HESI A2 A&P key terms

The standard reference stance: standing upright, face forward, arms at the sides with palms facing forward, and feet flat and parallel. Directional terms such as anterior, medial, and proximal are defined relative to this stance.

Directional terms describing vertical position. Superior means closer to the head or above; inferior means closer to the feet or below. The terms apply to the body as a whole or to structures within a region.

Directional terms for front and back. Anterior means toward the front or belly side; posterior means toward the back. In the feet and hands, dorsal corresponds to the top surface and plantar or palmar to the opposite surface.

Directional terms for position relative to the body's midline. Medial means closer to the midline; lateral means farther from it. For example, the ulna lies medial to the radius in anatomical position.

Directional terms describing distance along a limb from its attachment to the trunk. Proximal means nearer the point of attachment; distal means farther away. The elbow is proximal to the wrist.

A vertical plane that divides the body into right and left portions. A midsagittal plane splits the body into equal halves; other sagittal planes are parasagittal. Flexion and extension movements occur in this plane.

A vertical plane that divides the body into anterior and posterior portions, also called the coronal plane. Abduction and adduction occur in this plane, as does side-to-side bending of the trunk.

A horizontal plane that divides the body into superior and inferior portions. Rotation and twisting movements occur in this plane. A cross-section of a limb is typically taken in this plane.

Two major body cavities. The dorsal cavity lies along the back and contains the cranial and vertebral cavities, housing the brain and spinal cord. The ventral cavity contains the thoracic and abdominopelvic cavities.

Double-layered membranes that line closed body cavities and cover the organs within them. The parietal layer lines the cavity wall and the visceral layer covers the organ; serous fluid between them reduces friction during movement.

The central compartment of the thoracic cavity between the two pleural cavities. It contains the heart, great vessels, trachea, esophagus, and thymus. The heart sits within it in the pericardial sac.

The maintenance of relatively stable internal conditions despite external change. It relies on receptors detecting a variable, a control center comparing it to a set point, and effectors correcting the deviation through negative or positive feedback.

A control mechanism in which the response opposes the original deviation, bringing a variable back toward its set point. Most homeostatic regulation works this way, such as insulin lowering elevated blood glucose.

A control mechanism in which the response amplifies the original deviation rather than reversing it, driving a process to completion. Examples include labor contractions and the clotting cascade.

A tissue that covers body surfaces, lines cavities and ducts, and forms glands. Cells are closely packed with little extracellular matrix, are avascular, and rest on a basement membrane. Types are classified by shape and layering.

The most abundant tissue type, supporting, binding, and protecting other tissues. It is characterized by widely spaced cells within an abundant extracellular matrix of fibers and ground substance. Examples include bone, cartilage, and blood.

Tissue specialized for contraction, producing movement and generating heat. Three types exist: skeletal, which is striated and voluntary; cardiac, which is striated and involuntary; and smooth, which is nonstriated and involuntary.

Tissue that conducts electrical impulses for communication and control. It consists of neurons, which transmit signals, and neuroglia, which support, insulate, and protect neurons. It makes up the brain, spinal cord, and peripheral nerves.

The skin, hair, nails, and associated glands. It provides a protective barrier, regulates temperature, synthesizes vitamin D, and contains sensory receptors. The skin has an outer epidermis and a deeper dermis.

The two main layers of skin. The epidermis is the avascular outer layer of stratified squamous epithelium; the dermis is the deeper connective tissue layer containing vessels, nerves, and glands. A subcutaneous layer lies beneath the dermis.

The basic structural unit of compact bone, also called a Haversian system. It consists of concentric lamellae surrounding a central canal that carries blood vessels and nerves, with osteocytes housed in lacunae between the lamellae.

Two bone cells with opposing roles. Osteoblasts build new bone matrix and promote mineralization; osteoclasts break down bone matrix and release calcium into the blood. Their balance determines bone mass.

The central skeleton consisting of the skull, vertebral column, ribs, and sternum. It forms the body's longitudinal axis and protects the brain, spinal cord, heart, and lungs. It has 80 bones in the adult.

The bones of the limbs plus the girdles that attach them to the trunk. It includes the pectoral girdle, pelvic girdle, and the bones of the arms, legs, hands, and feet, and is built mainly for movement.

The smallest contractile unit of a muscle fiber, extending between two Z lines. It contains overlapping thick myosin and thin actin filaments. Shortening occurs when filaments slide past each other during contraction.

The synapse where a motor neuron's axon terminal meets a muscle fiber. Release of acetylcholine into the synaptic cleft depolarizes the muscle membrane and triggers contraction. It is the point where nerve signals become muscle activity.

A neurotransmitter released at neuromuscular junctions and by parasympathetic nerve endings. It binds receptors on the muscle membrane to initiate contraction and slows heart rate at the heart. It is broken down by acetylcholinesterase.

A nerve cell that receives, integrates, and transmits electrical signals. It has a cell body, dendrites that receive input, and an axon that carries impulses away. Many axons are wrapped in myelin to speed conduction.

The two divisions of the nervous system. The central nervous system is the brain and spinal cord, which integrate information. The peripheral nervous system is all nerves outside these structures, carrying signals to and from them.

The involuntary division of the peripheral nervous system that regulates smooth muscle, cardiac muscle, and glands. Its sympathetic branch mobilizes the body for stress; its parasympathetic branch promotes rest and digestion.

A chemical messenger secreted by an endocrine gland into the bloodstream, traveling to distant target cells with matching receptors. Hormones regulate metabolism, growth, reproduction, and fluid balance. Steroid and protein hormones differ in how they act on cells.

A hormone released by the parathyroid glands when blood calcium falls. It raises calcium by stimulating bone breakdown, increasing kidney reabsorption of calcium, and activating vitamin D to boost intestinal absorption. Its antagonist is calcitonin.

Blood vessels distinguished by direction of flow and structure. Arteries carry blood away from the heart under high pressure and have thick muscular walls; veins carry blood toward the heart under low pressure and contain valves.

The smallest blood vessels, consisting of a single layer of endothelium. They connect arterioles to venules and are the site of exchange of gases, nutrients, and wastes between blood and tissue fluid.

One complete sequence of heart contraction and relaxation. Systole is the contraction phase that ejects blood; diastole is the relaxation phase when chambers fill. The cycle produces the heart sounds heard through a stethoscope.

Tiny air sacs at the ends of respiratory bronchioles where gas exchange occurs. Their thin walls and surrounding capillary network allow oxygen to enter the blood and carbon dioxide to leave. Surfactant reduces surface tension to prevent collapse.

The dome-shaped skeletal muscle separating the thoracic and abdominal cavities. Its contraction flattens it and enlarges the thorax, causing inspiration. It is the primary muscle of quiet breathing.

The functional unit of the kidney, numbering about a million per kidney. It filters blood at the glomerulus and modifies the filtrate through reabsorption and secretion in the tubules, producing urine.

A protein that speeds a chemical reaction by lowering its activation energy without being consumed. Enzymes are specific to substrates, and their activity depends on temperature and pH. They are central to digestion and metabolism.