arrector pili A microscopic band of muscle tissue that connects a hair follicle to the dermis. When stimulated, the arrector pili contracts and causes the hair to become more perpendicular to the skin surface, thereby erecting the hair (causing the hair to stand on end). The arrector pili muscle plays a key role in forming…
aromatherapy A form of alternative medicine in which essential oils or other scents are inhaled to achieve therapeutic benefit. The mechanism of action in aromatherapy is unknown, but recent studies have shown that aromatherapy may be beneficial for some health problems.
Arnold Chiari malformation See Chiari malformation.
armed tapeworm See Taenia solium.
arm, wrist, and hand bones See bones of the arm, wrist, and hand.
arterial pressure The pressure of the blood within an artery. Also known as arterial tension and intra-arterial pressure.
arm In popular usage, the appendage that extends from the shoulder to the hand. However, the medical definition refers to the upper extremity extending from the shoulder only to the portion of the elbow, excluding the forearm, which extends from the elbow to the wrist. The arm contains one bone: the humerus.
argyria Silver poisoning, resulting in ashen, gray, discolored skin, and damage to other tissues of the body. Caused by long-term use of silver salts or other preparations containing silver.
arginine An essential amino acid and a key component of protein. Lack of arginine in the diet impairs growth, and in adult males it decreases the sperm count. Arginine is available in turkey, chicken, and other meats, and as L-arginine in supplements. Babies born without the enzyme phosphate synthetase have arginine deficiency syndrome; adding arginine…
areola 1 The small, darkened area around the nipple of the breast. 2 The colored part of the iris around the pupil of the eye. 3 Any small space in a tissue.
ARDS Acute respiratory distress syndrome.
arcus senilis A cloudy opaque arc or circle around the edge of the eye, often seen in the eyes of the elderly.