Glossary of Terms & Definitions
Listed Alphabetically - "R"

Radiation therapy:  The local use of radiation to destroy cancer cells or stop them from dividing and growing.

Radiographic anatomy: Diagnostic branch of anatomy that includes the use of x-rays.

Rami communicantes: Branches of a spinal nerve.  Singular is ramus communicans.

Randomized controlled trial (RCT):  A clinical trial that involves at least one test treatment and one control treatment, in which the treatments administered are selected by a random process (e.g., coin flips or a random-numbers table).

Randomized design:  An experiment in which participants are chosen for the experimental and control groups at random, in order to reduce bias caused by self-selection into experimental and control groups. This type of study design can provide evidence of causality.

RDA:  Recommended dietary allowance.  Set by the Food and Nutrition Board of the Institute of Medicine, the RDA is the average daily dietary intake level sufficient to meet the nutrient requirements of nearly all (97-98%) healthy individuals in a specific life stage and gender group (e.g., women from 19-50 years of age).  It is intended as a goal for daily intake of specific nutrients by individuals.

Reactivity: Ability of an antigen to react specifically with the antibody whose formation it induced.

Reactive oxygen species (ROS):  Highly reactive chemicals, containing oxygen, that react easily with other molecules, resulting in potentially damaging modifications.

Reactive nitrogen species (RNS):  Highly reactive chemicals, containing nitrogen, that react easily with other molecules, resulting in potentially damaging modifications.

Receptor:  A protein on or protruding from the cell surface to which select chemicals can bind. Binding of a specific molecule (ligand) may result in a cellular signal, or the internalization of the receptor and the ligand.

Recessive trait:  A trait that is expressed only when two copies of the gene responsible for the trait are present.

Rectum:  The last section of the large intestine (colon). It connects the sigmoid colon (above) to the anus (below).

Redox reaction:  Another term for an oxidation-reduction reaction. A redox reaction is any reaction in which electrons are removed from one molecule or atom and transferred to another molecule or atom. In such a reaction one substance is oxidized (loses electrons) while the other is reduced (gains electrons).

Reduction:  A chemical reaction in which a molecule or atom gains electrons.

Renal:  Refers to the kidneys.

Resorption:  The process of breaking down or assimilating something. With respect to bone, resorption refers to the breakdown of bone by osteoclasts that results in the release of calcium and phosphate (bone mineral) into the blood.

Response element:  A sequence of nucleotides in a gene that can be bound by a protein. Proteins that bind to response elements in genes are sometimes called transcription factors or binding proteins. Binding of a transcription factor to a response element regulates the production of specific proteins by inhibiting or enhancing the transcription of genes that encode those proteins. 

Retina:  The sensory membrane that lines most of the back of the eye. The retina is composed of several layers including one containing the rods and cones. It receives the image formed by the lens and converts it into chemical and nervous signals which reach the brain by way of the optic nerve.

Rheumatoid arthritis:  An autoimmune disease which causes chronic inflammation of the joints, the tissue around the joints, as well as other organs in the body. Because it can affect multiple other organs of the body, rheumatoid arthritis is a systemic illness and is sometimes called rheumatoid disease.

Riboucelotide:  A molecule consisting of a 5-carbon sugar (ribose), a nitrogen containing base, and one or more phosphate groups.

Rickets:  Often the result of vitamin D deficiency. Rickets affects children while their bones are still growing. It is characterized by soft and deformed bones, and is the result of a impaired incorporation of calcium and phosphate into the skeleton.

RNA:  Ribonuceic acid. a chain of nucleotides, which are composed of a nitrogen containing base, a 5-carbon sugar (ribose), and phosphate groups. RNA functions in the translation of the genetic information in DNA to protein synthesis.

Ruminant:  An animal that chews cud. Ruminant animals include cattle, goats, sheep, and deer.