Glossary

acid: a substance that is capable of donating hydrogen ions (protons or H+)

acidemia: a condition in which the pH of the blood is below normal

acidosis: a process that leads to the excess production of acid. The ultimate pH of the blood is determined by whether there one or multiple processes are present.

acinus (pulmonary lobule): an anatomical unit of lung distal to a terminal bronchiole; consists of respiratory bronchioles, alveolar ducts, and alveoli. The acinus is the site of gas exchange.

afferent information: neurological sensory messages that arise in the peripheral nervous system and are transmitted to the central nervous system, including information from flow, pressure, stretch, and irritant receptors of the lungs

afferent neural impulse: neural signal generated by the periphery (e.g., receptors in the lungs and chest wall) and sent to the central nervous system (see afferent information)

alkalemia: a condition in which the pH of the blood is above normal

alkalosis: a process that leads to excess production of base. The ultimate pH of the blood is determined by whether there one or multiple processes are present.

alveolar gas equation: an equation that describes the relationship between the concentration of inspired oxygen, the PACO2, and the PAO2; used to calculate the partial pressure of oxygen within the ideal alveolus

alveolar-to-arterial oxygen difference (A-aDO2): the difference between PAO2 and PaO2; determines whether there is a problem with the gas exchanger. The A-aDO2 increases in normal people as they age because of changes in ventilation that relate to loss of elastic recoil in the lungs.

alveoli (singular, alveolus): the thin-walled distal air sacs in the lungs across which gas exchange occurs. Oxygen from the atmosphere is picked up by hemoglobin in the red blood cells traversing alveolar capillaries, and carbon dioxide passes from the blood into the alveoli.

anaerobic threshold: the level of exercise at which the body begins to rely on anaerobic metabolism to meet energy needs such that lactic acid accumulates in the blood; expressed in terms of the level of oxygen consumption at which lactic acid production can be detected

anemia: a condition in which the amount of hemoglobin in the blood is below normal. Anemia reduces the oxygen content of the blood.

anion gap: the difference in concentrations between the commonly measured anions and cations in the blood. The anion gap can be altered by certain acids; thus, calculation of the anion gap serves as an aid to recognition and diagnosis of metabolic acidosis.

arterial blood gas: a test in which a sample of arterial blood is analyzed to determine the PaO2, PaCO2, and the pH of the blood

atelectasis: phenomenon that involves the collapse of lung units

A-VO2 difference: the difference between the oxygen content in the arterial blood and in the venous blood

base: a substance that is capable of accepting hydrogen ions (protons or H+)

body plethysmography: a method of measuring functional residual capacity. In this test, a person is seated within an airtight box. The body plethysmograph makes use of Boyle’s law to calculate the functional residual capacity based on changes in volume and pressure as the person attempts to inhale and exhale against a closed valve. This method, unlike helium diffusion, is not affected by poorly ventilated regions of the lungs.

bradypnea: abnormally slow breathing; a decrease in respiratory rate below the normal range (usually reserved for rates < 10 breaths/min)

bronchial circulation: delivery of oxygenated blood from the aorta to the lung tissue; serves as a major supply of oxygen and nutrients to the trachea, bronchi, and the visceral pleura (as well as the esophagus)

buffer: a substance that is able to accept or release hydrogen ions, resulting in minimal changes in the free hydrogen ion concentration and, hence, the pH. The primary intracellular buffers are proteins, phosphates, and hemoglobin in red blood cells, and the primary extracellular buffer is bicarbonate.

carbon dioxide content: the amount of carbon dioxide bound to hemoglobin plus the amount dissolved in the blood

carbon dioxide production (): the rate at which carbon dioxide is produced by the body. can be expressed as the difference between the amount of CO2 exhaled and the amount inhaled. Because there is virtually no CO2 in the gas humans inhale from the atmosphere, is equal to the amount of CO2 humans exhale each minute.

cardiac output (): the volume of blood pumped by the heart each minute

central pattern generator: the area of the brain, including the medulla, which is responsible for automatic control of respiration; it creates the inherent rhythm of breathing

C fibers: unmyelinated fibers in the lungs that carry information from a variety of receptors, including J receptors, and conduct sensory information that arises from inhalation of irritant substances. C fibers may also arise from receptors in the bronchi. The exact role of C fibers in the modulation of respiratory control is unknown.

chemoreceptors: specialized sensory bodies that contain nerve fibers that extend to the medulla in the region where inspiratory neurons are located; the chemoreceptors monitor oxygen and carbon dioxide levels as well as the pH of the blood. Two kinds of chemoreceptors are:

compliance: a measure of the stiffness of a closed container such as a balloon; the change in volume that occurs in the object divided by the change in pressure across the wall of the object. Compliance is mathematically expressed as .

conducting airways: respiratory passages through which air flows but no gas exchange takes place

corollary discharge: the simultaneous message that is sent to the sensory cortex as efferent (outgoing) neural messages are sent from the motor cortex to the ventilatory muscles. These messages from the motor cortex to the sensory cortex are believed to be responsible for the sense of effort, or perception of increased work of breathing, when ventilatory muscle activity is increased.

dead space: regions of the lungs (including the conducting airways) that receive air but do not participate in gas exchange (see physiological dead space)

dead space–to–tidal volume ratio (): a ratio used clinically to assess gas exchange. In an average person, a normal value is 175/500, or about one third. A ratio greater than or equal to 0.6 is indicative of respiratory failure.

driving pressure: the pressure change from one point to another along a tube through which a fluid flows. Driving pressure, or pressure decrease, is equal to flow multiplied by the resistance of the tube.

dynamic compression: the reversible narrowing of airways that results from changes in transmural pressure during exhalation

dynamics: the study of the respiratory system during conditions of flow of gas into and out of the lungs

dyspnea: breathing discomfort

efferent neural impulse: neural signal generated by the central nervous system (e.g., the respiratory controller) and sent to the periphery (e.g., receptors in the respiratory muscles)

efferent–reafferent dissociation: mismatch of outgoing (efferent) signals to the ventilatory pump and returning (reafferent) signals from receptors in the lungs and chest wall; increases the intensity of sensations such as “air hunger” and the effort or work of breathing

effort independence: condition that exists when increases in pleural pressure do not yield increased expiratory flow (see flow limitation)

elastic properties: the properties of a material or object that allow it to return to its original size and shape after being stretched or compressed. Both the lungs and the chest wall have elastic properties.

elastic recoil: in reference to the lungs, the force attributable to the elastic properties (e.g., attributable to stretching of elastin and collagen fibers) that act to return the lungs to their resting or unstressed position. The elastic recoil of the lungs provides a driving force for expiration.

equal pressure point (EPP): the point at which transmural pressure of the airway is 0, that is, the pressure inside and outside are the same. When the EPP is reached in peripheral, unsupported airways, further increases in pleural pressure do not result in increased flow (see flow limitation).

expiratory coving: the concave-up appearance of the expiratory flow–volume curve from the point of peak flow to that of residual volume; results from the rapid decrease of expiratory flow at lung volumes below total lung capacity attributable to narrowing of the airways, as in emphysema

expiratory reserve volume: the maximal volume of air that can be expelled from the lungs after a normal expiration; the difference in volume between the functional residual capacity and the residual volume

Fick equation: the relationship between cardiac output (Qt), oxygen consumption (), and arterial–venous oxygen content difference (A-VO2 difference). The oxygen consumption can be expressed as the difference between the oxygen delivered to the tissues and oxygen returned to the heart and is equal to the difference between the oxygen inhaled and the oxygen exhaled each minute. The Fick equation is used routinely in clinical practice for calculation of cardiac output. The equation for cardiac output is:

fixed acid: a nonvolatile acid, meaning, one that is unable to be excreted by the lungs. Fixed acids include the phosphates and sulfates produced by the metabolism of certain proteins. A functioning renal system is required for the elimination of fixed acids.

flow limitation: a condition characterized by an inability to increase flow at a given lung volume with greater expiratory efforts. When flow limitation is present, equal pressure point conditions exist, and the driving pressure that determines flow is the difference between alveolar pressure and pleural pressure; specifically, driving pressure at flow limitation is the elastic recoil pressure of the alveolus at that lung volume.

forced expiratory volume in 1 second (FEV1): the volume of gas that is exhaled in the first second of the forced vital capacity maneuver. People who have obstructive lung diseases such as emphysema and chronic bronchitis typically have an abnormally low FEV1.

forced vital capacity (FVC): the vital capacity measured with the subject exhaling as rapidly as possible

functional residual capacity (FRC): the volume of the lungs at the end of a normal, relaxed exhalation. At the FRC, the inward or deflating force of the lungs is balanced by the outward or expanding force of the chest wall.

gas exchanger: the alveoli and the pulmonary capillaries that are the site of exchange of oxygen and carbon dioxide. Oxygen from the atmosphere is picked up by hemoglobin in red blood cells traversing the alveolar capillaries, and carbon dioxide passes from the blood into the alveoli.

helium dilution: a technique commonly used to measure functional residual capacity (FRC). In this test, a person breathes into a tube connected to a container with a known volume and an initially known concentration of helium. By monitoring the concentration of helium as it distributes and comes to equilibrium across the combined volume of container plus lungs, the volume of the lungs (initially at FRC when connected to the container) can be calculated. This technique may lead to inaccurate measurements of lung volume when disease states characterized by regions of poorly ventilated lung are present.

hyperpnea: increased ventilation that meets the metabolic needs of the body, as reflected in the production of carbon dioxide. If ventilation is increasing in proportion to increases in carbon dioxide production, PaCO2 remains normal.

hyperventilation: increased ventilation greater than that required to meet metabolic needs, as reflected in the production of carbon dioxide, or increased alveolar ventilation relative to metabolic carbon dioxide production. If a person is hyperventilating, it means that his or her PaCO2 is less than normal, or less than about 38 mm Hg.

hypoventilation: decreased ventilation to a level that is less than that required to meet metabolic needs, as reflected in the production of carbon dioxide, or decreased alveolar ventilation relative to metabolic carbon dioxide production. If a person is hypoventilating, it means that his or her PaCO2 is greater than normal, or greater than about 42 mm Hg.

hypoxemia: a condition in which the PO2 of arterial blood is below normal

hypoxia: a condition in which the level of oxygen in inspired gases or tissues is below normal

hypoxic pulmonary vasoconstriction: narrowing of precapillary pulmonary vessels in response to hypoxia in corresponding poorly ventilated alveoli. This phenomenon aids in directing blood flow preferentially to well-ventilated regions of the lungs.

hysteresis: the separation of the inflation and deflation segments of the pressure-volume curves of the lungs. In other words, the pressure corresponding to a given volume is different for inflation versus deflation. This property is characteristic of air-filled lungs and can be eliminated in experiments in which the lungs are filled with saline (suggesting that hysteresis is caused by the surface forces associated with an air-liquid interface).

inspiratory capacity: the maximal volume of air that can be inspired after a normal expiration. The inspiratory capacity is the sum of the tidal volume and the inspiratory reserve volume.

intracavitary pressure: the pressure inside a closed container or object. In conjunction with the pressure outside the container, this pressure can be used to calculate the transmural pressure, which determines the volume of the object.

J receptors: a group of C fibers that arise deep within the lungs; these juxtacapillary receptors (hence their name) are located near the pulmonary capillaries. J receptors may be activated by increases in pulmonary capillary pressures or accumulation of interstitial fluid, as seen in individuals with congestive heart failure.

laminar flow: a condition in which the movement of fluid molecules in a tube is parallel to the direction of flow. Under conditions of laminar flow, changes in driving pressure are proportional to changes in the flow.

law of Laplace: the relationship between transmural pressure, wall tension, and radius of curvature of a concave surface. This law is mathematically expressed for a sphere as , where P is the pressure inside the sphere, T is the tension in the wall of the sphere, and r is the radius of the sphere.

load compensation: the process in which the controller, the centers in the brain that determine the rate and depth of breathing, makes an adjustment to the presence of increased airway resistance or a sudden change in the compliance of the respiratory system by instructing the muscles to contract more forcefully.

mechanoreceptors: specialized sensory bodies that are activated by mechanical distortion of the local environment.

medulla: the most caudal region of the brainstem, immediately continuous with the spinal cord. It contains neurons with both inspiratory and expiratory activity and is the primary source of automatic respiratory rhythm (also referred to as the medulla oblongata; see central pattern generator).

metaboreceptors: receptors that may play a role in stimulating ventilation during the metabolic phase of exercise. It is hypothesized that these receptors respond to local accumulation of metabolic byproducts in exercising skeletal muscles and send signals to the brain, leading to an increase in ventilation as well as the sensation of shortness of breath with which exercise is associated.

micelles: subsurface aggregates of surfactant molecules; formed when the alveolar radius diminishes and surfactant is extruded from the surface layer.

mixed expired gas: a combination of gases from perfused alveoli, from nonperfused alveoli (alveolar dead space), and from anatomic dead space; exhaled gas collected from a person over several minutes. Mixed expired gas is used to measure physiological dead space.

mixed venous blood: venous blood returning to the heart from all tissue beds. To accurately obtain a mixture of all blood returning to the heart, this blood must be drawn from the pulmonary artery.

muscle spindle: a component of skeletal muscle that responds to mechanical stimuli (contraction) and behaves like a slowly adapting receptor in the chest wall; the muscle spindle plays a role in monitoring the response of the ventilatory pump to neural output from the central controller.

nitrogen washout test: a method for measuring functional residual capacity (FRC). The subject breathes a gas containing no nitrogen (typically 100% oxygen), and the volume and decreasing concentration of nitrogen in the exhaled gas are measured. The volume of the lungs (initially at FRC when the test is started) can be calculated in a similar manner to the helium dilution test and has the same limitations as the helium dilution method for assessing functional reserve capacity.

oxygen consumption (): the rate at which oxygen is used by the body; a measure of metabolic activity and often given in mL O2/min. can be expressed as the difference between the amount of O2 inhaled and the amount of O2 exhaled.

oxygen content: the sum of the oxygen bound to hemoglobin and dissolved in the blood

oxygen saturation: the percentage of oxygen-binding sites on hemoglobin that are occupied by oxygen; the extent to which the entire oxygen-carrying capacity of hemoglobin is being used

parasympathetic nervous system: along with the sympathetic system, part of the autonomic nervous system; innervates the airways via cholinergic receptors. Stimulation of this system causes bronchoconstriction.

parietal pleura: the thin layer of membranous tissue lining the inside of the chest wall

perfusion: blood flow to an organ or tissue bed. In the lungs, the distribution of perfusion can be influenced by multiple factors, including localized hypoxia and gravity.

physiological dead space: all parts of the lungs (including the conducting airways) that receive air but do not participate in gas exchange; the total dead space. Physiological dead space is the sum of the anatomic dead space and the alveolar dead space, and it is about 175 mL in the average person.

pleural space: the potential space between the visceral pleura and the parietal pleura. This space normally only contains a few milliliters of fluid.

pons: a portion of the brainstem superior to the medulla containing neurons that play a role in respiration

pores of Kohn: microscopic passages that connect alveoli within a lobe of the lung, permitting transfer of gas between alveoli. These passages function to minimize the collapse of lung units if a more central airway is obstructed.

preload: the degree of stretch of the ventricular muscle before contraction of the heart

pulmonary circulation: the blood vessels that deliver deoxygenated blood from the right ventricle to the lungs and oxygenated blood from the alveoli to the left atrium

quasistatic: conditions that reflect changes in volume that occur so slowly as to be essentially static. Under these conditions, elastic recoil of the respiratory system is the primary force that must be overcome to achieve changes in volume (i.e., airway resistance is negligible).

rapidly adapting stretch receptors (RARs): myelinated stretch receptors in the lungs that change their firing rate quickly when they reach a new level of stretch; that is, they quickly resume their baseline rate of firing after the change in condition

reafferent: the afferent messages from receptors in the peripheral nervous system (e.g., in the lungs and chest wall) that are generated in response to efferent signals (e.g., from the motor cortex to the ventilatory muscles)

recruit: to make active, as when expiratory muscles are activated (recruited) or when blood vessels in the lung that are normally closed are opened up, making the alveoli they perfuse active participants in gas exchange

residual volume (RV): the volume remaining in the lungs at the end of a maximal expiration

respiratory bronchioles: the smallest bronchioles; these connect the terminal bronchioles to the alveolar ducts. The respiratory bronchioles have alveoli sporadically originating from their walls and are therefore the first site of gas exchange as air travels distally in the lungs.

respiratory controller: the elements of the central nervous system that tell humans how often and how deeply to breathe. These elements include a volitional component in the cerebral cortex and an automatic component in the brainstem. Both of these elements may be affected by a variety of stimuli from throughout the respiratory system.

respiratory failure: the inability of the respiratory system to sustain the metabolic needs of the body

respiratory quotient (RQ): ratio of carbon dioxide produced per unit of oxygen consumed by the body. The fuel used by the body determines this ratio: for carbohydrates, it is 1.0; for fats, it is 0.7; and for protein, it is 0.8. The RQ can be written as:

respiratory zone: the portion of the lungs containing alveoli

shunt: a condition in which deoxygenated blood in the venous system is directed to the arterial system without receiving oxygen from the lungs. A pulmonary shunt is an extreme form of ventilation/perfusion mismatch in which some of the alveoli of the lung completely collapse or fill with fluid. With a shunt, alveolar ventilation in the affected regions of the lung is 0. Conditions in which significant shunt is present are characterized by hypoxemia that does not respond to supplemental oxygen.

slowly adapting stretch receptors (SARs): myelinated stretch receptors in the lungs that alter their level of activity in response to a change in local conditions and maintain that new level of activity, or rate of firing, for an extended time after the new conditions have been established (i.e., they are slow to adapt to the new condition and reestablish their baseline firing rate)

spirometer: a device that measures the volume of air inspired or expired. One type, a water spirometer, is a device into which a subject breathes in a manner such that the expired gas is collected under a drum partially suspended under water. As the person inhales and exhales, the water spirometer transforms the motion of the drum into a graphical representation of the change in lung volume as a function of time.

Starling curve: the relationship between the stroke volume and the end-diastolic volume of the ventricle. In general, greater end-diastolic volume leads to increased force of contraction and therefore greater stroke volume and cardiac output.

statics: the study of the respiratory system when there is no air moving into or out of the lungs

stress adaptation: a phenomenon in which elastic recoil forces adjust over seconds after a change in volume of the lung; may play a small role in hysteresis of the lungs

stroke volume: the volume of blood ejected from a ventricle of the heart in a single beat; related to preload (filling or stretch of the ventricle before contraction) via the Starling curve, as well as other factors, including cardiac contractility (see Starling curve)

surface tension: the force with which the surface of a liquid contracts per unit length of surface; expressed in dynes/cm2. Surface tension occurs when a gas-liquid or an air-liquid interface is present and is caused by cohesive forces between molecules in the liquid surface layer.

surfactant: a substance that lowers surface tension within the lungs; produced by type II pneumocytes within the alveoli and becomes a component of the liquid layer lining the alveoli, thereby lowering surface tension and contributing to alveolar stability

sympathetic nervous system: along with the parasympathetic system, part of the autonomic nervous system. In the lungs, stimulation of sympathetic nervous system fibers, which terminate near the airways, causes bronchodilation.

systemic oxygen delivery: the total volume of oxygen transported to the tissues each minute; can be expressed as the product of the cardiac output and the arterial oxygen content

systemic vascular resistance: the resistance of the systemic (i.e., nonpulmonary) blood vessels; small muscular arteries (known as arterioles) represent the site of greatest resistance in the systemic circulation

tachypnea: rapid breathing; an increase in respiratory rate above the normal range (usually reserved for rates breaths/min)

terminal bronchioles: subdivisions of the bronchi that represent the last portions of the air conduction pathway before the site of gas exchange

tidal volume (): the volume of air inspired or expired in a normal breath. In the average person at rest, a normal tidal volume is approximately 450 to 500 mL.

time constant: a measure of how rapidly gas is moved into and out of the alveoli; provides information about the rate at which gas is replenished within an alveolus. The time constant may be expressed as the product of airway resistance and lung compliance; higher time constants are characteristic of poorly ventilated regions of the lungs.

total lung capacity (TLC): the volume of air in the lungs at the end of a maximal inspiration. The TLC is the sum of the residual volume and vital capacity.

transitional (marginally laminar) flow: generally laminar movement of fluid that involves generation of eddies at angles to the main direction of flow. Transitional flow is typically seen at branch points in tubes.

transitional zone: the zone marking the transition between the conducting airways and alveoli; consists of airways called the respiratory bronchioles

transmural pressure (PTM): the pressure across the wall of a structure; mathematically expressed as , where Pinside is the pressure on the inside of an enclosed structure (also known as intracavity pressure), and Poutside is the pressure on the outside the structure. Positive transmural pressures tend to expand or increase the volume of a structure, and negative transmural pressures tend to decrease the size or volume of a structure.

turbulent flow: a condition in which the movement of fluid molecules in a tube are in seemingly random directions relative to the main direction of flow. Under conditions of turbulent flow, changes in driving pressure are proportional to the square of the flow.

type II pneumocytes: alveolar cells interspersed among type I pneumocytes. Type II pneumocytes produce, absorb, and recycle surfactant.

vagus nerve: an important nerve that conveys information from sensory receptors in the lungs and other organs to the brain

ventilation (): movement of air into and out of the lungs. There is relatively more ventilation than perfusion at the lung apices. Total ventilation () is the sum of the alveolar ventilation and the dead space ventilation.

ventilation/perfusion () mismatch: an imbalance between alveolar ventilation and pulmonary capillary blood flow. Many disease states, including pneumonia, heart failure, asthma, and emphysema, can increase mismatch and lead to impaired gas exchange.

ventilatory pump: structures that perform the bellows function of the respiratory system; includes the muscles, bones, cartilage, and related soft tissue of the chest wall necessary to move the chest wall, the peripheral nerves that innervate the chest wall muscles, the conducting airways, and the pleura connecting the motion of the chest wall to the lungs

visceral pleura: the thin layer of membranous tissue surrounding the lungs

vital capacity (VC): the maximal amount of air that can be moved into or out of the lungs in a single breath. The VC is the difference between the total lung capacity and the residual volume.

volatile acid: acid that is able to be removed by elimination of CO2 via the respiratory system; generally refers to carbonic acid. Volatile acids are the products of the metabolism of carbohydrates and fats.