The blood is a type of connective tissue.  The cells of the blood are suspended in a liquid matrix.

Functions of the Blood:

The big function of the blood is to carry oxygen to the body's tissues.  The blood also plays parts in fighting infection, temperature regulation, fluid and electrolyte balance, and maintenance of optimal pH.

Blood Components:

The blood is made up of several different components and types of cells.   The stem cell for all blood cells is the hemocytoblast.

Erythrocytes: Red blood cells.  

Characteristics:  Erythrocytes are biconcave disks that are without a nucleus.  Each red blood cell contains Hemoglobin.  The iron in the hemoglobin molecule forms complexes with oxygen which allows the blood cell to transport the oxygen to the tissues.  Erythrocytes live about 120 days before being destroyed in the liver and spleen.  The iron in the red blood cells is conserved and recycled.

Formation:  Formed by a process called erythropoiesis.  Erythropoietin is a hormone released by the kidneys and liver which stimulates the differentiation of stem cells into proerythroblasts.  The formation of red blood cells take place in the red bone marrow.  This hormones release is controlled by a negative feedback cycle driven by low oxygen concentrations (hypoxia).

Leukocytes: White blood cells.  There are five types.  White blood cells are divided into two groups: Granulocytes and agranulocytes.  Granulocytes have granules visible inside the cell, and their names are derived from the color they stain with Wright Stain.  Agranulocytes have no visible granules.  The proportions of the different types of leukocytes are clinically important.  In order of most populous to least: neutrophils, lymphocytes, monocytes, eosinophils, basophils (Never Let Monkeys Eat Bananas).  Leukocytes are formed in the red bone marrow.  Agranulocytes undergo further development in the lymphatic tissue.

Neutrophils:  The most numerous of the white blood cells, normally making up about 65-70% of the total white blood cell count.  Neutrophils are granulocytes whose granules do not pick up a predominant color when stained with Wright Stain.   Neutrophils fuction as phagocytes.
Lymphocytes:  Make up 20-25% white blood cell count, and are easily distinguishable by the large nucleus which takes up almost all of the cell.  Lymphocytes are agranulocytes.  Lymphocytes are involved in immune responses and are further differentiated into T-lymphocytes and B-lymphocytes.
Monocytes:  Large cells with a kidney-shaped nucleus that make up about 4-8% of the total white blood cell population.  Monocytes function as phagocytes, and develop into macrophages.
Eosinophils:  Granulocytes that stain red with Wright Stain.   Nucleus looks like a droopy figure 8.  Eosinophils are involved in dissolving clots and also function in allergic reactions.  Make up 2-5% of white blood cells.
Basophils:  Granulocytes that stain blue with Wright Stain.  Basophils make up the smallest portion of the total white blood cells - 0.5 - 1%.  They release heparin and later develop into mast cells which are involved in the inflammatory response.  Basophils also release serotonin.

Thrombocytes:  Platelets.  Thrombocytes are actually fragments of cells (megakaryocytes).  They are formed in the red bone marrrow and live about 10 days.  Thrombocytes function in clotting.

Plasma:  The liquid portion of the blood.  Made up mostly of water, but also contains nutrients, electrolytes, hormones and cellular wastes.  Plasma carries gases as well (oxygen, carbon dioxide and nitrogen).  The plasma also contains three proteins which are made by the liver.  Albumin is a large protein which exerts osmotic pressure that pulls fluid into the blood.  Globulin transports lipid soluble vitamins (A, D, E and K) and has an antibody role.  Fibrinogen is a necessary protein in blood coagulation.

Hemostasis:  The body's bleeding control system.  When injury occurs, the body reacts in three steps.

1. Blood Vessel Spasm: Platelets secrete serotonin which causes the smooth muscle inside the blood vessels contract.

2. Platelet Aggregation:  Platelets stick to rough surface caused by injury.  The platelets form a plug.

3. Blood Coagulation:  The coagulation of blood involves many steps.  A couple of the steps are clinically important and are tested when medications like heparin or coumadin are given.  These steps involve the conversion of prothrombin to thrombin and the the conversion of fibrinogen to fibrin.   Prothrombin is an inactive protein in the blood.  Vitamin K is necessary to make this protein, and calcium is needed for it's coversion to thrombin.  Thrombin in turn is needed for the conversion of soluble fibrinogen to insoluble fibrin.  Fibrin forms a net which traps red blood cells and stops bleeding.

   To control bleeding follow these steps:

   1.  Apply direct pressure to the injured site.

   2.  Elevate the area above the level of the heart.

   3.  Apply indirect pressure at the nearest proximal pressure point.

   4.  Use tourniquet if other steps fail to control bleeding.

When we talk about someone's blood type, we're talking about what kind of antigens they have present on their red blood cell's surface.  Often when we speak of antigens we're talking about something foreign, but in this case the antigen is self.  Blood typing is important in preventing transfusion reactions.

ABO Blood Groups

Type A Has A antigen on the RBC surface, and Anti-B antibody in the plasma.
Type B Has B antigen on the RBC surface, and Anti-A antibody in the plasma.
Type AB Has both A and B antigens on the RBC surface, and neither of the antibodies in the plasma.  Because of the lack of antibodies in the plasma, type AB persons can recieve any type of blood as long as the Rh factor is matched.
Type O Has no antigen on the RBC surface, and both Anti-A and Anti-B in the plasma.  Because the cells do not have an antigen, type O blood can be given to anyone as long as the Rh factor is matched.

The Rh Blood Group

Another important antigen that we need to identify is the Rh group.  The blood of an Rh positive person has the antigen on the red blood cell surface, while the blood of an Rh- negative person does not.  Unlike the ABO blood groups, there is no antibody present in the plasma normally.  An antibody will be formed only if an Rh- negative person is exposed to Rh+ blood.

A good example of this is what happens with hemolytic disease of the newborn.  This is only a concern for an Rh- mom carrying an Rh+ baby.  If mom's and baby's blood mixes at birth, mom will form antibodies against the Rh antigen.  This becomes a problem for the next Rh+ baby.  Mom's antibodies will attack the second Rh+ baby's red blood cells causing agglutination.  This is a life threatening situation for the newborn.  To prevent this from happening, mom will be given a shot (Rhogam) after the birth of her first Rh+ baby, and then again after the births of any other Rh+ babies. Mom will also be given this shot after any procedures that may cause mixing of maternal and fetal blood (like an amniocentesis), or following a miscarriage.