Physiology, Boyle's Law


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Brian Kenny


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Updated:
2/9/2019 11:29:30 PM

Introduction

Boyle’s law is a gas law that describes the relationship between the pressure and volume of gas for a mass and temperature. This law is the mechanism by which the human respiratory system functions. Boyle’s law is equivalent to PV = K (P is pressure, V is volume, K is a constant), or one may state that pressure is inversely proportional to the volume.

Issues of Concern

The lungs do not follow Boyle’s law at all volumes.  In a resting state with a normal tidal volume, when the alveoli are not collapsed nor are the lungs at maximal capacity, the lungs follow proportional changes of volume and pressure in accordance to Boyle’s law. At low lung volumes, it takes a large pressure change to make small changes in the volume (low compliance of lung tissue). At high volumes within the lung, it takes a more negative pressure to expand the tissue, once again not in compliance with a direct relationship as Boyle’s law dictates. At low and high volumes, the lung has low compliance meaning that the ability of the tissue to expand or its elasticity decreases (compliance = [change in volume]/[change in pressure]).[1]

Organ Systems Involved

The primary organ system involved in the usage of Boyle’s law is the respiratory system. The human body brings air into the lungs by negative pressure. At baseline, the thoracic cavity is in static equilibrium with an intrapleural pressure near -5cmH2O. During inspiration, there is a contraction of inspiratory muscles (diaphragm, external intercostal muscles; additional muscles such as the scalene and sternocleidomastoid can take part under specific circumstances) that increases intrathoracic volume. Due to the combined motion of the lungs and the chest wall, the lungs will begin to expand as the thorax expands during inspiration. According to Boyle’s law, as the volume increases the pressure must decrease, therefore as the intrapleural volume increases, the intrapleural pressure decreases to about -8cm H2O occurs at end inspiration.[1]

At baseline (rest), the alveolar pressure is equal to the atmospheric pressure (0cm H2O), and during inspiration, this pressure will go to -1cmH2O as the volume expands within the alveoli. When the alveolar pressure drops below the atmospheric pressure, air will flow into the lungs for gas exchange.[1]

When the inspiratory muscles relax, the volume within the thorax will decrease, thus the pressure increases and forces out alveolar air back into the atmosphere. With inspiration: lung volume increases, intrapleural pressure decreases. With expiration: lung volume decreases, intrapleural pressure increases.[1]

Function

Intrapleural pressure is the term for pressure within the intrapleural space; alveolar pressure is pressure within the alveoli. As the intrapleural and alveolar pressure become increasingly negative due to the expansion of the chest cavity during inspiration, air from the atmosphere flows into the lungs which allow the lung volume to increase and participate in gas exchange.

Related Testing

Testing related to the mechanism that Boyle’s law works can be applied to the volume within the lung and equations to describe how much air is moving.

The minute ventilation, calculated as the product of tidal volume and respiratory rate, essentially is how much air is inhaled every minute. These two factors control ventilation, which directly depends on the thoracic cavity volume expanding and the decrease in pressure within the intrapleural space and alveoli, allowing for the lungs to fill with air, producing the tidal volume. If there is an adequate tidal volume, a normal respiratory rate will ensure. If the tidal volume is insufficient, there will be a compensatory increase in the respiratory rate in an attempt to maintain normal minute ventilation.[2] 

Minute alveolar ventilation is an equation that also depends on Boyle’s law and the inverse relationship of pressure and volume of the thoracic cavity. Alveolar ventilation is the amount of air that reaches the alveoli for gas exchange in each breath; calculated by subtracting the dead space from the tidal volume and then multiplying by the frequency of ventilation.[2]

Pathophysiology

With a pneumothorax or a hemothorax, there is increased pressure within the intrapleural space. Because of this increased pressure, it moves the resting state of about -5cmH2O to a higher value depending on the degree of disease. As this occurs, it would take a much more significant expansion of the thoracic cavity to create a negative pressure to bring air in from the atmosphere. In a tension pneumothorax, the pressure in the pleural space continually raises the intrapleural pressure, thus decreasing the volume in the lungs. Tension pneumothorax can generate enough pressure to cause a mediastinal shift which eventually interferes with venous return to the right side of the heart and cardiovascular demise.[3][4][5]

Clinical Significance

At birth, newborns are born with no air within their alveoli; thus the volume is zero. The compliance (elasticity of lung tissue) is low at birth. Therefore, the effort to create a negative intrapleural pressure during the initial breaths is high, however, with successive breaths, the lungs fill with air and become more compliant. As the lungs become more compliant, the newborn's lungs will follow Boyle’s law of the inverse relationship of pressure and volume.[1]

Pneumothorax is a clinical condition that can either be primary (typically from trauma) or secondary (patient has a predisposing condition such as COPD). Boyle’s law dictates how air draws into the lungs. As the intrathoracic pressure becomes increasingly negative, the intra-alveolar pressure decreases below atmospheric pressure, causing air to flow into the lungs. In a pneumothorax, there is increased pressure within the intrapleural space, thus causing the need for an increased force to create enough negative pressure for air to come into the lungs.[3][4][5]

Boyle’s law also applies when using a medical syringe. When the cylinder on the syringe is empty, it is said to be in a neutral state as there is no air in the syringe. As one pulls back on the plunger, the volume in the cylinder increases, therefore by Boyle’s law the pressure decreases. The liquid is thus drawn into the cylinder to balance the pressure within the syringe and outside of the syringe.

SCUBA divers must be cognizant of Boyle’s law as they descend and ascend to great depths. As a diver descends in the water, the pressure on the person’s lungs increases, and therefore according to Boyle’s law, the volume of air inside the lungs must decrease. As the diver ascends in the water and the pressure on the thoracic cage decreases, the volume of air increases. It is important to exhale steadily to release the volume of the gas if this does not occur the diver can experience pulmonary barotrauma which is overexpansion and alveolar rupture. The diver may have a pneumothorax (chest pain, dyspnea, unilateral decreased breath sounds) or pneumomediastinum (neck pain, pleuritic chest pain, dyspnea, coughing; there may be subcutaneous emphysema causing a crepitation on palpation).[6]


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Physiology, Boyle's Law - Questions

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How does Boyle's law describe pressure?



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The pressure of a contained ideal gas is inversely related to which of the following?



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Which of the following is not a correct statement in regards to Boyle's Law?



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As a 45-year-old deep sea diver with a past history of hypertension and hyperlipidemia gets ready for his plunge into the ocean, he has heard about 'the bends'. Hypothetically, if the temperature of the ocean were to remain constant throughout his descend, as he dives deeper and the pressure in his blood vessels becomes four times atmospheric pressure, what would happen to the volume of gas within his blood vessels?



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A 65-year-old man has a past medical history of myocardial infarction with a stent placement four years ago, chronic obstructive pulmonary disease, and osteoarthritis. He also had an ischemic stroke two years ago with no residual neurologic deficits. His chief complaint today is that over the past year he has gradually worsening shortness of breath. He states he can only walk for one block or climb one flight of stairs before he has to stop to rest. His last colonoscopy was six months ago, and he was told, "come back in 10 years". He denies any recent illness, chest pain, headache, nausea, vomiting, diarrhea, or blood in the stool. The patient states that he continues to smoke two packs of cigarettes per day and has no intention of quitting. On physical exam, there is wheezing throughout all lung fields. His blood pressure is 116/80 mmHg, heart rate is 62/min, respiratory rate is 12/min, and his SpO2 is 86%. What is the mechanism underlying the abnormality in the patient's vital signs?



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A 15-year-old male presents to the emergency department with shortness of breath and pain in his left chest that began abruptly about 30 minutes ago. The patient is 6 foot 5 inches and 155 pounds. He denies any trauma and said nothing like this has ever happened before. His vitals are T98.6, HR105, BP140/86, RR22, and SpO2 of 95%. X-ray shows a thin peripheral visceral line with no lung markings lateral to the line in the left lung field. How would this acute pathologic state affect pressure and volume in the alveoli in accordance with Boyles law?



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A 50-year-old SCUBA diver with a past history of hypertension, hyperlipidemia, and osteoarthritis. He is currently off the shores of Australia completing the final deep-sea dive of his vacation. One of his family members just ascended and is his severe respiratory distress, when questioned she stated that she wanted to see how long she could hold her breath on the way to the surface as her oxygen tank was running low. As the comes out of the water, she states that she had a sudden onset right-sided chest pain and is having difficulty catching her breath. Her respiratory rate is 24/min, and he is in visible distress. What is the mechanism behind her acute onset shortness of breath?



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While working in the emergency department basic labs are drawn from the patient recently brought in by emergency medical services (EMS). The patient called EMS due to chest pain for two hours after mowing his lawn. He admits to having angina after physical activity, but today is different because it did not go away with rest. CBC, comprehensive metabolic profile (CMP), brain natriuretic peptide (BNP), magnesium, phosphorus, and troponin levels were dawn. After collecting the IV access supplies and starting the line, you pull back on the syringe to draw blood into the syringe to distribute eventually into the individual blood containers. Which physical change in the syringe accounts for the fluid being drawn into the cylinder?



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Physiology, Boyle's Law - References

References

Mortola JP, How to breathe? Respiratory mechanics and breathing pattern. Respiratory physiology     [PubMed]
Tantucci C,Bottone D,Borghesi A,Guerini M,Quadri F,Pini L, Methods for Measuring Lung Volumes: Is There a Better One? Respiration; international review of thoracic diseases. 2016;     [PubMed]
Imran JB,Eastman AL, Pneumothorax. JAMA. 2017 Sep 12;     [PubMed]
Swierzy M,Helmig M,Ismail M,Rückert J,Walles T,Neudecker J, [Pneumothorax]. Zentralblatt fur Chirurgie. 2014 Sep;     [PubMed]
Arshad H,Young M,Adurty R,Singh AC, Acute Pneumothorax. Critical care nursing quarterly. 2016 Apr-Jun;     [PubMed]
Walker, III JR,Murphy-Lavoie HM, Diving, Gas Embolism 2018 Jan;     [PubMed]

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