Final answer:
Approximately 92 percent of plasma is composed of water, with the rest being proteins and other dissolved substances essential for the body's functions.
Step-by-step explanation:
Plasma, the liquid component of human blood, is comprised mostly of water. To answer the question, what percentage of plasma is composed of water—it is approximately 92 percent. The remaining volume includes crucial substances like plasma proteins (e.g., albumin and globulins), glucose, amino acids, fatty acids, urea, cholesterol, inorganic ions (like sodium, potassium, and chloride), and other solutes. While plasma makes up more than half of the blood's volume, proteins only constitute about 7 percent of plasma. The high water content in plasma is vital as it allows for the transport and distribution of nutrients, hormones, and waste products throughout the body.