Organization of the human body

Look at your hand. It seems simple, yet it is made of skin, bone, muscle, nerves, and blood vessels that work together so you can pick up a pencil. The body is organized in levels, from tiny to large, and knowing the levels is the key to everything else in this unit.

The smallest level you will study is the cell, the basic unit of life. Similar cells working together form a tissue. Different tissues combined into a structure with a specific job form an organ, such as the stomach or the heart. Organs that cooperate on a larger job form an organ system, and all the systems together make up the organism: you. Each level builds on the one below it and can do things the lower level cannot. A single heart cell cannot pump blood, but a heart can.

Scientists describe the body with two words. Anatomy is the study of body structure: what the parts are and where they sit. Physiology is the study of function: what the parts do and how. The two are always connected, which leads to one of the biggest ideas in biology: structure fits function. Your front teeth are thin and sharp for cutting, while your back teeth are broad and flat for grinding. Your thumb can swing across your palm so you can grip. When you meet a new body part, ask what its shape suggests about its job.

This unit follows eleven organ systems. The integumentary system (skin, hair, and nails) protects you. The skeletal system supports you and the muscular system moves you. The nervous system and the endocrine system carry messages, one with electrical signals and the other with hormones. The circulatory system moves blood, the respiratory system exchanges gases, the digestive system breaks down food, and the urinary system filters wastes from the blood. The immune and lymphatic system defends against disease, and the reproductive system makes new life possible.

A common mistake is to think each system works alone. None does. When you run, muscles move you, the respiratory and circulatory systems deliver oxygen, the nervous system coordinates everything, and your skin releases heat through sweat. As you move through the unit, keep asking how each system connects to the others.