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presenting part

Before delivery, the fetal anatomical structure nearest the internal cervical os, identified by sonogram or palpation during vaginal examination. SEE: presentation for illus.


(prē-zĕr′văt-ĭv) [L. praeservare, to watch beforehand] A substance added to medicines or foods to prevent them from spoiling. It may act by interfering with certain chemical reactions or with the growth of molds, fungi, bacteria, or parasites. Some common preservatives are sugar, salt, vinegar, ethyl alcohol, sulfur dioxide, and benzoic acid.

image In some cases preservatives may have undesirable effects, e.g., they may reduce the nutritional content of foods, or they may be carcinogenic or teratogenic.


(prĕs′or) [Fr. presser, to press] 1. Stimulating, increasing the activity of a function, esp. of vasomotor activity, as a nerve. 2. Inducing an elevation in blood pressure. 3. One of several drugs, such as dopamine, epinephrine, and norepinephrine, that are used to increase the blood pressure of patients in shock.


(prĕs″ō-rē-sĕp′tor) Baroreceptor.


(presh′ŭr) [L. pressura, crowding, throng, press,] 1. A compression. 2. Stress or force exerted on a body, as by tension, weight, or pulling. 3. In psychology, the quality of sensation aroused by moderate compression of the skin. 4. In physics, the quotient obtained by dividing a force by the area of the surface on which it acts.

airway opening p. The pressure at the access point to a patient's airway, (nose, mouth, or, for a mechanically ventilated patient, the trachea). When this pressure exceeds the alveolar pressure, gases tend to move into the lower airways and open the alveoli. When this pressure is less than the alveolar pressure, gases tend to move out of the lungs and into the atmosphere. SEE: alveolar p.

alveolar p. Air pressure in the alveoli and bronchial tree. When this pressure is positive, it is higher than atmospheric pressure; when negative, less. Gases flow from higher to lower pressures: when alveolar pressures are higher than atmospheric pressure, respiratory gases tend to be exhaled. When alveolar pressures are less than atmospheric pressures, gas flows into the lungs. SYN: intrapulmonic p.

arterial p. The pressure of the blood in the arteries. For a normal young person at physical and mental rest and in sitting position, systolic blood pressure averages about 120 mm Hg; diastolic pressure about 80 mm Hg. A wide range of normal variation is due to constitutional, physical, and psychic factors. For women, the figures are slightly lower. For older people, they are higher. Normally there is little difference in the blood pressure recorded in the two arms. SEE: blood pressure.

atmospheric p. The pressure of the weight of the atmosphere. At sea level it ...

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