Final answer:
Blood is part of the extracellular fluid and blood plasma, its fluid component, contains the highest concentration of the sodium ion.
Step-by-step explanation:
Blood is part of the extracellular fluid category, which specifically refers to the fluid outside of cells. Within extracellular fluid, there are various subtypes including blood plasma, interstitial fluid, and lymph. Blood plasma is the fluid component of blood and it plays a significant role in transporting nutrients, gases, and wastes, as well as defending the body and maintaining homeostasis. It is composed mainly of water, plasma proteins such as albumin, globulins, fibrinogen, and other dissolved solutes like glucose, electrolytes, and lipids. The sodium ion is predominant within the extracellular fluid, particularly in blood plasma.