What does oxygen primarily bind to during transport within the bloodstream?

Oxygen primarily binds to hemoglobin during its transport within the bloodstream. Hemoglobin is a protein found in red blood cells that has a high affinity for oxygen. Each hemoglobin molecule can bind up to four oxygen molecules at once, allowing for efficient oxygen transport from the lungs to the tissues throughout the body.

In addition to oxygen transport, hemoglobin also plays a role in the buffering of blood pH and facilitates the transport of carbon dioxide in the reverse process. Myoglobin, while it does bind oxygen, primarily serves to store oxygen in muscle tissues rather than transport it in the bloodstream. Serum albumin does assist in the transport of various molecules, but its role does not extend significantly to oxygen binding. Collagen is a structural protein and does not have a role in gas transport. Therefore, hemoglobin is the key player in oxygen transport in the circulatory system.