Peripheral blood is the blood circulating throughout the body’s network of arteries, veins, and capillaries. This fluid tissue acts as the primary transport system, delivering oxygen and nutrients to tissues while carrying away metabolic waste products. Unlike blood contained within specialized organs, peripheral blood represents the mature cells actively performing their functions in circulation. Because it is easily accessible, a sample of peripheral blood is the most common biological fluid collected and analyzed clinically.
The Primary Components of Peripheral Blood
Peripheral blood is a complex fluid composed of two main parts: a liquid matrix called plasma and the suspended cellular components known as formed elements. Plasma makes up approximately 55% of the total blood volume and is primarily water, containing dissolved proteins, electrolytes, hormones, and clotting factors like fibrinogen. This liquid environment facilitates the transport of substances and helps maintain the body’s fluid balance and temperature regulation.
The formed elements account for the remaining 45% of the blood volume and include three distinct types of cells or cell fragments. Erythrocytes, or red blood cells, are the most numerous, making up the majority of the cellular fraction. These biconcave discs contain the iron-rich protein hemoglobin, which binds and transports oxygen from the lungs to all other tissues.
Leukocytes, or white blood cells, are far less numerous but function as the core of the immune system. They are broadly categorized into granulocytes and agranulocytes, each with specialized roles in defense. Granulocytes, which include neutrophils, eosinophils, and basophils, contain distinct granules in their cytoplasm. These granules release chemicals to fight infection or mediate inflammation.
Agranulocytes consist of lymphocytes and monocytes. Lymphocytes play a targeted role in adaptive immunity, while monocytes differentiate into macrophages that engulf cellular debris and pathogens.
The third formed element is the thrombocyte, commonly known as the platelet. Platelets are fragments of larger cells that originate from megakaryocytes in the bone marrow. They are essential for hemostasis, the process of stopping bleeding, by sticking to the site of an injury and aggregating to form a plug that initiates blood clot formation.
Peripheral Blood Versus Bone Marrow Blood
The difference between peripheral blood and bone marrow blood lies in their function and cellular maturity. Bone marrow is the soft, spongy tissue inside certain bones and serves as the body’s factory for blood cell production, a process called hematopoiesis. This is where hematopoietic stem cells (HSCs) reside, continually dividing and maturing into all blood cell types.
Bone marrow blood contains a mix of stem cells, progenitor cells, and immature precursors, such as erythroblasts or myeloblasts, which are not typically found in high numbers in circulation. Once blood cells complete maturation, they are released from the marrow into the bloodstream to become the mature, functional components of the peripheral blood. The presence of immature cells in peripheral blood often signals an underlying disease process in the bone marrow.
Under normal conditions, hematopoietic stem cells remain confined to the bone marrow. However, these cells can be chemically induced to “mobilize” or move into the peripheral circulation using specific growth factors, such as granulocyte-colony stimulating factor (G-CSF). This technique is important for certain medical procedures, allowing for the collection of stem cells directly from the circulating peripheral blood rather than through a more invasive bone marrow harvest.
Medical Uses and Diagnostic Significance
Peripheral blood is the most frequently analyzed specimen in healthcare because it provides a non-invasive window into the body’s physiological status and immune function. The primary diagnostic test performed on a peripheral blood sample is the Complete Blood Count (CBC). The CBC quantifies the circulating levels of red blood cells, white blood cells, and platelets, providing data that can indicate conditions like anemia, infection, or a bleeding disorder.
A CBC is often complemented by a peripheral blood smear, where a drop of blood is spread thinly on a slide and examined under a microscope. This visual inspection allows a hematologist to assess the morphology—the size, shape, and structure—of the cells, which automated machines cannot fully evaluate. Abnormal cell shapes, like the sickle shape seen in sickle cell disease, or the presence of blast cells indicative of leukemia, provide specific diagnostic clues.
In addition to diagnostics, peripheral blood is used for therapeutic purposes, most notably in Peripheral Blood Stem Cell (PBSC) donation and transplantation. After a donor is treated with mobilizing agents to push stem cells into the circulation, the blood is collected through apheresis. This procedure separates and harvests the circulating stem cells, which are then infused into a patient to restore their blood-producing capacity following high-dose chemotherapy. The use of PBSCs for transplantation has become increasingly common due to the less invasive nature compared to traditional bone marrow aspiration.