What is the pale yellow liquid portion of blood called?

The pale yellow liquid portion of blood is called plasma.

Plasma is the fluid medium that carries red blood cells, white blood cells, and platelets through the circulatory system. It makes up roughly 55% of whole blood and consists mostly of water, along with proteins, electrolytes, nutrients, hormones, gases, and waste products.

Its yellow or light amber color mainly comes from dissolved substances such as bilirubin and carotenoids. Plasma proteins have vital jobs: albumin helps maintain the blood’s osmotic balance, antibodies support immunity, and fibrinogen participates in clot formation. Plasma also transports carbon dioxide and helps distribute heat around the body.

Plasma is not the same as serum. Serum is the liquid left after blood has clotted, so it lacks many of the clotting factors present in plasma. In a laboratory, plasma can be separated from blood by adding an anticoagulant and spinning the sample in a centrifuge. Donated plasma is also used in transfusion medicine and in therapies for certain bleeding or immune-related conditions.