The Hematologic System

Introduction

The average volume of blood in the human body is approximately five to seven percent of the total body weight. Blood is essential to transport necessary elements to and from the different body systems. Think of blood like the body's gasoline; a car can't run without gasoline, just as the human body can't operate without blood.

 

Blood

Blood is a thick and dark substance that's made up of a wide variety of particles that serve diverse functions in the body. Three main types of blood cells are

1. Red blood cells
2. White blood cells
3. Platelets 

Collectively, these cells are called the formed elements of blood. Blood cells are produced in the bone marrow, and the process of formation of blood cells is called hematopoiesis.

The basic foundation of blood, however, is plasma. Plasma is chiefly made of water, but it also contains other particles.

 

Red blood cells (RBCs) are also referred to as erythrocytes. RBCs give blood its dark red color, and their main function is to deliver oxygen to the body and remove waste products. As you previously learned, oxygen molecules from the lungs cross over into the bloodstream in lungs. But that's only part of the story.

 

The oxygen doesn't merely dissolve into the blood plasma. Each RBC contains millions of molecules of hemoglobin, a complex protein designed to bind and release oxygen. When hemoglobin binds with oxygen, it's called oxyhemoglobin.

RBCs take oxygen to body tissues, where the oxygen is released. As the tissues use oxygen, they produce the waste product carbon dioxide, which is picked up by more RBCs and brought back to the lungs for release out of the body via exhalation. 

RBCs don't have nuclei, so they don't contain chromosomes and can't replicate by mitosis like other cells. Bone marrow generates red blood cells through the process of erythropoiesis. The cells are born in the bone marrow and, after maturation, are released into the bloodstream. In a typically functioning body, the spleen and liver destroy RBCs after about 120 days.

 

Platelets are the third and final type of blood cell. Platelets are also known as thrombocytes because they're involved in the first step of thrombosis, or blood clotting. When your arm gets cut, blood wells up in the cut because the small blood vessels have been damaged. Platelets stick to the damaged edges of the blood vessel and to each other, forming a platelet plug. This plug represents the "quick fix" patch that quickly stops blood flow until a more durable blood clot can form. Without an adequate number of platelets or properly functioning platelets, the cut would continue to bleed instead of stopping after a few minutes.

 

Types of Blood Cells

Platelets or Thrombocytes 

• Normal count = 150,000—450,000 cells/microliter blood
• The main function is to promote blood clotting to stop bleeding
• Low platelet count increases the risk of uncontrolled bleeding
• Common disorder is thrombocytopenia 

WBCs or Leukocytes 

• Normal count = 4,500–10,000 cells/microliter blood
• The main function is to produce an immune response and protect the body against foreign pathogens
• Increased WBC count is an indicator of infection
• Two main types are granulocytes and agranulocytes: (1) Granulocytes are further divided into three categories: neutrophils, eosinophils, and basophils. (2) Agranulocytes are further classified as lymphocytes and monocytes.
• Common disorders are leukemia and lymphoma 

RBCs or Erythrocytes 

• Normal count = 4–6 million cells/microliter blood
• The main function is to transport hemoglobin that carries oxygen from the lungs to the entire body
• Life span is 120 days
• Common disorder is anemia

 

More than half of our blood volume is plasma, and 92 percent of that plasma is water.

Plasma is a straw-colored fluid when separated from the formed elements. Besides water, plasma consists of proteins, nutrients, electrolytes, hormones, vitamins, enzymes, and waste products of metabolism. The three important proteins in plasma are fibrinogen, albumin, and globulin. 

Fibrinogen: Blood-clotting protein made in the liver.

Albumin: Made in the liver and is the most plentiful protein in plasma. It maintains the pressure needed to pull water from tissues back into blood vessels.

Globulin: Formed both in the liver and in the lymphatic system. It can be found as two different forms in plasma. Gamma globulin helps form antibodies, and prothrombin helps blood coagulation. 

 

Coagulation

Coagulation, or blood clotting, is complex and still not entirely understood. However, it's known that the process depends on platelets. When tissue is injured, platelets and the injured tissue release thromboplastin, an enzyme that works with calcium and other factors to convert prothrombin in the plasma into thrombin. The thrombin then acts as an enzyme to convert the fibrinogen in the plasma into fibrin, which forms a mesh over the area and traps red blood cells, platelets, and plasma to form a clot. When the clot dries, it forms a scab, and the bleeding stops.

Coagulation is complex process that requires the concerted action of several blood elements. If any portion of this coagulation cascade is missing, deficient, or malfunctioning, then blood clotting may be impaired. This could result in varying degrees of hemorrhage, or prolonged or absent blood clotting. Hemorrhages can range from minor bleeding—such as that after minor cuts or procedures—to spontaneous bleeding within the body, such as the cranium, chest, or abdominal cavities. 

A drug or substance that promotes coagulation is a pro-coagulant, whereas a substance that inhibits coagulation is an anti-coagulant.

 

Blood Types

A blood type is defined by genetic markers and depends on a blood protein called agglutinogen, or antigen, on the surface of the red blood cell. 

There are four blood types: A, B, AB, and O. There are two types of antigen: A and B.

If you have the A antigen, your blood will be type A; if you have the B antigen, your blood will be type B. If you have both A and B, then your blood will be type AB, and if you have neither of them, your blood will be type O.

 

Besides the A, B, AB, and O blood types, human blood is also characterized according to the presence or absence of the Rh factor—another antigen against which the body can make antibodies. Individuals with the Rh factor are called Rh positive (or RH+). Those without the Rh factor are Rh negative (or Rh–). Thus, human blood can be categorized into eight different types:

1. Type A+ (Type A, Rh+)
2. Type B+
3. Type AB+
4. Type O+
5. Type A– (Type A, Rh–)
6. Type B–
7. Type AB–
8. Type O–

Individuals with Type AB+ blood are universal recipients, which means that they can receive any of the four blood types. They have no antibodies against A, B, or Rh factor antigens because these antigens are present in their own blood and are considered normal.

 

Key Points

• Red blood cells are responsible for transportation of oxygen to various body organs.
• White blood cells are the main defense system against foreign pathogens entering our body.
• Platelets play a crucial role to stop bleeding by clumping at the site of bleeding.
• Coagulation is a complicated process that involves coordination of various blood cells and coagulation factors to stop bleeding after a trauma.
• The following is a list of different blood types:                                                                                                                                                                                *A– and A+                                                                                                                                                                                                                                            *B– and B+                                                                                                                                                                                                                                            *AB– and AB+                                                                                                                                                                                                                                      *O– and O+