27 The Human Body-Mid

The Human Body Written by Kira Freed

Introduction Your body is like a busy city. A city has homes, stores, streets, a power station, and much more. Like a city, your body has many systems that work together to keep you alive. The systems are working all the time, and they all need to stay healthy to do their jobs. Do You Know? Check your knowledge before reading this book. What do you know about each of the ten 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 pumps blood through your body. Messages move to and from your brain, and your lungs carry oxygen to cells. Your body kills bad germs, controls your temperature, and gets rid of waste products. It also does thousands more jobs— even while you are sleeping! In this book, you will learn about seven important body systems. As you read, think about all the jobs each body system does 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 support all the other systems. Without bones, you would be a mushy blob! Bones also help you to move, and they protect your organs. Your spine, the main support, is a long column of twenty-six bones. Your skull protects your brain from getting hurt. Your ribs wrap around your chest, protecting your heart and lungs. Your pelvis helps to protect the organs in your belly.

Long, strong bones support your arms and legs. Your hands, wrists, feet, and ankles have many smaller bones. Each of the 206 bones in your body has a special job. Did you realize your bones are alive? Bones are made of living cells. This is why bones grow. Spongy stuff 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 works with the skeletal system so you can move. Unlike other body tissues, your muscles can contract (become shorter and tighter) or relax (become longer and looser). The muscles that bend your joints work in pairs. When one contracts, its partner relaxes. Then they switch jobs. Your body has three different types of muscles— skeletal, smooth, and cardiac. Wowser! Your body has more than 650 muscles! Almost half your weight is muscles.