Blood, the Lymphatic System and Immunity

Blood Composition, Plasma, and Formed Elements

Blood is a specialized fluid connective tissue that circulates throughout the cardiovascular system. It is composed of liquid plasma and cellular formed elements. Plasma makes up roughly 55 percent of total blood volume and is predominantly water. Suspended within this fluid are major plasma proteins, including albumin, globulins, and fibrinogen. Albumin is the most abundant plasma protein and maintains blood osmotic pressure to keep fluid balanced between the bloodstream and surrounding spaces. Fibrinogen is an essential plasma protein required for blood clot formation, ensuring the body can seal tears in blood vessels.

The formed elements of blood comprise erythrocytes, leukocytes, and thrombocytes. Erythrocytes, commonly known as red blood cells, transport oxygen from the lungs to tissues throughout the body. To maximize internal space for hemoglobin, mature erythrocytes lack nuclei and organelles. Hemoglobin is an iron-bearing protein that binds reversibly to oxygen molecules, allowing efficient pickup in the respiratory tract and release at metabolically active tissues. Because mature red blood cells cannot divide or synthesize new proteins and have limited capacity to repair damage, they must be continually replaced. Erythropoietin is a hormone produced by the kidneys that stimulates erythrocyte production in red bone marrow.

Thrombocytes, also called platelets, are membrane-bound cell fragments derived from megakaryocytes. When a vessel suffers damage, platelets initiate formation of a platelet plug by adhering to exposed tissue beneath the damaged blood vessel lining. Following this immediate response, the coagulation cascade converts soluble fibrinogen into insoluble fibrin strands to stabilize blood clots, creating a strong meshwork that halts fluid loss and permits repair.