The circulatory system exists to carry oxygenated blood from the lungs to the body tissues, via red blood cells (and the hemoglobin on the RBCs). The blood also carries nutrients, water, and wastes around the body.

Airborne lead is in the form of aerosol particles. These may come from decaying leaded paints, leaded gasoline (not so much a problem these days), industrial ore smelters, and coal-fired power plants (the lead is in the coal). The way lead usually gets into the body is not usually from directly breathing in particles (although it's certainly possible for lead to leach from inhaled particles into the blood), but by the lead particles settling out into the soil or onto plants, which allows the lead to get into the food or water supply.

Carbon monoxide (CO) displaces oxygen on red blood cells, causing symptoms of oxygen deprivation. The next screen has more detailed information about CO in the blood.