Antigen(抗原)
A protein coat on the surface of red blood cells; a red blood cell may have a protein coat of type A, B, or AB. If the cell has no antigens, it is called type O. The presence of a foreign antigen in a body will cause blood to clot.
Aorta(大动脉)
The largest artery in the body; carries oxygenated blood from the left ventricle of the heart.
aphotic zone(无光带)
Literally, zone without light. The aphotic zone is part of the marine pelagic zone and begins 600 feet below the surface of the ocean. Only chemosynthetic organisms, scavengers, and predators are able to survive in this habitat.
Appendage(附属肢体)
A structure that extends from the trunk of an organism and is capable of active movements.
Artery(动脉)
Vessel that carries blood away from the heart and has thick, elastic, muscular walls that can dilate or contract to control blood pressure within the vessels. Blood in arteries is oxygenated, with the exception of the blood in the pulmonary artery(肺动脉，把心脏中的血输送到肺部的动脉).
autonomic nervous system(自主神经系统)
The involuntary half of the peripheral nervous system. The autonomic nervous system is in two antagonistic parts: the sympathetic(交感神经的) and parasympathetic(副交感神经的) nervous systems. Their interactions control smooth and cardiac muscle, glands, and organs and processes such as heartbeat, the movements of the digestive tract, and the contraction of the bladder.
Autosome(正染色体，非性染色体)
Any chromosome that is not a sex chromosome. Humans have 44 autosomes, in 22 homologous pairs. The two sex chromosomes are the twenty-third pair of chromosomes.
Autotroph(自养生物)
An organism that can produce the organic molecules and energy necessary for life through the processes of photosynthesis or chemosynthesis. Autotrophs do not rely on other organisms for food. In a food web, autotrophs are producers.
Auxin(茁长素) ：一种植物激素，刺激细胞伸长
One in a class of plant hormones that stimulates (among other things) cell elongation, secondary tissue growth, and fruit development.