Blood|Erythrocytes(red blood cells),Leukocytes(white blood cells),Platelets(thrombocytes) - Medical world

Blood

Blood is composed of a clear , straw-coloured, watery fluid called plasma.

Plasma

The constitution of plasma are water and dis-solved and suspended substance, including: plasma protein, inorganic salts,nutrients,  waste material, hormones and gases.

Cellular content of blood: 

There are three types of blood cell:

1- Erythrocytes

2- Platelets

3- Leukocytes

Blood cells are synthesised mainly in red bone marrow. Some lymphocytes, additionally, are produced in lymphoid tissue. In the bone marrow, all blood cells originate from pluriotent stem cells and go through several development stages before entering the blood.

Erythrocytes (red blood cells )

Red blood cells are by far the most abundant type of blood cell; 99% of all blood cells are Erythrocytes. They are biconcave discs with no nucleus, and their diameter is about 7 micro miter .

Their main function is in gas transport, mainly of oxygen, but they also carry some carbon dioxide. 

Life span and functions of Erythrocytes :

They have no nucleus,  erythrocytes cannot divide and so need to be continually replaced by new cells from the red bone marrow, which is present in the ends of long bones and in flat and irregular bones. Their life span in the circulation is about 120 days. There are approximately 30 trillion red blood cells in the average human body, about 25% of the body's total cell count and around 1%, mainly older cells, are cleared and destroyed daily. 

Vitamin B12 and folic acid are required for red blood cells synthesis. They are absorbed in the intestines, although vitamin B12 must be bound to intrinsic factor to allow absorption to take place.  

Haemoglobin :

Haemoglobin is a large,  complex molecule containing a globular protein and a pigmented iron- containing complex called haem. Each haemoglobin molecule contains four globin chains and four haem units, each with one atom. 

Normal haemoglobin in female 11.5- 15 and in males 13-18

 .

Oxygen transport :

When all four oxygen- binding sites on a haemoglobin molecule are full, it is described as saturated.  

Haemoglobin binds reversibly to oxygen to form oxyhaemoglobin according to the equation:

Haemoglobin+oxygen=oxyhaemoglobin

As the oxygen content of blood increase, its colour changes too. Blood rich in oxygen is bright red because of the high level of oxyhaemoglobin.

Where oxygen levels are low, oxyhaemoglobin breaks down , releasing oxygen. In the tissue,  which constantly consume oxygen,  oxygen levels are always low.

Leukocytes (white blood cells) 

These cells have an important function in defence and immunity.  They detect foreign or abnormal material and destory it. Leukocytes are the largest blood cells but they account for only about 1% of the blood volume. They contain nuclei and some have granules in their cytoplasm.

Granulocytes - it is also known as polymorphonuclear Leukocytes and its include: neutrophils,eosinophils and basophils.

Agranulocytes - monocytes and  .                                                   lymphocytes 

Granulocytes- During their formation, granulopoieses, they follow a common line of development through myeloblast to myelocyte before differentiating into the three types-

1-). Neutrophils:  neutrophils are highly mobile and squeeze through the capillary walls in the affected area by diapedesis Their numbers rise very quickly in an area of damaged or infected tissue once there, they engulf and kill bacteria by phagocytosis.

2-) Eosinophils:  Eosinophils, although capable of phagocytosis, are less active in this than neutrophils; their specialised role appears to be in the elimination of parasites, they are equipped with certain toxic chemical,  stored in their granules , which they release when the eosinophil binds to an infecting organism.

Local accumulation of eosinophils may occur in allergic inflammation, sach as the asthmatic airway and skin allergies.

3: Basophils: Basophils,  which are closely associated with allergic reactions, contain cytoplasmic granules packed with heparin, histamine and other substances that promote inflammation. Usually the stimulus that causes basophils to release the contents of their granules is an allergen of some type. This binds to antibody -type receptors on the basophil membrane.

Agranulocytes:  The monocytes and lymphocytes make up 25 to 50 % of the total Leukocyte count. They have a large nucleus and no cytoplasmic granules.

1-) Monocytes: These are the largest of the white blood cells. Some circulate in the blood and are actively motile and phagocytic while others migrate into the tissue where they develop into macrophages, both types pf cell produce interleukin.

2-) Lymphocytes:  it is smaller than monocytes and have large nuclei some circulate in the blood but most are found in tissue,  including lymphatic tissue such as lymph nodes and the spleen. Lymphocytes develop from pluriotent stem cells in red bone marrow and from precursor in lymphoid tissue.

The production of two distinct type of lymphocytes- T lymphocytes and B lymphocytes 

Platelets (Thrombosis)

These are very small discs, 2-4 micro-miter in diameter, it is derived from the cytoplasm of megakaryocytes in red bone marrow. Although they have no nucleus, their cytoplasm is packed with granules containing a variety of substances that promote blood clotting, which causes haemostasis.

  Platelets is also known as thrombosis. The mechanism that regulate platelet number are not fully understood but the hormone "thrombopoeitin from the liver stimulates platelet production.

    The life span of platelets is between 8 and 11 days and those not used in haemostasis are destroyed by macrophages, mainly in the spleen.

Haemostasis: 

when blood vessel is damaged,  loss of blood is stopped and healing occurs in a series of overlapping processes in which platelets play a vital part. The more badly damaged the vessels wall is,the faster coagulation begin sometimes as quickly as 15 seconds after injury.

1- Vasoconstriction- when platelets come into contact with a damaged blood vessels, their surface becomes sticky and they adhere to the damaged wall. They then 'release serotonin' , which constricts the vessel , reducing or stopping blood flow through  it. Other chemicals that cause vasoconstriction, 

e.g. thromboxanes, are released by the damaged vessels itself.

2- platelet plug formation:  The adherent platelets clump to each other and release other substances, including ADP , which attract more platelets to the site.

Platelet plug formation is usually complete within 6 minutes of injury.

3- Coagulation (blood clotting):  This is a complex process that also involves a positive feedback system and only a few stages are included here. 

The blood clotting factors includes:

a-).  Fibrinogen,

 b-)  Prothrombin

. c-)  Tissue factor

. d-)   Calcium

. e-)  Labile factor, proaccelerin,Ac- globulin

. g-)    Stable factor

.  f-) Antihaemophilic globulin.

            Antihaemophilic factor

   h-) Christmas factor, plasma.                             

    i-)    Stuart prower factor

.   j-)     Antihaemophilic factor C

.   k-)     Hageman factor

.   l-)        Fibrin stabilising factor

The final common pathway can be initiated by two processes which often occur together the extrinsic and intrinsic pathways.

The 'extrinsic pathway' is activated rapidly following tissue damage.

Damaged tissue releases a complex of chemicals called "thromboplastin"  .

The 'intrinsic pathway ' is slower and is triggered when blood comes into contact with damaged blood vessels lining .

4-) Fibrinolysis: After the clot has formed , the process of removing it and healing the damaged blood vessels begins. 

The breakdown of the clot, or Fibrinolysis,  is the first stage.