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Medical Dictionary


pressosensitivity (pres′o-sen-si-tiv′i-te)
The state of being able to perceive changes in pressure. SEE ALSO: pressoreceptive. reflexogenic p. p. also capable of initiating the regulation of heart rate, vascular tone, and blood pressure.

pressure (P, P) (presh′ur)
1. A stress or force acting in any direction against resistance. 2. (P, frequently followed by a subscript indicating location)In physics and physiology, the force per unit area exerted by a gas or liquid against the walls of its container or that would be exerted on a wall immersed at that spot in the middle of a body of fluid.The p. can be considered either relative to some reference p., such as that of the ambient atmosphere (imagined to be on the other side of the wall), or in absolute terms (relative to a perfect vacuum). [L. pressura, fr. premo, pp. pressus, to press] abdominal p. p. surrounding the bladder; estimated from rectal, gastric, or intraperitoneal p.. absolute p. p. measured with respect to zero p.. Cf.:gauge p.. acoustic p. in ultrasound, the instantaneous value of the total p. minus the ambient p.; unit is pascal (Pa). atmospheric p. SYN: barometric p.. back p. p. exerted upstream in the circulation as a result of obstruction to forward flow, as when congestion in the pulmonary circulation results from stenosis of the mitral valve or failure of the left ventricle. barometric p. (PB) the absolute p. of the ambient atmosphere, varying with weather, altitude, etc.; expressed in millibars (meteorology) or mm Hg or torr (respiratory physiology); at sea level, one atmosphere (atm, 760 mm Hg or torr) is equivalent to: 14.69595 lb/sq in, 1013.25 millibars, 1013.25 × 106 dynes/cm2, and, in SI units, 101,325 pascals (Pa). SYN: atmospheric p.. biting p. SYN: occlusal p.. blood p. (BP) the p. or tension of the blood within the systemic arteries, maintained by the contraction of the left ventricle, the resistance of the arterioles and capillaries, the elasticity of the arterial walls, as well as the viscosity and volume of the blood; expressed as relative to the ambient atmospheric p.. SYN: piesis. central venous p. (CVP) the p. of the blood within the venous system in the superior and inferior vena cava cephalad to the diaphragm, normally between 4 and 10 cm of water; it is depressed in circulatory shock and deficiencies of circulating blood volume and increased with cardiac failure and congestion of the venous circulation. cerebrospinal p. the p. of the cerebrospinal fluid, normally 100–150 mm of water, relative to the ambient atmospheric p.. continuous positive airway p. (CPAP) a technique of respiratory therapy, in either spontaneously breathing or mechanically ventilated patients, in which airway p. is maintained above atmospheric p. throughout the respiratory cycle by pressurization of the ventilatory circuit. coronary perfusion p. the p. at which blood proceeds through the coronary circulation, mainly in diastole. critical p. the minimum p. required to liquefy a gas at the critical temperature. detrusor p. that component of intravesical p. created by the tension (active and passive) exerted by the bladder wall; the transmural p. across the bladder wall estimated by subtracting abdominal p. from intravesical p.. diastolic p. the intracardiac p. during or resulting from diastolic relaxation of a cardiac chamber; the lowest arterial blood p. reached during any given ventricular cycle. differential blood p. the arterial blood p. at corresponding points on the two sides of the body. Donders p. an increase of about 6 mm Hg shown by a manometer connected with the trachea when the thorax of a dead body is opened; it is caused by the collapse of the lungs when air is admitted to the thorax. effective osmotic p. that part of the total osmotic p. of a solution that governs the tendency of its solvent to pass across a boundary, usually a semipermeable membrane; it is commonly represented by the product of the total osmotic p. of the solution and the ratio (corrected for activities) of the number of dissolved particles that do not permeate the bounding membrane to the total number of particles in the solution; equivalent in meaning to tonicity; commonly expressed in equivalent units of osmolality rather than p. per se. gauge p. p. measured relative to ambient atmospheric p.; at sea level, it is 1 atm less than the p. in the atmosphere. Cf.:absolute p.. hydrostatic p. the p. exerted by a liquid as a result of its potential energy, ignoring its kinetic energy; frequently used to distinguish a true p. from an osmotic p. or to emphasize the variation in p. in a column of fluid due to the effect of gravity. intracranial p. (ICP) p. within the cranial cavity. intraocular p. the p. (usually measured in millimeters of mercury) of the intraocular fluid within the eye, measured by means of a manometer. leak point p. storage p. in bladder at which leakage occurs passively, usually in patients with neuropathic bladder. negative p. p. less than that of the ambient atmosphere. negative end-expiratory p. (NEEP) a subatmospheric p. at the airway at the end of expiration. occlusal p. any force exerted upon the occlusal surfaces of teeth. SYN: biting p.. oncotic p. osmotic p. exerted by colloids in solution. osmotic p. (Π) the p. that must be applied to a solution to prevent the passage into it of solvent when solution and pure solvent are separated by a membrane permeable only to the solvent (sometimes less correctly viewed as the force with which the solution attracts solvent through the semipermeable membrane). partial p. (P) the p. exerted by a single component of a mixture of gases, commonly expressed in mm Hg or torr; for a gas dissolved in a liquid, the partial p. is that of a gas that would be in equilibrium with the dissolved gas. Formerly, symbolized by p, followed by the chemical symbol in capital letters ( e.g., pCO2, pO2); now, in respiratory physiology, P, followed by subscripts denoting location and/or chemical species ( e.g., Pco2, Po2, Paco2. pleural p. the p. in the pleural space between the visceral and parietal pleurae. positive end-expiratory p. (PEEP) a technique used in respiratory therapy in which airway p. greater than atmospheric p. is achieved at the end of exhalation by introduction of a mechanical impedance to exhalation. So-called “auto-PEEP” occurs when increased time is needed for expiration during mechanical ventilation and the next breath is delivered before the system p. has dropped to zero; this can be a dangerous phenomenon, which may lead to barotrauma and hypotension. pulmonary p. the blood p. in the pulmonary artery. pulmonary capillary wedge p. (PCWP) the p. obtained when a catheter is passed from the right side of the heart into the pulmonary artery as far as it will go and “wedged” into an end artery. PCWP is measured by letting pulmonary blood flow guide a balloon-flotation catheter into a small pulmonary end artery. The p. distal to the wedged catheter is an approximation of left ventricular end diastolic p.. The p. recorded with the balloon deflated is pulmonary artery p.. pulp p. the p. in the dental pulp cavity associated with extracellular fluid p., but showing pulsatile variations during the cardiac cycle because of the encasement of the pulp within the tooth. pulse p. the variation in blood p. occurring in an artery during the cardiac cycle; it is the difference between the systolic or maximum and diastolic or minimum pressures. selection p. impact of effective reproduction due to environmental impact on the phenotype. solution p. the force driving atoms or molecules to leave a solid particle and enter into solution ( i.e., to dissolve). standard p. the absolute p. to which gases are referred under standard conditions (STPD), i.e., 760 mmHg, 760 torr, or 101,325 N/m2 ( i.e., 101,325 Pa). systolic p. the intracardiac p. during or resulting from systolic contraction of a cardiac chamber; the highest arterial blood p. reached during any given ventricular cycle. transmural p. p. across the wall of a cardiac chamber or of a blood vessel. In the heart, transmural p. is the resultant of the intracavitary p. minus the extracavitary ( i.e., pericardial) p. and is the distending, i.e., true filling, p. of the cardiac chamber of measurement when this is done during diastole. Since the pericardial p. normally approximates zero, the filling p. usually equals ventricular diastolic mean p., obviating the complexities of measuring pericardial p.. transpulmonary p. the difference between the p. of the respired gas at the mouth and the pleural p. around the lungs, measured when the airway is open; thus, it includes not only the transmural p. of the lung but also any drop in p. along the tracheobronchial tree during flow. transthoracic p. the p. in the pleural space measured relative to the p. of the ambient atmosphere outside the chest; the transmural p. across the chest wall. vapor p. the partial p. exerted by the vapor phase of a liquid. ventricular filling p. the p. in the ventricle as it fills with blood, ordinarily equivalent to the mean atrial p. when there is no AV valvular gradient. Atrial p. can be used in place of transmural p. because pericardial p. usually varies between −2 and +2 mm Hg and hence is negligible. During cardiac tamponade, pericardial and atrial pressures equilibrate so that transmural p. is zero and the high atrial presures cannot be “filling” pressures. wedge p. the intravascular p. reading obtained when a fine catheter is advanced until it completely occludes a small blood vessel or is sealed in place by inflation of a small cuff; commonly measured in the lung (pulmonary artery) to estimate left atrial p.. zero end-expiratory p. (ZEEP) airway p. that, at the end of expiration, equals atmospheric p..

presternum (pre′ster′num)
SYN: manubrium of sternum.

presuppurative (pre-sup′u-ra-tiv)
Denoting an early stage in an inflammation prior to the formation of pus.

presynaptic (pre′si-nap′tik)
Pertaining to the area on the proximal side of a synaptic cleft.

presystole (pre-sis′to-le)
That part of diastole immediately preceding systole. SYN: late diastole.

presystolic (pre-sis-tol′ik)
Late diastolic, relating to the interval immediately preceding systole.

pretarsal (pre-tar′sal)
Denoting the anterior, or inferior, portion of the tarsus.

pretecta (pre-tek′ta)
Orad to the hidden part of the duodenum.

pretectum (pre-tek′tum)
SYN: pretectal area.

prethyroid, prethyroideal, prethyroidean (pre-thi′royd, -thi-roy′de-al, -thi-roy′de-an)
Anterior to or preceding the thyroid gland or cartilage.

pretibial (pre-tib′e-al)
Relating to the anterior portion of the leg; denoting especially certain muscles.

pretracheal (pre-tra′ke-al)
Anterior to the trachea; denoting especially the middle layer of deep cervical fascia.

pretrematic (pre-tre-mat′ik)
Relating to the cranial surface of a branchial cleft. [pre- + G. trema, perforation]

pretympanic (pre-tim-pan′ik)
Anterior to the drum of the ear.

prevalence (prev′a-lens)
The number of cases of a disease existing in a given population at a specific period of time (period p.) or at a particular moment in time (point p.).

preventive (pre-ven′tiv)
SYN: prophylactic (1) . [L. prae-venio, pp. -ventus, to come before, prevent]

prevertebral (pre-ver′te-bral)
Anterior to the body of a vertebra or of the vertebral column; denoting especially the deepest layer of deep cervical fascia and the muscles on the anterior aspect of the vertebral column.

prevesical (pre-ves′i-kal)
Anterior to the bladder; denoting especially the retropubic space. [pre- + L. vesica, bladder]

Prevotella (prev′o-tel′ah)
Genus of Gram-negative, nonmotile, nonsporeforming, obligately anaerobic, chemoorganotrophic, and pleomorphic rods; contains many species previously classified in the genus Bacteroides. P. bivia the species of P. in highest concentration in the human vaginal tract. P. denticola a bacterial species found in the human mouth; a cause of infections of the oral cavity and adjacent structures. P. disiens a bacterial species associated with human infections, primarily of the female genital tract. SYN: Bacteroides disiens. P. heparinolytica a bacterial species associated with human periodontal disease. P. intermedia a species found in gingival crevices, especially associated with gingivitis, and other oral infections. P. melaninogenica a species found in the mouth, feces, infections of the mouth, soft tissue, respiratory tract, urogenital tract, and intestinal tract; implicated in periodontal disease; seen in aspiration. The type species of Pretovella. SYN: Bacteroides melaninogenicus. P. oralis a bacterial species found in the gingival crevice of humans and in infections of the oral cavity and upper respiratory and genital tracts. P. oris a bacterial species isolated from the gingival crevice, systemic infections, face, neck, and chest abscesses, wound drainages, and blood and various bodily fluids.

prezone (pre′zon)
SYN: prozone.

priapism (pri′a-pizm)
Persistent erection of the penis, accompanied by pain and tenderness, resulting from a pathologic condition rather than sexual desire; a term loosely used as a synonym for satyriasis. [see priapus]

priapus (pri′a-pus)
SYN: penis. [L. fr. P. (G. Priapos), god of procreation]

Pribnow (prib′now)
David, 20th-century U.S. molecular biologist. See P. box.

Ernest Arthur, English biochemist, *1882. See Carr-P. reaction.

Cecil, English hematologist, 1863–1943. See Price-Jones curve.

John Gillies, British physiologist, 1880–1941. See Haldane-P. sample.

prilocaine hydrochloride (pril′o-kan)
A local anesthetic of the amide type, related chemically and pharmacologically to lidocaine hydrochloride; used for peridural, caudal, and nerve blocks, and for regional and infiltration anesthesia. SYN: propitocaine hydrochloride.

primacy (pri′ma-se)
The state of being primary, or foremost in rank or importance. [see primary] genital p. in psychoanalysis, the primary characteristic of the genital phase of psychosexual development, i.e., the libido becomes preponderantly concentrated in the penis. oral p. in psychoanalysis, the primary characteristic of the oral phase of psychosexual development, i.e., the libido is concentrated mainly in the oral zone.

primal (pri′mal)
1. First or primary. 2. SYN: primordial (2) .

primaquine phosphate (pri′ma-kwin)
An antimalarial agent especially effective against Plasmodium vivax, terminating relapsing vivax malaria; usually administered with chloroquine. p. sensitivity a sensitivity to p. observed in individuals with glucose-6-phosphate dehydrogenase deficiency.

primary (pri′mar-e)
1. The first or foremost, as a disease or symptoms to which others may be secondary or occur as complications. 2. Relating to the first stage of growth or development. See primordial. [L. primarius, fr. primus, first]

primary reninism (ren′in-izm)
Overproduction of renin by juxtaglomerular cells in the absence of a stimulus (such as decreased renal perfusion); leads to hyperaldosteronism, hypertension, hypokalemia, and edema.

primase (pri′maz)
A polymerase that acts on a template DNA strand to produce RNA, resulting in the formation of an RNA primer needed in RNA replication. SYN: dnaG. [primer + -ase]

primate (pri′mat)
An individual of the order Primates. [L. primus, first]

Primates (pri-ma′tez)
The highest order of mammals, including humans, monkeys, and lemurs. [L. primus, first]

primer (pri′mer)
1. A molecule (which may be a small polymer) that initiates the synthesis of a larger structure. SYN: starter. 2. A pheromone that causes a long-term physiologic change.

primerite (pri′me-rit)
SYN: protomerite. [L. primus, first, + G. meros, part]

primidone (pri′mi-don)
An anticonvulsant drug used in the management of generalized tonic clonic and complex partial epilepsy.

primigravida (pri-mi-grav′i-da)
See gravida. [L. fr. primus, first, + gravida, a pregnant woman] elderly p. dated term referring to a woman older than 35 years who is pregnant for the first time.

primipara (pri-mip′a-ra)
See para. [L. fr. primus, first, + pario, to bring forth]

primiparity (pri-mi-par′i-te)
Condition of being a primipara.

primiparous (pri-mip′a-rus)
Denoting a primipara.

primite (pri′mit)
The anterior member of a pair of gregarine gamonts in syzygy.

primitive (prim′i-tiv)
SYN: primordial (2) . [L. primitivus, fr. primus, first]

primordia (pri-mor′de-a)
Plural of primordium.

primordial (pri-mor′de-al)
1. Relating to a primordium. 2. Relating to a structure in its first or earliest stage of development. SYN: primal (2) , primitive.

primordium (pri-mor′-de-um)
An aggregation of cells in the embryo indicating the first trace of an organ or structure. SYN: anlage (1) . [L. origin, fr. primus, first, + ordior, to begin] genital p. ovoid clump of cells seen in the rhabditiform larvae of Strongyloides stercoralis and hookworm that becomes the reproductive system.

primosome (pri-mo-som)
A complex of proteins that bind with primase at specific sequences of DNA that serve as the sites for the formation of RNA primers; a part of the replisome. [primer + -some]

primula (prim′u-la)
The rhizome and roots of a number of species of P. (family Primulaceae), primrose or cowslip; has been used as expectorant, diuretic, and anthelmintic. In some sensitive persons contact with the plant causes a rash. [Mediev. L. primrose, fem. of L. primulus, first]

primulin (pri′mu-lin) [C.I. 49000]
An acid yellow thiazole dye used as a fluorescent vital stain.

primus (pri′mus)
First; denoting the first of a series of similar structures. [L.]

princeps, pl .principes (prin′seps, -si-pez)
Principal; in anatomy, term used to distinguish the largest and most important of several arteries. [L. chief, fr. primus, first, + capio, to take, choose] p. cervicis SYN: descending branch of occipital artery. p. pollicis SYN: p. pollicis artery.

L.R., French physician, *1884. See P. tubercle.

principle (prin′si-pl)
1. A general or fundamental doctrine or tenet. SEE ALSO: law, rule, theorem. 2. The essential ingredient in a substance, especially one that gives it its distinctive quality or effect. [L. principium, a beginning, fr. princeps, chief] active p. a constituent of a drug, usually an alkaloid or glycoside, upon the presence of which the characteristic therapeutic action of the substance largely depends. antianemic p. the material in liver (and certain other tissues) that stimulates hemopoiesis in pernicious anemia; for practical purposes, the antianemic effect of extracts from such tissues is approximately equivalent to the content of vitamin B12. Bernoulli p. SYN: Bernoulli law. bitter principles a class of plant substances with a bitter taste that produce a reflexive increase in saliva secretion as well as secretion of digestive juices. closure p. in psychology, the p. that when one views fragmentary stimuli forming a nearly complete figure ( e.g., an incomplete rectangle) one tends to ignore the missing parts and perceive the figure as whole. See gestalt. consistency p. in psychology, the desire of the human being to be consistent, especially in attitudes and beliefs; theories of attitude formation and change based on the consistency p. include balance theory, which suggests that one seeks to avoid incongruity in one's various attitudes. SEE ALSO: cognitive dissonance theory. Fick p. SYN: Fick method. follicle-stimulating p. SYN: follitropin. founder p. the conditional probabilities of the frequencies of a set of genes at any future date depend on the initial composition of the founders of the population and have in general no tendency to revert to the composition of the population from which the founders were themselves derived. hematinic p. the p. previously thought to be produced by the action of Castle intrinsic factor upon an extrinsic factor in food, now recognized as vitamin B12. Huygens p. used in ultrasound technology; the p. that any wave phenomenon can be analyzed as the sum of many simple sources properly chosen with regard to phase and amplitude. p. of inertia SYN: repetition-compulsion p.. Le Chatelier p. SYN: Le Chatelier law. luteinizing p. SYN: lutropin. mass action p. the fundamental p. in epidemic theory: the incidence of an infectious disease is determined by the product of the current prevalence and the number of susceptibles in the population. SEE ALSO: serial interval, infection transmission parameter. melanophore-expanding p. SYN: melanotropin. Mitrofanoff p. use of a catheterizable channel (appendix, bowel, ureter) to drain the bladder as an alternative to the urethra. SEE ALSO: appendicovesicostomy. nirvana p. in psychoanalysis, the p. that expresses the tendency to attain a conflict-free state of freedom from pain or worry. organic p. SYN: proximate p.. pain-pleasure p. a psychoanalytic concept that, in human psychic functioning, the person tends to seek pleasure and avoid pain; a term borrowed by experimental psychology to denote the same tendency of an animal in a learning situation. SYN: pleasure p.. Pauli exclusion p. the theory limiting the number of electrons in the orbit or shell of an atom; that it is not possible for any two electrons to have all four quantum numbers identical. pleasure p. SYN: pain-pleasure p.. proximate p. in chemistry, an organic compound that may exist already formed as a part of some other more complex substance ( e.g., various sugars, starches, and albumins). SYN: organic p.. reality p. the concept that the pleasure p. in personality development is modified by the demands of external reality; the p. or force that compels the growing child to adapt to the demands of external reality. repetition-compulsion p. in psychoanalysis, the impulse to redramatize or reenact earlier emotional experiences or situations. SYN: p. of inertia. ultimate p. one of the chemical elements.

John J., English dermatologist, 1855–1922. See P. disease, Bourneville-P. disease.

Myron, U.S. cardiologist, 1908–1994. See P. angina.

prion (pri′on)
Small, infectious proteinaceous particle, of nonnucleic acid composition because of its resistance to nucleases; the causative agent, either on a sporadic, genetic, or infectious basis, of six neurodegenerative diseases in animals, and four in humans; the latter include the spongiform encephalopathies of kuru, Creutzfeldt-Jakob disease, Gerstmann-Straussler-Scheinker syndrome and fatal familial insomnia. The gene encoding for the PrP is found on chromosome 20. SYN: p. protein. [proteinaceous infectious particle] Stanley B. Prusiner received the Nobel Prize in Physiology or Medicine in 1997 for his discovery of prions. Prusiner began his research in 1972 to identify the infectious agent of Creutzfeldt-Jakob disease. In 1982 he and his colleagues isolated a protein that was capable of transmitting infection but, unlike all other known pathogens, contained neither DNA nor RNA. Prusiner's term for this protein, p., was derived from the phrase proteinaceous infectious particle. A gene encoding this protein has been found in all animals tested, including humans. The p. protein can occur in either of 2 structural conformations, one that is normal (but of unknown function), designated PrPc, and one that results in disease, called PrPSc. The normal p. protein is a component of lymphocytes and other cells and is particularly abundant on the cell membranes of CNS neurons. The PrPSc p. protein is extremely stable and is resistant to proteolysis, organic solvents, and high temperatures. Once produced or acquired by a suitable host, it can initiate a chain reaction whereby normal PrPc protein is converted into the more stable PrPSc form. After a long, asymptomatic incubation period, the disease-causing PrPSc accumulates to reach neurotoxic levels. Symptoms of p. diseases vary with the parts of the brain affected. All known p. diseases lead to the death of those affected. P. diseases are called spongiform encephalopathies because of the histologic appearance of affected cerebral cortex and cerebellum, which display large vacuoles. Probably most mammalian species develop these diseases. Prions are not living, are smaller than viruses, and do not elicit an immune response in either their normal or disease-causing form. P. diseases besides Creutzfeldt-Jakob disease include kuru (once prevalent among the Fore people of New Guinea, who practiced ritual cannibalism), bovine spongiform encephalopathy (BSE, mad cow disease), and scrapie, a disease of sheep. A new variant of CJD may have arisen through transmission of prions to human beings from cattle infected with BSE. P. diseases are unique in being both infectious and hereditary. Hereditary forms are due to transmitted mutations in the p. gene, located on chromosome 20 in human beings. Gertsmann-Sträussler-Scheinker (GSS) disease is a hereditary dementia resulting from a mutation in this gene. Approximately 50 families with GSS mutations have been identified. About 10–15% of cases of CJD are caused by inherited mutations in the p. protein gene. Strains of mice from which this gene has been abolished are immune to p.-caused disease. See Creutzfeldt-Jakob disease, bovine spongiform encephalopathy.

prism (prizm)
A transparent solid, with sides that converge at an angle, that deflects a ray of light toward the thickest portion (the base) and splits white light into its component colors; in spectacles, a p. corrects ocular muscle imbalance. [G. prisma] enamel prisms SYN: prismata adamantina, under prisma. Fresnel p. a p. composed of concentric annular rings. Nicol p. a p. that transmits only polarized light. Risley rotary p. a p. with a circular base that is rotated in a metal frame marked with a scale; used in examination of ocular muscle imbalance.


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