28 The Human Body-High

The Human Body Written by Kira Freed

Introduction Your body is like a busy city. A city has homes, stores, streets, a power station, waste management, and much more. Like a city, your body has different systems that work together to keep you alive. All the systems are working all the time, and they all need to remain healthy to do their jobs. Do You Know? Check your knowledge before reading this book. What do you know about each of the systems in your body? skeletal system muscular system nervous system respiratory system circulatory system digestive system excretory system reproductive system endocrine system immune system

Every minute of your life, your heart is pumping blood throughout your body. Messages are traveling to and from your brain, and your lungs are carrying life-giving oxygen to cells. Your body is killing harmful germs, controlling your temperature, and removing waste products. It is also performing thousands more tasks— even while you are sleeping! In this book, you will learn about seven of these important body systems. As you read, think about all the tasks each body system is performing to keep you alive and healthy.

The Skeletal System The bones of your skeletal system support the rest of your body. Like the steel beams inside the walls of buildings, they provide support for all the other systems. Without bones, your body would collapse into a mushy blob! Bones also protect organs and play an important role in your body's movement. Your spine, the main support, is a long column of twenty-six bones. Your skull, a bony case at the top of the column, protects your brain from injury. Some of the bones of your spine have ribs attached to them. Your ribs wrap around your chest, protecting your heart and lungs.

Your pelvis helps to protect the organs of your abdomen. Muscles attached to bones in your arms and legs help you move. Long, strong bones provide support for your arms and legs. Your hands, wrists, feet, and ankles contain many smaller bones. Each of the 206 bones in your body has a special job. The place where two bones meet is called a joint. Some joints bend or rotate, allowing you to move in many ways. Did you realize that your bones are alive? Bones are made of living cells. This is why bones grow. Spongy material in the center of bones, called bone marrow, makes new blood cells. Do You Know? Can you guess where the smallest bones in your body are? They're in your middle ears! Three tiny bones carry vibrations from your eardrums to your inner ears.

The Muscular System The muscular system, along with the skeletal system, allows your body to move. Unlike other body tissues, your muscles can contract (become shorter and tighter) or relax (become longer and looser). Muscles use the energy from the food you eat to do this work. The muscles that bend your joints never push— they only pull, which is why they must work in pairs. When one contracts, its partner relaxes. Then they switch jobs. Your body has three types of muscles— skeletal, smooth, and cardiac. Wowser! Your body has more than 650 muscles! Almost half your weight is muscles.