The 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.
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 are 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.
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.
All 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.
The 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.
A 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.
Bones 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. Small 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.
The bones of your skeleton are both strong and lightweight. Bones are hard because they are made up of two minerals-phosphorus and calcium. New bone tissue forms continually throughout your life.
A 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.
There are about 600 muscles in your body. The muscles that are not under your conscious control are called involuntary muscles. Involuntary muscles are responsible for activities such as breathing and digesting food. The muscles that are under your control are called voluntary muscles. Smiling and turning the pages in a book are actions of voluntary muscles.
Your body has three types of muscle tissue-skeletal muscle, smooth muscle, and cardiac muscle. Some of these muscle tissues are involuntary, and some are voluntary. Skeletal muscles are attached to the bones of your skeleton. At the end of a skeletal muscle is a tendon. A tendon is a strong connective tissue that attaches muscle to bone. Because you have conscious control of skeletal muscles, they are classiﬁed as voluntary muscles. These muscles provide the force that moves your bones. Skeletal muscles react quickly and tire quickly. Skeletal muscle cells appear banded, or striated. For this reason, they are sometimes called striated muscles.
Smooth muscles are called involuntary muscles because they work automatically. They are inside many internal organs of the body, and control many types of movements inside your body, such as those involved in the process of digestion. Smooth muscles react more slowly and tire more slowly than skeletal muscles. Cardiac muscles are involuntary muscles found only in the heart. Cardiac muscles do not get tired.
Muscles work by contracting, or becoming shorter and thicker. Because muscle cells can only contract, not extend, skeletal muscles must work in pairs. While one muscle contracts, the other muscle in the pair relaxes to its original length. For example, in order to move the lower arm, the biceps muscle on the front of the upper arm contracts to bend the elbow. This lifts the forearm and hand. As the biceps contracts, the triceps on the back of the upper arm returns to its original length. To straighten the elbow, the triceps muscle contracts while the biceps returns to its original length.
Exercise is important for maintaining both muscular strength and ﬂexibility. Exercise makes individual muscle cells grow wider, thicker, and stronger. Sometimes, muscle injuries such as strains and cramps, can occur. Resting the injured area can help it heal.
The skin performs several major functions in the body. The skin covers and protects the body from injury, infection, and water loss. The skin also helps regulate body temperature, eliminate wastes, gather information about the environment, and produce vitamin D.
The skin is organized into two main layers, the epidermis and the dermis. The epidermis is the outermost layer of the skin. The epidermis does not have nerves or blood vessels. Cells in the epidermis have a deﬁnite life cycle. New cells form deep in the epidermis, gradually mature, and move upward. When these cells die, they become part of the surface layer of the epidermis. Soon, these cells are shed and replaced by the dead cells below them. Cells deep in the epidermis produce melanin, a pigment, or colored substance, that gives skin its color. The more melanin in your skin, the darker it is. Melanin production helps to protect the skin from burning.
The dermis is the inner layer of the skin. It contains nerves, blood vessels, sweat glands, hairs, and oil glands. Sweat glands produce perspiration, which reaches the surface through openings called pores. Strands of hair grow within the dermis in structures called follicles. Oil produced in glands around the hair follicles waterproofs the hair. In addition, oil helps to keep the skin moist.
Three simple habits can help you keep your skin healthy. Eat a healthful diet. Keep your skin clean and dry. Limit your exposure to the sun.
Eating a well-balanced diet and drinking plenty of water are important for healthy skin. Good washing habits can get rid of dirt and harmful bacteria, and can help control oiliness.
Repeated exposure to sunlight can damage skin cells and cause them to become cancerous. Cancer is a disease in which some body cells divide uncontrollably. Wearing sunscreen and limiting sun exposure can protect skin from sun damage.
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