Blood: physiology, roles in oxygen transport, hemostasis, immune defense, and key disorders affecting hematology

Blood is a specialized connective tissue that circulates through the cardiovascular system, enabling transport of gases, nutrients, and waste products while coordinating hemostasis and immune surveillance. It consists of plasma (the liquid portion) and formed elements, including red blood cells (RBCs), white blood cells (WBCs), and platelets. In clinical medicine, “blood” is not only a laboratory specimen but also an active organ system whose composition and rheology influence oxygen delivery, coagulation balance, inflammation, and host defense.

The primary physiologic function of blood is oxygen transport. RBCs contain hemoglobin, a metalloprotein that binds oxygen in the lungs and releases it in peripheral tissues according to oxygen partial pressure, pH, and temperature (the Bohr effect). Hemoglobin also carries a portion of carbon dioxide indirectly via bicarbonate buffering in plasma and directly through carbaminohemoglobin. Blood flow and microvascular perfusion determine how effectively oxygen is delivered, and dysregulation can present clinically as dyspnea, fatigue, or ischemic symptoms.

Plasma is predominantly water with dissolved proteins, electrolytes, metabolites, hormones, and lipids. Key plasma proteins include albumin, which maintains oncotic pressure; immunoglobulins, which mediate humoral immunity; and clotting factors, which are essential components of the coagulation cascade. The coagulation system maintains a delicate equilibrium between bleeding prevention and controlled dissolution of clots. Platelets are small anucleate cell fragments derived from megakaryocytes. When vascular injury exposes subendothelial matrix, platelets adhere, become activated, and aggregate via receptors such as glycoprotein IIb/IIIa, forming the initial platelet plug.

Coagulation proceeds via intrinsic and extrinsic pathways that converge on thrombin generation. Thrombin converts fibrinogen into fibrin, stabilizing the clot. Fibrinolysis then removes clots through plasmin, which is regulated by plasminogen activators and inhibitors such as plasminogen activator inhibitor-1 (PAI-1). When these systems fail—either toward hypercoagulability or bleeding—patients may develop thromboembolism (e.g., deep venous thrombosis, pulmonary embolism) or hemorrhagic disorders (e.g., hemophilia, von Willebrand disease). Laboratory evaluation often includes platelet count, prothrombin time (PT/INR), activated partial thromboplastin time (aPTT), fibrinogen, and D-dimer depending on clinical context.

Blood is also central to immune defense. WBCs include neutrophils, lymphocytes, monocytes/macrophages, eosinophils, and basophils. Neutrophils provide rapid innate immune responses through phagocytosis and production of reactive oxygen species and antimicrobial peptides. Monocytes migrate into tissues and differentiate into macrophages and dendritic cells, driving antigen presentation and cytokine signaling. Lymphocytes coordinate adaptive immunity: B cells produce antibodies, while T cells provide cell-mediated responses including cytotoxic killing and cytokine regulation. Dysfunctions in WBC number or function can increase susceptibility to infection, drive chronic inflammation, or contribute to malignancies such as leukemias and lymphomas.

Hematologic disorders span abnormalities in RBCs, WBCs, platelets, and clotting factors. Anemia commonly results from decreased RBC production, increased destruction, or blood loss. Clinical classification includes iron deficiency anemia, anemia of chronic disease, hemolytic anemias, and megaloblastic anemia due to vitamin B12 or folate deficiency. Symptoms often reflect reduced oxygen delivery and can include pallor, tachycardia, dizziness, and exercise intolerance; severe cases may cause syncope or cardiac stress.

Thrombocytopenia (low platelet count) may lead to petechiae, mucosal bleeding, or easy bruising and can be caused by decreased production, increased destruction (immune thrombocytopenia), or consumption (disseminated intravascular coagulation). Conversely, thrombocytosis (high platelet count) can be reactive (inflammation, iron deficiency) or clonal (myeloproliferative neoplasms), sometimes increasing thrombotic risk.

Leukocytosis and leukopenia reflect changes in WBC number that can arise from infection, stress responses, medications, autoimmune disease, or bone marrow disorders. Hematologic malignancies often present with persistent cytopenias, abnormal cell morphology, “B symptoms” (fever, night sweats, weight loss), lymphadenopathy, or unexplained bruising and infections.

From a diagnostic perspective, “blood” evaluation integrates complete blood count (CBC), peripheral smear, reticulocyte count, iron studies, coagulation testing, inflammatory markers, and targeted hematologic workup when indicated. Bone marrow examination and molecular testing may be required to distinguish benign reactive processes from clonal disorders. Management is condition-specific: iron supplementation for iron deficiency, transfusion or disease-directed therapy for severe anemia, immunomodulation for immune-mediated cytopenias, and anticoagulation or hemostatic agents depending on clotting status.

In summary, blood is a multifunctional tissue: it transports oxygen via hemoglobin, maintains oncotic and protein-mediated plasma functions, orchestrates coagulation through platelet activation and fibrin formation, and enables innate and adaptive immunity through diverse leukocyte populations. Understanding blood physiology provides the foundation for interpreting common laboratory abnormalities and for diagnosing and treating hematologic and hemostatic diseases.

Source: [@boopaonsol] (original post: “GM Blood”)