Dictionary


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A B C D E F G H I L M O P R S T V

Air Flow
Air will flow from an area of higher pressure to one of lower pressure; during inspiration, the pressure in the alveoli must be less than the pressure at the mouth for air to flow in, and during expiration, the reverse is true. Air flow may be laminar, turbulent or transitional, depending on the velocity of flow and on the diameter and configuration of the tube.

Airway
The anatomical structures through which air passes on its way to or from the alveoli; the nasopharynx and oropharynx, the larynx, the trachea, bronchi, and bronchioles.

Airway Resistance
Driving pressure divided by flow (P/V); or, the opposition to motion caused by the forces of friction, which is a function of flow rate, airway caliber, nature of gas breathed, and type of flow (laminar vs. turbulent).

Alveolar Pressure
The pressure within the alveoli, conventionally given in cm H20, with reference to an atmospheric pressure of zero. Thus, a negative alveolar pressure indicates that the pressure is lower than atmospheric; a positive alveolar pressure indicates that the pressure is above atmospheric.

Alveoli
The air sacs that act as the primary gas exchange units of the lung.

Asthma

A condition characterized by increased tone of the smooth muscle surrounding the bronchi and by bronchial inflammation and excess mucous secretion. An individual with acute asthma will present with an obstructive profile on respiratory function tests.

Atmospheric Pressure
Ambient air pressure, averages 760 mm Hg at sea level. In pulmonary calculations, atmospheric pressure is taken as the reference value, 0 cm H20. Pressures higher than atmospheric pressure then are positive; those lower than atmospheric pressure are negative.

Body Plethysmograph
A large airtight box used for measuring lung volumes; the subject sits in the box where pressure and volume changes are measured by Boyle’s Law during respiratory efforts.

Boyle's Law
P1V1=P2V2; the principle that at constant temperature the volume of a gas varies inversely as the absolute pressure applied to the gas.

Bronchitis
A clinical condition marked by airway inflammation and excess mucus secretion, manifested by couth and sputum production. It may cause narrowing of the airways and increase their resistance; this results in an obstructive ventilatory defect.

Chest Wall
The anatomical structures that border the parietal pleura, including the ribs with intercostal muscles, and diaphragm; when the muscles of the chest wall are relaxed, the chest wall acts in an elastic fashion comparable to the lung, responding passively to the pressure differences around it.

Compliance
Volume change per unit of pressure change across an elastic structure.

Compliance Curve
The pressure-volume curve for the lung or relaxed chest wall; plotting volume as a function of pressure inside minus pressure outside. The slope of this curve is the compliance.

Dead Space
The portion of each breath that does not participate in gas exchange. Anatomic dead space is the volume of the conducting airways; physiologic dead space also includesthe contribution of alveoli that are well-ventilated but poorly perfused.

Diaphragm
A thin, dome-shaped sheet of muscle that inserts into the lower ribs; it is the most important muscle of inspiration—when it contracts, it lowers pleural pressure.

Distending Pressure
The inside pressure minus the outside pressure of an elastic structure; for the lung, this is also referred to as the transpulmonary pressure or the recoil pressure of the lung.

Elastance
The reciprocal of compliance; a measure of the change in pressure achieved per unit change in volume, or stiffness.

Elastic Recoil of the Chest Wall
Pleural pressure minus pressure at the body surface (Ppl-Pbs).

Elastic Recoil of the Lung
Alveolar pressure minus pleural pressure (Palv-Ppl).

Emphysema
A condition characterized by dilation and destruction of alveolar walls; it produces airflow obstruction as determined by pulmonary function testing.

ERV
Expiratory reserve volume, the difference between FRC and RV. This is the maximal amount of air that can be expired starting at FRC.

Esophageal Balloon
A thin walled balloon positioned in the lower esophagus and attached to a strain gauge for estimating pleural pressure.

FEV1
Forced expiratory volume in one second; the volume that a subject can exhale in the first second during a forced expiration test.

FEV1/FVC
The volume that a subject can forcibly expel in one second (FEV1) divided by the total volume that can be expelled (FVC); this result can be especially useful in diagnosing obstructive and restrictive disorders.

Forced Expiration
The recording of a maximal expiration from Total Lung Capacity (TLC). This permits the of forced vital capacity (FVC) and various of air flow.

FRC
Functional Residual Capacity Also known as FRC, this is the lung volume at the end of a normal expiration, when the muscles of respiration are completely relaxed; at FRC and at FRC only, the tendency of the lungs to collapse is exactly balanced by the tendency of the chest wall to expand.

Functional Residual Capacity
Also known as FRC, this is the lung volume at the end of a normal expiration, when the muscles of respiration are completely relaxed; at FRC and at FRC only, the tendency of the lungs to collapse is exactly balanced by the tendency of the chest wall to expand.

FVC
Forced vital capacity; the total volume of air that can be exhaled from the lungs during a forced expiration following a maximal inspiration.

Gas Dilution
A method of ascertaining functional residual capacity (FRC) and residual volume (RV) by mixing the unknown volume of gas in the lungs with a known volume of gas containing a known concentration of a poorly soluble gas like helium.

Glottis
The true vocal cords; when one closes the glottis, no air can escape from the lungs.

Histeresis
The difference in the pressure-volume curves of the lung during inflation and deflation (the lung volume at any given pressure during deflation is larger than during inflation).

IRV
Inspiratory reserve volume; the difference between VC and FRC. This is the maximal amount of air that can be inspired starting at FRC.

Laminar Flow
Air flow in the lungs which is streamlined, low velocity, and obeys Poiseuille's Law; generally it is confined to the small peripheral airways.

Laplace's Law
Equation expressing the relationship between the surface tension of a sphere and the resultant pressure: P=2T/r, where P=pressure, T=surface tension, and r=radius (for a soap bubble or sphere with two surfaces, P=4T/r)

Muscles of Respiration
During quiet inspiration: diaphragm and external intercostals During active inspiration: the muscles of quiet inspiration plus the scalenes and sternomastoids During quiet expiration: passive active expiration: abdominal muscles, internal intercostals

Obstructive Disease
A respiratory abnormality characterized by delay in forced expiration of air from the lungs.

Palv
Alveolar pressure The pressure within the alveoli, conventionally given in cm H20, with reference to an atmospheric pressure of zero. Thus, a negative alveolar pressure indicates that the pressure is lower than atmospheric; a positive alveolar pressure indicates that the pressure is above atmospheric. Asthma, bronchitis, and emphysema are all considered obstructive conditions.

Parietal Pleura
The portion of the pleural membrane that lines the thoracic cavity.

Pbs
The pressure at the body surface, usually atmospheric pressure.

Pes
Esophageal pressure.

PL
Transpulmonary pressure The pressure difference across the lung. Alveolar pressure minus pleural pressure (Palv-Ppl), which is also known as the elastic recoil pressure of the lung.

Pleural Pressure
The pressure within the pleural space. Its value is generally given with reference to an atmospheric pressure of zero, and it is measured in cm H2O. Its symbol is Ppl.

Pleural Space
The tiny fluid-filled “space” between the visceral and parietal pleura; if air should enter this normally non-communicating space, a pneumothorax will result

Pneumothorax
The presence of air in the pleural cavity, caused either by a rupture of the visceral pleural membrane or by a communication through the chest wall (i.e., a gunshot wound).

Poiseuille's Law
An equation which describes laminar flow in a straight tube. V=PĻr4/8nl, where V= flow P= driving pressure r= radius of tube n= fluid viscosity l= length of tube

Ppl
Pleural pressure.

Pulmonary Fibrosis
A condition characterized by deposition of fibrous tissue in the lung. It decreases lung compliance and results in a restrictive ventilatory defect as seen on pulmonary function testing.

Pw
Pleural pressure minus pressure at the body surface (Ppl-Pbs), also known as the elastic recoil pressure of the chest wall.

Recoil Pressure
For the lung, the difference between alveolar pressure and pleural pressure (Palv-Ppl)-- same as PL and transpulmonary pressure. For the chest wall, the difference between the pleural pressure and the body surface pressure (Ppl-Pbs)-- same as PW.

Restrictive Ventilatory Defect
A condition characterized by a reduction in total lung capacity and vital capacity. Restrictive disorders may be caused by stiffening of the chest wall, stiffening of the lung itself, or by muscle weakness.

Residual Volume
Also known as RV, this is the volume of the lungs after a maximal expiration.

RV Residual Volume. Also known as RV, this is the volume of the lungs after a maximal expiration-- the lowest voluntary volume attainable.

Spirometry
A simple lung function test that measures lung volume as a function of time; it can be used to ascertain lung volumes or to gain information about maximal expiratory flow rates.

Statics
That part of pulmonary mechanics dealing with pressure, volume and flow relationships during apnea or quiet breathing.

Surface Tension
The force of attraction between adjacent molecules of a liquid.

Surfactant
A phospholipid, predominantly dipalimtoyl lecithin, secreted by the Type II alveolar cells. It not only decreases surface tension, but decreases it most at low volumes and least at high volumes, contributing to the overall stability of the alveolar units.

Tidal Volume
Also known as VT, this is the volume of an individual breath during quiet breathing. It averages about 500 ml.

TLC
Total Lung Capacity. Also known as TLC, this is the volume of the lungs after a maximal voluntary inspiration.

Total Lung Capacity Also known as TLC, this is the volume of the lungs after a maximal voluntary inspiration.

Transitional Flow
An intermediate type of airflow which has characteristics of both laminar flow and of turbulent flow. See Turbulent Flow.

Transpulmonary Pressure
The pressure difference across the lung. Alveolar pressure minus pleural pressure (Palv-Ppl), which is also known as the elastic recoil pressure of the lung.

Turbulent Flow
Air flow characterized by disorganized movement of gas molecules and/or eddy formation; it occurs when velocity of flow exceeds a limiting value or when irregularities in the configuration of the tube preclude laminar flow.

TV
Tidal Volume. Also known as TV, this is the volume of a normal inspiration (or expiration) during quiet breathing, and averages about 500 ml.

VC
Vital Capacity. Also known as VC, this is the difference between Total Lung Capacity (TLC) and Residual Volume (RV); i.e, it is the maximum volume of air which can be exhaled starting at full lung inhalation.

Visceral Pleura
The portion of the pleural membrane that covers the lung.

Vital Capacity
Also known as VC, this is the difference between Total Lung Capacity (TLC) and Residual Volume (RV); i.e, it is the maximum volume of air that can be exhaled starting at full lung inflation.

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Last modified, November 8, 1995
© Johns Hopkins School of Medicine, 1994
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