Published June 26, 2023 by

Exchange Of Gas In The Lungs



Respiration involves the transport of oxygen to the pulmonary alveoli and the elimination of carbon dioxide from the alveoli to the outside.

The air enters through the nostrils, where it is filtered and warmed;  it can also enter through the mouth.  The air passes through the respiratory tubes (pharynx, larynx and trachea) and from the trachea it branches to the two bronchi and passes through the bronchioles to reach the lungs.  In the lungs it reaches the alveoli, which is where gas exchange occurs: oxygen is released from the air and carbon dioxide is collected.  The air with carbon dioxide now makes the reverse journey to be expelled.

To carry out this entire process of entering and leaving air in the lungs, respiratory movements called inspiration and expiration are carried out.

Inspiration and expiration movements

Inspiratory movement: When the diaphragm contracts and moves downward, the pectoral and intercostal muscles push the ribs outward, the chest cavity expands, and air enters the lungs through the trachea.

Expiration movement: when the diaphragm relaxes, it assumes its normal position, curved upwards, and the lungs contract, expelling the air.

Types of breathing

Breathing can be of three types:

1. Clavicular or high breathing: The upper part of the chest intervenes, it is the one that takes the least amount of air.  This type of breathing occurs in moments of nervousness or anxiety.

2. Thoracic and average breathing: It is the most frequent type of breathing.  On inspiration, the intercostal muscles cause elevation of the ribs, increasing the capacity of the thoracic cage.  On expiration, the thorax performs the opposite movement, relaxing the muscles.

3. Abdominal breathing: It is the type of breathing that is used during sleep and is the one used in relaxation exercises.  With it we move the abdomen independently of the thorax.  When we inhale we relax the abdominal muscles and when we exhale we contract the abdominals.

Our Respiratory System: Anatomy and Function

The respiratory system is made up of the airways and the lungs.  Through the airways the air circulates in the direction of the lungs and it is in these organs where gas exchange takes place.

In the airways we differentiate the upper airway, which goes from the nose and mouth to the vocal cords, and includes the pharynx and larynx, and the lower airway, formed by the trachea, the bronchi and their ramifications inside. of the lungs, the bronchioles.

The trachea is the tube that goes from the larynx to the main bronchi.  These, in turn, penetrate the interior of each lung and divide into smaller branches (bronchioles).  Finally, as they are introduced into the lungs, they end up in bags or sacs called alveoli.

On the walls of the trachea and the thickest bronchi there are several layers that from the outside in are the cartilage, which gives it structure and consistency, a muscular layer and a more internal covering, which is the mucosa.

Breathing is the primary function of the respiratory system. The process involves transferring oxygen to the blood and removing carbon dioxide (CO2) from the air. The lungs are the location of this gas exchange.

The alveoli, where gas exchange takes place, receive air from the nose or mouth through the airways. As a result, oxygen enters the blood and reaches every cell. In turn, the cells' production of carbon dioxide (CO2) is carried to the lungs for elimination.

Gas Exchange

Since human cells rely primarily on aerobic metabolism, it is vitally important to obtain oxygen from the environment and deliver it to the tissues efficiently, while excreting the byproduct of cellular respiration carbon dioxide (CO2).  Breathing involves both the respiratory and circulatory systems.  There are 4 processes that supply oxygen (O2) to the body and remove CO2.  The respiratory system is engaged in pulmonary ventilation and external respiration, whereas the circulatory system is in charge of movement and internal respiration.  Air entering and leaving the lungs is symbolized by pulmonary ventilation (breathing).  The exchange of O2 and CO2 between the lungs and the blood symbolizes external respiration, also known as gas exchange.

The gas that fills the lungs on inspiration is air rich in O2 and low in O2, while the air that comes out on expiration is air rich in CO2 and depleted in O2.

The pulmonary alveoli are surrounded by blood capillaries, with walls so thin that they allow the exchange of gases (O2 and CO2) between air and blood.

The exchange of gases is carried out through a physical process called diffusion, in which the molecules move from where there is more concentration to where there is less until they equalize.

Oxygen is carried out by red blood cell throughout the body. Instead, the plasma, the liquid component of the blood, is used to carry carbon dioxide in dissolved form.

The air that reaches the lungs with inspiration is rich in oxygen (O2) and also contains some carbon dioxide (CO2).  The air that leaves the lungs with expiration is rich in carbon dioxide and poor in oxygen.

The pulmonary alveoli are surrounded by blood capillaries in which gas exchange between air and blood occurs.  The movement of oxygen throughout the body is carried out by red blood cells.

Measurement and Abnormalities related to gas exchange :

Gas exchange is measured by various means, such as

 Diffusing capacity for carbon monoxide

• pulse oximetry

• Arterial blood gas sample

• Diffusion capacity of carbon monoxide

The ability of gas to traverse the capillary endothelium and alveolar epithelium to pass from alveoli to erythrocytes is measured by the carbon monoxide diffusing capacity (DLCO).  DLCO is influenced by both the amount of blood in the pulmonary capillaries and the area and thickness of the blood-gas barrier.

Diseases that reduce DLCO

Conditions that compromise the pulmonary vasculature, such as primary pulmonary hypertension and pulmonary embolism, decrease DLCO.  Conditions that diffusely affect the lungs, such as emphysema and pulmonary fibrosis, decrease DLCO and alveolar ventilation (AV).  The reduction in DLCO also occurs in patients with previous lung resection, as the total lung volume is smaller, but the DLCO is corrected or even exceeds the normal value when adjusted for AP, because there is recruitment of additional areas of vascular surface in the lung remaining.  Patients with anemia have lower DLCO values, which are corrected when adjusting for hemoglobin values.

Diseases that increase DLCO

Some diseases that cause the DLCO to be higher than predicted are

• Cardiac insufficiency

• Polycythemia

• Alveolar hemorrhage

• Asthma

Due to the recruitment of more pulmonary microvessels as a result of elevated pulmonary arterial and venous pressures, DLCO rises in heart failure.  Because of the rise in hematocrit and vascular recruitment brought on by raised pulmonary pressures from increased viscosity, DLCO rises in polycythemia.  Erythrocytes in the alveolar space may also bind carbon monoxide during alveolar bleeding, raising DLCO.  Although some evidence points to the possibility of neovascularization induced by growth factors, the rise in DLCO in asthma is typically attributed to supposed vascular recruitment.

Pulse Oximetry

Transcutaneous pulse oximetry estimates the oxygen saturation (SpO2) of blood capillaries based on light absorption from light-emitting diodes positioned on a finger clip or on a probe attached with an adhesive strip.  Estimates are generally quite accurate, correlating within 5% of the measured arterial oxygen saturation (SaO2). Patients  may have less reliable results in case of

• Highly pigmented skin

• Arrhythmias

• Hypotension

• Deep systemic vasoconstriction

Pulse oximetry results are also less accurate in patients who use nail polish.

Pulse oximetry is only able to detect oxyhemoglobin or reduced hemoglobin;  other types of hemoglobin (eg, carboxyhemoglobin, methemoglobin) are identified as oxyhemoglobin, falsely raising the oxygen saturation (SpO2) value using pulse oximetry.

Carbon dioxide

Normally, PCO2 is kept between 35 and 45 mmHg.  While the dissociation curve for carbon dioxide resembles that of oxygen, it is virtually linear across the healthy range of PaCO2.  When PCO2 is abnormal, alterations in acid-base balance are invariably connected to ventilation problems (unless they are compensating for a metabolic problem).



Hypercapnia is PCO2 > 45 mmHg.  The causes of hypercapnia are the same as those of hypoventilation (e.g., disorders that decrease respiratory rate or depth or increase the fraction of dead space ventilation in patients who are already at their maximum ventilation threshold).  When combined with an inability to increase ventilation, disorders that increase carbon dioxide production (eg, hyperthyroidism, fever) also cause hypercapnia.


Hypocapnia is PCO2 35 mmHg.  Hypocapnia is always caused by hyperventilation due to lung (e.g., pulmonary edema or pulmonary embolism), cardiac (e.g., heart failure), metabolic (e.g., acidosis), drug-induced (e.g., heart failure) diseases. eg, acetylsalicylic acid and progesterone), central nervous system (e.g., infection, tumor, bleeding, and increased intracranial pressure), or physiological (e.g., pain, pregnancy).  It is assumed that hypocapnia directly increases bronchoconstriction and lowers the threshold for cerebral and myocardial ischemia, perhaps due to its effects on acid-base status.


Carbon monoxide binds to hemoglobin with an affinity 210 times greater than that of oxygen and prevents the transport of oxygen.  Clinically toxic levels of carboxyhemoglobin are most often the result of exposure to exhaust fumes or smoke inhalation, although cigarette smoke does have detectable levels.

Patients with carbon monoxide poisoning may have nonspecific symptoms such as malaise, headache, and nausea.  As poisoning often occurs during the colder months (due to the use of heaters that use flammable fuels), the symptoms can be confused with a viral syndrome such as influenza.  Clinicians should be alert to the possibility of carbon monoxide poisoning and measure carboxyhemoglobin levels when indicated.  Blood from the veins can be used to assess carboxyhemoglobin; an arterial sample is not required.  Pulse oximetry will show normal oxygen saturation and cannot be used to check for carbon monoxide overdose.  Co-oximetry can be used to measure carboxyhemoglobin.

100% oxygen is given as part of the treatment, which reduces the half-life of carboxyhemoglobin. Hyperbaric oxygen therapy is also occasionally used.