Organisation of the body
Anatomy is the study of body structure; physiology is the study of how it works. The two go together, because structure determines function.
Levels of organisation
The body is built in levels: chemicals form cells; cells form tissues; tissues form organs; organs form organ systems; and systems together make the organism. There are 11 organ systems, from the integumentary (skin) to the reproductive system, all interdependent.
Anatomical position and directions
Anatomists describe the body in the anatomical position: standing upright, facing forward, arms at the sides, palms forward. Directional terms are relative to this: superior (toward the head) and inferior (toward the feet), anterior (front) and posterior (back), medial (toward the midline) and lateral (away from it), proximal (near a limb's attachment) and distal (farther away).
Body planes and cavities
A sagittal plane divides the body into left and right; a frontal (coronal) plane into front and back; a transverse plane into upper and lower parts, as in a CT scan slice. The dorsal cavity holds the brain and spinal cord; the ventral cavity holds the thoracic organs (heart and lungs) and abdominopelvic organs, separated by the diaphragm.
Homeostasis revisited
Every organ system helps keep internal conditions stable. A homeostatic control loop has a receptor that detects change, a control centre (often the brain) that compares it with a set point, and an effector that responds. Disease often reflects a failure of these loops.
The four tissue types
Tissues are groups of similar cells with a common function, embedded in an extracellular matrix. There are four primary types: epithelial tissue covers and lines; connective tissue supports and binds; muscle tissue contracts; and nervous tissue conducts signals. Histology is the study of tissues under the microscope.
Membranes
Body membranes combine epithelium with connective tissue. Mucous membranes line passages open to the outside, such as the digestive and respiratory tracts. Serous membranes line closed cavities and cover organs, producing slippery fluid that reduces friction: the pleura around the lungs, the pericardium around the heart, and the peritoneum in the abdomen.
Worked example
Describe the wrist relative to the elbow: the wrist is distal to the elbow. The nose is medial to the eyes and anterior to the ears. The heart is superior to the diaphragm, in the thoracic cavity. Using these terms lets clinicians describe locations precisely and unambiguously.
Studying anatomy
Gross anatomy studies structures visible to the naked eye; microscopic anatomy includes cytology (cells) and histology (tissues). Imaging such as X-rays, CT, MRI, and ultrasound shows internal anatomy in living people. Regional anatomy studies body areas; systemic anatomy studies one system at a time.
Key takeaways
The body is organised from chemicals to organism, with 11 organ systems. Directional terms, planes, and cavities describe locations relative to the anatomical position. Homeostasis uses receptors, control centres, and effectors. The four tissue types are epithelial, connective, muscle, and nervous. Mucous and serous membranes line passages and cavities.
Flashcard
Study of body structure
Check your understanding
The knee is ___ to the ankle.