In the hematologic system, blood performs a variety of essential functions. It continuously transports oxygen, nutrients, hormones, antibodies, and other substances around the body for use. It also carries cellular-metabolism wastes to sites where they are transformed or eliminated from the body. While circulating through the vascular system, blood helps regulate fluid, electrolyte, and acid‒base balance. It can also protect the body with its clotting capability and by fighting infections.

The two major components of the blood are plasma and blood cells. Plasma is the liquid portion of the blood. It consists primarily of water, but also includes proteins (e.g., albumin, globulin), clotting factors (e.g., fibrinogen), electrolytes, nutrients, wastes, and other substances. Within the vascular system, the protein albumin plays an important role in maintaining fluid balance. The presence of sufficient albumin in the plasma creates an osmotic force (colloidal or oncotic pressure) that offsets the hydrostatic pressure and pulls fluid into the vascular system.

The blood cells include erythrocytes (red blood cells [RBCs]), leukocytes (white blood cells [WBCs]), and thrombocytes (platelets). These cells primarily originate from hematopoietic (blood cell‒producing) stem cells in the bone marrow—for example, in the ribs and the ends of long bones. Stem cells, which exist in both embryonic and adult forms, are primitive cells not only capable of differentiating into other cells, but also of self-replicating to ensure their continuous supply. The hematopoietic stem cells found in the bone marrow are able to differentiate into either myeloid or lymphoid stem cells when specifically stimulated to do so. Lymphoid stem cells are able to produce either T or B lymphocytes, whereas myeloid stem cells can differentiate into RBCs, WBCs, and platelets.

The functions of RBCs include transporting oxygen (O₂) and carbon dioxide (CO₂), and helping maintain acid‒base balance. The hemoglobin in RBCs contains heme, an iron compound, and globin, a protein. In the capillaries within the lung, oxygen binds with the iron on this hemoglobin; the oxygen-laden RBCs then flow to body tissues, where the body's cells receive this oxygen supply. Carbon dioxide attaches to the globin protein when CO₂ diffuses from tissue cells into the capillaries; from there, it is carried in the blood to the lungs to be expelled. Hemoglobin buffers excessive acids in venous blood by combining with hydrogen ions, which are produced by cellular metabolism.

Most of the heme found in RBCs is ultimately converted into bilirubin, a yellowish or orange-colored pigment in the bile, which is eventually excreted mostly through the feces. Pathologic bilirubin accumulation leads to jaundice, which may be evidenced by a yellowish tone of the patient's skin or sclera.

Erythropoiesis (production of RBCs) within the bone marrow is stimulated by erythropoietin, a hormone primarily produced and released by the kidney. This process requires many essential nutrients, including folic acid; vitamins B₁₂, B₂, and B₆; protein; and iron. Destruction of RBCs normally occurs in the bone marrow, liver, and spleen after approximately 120 days—the RBC's average life span.

Thrombocytes (platelets) function primarily to promote blood coagulation by initiating the formation of a platelet “plug” and the clotting process. This series of events can close an opening in the capillary wall to stop bleeding.

Leukocytes (WBCs) play an important role in the body's defensive and reparative mechanism. Granulocytes—that is, WBCs with granules in their cytoplasm—include neutrophils, eosinophils, basophils, and band cells (less mature granulocytes, the presence of which increases in infection). Eosinophils and basophils are said to be involved in hypersensitivity or allergic reactions. Agranulocytes—that is, WBCs without granules in their cytoplasm—include lymphocytes and monocytes.

The two subtypes of lymphocytes are B cells (derived from bone marrow) and T cells (mostly derived from the thymus). B cells provide humoral immunity, whereas T cells provide cell-mediated immunity. Both types of immunity are essential for maintaining human health, though their mechanisms can be complicated. In humoral (antibody-mediated) immunity, ­antibodies are produced by the B lymphocytes (differentiated B cells) found in plasma (the Greek word humor means “body fluid”). Humoral immunity is believed by researchers to need the “help” of T cells in recognizing some antigens and triggering antibody formation. In contrast, in cell-mediated immunity, T cells directly attack antigens—the foreign invaders, such as bacteria or viruses—instead of producing antibodies.

The immune system can be affected by a broad spectrum of factors, including a person's physical or emotional status, diet, or medications; thus, various types of immune system dysfunctions can occur anytime in the course of life. Normally, this system functions primarily to recognize the initial invasion of foreign (non-self) substances, such as microorganisms. It may subsequently develop antibodies and sensitize lymphocytes to mount a specific reaction (the immune response) to ward off repeated invasions of the foreign substance. ­Over-­reaction of the immune system may result in hypersensitivity or allergy. When the ability to accept self-antigen or one's own tissues is impaired, autoimmune disorders may set in.

Immunity may be naturally developed or acquired. Natural immunity involves no prior contact with an antigen. For example, humans are immune to certain infectious agents that cause illness in other species. Acquired immunity can be classified into two types: actively or passively acquired immunity. Actively acquired immunity may develop after a person has a disease or through immunization (i.e., vaccination with a less virulent antigen). Passively acquired immunity develops after a person receives antibodies to an antigen rather than synthesizing antibodies; for example, an infant may obtain antibodies through the mother's breastmilk or through an injection with hepatitis B immune globulin (serum antibodies). Passive immunity is usually short-lived, as it does not lead to the production of memory cells that might offer long-term protection to the individual against future encounters with the antigen.

A blood typing test is usually done before a person donates or receives blood, and for assessing the risk of Rh (Rhesus factor) incompatibility between an expectant mother and her fetus. The ABO blood typing system is based on whether specific blood group antigens (i.e., A and B) are present on RBCs' surface membranes. Group A blood has antigen A and anti-B antibodies, whereas group B blood has antigen B and anti-A antibodies. Group AB blood has both antigens (A and B) and no antibodies to react to the transfused blood; for this reason, individuals with type AB blood are known as universal recipients. Group O blood has neither antigen on its RBCs; individuals with group O blood are known as universal donors because their blood has neither antigen A nor antigen B, but does have both antibodies.

Blood clumping will occur when a patient with the A blood type receives a donor's blood containing B antigens (in either type B or type AB blood), and when a patient with the B blood type receives donated blood containing A antigens. Such a mismatch results in hemolysis of RBCs. Before administering a blood transfusion, it is imperative to confirm that the correct blood type is being given to the correct patient. If a patient who is receiving a blood transfusion starts to feel a vague sense of uneasiness, or have signs or complaints such as nausea, sweating, chills, shortness of breath, or low back pain, the first nursing action is to immediately stop the transfusion and act per protocol; a potential blood type mismatch may be to blame.

The spleen, which is the largest lymphoid organ, is located in the left upper quadrant of the abdomen. It filters the blood and performs many functions, including producing RBCs in the fetus and when bone marrow is damaged in adults, removing old and defective RBCs, recycling iron, filtering circulating bacteria, and storing RBCs and platelets. In addition, the spleen has some immunologic functions, such as forming lymphocytes and monocytes. Nevertheless, this organ is not considered essential to survival.

The spleen is highly vascular. A penetrating injury to it may necessitate a splenectomy to prevent hemorrhage, septicemia, or peritonitis. After a splenectomy, the patient may have immunologic deficiencies; measures should be taken to prevent infection.