12.1 Introduction
Blood is a type of connective tissue whose cells are suspended in a liquid extracellular matrix. It transports substances between body cells and the external environment, and helps maintain a stable internal environment,

12.2 Blood and Blood Cells
Blood contains red blood cells, white blood cells, and platelets.
1. Blood volume and composition
a. Blood volume varies with body size, fluid and electrolyte balance, and adipose tissue content.
b. Blood can be separated into formed elements and liquid portions.
1. The formed elements portion is mostly red blood cells.
2. The liquid plasma includes water, gases, nutrients, hormones, electrolytes, and cellular wastes.
2. Characteristics of red blood cells
a. Red blood cells are biconcave discs with shapes that increase surface area.
b. Red blood cells contain hemoglobin, which combines with oxygen.
3. Red blood cell counts
a. The red blood cell count equals the number of cells per cubic millimeter of blood.
b. The average count ranges from approximately 4 to 6 million cells per cubic millimeter of blood.
c. Red blood cell count is directly proportional to the oxygen-carrying capacity for the blood. It is used to diagnose and evaluate the courses of certain diseases.
4. Red blood cell production and its control
a. Red bone marrow produces red blood cells.
b. In health, the number of red blood cells remains relatively stable.
c. Erythropoietin controls the rate of red blood cell formation by negative feedback.
5. Dietary factors affecting red blood cell production
a. Availability of vitamin B12 and folic acid influences red blood cell production.
b. Hemoglobin synthesis requires iron.
6. Destruction of red blood cells
a. Macrophages in the liver and spleen phagocytize damaged red blood cells.
b. Hemoglobin molecules decompose, and some of the iron they contain is recycled.
c. Hemoglobin releases biliverdin and bilirubin pigments.
7. Types of white blood cells
a. White blood cells develop from hematopoietic stem cells, in response to interleukins and colony-stimulating factors, to protect against disease.
b. Granulocytes include neutrophils, eosinophils, and basophils.
8. Functions of white blood cells
a. Neutrophils and monocytes phagocytize foreign particles.
b. Eosinophils kill parasites and help control inflammation and allergic reactions.
c. Basophils release heparin, which inhibits blood clotting, and histamine to increase blood flow to injured tissues.
d. Lymphocytes produce antibodies that attack specific foreign substances.
9. White blood cell counts
a. Normal total white blood cell counts vary from 5,000 to 10,000 cells per mm3 of blood.
b. The number of white blood cells may change in response to abnormal conditions, such as infections, emotional disturbances, or excessive loss of body fluids.
c. A differential white blood cell count indicates the percentages of various types f leukocytes present.
10. Blood platelets
a. Blood platelets, which develop in the red bone marrow in response to thrombopoietin, are fragments of giant cells.
b. The normal platelet count varies from 130,000 to 360,000 platelets per mm3 of blood.
c. Platelets help close breaks in blood vessels.

12.3 Blood Plasma
Plasma transports gases and nutrients, helps regulate fluid and electrolyte balance, and helps maintain stable pH.
1. Plasma proteins
a. Plasma proteins remain in blood and interstitial fluids, and are not normally used as energy sources.
b. Three major groups exist.
1. Albumins help maintain the colloid osmotic pressure.
2. Globulins include antibodies. They provide immunity and transport lipids and fat-soluble vitamins.
3. Fibrinogen functions in blood clotting.
2. Gases and nutrients
a. Gases in plasma include oxygen, carbon dioxide, and nitrogen.
b. Plasma nutrients include simple sugars, amino acids, and lipids.
1. The liver stores glucose as glycogen and releases glucose whenever blood glucose concentration falls.
2. Amino acids are used to synthesize proteins and are deaminated for use as energy sources.
3. Lipoproteins function in the transport of lipids.
3. Nonprotein nitrogenous substances
a. Nonprotein nitrogenous substances are composed of molecules that contain nitrogen atoms but are not proteins
b. They include amino acids, urea, and uric acid.
4. Plasma electrolytes
a. Plasma electrolytes include ions of sodium, potassium, calcium, magnesium, chloride, bicarbonate, phosphate, and sulfate.
b. Bicarbonate ions are important in maintaining the osmotic pressure and pH of plasma.

12.4 Hemostasis
Hemostasis is the stoppage of bleeding.
1. Blood vessel spasm
a. Smooth muscles in blood vessel walls reflexly contract following injury.
b. Platelets release serotonin, which stimulates vasoconstriction and helps maintain vessel spasm.
2. Platelet plug formation
a. Platelets adhere to rough surfaces and exposed collagen.
b. Platelets adhere to each other at injury sites and form platelet plugs in broken vessels.
3. Blood coagulation
a. Blood clotting is the most effective means of Hemostasis.
b. Clot formation depends on the balance between factors that promote clotting and those that inhibit clotting.
c. The basic event of coagulation is the conversion of soluble fibrinogen into insoluble fibrin.
d. Biochemicals that promote clotting include prothrombin activator, prothrombin, and calcium ions.
e. A thrombus is an abnormal blood clot in a vessel. An embolus is a clot or fragment of a clot that moves in a vessel.

12.5 Blood Groups and Transfusions
Blood can be typed on the basis of cell surface antigens.
1. Antigens and antibodies
a. Agglutination is the clumping of red blood cells following a transfusion reaction.
b. Red blood cell membranes may contain specific antigens, and blood plasma may contain antibodies against certain of these antigens
2. ABO blood group
a. Blood is grouped according to the presence or absence of antigens A and B.
b. Mixing red blood cells that contain an antigen with plasma that contains the corresponding antibody results in an adverse transfusion reaction.
3. Rh blood group
a. Rh antigens are present on the red blood cell membranes of Rh-positive blood. They are absent in Rh-negative blood.
b. Mixing Rh-positive red blood cells with plasma that contains anti-Ph antibodies agglutinates the positive cells.
c. Anti-Ph antibodies in maternal blood may cross the placental tissues and react with the red blood cells of an Rh-positive fetus.