, and the hypodermis, as you saw in Figure 2.2.

The outermost layer of the skin is the epidermis. The epidermis, which consists of of its own, is the thinnest layer of the skin. The main function of the is to protect the body from infection by stopping foreign substances from entering into the body.

The middle layer of the skin is the . This layer contains two proteins, collagen and elastin. These proteins provide support to the skin and give skin the ability to stretch and return to its normal shape. With age, the body creates less of these proteins, which leads to the appearance of wrinkles and sagging skin.

The innermost layer of the skin is the . This layer consists of fat, , and nerve endings. It also connects the skin to the bone and muscle underneath. Additional functions of each layer of skin are listed in Figure 2.3.

Hair and Nails

Hair and nails are made by cells in the skin. Both are made of the protein keratin. Hair grows on all skin surfaces except the palms, soles, lips, nipples, and some areas of the genitals. Each hair grows from a specialized cell called a hair follicle. Hair, like the skin, helps the body. For example, eyelashes and eyebrows shield the eyes. Nose hair prevents dust and particles from entering the airways. Head hair helps regulate temperature and protects the head from .

protect the ends of fingers and toes. They grow on the upper sides of fingers and toes near the ends. As they grow, the older are pushed out, making the nails longer.

Skeletal System

The system is the body system made up of that provides structure, shape, and protection to the body (Figure 2.4). For example, you can stand upright because of your sturdy and backbone. You can walk because your are strong and movable. The ribs protect the heart, lungs, and other internal organs. The and protect the brain and spinal cord. Bones also make movement possible when they are attached to muscles.

Bone Tissue

develop, grow, and change throughout life. They are made of minerals, , and living cells. Bone hardness comes from the minerals calcium and phosphate. Bone flexibility comes from the protein . To envision bone tissue, think of a gelatin dessert with marbles in it. The marbles represent the hard . The firm, flexible gelatin represents collagen. Together, the gelatin and marbles have hardness and flexibility, like bones.

Bones can grow and change because they contain living . Some bone cells can make more bones. Other cells dissolve bones. Weight-bearing physical activities, such as and lifting weights, put stress on bones. That stress pushes bone-making cells to make more bone tissue.

Bone Structure

Bones come in many and sizes (Figure 2.5). For example, bones of the arms and legs are long. Bones of the skull, , and backbone are flat or have an irregular shape. Most bones have a dense outer tissue. Inside that dense tissue is a softer, bone tissue. This spongy tissue contains cells that can make blood cells. The long bones of the arms and legs also have a hollow space filled with fat.

The ends of some bones are covered with , which is not as hard as bone. The cartilage slowly turns into bone tissue until it is all used up, causing growth. This growth stops at different ages in different people. Some people stop in their mid-teens. Others continue growing into their mid-twenties.

The joint and other moving joints are kept moist by a special fluid. This reduces the amount of friction between moving bones. It also cushions the bones that meet in the joint. In a that moves, cartilage covers the bones’ surfaces. This tissue absorbs shock in the joint and protects the ends of the bones.

Muscular System

The system helps the body move and plays important parts in other body systems (Figure 2.6). For example, the skeletal and muscular systems work together to move the body. A muscular system can help you achieve tasks and prevent injury.

Muscles are made of tissue that can shorten and stretch. Muscle that is attached to a bone can make the bone move.

Muscles are to bones by tendons made of tough tissues. Like muscles, tendons can shorten or lengthen. Muscles, bones, and tendons work closely together.

Muscle Tissue

Muscles are built from bundles of muscle cells. Different muscle cell arrangements create different types of muscle. Following are the three types of muscle tissue:

• are attached to bones and can be controlled. For example, you can choose to move your legs to walk or move them faster to run.

• cannot be controlled. These muscles do very important work without you realizing it. For example, the smooth muscles in your intestines digest food.

• is found in the heart. This muscle pumps blood through the body.

Muscle Pairs

Muscles can contract (become shorter and tighter) and relax. Most skeletal muscles contract to move the bones of the body. When muscles relax, body parts return to an original state. Throughout the body, most skeletal muscles work in pairs to move certain body parts. Some examples of muscle pairs include the following:

• The contracts to bend the arm at the elbow. The triceps muscle contracts to straighten the arm at the elbow (Figure 2.7).

• The contracts to bend the leg at the knee. The quadricep muscle contracts to extend the leg at the knee.

• The contracts to bend the leg at the hip. The gluteus maximus muscle contracts to extend the leg at the hip.