Final answer:
Plasma is the liquid part of blood and makes up approximately 55% of blood's volume. It is predominantly water that carries various substances and is critical for multiple physiological functions.
Step-by-step explanation:
When considering the composition of blood, plasma is a major component which plays a critical role in its function. Plasma is the liquid part of blood and is vital for transporting various substances throughout the body. It carries water, electrolytes, nutrients, hormones, and waste products, as well as being the medium in which blood cells are suspended.
Blood plasma is primarily water, which accounts for about 92%, and contains multiple elements dissolved within, including a significant amount of proteins. These proteins serve various functions like maintaining osmotic balance, providing immunity, and serving as carriers for certain molecules. Although there are numerous dissolved substances, most are present in very small quantities.
When we centrifuge blood, as shown in Figure 17.5.2, plasma separates from the other components due to its lower density (such as leukocytes, platelets, and erythrocytes). This process visually demonstrates that plasma indeed makes up a substantial proportion of blood volume. Answering the students' question, the percentage of blood made up of plasma is approximately 55%, which corresponds to option (1). Therefore, the correct response is that plasma constitutes about 55% of the blood's volume.