What is the liquid portion of blood known as?

The liquid portion of blood is known as plasma. Plasma is a yellowish fluid that constitutes about 55% of total blood volume and serves several critical functions. It acts as a transport medium for glucose, proteins, hormones, and waste products, facilitating their movement throughout the body. Additionally, plasma contains electrolytes, antibodies, and other substances essential for maintaining homeostasis and supporting the immune system.

Serum is often confused with plasma, but while serum is essentially plasma from which clotting factors have been removed, it does not contain fibrinogen and some other proteins found in plasma. The term "fluid" is too vague and does not specifically refer to the components of blood. Hemoglobin is a protein found in red blood cells that is responsible for transporting oxygen but is not a liquid portion of blood itself. Thus, plasma is the accurate answer as it specifically refers to the liquid portion that carries various components crucial for bodily functions.