What Is Blood?

Blood is a specialized connective tissue and biological fluid that circulates continuously through the cardiovascular system, transporting oxygen, nutrients, hormones, and metabolic waste products between organ systems. In an average adult human, approximately five liters of blood are in constant motion, driven by the pumping action of the heart through a closed network of arteries, capillaries, and veins. The study of blood and its properties is central to physiology, hematology, and biomedical engineering, where understanding blood behavior informs the design of diagnostic devices, therapeutic equipment, and implantable materials.

Blood consists of two primary fractions: a liquid plasma phase and a cellular phase. Plasma, which accounts for roughly 55 percent of total blood volume, is an aqueous solution of proteins (primarily albumin, globulins, and fibrinogen), electrolytes, glucose, and dissolved gases. The remaining volume comprises formed elements: red blood cells (erythrocytes), white blood cells (leukocytes), and platelets (thrombocytes). This composite structure gives blood both its rheological complexity and its capacity to perform multiple physiological roles simultaneously.

Composition and Cellular Components

Red blood cells are the most numerous formed element, numbering approximately 25 trillion in the body. Their primary function is oxygen transport via the iron-containing protein hemoglobin, which binds oxygen in the lungs and releases it in peripheral tissues. White blood cells, present in far smaller quantities, constitute the cellular arm of the immune response, identifying and neutralizing pathogens. Platelets, the smallest cellular component, are produced from large precursor cells called megakaryocytes and initiate the clotting cascade when vascular injury occurs. IEEE Transactions on Biomedical Engineering regularly publishes research on the behavior of these cellular components under flow, under centrifugal separation, and in contact with artificial surfaces.

Rheological Properties

Blood does not behave as a simple Newtonian fluid. At low shear rates, red blood cells stack into cylindrical aggregates called rouleaux, increasing apparent viscosity. At high shear rates, as occur in large arteries, cells align with the flow and viscosity decreases. This shear-thinning behavior, combined with the elastic deformability of individual red blood cells (which must squeeze through capillaries narrower than their resting diameter), makes blood a non-Newtonian viscoelastic fluid. These properties are clinically significant: conditions such as sickle cell disease alter cell deformability, and the resulting change in flow dynamics contributes to vascular occlusion. Biomedical engineers must account for these rheological characteristics when designing blood-contacting devices such as ventricular assist devices, dialysis membranes, and oxygenators. The American Physiological Society supports interdisciplinary education that connects these principles across engineering and medicine.

Blood-Material Interactions

When blood contacts a foreign material surface, a rapid and ordered response occurs: plasma proteins adsorb within seconds, followed by platelet adhesion and activation, and then engagement of the coagulation cascade. This sequence, well characterized in the biomaterials literature, determines whether an implanted device will be tolerated or will trigger thrombosis. Surface chemistry, roughness, and charge all influence which proteins adsorb preferentially, and therefore which downstream cellular events follow. Research published in PMC documents how proteins, platelets, and coagulation interact at biomaterial interfaces, providing a foundation for surface modification strategies used in vascular grafts, stents, and biosensors. Understanding this cascade at the molecular level is essential for the rational design of hemocompatible materials.

Applications

Blood, as an object of engineering study, has applications in a wide range of fields, including:

• Biomedical device design (dialysis equipment, ventricular assist devices, oxygenators)
• Point-of-care diagnostics and clinical laboratory instrumentation
• Transfusion medicine and blood banking technology
• Computational fluid dynamics modeling of cardiovascular pathologies
• Wearable biosensors for continuous monitoring of blood analytes