Body Organization.

Atoms are the smallest chemical units of an element. They are the building blocks of all matter. Each atom contains three components: negatively charged electrons, positively charged protons, and neutrally charged neutrons. A molecule is a chemical combination of two or more atoms. A cell is the basic unit of life, not of a chemical element. An organelle is a specialized structure within a cell.

Chemical compounds are made up of molecules. A molecule is a chemical combination of two or more atoms. Water, for example, is composed of two hydrogen atoms and one oxygen atom. A molecule is not a physical or biologic combination of atoms. Bonds in a molecule may be covalent or ionic.

Cells are the smallest and most fundamental unit of life. Every structure in the human body is made of cells. Cells combine to form tissues, tissues form organs, and organs make up organ systems. Organelles are specialized structures within a cell (such as mitochondria or the nucleus), not the basic building blocks of the body. Tissues are groups of similar cells working together. Organs are made up of multiple tissue types performing complex functions.

A cell consists of several organelles. Several cells performing a similar function make up a tissue. A collection of tissues performing a specialized function make up an organ. Several organs work together to make up a system. The other sequences have the organizational levels out of order. Remember: organelles live inside cells, cells group into tissues, tissues build organs, organs form systems.

Epithelial tissues perform specialized functions including excretion, absorption, secretion, diffusion, and protection. Connective tissues provide a supporting network for organs and muscles. Nerve tissues control and coordinate body activities. Muscle tissue mediates movement. None of these perform excretion.

Several organs working together to carry out a specialized function make up a system. For example, the respiratory system includes the nose, pharynx, larynx, trachea, bronchi, bronchioles, alveoli, and lungs. A cell consists of organelles. Several cells working together form a tissue. Several tissues working together form an organ. The system is the highest organizational level.

A transverse plane (also called a horizontal plane) divides the body into upper (superior) and lower (inferior) portions. A frontal (coronal) plane divides the body into anterior and posterior portions. A sagittal plane divides the body into left and right portions. A median (midsagittal) plane runs through the midline, dividing the body into equal left and right halves.

The dorsal cavity contains two subdivisions: the cranial cavity and the spinal (vertebral) cavity. The thoracic, abdominal, and pelvic cavities are all part of the ventral cavity. Body cavities are hollow spaces containing organs, divided into dorsal and ventral compartments.

Posterior is an anatomical term referring to the back of the body or a body part. It is the opposite of anterior. Medial describes a position closer to the midline (opposite of lateral). Lateral refers to the sides of the body. Anterior describes the front of the body.

Inferior means below. The abdomen lies below the chest. Superior means above, and the abdomen is not above the chest. Ventral means front. Dorsal means back or behind. Neither describes the relationship between abdomen and chest.