SQA National 5 Biology Practice Exam 2025 – The Complete Study Guide for Exam Success!

Erythrocytes, or red blood cells, are primarily responsible for transporting oxygen in the blood. They contain a protein called hemoglobin, which binds to oxygen molecules in the lungs and carries them to tissues throughout the body. The structure of erythrocytes allows for a large surface area for gas exchange, facilitating efficient oxygen transport.

In contrast, leukocytes, or white blood cells, are part of the immune system and primarily function to fight infections and protect the body against foreign invaders. Platelets are small cell fragments involved in blood clotting and play a crucial role in wound healing, but they do not have a role in oxygen transport. Plasma, the liquid component of blood, carries nutrients, hormones, and waste products but does not transport oxygen in the same specific and efficient manner as erythrocytes. Thus, erythrocytes stand out as the key component of blood responsible for delivering oxygen to the body.