Levels of organization of the human body

What are the levels of organization of the human body?

At the chemical level, the simplest level, atoms join to form molecules. Atoms are the fundamental units of matter; molecules are groups of atoms that carry out specific functions. These molecules build the cell, the smallest unit of life, which defines the cellular level. The body has many cell types, each with a defined task. Similar cells group into tissues at the tissue level, and different tissues combine into organs at the organ level. Organs that share a broad function form organ systems, and all the organ systems together make up the organism, the complete living human body.

Chemical level

Atoms and molecules make up the chemical level, the simplest level of organization. Atoms are the basic building blocks of matter and of the human body, and each one consists of three subatomic particles. Protons and neutrons sit together in the nucleus, while electrons occupy orbits around it. The most abundant atoms in the body are hydrogen (H), oxygen (O), carbon (C), and nitrogen (N), which together account for about 99% of its atoms by number. Around 60 elements can be detected in the body, of which roughly 25 are essential. Most of the others are trace elements, present in very small amounts. Examples include iron (Fe), zinc (Zn), and copper (Cu).

The atoms mentioned above are combined in many different ways, producing vital molecules for the body. For example, water (H₂O), a significant molecule for the organism, comprises about 60% of the body. Another example is the so-called macromolecules—larger molecules such as deoxyribonucleic acid (DNA) or proteins, with multiple and significant functions.

Cellular and tissue levels

The cellular level begins with the cell, the smallest unit of living matter that functions on its own. This structure is the result of various organelles distributed throughout a water-based fluid, the cytoplasm, surrounded by a membrane. The cell membrane, a phospholipid bilayer, is the boundary of the cell, creating an intracellular environment consisting of multiple structures important for the cell function. The main structural components of the cells are:

• cell membrane,
• nucleus,
• golgi,
• endoplasmic reticulum,
• mitochondria,
• ribosomes,
• lysosomes,
• peroxisomes,
• endosomes and
• cytoskeleton.

The human body contains more than 30 trillion cells and over 200 different types, each carrying out a defined task. For example, some human cell types are blood cells, muscle cells, and nerve cells. At the tissue level, large numbers of cells with a common origin form tissues. The main types of tissue are:

• epithelial,
• connective, 
• nervous,
• muscle.

Organs and organ systems

An even bigger, anatomically defined structure of the human body is an organ, which consists of two or more different types of tissue and has a specific functional role. The largest organ of the human body is the skin, and other vital organs include the heart, the brain, and the lungs.

The coordinated response of different organs forms the organ systems. Specifically, there are 11 different organ systems present in the human body. These systems are the following:

• integumentary system,
• skeletal system,
• muscular system,
• nervous system,
• endocrine system,
• cardiovascular system,
• lymphatic system,
• respiratory system,
• digestive system,
• urinary system,
• reproductive system.

The proper functioning and coordination of all the above ensure that the biological needs of the human organism are met.