The statement that oxygen is mostly carried by a pigment in our blood, while carbon dioxide is transported in dissolved form, is accurate and can be explained by the following two reasons:

1. Haemoglobin’s Role in Oxygen Transport:
Haemoglobin is the pigment responsible for carrying oxygen in our blood. It is a protein found in red blood cells, and each haemoglobin molecule can bind to four oxygen molecules. This allows for efficient transport of oxygen from the lungs (where oxygen is in high concentration) to tissues and organs throughout the body. The oxygen-haemoglobin complex gives blood its characteristic red colour when oxygenated.

2. Carbon Dioxide’s Solubility in Blood Plasma:
While a small portion of carbon dioxide is transported by haemoglobin (as carbaminohaemoglobin), the majority of carbon dioxide is transported in dissolved form in the blood plasma. Carbon dioxide is more soluble in water than oxygen, and blood plasma contains water. As a result, carbon dioxide can dissolve directly into the plasma and be carried in the form of dissolved carbon dioxide or as bicarbonate ions (HCO3-). This allows for efficient removal of carbon dioxide from tissues to the lungs, where it can be exhaled.

In summary, we can say, haemoglobin in red blood cells serves as the primary carrier of oxygen, forming an oxygen-haemoglobin complex. Meanwhile, carbon dioxide is transported mainly in dissolved form in the blood plasma, facilitated by its solubility in water. These mechanisms ensure the effective exchange of gases (oxygen and carbon dioxide) between the lungs and tissues in the body.