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..

* * *

pres·sure 'presh-ər n
1) the burden of mental or physical distress esp. from grief, illness, or adversity
2) the application of force to something by something else in direct contact with it: COMPRESSION
3 a) the action of a force against some opposing force: a force in the nature of a thrust distributed over a surface
b) the force or thrust exerted over a surface divided by the area of the surface
6) a touch sensation aroused by moderate compression of the skin

* * *

pres·sure (P) (preshґər) [L. pressura] force per unit area; the SI unit is the pascal.

Medical dictionary. 2011.

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