- The fluid component of circulating blood is known as plasma.
- Plasma is the yellowish, straw-colored liquid that makes up about 55% of total blood volume.
- It serves as the medium in which the blood cells (red blood cells, white blood cells, and platelets) are suspended and transported throughout the body.
- Plasma contains water (about 90-92%), proteins (such as albumin, globulins, and fibrinogen), electrolytes, nutrients, hormones, and waste products.

- In contrast, serum is the fluid obtained after blood has clotted and is devoid of clotting factors like fibrinogen.
- Interstitial fluid is the fluid that surrounds tissue cells but is not within the blood vessels.
- Lymph is a similar fluid but found in the lymphatic system, not in circulating blood.

Therefore, the correct answer is plasma, as it is the main fluid component of circulating blood.

Reference: Ganong's Review of Medical Physiology, 26th Edition, Chapter 21: Blood