radiodensity: The relative resistance to the passage of X-rays.
radiographic techniques: Methods of visualizing internal structures by using various forms of radiational energy.
radiopaque: Having a high radiodensity.
rami communicantes: Axon bundles that link the spinal nerves with the ganglia of the sympathetic chain.
ramus/rami: A branch.
raphe: A seam.
receptive field: The area monitored by a single sensory receptor.
recessive trait: An allele that affects the phenotype only when the individual is homozygous for that trait.
recombinant DNA: DNA created by splicing together a specific gene from one organism into the DNA strand of another organism.
rectal columns: Longitudinal folds in the walls of the anorectal canal.
rectouterine pouch: The peritoneal pocket between the anterior surface of the colon and the posterior surface of the uterus.
rectum: The inferior 15 cm (6 in.) of the digestive tract.
rectus: Straight.
red blood cell (RBC): See erythrocyte.
reduction: The gain of hydrogen atoms or electrons or the loss of an oxygen molecule.
reductional division: The first meiotic division, which reduces the chromosome number from 46 to 23.
reflex: A rapid, automatic response to a stimulus.
reflex arc: The receptor, sensory neuron, motor neuron, and effector involved in a particular reflex; interneurons may be present, depending on the reflex considered.
refraction: The bending of light rays as they pass from one medium to another.
refractory period: The period between the initiation of an action potential and the restoration of the normal resting potential; during this period, the membrane will not respond normally to stimulation.
relaxation phase: The period after a contraction when the tension in the muscle fiber returns to resting levels.
relaxin: A hormone that loosens the pubic symphysis; secreted by the placenta.
renal: Pertaining to the kidneys.
renal corpuscle: The initial portion of the nephron, consisting of an expanded chamber that encloses the glomerulus.
renin: The enzyme released by cells of the juxtaglomerular apparatus when renal blood flow declines; converts angiotensinogen to angiotensin I.
rennin: A gastric enzyme that breaks down milk proteins.
replication: Duplication.
repolarization: The movement of the transmembrane potential away from a positive value and toward the resting potential.
residual volume: The amount of air remaining in the lungs after maximum forced exhalation.
respiration: The exchange of gases between cells and the environment; includes pulmonary ventilation, external respiration, internal respiration, and cellular respiration.
respiratory minute volume VE: The amount of air moved into and out of the respiratory system each minute.
respiratory pump: A mechanism by which changes in the intrapleural pressures during the respiratory cycle assist the venous return to the heart; also called thoracoabdominal pump.
resting potential: The transmembrane potential of a normal cell under homeostatic conditions.
rete: An interwoven network of blood vessels or passageways.
reticular activating system (RAS): The mesencephalic portion of the reticular formation; responsible for arousal and the maintenance of consciousness.
reticular formation: A diffuse network of gray matter that extends the entire length of the brain stem.
reticulospinal tracts: Descending tracts of the medial pathway that carry involuntary motor commands issued by neurons of the reticular formation.
retina: The innermost layer of the eye, lining the vitreous chamber; also called neural tunic.
retinal: A visual pigment derived from vitamin A.
retraction: Movement posteriorly in the horizontal plane.
retroflexion: A posterior tilting of the uterus that has no clinical significance.
retrograde flow: The transport of materials from the telodendria to the cell body of a neuron.
retroperitoneal: Behind or outside the peritoneal cavity.
reverberation: A positive feedback along a chain of neurons such that they remain active once stimulated.
rheumatism: A condition characterized by pain in muscles, tendons, bones, or joints.
Rh factor: An agglutinogen that may be present (Rh-positive) or absent (Rh-negative) from the surfaces of red blood cells.
rhizotomy: The surgical transection of a dorsal root, normally performed to relieve pain.
rhodopsin: The visual pigment in the membrane disks of the distal segments of rods.
rhythmicity center: A medullary center responsible for the pace of respiration; includes inspiratory and expiratory centers.
ribonucleic acid: A nucleic acid consisting of a chain of nucleotides that contain the sugar ribose and the nitrogenous bases adenine, guanine, cytosine, and uracil.
ribose: A five-carbon sugar that is a structural component of RNA.
ribosome: An organelle that contains rRNA and proteins and is essential to mRNA translation and protein synthesis.
rigor mortis: The extended muscular contraction and rigidity that occurs after death; the result of calcium ion release from the sarcoplasmic reticulum and the exhaustion of cytoplasmic ATP reserves.
rod: A photoreceptor responsible for vision in dim lighting.
rough endoplasmic reticulum (RER): A membranous organelle that is a site of protein synthesis and storage.
round window: An opening in the bony labyrinth of the inner ear that exposes the membranous wall of the tympanic duct to the air of the middle ear cavity.
rubrospinal tracts: Descending tracts of the lateral pathway that carry involuntary motor commands issued by the red nucleus of the mesencephalon.
Ruffini corpuscles: Receptors sensitive to tension and stretch in the dermis of the skin.
rugae: Mucosal folds in the lining of the empty stomach that disappear as gastric distension occurs.