{receptor}{}{A cell or other target that receives a signal such as a hormone or other cellular mediator.}
{recessive trait}{}{A trait that can only be expressed if both copies of the gene present in the individual are of the recessive allele.}
{red marrow}{}{Blood cell-forming tissue in spaces within bones.}
{refractory period}{}{Period following effective stimulation during which excitable tissue such as heart muscle fails to respond to a stimulus of threshold intensity.}
{renal capsule}{}{Fibrous connective tissue layer enveloping each kidney.}
{renal cortex}{}{Outer portion of a kidney.}
{renal fascia}{}{Thin layer of connective tissue anchoring the kidneys and their adipose tissue to the abdominal wall.}
{renal fat pad}{}{Fatty deposit around the kidney.}
{renal medulla}{}{Inner portion of a kidney.}
{renal pelvis}{}{Cavity in a kidney that channels urine to the ureter.}
{repolarization}{}{Phase of the action potential in which the membrane potential moves from its maximum degree of depolarization toward the value of the resting membrane potential.}
{reproduction}{}{Offspring formation}
{respiration}{}{Process of life in which oxygen is used to oxidize organic fuel molecules, providing a source of energy, carbon dioxide, and water. Movement of air into and out of the lungs, the exchange of gases with blood, the transportation of gases in the blood, and gas exchange between the blood and the tissues.}
{respiratory system}{}{Group of organs that carries out the process of respiration.}
{responsiveness}{}{The ability to act as a result of a stimulus.}
{resting membrane potential}{}{Electric charge difference inside a cell membrane, measured relative to just outside the cell membrane.}
{reticular formation}{}{Complex network of nerve fibers within brain stem that arouses cerebrum.}
{retraction}{}{Movement in the posterior direction.}
{ribonucleic acid [RNA]}{}{Nucleic acid containing ribose as the sugar component; found in all cells in both nuclei and cytoplasm; helps direct protein synthesis.}
{ribosomal RNA [rRNA]}{}{RNA that is associated with certain proteins to form ribosomes.}
{ribosome}{}{Small, spherical, cytoplasmic organelle where protein synthesis occurs.}
{rotation}{}{Movement of a structure about its axis.}
{rough endoplasmic reticulum}{}{Membranous network of ribosome-covered tubes within the cytoplasm.}
{Ruffini's end organ}{}{Named for Angelo Ruffini, Italian histologist, 1864-1929; receptor located deep in the dermis and responding to continuous touch or pressure.}
