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Blood is a liquid connective tissue consisting of plasma (~55% by volume) and formed elements (~45%). Plasma is approximately 90% water and carries dissolved proteins (albumin maintains osmotic pressure; clotting factors; immunoglobulins), nutrients, hormones, waste products, and dissolved gases. Erythrocytes (red blood cells) contain hemoglobin for O2 and CO2 transport and lack nuclei in their mature form. Leukocytes (white blood cells) — neutrophils, lymphocytes, monocytes, eosinophils, basophils — constitute the cellular arm of immunity. Platelets (cell fragments from megakaryocytes) initiate hemostasis. All formed elements arise continuously from hematopoietic stem cells in red bone marrow.
Study a labeled blood smear, identifying each formed element by size and morphology. Then link each to its function: erythrocyte → O2/CO2 transport; neutrophil → acute bacterial phagocytosis; lymphocyte → specific immunity; monocyte → tissue macrophage precursor; eosinophil → parasite defense and allergy; basophil → inflammatory mediator release; platelet → clotting initiation.
From your study of the cardiovascular system, you know that blood circulates continuously through the heart and vessels. But blood is more than a carrier fluid — it is a complex liquid tissue performing gas exchange, immune surveillance, nutrient delivery, waste removal, pH buffering, and hemostasis simultaneously. Understanding what blood is made of is the foundation for understanding how all of these functions are coordinated.
Blood is roughly 55% plasma — the liquid matrix — and 45% formed elements (cells and cell fragments). Plasma is mostly water (~90%), but the dissolved proteins are what give it distinct physiological power. Albumin maintains the osmotic pressure that keeps fluid inside blood vessels; when albumin is low, water leaks into tissues and edema results. Immunoglobulins (antibodies) circulate in plasma as part of the adaptive immune response. Clotting factors (fibrinogen, prothrombin, and others) are always present in inactive form, ready to cascade into a clot when vessel damage is detected.
The most abundant formed elements are erythrocytes (red blood cells), which number around 5 million per microliter of blood. Each one is packed with hemoglobin, the iron-containing protein that binds O₂ in the lungs and releases it in peripheral tissues where O₂ partial pressure is lower. CO₂ is transported partly by hemoglobin but mostly as bicarbonate dissolved in plasma. Critically, mature erythrocytes have no nucleus — they cannot synthesize new proteins or divide. This makes them metabolically simple but structurally vulnerable; they are continuously replaced (about 1% per day) by erythropoiesis in red bone marrow.
Leukocytes (white blood cells) are the cellular immune workforce. The major types differ in origin, morphology, and function: neutrophils are rapid-response phagocytes targeting bacteria; lymphocytes (B and T cells) orchestrate specific adaptive immunity; monocytes differentiate into macrophages in tissues; eosinophils target parasites and mediate allergic responses; basophils release histamine and other inflammatory mediators. A key reality check: most leukocytes live in tissues, not blood. A blood count captures only the circulating fraction — perhaps 2–3% of the total leukocyte pool.
Platelets are not full cells but small membrane-bound fragments shed by large bone marrow cells called megakaryocytes. Their role is to initiate hemostasis — the sealing of vessel damage. On contact with damaged endothelium, platelets activate, change shape, aggregate at the wound site, and release chemical signals that trigger the clotting cascade. Understanding platelets as the first responders to vascular injury will prepare you for deeper study of the coagulation pathway and immune activation, topics you will encounter when studying the innate immune response.