Hemoglobin is a protein designed to carry oxygen from the lungs to the body tissues.  Hemoglobin is found in red blood cells.  In fact, there are about 300 million hemoglobin molecules in each red blood cell, and each hemoglobin molecule can carry 4 oxygen molecules. 

So, for those of you interested in math...

An average adult male has about...

• 6 liters of blood
• 1 million microliters per liter
• 5 million red blood cells per microliter of blood
• 300 million hemoglobin molecules per red blood cell
• 4 oxygen molecules per hemoglobin molecule
• Which equals 36,000,000,000,000,000,000,000 oxygen molecules per person.

What is the hematocrit?

The hematocrit is the proportion of the blood that is made up of red blood cells.  Generally, the hematocrit ranges between 38% to 48%.  Which means that 38% to 48% of blood is made of red blood cells.  A simple way to determine the hematocrit is to take a blood sample in a test tube and spin it in a cetrifuge.  All the cells move to the bottom of the tube and the liquid portion of blood (plasma) stays on the top. 

Doctors use the hematocrit to determine two major factors: 1) are there enough red blood cells and 2) what is the water content of blood.

A low hematocrit can be due to a low number of red blood cells or a high proportion of water in the blood.

A high hematocrit can be due to a high number of red blood cells or a low proportion of water in the blood.

What happens to the hematocrit after a large amount of blood is lost?

A large amount of abnormal bleeding is called a hemorrhage.  This may occur after an accident or due to a clotting disorder.   Interestingly, the hematocrit does not immediately change after an acute hemorrhage.  Bleeding that happens quickly results in the loss of whole blood, which includes both red blood cells and water.  Imagine spilling part of your bowl of cereal.  After a spill, the cereal in the bowl still contains the same amount of cereal per volume of milk!

A large hemorrhage will cause a drop in the blood pressure and an increase in the heart rate. Shortly after a hemorrhage, the body begins working on increasing the volume of blood (in order to increase the blood pressure).  This is most quickly accomplished by increasing the amount of water in the blood (in the cereal example, this would be like adding more milk to the bowl).  Water is conserved by the kidneys and flows from the body tissues into the bloodstream.  This causes the hematocrit to drop as the blood becomes more dilute.

Over several days, the bone marrow will produce red blood cells at a higher rate and this will return the hematocrit to a normal level.

What is hemoglobin?

Hemoglobin is a protein found in red blood cells which carries oxygen from the lungs to the body tissues.  Each hemoglobin molecule has 4 iron ions.  These iron ions are the attachment points for oxygen.  The complex formed by hemoglobin-iron-oxygen gives blood its red color.  Removing the oxygen makes hemoglobin have a dark red appearance.  Venous blood contains hemoglobin with low amounts of oxygen.  This blood is dark red but veins seen through the skin may look blue or purple.

Does hemoglobin carry carbon dioxide?

Yes... but only a small portion of the carbon dioxide removed from the body tissues is attached to hemoglobin (about 10% of the total).  Most carbon dioxide removed from the body travels in the form of bicarbonate ions in the plasma.

What does carbon monoxide do to hemoglobin?

Carbon monoxide can attach tightly to the iron atoms in hemoglobin.  This blocks the sites of attachment for oxygen.  Carbon monoxide poisioning occurs when too many of the oxygen binding sites are blocked by carbon monoxide.  This can lead to inadequate delivery of oxygen to the tissues and can be very dangerous to your health!

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