Blood

Blood is a specialized connective tissue in fluid form. Blood, a component of the cardiovascular system, supplies organs and tissues with the substances they need for life. It consists of liquid with cells, proteins, electrolytes and other substances either dissolved or suspended in the liquid.

Plasma and serum are particular liquid components of blood. Plasma is liquid blood with the cells removed, and serum is plasma with the clotting proteins removed.

An electron microscopic image of blood showing red cells (large, dimpled disks) and platelets (small disks in background and foreground). Source: National Cancer Institute via Wikimedia Commons.

Types

Human blood is classified into four types: A, B, AB, or O. This designation is based on the specific proteins (antigens) that are on the surface of the blood cells. Type A blood, for example, has different proteins than does type B. Type AB has both type A and type B proteins, whereas type O does not have either A or B proteins.

Every person's blood is also either Rhesus factor (Rh) positive or Rh-negative. This means that either the blood cells have the Rhesus factor protein, or they do not. The ABO type and the Rh factor are used together to classify blood. For example, type A blood is either A positive or A negative.

People with type O blood are called universal donors because O can be safely transfused into most recipients, regardless of the recipient’s blood type. Since type O blood cells don’t have A or B proteins, the recipient’s immune system doesn't "see" any foreign proteins and therefore doesn't attack the new blood. About 40% of the population has type O blood.^[1] People with type AB blood are called universal recipients. They can get any type of blood because their immune system recognizes both type A and type B blood proteins as familiar. AB donors can only safely donate to other people with AB blood because the presence of the A protein will elicit an immune response in recipients with type B blood, and the B protein will elicit an immune response in recipients with type A blood.

Description

A bag of platelets, ready to be transfused. Source: Wikimedia Commons

Blood is a complex mixture of plasma and other substances referred to as formed elements.

Plasma

Plasma is the liquid component of blood, usually taking up about 55% of the volume of whole blood. It is easily separated from whole blood, since the formed elements (which include red blood cells, white blood cells, and platelets) are denser and sink quickly to the bottom of a tube during centrifugation (rapid spinning). Plasma is distinguished from serum because plasma contains proteins that promote blood clotting(coagulation factors) that are soluble (able to be dissolved), while serum does not. Plasma is usually transparent and straw-colored and contains the following:

• Water: Approximately 90% of plasma is water.
• Proteins: Approximately 9% of plasma is protein. Three types of protein predominate:
  • Albumins, which make up about 60% of the total mass of serum proteins and have roles in maintaining the flow of water and compounds into and out of blood (osmotic pressure) and transporting large molecules called anions. Albumin also binds to and transports numerous drugs.
  • Globulins
    • α-globulins
    • β-globulins
    • γ-globulins (also known as immunoglobulins or antibodies)
  • Fibrinogen, which plays a key role in blood clotting
• Hormones
• Vitamins
• Cholesterol
• Glucose
• Electrolytes, including sodium, potassium, calcium, and magnesium.
• Carbon dioxide

Formed elements (blood cells)

The formed elements of the blood are sometimes collectively referred to as blood cells, though they aren't all true cells.

• Red blood cells (RBCs, erythrocytes) are the most numerous of the formed elements. They aren't true cells because they lack several key components of cells, like nuclei, mitochondria, and ribosomes. They consist mostly of the protein hemoglobin, which carries oxygen. Each RBC contains 200 million to 300 million molecules of hemoglobin.
• White blood cells (WBCs, leukocytes) are key components of the immune system. There are many types of WBCs, including neutrophils, eosinophils, basophils, lymphocytes, monocytes, and macrophages.
• Platelets (thrombocytes) are small, irregular bodies that lack nuclei and are formed by fragmentation of megakaryocytes in the bone marrow. They are important in blood clotting.

A normal microliter of blood may contain about 5 million RBCs, 250,000 platelets, and 5,000 white blood cells.

Role of Blood in the Body

The heart pumps blood through a network of arteries and veins throughout the body, the circulatory system.

Blood has many vital jobs in the body:

• It carries oxygen and other nutrients to the body's organs and tissues and transfers waste products like carbon dioxide to areas where they can be excreted.
• It carries cells of the immune system, as well as hormones like insulin.
• It helps regulate body temperature and pH (acidity).
• It carries components needed for wound healing.

How It Works

• RBCs are the part of the blood that carries oxygen to the rest of the body and removes waste products such as carbon dioxide. They are similar to trucks shipping containers of oxygen. The containers are the iron-rich molecule within RBCs, hemoglobin, which carries oxygen to tissues throughout the body. Thus, RBCs are the reason blood can carry oxygen.

• White blood cells (WBCs) are part of the immune system. Different types of WBCs have different roles in the immune system. For example, eosinophils are the main cells that work in allergic reactions, and lymphocytes make antibodies.

• Platelets are tiny cell-like structures that contribute to the blood clotting process by clumping together to form a plug at the site of bleeding.

Bleeding and clotting

Plasma contains proteins called clotting factors, which help the blood to clot.

Diseases of the Blood

Diseases of the blood include many kinds of anemia, cancers, genetic diseases, and clotting problems, among others. Examples include:

• Anemia
• Disseminated intravascular coagulation
• HELLP syndrome
• Hemochromatosis
• Hemophilia
• Leukemia
• Thalassemia
• Sickle cell anemia

Procedures

Procedures performed to treat diseases of the blood include the following:

• Blood and Marrow Stem Cell Transplant
• Blood transfusion
• Phlebotomy

Related Professions

• A hematologist is a medical doctor with special training in diseases of the blood. In the US, hematologists are usually trained in oncology, or cancer care, as well.
• Pathologists, including both clinical and anatomical pathologists, study blood and other body tissues in the laboratory to help arrive at diagnoses.
  • Hematopathology is concerned chiefly with blood.
  • Blood Bank-Transfusion medicine physicians are concerned with blood storage and transfusions.

History

How blood was discovered

Blood has been described since the time of Hippocrates. Hippocrates classified blood as one of four "humors," along with phlegm, black bile, and yellow bile. Health was thought to be achieved by balancing the humors. Disease occurred when the humors became imbalanced.^[2]

The Arab physician Ibn al-Nafis in the 13th century, as well as the Spanish physician Servetus in the 16th, demonstrated that blood circulates in a circuit between heart and lungs.

William Harvey demonstrated in 1628 that blood circulates through the body by being pumped by the heart. Before that, people had subscribed to ideas promulgated by Galen that dark (venous) blood and bright (arterial) blood came from different parts of the body.

Etymology

The word blood has been in use since before the 12th century.^[3]

References

1. ↑ Dutton RP, Shih D, Edelman BB, Hess J, Scalea TM. Safety of uncrossmatched type-O red cells for resuscitation from hemorrhagic shock. J Trauma. 2005 Dec;59(6):1445-9. Abstract
2. ↑ Boylan M. Hippocrates. The Internet Encyclopedia of Philosophy.
3. ↑ Merriam-Webster Online. Blood.

External Links

American Red Cross