Blood

Source ¹  licensed under Creative Commons Attribution 4.0 International License.

Anatomy

• Blood is a connective tissue made up of cellular elements and an extracellular matrix.
• Include red blood cells (RBCs), white blood cells (WBCs), and platelets.
• In the laboratory, blood samples are often centrifuged in order to separate the components of blood from one another.
• Erythrocytes (red blood cells) are the heaviest elements in blood and settle at the very bottom of the tube.
  • Are basically sacs packed with an oxygen-carrying compound called hemoglobin.
  • Transport oxygen and some carbon dioxide between tissues and lungs.
• Above the erythrocyte layer we see the buffy coat, a pale, thin layer of leukocytes and thrombocytes, which together make up less than 1% of the sample of whole blood.
  • Leukocytes function in the body’s defenses.
• Above the buffy coat is the blood plasma, normally a pale, straw-colored fluid, which constitutes the remainder of the sample.
  • Like other fluids in the body, plasma is composed primarily of water. In fact, it is about 92% water. Dissolved or suspended within this water is a mixture of substances, most of which are proteins.
• In normal blood, about 45 percent of a sample is erythrocytes, which is referred to as the hematocrit.

White Blood Cells

Never Let Monkeys Eat Bananas.

• Neutrophils are phagocytic and particularly effective against bacteria. They release cytotoxic chemicals.
• Lymphocytes are primarily specific (adaptive) in immunity.
  • T cells directly attack other cells (cellular immunity)
  • B cells release antibodies (humoral immunity) and are natural killer cells similar to T cells but nonspecific.
  • Plasma cells (not to be confused with blood plasma), is a type of B cell, produces antibodies or immunoglobulins that bind to specific foreign or abnormal components of plasma membranes.
  • Natural killer (NK) cells are capable of recognizing cells that do not express “self” proteins on their plasma membrane or that contain foreign or abnormal markers. These “nonself” cells include cancer cells, cells infected with a virus, and other cells with atypical surface proteins.
  • Memory cells are a variety of both B and T cells that form after exposure to a pathogen and mount rapid responses upon subsequent exposures. Unlike other leukocytes, memory cells live for many years.
• Monocytes are very effective phagocytic cells that engulf pathogens or worn-out cells. Serve as antigen-presenting cells (APCs) for other components of the immune system.
• Eosinophils are also phagocytic cells and are particularly effective with antigen-antibody complexes. They release antihistamines and increase with allergies and parasitic infections.
• Basophils promote inflammation.

Platelets

• Are cellular fragments surrounded by a plasma membrane.
• Key players in hemostasis, the process by which the body seals a ruptured blood vessel and prevents further loss of blood.
• Assists with hemostasis and release growth factors for repair and healing of tissue.
• There are three steps to the process: vascular spasm, the formation of a platelet plug, and coagulation (blood clotting) to prevent hemorrhage.

Hemopoiesis

• The lifespan of the formed elements is very brief.
• Although one type of leukocyte (memory cells) can survive for years, most erythrocytes, leukocytes, and platelets normally live only a few hours to a few weeks.
• Thus, the body must form new blood cells and platelets quickly and continuously, a process known as hemopoiesis.
• In children, hemopoiesis can occur in the medullary cavity of long bones.
• In adults, the process is largely restricted to the cranial and pelvic bones, the vertebrae, the sternum, and the proximal epiphyses of the femur and humerus.
• Throughout adulthood, the liver and spleen maintain their ability to generate the formed elements. This process is referred to as extramedullary hemopoiesis.
• All formed elements arise from stem cells of the red bone marrow, called hemopoietic stem cell, or hemocytoblast.
• Hemopoiesis begins when the hemopoietic stem cell is exposed to appropriate chemical stimuli collectively called hemopoietic growth factors, which prompt it to divide and differentiate.
• One daughter cell remains a hemopoietic stem cell, allowing hemopoiesis to continue.
• The other daughter cell becomes either of two types of more specialized stem cells.
• Production of erythrocytes in the red bone marrow occurs at the staggering rate of more than 2 million cells per second.

Function of Blood

• Transportation
• Defense
• Hemostasis

Blood Types

• Blood Type A – People whose erythrocytes have A antigens on their erythrocyte membrane surface.
• Blood Type B – People whose erythrocytes have B antigens.
• Blood Type AB – have both A and B antigens on their erythrocytes.
• Blood Type O – have neither A nor B antigens.
• Rh blood group is classified according to the presence or absence of a second erythrocyte antigen identified as Rh.
  • Those who have the Rh D antigen present on their erythrocytes are described as Rh positive (Rh+).
  • Those who lack it are Rh negative (Rh−).

Blood Transfusions

To avoid transfusion reactions, it is best to transfuse only matching blood types; that is, a type B+ recipient should ideally receive blood only from a type B+ donor and so on. That said, in emergency situations, when acute hemorrhage threatens the patient’s life, there may not be time for cross-matching to identify blood type. In these cases, blood from a universal donor may be transfused.

1. Betts, J.G., Young, K.A., Wise, J.A., Johson, E., Poe, B., Kruse, D. H., Korol, O., Johnson, J.E., Womble, M. & DeSaix, P. (2013). Anatomy and Physiology. OpenStax. http://cnx.org/content/col11496/latest/[↩]