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Respiratory Physiology
Gas Exchange, Lung Volumes, and V/Q Matching
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Respiratory physiology explains how the lungs bring oxygen into the body and remove carbon dioxide from the blood. This process matters because every tissue depends on a steady oxygen supply for aerobic metabolism and on carbon dioxide removal to help maintain acid-base balance. Understanding gas exchange also helps students connect normal anatomy to common diseases such as asthma, pneumonia, pulmonary embolism, and emphysema. Lung volumes and ventilation-perfusion relationships are core ideas used in physiology, medicine, and critical care.
Gas exchange occurs across the thin alveolar-capillary membrane, where oxygen diffuses from alveoli into blood and carbon dioxide diffuses in the opposite direction. Lung volumes describe how much air moves in and out during breathing and how much remains after exhalation, which helps assess mechanics and reserve. Ventilation-perfusion matching compares airflow to blood flow in different lung regions and determines how efficiently oxygen enters the blood. When diffusion, ventilation, or perfusion is disrupted, arterial oxygenation falls and characteristic clinical patterns appear.
Key Facts
- Dalton's law: Ptotal = P1 + P2 + P3 + ...
- Fick's law: Gas diffusion is proportional to (A x D x deltaP) / T
- Minute ventilation = tidal volume x respiratory rate
- Alveolar ventilation = (tidal volume - dead space) x respiratory rate
- Vital capacity = IRV + TV + ERV
- V/Q ratio = alveolar ventilation / pulmonary blood flow, normal whole-lung value is about 0.8
Vocabulary
- Alveolus
- An alveolus is a tiny air sac in the lung where oxygen and carbon dioxide are exchanged with capillary blood.
- Tidal volume
- Tidal volume is the amount of air inhaled or exhaled during a normal quiet breath.
- Residual volume
- Residual volume is the air left in the lungs after a maximal exhalation and cannot be measured by simple spirometry.
- Dead space
- Dead space is the portion of inspired air that does not participate in gas exchange, either because it stays in conducting airways or reaches unperfused alveoli.
- Ventilation-perfusion matching
- Ventilation-perfusion matching is the relationship between airflow and blood flow in the lungs that determines how efficiently gas exchange occurs.
Common Mistakes to Avoid
- Confusing ventilation with perfusion, which is wrong because ventilation refers to airflow into alveoli while perfusion refers to blood flow through pulmonary capillaries.
- Assuming all lung volumes can be measured by spirometry, which is wrong because residual volume, functional residual capacity, and total lung capacity require methods beyond simple spirometry.
- Thinking oxygen and carbon dioxide move by active transport across the alveolar membrane, which is wrong because both gases normally move by passive diffusion down partial pressure gradients.
- Assuming a normal total ventilation always means normal gas exchange, which is wrong because poor V/Q matching or increased dead space can reduce oxygenation even when breathing rate looks adequate.
Practice Questions
- 1 A student has a tidal volume of 500 mL, an anatomic dead space of 150 mL, and a respiratory rate of 12 breaths per minute. What is the alveolar ventilation in mL/min?
- 2 A patient breathes with a tidal volume of 450 mL at 16 breaths per minute. Calculate the minute ventilation in L/min.
- 3 A region of lung is well ventilated but receives almost no blood flow because of a pulmonary embolism. Describe how the V/Q ratio changes in that region and explain why gas exchange becomes inefficient.