What substance in the blood has low concentrations of oxygen and high concentrations of carbon dioxide as it travels next to the alveoli?

The correct answer identifies deoxygenated blood as the substance in the bloodstream that has low concentrations of oxygen and high concentrations of carbon dioxide when it is near the alveoli in the lungs. This blood is typically returning from the body's tissues, where oxygen has been delivered and used for cellular respiration, producing carbon dioxide as a waste product.

When deoxygenated blood arrives at the alveoli, it interacts with the air in the alveolar sacs, which has a higher concentration of oxygen and a lower concentration of carbon dioxide compared to the blood. This sets up a diffusion gradient, driving oxygen into the blood and carbon dioxide out of the blood to be exhaled.

In contrast, red blood cells carry both oxygen and carbon dioxide, but it is the state of the blood (whether it is oxygenated or deoxygenated) that specifically describes its concentration levels of these gases. Oxygenated blood, which flows away from the lungs, is rich in oxygen and low in carbon dioxide. Plasma, while it can transport gases, does not typically serve as a distinguishing factor in describing the gas concentrations compared to the entirety of blood flow dynamics. Thus, deoxygenated blood is the best describing term for the conditions presented near the alveoli.