Gas exchange
Gas exchange is a process in biology where gases contained in an organism and atmosphere transfer or exchange. In human gas-exchange, gases contained in the blood of human bodies exchange with gases contained in the atmosphere. Human gas-exchange occurs in the lungs
Human lung
The human lungs are the organs of respiration in humans. Humans have two lungs, with the left being divided into two lobes and the right into three lobes. Together, the lungs contain approximately of airways and 300 to 500 million alveoli, having a total surface area of about in...

. In unicellular organism
Unicellular organism
A unicellular organism, also known as a single-celled organism is an organism that consists of only one cell, in contrast to a multicellular organism that consists of multiple cells. Historically simple single celled organisms have sometimes been referred to as monads Prokaryotes, most protists,...

s the respiratory surface and gas-exchange is governed by Fick's law.


In humans and mammals, respiratory gas exchange is carried out by mechanisms of the heart and lungs. Ventilation
Ventilation is movement of air in and out of an enclosed space, including a body. It is used in the following contexts:* Ventilation * Ventilation * Ventilation * Ventilation * Ventilation...

 is the process of air movement into and out of the lungs. Once air enters the lungs, diffusion
Molecular diffusion, often called simply diffusion, is the thermal motion of all particles at temperatures above absolute zero. The rate of this movement is a function of temperature, viscosity of the fluid and the size of the particles...

 of O2 and CO2 occurs in the alveoli. The oxygenated blood is then perfused
In physiology, perfusion is the process of nutritive delivery of arterial blood to a capillary bed in the biological tissue. The word is derived from the French verb "perfuser" meaning to "pour over or through."...

 throughout the body where gas exchange occurs in the capillary
Capillaries are the smallest of a body's blood vessels and are parts of the microcirculation. They are only 1 cell thick. These microvessels, measuring 5-10 μm in diameter, connect arterioles and venules, and enable the exchange of water, oxygen, carbon dioxide, and many other nutrient and waste...

 beds. The blood is subjected to a transient electric field (QRS waves of the EKG) in the heart, which dissociates molecules of different charge. The blood, being a polar fluid, aligns dipoles with the electric field, is released, a then oscillates in a damped driven oscillation to form Y or Osborn Waves, V, U, and Y waves. The electric field exposure and subsequent damped driven oscillation dissociate gas from hemoglobin, primarily CO2, but more important, BPG
2,3-Bisphosphoglyceric acid is a three-carbon isomer of the glycolytic intermediate 1,3-bisphosphoglyceric acid . 2,3-BPG is present in human red blood cells at approximately 5 mmol/L...

, which has a higher affinity for hemoglobin than does oxygen, due in part to its opposite charge. Completely dissociated hemoglobin (which will even effervesce if the electric field is too strong — the reason defibrillation joule
The joule ; symbol J) is a derived unit of energy or work in the International System of Units. It is equal to the energy expended in applying a force of one newton through a distance of one metre , or in passing an electric current of one ampere through a resistance of one ohm for one second...

s are limited, to avoid bubble emboli that may clog vessels in the lung) enters the lung in red blood cells ready to be oxygenated.

Pulmonary physics

The primary force applied in the respiratory tract is supplied by atmospheric pressure
Atmospheric pressure
Atmospheric pressure is the force per unit area exerted into a surface by the weight of air above that surface in the atmosphere of Earth . In most circumstances atmospheric pressure is closely approximated by the hydrostatic pressure caused by the weight of air above the measurement point...

. Total atmospheric pressure at sea level
Sea level
Mean sea level is a measure of the average height of the ocean's surface ; used as a standard in reckoning land elevation...

 is 760 mmHg (101 kPa), with oxygen (O2) providing a partial pressure (pO2) of 160 mmHg (21,331.6 Pa), 21% by volume, at the entrance of the nares, a partial pressure of 150 mmHg (19,998.4 Pa) in the trachea due to the effect of partial pressure of water vapor, and an estimated pO2 of 100 mmHg (13,332.2 Pa) in the alveoli sac, pressure drop due to conduction loss as oxygen
Oxygen is the element with atomic number 8 and represented by the symbol O. Its name derives from the Greek roots ὀξύς and -γενής , because at the time of naming, it was mistakenly thought that all acids required oxygen in their composition...

 travels along the transport passageway. Atmospheric pressure decreases as altitude increases, making effective breathing more difficult at higher altitudes. Higher BPG levels in the blood are also seen at higher elevations, as well.


In similar manner, CO2, which is a result of tissue cellular respiration, is also exchanged. The pCO2 changes from 45 millimetre of mercury in the alveoli. The concentration of this gas in the breath can be measured using a capnograph
A capnograph is an instrument used to measure the carbon dioxide concentration in an air sample. It does this by measuring the absorption of infrared light, which is absorbed particularly well by carbon dioxide....

. As a secondary measurement, respiration rate can be derived from a CO2 breath waveform.

Gas exchange occurs only at pulmonary and systemic
Systemic circulation
Systemic circulation is the part of the cardiovascular system which carries oxygenated blood away from the heart to the body, and returns deoxygenated blood back to the heart. This physiologic theory of circulation was first described by William Harvey...

 capillary beds, but anyone can perform simple experiments with electrodes in blood on the bench-top to observe electric field-stimulated effervescence.

Trace gases

Trace gases present in breath at levels lower than a part per million are ammonia
Ammonia is a compound of nitrogen and hydrogen with the formula . It is a colourless gas with a characteristic pungent odour. Ammonia contributes significantly to the nutritional needs of terrestrial organisms by serving as a precursor to food and fertilizers. Ammonia, either directly or...

, acetone
Acetone is the organic compound with the formula 2CO, a colorless, mobile, flammable liquid, the simplest example of the ketones.Acetone is miscible with water and serves as an important solvent in its own right, typically as the solvent of choice for cleaning purposes in the laboratory...

, isoprene
Isoprene , or 2-methyl-1,3-butadiene, is a common organic compound with the formula CH2=CCH=CH2. Under standard conditions it is a colorless liquid...

. These can be measured using selected ion flow tube mass spectrometry.


Blood carries oxygen, carbon dioxide, and hydrogen
Hydrogen is the chemical element with atomic number 1. It is represented by the symbol H. With an average atomic weight of , hydrogen is the lightest and most abundant chemical element, constituting roughly 75% of the Universe's chemical elemental mass. Stars in the main sequence are mainly...

An ion is an atom or molecule in which the total number of electrons is not equal to the total number of protons, giving it a net positive or negative electrical charge. The name was given by physicist Michael Faraday for the substances that allow a current to pass between electrodes in a...

s between tissues and the lungs. The majority of CO2 transported in the blood is dissolved in plasma (primarily as dissolved bicarbonate
In inorganic chemistry, bicarbonate is an intermediate form in the deprotonation of carbonic acid...

; 60%). A smaller fraction is transported in red blood cells combined with the globin portion of haemoglobin as carbaminohaemoglobin. This is the chemical portion of the red blood cell that aids in the transport of oxygen and nutrients around the body, but, this time, it is carbon dioxide that is transported back to the lung.

As CO2 diffuses into the blood stream, it is absorbed by red blood cells before the majority is converted into H2CO3 by carbonic anhydrase
Carbonic anhydrase
The carbonic anhydrases form a family of enzymes that catalyze the rapid interconversion of carbon dioxide and water to bicarbonate and protons , a reversible reaction that occurs rather slowly in the absence of a catalyst...

, an enzyme that is not present in the plasma. The H2CO3 dissociates into H+ and HCO. The HCO moves out of the red blood cells in exchange for Cl (chloride shift). The hydrogen ions are removed by buffers in the blood (Hb).


The hemoglobin molecule is a protein with four subunits. Each subunit is made up of a heme (a molecular group) with a polypeptide attached. Heme contains one

Control of respiration

Control of respiration is due to rhythmical breathing generated by the phrenic nerve
Phrenic nerve
The phrenic nerve originates mainly from the 4th cervical nerve, but also receives contributions from the 5th and 3rd cervical nerves in humans....

 in order to stimulate contraction and relaxation of the diaphragm during inspiration
Inhalation is the movement of air from the external environment, through the air ways, and into the alveoli....

 and expiration
Exhalation is the movement of air out of the bronchial tubes, through the airways, to the external environment during breathing....

. Ventilation is controlled by partial pressures of oxygen and carbon dioxide and the concentration of hydrogen ions. The control of respiration can vary in certain circumstances such as during exercise.

External links

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