Back to the Basics: Hematology, Part 1

“You know you are old if they have discontinued your blood type.”- Phyllis Diller

Hematology: the subject in EMT or paramedic school where you thought, “Why do I need to know this?” I am even willing to admit it is not one of my favorite sections to teach. Interestingly though, one of the consistent issues is that many patients with a hematological disorder have complicated medical histories. As such, when they experience an acute exacerbation, their condition requires EMS professionals to perform a more thorough-than-normal assessment and physical exam. A thorough understanding of the pathophysiology of these diseases, early recognition, and prompt transport can reduce patient morbidity and mortality.

Blood Components

Blood has two components – plasma (the liquid portion of the blood) and formed elements (red cells, white cells, and platelets). Plasma and the Formed Elements…sounds like a rock band, doesn’t it? Anyway, plasma is the largest component of the blood, making up more than 55 percent of its overall content. When separated from the formed elements, it appears as a light-yellow liquid and is about 92 percent water. About seven percent of the components are coagulants (fibrinogen and prothrombin), plasma proteins (albumins), and globulins (lipoproteins and antibodies). The remaining one-ish percent is a combination of salts (sodium, calcium, potassium, magnesium, chloride, bicarbonate, sulfates, and phosphates), lipids, glucose, amino acids, hormones, vitamins, oxygen, carbon dioxide, and nitrogenous waste.

As plasma forms the liquid base of blood, the functions of plasma and blood tend to overlap. Plasma’s multitude of functions include¹

  • Coagulation: fibrinogen plays a major role in blood clotting, along with thrombin and factor ten.
  • Defense: immunoglobulins in plasma play an important role in the body’s defense against bacteria, viruses, fungi, and parasites.
  • Maintenance of Osmotic Pressure: maintains colloidal osmotic pressure by the presence of plasma proteins.
  • Nutrition: transportation of nutrients absorbed from the digestive tract to different parts of the body.
  • Respiration: transportation of oxygen to the various organs and carrying carbon dioxide back to the lungs for excretion.
  • Excretion: the blood removes nitrogenous waste products produced after cellular metabolism and transports them to the kidney, lungs, and skin for excretion.
  • Hormones: hormones are released into the blood and transported to their target organs.
  • Regulation of Acid-Base Balance: plasma proteins contribute to acid-base balance through their buffering action.

The remaining 45 percent of the blood is composed of formed elements. Red blood cells (erythrocytes) make up approximately 98-99 percent of the formed elements, with four to six million per mm3 (basically, one drop of blood). This percentage of red cells within the blood is the hematocrit. Normal hematocrit levels vary based on age and race. Normal levels for adult men range from 41 to 50 percent, whereas adult women range between 36 and 44 percent.2 Red blood cells are produced by red bone marrow within the medullary cavity of flat bones (pelvis, scapula, skull, and sternum) and have a lifespan that can reach 120 days. Each red cell contains approximately 270 million hemoglobin molecules. Hemoglobin enables red cells to transport oxygen and carbon dioxide via four iron atoms within each molecule.

White blood cells (leukocytes) make up about one percent of the blood, consisting of neutrophils, basophils, eosinophils, lymphocytes, and monocytes. Neutrophils comprise the largest percentage of white cells and engulf and digest foreign materials. Basophils release histamine and heparin, which increases blood flow to injured tissue. Eosinophil functions include movement to inflamed areas, trapping substances, killing cells, anti-parasitic and antibacterial activity, and participating in immediate allergic reactions.3 Monocytes leave the bone marrow and develop into macrophages, which envelop and destroy bacteria, other foreign material, and cellular debris. Lymphocytes are part of the immune defense, consisting of T cells, B cells, and natural killer (NK) cells. These cells act together to recognize antigens, produce antibodies, and destroy foreign or domestic cells that could damage bodily functions.

Platelets are fragments from larger cells called megakaryocytes and are important for blood clotting (hemostasis). The process by which they form a clot is called adhesion. If you accidentally cut your finger and rupture a blood vessel, it will start to bleed. To stop the bleeding, platelets within that broken vessel adhere to the site of injury and send out chemical signals for more help. More platelets answer the call and begin to connect to form a plugin, a process called aggregation. Once a plug or clot is formed in the blood vessel wall, the clotting cascade is activated, adding fibrin (a structural protein) to the clot to knit it together.(5)

Blood Types

Blood types are determined by the presence or absence of certain antigens – substances that can trigger an immune response if they are foreign to the body. There are four major blood groups determined by the presence or absence of two antigens, A and B, on the surface of red blood cells. Depending on which antigen is on the surface of a red cell, the patient’s plasma will contain the opposite antibody (6).

  • A type blood has A surface antigens and has B antibodies
  • B type blood has B surface antigens and has A antibodies
  • AB blood has both A and B surface antigens and no antibodies
  • O type blood has no surface antigens, and A and B antibodies

Donors with type O red blood cells are universal donors, and their cells can be given to any other blood type. When O blood is infused, since its red cells have no surface antigens, the receiving patient’s blood antibodies have nothing to latch onto and attack. Thus, there is no reaction and destruction of the infused red cells. Donors with type AB blood are universal recipients and can receive red blood cells from any other blood type. This blood type has no circulating plasma antibodies because the red cells have both A and B surface antigens.

In addition to the A and B antigens, there is a protein called the Rh factor, which can be either present (+) or absent (–), creating the eight most common blood types (A+, A-, B+, B-, O+, O-, AB+, AB-). Rh blood type is even more important for pregnant women. Rh incompatibility can occur when a woman with Rh-negative blood becomes pregnant with a fetus with Rh-positive blood. When the blood of an Rh-positive baby is mixed with the blood of an Rh-negative mother during pregnancy or delivery, the mother’s immune system will make antibodies. This antibody response is called Rh sensitization. It can destroy the baby’s red blood cells depending on when it occurs.

Rh sensitization does not generally affect the baby’s health during the first pregnancy in which the sensitization occurs. However, a baby’s health with Rh-positive blood during future pregnancies is more likely to be affected. After sensitization, the baby can develop mild to severe problems (called erythroblastosis fetalis). In rare cases, if Rh disease is not treated, the baby may die (7).


  1. Mathew J, Sankar P, Varacallo M. Physiology, Blood Plasma. [Updated 2021 Apr 28]. In: StatPearls [Internet]. Treasure Island (FL): StatPearls Publishing; 2021 Jan-. Available from:
  2. American Red Cross. (2021). What Does Hematocrit Mean? Hematocrit and Hemoglobin. Retrieved December 9, 2021, from
  3. Cincinnati Center for Eosinophilic Disorders. (2021). Eosinophilic disorders: What is an eosinophil? Eosinophilic Disorders | What is an Eosinophil? Retrieved December 9, 2021, from,reactions%2C%20and%20modulating%20inflammatory%20responses.
  4. What are platelets, and why are they important? Johns Hopkins Medicine. (n.d.). Retrieved December 9, 2021, from
  5. Suzanne Dixon, M. P. H. (2021, September 2). What do your blood platelets do? Verywell Health. Retrieved December 10, 2021, from
  6. American Red Cross. (2021). What Does Hematocrit Mean? Hematocrit and Hemoglobin. Retrieved December 9, 2021, from
  7. Thompson, E. G., Husney, A., & Gabica, M. J. (2021, June 17). Blood type test. Retrieved December 10, 2021, from