Peripheral blood is the fluid connective tissue circulating throughout the body within the vascular system. This is distinct from the blood-forming tissues located inside bone marrow. An average adult has a volume of roughly 5 liters, which accounts for about 7% of the human body weight. This circulating fluid is a complex mixture responsible for transporting necessary substances to and from cells.
Composition of Peripheral Blood
Plasma
The largest component of peripheral blood by volume, at about 55%, is plasma. This straw-yellow liquid is the medium in which all other blood components are suspended. Water makes up approximately 92% of plasma, with the remainder consisting of a complex mix of proteins, electrolytes, hormones, and dissolved nutrients. Important proteins include albumin, which helps maintain osmotic pressure; globulins, which include antibodies for immune defense; and fibrinogen, a precursor to clot-forming fibers.
Red Blood Cells (Erythrocytes)
Constituting about 45% of whole blood volume, red blood cells are the most numerous cells in circulation. These cells are unique in that they are non-nucleated, biconcave discs, a shape that maximizes their surface area for gas exchange. Their primary role is oxygen transport, made possible by the iron-containing protein hemoglobin. Hemoglobin binds to oxygen in the lungs and releases it to tissues throughout the body, giving oxygenated blood its characteristic bright red color.
White Blood Cells (Leukocytes)
White blood cells are a diverse group of cells that are part of the immune system, making up only about 0.7% of total blood volume. They are categorized into two main groups: granulocytes and agranular leukocytes. Granulocytes, which include neutrophils, eosinophils, and basophils, are involved in responses to bacterial infections and allergic reactions. Agranular leukocytes include lymphocytes, which orchestrate targeted immune responses, and monocytes, which can develop into macrophages that engulf cellular debris and pathogens.
Platelets (Thrombocytes)
Platelets are not complete cells but rather small, irregular fragments of larger cells originating in the bone marrow. They are suspended in the plasma alongside red and white blood cells. Their main function is to initiate the blood clotting process, known as hemostasis. When a blood vessel is damaged, platelets adhere to the site of injury and form a temporary plug, while also releasing factors that start the cascade of reactions leading to a stable clot.
Core Functions in the Body
The components of peripheral blood work in concert to perform several functions. The primary roles of transport, immune defense, and clotting are carried out simultaneously as blood circulates. Plasma carries nutrients and hormones, red blood cells transport oxygen, white blood cells patrol for pathogens, and platelets initiate clotting at injury sites. This integrated system ensures that tissues receive necessary supplies while being protected from threats and blood loss.
Diagnostic Use of Peripheral Blood
Because of its systemic circulation, peripheral blood is one of the most valuable tissues for diagnostic purposes. A small sample can provide a wealth of information about a person’s overall health. The analysis of this blood is a tool for detecting and monitoring a wide range of conditions.
A common and informative test is the complete blood count, or CBC. This automated test quantifies the major components of blood, including the number of red blood cells, white blood cells, and platelets. The CBC also measures hemoglobin concentration and hematocrit, which is the proportion of blood volume occupied by red cells. Abnormalities in these counts can signal various issues; for example, a low red blood cell count may indicate anemia, while an unusually high white blood cell count could suggest an underlying infection.
For a more detailed assessment, a peripheral blood smear is performed. In this procedure, a drop of blood is spread thinly on a microscope slide, stained, and examined by a laboratory professional. This analysis provides information that a CBC cannot, such as the size, shape, and specific features of individual blood cells. It is useful for identifying malformed red blood cells, as seen in sickle cell anemia, or detecting immature or abnormal cells that may indicate leukemia or other hematological disorders.
Role in Medical Treatments and Donation
Peripheral blood also plays a direct role in various medical treatments. The most well-known application is blood transfusion, where a patient receives whole blood or specific components. Transfusions of packed red blood cells are administered to treat severe anemia, while platelet transfusions are given to patients with clotting deficiencies to prevent or stop bleeding.
A more specialized application involves the collection of peripheral blood stem cells (PBSCs) for transplantation. These are the same stem cells found in bone marrow but can be encouraged to move into the circulating blood with medication. A donor’s blood is drawn and passed through a machine using a procedure called apheresis. This machine separates out the target cells, and the remaining blood is returned to the donor.
These collected PBSCs are used to treat certain cancers, such as leukemia and lymphoma, as well as other bone marrow diseases. After a patient undergoes high-dose chemotherapy or radiation to destroy cancerous cells, the transplanted stem cells are infused into their bloodstream. These healthy stem cells then travel to the bone marrow and begin to produce new blood cells, rebuilding the patient’s blood and immune system.