Anatomy, the study of the body’s structure, is closely linked to physiology, the study of function. An anatomical structure is any identifiable part of the body, from the smallest chemical compound to the largest organ system. Structure dictates function at every organizational level. Understanding the human body requires exploring how these individual parts are arranged into a cohesive whole.
Defining the Standard Reference Point
To describe structure location accurately, anatomists rely on the anatomical position. This standard posture involves the body standing upright, feet parallel, and arms held out with palms facing forward. All descriptions of location are made as if the person were in this fixed position, which removes ambiguity in communication.
This standard allows the use of directional terms that precisely locate one structure relative to another. For example, superior means toward the head, while inferior means away from the head, toward the feet. Structures positioned closer to the midline of the body are medial, and those farther away are lateral. When discussing the limbs, proximal indicates a position closer to the trunk’s point of attachment, and distal refers to a position farther away.
The body can also be visualized as being cut by three imaginary flat surfaces called planes. The sagittal plane divides the body vertically into right and left portions. The frontal, or coronal, plane also runs vertically but divides the body into an anterior (front) section and a posterior (back) section. Finally, the transverse, or horizontal, plane divides the body into superior (upper) and inferior (lower) parts.
Hierarchical Organization of Structures
The body’s organization follows a structured hierarchy, beginning with the simplest components and building up to the most complex living organism. The chemical level involves atoms (like carbon and oxygen) that combine to form molecules (such as proteins and DNA). These molecules then aggregate and interact to form the tiny working machinery inside cells, establishing the cellular level.
The cell is the smallest independently functioning unit of life, with different types specialized to perform unique tasks. Groups of similar cells working together define the tissue level of organization. The body contains four basic tissue types: epithelial tissue covers surfaces, connective tissue provides support, muscle tissue enables movement, and nervous tissue allows for communication and control.
Multiple types of tissues combine to form the organ level, executing complex functions. The stomach, for instance, is an organ composed of epithelial tissue, muscle tissue for churning food, and connective and nervous tissues for support and control. When organs are integrated, they form an organ system, such as the digestive system. The highest level is the organism, the entire living being where all organ systems function cooperatively to maintain life.
Major Functional Systems
At the level of the organism, 11 distinct organ systems work together to perform the biological functions required for life. The integumentary system, composed of the skin, hair, and nails, serves as the body’s external protective barrier and helps regulate temperature. Beneath the surface, the skeletal system (bones, ligaments, and cartilage) provides structural support, protects internal organs, and stores minerals.
The muscular system (skeletal, smooth, and cardiac muscles) generates movement, maintains posture, and produces body heat. Coordination is managed by the nervous system, a network of the brain, spinal cord, and nerves that rapidly transmit electrical signals. The endocrine system complements this control by using glands (like the pituitary and thyroid) to secrete hormones, which are slower-acting chemical messengers regulating processes like metabolism and growth.
The cardiovascular system (heart, blood, and blood vessels) acts as the body’s transport network, circulating oxygen, nutrients, hormones, and waste products. The lymphatic system (lymph vessels and nodes) drains excess tissue fluid and houses immune cells to defend against disease. Gas exchange is the function of the respiratory system, involving the lungs and airways, which brings in oxygen and eliminates carbon dioxide.
The digestive system (mouth, esophagus, stomach, intestines, liver, and pancreas) breaks down food into absorbable nutrients and eliminates solid waste. Waste products dissolved in the blood are filtered out by the urinary system (kidneys, ureters, bladder, and urethra) to maintain fluid, electrolyte, and pH balance. Finally, the reproductive system (ovaries and testes) is required for the continuation of the species, though not necessary for individual survival.