Conditions for effective gas exchange
For gas exchange between the alveoli and blood to occur fully, three key conditions are necessary:
- Continuous alveolar ventilation. This is required to keep the gas composition inside the alveoli constant.
- Gas diffusion. The transfer of molecules across the alveolar-capillary membrane. The driving force here is the difference (gradient) in the partial pressure of the gases.
- Adequate perfusion. The blood supply to the pulmonary capillaries must strictly correspond to the level of their ventilation. This is known as the ventilation-perfusion ratio.
Mechanisms of gas diffusion
Diffusion of oxygen and carbon dioxide always follows the partial pressure (tension) gradient — from an area of high pressure to an area of low pressure.
In the lungs
Here, gas exchange takes place between alveolar air and venous blood flowing into the capillaries:
- Oxygen (O2): its pressure in the alveoli is 102 mmHg, and in venous blood it is 40 mmHg. Due to this gradient ($102 \rightarrow 40$), oxygen passes into the blood.
- Carbon dioxide (CO2): its tension in the blood is 48 mmHg, and in the alveoli it is 40 mmHg. Carbon dioxide diffuses into the alveolus ($48 \rightarrow 40$).
In the tissues
Gas exchange occurs between arterial blood and cells:
- Oxygen: in tissue capillaries, O2 pressure is about 90 mmHg, while in cells actively consuming it, it drops to 0–20 mmHg. Oxygen tends to move from the blood into the cells.
- Carbon dioxide: cells actively produce CO2, and its pressure there reaches 60 mmHg. In the inflowing arterial blood, it is about 46 mmHg. Carbon dioxide passes into the blood.
Ventilation-perfusion relationship
The lungs possess local autoregulation mechanisms to make gas exchange as efficient as possible. The body redistributes resources so as not to waste blood flow on unventilated areas, and vice versa.
- If blood flow drops (hypoperfusion). The lumen of the bronchioles reflexively decreases in this area. Ventilation decreases, and air is directed to those areas of the lungs that are better perfused.
- If ventilation drops (hypoventilation). Oxygen shortage (hypoxia) and excess carbon dioxide (hypercapnia) develop. This causes local vasoconstriction. Blood flow decreases, and blood is redirected to well-ventilated alveoli.
There is also a vertical gradient in the lungs. Both ventilation and perfusion change from top to bottom, but the blood flow gradient is more pronounced. This creates different ventilation-perfusion ($V/Q$) ratios in different parts of the lungs.
Barrier and blood flow characteristics
The blood-air barrier through which gases diffuse has a total surface area of 70–80 m² and a very small thickness — from 0.3 to 2.0 µm.
Surfactants play an important role in diffusion. They not only regulate water evaporation from the alveolar surface but also control the rate of oxygen absorption at the gas-liquid interface.
The rate of oxygen diffusion into the blood strongly depends on the pressure gradient. If the gradient is only 1 mmHg, 25–60 ml/min of O2 is transferred. With a gradient of 60 mmHg, the rate reaches 3,600 ml/min (while the average resting body requirement is about 300 ml/min). Carbon dioxide has a much higher penetrating capacity and diffuses 25 times faster than oxygen.
Briefly: answers to topical questions
What structural layers make up the blood-air barrier?
The blood-air barrier includes the structures of the alveolar-capillary membrane through which gas exchange occurs:
- Alveolar epithelium / pneumocytes.
- Pulmonary interstitium — extracellular matrix and interstitial cells.
- Alveolar capillary endothelium.
What physical parameters does the diffusing capacity of the lungs depend on (according to Fick's law)?
The rate of passive gas diffusion across the blood-air barrier is described by Fick's law and depends on the properties of the membrane and the diffusing substance. The process is determined by the following physical parameters:
- Partial pressure gradient (concentration gradient) — the main driving force of gas exchange.
- Surface area — direct proportionality (the larger the area, the faster the diffusion).
- Membrane thickness — inverse proportionality (the thicker the barrier, the slower the process).
- Molecular weight — the rate is inversely proportional to the square root of the molecular mass.
- Diffusion coefficient — determined primarily by lipophilicity.
Additionally, the rate is affected by ambient temperature and molecular shape.
Class and exam questions
Oxygen diffuses from the alveoli into the pulmonary capillaries because the partial pressure of O2 is higher in the alveoli (102 mmHg) than in the inflowing venous blood (40 mmHg).
Oxygen passes from the alveoli into the blood of the pulmonary capillaries. This occurs along the partial pressure gradient: in the alveoli it is 102 mmHg, and in venous blood it is 40 mmHg.
What happens to local blood flow if ventilation decreases in a lung segment?
Hypoventilation will lead to hypoxia and hypercapnia. This will cause local vasoconstriction, blood flow will decrease, and blood will be redirected to better-ventilated areas.
Which gas diffuses faster: oxygen or carbon dioxide?
Carbon dioxide diffuses across the alveolar-capillary barrier 25 times faster than oxygen.
What else to study on this topic
- Structure and components of the blood-air barrier
- Physicochemical properties of surfactant
- Formula for calculating the ventilation-perfusion ratio (V/Q)
- Effect of gravity on the regional distribution of pulmonary blood flow
- Oxygen-hemoglobin dissociation curve
Master the entire topic in the "Physiology" course
This is a summary. The course covers everything tested in classes and exams.
- Complete study guide on "Diffusion and Gas Exchange in the Lungs" and 4,128 more pages of the subject
- Assignments and quizzes after every topic — just like in a colloquium
- AI tutor answers questions on the topic anytime
- 114 video lessons on the subject
Other topics in the "Respiration" section
Free video lessons: PhysiologyShort lessons with study notes — no registration required.
Prepared by WhiteCoat Pro faculty in accordance with the medical school curriculum. For educational purposes only.