22.3
Composition of Blood
The blood in our bodies comprises three major components: blood plasma, formed elements, and the extracellular matrix. Blood plasma is a yellowish flu…
Whole blood consists of the blood plasma, a fluid extracellular matrix, and formed elements, including cells and cell fragments.
Blood plasma, which makes up about 55% of total blood volume, is a yellowish fluid. It is predominantly water dissolved with vital substances, such as electrolytes and proteins.
It also acts as a medium for carrying nutrients, enzymes, antibodies, hormones, gasses, and blood cells.
Red blood cells, white blood cells, and platelets, collectively known as the formed elements, make up 45% of the blood's volume.
The red blood cells or erythrocytes comprise about 99% of the formed elements; their percentage in the total volume of a blood sample is known as the hematocrit.
The mature erythrocytes are anucleated and contain hemoglobin — a specialized protein that binds and transports oxygen and carbon dioxide between lungs and tissues.
The white blood cells or leukocytes are a group of nucleated cells involved in immune responses.
Lastly, platelets are enucleated cell fragments essential for clotting and repairing damaged blood vessels.
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Q1: What are the main components that make up whole 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.
Q2: What is the hematocrit and why is it clinically important?
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.
Q3: How do red blood cells transport oxygen and carbon dioxide?
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.
Q4: What role do white blood cells play in the body?
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.
Q5: What functions do platelets serve in blood?
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.
Q6: What substances does blood plasma carry besides cells?
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.
Q7: Why are mature red blood cells anucleated while white blood cells retain a nucleus?
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.