What does haemoglobin in red blood cells primarily carry?

Haemoglobin in red blood cells primarily carries oxygen, which is crucial for the survival of cells and the overall functioning of the body. Each haemoglobin molecule can bind to four oxygen molecules, allowing for efficient transport from the lungs, where oxygen is inhaled, to the tissues and organs that require it for cellular respiration. This process plays a vital role in maintaining the body’s metabolic functions and energy production.

In the context of the other options, nitrogen is a gas that is not effectively transported by haemoglobin in the blood, and while water is essential for various biological processes, it is not carried by haemoglobin. Glucose is a simple sugar that serves as an energy source for cells but is transported in the bloodstream in a dissolved state rather than being carried by haemoglobin. Therefore, the primary function of haemoglobin is its oxygen-carrying capacity, which is fundamental to the body's respiratory and circulatory systems.