Organized Structure of the Body

In all multicellular organisms, such as animals, the body consists of specialized cells. These cells work together and perform certain bodily functions. When specialized cells are grouped together to perform a common function, they form a tissue. For example, a specific type of epithelial cell aggregates to form the elastic epithelial tissue that lines blood vessels, and muscle cells group together to form muscles that move body structures. An organ is a structure, formed from more than one tissue, with a specific function. There are many different types of organs, such as the heart, liver, and brain. When groups of organs share a common function, they are an organ system. There are several different organ systems, including the circulatory, digestive, excretory, respiratory, immune, skeletal, and nervous systems. All the organ systems work together to keep an animal's body functioning.

For example, a deer walking and grazing in a meadow relies on its skeletal and muscular system to walk. It uses its respiratory system to breathe and its cardiovascular system to pump blood throughout its body. The deer needs its digestive system to collect nutrients from the food on which it grazes. Animals have a complex system of interdependent parts, or body systems, that work together. Knowing how these body systems relate to each other provides greater insight into how the body functions as a whole.

The tissues of the body are grouped into four main categories: muscle, nervous, epithelial, and connective tissue. Within these groups are several different types of tissues with specific functions related to their individual structures.

Muscle tissue is responsible for body movement as well as moving substances through the body. Muscle cells are able to change their length through contraction, resulting in the movement of structures to which the muscles are attached. Walking, running, and any form of movement by the body involves the use of muscle tissue. The three types of muscle tissue are smooth, skeletal, and cardiac.

• Smooth muscle tissue is commonly found in the inner lining of organs. The contraction of smooth muscle helps move blood through blood vessels and food through the intestines. The cells are elongated, with pointed ends and a single nucleus. Smooth muscle contraction is under involuntary control; the organism does not need to think about making smooth muscle contract and cannot induce or stop the contraction by will.

• Skeletal muscle tissue is attached to the bones of an animal's skeleton. The contraction of skeletal muscle moves the bones to which the muscles are attached. The cells are cylindrical in shape, with repeating dark and light bands and multiple nuclei. Skeletal muscle is under voluntary control; the organism can induce or stop contraction by will, and most movement must be initiated by thought. For example, the muscle of a human's upper arm is the bicep, contraction of which moves the forearm toward the upper arm.

• Cardiac muscle tissue is found in the walls of the heart. The contraction of cardiac muscle allows the heart to move blood. The cells are branching, with alternating light and dark bands and one nucleus. Cardiac muscle is under involuntary control.

Nervous tissue is responsible for receiving and transmitting information throughout the body. Nervous tissue is made of neurons, which are cells that receive and transmit impulses. Collectively, these cells form the nervous system. Nervous tissue forms the structure of the brain and the network of nerves throughout the body, and it tells the body how to respond to various external or internal stimuli. For example, when an animal hears a noise, this information is transmitted to its brain as a type of signal. The brain receives this signal and tells the body how to respond.

Epithelial tissue covers the surface of the body and various internal organs and lines body cavities. It consists of layers formed from cells packed tightly together in sheets. This tissue provides a barrier between the organ and its surrounding environment. It functions in protection, absorption, and secretion. The skin is a type of epithelial tissue that protects the interior of the body from the environment.

Connective tissue is made of many different cell types. All of the cells collectively help with structure and support for the body. This tissue holds other tissues together. It also protects and insulates organs. Examples of connective tissues are bone, fat, cartilage, and blood. Because of the many cell types it can contain, connective tissue can be either densely packed or loosely packed together.