In the healthy lung the airsacs look like a bunch of grapes. Ventilation is the movement of air in and out of these airsacs. Each airsac is surrounded by blood vessels. Perfusion is the movement of blood through these vessels. The area where the airsacs and blood vessels meet is where the exchange of oxygen and carbon dioxide occur. Air is inhaled. The oxygen moves from the airsacs of the lung to the blood. It then travels to the cells of the body. Carbon dioxide travels from the cells of the body to the airsacs of the lung through the blood.
The term COPD is an abbreviation for Chronic Obstructive Pulmonary Disease. COPD is a general term used to describe the chronic lung disease linked with cigarette smoking. When the lungs are affected by COPD there is a relatively irreversible decrease in airflow and permanent destruction of the airsacs in the lung. Over time this creates some areas where there is a blood supply without any airsac. This is considered a ventilation-perfusion mismatch.
When airsacs of the lung are destroyed, there is less surface area for oxygen to get from the lungs into the blood and for carbon dioxide to get from the blood into the lungs to be exhaled. This can reach a point where the amount of oxygen in the blood is low. This is called hypoxemia. This can also reach a point where the amount of carbon dioxide in the blood is high. This is called hypercarbia or hypercapnia.