Introduction to Human Anatomy and Physiology

Foundations of Anatomy and Physiology

Anatomy is defined as the study of the structure and shape of the body and its parts, derived from the Greek words meaning to cut ($tomy$) apart ($ana$). It is divided into gross anatomy, focusing on large structures visible to the naked eye, and microscopic anatomy, encompassing cytology and histology. Physiology is the study of how the body and its parts function. The internal structure of an organ determines its function; for instance, the thin walls of the lungs facilitate gas exchange, whereas the muscular heart pumps blood.

Structural Organization and Body Systems

The human body exhibits $6$ levels of structural organization: the chemical level (atoms and molecules), cellular level, tissue level, organ level, organ system level, and the organismal level. The human body contains $11$ distinct organ systems: Integumentary, Skeletal, Muscular, Nervous, Endocrine, Circulatory (Cardiovascular), Lymphatic, Respiratory, Digestive, Urinary (Excretory), and Reproductive. These systems work in an interrelated fashion; for example, the digestive and respiratory systems provide nutrients and oxygen, respectively, which the circulatory system then distributes to cells.

Necessary Life Functions and Survival Needs

To maintain life, organisms must perform $8$ functions: maintaining boundaries, movement (specifically contractility at the cellular level), responsiveness (irritability), digestion, metabolism (catabolism of substances and anabolism of structures), excretion, reproduction, and growth. Survival depends on the presence of $5$ factors in appropriate amounts: nutrients (carbohydrates, proteins, fats, vitamins, and minerals), oxygen (which makes up approximately $20\%$ of air), water ($60-80\%$ of body weight), a normal body temperature of $37\,^\circ\text{C}$ ($98.6\,^\circ\text{F}$), and appropriate atmospheric pressure for gas exchange.

Homeostasis and Control Systems

Homeostasis is the body's ability to maintain a dynamic state of equilibrium even as external conditions change. It is regulated primarily by the nervous and endocrine systems through homeostatic control mechanisms consisting of a receptor (sensor for stimuli), a control center (analyzer of input), and an effector (provider of response). Information travels via the afferent pathway to the control center and the efferent pathway to the effector. There are two types of feedback mechanisms: negative feedback, which reduces or shuts off the original stimulus (e.g., body temperature regulation), and positive feedback, which enhances the stimulus (e.g., blood clotting and labor contractions). Homeostatic imbalance relates to many diseases and the effects of aging.

Anatomical Language and Body Orientation

Standard anatomical position is described as standing erect, feet parallel, and arms at the sides with palms facing forward. Directional terminology includes superior/inferior, anterior (ventral)/posterior (dorsal), medial/lateral, proximal/distal, and superficial/deep. The body can be divided by sagittal (left/right), frontal/coronal (anterior/posterior), or transverse (superior/inferior) planes. Internal organs are housed in two main cavities: the dorsal body cavity (cranial and spinal) and the ventral body cavity (thoracic and abdominopelvic, separated by the diaphragm). The abdominopelvic cavity is further divided into $4$ quadrants (RUQ, LUQ, RLQ, LLQ) or $9$ specific regions (umbilical, epigastric, hypogastric, iliac/inguinal, lumbar, and hypochondriac).