Medical Dictionary banner
Medical Dictionary - Dictionary of Medicine and Human Biology

Medical Dictionary


rhopheocytosis (ro′fe-o-si-to′sis)
Formation of vacuoles at a cell surface without prior formation of cytoplasmic projections, by which the cell appears to aspirate surrounding material. SEE ALSO: pinocytosis. [G. rhopheo, to gulp down, or aspirate, + kytos, cell, + -osis, condition]

rhoptry, pl .rhoptries (rop′tre, -tres)
Electron-dense club-shaped, tubular or saccular organelles, extending back from the anterior end of sporozoites and other stages of certain sporozoans in the subphylum Apicomplexa. SYN: paired organelles, toxoneme. [G. rhopalon, club]

rhotacism (ro′ta-sizm)
Mispronunciation of the “r” sound. [G. rho, the letter r]

rhubarb (roo′barb)
Any plant of the genus Rheum (family Polygonaceae), especially R. rhaponticum, garden r., and R. officinale or R. palmatum; the last two species or their hybrids, deprived of periderm tissues, dried, and powdered, are used for their astringent, tonic and laxative effects.

Rhus (roos, rus)
A genus of vines and shrubs (family Anacardiaceae) containing various species that are used for their ornamental foliage; formerly used in tanning. Certain poisonous species are classified as Toxicodendron. [L., fr. G. rhous, sumac]

rhypophobia (ri-po-fo′be-a)
SYN: mysophobia. [G. rhypos, filth, + phobos, fear]

rhythm (rith′um)
1. Measured time or motion; the regular alternation of two or more different or opposite states. 2. SYN: r. method. 3. Regular or irregular occurrence of an electrical event in the electrocardiogram or electroencephalogram. SEE ALSO: wave. 4. Sequential beating of the heart generated by a single beat or sequence of beats. [G. rhythmos] agonal r. an idioventricular r., characterized by unusually wide and bizarre ventricular complexes, often seen in moribund patients. alpha r. 1. a wave pattern in the encephalogram in the frequency band of 8–13 Hz; 2. the posterior dominant 8–13 Hz r. in the awake, relaxed person with closed eyes, that attenuates with eye opening. SYN: alpha wave, Berger r.. atrioventricular junctional r. the cardiac r. when the heart is controlled by the AV junction (including node); arising in the AV junction, the impulse ascends to the atria and descends to the ventricles, each at varying speeds depending on the site of the pacemaker; only descends to the ventricles in the common form of atrioventricular dissociation and in idiojunctional r.. SYN: AV junctional r., nodal bradycardia, nodal r.. AV junctional r. SYN: atrioventricular junctional r.. basic electrical r. (BER) a slow wave of depolarization of smooth muscle from the fundus to the pylorus that coordinates gastric peristalsis and emptying. Berger r. SYN: alpha r.. beta r. a wave pattern in the electroencephalogram in the frequency band of 18–30 Hz. SYN: beta wave. bigeminal r. that cardiac r. when each beat of the dominant r. (sinus or other) is followed by a premature beat, with the result that the heartbeats occur in pairs (bigeminy). SYN: coupled r.. cantering r. SYN: gallop. chaotic r. completely irregular cardiac r. at varying rates. SEE ALSO: arrhythmia. circadian r. circadian. circus r. SYN: circus movement. coronary nodal r. formerly applied by some authorities to the electrocardiographic pattern of normal upright P waves in leads I and II with a short P-R interval. coronary sinus r. an ectopic atrial r. supposedly originating from a pacemaker at the mouth of the coronary sinus; recognized in the electrocardiogram by P-waves that are inverted in leads II, III, and aVF with a normal or prolonged P-R interval; an ectopic (“lower”) atrial r.. coupled r. SYN: bigeminal r.. delta r. a wave pattern in the electroencephalogram in the frequency band of 1.5–4.0 Hz. diurnal r. diurnal. ectopic r. any cardiac r. arising from a center other than the normal pacemaker, the sinus node. escape r. three or more consecutive impulses at a rate not exceeding the upper limit of the inherent pacemaker; extreme range of impulse formation at the sinoatrial node is between 40 and 180 impulses per minute, that of the atrioventricular junction is normally 40–60 impulses per min, and the normal rate of the ventricular myocardium (idioventricular r.) is 20–40 impulses per min. gallop r. SYN: gallop. idiojunctional r. SYN: idionodal r.. idionodal r. an independent r., the ventricles being under control of the AV node (AV junction). SYN: idiojunctional r.. idioventricular r. a slow independent ventricular r. under control of a ventricular center (which is, by definition, ectopic). SYN: ventricular r.. junctional r. rhythms originating anywhere within the AV junction. Formerly, “AV nodal” or simply “nodal” rhythms. nodal r. SYN: atrioventricular junctional r.. pendulum r. SYN: embryocardia. quadrigeminal r. a cardiac arrhythmia in which the heartbeats are grouped in fours, each usually composed of one sinus beat followed by three extrasystoles, but a repetitive group of four of any composition is quadrigeminal. SYN: quadrigeminy. quadruple r. a quadruple cadence to the heart sounds due to the easy audibility of both third and fourth heart sounds, indicative of serious myocardial disease. SYN: trainwheel r.. reciprocal r. a cardiac arrhythmia in which the impulse arising in the AV junction descends to and activates the ventricles on one intrajunctional pathway and simultaneously ascends toward the atria in parallel pathways; before reaching the atria, however, the impulse is reflected downward and again activates the ventricles, producing an echo or reciprocal beat; recognized in the electrocardiogram by the presence of an inverted P wave in lead aVF and usually II sandwiched between two ventricular complexes aberrantly, both of which may be normal or one of which may be conducted. reciprocating r. a cardiac arrhythmia initiated by an AV junctional beat followed in turn by a reciprocal beat; the descending impulse of the reciprocal beat, before reaching the ventricles, is also reflected backward to the atria, but before reaching the atria is reflected downward again to the ventricles, so that there is both retrograde atrial activation and orthograde ventricular activation. reversed reciprocal r. a cardiac arrhythmia in which a normal sinus impulse, before reaching the ventricles, is reflected backward to the atria; thus in the electrocardiogram a ventricular complex is sandwiched between a normal sinus P wave and a retrograde P wave; if the dysrhythmia continues, subsequent cycles are similar to those of reciprocating r.. sinus r. normal cardiac r. proceeding from the sinoatrial node; in healthy adults its rate is 60–90 beats/min. systolic gallop r. obsolete term for extra sounds, usually clicks, heard during systole. theta r. a wave pattern in the electroencephalogram in the frequency band of 4–7 Hz. SYN: theta wave. tic-tac r. SYN: embryocardia. trainwheel r. SYN: quadruple r.. trigeminal r. a cardiac arrhythmia in which the beats are grouped in trios, usually composed of a sinus beat followed by two extrasystoles. SYN: trigeminy. triple r. a triple cadence to the heart sounds at any heart rate, due to the easy audibility of a third (S3) (usually) or fourth (S4) heart sound, or at faster rates a summation sound due to coincidence of the third and fourth heart sounds (“S7” = S3 + S4). ultradian r. ultradian. ventricular r. SYN: idioventricular r..

rhytide (ri′tid)
A skin wrinkle. [[G. rhytis, -idos, wrinkle]]

rhytidectomy (rit-i-dek′to-me)
Elimination of wrinkles from, or reshaping of, the face by excising any excess skin and tightening the remainder; the so-called face-lift. SYN: face-lift, rhytidoplasty. [G. rhytis (rhytid-), a wrinkle]

rhytidoplasty (rit′i-do-plas-te)
SYN: rhytidectomy. [G. rhytis, a wrinkle, + plastos, formed]

rhytidosis (rit-i-do′sis)
1. Wrinkling of the face to a degree disproportionate to age. 2. Laxity and wrinkling of the cornea, an indication of approaching death. SYN: rutidosis. [G. a wrinkling, fr. rhytis, a wrinkle, + -osis, condition] r. retinae retinal wrinkling.

Abbreviation for radioimmunoassay.

Symbol for ribose. SYN: os costale.

See ribo-.

ribavirin (ri′ba-vi-rin)
A synthetic nucleoside antiviral agent that, by its inhibitory effect on the synthesis of guanosine 5′-phosphate, inhibits both DNA and RNA synthesis; used for treatment of viral pneumonia caused by respiratory syncytial virus.

ribbon (rib′on)
A r.-shaped structure. [M. E. riban] Reil r. SYN: medial lemniscus.

François, French physician, 1765–1845. See R. ganglion.

ribitol (ri′bi-tol)
Reduction product of ribose (–CHO at position 1 of ribose reduced to –CH2OH). SYN: adonitol.

ribityl (ri′bi-til)
The radical of ribitol; a constituent of riboflavin.

rib [I–XII]
One of the 24 elongated curved bones forming the main portion of the bony wall of the chest. SYN: costa (1) . [A.S. ribb] bicipital rib fusion of first thoracic rib with cervical vertebra. bifid rib one in which the body bifurcates. cervical rib [TA] a supernumerary rib articulating with a cervical vertebra, usually the seventh, but not reaching the sternum anteriorly. SEE ALSO: cervical rib syndrome. SYN: costa cervicalis [TA] . false ribs five lower ribs on either side that do not articulate with the sternum directly. SYN: costae spuriae [VII–XII] [TA] , vertebrochondral ribs. first rib [I] [TA] atypical rib having a single facet on its head, for articulation with the T1 vertebra, and the broadest, shortest and most sharply curved shaft; it also bears two transverse grooves on its superior surface for the subclavian vessels, separated by the scalene tubercle and ridge. SYN: costa prima [I] [TA] . floating ribs [XI–XII] the two lower ribs on either side that are not attached anteriorly. SYN: costae fluctuantes [XI–XII], costae fluitantes, vertebral ribs. lumbar rib [TA] an occasional rib articulating with the transverse process of the first lumbar vertebra. rib notching a smooth defect in the lower border of one or more upper ribs caused by enlarged intercostal collateral vessels, most often a sign of coarctation of the aorta. slipping rib subluxation of a rib cartilage, with costochondral separation. true ribs [I–VII] seven upper ribs on either side whose cartilages articulate directly with the sternum. SYN: costae verae [I–VII] [TA] , vertebrosternal ribs. vertebral ribs SYN: floating ribs [XI–XII]. vertebrochondral ribs SYN: false ribs. vertebrosternal ribs SYN: true ribs [I–VII].

1. Ribose. 2. As an italicized prefix to the systematic name of a monosaccharide, r. indicates that the configuration of a set of three consecutive, but not necessarily contiguous, CHOH (or asymmetric) groups is that of ribose; e.g., d-ribose, a trivial name, is d-r.pentose in systematic nomenclature. [German Ribose]

riboflavin, riboflavine (ri′bo-fla-vin)
A heat-stable factor of the vitamin B complex whose isoalloxazine nucleotides are coenzymes of the flavodehydrogenases. The daily human requirement is 1.7 mg for adult men and 1.3 mg for adult women, with a higher daily requirement during pregnancy and lactation; dietary sources include green vegetables, liver, kidneys, wheat germ, milk, eggs, cheese, and fish. SYN: flavin (1) , flavine, lactoflavin (2) , vitamin B2 (1) . r. kinase a cytosolic enzyme catalyzing the formation of flavin mononucleotide (r. phosphate) from r., utilizing ATP as phosphorylating agent. SYN: flavokinase. methylol r. a mixture of methylol derivatives of r. formed by the action of formaldehyde on r. in weakly alkaline solution; it has the same action as r., but is preferred for parenteral administration.

riboflavin 5′-phosphate
SYN: flavin mononucleotide.

ribofuranose (ri-bo-foor′a-nos)
The 1,4 cyclic furan form of ribose.

ribofuranosylthymine (ri′bo-foor-an′o-sil-thi′men)
SYN: ribothymidine.

SYN: psicose.

ribonuclease (RNase) (ri-bo-noo′kle-as)
A transferase or phosphodiesterase that catalyzes the hydrolysis of ribonucleic acid. SEE ALSO: r. (pancreatic), r. (Bacillus subtilis). SYN: ribonucleinase. RNase A r. (pancreatic). alkaline RNase r. (pancreatic). RNase α an enzyme catalyzing endonucleolytic cleavage of O-methylated RNA yielding 5′-phosphomonoesters. r. D (RNase D) an enzyme (endonuclease) that trims the extra 3′ nucleotides from immature tRNA. Escherichia coli RNase I SYN: RNase T2. RNase I r. (pancreatic). RNase II an enzyme cleaving RNA exonucleolytically in the 3′ to 5′ direction, yielding 5′-phosphomononucleotides. SEE ALSO: microbial RNase II. RNase III an enzyme catalyzing endonucleolytic cleavage of double-stranded RNA, yielding 5′-phosphomonoesters. microbial RNase II SYN: RNase T2. RNase N1 SYN: RNase T1. RNase N2 SYN: RNase T2. RNase P an enzyme catalyzing the endonucleolytic cleavage tRNA precursors to yield 5′-phosphomonoesters. pancreatic RNase r. (pancreatic). plant RNase SYN: RNase T2. RNase T1 a nuclease endonucleolytically cleaving ribonucleic acids at the 3′-5′ link of a guanosine 3′-phosphate residue, producing oligonucleotides terminating in this nucleotide; a transferase (endonuclease) in the first (cyclizing) step, a phosphodiesterase on the second (hydrolyzing) step. SYN: guanyloribonuclease, RNase N1. RNase T2 an enzyme endonucleolytically cleaving RNA to 3′-nucleotides with 2′,3′-cyclic nucleotides as intermediates. SYN: Escherichia coli RNase I, microbial RNase II, plant RNase, RNase N2. RNase U2 an enzyme endonucleolytically cleaving RNA to 3′-phosphomono- and 3′-phosphooligonucleotides ending in adenylate or guanylate residues with 2′,3′-cyclic phosphate intermediates. RNase U4 SYN: yeast RNase. yeast RNase an enzyme catalyzing the exonucleolytic cleavage of RNA to yield 3′-phosphomononucleotides. SYN: RNase U4.

ribonuclease (Bacillus subtilis)
1. Ribonuclease (Azotobacter agilis); ribonuclease (Proteus mirabilis); an enzyme catalyzing the endonucleolytic cleavage of RNA to yield 2′,3′-cyclic nucleotides. 2. Ribonuclease T1.

ribonuclease (pancreatic)
An enzyme isolated from the pancreas of ruminants that transfers the 3′-phosphate of a pyrimidine ribonucleotide residue in a polynucleotide from the 5′-position of the adjoining nucleotide to the 2′-position of the pyrimidine nucleotide itself (a transferase, endonuclease action), thus breaking the chain and forming a pyrimidine 2′,3′-cyclic phosphate, then (or independently) hydrolyzing this phosphodiester to leave a pyrimidine nucleoside 3′-phosphate residue (phosphodiesterase action); used in cytochemistry to selectively degrade and remove RNA as a control for staining of RNA.

ribonucleic acid (RNA) (ri′bo-noo-kle′ik)
A macromolecule consisting of ribonucleoside residues connected by phosphate from the 3′-hydroxyl of one to the 5′-hydroxyl of the next nucleoside. RNA is found in all cells, in both nuclei and cytoplasm and in particulate and nonparticulate form, and also in many viruses; polynucleotides made in vitro are generally called such. Various RNA fractions are identified by location, form, or function. acceptor RNA SYN: transfer RNA. antisense RNA the transcription product of the DNA antisense strand; it can play a role in the inhibition of translation. SEE ALSO: antisense DNA. chromosomal RNA RNA associated with the chromosome (not mRNA, tRNA, or rRNA) that may have a role in transcription. heterogeneous nuclear RNA (hnRNA) an ill-defined form of RNA, of high molecular weight, that never leaves the nucleus and is thought to be the precursor of messenger RNA. informational RNA SYN: messenger RNA. initiation tRNA tRNA in prokaryotes containing a formyl-methionyl residue that initiates translation. SYN: formyl-methionyl-tRNA, starter tRNA. messenger RNA (mRNA) the RNA reflecting the exact nucleoside sequence of the genetically active DNA and carrying the “message” of the latter, coded in its sequence, to the cytoplasmic areas where protein is made in amino acid sequences specified by the mRNA, and hence primarily by the DNA; viral RNAs are considered to be natural messenger RNAs. SYN: informational RNA, template RNA. messengerlike RNA (mlRNA) heterogeneous nuclear RNA. nuclear RNA (nRNA) rNA found in nuclei, or associated with DNA, or with nuclear structures (nucleoli). RNA polymerase nucleotidyltransferases. ribosomal RNA the RNA of ribosomes and polyribosomes. small nuclear RNA (snRNA) small RNA ( i.e., about 90–300 nucleotides long) in the nucleus believed to have a role in RNA processing and cellular architecture. soluble RNA (sRNA) SYN: transfer RNA. [soluble in molar salt] starter tRNA SYN: initiation tRNA. suppressor tRNA the tRNA associated with a suppressor mutation. template RNA SYN: messenger RNA. transfer RNA (tRNA) short-chain RNA molecules present in cells in at least 20 varieties, each variety capable of combining with a specific amino acid (see aminoacyl-tRNA). By joining (through their anticodons) with particular spots (codons) along the messenger RNA molecule and carrying their amino acyl residues along, they lead to the formation of protein molecules with a specific amino acid arrangement—the one ultimately dictated by a segment of DNA in the chromosomes. Each tRNA has about 80 nucleotides (MW about 25,000); most of the 20 varieties occur in multiple “isoacceptor” forms, separable by chromatography. Further subvarieties exist in, e.g., different strains of an organism, in subcellular organelles, and in different metabolic states. Cognate tRNAs are the tRNAs recognized by the specific amino acyl-tRNA synthetases. SYN: acceptor RNA, soluble RNA.

ribonucleinase (ri-bo-noo′kle-i-nas)
SYN: ribonuclease.

ribonucleoprotein (RNP) (ri′bo-noo′kle-o-pro′ten)
A combination of ribonucleic acid and protein.

ribonucleoside (ri-bo-noo′kle-o-sid)
A nucleoside in which the sugar component is ribose; the common ribonucleosides of RNA are adenosine, cytidine, guanosine, and uridine.

ribonucleotide (ri-bo-noo′kle-o-tid)
A nucleotide (nucleoside phosphate) in which the sugar component is ribose; the major ribonucleotides of RNA are adenylic acid, cytidylic acid, guanylic acid, and uridylic acid. r. reductase a protein complex that converts r. diphosphates (NDPs) such as ADP and CDP to 2′-deoxyribonucleotide diphosphates (dNDPs) such as dADP and dCDP. This complex requires thioredoxin, thioredoxin reductase, and NADPH. It is crucial for DNA synthesis.

ribophorins (ri′-bo-for′inz)
Ribosome receptor proteins that interact specifically with the large ribosomal subunit and aid in translocation of newly synthesized proteins across the endoplasmic reticulum. [ribonucleic acid + G. phoros, carrying, + -in]

ribopyranose (ri-bo-pir′a-nos)
The 1,5-cyclic form of ribose.

ribose (Rib) (ri′bos)
The pentose that, as the d-isomer, is present in ribonucleic acid; epimers of d-r. are d-arabinose, d-xylose, and l-lyxose.

Ribose phosphorylated on carbon-5; an intermediate in the pentose phosphate pathway. ribose 5-phosphate isomerase an enzyme catalyzing interconversion of d-ribose 5-phosphate and d-ribulose 5-phosphate; of importance in ribose metabolism and in the pentose phosphate pathway. SYN: phosphopentose isomerase, phosphoriboisomerase.

riboside (ri′bo-sid)
The product formed by replacement of the H of the C-1 OH of ribose by an alcohol residue (which may be another sugar); differs from ribosyl compounds and does not occur in ribonucleic acids, where the radical is a ribosyl (1-OH missing entirely). See structure for methyl β-d-ribofuranoside below.

ribosome (ri′bo-som)
A granule of ribonucleoprotein, 120–150 Å in diameter, that is the site of protein synthesis from aminoacyl-tRNAs as directed by mRNAs. SYN: Palade granule.

ribosuria (ri-bo-soo′re-a)
The enhanced urinary excretion of d-ribose; commonly one manifestation of muscular dystrophy. [ribose + G. ouron, urine]

ribosyl (ri′bo-sil)
The radical formed by loss of the hemiacetal OH group from either of the two cyclic forms of ribose (yielding ribofuranosyl and ribopyranosyl compounds), by combination with an H of an –NH– or a –CH– group; the natural nucleosides are r. compounds, not ribosides, as the bond between ribose and aglycon is C–N or C–C, not –C–O–X–.

ribosylation (ri-bo-sil-a-shun)
The covalent attachment of one or more ribosyl groups to a molecule (usually a macromolecule). ADP r. covalent attachment of an ADP-ribosyl moiety to a macromolecule; E.G., the action of diphtheria toxin.

1-ribosylorotate (ri′bo-sil-or′o-tat)
SYN: orotidine.

ribosylpurine (ri′bo-sil-pur′en)
SYN: nebularine.

SYN: ribothymidine.

ribothymidine (T, Thd) (ri-bo-thi′mi-den)
5-Methyluridine;the ribosyl analog of thymidine (deoxyribosylthymine); a nucleoside found in small amounts in ribonucleic acids. SYN: ribofuranosylthymine, ribosylthymidine.

ribothymidylic acid (rTMP, TMP) (ri′bo-thi-mi-dil′ik)
Ribothymidine 5′-phosphate;the ribose analog of thymidylic acid; a rare component of transfer RNAs.

ribotide (ri′bo-tid)
A corruption of riboside, by analogy with nucleoside-nucleotide, to mean ribonucleotide.

ribovirus (ri′bo-vi′rus)
SYN: RNA virus.

ribozyme (ri′bo-zim)
A nonprotein biocatalyst; several cleave precursors of tRNA to yield functional tRNAs; others act on rRNA; plays a key role in intron splicing events. SYN: organic catalyst (1) , RNA enzyme. [ribonucleic acid + -zyme]

ribulose (ri′bu-los)
The 2-keto isomer of ribose. As the 5-phosphate, it participates in the pentose monophosphate shunt; as the 1,5-bisphosphate, it combines with CO2 at the start of the photosynthetic process in green plants (“carbon dioxide trap”); d-r. is the epimer of d-xylulose.

ribulose-1,5-bisphosphate carboxylase
A dimerizing carboxy-lyase; an enzyme that catalyzes the addition of carbon dioxide to d-ribulose 1,5-bisphosphate and the hydrolysis of the addition product to two molecules of 3-d-phosphoglyceric acid, a key reaction in the fixation of CO2 in photosynthesis. SYN: carboxydismutase.

ribulose-phosphate 3-epimerase
An enzyme catalyzing the reversible interconversion of d-xylulose 5-phosphate and its epimer, d-ribulose 5-phosphate; a step in the nonoxidative phase of the pentose phosphate pathway. SYN: phosphoribulose epimerase.

Annibale, Italian astrophysicist, 1844–1919. See R. law.

rice (ris)
The grain of Oryza sativa (family Gramineae), the r. plant; a food; also used, finely pulverized, as a dusting powder. [G. oryza]

Arnold R., U.S. pathologist, 1893–1968. See Hamman-R. syndrome.

Barry Wyndham, 20th century English physician. See R.-Rundle syndrome.

John Clifford, Canadian neurologist, *1909. See Steele-R.-Olszewski disease, Steele-R.-Olszewski syndrome.

August G., German surgeon, 1742–1812. See R. hernia, R.-Monro line, Monro-R. line.

Maurice N., U.S. pathologist, *1897. See R. syndrome.

ricin (ri′sin, ris′in)
A highly toxic lectin and hemagglutin occurring in the seeds (castor beans) of the castor oil plant, Ricinus communis; if eaten, acts as a violent irritant and may be fatal; an N-glycosidase that acts on the GOS subunit of rRNA.


. . . Feedback