Based on the same structure found in other mammals, human anatomy is characterized by bilateral symmetry. However, unlike other mammals, humans walk upright. This difference has caused significant evolutionary adaptations that sethumans apart anatomically from the rest of the animal world. These changes mainly concern the pelvis, which has to bear more of the organs' weight and serve as the trunk's center of balance; the feet, which have a greater load to carry; the arms, which no longer have to bear the upper body's weight; the skull, which has grown to accommodate a larger brain that has also moved more toward the body's center line; and the femur and its associated musculature.
The building blocks of human anatomy, like those of the other mammals, are four kinds of tissues, each of which serves a specific purpose. Nerve tissues comprise the nervous system and send and receive the brain's electrical messages; connective tissues, which hold together the body's different structures (i.e, the ligaments that connect muscle to bone); epithelial tissues, including the skin, that line cavities and cover organs; and muscle tissues, which make up the body's musculature.
These building blocks all come together in the organs, discrete structures that perform certain specialized functions. The human body has nine major organsystems: the respiratory system (lungs and airways); the digestive system (stomach, intestines, esophagus); the musculoskeletal system (bones and skeletal muscles); the reproductive system (ovaries or testes, female or malegenital); the integumentary system (skin); the circulatory system (blood vessels, heart, blood); the excretory system (kidneys, bladder, urethra, ureters); the nervous system (brain, spinal cord, nerves); and the endocrine system(hormone producing glands and tissues).
Certain terms exist that allow physicians, surgeons, and others to discuss human anatomy in a way that eliminates misunderstanding and ambiguity. For instance, the words "trunk" (neck, chest, and upper abdomen), "pelvis" (lower abdomen), "perineum" (below the abdomen), and "vertebral column" (spine) describe the main regions of the body. These and all of the more specific terms relyon the body always being in the same position so that they mean the same thing to everyone at all times. Even if a particular body is not in this position, the words medical professional use to describe it will correspond to the "anatomical position." This consists of a body standing upright with its armsat its side, palms facing out, feet together, and head facing forward. From there, the body is divided by the median plane and two subplanes (sagittal orcoronal); four directions (superior or inferior, posterior or anterior), andtwo directions relative to the medial plan (medial and lateral). In addition,the terms "superficial" and "deep" refer to location relative to the skin surface, while "proximal" and "distal" indicate location relative to the structure's root. The hands and feet each have their own descriptors as well (i.e.,palmar/dorsal, dorsal/plantar, respectively).