By Wanda Lockwood | Updated Mar 24, 2022
Blood is roughly 78 % fluid and 22 % solid. Its main constituents are plasma, red blood cells (erythrocytes), white blood cells (leukocytes), and platelets. All of these cells originate in bone marrow, especially the red marrow found in the long bones of the arms, legs, spine, and sternum. Yellow marrow stores fat, while red marrow houses hematopoietic stem cells that give rise to new blood cells.
Plasma is the liquid phase that transports red and white cells, antibodies, vitamins, minerals, electrolytes (such as sodium, calcium, and potassium), proteins, and clotting factors throughout the body. After you eat or drink, nutrients and fluids are absorbed in the intestines and replenish plasma, allowing it to circulate blood cells and nourish every organ system.
Red blood cells constitute about 40 % of the blood volume. They produce hemoglobin, the oxygen‑carrying protein that delivers oxygen from the lungs to tissues and shuttles carbon dioxide back to the lungs for exhalation. Formed in bone marrow, erythrocytes become biconcave disks, enabling them to squeeze through capillaries as narrow as 5 µm. They have a lifespan of roughly 120 days. When oxygen levels drop, the kidneys release erythropoietin, stimulating the marrow to produce more red cells. In anemia, the marrow may release immature reticulocytes to compensate.
White blood cells (leukocytes) account for only about 1 % of blood volume, yet they surge in response to infection, injury, or allergy. Each leukocyte type—lymphocytes, monocytes, eosinophils, basophils, and neutrophils—has a specialized defense role. Most mature outside bone marrow in lymphoid tissues such as the spleen, thymus, and adenoids, remaining dormant until activated by a threat.
Platelets, or thrombocytes, are fragments shed from large cells called megakaryocytes within bone marrow. When tissue injury occurs, platelets change shape, extend pseudopodia, and aggregate to form a plug that initiates clotting and stops bleeding.
