What is the fluid part of the blood called?

The fluid part of the blood is referred to as plasma. Plasma is a yellowish liquid that makes up about 55% of total blood volume and is primarily composed of water, electrolytes, proteins, hormones, and waste products. It serves several critical functions, including transporting nutrients, hormones, and proteins to the parts of the body that need it, and also playing a vital role in maintaining blood pressure and volume.

Serum, while it is a fluid, is actually derived from plasma but does not contain clotting factors. It is obtained after the blood has clotted and is the liquid that remains. Hemolymph is a fluid found in invertebrates with an open circulatory system and is not relevant to human blood. Lymph is a clear fluid that circulates in the lymphatic system and is involved in immune response, but it is distinct from blood and its components. Thus, plasma is the correct answer, as it directly pertains to the composition and function of blood in the human body.