What to know about Composition of blood

Composition of Blood 
Blood is classified as a connective tissue because it consists of cells separated by a matrix, the liquid called plasma. An adult male of average size (70 kilograms or 154 pounds) has a blood volume of about 5 liters (5.2 quarts).
A blood sample that is transferred to a test tube and prevented from clotting separates into two layers. The up­per layer, plasma, contains a variety of inorganic and or­ganic substances dissolved or suspended in water. Plasma accounts for about 55% of the volume of whole blood. The lower layer comprises about 45% of the volume of whole blood and is composed mainly of red blood cells; this per­centage is called the hematocrit (HCT). Red blood cells (erythrocytes) are one of the formed elements in blood. The other formed elements­white blood cells (leukocytes) and blood platelets (thrombocytes)-are found in a narrow layer between the plasma and the red blood cells. Blood is a liquid connective tissue. The liquid portion is termed plasma, and the solid portion consists of the formed elements.
Red Blood Cells (Erythrocytes) 
The red blood cells (RBCs), or erythrocytes, are small, bi­concave, disk-shaped cells without nuclei. They contain hemoglobin (he"mo-glo'bin), a protein carrier for oxygen. Hemoglobin binds with oxygen in the cool, neutral conditions of the lungs and readily gives up oxygen under the warm, more acidic conditions of the tissues. Oxyhemoglobin (hemoglobin plus oxygen) is bright red. Deoxyhemoglobin (hemoglobin minus oxy­gen) is dark purple. The presence or absence of oxygen at­tached to hemoglobin accounts for the difference in color between arteries and veins.
The number of red blood cells in a cubic millimeter (mm3) of blood is called the red blood cell count. Males usually have 4.6 to 6.2 million cells per cubic millimeter, and females have 4.2 to 5.4 million cells per cubic mil­limeter. The Medical Focus reading on page 210 examines disorders resulting from abnormal red and white blood cell counts. Each cell contains about 200 million hemoglo­bin molecules. If this much hemoglobin were suspended within the plasma, rather than being enclosed within the cells, the blood would be so thick that the heart would have difficulty pumping it.
Red blood cells live only about 120 days and are de­stroyed chiefly in the liver and spleen, where they are en­gulfed by large phagocytic cells. When red blood cells are broken down, the hemoglobin is released and broken down into heme (hem), the iron-containing portion, and globin (glo'bin), a protein. The iron is recovered and returned to the red bone marrow for reuse. The rest of the heme undergoes further chemical degradation and is ex­creted by the liver in the bile as bile pigments, which are primarily responsible for the color of feces.
Red blood cells do not have nuclei but they do have hemoglobin. vvhich carries oxygen.
White Blood Cells (Leukocytes) 
White blood cells (WBCs), or leukocytes, differ from red blood cells in that they are usually larger, have a nucleus, lack hemoglobin, and without staining, appear to be white. White blood cells are not as numerous as red blood cells. The white blood cell count is usually 5,000-11,000 per cubic millimeter of blood. White blood cells fight in­fection in ways, which concerns immunity.
Red blood cells are confined to the blood, but white blood cells are also found in lymph and tissue fluid. They are able to squeeze through pores in the capillary wall). Some are always found in small numbers in the tissues, but their number greatly increases when there is an infection. Many white blood cells live only a few days-they probably die while engaging pathogens. Oth­ers live months or even years.
Types of White Blood Cells
White blood cells are classified as granular leukocytes or agranular leukocytes. Both types of cells have granules in the cytoplasm surrounding the nucleus, but the granules are more prominent in granular leukocytes. The granules contain various enzymes and antibiotic-like proteins that help white blood cells defend the body. There are three types of granular leukocytes and two types of agranular leukocytes. They differ somewhat by the size of the cell and the shape of the nucleus.
Granular Leukocytes 
eutrophils are the most abundant of the white blood cells. They have a multilobed nucleus joined by nuclear threads; therefore, they are also called polymor­phonuclear. Their granules do not significantly take up the stain eosin (a pink to red stain) or a basic stain that is blue to purple. This accounts for the name neutrophil.
Neutrophils are the first type of white blood cells to re­spond to an infection, and they engulf bacteria and cell de­bris via phagocytosis. Eosinophils have a bilobed nucleus.
Their granules take up eosin red stain, which accounts for the name eosinophil, meaning red-loving. The function of eosinophils is not completely understood, but they are known to increase in number during a parasitic worm in­fection or an allergic reaction. Basophils have a U-shaped or lobed nucleus. The granules take up the basic stain and turn dark blue, which accounts for the name basophil. Basophils move to sites of injury and enter the tissues, where they release histamine, a chemical associated with allergic reactions. Histamine dilates blood vessels and causes contraction of smooth muscle in the walls of air passageways.
Agranular Leukocytes 
Agranular leukocytes (monocytes and lymphocytes) typi­cally have a spherical or kidney-shaped nucleus. Monocytes are the largest of the white blood cells, and after taking up residence in the tissues, they dif­ferentiate into even larger macrophages. Macrophages phagocytize pathogens and stimulate other white blood cells to defend the body.
Lymphocytes are of two types, but both have the same appearance. B lymphocytes are responsible for antibody-mediated immunity-that is, they produce anti­bodies, proteins that combine with antigens. T lymphocytes are responsible for cell­mediated immunity-that is, they directly destroy any cell that bears foreign antigens.
White blood cells are divided into the granular leukocytes and the agranular leukocytes. Each type of vvhite blood cell has a specific role to play in defending the body against disease.
Platelets (Thrombocytes) 
Platelets (plat'lets) (thrombocytes) result from fragmenta­tion of certain large cells, called megakaryocytes, in the red bone marrow. The platelet count in blood is normally 150,000 to 300,000 per cubic millimeter. Thrombocytopenia is an insufficient number of platelets (less than 50,000 per cubic millimeter).
Platelets do not have nuclei and are about half the size of red blood cells. They live only about 10 days and are in­volved in repairing damaged blood vessels and in initiating the process of blood clotting.
Platelets are fragments of cells that are involved in vessel repair and blood clotting.
Hematopoiesis 
The process by which blood cells are formed is called hematopoiesis. Blood cells are continuously produced in the red bone marrow of the skull, ribs, and vertebrae, and in the ends of long bones. The bone marrow contains stem cells that constantly divide, producing cells that eventually become the various types of formed elements.  At the top is a multipotent stem cell (hemocytoblast) that divides, producing two other types of stem cells. The myeloid stem cell gives rise to the cells that go through a number of stages to become red blood cells, platelets, granular leuko­cytes, and monocytes. The lymphoid stem cell produces the lymphocytes.
As red blood cells pass through a number of stages, they lose their nuclei and gain hemoglobin. About 2.5 mil­lion erythrocytes are produced every second, and an equal number is continuously destroyed by the spleen and liver. Red blood cell production increases when a growth factor called erythropoietin is released by the kidneys (and prob­ably other organs as well). Erythropoietin stimulates stem cells to divide and produce cells that become red bloar! cells.
Ordinarily, only mature blood cells enter the blood­stream. Blood entering the bone goes into blood sinuses, where the mature blood cells collect and from which they enter the circulatory system.
Each type of white blood cell seems capable of pro­ducing specific growth factors that circulate back to the bone marrow and stimulate increased production of white blood cells. The best known growth factor is Gm-CSH (granulocyte- macrophage colony-stimulating hormone).
Blood cells are formed through the process of hematopoiesis. Stem cells in the red bone marrovv continuously divide, giving rise to cells that become the formed elements.
Plasma 
Plasma, the liquid portion of blood, is approximately 92% water but contains many different types of molecules, in­cluding the plasma proteins. Two types of plasma proteins have special functions. One type, the gamma globulins (glob'u-linz), are antibodies that help fight infection. The other is the protein fibrinogen (fi­brin'o-jen), which is converted to fibrin threads when blood clotting occurs. Both of these proteins are synthe­sized in the liver.
Plasma proteins, along with electrolytes, create an os­motic pressure that draws water from the tissues into the blood. This function of the plasma proteins is particularly associated with albumin, the smallest and most plentiful of the plasma proteins.
There are several different plasma proteins, each with specific functions.