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Information about Alveolar Gas Equation

The alveolar pO2 is not routinely measured but is calculated from blood gas measurements by the Alveolar gas equation:



where:
  • R is the Respiratory quotient (normally about 0.8)
  • pAO2 is the Alveolar pO2
  • pIO2 is the Inspired pO2, equal to about 150 mm Hg (0.21 x 713 mmHg at sea level). The given pressure at sea level is due to atmospheric pressure (760 mmHg) minus the partial pressure of water vapor (47 mmHg), as alveolar gas is completely saturated with water. The mole fraction of oxygen is about 0.21 in dry atmospheric gas.
  • pACO2 is the Alveolar pCO2 (assumed to be equal to the measured arterial pCO2)
  • F is a correction factor (usually less than 2 mmHg)

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Respiratory system, physiology: respiratory physiology
Volumeslung volumes - vital capacity - functional residual capacity - respiratory minute volume - dead space - spirometry - body plethysmography - peak flow meter
Airwaysventilation (V) (positive pressure) - breath (inhalation, exhalation) -respiratory rate - respirometer - pulmonary surfactant - compliance - hysteresivity - airway resistance
Bloodpulmonary circulation - perfusion (Q) - hypoxic pulmonary vasoconstriction - pulmonary shunt
Interactionsventilation/perfusion ratio (V/Q) and scan - zones of the lung - gas exchange - pulmonary gas pressures - alveolar gas equation - hemoglobin - oxygen-haemoglobin dissociation curve (2,3-DPG, Bohr effect, Haldane effect) - carbonic anhydrase (chloride shift) - oxyhemoglobin - respiratory quotient - arterial blood gas - diffusion capacity - Dlco
Control of respirationpons (pneumotaxic center, apneustic center) - medulla (dorsal respiratory group, ventral respiratory group) - chemoreceptors (central, peripheral) - pulmonary stretch receptors - Hering-Breuer reflex
Insufficiencyhigh altitude - oxygen toxicity - hypoxia
The respiratory quotient (or RQ or respiratory coefficient), is a unitless number used in calculations of basal metabolic rate (BMR) when estimated from carbon dioxide production. Such measurements, like measurements of oxygen uptake, are forms of indirect calorimetry.
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respiratory system generally includes tubes, such as the bronchi, used to carry air to the lungs, where gas exchange takes place. A diaphragm pulls air in and pushes it out. Respiratory systems of various types are found in a wide variety of organisms.
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Human physiology is the science of the mechanical, physical, and biochemical functions of humans in good health, their organs, and the cells of which they are composed. The principal level of focus of physiology is at the level of organs and systems.
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Respiratory physiology is the branch of human physiology focusing upon respiration.

Topics include:

Volumes

  • lung volumes
  • vital capacity
  • functional residual capacity
  • dead space
  • spirometry
  • body plethysmography
  • peak flow meter


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Larger volumes Smaller volumes
males females
taller people shorter people
non-smokers heavy smokers
professional athletes[1] non-athletes
people living at high altitudes people living at low altitudes
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Vital capacity is the maximum volume of air that a person can exhale after maximum inhalation. It can also be the maximum volume of air that a person can inhale after maximum exhalation.
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Functional Residual Capacity (FRC) is the volume of air present in the lungs at the end of passive expiration. At FRC, the elastic recoil forces of the lungs and chest wall are equal but opposite and there is no exertion by the diaphragm or other respiratory muscles.
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Respiratory minute volume (or minute ventilation, or flow of gas) is the volume of air which can be inhaled (inhaled minute volume) or exhaled (exhaled minute volume) from a person's lungs in one minute.
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dead space is air that is inhaled by the body in breathing, but does not partake in gas exchange.

In adults, it is usually in the range of 150ml.[1]

Understanding dead space


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Spirometry (meaning the measuring of breath) is the most common of the Pulmonary Function Tests (PFTs), measuring lung function, specifically the measurement of the amount (volume) and/or speed (flow) of air that can be inhaled and exhaled.
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A peak flow meter is a small, hand-held device used to manage asthma by monitoring airflow through the bronchi and thus the degree of restriction in the airways. The peak flow meter measures the patient's maximum ability to expel air from the lungs, or
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ventilation (or ventilation rate) is the rate at which gas enters or leaves the lung. It is categorised under the following definitions:

Measurement Equation Description
Minute ventilation = tidal volume * respiratory rate[1]
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Positive pressure ventilators help patients with respiratory problems to breathe easier. They use high pressure gas at the opening of the patients lungs in order to mobilize oxygen flow down the pressure gradient, and into the patient's lungs.
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Breathing transports oxygen into the body and carbon dioxide out of the body. Aerobic organisms require oxygen to create energy via respiration, in the form of energy-rich molecules such as glucose. The medical term for normal relaxed breathing is eupnoea.
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Inhalation (also known as inspiration) is the movement of air from the external environment, through the airways, into the alveoli during breathing.

Inhalation begins with the onset of contraction of the diaphragm, which results in expansion of the intrapleural space
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Exhalation (or expiration) is the movement of air out of the bronchial tubes, through the airways, to the external environment during breathing.

Exhaled air is rich in carbon dioxide, a waste product of cellular respiration during the production of ATP.
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respiratory rate (or respiration rate) is the number of breaths a living being, such as a human, takes per minute.

There is only limited research on monitoring alien respiratory rate, and these studies have focused on such issues as the inaccuracy of respiratory rate
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A respirometer is a device used to measure the rate of respiration of a living organism by measuring its rate of exchange of oxygen and carbon dioxide. They allow investigation into how factors such as age or the effect of light affect the rate of respiration.
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Pulmonary surfactant is a surface-active lipoprotein complex formed by type II alveolar cells. The proteins and lipids that comprise surfactant have both a hydrophilic region and a hydrophobic region.
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158 (4): 1052-60. PMID 9769260.
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“Hysteresivity” derives from “hysteresis”, meaning “lag”. It is the tendency to react slowly to an outside force, or to not return completely to its original state.
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Airway resistance is a concept used in respiratory physiology to describe mechanical factors which limit the access of inspired air to the pulmonary alveoli, and thus determine airflow.
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Pulmonary circulation is the portion of the cardiovascular system which carries oxygen-depleted blood away from the heart, to the lungs, and returns oxygenated blood back to the heart. The term is contrasted with systemic circulation.
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In physiology, perfusion is the process of nutritive delivery of arterial blood to a capillary bed in the biological tissue.

Tests of adequate perfusion are a part of patient triage performed by medical or emergency personnel in a mass casualty incident.
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Hypoxic pulmonary vasoconstriction is a physiological phenomenon in which pulmonary arteries constrict in the presence of hypoxia (low oxygen levels) without hypercapnia (high carbon dioxide levels), redirecting blood flow to alveoli with higher oxygen tension.
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Pulmonary shunts exist when there is normal perfusion to an alveolus, but ventilation fails to supply the perfused region.

This will lead to a situation where the blood supply leaving a shunted area of the lung will have lower levels of oxygen and higher levels of carbon
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In respiratory physiology, the ventilation/perfusion ratio (or V/Q ratio) is a measurement used to the efficiency and adequacy of the matching of two variables:[1]
  • "V" - ventilation - the air which reaches the lungs

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A ventilation/perfusion scan, also called a V/Q scan, is a medical test to measure the circulation of air and blood within a patient's lungs. The ventilation part of the test evaluates the ability of air to reach all parts of the lungs, while the perfusion part measures how well
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The zones of the lung proposed by West in 1964,[1] divide the lung into three vertical regions, based upon the relationship between the pressure in the alveoli (PA), in the arteries (Pa), and the veins (Pv):
  • #1: alveolar > arterial > venous
  • #2: arterial >

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