Blood Constituents

[Princess Royal]

BLOOD

The blood is divided into two main components. The liquid portion is the plasma. The formed elements, which include cells and cell fragments, fall into these three categories, as follows:

• Erythrocytes (eh-RITH-ro-sites), from erythro, meaning "red," are the several types of red blood cells, which transport oxygen.
• Leukocytes (LU-ko-sites), from leuko, meaning "white," are the several types of white blood cells, which protect against infections.
• Platelets, also called thrombocytes (THROM-bo-sites), are cell fragments that participate in blood clotting.

Blood Plasma

About 55% of the blood volume is plasma. The plasma itself is 91% water. Many different substances, dissolved or suspended in the water, make up the other 9% by weight. The plasma content may vary somewhat because substances are removed and added as the blood circulates to and from the tissues. However, the body tends to maintain a fairly constant level of most substances. For example, the level of glucose, a simple sugar, is maintained at a remarkable constant level of about one tenth of one percent (0.1%) in solution.

After water, the next largest percentage (about 8%) of material in the plasma is protein. The plasma proteins include the following:

*Albumin (al-BU-min), the most abundant protein in plasma, is important for maintaining the osmotic pressure of the blood. This protein is manufactured in the liver.

*Clotting factors, necessary for blood coagulation, are also manufactured in the liver.

*Antibodies combat infection. Antibodies are made by certain white blood cells.

*Complement consists of a group of enzymes that helps antibodies in their fight against pathogens.

The remaining 1% of the plasma consists of nutrients electrolytes, and other materials that must be transported.

With regards to the nutrients, the principal carbohydrate found in the plasma is glucose. This simple sugar is absorbed from digested foods in the intestine. It is also stored as glycogen, mainly in the liver, and released as needed into the blood to supply energy to the cells. Amino acids, the products of protein digestion, also circulate in the plasma. Lipids constitute a small percentage of blood plasma. Lipids components include fats, cholestrol, and lipoprotein, which are proteins bound to cholestrol.

The electrolytes in the plasma appear primarily as chloride, carbonate, or phosphate salts of sodium, potassium, calcium, and magnesium. These salts have a variety of functions, including the formation of bone (calcium and phosphorus), the production of certain hormones (such as iodine for the production of thyroid hormones), and the maintenance of the acid-base balance (such as sodium and potassium carbonates and phosphates present in buffers).

Other materials transported in plasma include vitamins, hormones, waste products, drugs, and dissolved gases, primarily oxygen and carbon dioxide.

The Formed Elements

All of the blood's formed elements are produced in red bone marrow, which is located in the ends of long bones and in the inner mass of all other bones. The ancestors of all the blood cells are called hematopoietic (blood-forming) stem cells. These cells have the potential to develop into any of the blood cell types produced within the red marrow.
In comparison with other cells, most of those are short lived. The need for constant blood cell replacement means that normal activity of the red bone marrow is absolutely essential to life.

Erythrocytes:-
Erythrocytes, the red blood cells measureabout 7 um in diameter. They are disk-shaped bodies with a depression on both sides. This biconcave creates a central area that is thinner then the edges. Erythrocytes are different from other cells in that the mature form found in the circulating blood lacks a nucleus and also lacks most of the organelles commonly found in cells. As red cells mature, these components are lost, providing more space for the cells to carry oxygen. This vital gas is bound in the red cells to hemoglobin, a protien that contains iron (Hemoglobin: Door to door oxygen delivery). Hemoglobin, combined with oxygen, gives the blood its characteristic red colour. The more oxygen carried by the hemoglobin, the brighter is the red colour of the blood. Therefore, the blood that goes from the lungs to the tissues is a brighter red because it carries a great supply of oxygen; in contrast, the blood that returns to the lungs is a much darker red because it has given up much of its oxygen to the tissues.

Hemoglobin has two lesser functions in addition to the transport of oxygen. Hemoglobin that has given up its oxygen is able to carry hydrogen ions. In this way, hemoglobin acts as a buffer and plays an important role in acid-base balance. Hemoglobin also carries some carbon dioxide from the tissues to the lungs for elimination. The carbon dioxide is bound to a different part of the molecule than the parts that holds oxygen, so that it does not interfere with oxygen transport.

Hemoglobin's ability to carry oxygen can be blocked by carbon monoxide. This odorless and colourless but harmful gas combines with hemoglobin to form a stable compound that can severly restrict the erythrocyte's ability to carry oxygen. Carbon monoxide is a byproduct of the incomplete burning of fuels, such as gasoline and other petroleum products and coal, wool, and other carbon-containing materials. It also occurs in cigarette smoke and automobile exhaust.

Erythrocytes are by far the most numerous of the blood cells, averaging from 4.5 to 5 million per microlitre of blood. Because mature red cells have no nucleus and cannot divide, they must be replaced constantly. After leaving the bone marrow, they circulate in the bloodstream for about 120 days before there membranes deteriorate and they are destroyed by liver or spleen. Red cell production is stimulated by the hormone erythropoietin ,EPO, which is released from the kidney in response to a decrease in its oxygen supply. The constant production of a red cell requires and adequate supply of nutrients, particularly protein, the B vitamins B12 and folic acid, required for the production of DNA, and the mineral iron and copper for the production of hemoglobin. Vitamin C is also important for the proper absorption of iron from the small intestine.

Leukocytes:-
The leukocytes, or white blood cells, are different from the erythrocytes in apperance, quantity, and function. The cells themselves are round, but they contain prominent nuclei of varying shapes and sizes. Occuring at a concentration of 5,000 to 10,000 per cubic milimeter of blood, leukocytes are outnumbered by red cells by about 700 to 1. Although the red cells have a definite colour, the leukocytes tend to be colourless.

The different types of white cells are identified by their size, the shape of the nucleus, and the apperance of granules in the cytoplasm when the cells are stained. The stain commonly used for blood is Wright stain, which is a mixture of dyes that differentiates the various blood cells. The granules in the white cells are actually lysosomes and other secretory vesicles. They are present in all white cells, but they are more easily stained and more visible in some cells than in others. The relative percentage of the different types of leukocytes is a valuable clue in arriving at a medical diagnosis.

The granular leukocytes, or granulocytes, are so named because they show visible granules in the cytoplasm when staineD. Each has a very distincttive, highly segmented nucleus. The different types of granulocytes are named for the type of dyes they take up when stained. They include the following:

*Neutrophils stain with either acidic or basic dyes and show lavender granules.

*Eosinophils stain with acidic dyes (eosin one) and have beadlike, bright pink granules.

*Basophils stain with basic dyes and have large, dark blue granuels that often obscure the nucleus.

*Lymphocytes are the second most numerous of the white cells. Although lymphocytes originate in the red bone marrow, they develop to maturity in lymphoid tissue and can multipy in this tissue as well. They circulate in the lymphatic system and are active in immunity.

*Monocytes are the largest in size. They average about 5% of the leukocytes.

Functions of Leukocytes
Leukocytes clear the body of foreign material and cellular debris. Most importantly they destroy pathogens that may invade the body. Neutrophils and monocytes engage in phagocytes, the engulfing of foreign matter. Whenever pathogens enter the tissues, as through a wound, they are attracted to the area. They squeeze between the cells of the capillary walls and proceed by ameboid, or amebalike, motion to the area of infection where they engulf the invaders. Lysosomes in the cytoplasm then digest the foreign organisms and the cells eliminate the waste products.

When foreign organisms invade, the bone marrow and lymphoid tissue go into emergency production of white cells, and their number increases enormously as a result. Detection of an abnormally large number of white cells in the blood is an indication of infection. In battling pathogens, leukocytes themselves may be destroyed. A mixture of dead and living bacteria, together with dead and living leukocytes, forms pus. A collection of pus localized in one area is known as an abscess.

Some monocytes enter the tissues, enlagre, and mature into macrophagus, which are highly active in disposing of invaders and foreign material. Although most circulating lymphocytes live only 6 to 8 hours, those that enter the tissue may survive for longer periods-days, months, or even years.

Some lymphocytes become plasma cells, active in the production of circulating antibodies needed for immunity.

Platelets:-
The blood platelets (thrombocytes) are the smallest of all the formed elements. These tiny structures are not cells in themselves but rather fragments constantly released from giant bone marrow cells called megakaryocytes. Platelets do not have nuclei or DNA, but they do contain active enzymes and mitochondria. The number of platelets in the circulating blood has been estimated to range from 150,000 to 450,000 per mm3. They have a life-span of about 10 days.

Platelets are essential to blood coagulation (clotting). When blood comes in contact with any tissue other than the smooth lining of the blood vessels, as in the case of injury, the platelets stick together and form a plug that seals the wound. The platelets then release chemicals that participate in the formarion of a clot to stop blood loss.