The remaining 40 to 50 per cent of the blood is composed of the formed elements called blood cells. Three main types of cells are found in human blood:

• red blood cells (erythrocytes)
• white blood cells (leukocytes)
• platelets

If one takes a sample of blood, treats it with an agent to prevent clotting, and spins it in a centrifuge, the red cells settle to the bottom and the white cells settle on top of them forming the "buffy coat".

The fraction occupied by the red cells is called the hematocrit. The hematocrit can be calculated by using the following equation:

Hematocrit = red blood cell volume/total blood volume x 100%

Normally the hematocrit is approximately 45% (45 per cent of the total blood volume is red blood cells). Values much lower than this are a sign of anemia. Values much higher than this are a sign of increased red blood cell production.

Byron and the other members of the expedition will develop a higher hematocrit as their bodies acclimatize. Although the higher hematocrit will increase the oxygen carry capacity of the blood it also causes the blood to thicken. The increased viscosity may limit the flow of blood.

Red Blood Cells
Human red blood cells (erythrocytes) are biconcave, disc-shaped cells. The electron micrograph shows the shape of the red blood cells. This shape gives the red blood cell a very large surface area.

There are normally about five million of them per cubic millimetre of blood. Though the number of red blood cells remains amazingly constant from day to day, continual destruction of some and the formation of new ones goes on; the normal life span of a red blood cell is 120 days.

Unlike all other cells in the human body, red blood cells do not have a nucleus. In adults, the erythrocytes are formed in the red bone marrow, in bones such as the sternum and the upper ends in the long bones of the body. During their development, these cells do have a nucleus.

However, as the red blood cells mature, the nucleus disintegrates and they acquire the red oxygen-carrying pigment called hemoglobin. Hemoglobin is a protein that contains iron. A single human erythrocyte contains about 280 million molecules of hemoglobin. Red blood cells are an effective and efficient mechanism for packaging large amounts of hemoglobin.

Function of Red Blood Cells
The hemoglobin molecule has four sites to transport gases. One of the primary functions of red blood cells is to transport oxygen from the lungs to all of the cells of the body. As well, the erythrocytes pick up carbon dioxide from every cell and transport it to the lungs where much of it is removed from the body. The hemoglobin molecule is responsible for transporting both oxygen and carbon dioxide.

If the concentration of oxygen is high, as it is in the lungs, nearly all of the hemoglobin sites will be filled with oxygen. On the other hand, if the concentration of oxygen is low, the oxygen molecules will shake loose from the hemoglobin. The oxygen is released from the hemoglobin and diffuses into the cells.

Carbon dioxide (CO2) is the waste product produced by cells. When the CO2 enters the blood stream, about 27% of it attaches directly to the hemoglobin molecules. The red blood cells transport the carbon dioxide to the lungs. In the lungs, the O2 molecules combine with the hemoglobin, which then release the carbon dioxide molecules. The carbon dioxide is removed from the body. This will be covered in more detail when we discuss the respiratory system.

Blood Cell Production
The rate of production of red blood cells is not rigidly fixed. It depends on the oxygen concentration in the blood. Whenever the oxygen content of the blood is low, the production of red blood cells increases. This provides the body with an effective mechanism for regulation of blood oxygen.

When you climb a mountain, for example, as you go higher the air becomes less dense. This means that less and less oxygen is reaching your lungs so the oxygen content in the blood drops. You would likely find yourself "short of breath." However, after a few weeks of living at high altitude, a process of acclimatization occurs.

The process of acclimatization is regulated by a protein called erythropoietin (EPO). It acts on the bone marrow to increase the production of red blood cells. An oxygen shortage (such as found at high altitude) stimulates the kidney to release erythropoietin (EPO) which enhances the production of red blood cells in the bone marrow.

Each millilitre of blood now contains more red blood cells and, therefore, a greater concentration of hemoglobin, which can carry oxygen. The oxygen content of the blood has risen back toward normal. The following diagram illustrates this process.

Red blood cell production is stimulated by the lack of oxygen. The feedback loop shows how this response helps to maintain the oxygen content in the blood.

It is important to emphasize that this response (increased red blood cell production) to a decreased oxygen content in the atmosphere is a very slow one. It takes many days before the increased cell production fully compensates for the lower oxygen levels.

This process helps to explain why Byron Smith and the members of the expedition must take their time acclimatizing. The process of acclimatization occurs as the members of the expedition trek from Katmandu to Base Camp. It continues as the climbers spend many weeks climbing up to various camps on the mountain and then back down to lower altitudes. The process of acclimatization is critical to prevent potentially fatal disorders including Acute Mountain Sickness.

Advanced Concepts: Oxygen Transport
The hemoglobin (Hb) molecule consists of four polypeptide molecules. Each of these is attached to a heme group. There is one atom of iron at the center of each heme molecule. One molecule of oxygen can bind to each heme. The reaction of oxygen binding to the heme group is reversible - this process allows for the release of oxygen in the tissues.

Under the conditions of lower temperature, higher pH, and increased oxygen pressure in the capillaries of the lungs, the reaction proceeds to the right. The purple-red deoxygenated hemoglobin of the venous blood becomes the bright-red oxyhemoglobin of the arterial blood.

Under the conditions of higher temperature, lower pH, and lower oxygen pressure in the tissues, the reverse reaction is promoted and oxyhemoglobin gives up its oxygen.

Byron and the other members of the expedition will have to acclimatize to the high altitude.

The circulatory system's response to low levels of oxygen is to produce more red blood cells.

Eventually, the red blood cell mass increases. Whether an increased hematocrit is helpful physiologically is still somewhat controversial.

The hematocrit rises from 40 to 45 per cent red blood cells to a maximum level of 60 per cent red blood cells. These effects take several weeks to start to develop. Eventually, tissue becomes more efficient at extracting oxygen, by mechanisms that are not yet understood.

Blood and Tissue Oxygen Transfer at High Altitude

Graph provided by Wilderness Medical Society

As the hematocrit increases with altitude, the oxygen carrying capacity of a given volume of blood increases. This is demonstrated by the following diagram of blood in two capillary tubes, however, the increased mass of red blood cells makes the blood more viscous which may limit its ability to flow.

In the second part, hemoglobin is represented inside an erythrocyte by an oxygen-hemoglobin dissociation curve. At high altitude, the tendency of hemoglobin to bind oxygen more avidly (left shift) due to alkalosis (more alkaline pH) is offset by the increased release of hormones. At extreme altitude, the curve shifts to the left; the increased carrying capacity of the hemoglobin may outweigh the correspondingly decreased tendency of oxyhemoglobin to give up oxygen in the peripheral circulation and result in better oxygen delivery to the tissues.

Interesting Question
Are there any physiological differences between people living at high altitudes compared to those living at lower altitudes? This might be a question you would like to explore with members of the Everest 2000 expedition.

One article indicates there are physical differences between people who live at high altitudes (17,500 feet - altitude of Basecamp on Everest) and those living at a lower altitude. These differences include increased chest size; decreased body mass and increased size of the right side of the heart.

Based on these observations, one could infer that:

People who live at higher altitudes also have a larger quantity of hemoglobin, which enables a higher percentage of oxygen in the blood even when the air pressure (the pressure of oxygen) is much lower.