Gas exchange in the alveoli is the process that moves oxygen from inhaled air into the blood and removes carbon dioxide from the blood to be exhaled. It matters because every cell needs oxygen for aerobic respiration, while carbon dioxide must be cleared to help maintain blood pH. The alveoli provide a huge surface area, a very thin barrier, and a moist surface that allow rapid diffusion.
Their close contact with capillaries makes the lungs an efficient exchange organ.
Understanding Biology: Gas Exchange in the Alveoli
Each alveolus must stay open and stable for gas exchange to continue. A thin watery lining allows gases to dissolve before crossing the cells. This lining creates surface tension, which tends to make tiny air spaces collapse.
Lung cells release surfactant, a soapy substance that lowers surface tension. Surfactant is especially important when breathing out, because the alveoli become smaller at that point. Premature babies can have too little surfactant, making their lungs hard to inflate.
This shows that efficient exchange depends on more than a thin wall. The air spaces must remain open, moist, and well supplied with blood.
Breathing maintains the conditions needed for diffusion. When the diaphragm contracts, the chest volume increases and fresh air enters the lungs. Fresh air keeps the oxygen level in the alveoli relatively high.
When the diaphragm relaxes, air leaves and removes carbon dioxide. Blood flow through the capillaries must match airflow around the alveoli. If an alveolus receives air but little blood, its oxygen cannot be collected effectively.
If blood reaches an alveolus with little air, that blood cannot gain enough oxygen. Doctors call this balance ventilation and perfusion. It becomes less even in some lung diseases and during changes in body position.
Most oxygen travels in the blood attached to hemoglobin inside red blood cells. Only a small amount is dissolved directly in the liquid part of blood. Hemoglobin loading helps keep dissolved oxygen low in the lung capillaries, so oxygen can continue entering from the alveoli.
In active muscles, conditions encourage hemoglobin to release its oxygen. These conditions include a lower oxygen level, a higher carbon dioxide level, a higher temperature, and greater acidity. This is useful during exercise because the cells with the greatest energy demand receive more oxygen.
Carbon dioxide has a different transport story. Some carbon dioxide is carried dissolved in blood, and some attaches to hemoglobin. Most is converted into bicarbonate ions in red blood cells.
This conversion allows the blood to carry much more carbon dioxide from body tissues to the lungs. In the lungs, the process reverses and carbon dioxide is produced again. It then enters the alveolar air before being breathed out.
This link matters because carbon dioxide affects blood acidity. A rise in carbon dioxide can make blood more acidic, which can disturb enzyme activity and normal cell function.
Several real health problems show why the alveoli need their special structure. Smoking can damage alveolar walls and reduce the total exchange surface, as happens in emphysema. Pneumonia can fill alveoli with fluid or immune cells, increasing the distance gases must cross.
Asthma mainly narrows airways, so less fresh air reaches some alveoli. At high altitude, the air contains a lower oxygen pressure, so loading oxygen into blood becomes harder even with healthy lungs. When learning this topic, separate air movement from gas movement.
Ventilation moves bulk air through airways. Diffusion moves individual gas molecules across the respiratory membrane. Both processes are needed for normal oxygen delivery.
Key Facts
- Oxygen diffuses from alveolar air into blood because PO2 is higher in the alveolus than in the deoxygenated capillary blood.
- Carbon dioxide diffuses from blood into alveolar air because PCO2 is higher in the deoxygenated capillary blood than in the alveolus.
- Diffusion rate increases with larger surface area, steeper concentration gradient, and thinner membrane: rate is proportional to A × gradient / thickness.
- The respiratory membrane includes alveolar epithelium, fused basement membrane, and capillary endothelium, and is often less than 1 micrometer thick.
- Hemoglobin binds oxygen in red blood cells: Hb + O2 ⇌ HbO2.
- The oxygen dissociation curve shows that hemoglobin loads oxygen at high PO2 in the lungs and unloads oxygen at lower PO2 in body tissues.
Vocabulary
- Alveolus
- A tiny air sac in the lung where oxygen and carbon dioxide are exchanged with the blood.
- Partial pressure
- The pressure contributed by one gas in a mixture, which helps determine the direction of gas diffusion.
- Diffusion
- The net movement of particles from an area of higher concentration or partial pressure to an area of lower concentration or partial pressure.
- Hemoglobin
- An iron-containing protein in red blood cells that binds and transports oxygen.
- Oxygen dissociation curve
- A graph showing how hemoglobin oxygen saturation changes as the partial pressure of oxygen changes.
Common Mistakes to Avoid
- Saying oxygen is actively pumped into the blood, which is wrong because oxygen crosses the alveolar membrane by diffusion down its partial pressure gradient.
- Treating oxygen and carbon dioxide as moving in the same direction, which is wrong because oxygen moves into the blood while carbon dioxide moves into the alveolus.
- Ignoring membrane thickness, which is wrong because a thicker respiratory membrane slows diffusion and can reduce gas exchange efficiency.
- Thinking hemoglobin is only for storage, which is wrong because hemoglobin keeps dissolved oxygen levels low enough to maintain diffusion into the blood and then releases oxygen in tissues.
Practice Questions
- 1 In an alveolus, PO2 is 100 mmHg and the incoming capillary blood PO2 is 40 mmHg. What is the oxygen partial pressure gradient, and which direction will oxygen move?
- 2 A student compares two respiratory membranes. Membrane A has thickness 0.5 micrometers, and Membrane B has thickness 1.0 micrometers, with the same surface area and gradient. Using rate proportional to 1 / thickness, how does the diffusion rate of A compare with B?
- 3 Explain why exercise can increase oxygen unloading from hemoglobin in active muscles even though the lungs are still loading hemoglobin with oxygen.