What substance is primarily responsible for carrying oxygen in the blood?

Hemoglobin is the primary substance responsible for carrying oxygen in the blood. It is a protein found in red blood cells (erythrocytes) and has a unique ability to bind oxygen in the lungs, where oxygen concentration is high, and release it in tissues where oxygen is needed. Each hemoglobin molecule can bind up to four oxygen molecules, making it an efficient transport system for oxygen throughout the body.

In contrast, myoglobin, which is mentioned in the first option, is primarily found in muscle tissues and serves as a storage form of oxygen to supply muscles during strenuous exercise. Platelets are crucial for blood clotting and do not play a role in transporting oxygen. Plasma proteins, which include albumins and globulins, serve various functions such as maintaining osmotic pressure and immune responses, but they are not involved in oxygen transport. Thus, hemoglobin stands out as the key player in oxygen transport within the bloodstream.