Ventilation, Perfusion & V/Q

Inside one air sac, where air meets blood · Week 12
Gas exchange needs two things to line up at the air sac. Ventilation (V) is air getting in. Perfusion (Q) is blood flowing past. When one of them is missing, you have a V/Q mismatch. In a shunt there is blood but no air. In dead space there is air but no blood. Click each one to look inside the air sac, the alveolus.

One air sac and its capillary

Air comes down the airway. Blood flows through the capillary. Watch to see if the oxygen (red) really loads onto the blood.
Pick a state to see if gas exchange works.
Pick a stateNormal · Shunt · Dead space

👆 What's the matchup?

What's happening

The three parts of respiration

Ventilation

moving air in & out of the air sacs. Blocked by asthma, COPD, OSA, CF.

Diffusion

gas crossing the alveolar-capillary wall, the thin wall between air and blood. Blocked when the sacs fill with fluid, as in pneumonia, edema, ARDS.

Perfusion

blood flowing past the air sacs. Blocked by a pulmonary embolism or pulmonary HTN.
Memory trick: Shunt keeps the blood but Stops the air. That is perfusion without ventilation. Dead space keeps the air but Drops the blood. That is ventilation without perfusion.