Anemia simply means that you don't have enough blood. The main function of blood (but not the only one) is to carry oxygen from the lungs to the tissues where it is used. It does this with the red stuff in blood, which is known as hemoglobin. On meeting oxygen the hemoglobin undergoes a subtle change to oxy-hemoglobin, and when it releases the oxygen it changes back. Oh, and by the way, the abbreviation for hemoglobin is Hb (not Hg. Hg is the chemical symbol for mercury).
The body has sensors for how much oxygen it is getting and it responds to a deficiency by sending a signal to make more blood. More about that later.
Everyone needs slightly different amounts of blood, which is why everybody has a different hemoglobin (Hb) level. Reasons why you might need more Hb than other people include living at altitude, your Hb molecule is slightly different so that it doesn't release oxygen to the tissues so well, having something wrong with your lungs so they can't transmit oxygen to the blood so well, and smoking, so that some of your Hb is constantly bound to carbon monoxide, rendering it unavailable to oxygen. A reason why your Hb might be set lower than other people's is that your Hb molecule is ultra-good at releasing oxygen to the tissues.
So what is a normal Hb? No-one knows what is normal for you, but we recognise that there is a wide distribution of values and we talk about a reference range. If your level falls below the reference range then you are anemic. But not everybody who has a level below the reference range is anemic for them. Very rare individuals have a low Hb normally (I'll tell you why later).
For man the reference range for Hb is 13.5-18.0 grams per deciliter (g/dl) for men and 11.5-16.0 g/dl for women. Many people are surprised at how wide the range is and some reference ranges for different laboratories have a tighter spread than this. In the past 20 years it has been customary to give Hb values in grams per liter in many laboratories so teh ranges would then be 135 to 180 and 115 to 160 g/L.
Why do women have so much lower Hbs than men? It's because of the testosterone which enhances other chemicals that stimulate the bone marrow.
As far as CLL patients are concerned I have always thought it strange that Rai stage 3 and Binet stage C patients are defined by the same reduced Hb level in both men and women, when plainly a man has to be much more anemic to reach these stages than a woman does. I wonder if this is why women seem to do better than men?
There are dozens of causes of anemia and many different ways of classifying them. I prefer to use the size of the red cell to do this. Red cells are by far the most numerous cell in the blood. In a teaspoonful of blood there are 25 trillion of them (or if you are English 25 billion; ie 25 million million). Red cells are round, flat discs with a dimple on each side, and red in color. Their diameter is about 7 microns and their average volume is between 80 and 96 fl. We call this the mean cell volume (MCV). Again different labs have their own reference range (I have seen 78-92 and 80-100). What does fl stand for? Femto-litres which means 10 to the power of minus 15 litres or 0.00000000000001 litres. That's pretty small!
We could call red cells an MCV lower than the reference range microcytic; those with an MCV greater than the reference range macrocytic; and those with MCVs within the reference range normocytic.
A red cell is 99% hemoglobin (the rest is membrane and a few enzymes); so a small red cell has a deficiency of hemoglobin. Here is a link to a good picture of Hb.
In my picture
the green, yellow, blue, and gray colors make up the four polypeptide subunits of hemoglobin. These polypeptides are collectively known as the globin chains and in common or garden hemoglobin (known as HbA) there are two alpha chains and two beta chains. Each subunit has its own heme group (shown in red.)
I have also reproduced a chemical structure for heme. I don't expect any but chemistry students to follow this. Take my word for it that it is made up of four porphyrin rings, but notice, right at the center the letters 'Fe'. Fe is the chemical formula of iron and it is here that the iron fits in.
The availability of iron controls the production of hemoglobin, and anemias with deficiencies of hemoglobin (microcytic anemias) are microcytic becuse they have too little iron or too little globin.