2The levels of organization in the human body consist of cells, tissues, organs, and organ systems. A cell is the basic unit of structure and function in a living thing. Cells perform the basic processes that keep organisms alive. Most cells are too small to see without a microscope. In most animal cells, a cell membrane forms the outside boundary of the cell. Inside the cell membrane is a large structure called the nucleus. The nucleus is the control center that directs the cell’s activities and contains information that determines the cell’s characteristics. The area between the cell membrane and the nucleus is called the cytoplasm
3A tissue is a group of similar cells that perform the same function A tissue is a group of similar cells that perform the same function. The human body contains four basic types of tissue: muscle tissue, nerve tissue, connective tissue, and epithelial tissue. Muscle tissue can contract, or shorten. This tissue is what makes parts of your body move. Nervous tissue carries messages back and forth between the brain and every other part of the body. It directs and controls the body. Connective tissue provides support for your body and connects all its parts. Bone, fat, and blood ar e all connective tissues. Epithelial tissue covers the surfaces of your body. The skin and the lining of the digestive system are examples of epithelial tissue.
4An organ is a structure that is composed of different kinds of tissue An organ is a structure that is composed of different kinds of tissue. Like a tissue, an organ performs a specific job. An organ’s job is usually more complex than that of a tissue. The heart is an organ that pumps blood throughout the body.Each organ in your body is part of an organ system. An organ system is a group of organs that work together to perform a major function. Your heart is part of an organ system called the circulatory system. The blood vessels are also part of the circulatory system. The different organ systems work together and depend on one another.
5All the systems of the body work together to maintain homeostasis, the body’s tendency to keep an internal balance. Homeostasis is the process by which an organism’s internal environment is kept stable in spite of changes in the external environment. Sometimes, things can happen to throw off homeostasis. As a result, your heart may beat more rapidly or your breathing may quicken. These are signs of stress, the reaction of your body to potentially threatening, challenging, or disturbing events. When the stress is over, homeostasis is restored, and the body returns to its normal state.
6The Skeletal System Guide for Reading ■ What are the functions of the skeleton? ■ What role do joints play in the body? ■ What are the characteristics of bone, and how can you keep your bones strong and healthy?
7What the Skeletal System Does Your skeleton has five major functions. It provides shape and support, enables you to move, protects your organs, produces blood cells, and stores minerals and other materials until your body needs them.
8The Skeletal SystemThe skeleton is made up of all the bones in one’s body. Your skeleton has five major functions. It provides shape and support, enables you to move, protects your organs, produces blood cells, and stores minerals and other materials until your body needs them. The backbone, or vertebral column, is the center of the skeleton. The backbone is made up of 26 small bones, or vertebrae (singular vertebra). If your backbone were just one bone, you would not be able to bend or twist.
9Joints of the Skeleton - The Skeletal System A joint is a place in the body where two bones come together. Joints allow bones to move in different ways.
10The Skeletal SystemA joint is a place in the body where two bones come together. Joints allow bones to move in different ways. Immovable joints connect bones in a way that allows little or no movement. Movable joints allow the body to make a wide range of movements. The bones in movable joints are held together by a strong connective tissue called a ligament. Cartilage is a connective tissue that is more flexible than bone
11The Skeletal SystemBones are complex living structures that undergo growth and development. A thin, tough membrane covers all of a bone except the ends. Blood vessels and nerves enter and leave the bone through the membrane. Beneath the membrane is a layer of compact bone, which is hard and dense, but not solid.
12The Skeletal SystemSmall canals run through the compact bone, carrying blood vessels and nerves from the bone’s surface to the living cells within the bone. Just inside the compact bone is a layer of spongy bone, which has many small spaces within it. Spongy bone is also found at the ends of the bone. The spaces in bone contain a soft connective tissue called marrow. There are two types of marrow—red and yellow. Red bone marrow produces blood cells. Yellow marrow stores fat that serves as an energy reserve.
13The Skeletal SystemA combination of a balanced diet and regular exercise are important for a lifetime of healthy bones. As people become older, their bones begin to lose some minerals. Mineral loss can lead to osteoporosis, a condition in which the body’s bones become weak and break easily. Regular exercise and a diet rich in calcium can help prevent osteoporosis.
14Bones—Strong and Living - The Skeletal SystemBones—Strong and LivingBones are complex living structures that undergo growth and development.
15Asking Questions - The Skeletal System Before you read, preview the red headings. In a graphic organizer like the one below, ask a what or how question for each heading. As you read, write answers to your questions.QuestionAnswerWhat does the skeleton do?The skeleton provides shape and support, helps you to move, protects organs, produces blood cells, and stores minerals and other materials.How do joints move?Joints can move forward or backward, in a circle, in a rotating motion, and in a gliding motion.How strong are bones?Bones can absorb more force without breaking than granite or concrete.What can I do to care for my bones?Eat a well balanced diet and getplenty of exercise.