Final answer:
Plasma is primarily made up of water, accounting for about 92 percent of its composition. The rest includes plasma proteins, electrolytes, and other dissolved substances essential for bodily functions.
Step-by-step explanation:
The substance that plasma is mostly composed of is water. Plasma, which is the liquid component of human blood, comprises about 55 percent of blood by volume. It is vital to understand that plasma is approximately 92 percent water.
The remainder of plasma consists of dissolved substances such as plasma proteins, electrolytes like sodium (Na), potassium (K), and calcium (Ca) ions, as well as gases, nutrients, hormones, and wastes.
These nonprotein solutes, including glucose, amino acids, fatty acids, urea, cholesterol, and phosphates, play key roles in various physiological functions. It is primarily the water in plasma that provides the medium for these substances to be transported throughout the body.