# 74 Blood cells - structure and functions

Blood consists of cells floating in plasma. 
Most of the cells are red blood cells. 
A much smaller number are white blood cells.

There are also fragments formed from special cells in the bone marrow, called platelets. 

Red and white blood cells as seen under a light microscope.

Blood as seen through a microscope:

• The largest cells are white cells. 
• The others are all red cells. 
• There are also a few platelets. 

Functions of blood cells 

• Red blood cells transport oxygen.
• White blood cells protect against disease.
• Blood platelets help the blood to clot.

1.Red blood cells (erythrocytes)

• Made in the bone marrow of some bones, including ribs, vertebrae and some limb bones. Produced at a very fast rate – about 9000 million per hour!

• Transport O₂ from lungs to all respiring tissues. Prepare CO₂ for transport from all rerspiring tissues to lungs.

• Contain haemoglobin (Hb), a red iron-containing pigment which can carry O_2. In the lungs, Hb combines with O₂ to form oxyhaemoglobin. In other organs, oxyhaemoglobin splits up into Hb and O₂

• Have no nucleus à can fit more Hb inside the cytoplasm, but can lives only for about 4 months. 

• Have a special biconcave disc shape à increases the surface area and makes the diffusion of oxygen into & out of the cell easier.

• Old red blood cells are broken down in the liver, spleen and bone marrow. Some of the iron from the Hb is stored, and used for making new Hb, some of it is turned into bile pigment and excreted. 

2. White blood cells (leukocytes)

• Made in the bone marrow and in the lymph nodes.

• Have a nucleus, often large and lobed.

• Can move around and squeeze out through the walls of blood capillaries into all parts of the body.

• There are many different kinds of white blood cells. They all have the function of fighting pathogens (disease-causing bacteria and viruses) and to clear up any dead body cells in your body: 

a.Phagocytes: 

• Have lobed nuclei and granular cytoplasm.

• Can move out of capillaries to the site of an infection. 

• Remove any microorganisms that invade the body and might cause infection, engulf (ingest) and kill them by digesting them. 

   b. Lymphocytes: produce antibodies to fight bacteria and foreign     materials.   

• Have large nuclei
• Responsible for immunity
• There are two different types of lymphocytes:

       B-lymphocytes: secrete special proteins called antibodies in response to contact with their particular antigen, which may be an invading pathogen or a foreign tissue that has been transplanted. 

      T-lymphocytes attack foreign or infected cells and kill them.

3. Platelets (thrombocytes) 

• Small fragments of cells, with no nucleus. 

• Made in the red bone marrow.

• Involve in blood clotting: form blood clot, which stop blood loss at a wound and prevent the entry of germs into the body.

Functions of the blood

• Transportation of R.B.C’s, W.B.C’s, oxygen, food nutrients, hormones, and waste products.

• Defence against disease, by white blood cells phagocytosis and production of antibodies.

• Supplying cells with glucose to respire and keep a constant temperature.