Copyright © 2007 Dorling Kindersley
Our body structures are arranged into several different systems, each with its own specific function. The smallest units in the body are CELLS, which share certain characteristics. These tiny structures are collected into TISSUES, which are themselves arranged into ORGANS. Different body systems consist of collections of cells, tissues, and organs with a common purpose.
The skin, hair, and nails form the body’s outer covering, or integument. They help to protect the body’s internal parts from damage and provide a barrier to invasion by infectious organisms. An adult’s skin covers an area of about 2 m2 or 22 sq ft.
The muscular system consists of layers of muscles that cover the bones of the skeleton, extend across joints, and can contract and relax to produce movement.
The skeleton is a strong yet flexible framework of bones and connective tissue. It provides support for the body and protection for many of its internal parts.
This system consists of the heart and a network of vessels that carry blood. It supplies oxygen and nutrients to the body’s cells and removes waste products.
The nervous system is the body’s main control system. It consists of the brain, the spinal cord, and a network of nerves that extend out to the rest of the body.
This system is a network of vessels that collects fluid from tissues and returns it to the blood. It also contains groups of cells that protect the body against infection.
The respiratory system is centred on the lungs, which work to get life-giving oxygen into the blood. They also rid the body of a waste product, carbon dioxide.
Many body processes, such as growth and energy production, are directed by hormones. These chemicals are released by the glands of the endocrine system.
The digestive system takes in the food the body needs to fuel its activities. It breaks the food down into units called nutrients and absorbs the nutrients into the blood.
The body’s cells produce waste products, many of which are eliminated in urine. The job of the urinary system is to make urine and expel it from the body.
The male and female parts of the reproductive system produce the sperm and eggs needed to create a new person. They also bring these tiny cells together.
The basic building blocks of the body are tiny structures called cells. The human body contains trillions of cells, which fall into several types – nerve cells, muscle cells, fat cells, liver cells, and so on – each with a different function. A typical cell has a central nucleus surrounded by some jellylike material called cytoplasm. Covering the cytoplasm is the plasma membrane. This controls the movement of substances into and out of the cell.
Nerve cells, or neurons, are one of the most numerous types of body cell. Each nerve cell has a central body, containing the cell nucleus, and fibrelike projections, which can be up to 1 m (3 1/3 ft) long. The nervous system contains billions of neurons, which collect and transmit information around the body. The adult brain alone may contain as many as 25 billion neurons.
Cells come in different shapes and sizes, but all have features in common. Most cells have a nucleus. This contains genetic material, which directs the cell’s activities. The cytoplasm contains small structures called organelles. There are several types of organelle, each with a specific job. Mitochondria, for example, produce energy for the cell.
Cells group together to form tissues, each with specific functions. Connective tissue is the most widespread; it separates and supports other tissues and organs, and includes cartilage and bone. Adipose tissue is packed with fat cells, which provide energy storage and insulation. Epithelial tissue protects and lines the surfaces of many body organs. Other types include muscle and nervous tissue.
Tissues are grouped together in the body to form organs. These include the brain, heart, lungs, kidneys, and liver. Each body organ has a specific shape and is made up of different types of tissue that work together. For example, the heart consists mainly of a specialized type of muscle tissue, which contracts rhythmically to provide the heart’s pumping action. But it also contains nervous tissue, which carries the electrical signals that bring about the contractions, and is lined with epithelial tissue.