Composition of Blood

Whole blood consists of blood plasma and formed elements. Blood plasma, a yellowish fluid, comprises 55% of total blood volume and is predominantly water with dissolved electrolytes and proteins. Formed elements—red blood cells, white blood cells, and platelets—make up the remaining 45% of blood volume. Together, these components work to transport oxygen, nutrients, hormones, and immune cells throughout the body.

The hematocrit is the percentage of red blood cells, or erythrocytes, in the total volume of a blood sample. Since erythrocytes comprise about 99% of all formed elements, the hematocrit essentially measures the proportion of red blood cells relative to plasma. This measurement is clinically important because it indicates oxygen-carrying capacity and can reveal anemia or other blood disorders.

Red blood cells contain hemoglobin, a specialized protein that binds and transports oxygen and carbon dioxide between the lungs and tissues. Mature erythrocytes are anucleated, meaning they lack a nucleus, which allows them to carry more hemoglobin and maximize oxygen transport. This efficient gas exchange is essential for delivering oxygen to all body tissues and removing metabolic waste.

White blood cells, or leukocytes, are nucleated cells involved in immune responses and defense against infections. Unlike red blood cells, leukocytes retain their nucleus, which enables them to perform complex immune functions. These cells circulate in blood plasma and work to identify and eliminate pathogens, protecting the body from disease.

Platelets are small, enucleated cell fragments essential for hemostasis—the process of stopping bleeding. They initiate clotting when blood vessels are damaged and help repair vessel walls to prevent blood loss. Though platelets lack a nucleus, they contain specialized proteins and organelles that enable them to respond rapidly to vascular injury.

Blood plasma carries numerous vital substances dissolved in water, including electrolytes, proteins, nutrients, enzymes, antibodies, hormones, and gases like oxygen and carbon dioxide. This yellowish fluid acts as a transport medium, delivering these essential molecules to tissues and removing metabolic waste products. The composition of plasma makes it crucial for maintaining homeostasis and supporting all physiological functions.

Mature red blood cells are anucleated to maximize their hemoglobin content and oxygen-carrying capacity, making them highly efficient at gas transport. White blood cells retain their nucleus because they require it to perform complex immune functions, including pathogen recognition and elimination. This structural difference reflects each cell type's specialized role in blood function.