Lung volumes

Lung volumes and lung capacities refer to the volume of air in the lungs at different phases of the respiratory cycle.

The average total lung capacity of an adult human male is about 6 litres of air.

Tidal breathing is normal, resting breathing; the tidal volume is the volume of air that is inhaled or exhaled in only a single such breath.

The average human respiratory rate is 30-60 breaths per minute at birth,[1] decreasing to 12-20 breaths per minute in adults.[2]

Lungvolumes Updated

Factors affecting volumes

Several factors affect lung volumes; some can be controlled and some cannot be controlled. Lung volumes vary with different people as follows:

Larger volume Smaller volumes
taller people shorter people
people who live at higher altitudes people who live at lower altitudes
fit obese[3]

A person who is born and lives at sea level will develop a slightly smaller lung capacity than a person who spends their life at a high altitude. This is because the partial pressure of oxygen is lower at higher altitude which, as a result means that oxygen less readily diffuses into the bloodstream. In response to higher altitude, the body's diffusing capacity increases in order to process more air. Also, due to the lower environmental air pressure at higher altitudes, the air pressure within the breathing system must be lower in order to inhale; in order to meet this requirement, the thoracic diaphragm has a tendency to lower to a greater extent during inhalation, which in turn causes an increase in lung volume.

When someone living at or near sea level travels to locations at high altitudes (e.g., the Andes; Denver, Colorado; Tibet; the Himalayas) that person can develop a condition called altitude sickness because their lungs remove adequate amounts of carbon dioxide but they do not take in enough oxygen. (In normal individuals, carbon dioxide is the primary determinant of respiratory drive.)

Lung function development is reduced in children who grow up near motorways[4][5] although this seems at least in part reversible[6]. Air pollution exposure affects FEV1 in asthmatics, but also affects FVC and FEV1 in healthy adults even at low concentrations.[7]

Specific changes in lung volumes also occur during pregnancy. Functional residual capacity drops 18–20%,[8] typically falling from 1.7 to 1.35 litres, due to the compression of the diaphragm by the uterus. The compression also causes a decreased total lung capacity (TLC) by 5%[8] and decreased expiratory reserve volume by 20%.[8] Tidal volume increases by 30–40%, from 0.5 to 0.7 litres,[8] and minute ventilation by 30–40%[8][9] giving an increase in pulmonary ventilation. This is necessary to meet the increased oxygen requirement of the body, which reaches 50 mL/min, 20 mL of which goes to reproductive tissues. Overall, the net change in maximum breathing capacity is zero.[8]

Values

Average lung volumes in healthy adults[10]
Volume Value (litres)
In men In women
Inspiratory reserve volume (IRV) 3.0 1.9
Tidal volume (TV) 0.5 0.5
Expiratory reserve volume (ERV) 1.1 0.7
Residual volume (RV) 1.2 1.1
Lung capacities in healthy adults[10]
Volume Average value (litres) Derivation
In men In women
Vital capacity 4.6 3.1 IRV + TV + ERV
Inspiratory capacity 3.5 2.4 IRV + TV
Functional residual capacity 2.3 1.8 ERV + RV
Total lung capacity 5.8 4.2 IRV + TV + ERV + RV

The tidal volume, vital capacity, inspiratory capacity and expiratory reserve volume can be measured directly with a spirometer. These are the basic elements of a ventilatory pulmonary function test.

Determination of the residual volume is more difficult as it is impossible to "completely" breathe out. Therefore, measurement of the residual volume has to be done via indirect methods such as radiographic planimetry, body plethysmography, closed circuit dilution (including the helium dilution technique) and nitrogen washout.

In absence of such, estimates of residual volume have been prepared as a proportion of body mass for infants (18.1 mL/kg),[11] or as a proportion of vital capacity (0.24 for men and 0.28 for women)[12] or in relation to height and age ((0.0275* Age [Years]+0.0189*Height [cm]-2.6139) litres for normal-mass individuals and (0.0277*Age [Years]+0.0138*Height [cm]-2.3967) litres for overweight individuals).[13] Standard errors in prediction equations for residual volume have been measured at 579 mL for men and 355 mL for women, while the use of 0.24*FVC gave a standard error of 318 mL.[14]

Online calculators are available that can compute predicted lung volumes, and other spirometric parameters based on a patient's age, height, weight, and ethnic origin for many reference sources.

Weight of Breath

The mass of one breath is approximately a gram (0.5-5g). A Litre of air weighs about 1.2 grams (1.2 kg per cubic metre).[15] A half Litre ordinary tidal breath weighs just over half a gram; a maximal four Litre breath weighs almost five grams.

Restrictive and obstructive

Lung volumes in restricted, normal and obstructed lung
Scheme of changes in lung volumes in restricted and obstructed lung in comparison with healthy lung.

The results (in particular FEV1/FVC and FRC) can be used to distinguish between restrictive and obstructive pulmonary diseases:

Type Examples Description FEV1/FVC
restrictive diseases pulmonary fibrosis, Infant Respiratory Distress Syndrome, weak respiratory muscles, pneumothorax volumes are decreased often in a normal range (0.8 - 1.0)
obstructive diseases asthma, COPD, emphysema volumes are essentially normal but flow rates are impeded often low (asthma can reduce the ratio to 0.6, emphysema can reduce the ratio to 0.78 - 0.45)

Increasing lung capacity

Lung capacity can be expanded through flexibility exercises such as yoga, breathing exercises, and physical activity. A greater lung capacity is sought by people such as athletes, freedivers, singers, and wind-instrument players. A stronger and larger lung capacity allows more air to be inhaled into the lungs. In using lungs to play a wind instrument for example, exhaling an expanded volume of air will give greater control to the player and allow for a clearer and louder tone.

See also

References

  1. ^ Scott L. DeBoer (4 November 2004). Emergency Newborn Care. Trafford Publishing. p. 30. ISBN 978-1-4120-3089-2.
  2. ^ Wilburta Q. Lindh; Marilyn Pooler; Carol Tamparo; Barbara M. Dahl (9 March 2009). Delmar's Comprehensive Medical Assisting: Administrative and Clinical Competencies. Cengage Learning. p. 573. ISBN 978-1-4354-1914-8.
  3. ^ Jones RL, Nzekwu MM. The effects of body mass index on lung volumes. Chest. 2006 Sep; 130 (3) :827–33. PubMed PMID 16963682.
  4. ^ Living Near Freeways Hurts Kids' Lungs https://www.washingtonpost.com/wp-dyn/content/article/2007/01/26/AR2007012600568.html
  5. ^ Gauderman, W (2007). "Effect of exposure to traffic on lung development from 10 to 18 years of age: a cohort study". The Lancet. 369 (9561): 571–577. CiteSeerX 10.1.1.541.1258. doi:10.1016/S0140-6736(07)60037-3. PMID 17307103.
  6. ^ https://healthstudy.usc.edu/study-findings/
  7. ^ Int Panis, L (2017). "Short-term air pollution exposure decreases lung function: a repeated measures study in healthy adults". Environmental Health. 16. doi:10.1186/s12940-017-0271-z.
  8. ^ a b c d e f Simpson, Kathleen Rice; Patricia A Creehan (2007). Perinatal Nursing (3rd ed.). Lippincott Williams & Wilkins. pp. 65–66. ISBN 978-0-7817-6759-0.
  9. ^ Guyton and hall (2005). Textbook of Medical Physiology (11 ed.). Philadelphia: Saunders. pp. 103g. ISBN 978-81-8147-920-4.
  10. ^ a b Ganong, William. "Fig. 35-7". Review of Medical Physiology (21st ed.).
  11. ^ Morris, Mohy G. (2010). "Comprehensive integrated spirometry using raised volume passive and forced expirations and multiple-breath nitrogen washout in infants". Respiratory Physiology & Neurobiology. 170 (2): 123–140. doi:10.1016/j.resp.2009.10.010. ISSN 1569-9048. PMC 2858579. PMID 19897058.
  12. ^ Wilmore, J. H. (1969). "The use of actual predicted and constant residual volumes in the assessment of body composition by underwater weighing". Med Sci Sports. 1 (2): 87–90. doi:10.1249/00005768-196906000-00006.
  13. ^ MILLER, WAYNE C.; SWENSEN, THOMAS; WALLACE, JANET P. (February 1998). "Derivation of prediction equations for RV in overweight men and women". Medicine & Science in Sports & Exercise. 30 (2): 322–327. doi:10.1097/00005768-199802000-00023. PMID 9502364.
  14. ^ Morrow JR Jr; Jackson AS; Bradley PW; Hartung GH. (Dec 1986). "Accuracy of measured and predicted residual lung volume on body density measurement". Med Sci Sports Exerc. 18 (6): 647–52. doi:10.1249/00005768-198612000-00007. PMID 3784877.
  15. ^ Atmosphere of Earth#Density and mass

External links

Bubble CPAP

Bubble CPAP is a non-invasive ventilation strategy for newborns with infant respiratory distress syndrome (IRDS). It is one of the methods by which continuous positive airway pressure (CPAP) is delivered to a spontaneously breathing newborn to maintain lung volumes during expiration. With this method, blended and humidified oxygen is delivered via short binasal prongs or a nasal mask and pressure in the circuit is maintained by immersing the distal end of the expiratory tubing in water. The depth to which the tubing is immersed underwater determines the pressure generated in the airways of the infant. As the gas flows through the system, it “bubbles” out and prevents buildup of excess pressures.

Bubble CPAP is appealing because of its simplicity and low cost. It is also associated with a decreased incidence of bronchopulmonary dysplasia (BPD) compared to mechanical ventilation. Not all infants with IRDS are candidates for initial treatment with CPAP and not all those who are given CPAP can be successfully managed with this modality.

Closing capacity

The closing capacity (CC) is the volume in the lungs at which its smallest airways, the respiratory bronchioles, collapse. It is defined mathematically as the sum of the closing volume and the residual volume. The alveoli lack supporting cartilage and so depend on other factors to keep them open. The closing capacity is greater than the residual volume (RV), the amount of gas that normally remains in the lungs during respiration, and specifically, after forced expiration. This means that there is normally enough air within the lungs to keep these airways open throughout both inhalation and exhalation. As the lungs age, there is a gradual increase in the closing capacity (i.e. The small airways begin to collapse at a higher volume/before expiration is complete). This also occurs with certain disease processes, such as asthma, chronic obstructive pulmonary disease, and pulmonary edema. Any process that increases the CC by increasing the functional residual capacity (FRC) can increase an individual's risk of hypoxemia, as the small airways may collapse during exhalation, leading to air trapping and atelectasis.

A mnemonic for factors increasing closing capacity is ACLS-S: Age, Chronic bronchitis, LV failure, Smoking, Surgery. Of note supine positioning will decrease functional residual capacity (FRC) but has no effect on closing capacity.

Functional residual capacity

Functional Residual Volume (FRC) is the volume of air that will remain in the lungs after a normal expiration.

It can be also derived as

ERV + RV

Functional Residual Capacity (FRC) is the volume of air present in the lungs at the end of passive expiration. At FRC, the opposing elastic recoil forces of the lungs and chest wall are in equilibrium and there is no exertion by the diaphragm or other respiratory muscles.

FRC is the sum of Expiratory Reserve Volume (ERV) and Residual Volume (RV) and measures approximately 2100 mL in a 70 kg, average-sized male (or approximately 30ml/kg) .It cannot be estimated through spirometry, since it includes the residual volume. In order to measure RV precisely, one would need to perform a test such as nitrogen washout, helium dilution or body plethysmography.

A lowered or elevated FRC is often an indication of some form of respiratory disease. For instance, in emphysema, FRC is increased, because the lungs are more compliant and the equilibrium between the inward recoil of the lungs and outward recoil of the chest wall is disturbed. As such, patients with emphysema often have noticeably broader chests due to the relatively unopposed outward recoil of the chest wall. Total lung capacity also increases, largely as a result of increased functional residual capacity. In healthy humans, FRC changes with body posture. Obese patients will have a lower FRC in the supine position due to the added tissue weight opposing the outward recoil of the chest wall.

Positioning plays a significant role in altering FRC. It is highest when in an upright position and decreases as one moves from upright to supine/prone or Trendelenburg position. The greatest decrease in FRC occurs when going from 60° to totally supine at 0°. Interestingly, there is no significant change in FRC as position changes from 0° to Trendelenburg of up to -30°. However, beyond -30°, the drop in FRC is considerable.The helium dilution technique and pulmonary plethysmograph are two common ways of measuring the functional residual capacity of the lungs.

The predicted value of FRC was measured for large populations and published in several references. FRC was found to vary by a patient's age, height, and sex. Functional residual capacity is directly proportional to height and indirectly proportional with obesity. It is reduced in the setting of obesity primarily due to a reduction in chest wall compliance. An online calculator exists that will calculate FRC for a patient using these references.

Helium dilution technique

The helium dilution technique is the way of measuring the functional residual capacity of the lungs (the volume left in the lungs after normal expiration).

This technique is a closed-circuit system where a spirometer is filled with a mixture of helium (He) and oxygen. The amount of He in the spirometer is known at the beginning of the test (concentration × volume = amount). The patient is then asked to breathe (normal breaths) in the mixture starting from FRC (functional residual capacity), which is the gas volume in the lung after a normal breath out. The spirometer measures helium concentration. The helium spreads into the lungs of the patient, and settles at a new concentration (C2). Because there is no leak of substances in the system, the amount of helium remains constant during the test, and the FRC is calculated by using the following equation:

C1×V1 = C2×V2

C1×V1 = C2×(V1+FRC)

FRC = ((C1xV1)/C2) - V1

V2 = total gas volume ( FRC + volume of spirometer)

V1 = volume of gas in spirometer

C1 = initial (known) helium concentration

C2 = final helium concentration (measured by the spirometer)

Note: to measure FRC the patient is connected to the spirometer directly after a normal breath (when the lung volume equals FRC), if the patient is initially connected to the spirometer at a different lung volume (like TLC or RV) the measured volume will be the initial volume started from and not FRC. In patients with obstructive pulmonary diseases the measurements of the helium dilution technique are not reliable because of incomplete equilibration of the helium in all areas of the lungs. In such cases it is more accurate to use a body plethysmograph.

A simplified helium dilution technique may be used as an alternative to quantitative CT scans to assess end-expiratory lung volumes (EELV) among patients who are on mechanical ventilation with diagnosis of ALI/ARDS according to a cross-sectional study. The results show a good correlation [EELV(He)=208+0.858xEELV(CT), r=0.941, p<0.001] between the two methods, and the helium dilution technique offers the advantages of lower cost, decreased transportation of critically ill patients, and reduced radiation exposure. This study's results may have limited generalizability due to its specificity to the ALI/ARDS population and its small sample size (21 patients).

Idiopathic pulmonary fibrosis

Idiopathic pulmonary fibrosis (IPF) is a type of chronic lung disease characterized by a progressive and irreversible decline in lung function. Symptoms typically include gradual onset of shortness of breath and a dry cough. Other changes may include feeling tired and nail clubbing. Complications may include pulmonary hypertension, heart failure, pneumonia, or pulmonary embolism.The cause is unknown. Risk factors include cigarette smoking, certain viral infections, and a family history of the condition. The underlying mechanism involves scarring of the lungs. Diagnosis requires ruling out other potential causes and may be supported by a CT scan or lung biopsy. It is a type of interstitial lung disease (ILD).People often benefit from pulmonary rehabilitation and supplemental oxygen. Certain medications like pirfenidone or nintedanib may slow the progression of the disease. Lung transplantation may also be an option.About 5 million people are affected globally. The disease newly occurs in about 12 per 100,000 people per year. Those in their 60s and 70s are most commonly affected. Males are affected more often than females. Average life expectancy following diagnosis is about four years.

Insufflation (medicine)

Insufflation (Latin: insufflare, lit. 'to blow into') is the act of blowing something (such as a gas, powder, or vapor) into a body cavity. Insufflation has many medical uses, most notably as a route of administration for various drugs.

Julius Jeffreys

Julius Jeffreys (1800–1877) was a British surgeon and writer, was the inventor of the respirator, and was a pioneer in the development of early air conditioning systems.

Lung compliance

Lung compliance, or pulmonary compliance, is a measure of the lung's ability to stretch and expand (distensibility of elastic tissue). In clinical practice it is separated into two different measurements, static compliance and dynamic compliance. Static lung compliance is the change in volume for any given applied pressure. Dynamic lung compliance is the compliance of the lung at any given time during actual movement of air.

Low compliance indicates a stiff lung (one with high elastic recoil) and can be thought of as a thick balloon – this is the case often seen in fibrosis. High compliance indicates a pliable lung (one with low elastic recoil) and can be thought of as a grocery bag – this is the case often seen in emphysema. Compliance is highest at moderate lung volumes, and much lower at volumes which are very low or very high. The compliance of the lungs demonstrate lung hysteresis; that is, the compliance is different on inspiration and expiration for identical volumes.

Medical sign

A medical sign is an objective indication of some medical fact or characteristic that may be detected by a patient or anyone, especially a physician, before or during a physical examination of a patient. For example, whereas a tingling paresthesia is a symptom (only the person experiencing it can directly observe their own tingling feeling), erythema is a sign (anyone can confirm that the skin is redder than usual). Symptoms and signs are often nonspecific, but often combinations of them are at least suggestive of certain diagnoses, helping to narrow down what may be wrong. In other cases they are specific even to the point of being pathognomonic.

Some signs may have no meaning to the patient, and may even go unnoticed, but may be meaningful and significant to the healthcare provider in assisting diagnosis.

Examples of signs include elevated blood pressure, a clubbing of the ends of fingers (which may be a sign of lung disease, or many other things), a staggering gait and arcus senilis of the eyes.

The term sign is not to be confused with the term indication, which in medicine denotes a valid reason for using some treatment.

Nitrogen washout

Nitrogen washout (or Fowler's method) is a test for measuring anatomic dead space in the lung during a respiratory cycle, as well as some parameters related to the closure of airways.

Pulmonary function testing

Pulmonary function test (PFT) is a complete evaluation of the respiratory system including patient history, physical examinations, and tests of pulmonary function. The primary purpose of pulmonary function testing is to identify the severity of pulmonary impairment. Pulmonary function testing has diagnostic and therapeutic roles and helps clinicians answer some general questions about patients with lung disease. PFTs are normally performed by a respiratory therapist, physiotherapist and/or GP

Spirometer

A spirometer is an apparatus for measuring the volume of air inspired and expired by the lungs. A spirometer measures ventilation, the movement of air into and out of the lungs. The spirogram will identify two different types of abnormal ventilation patterns, obstructive and restrictive. There are various types of spirometers which use a number of different methods for measurement (pressure transducers, ultrasonic, water gauge).

Spirometry

Spirometry (meaning the measuring of breath) is the most common of the pulmonary function tests (PFTs). It measures lung function, specifically the amount (volume) and/or speed (flow) of air that can be inhaled and exhaled. Spirometry is helpful in assessing breathing patterns that identify conditions such as asthma, pulmonary fibrosis, cystic fibrosis, and COPD. It is also helpful as part of a system of health surveillance, in which breathing patterns are measured over time.Spirometry generates pneumotachographs, which are charts that plot the volume and flow of air coming in and out of the lungs from one inhalation and one exhalation.

Tidal volume

Tidal volume (symbol VT or TV) is the lung volume representing the normal volume of air displaced between normal inhalation and exhalation when extra effort is not applied. In a healthy, young human adult, tidal volume is approximately 500 mL per inspiration or 7 mL/kg of body mass.

Vascular resistance

Vascular resistance is the resistance that must be overcome to push blood through the circulatory system and create flow. The resistance offered by the systemic circulation is known as the systemic vascular resistance (SVR) or may sometimes be called by the older term total peripheral resistance (TPR), while the resistance offered by the pulmonary circulation is known as the pulmonary vascular resistance (PVR). Systemic vascular resistance is used in calculations of blood pressure, blood flow, and cardiac function. Vasoconstriction (i.e., decrease in blood vessel diameter) increases SVR, whereas vasodilation (increase in diameter) decreases SVR.

Units for measuring vascular resistance are dyn·s·cm−5, pascal seconds per cubic metre (Pa·s/m³) or, for ease of deriving it by pressure (measured in mmHg) and cardiac output (measured in l/min), it can be given in mmHg·min/l. This is numerically equivalent to hybrid resistance units (HRU), also known as Wood units (in honor of Paul Wood, an early pioneer in the field), frequently used by pediatric cardiologists. The conversion between these units is:

Vital capacity

Vital capacity (VC) is the maximum amount of air a person can expel from the lungs after a maximum inhalation. It is equal to the sum of inspiratory reserve volume, tidal volume, and expiratory reserve volume.

A person's vital capacity can be measured by a wet or regular spirometer. In combination with other physiological measurements, the vital capacity can help make a diagnosis of underlying lung disease. Furthermore, the vital capacity is used to determine the severity of respiratory muscle involvement in neuromuscular disease, and can guide treatment decisions in Guillain–Barré syndrome and myasthenic crisis.A normal adult has a vital capacity between 3 and 5 litres. A human's vital capacity depends on age, sex, height, mass, and ethnicity.Lung volumes and lung capacities refer to the volume of air associated with different phases of the respiratory cycle. Lung volumes are directly measured, whereas lung capacities are inferred from volumes.

Yellow nail syndrome

Yellow nail syndrome, also known as "primary lymphedema associated with yellow nails and pleural effusion", is a very rare medical syndrome that includes pleural effusions, lymphedema (due to under development of the lymphatic vessels) and yellow dystrophic nails. Approximately 40% will also have bronchiectasis. It is also associated with chronic sinusitis and persistent coughing. It usually affects adults.

Zones of the lung

The zones of the lung divide the lung into four vertical regions, based upon the relationship between the pressure in the alveoli (PA), in the arteries (Pa), in the veins (Pv) and the pulmonary interstitial pressure (Pi) :

Zone 1: PA > Pa > Pv

Zone 2: Pa > PA > Pv

Zone 3: Pa > Pv > PA

Zone 4: Pa > Pi > Pv > PAThis concept is generally attributed to an article by West et al. in 1964, but was actually proposed two years earlier by Permutt et al. In this article, Permutt suggests "The pressure in the pulmonary arteries and veins is less at the top than at the bottom of the lung. It is quite likely that there is a portion of the lung toward the top in an upright subject in which the pressure in the pulmonary arteries is less than alveolar pressure."

The concept is as follows:

Alveolar pressure (PA) at end expiration is equal to atmospheric pressure (0 cm H20 differential pressure, at zero flow), plus or minus 2 cm H2O (1.5 mmHg) throughout the lung. On the other hand, gravity causes a gradient in blood pressure between the top and bottom of the lung of 20 mmHg in the erect position (roughly half of that in the supine position). Overall, mean pulmonary venous pressure is ~5 mmHg. Local venous pressure falls to -5 at the apexes and rises to +15 mmHg at the bases, again for the erect lung.

Pulmonary blood pressure is typically in the range 25–10 mmHg with a mean pressure of 15 mmHg. Regional arterial blood pressure is typically in the range 5 mmHg near the apex of the lung to 25 mmHg at the base.

Zone 1 is not observed in the normal healthy human lung. In normal health pulmonary arterial (Pa) pressure exceeds alveolar pressure (PA) in all parts of the lung. It is generally only observed when a person is ventilated with positive pressure or hemorrhage. In these circumstances, blood vessels can become completely collapsed by alveolar pressure (PA) and blood does not flow through these regions. They become alveolar dead space

Zone 2 is the part of the lungs about 3 cm above the heart. In this region blood flows in pulses. At first there is no flow because of obstruction at the venous end of the capillary bed. Pressure from the arterial side builds up until it exceeds alveolar pressure and flow resumes. This dissipates the capillary pressure and returns to the start of the cycle. Flow here is sometimes compared to a starling resistor or waterfall effect.

Zone 3 comprises the majority of the lungs in health. There is no external resistance to blood flow and blood flow is continuous throughout the cardiac cycle. Flow is determined by the Ppa-Ppv difference (Ppa - Ppv), which is constant down this portion of the lung. However, transmural pressure across the wall of the blood vessels increases down this zone due to gravity. Consequently the vessels wall are more stretched so the caliber of the vessels increases causing an increase in flow due to lower resistance.

Zone 4 can be seen at the lung bases at low lung volumes or in pulmonary edema. Pulmonary interstitial pressure (Pi) rises as lung volume decreases due to reduced radial tethering of the lung parenchyma. Pi is highest at the base of the lung due to the weight of the above lung tissue. Pi can also rise due to an increased volume of 'leaked' fluid from the pulmonary vasculature (pulmonary edema). An increase in Pi causes extraalveolar blood vessels to reduce in caliber, in turn causing blood flow to decrease (extraalveolar blood vessels are those blood vessels outside alveoli). Intraalveolar blood vessels (pulmonary capillaries) are thin walled vessels adjacent to alveoli which are subject to the pressure changes described by zones 1-3. Flow in zone 4 is governed by the arteriointerstitial pressure difference (Pa − Pi). This is because as Pi rises, the arterial caliber is reduced, thereby increasing resistance to flow. The Pa/Pv difference remains unchanged since Pi is applied over both vessels.

The ventilation/perfusion ratio is higher in zone #1 (the apex of lung) when a person is standing than it is in zone #3 (the base of lung) because perfusion is nearly absent. However, ventilation and perfusion are highest in base of the lung, resulting in a comparatively lower V/Q ratio.

Respiration
Control
Lung volumes
Circulation
Interactions
Insufficiency

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