The rate at which an aerosol particle settles in still air as a result of gravity depends upon the particle's shape, its size, and its density, as well as viscous resistance arising from moving through the air. The aerodynamic equivalent diameter (AED) of a particle is a concept based on the assumption that such a particle velocity can be measured. The aerodynamic equivalent diameter is the diameter of a sphere of unit density (1 g per cm³) which has the same settling velocity in the same gas. Particles with the same AED behave dynamically identically.
An agonist is a substance that can set a process into motion. Histamine and methacholine, for example, can stimulate glands, smooth muscles and nerves. Antagonists can attenuate the effects by competing with the agonist for an available receptor site.
"Normal" transmission in neurons is from the excitatory organ to the end organ. Thus if lung stretch receptors are stimulated, the sensory signal travels up the vagus nerve. In the case of a motor neuron the information travels down from the excitatory organ to end organ; e.g. respiratory motor impulses generated in the medulla oblongata, travel down and are transmitted to the diaphragm via the phrenic nerve.
In the case of antidromic transmission
excitatory signals travel in the opposite direction.
A clinical syndrome characterised by increased responsiveness of the tracheobronchial tree to a variety of stimuli. The primary physiological manifestation of this bronchial hyperresponsiveness is variable airways obstruction.
Chronic obstructive pulmonary disease, a set of possibly related syndromes comprising chronic (obstructive) bronchitis, peripheral airways disease and pulmonary emphysema. Each of these syndromes is related to tobacco smoking.
Clinically a condition in which the right ventricle of the heart is overloaded secondary to pulmonary causes, i.e. not arising from a primary cardiac defect. In the compensated condition the right ventricle adequately maintains the circulation in spite of the increased load. If the right ventricle can no longer meet the circulatory demands due to the increased afterload, there is right ventricular failure. Cor pulmonale is also defined in pathological terms, based on the thickness of the right ventricular wall, and the ratio between the right and ventricular masses. This information is not usually obtained in vivo and hence not a clinically useful criterion. In addition, right ventricular failure may develop acutely in the case of a massive pulmonary embolus, and not be associated with any ventricular hypertrophy.
Particles have kinetic energy which causes them to move; the movement due to this process is called diffusion or Brownian motion. The motion leads to collision with neighbouring particles, so that the direction of movement is erratic. If overall mass flow is almost zero then the erratic movement of particles due to diffusion may lead to the particles settling on the mucosal surface of the airway walls.
A condition characterized by an abnormally high partial pressure of carbon dioxide in arterial blood (Pa,CO2). The upper limit of a normal Pa,CO2 is 6 kPa (45 mmHg).
A condition characterised by an abnormally low partial pressure of oxygen in arterial blood (Pa,O2). What constitutes a low oxygen tension in healthy subjects depends on age and on environmental conditions. In a newborn a Pa,O2 of 9 kPa (68 mmHg) is quite satisfactory; it is about 12-13 kPa (90-97 mmHg) in a young adult, and declines somewhat with ageing so that it is on average about 10 kPa (75 mmHg) in a 70 year old healthy subject. This applies at sea level in the awake and upright subject. At altitude, where the environmental oxygen pressure is less, and in the supine position during wakefulness and even more so during sleep, the Pa,O2 is lower.
The term impaction implies that particles, due to their speed and mass inertia, collide with the wall of the airways. The larger the particle's mass, the larger its mass inertia. Therefore only the smallest particles with the smallest mass will be capable of following the main stream of airflow, while the larger particles impact into the airway walls as the flow is deflected in branching airways.
Irritant receptors lie between airway epithelial cells. They are stimulated by mechanical stimuli, inhaled dusts, cold air, noxious gases and cigarette smoke. The receptors show rapid adaptation when they are continuously stimulated. Impulses travel via myelinated fibers in the vagus nerve.
Irritant receptors in more peripheral bronchi are insensitive to mechanical irritants, but they are sensitive to chemical stimuli; the pattern is reversed in the central airways. Reflex effects include bronchoconstriction and hyperpnoea.
Irritant receptors are also sensitive to histamine and can thus contribute to the bronchoconstrictor response in asthmatic subjects in whom histamine is released from mast cells or other cells.
Juxta-capillary or J-receptors are believed to be in the alveolar walls and in close contact with the capillaries. The receptors respond to the accumulation of interstitial fluid in the lung parenchyma (lung edema) and to pulmonary capillary engorgement. Impulses travel up the vagus nerve via slowly conducting nonmyelinated fibers and may induce rapid shallow breathing.
J-receptors are believed to play a role
in the sensation of dyspnoea and in rapid shallow breathing in patients
with interstitial lung disease and with left ventricular heart failure,
as well as in exercise dyspnoea. The dyspnoea is associated with
a sensation of choking or pressure in the neck and upper thorax.
Stronger stimulation of J-receptors also leads to a dry cough.
Paintal, AS - Sensations from J-receptors. NIPS 1995; 10: 238-243
By convention upper airways are extrathoracic airways (nasopharynx, mouth, larynx and extrathoracic trachea). Lower airways are intrathoracic. The intrathoracic airways are commonly divided into large and small airways. The small airways have a diameter of <2 mm.
These receptors are believed to lie within airway smooth muscle. They discharge in response to lung inflation. As their activity is more or less maintained when the lung is distended, they are called "slowly adapting" stretch receptors. The impulses travel via myelinated fibers in the vagus nerve and project onto the ventrolateral nucleus of the solitary tract in the medulla oblongata. Stimulating these receptors leads to slowing of respiratory frequency due to an increase in expiratory time. This reflex effect is on record as the Hering-Breuer inflation reflex. This reflex is important in neonates and infants, but is inactive in adults at normal tidal volumes.
Mass flow or convection is the process whereby
a fluid (bear in mind that physically gas is also a fluid) moves
from one site to another due to a pressure gradient. All molecules
or particles move in the same direction, unlike in diffusive processes
(Brownian motion), where movement is erratic.
The maximal response is the largest decrease in airway calibre obtained during challenge with an agonist. In practice the maximal response is often limited by symptoms related to severe airways obstruction in asthmatic subjects.
The PC20 is the concentration of a substance administered by aerosol which causes the FEV1 to fall by 20%.
The PC200 is the concentration of a substance administered by aerosol which causes airways resistance to double.
The PD20 is the dose of a substance administered by aerosol which causes the FEV1 to fall by 20%.
By weighing the nebulizer prior to and after nebulization one can establish how many mL have been adminstered; taking into account the concentration, one can now compute the dose of agonist administered.
Reactivity is the slope, or the rate of change, of the dose-response curve. Hyperreactivity, therefore, means that when airways are challenged with an agonist the resulting slope of the dose-response curve is abnormally steep.
Sedimentation of particles occurs because they are subject to gravitational forces. Thus particles tend to move in a downward direction. This gravitational pull is not significant at high velocities, but is very important at very low velocities. The smallest airways have a large total cross-sectional area; here sedimentation contributes to particles settling on the mucosal surface.
Sensitivity denotes the dose of agonist required to obtain a specific
response in a dose-response curve. Hypersensitivity implies that
a specific response occurs at a lower concentration of the agonist
compared to normal sensitivity, and therefore specifically refers
to a leftward shift of the dose-response curve.
The Starling-Landis mechanism sums up the factors that govern the transport of water across the capillary membrane (i.e. between plasma and interstitial spaces). It is depicted for the trajectory between arteriole and venule. The following factors play a role:
- the capillary and interstitial hydrostatic pressure,
- the capillary and interstitial colloidosmotic pressure,
- the hydraulic conductance (the reciprocal of the resistance of the capillary to the flow of water), and
- Staverman's reflection coefficient, an index to which extent the capillary membrane is an ideal semipermable membrane with respect to plasma proteins. The reflection coefficient is not incorporated in the equation shown.