Receptor The site within the body that detects or receives the stimulus, senses the change from normal, and sends signals to the control center. Reproductive System The organ system that produces germ cells (eggs and sperm) and provides the environment for growth of the fetus (women).  The main reproductive organs are the ovaries, uterus, mammary glands, testes, prostate gland, and the external genitalia. Respiratory System The organ system responsible for oxygen and carbon dioxide exchange.  The main organs are the lungs, trachea, larynx, nasal cavities, and pharynx. Reversible Cell Damage A type of cellular damage in which the response of the cell to toxic injury may be transient and once the stress has been removed or the compensatory cellular changes made, it returns to full capability. Ribonucleic Acid (RNA) A nucleic acid consisting of a chain of nucleotides that contain the sugar ribose and the nitrogen bases adenine, guanine, cytosine, and uracil. Ribosomes Very small cell organelles that consist of RNA and proteins, and function in protein synthesis.