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Archived by Raymond J. Noonan, Ph.D., Health and Physical Education Department, Fashion Institute of Technology of the State University of New York (FIT-SUNY), and SexQuest/The Sex Institute, NYC, for the benefit of students and other researchers interested in the human aspects of the space life sciences. Return to first page for background information on these pages.
Space Biomedical Research Institute



Blood supplies oxygen and nutrients to all of the parts of the body, and it carries away waste products from cells. The blood vessels serve as the transportation pathways in the body for the flow of fluid, blood cells, platelets, gases, waste products, nutrients, lipids, sugars, amino acids, vitamins, proteins, hormones, electrolytes, and other substances. Blood contains all of these elements. One could easily enlarge this list, and specify what kind of fluids, what kind of cells, what kind of gases, and what kind of waste products, etc. are found in the blood. It is beyond the scope of this discussion to present a full description of every component that we have mentioned; however, we will attempt to characterize the general nature of blood and how this nature is changed when a human enters the microgravity environment.

Figure 1. Blood can be separated into its components by putting it into a centrifuge and "spinning it down." The parts separate according to their relative "weights." This shows the components of blood in their relative ratios. It shows a hematocrit of 40% because the RBCs and "buffy coat" make up 40% of the total volume of centrifuged blood (4 of 10 ml.).

Blood has two distinct parts to it. Plasma, the fluid part of the blood, takes up about 55% of the blood volume. Plasma is 91% water with various other materials in solution and suspension, such as dissolved proteins, nutrients, electrolytes, hormones, and metabolic wastes. By analyzing plasma, medical doctors can find out what types of nutrients are circulating throughout the body, and they can measure the levels of hormones and other constituents that plasma helps to transport.

The cellular portion of blood normally makes up about 45% of the blood volume and it consists primarily of white blood cells (WBCs), platelets, and red blood cells (RBCs). The white blood cells are the mobile elements of the body's defense system. Platelets are small cell fragments which play an important part in blood clotting. For our discussion here, we will concentrate on the most abundant of all the cells of the body, namely the RBC, often referred to as an erythrocyte. The study of the activity of the red blood cells is called erythrokinetics (kinetic = movement or activity). Erythrokinetics involves looking at the entire lifetime of a red blood cell from its "birth" (it is born in the bone marrow), through its passage around the body (each RBC travels around the body about once per minute), all the way through its destruction. A normal lifetime for each RBC is 90 to 120 days.

Figure 2. Regulation of red blood cell production (erythropoiesis).

RBCs carry vital oxygen throughout the body, and they do so continuously in order to meet the oxygen requirements of the cells and tissues in the body. The oxygen-carrying component of the RBC is known as hemoglobin. If the number or volume of RBCs decreases below normal, the hemoglobin oxygen-carrying capacity decreases. This condition is called anemia. Conversely, if the number or volume of RBCs increases above normal, the hemoglobin and oxygen-carrying capacity increases. This occurs as an adaptation to life at a high altitude.

Normal RBCs are biconcave disks that are capable of changing their shape as they pass through capillaries. Actually, the RBC is a "bag" that can be deformed into almost any shape without rupturing the cell. They are remarkably flexible and remarkably small. In normal men the average number of RBCs per cubic millimeter is 5,200,000 (+/– 300,000) and in normal women 4,700,000 (+/– 300,000). The number of RBCs varies in the two sexes a different ages. Also, the altitude at which a person lives affects the number of RBCs present in his or her system.

A common laboratory test can tell a physician a great deal about the volume of red cells in a blood sample. The volume of RBCs refers to the amount of space that the RBCs occupy within the blood. If whole blood (the cellular portion together with the plasma) is placed in a special hematocrit tube and then "spun down" in a centrifuge, the heavier formed elements will quickly settle to the bottom of the tube. During the hematocrit procedure, the RBCs are forced to the bottom of the tube first because they are the heaviest element in the blood. The white blood cells and platelets then settle out in a layer called the buffy coat (see Figure 1). Above the buffy coat is the plasma. From the hematocrit tube, one can approximate the percentage of space that the RBCs occupy in the total sample. At sea level, the hematocrit of a normal adult male averages about 47 (which means that 47% of the blood volume is RBCs), while that of a normal adult female is 42.

The total mass of RBCs in the circulatory system is regulated within very narrow limits so that an adequate number of red cells is always available to provide sufficient tissue oxygenation, and, yet, so that the cells do not become so concentrated that they impede blood flow. The term erythropoiesis (erythro=RBC, and, poiesis=to make) is used to describe the process of RBC formation. In humans, erythropoiesis occurs almost exclusively in the bone marrow. The bone marrow of essentially all bones produces RBCs from birth to about 5 years of age. Above age 20 most RBCs are produced primarily in the marrow of the vertebrae, the sternum the ribs, and the pelvis. Our body produces RBCs every day to replace the old RBCs. (The old RBCs are "eaten" by the spleen and the liver and most of the leftover components are recycled to form new RBCs.) The body can, however, increase production in response to special needs. For instance, if you were to travel to a high altitude area, you would feel dizzy because the oxygen levels at high altitudes are lower (the air is thinner), and your brain is not receiving the amount of oxygen that it is accustomed to receiving. That is, your normal body oxygen needs are not being met. Therefore, your body responds by increasing the production of RBCs to increase the oxygen-carrying capacity of the blood.

The organ responsible for "turning on the faucet" of RBC production is the kidney. The kidney responds to low levels of oxygen by stimulating the release of a hormone called erythropoietin which, in turn, acts on the bone marrow to increase the rate of RBC production. At this point, the number of RBCs increases (and so does the hematocrit), thereby increasing the oxygen-carrying capacity of the blood. When the oxygen level in the blood is sufficient, the kidneys suppress the erythropoietin activity until the next need arises.

What has surprised scientists is the fact that, for space travelers, the "space normal" hematocrit is about the same as the "Earth-normal" hematocrit. Why is this surprising? Remember that as humans are exposed to the microgravity of space, there is a loss of body fluid due to the headward fluid shift. One would imagine that as the fluid level is decreased, the percentage of RBCs per unit volume of blood would increase. That is, the hematocrit would rise. Data has indicated that the pre-flight and in-flight hematocrit measurements for astronauts do not change to any appreciable extent. Since there is a decrease in fluid, and since the hematocrit does not change, this suggests that the number of RBCs must decrease. We call this reduction in RBCs "space anemia."

There are different theories to explain this decrease in the number of RBCs. One, called the "hemoconcentration" theory, suggests that, while in space, the body detects an overabundance of fluids in the upper part of the body. The kidney is then stimulated to remove this excess fluid, and the blood becomes "thicker," because there is a higher percentage of RBCs per volume of blood. This causes an overabundance of oxygen-carrying ability; the kidney is not stimulated to produce erythropoietin, which, in turn, suppresses RBC formation. Data suggests that this is not the only theory to consider.

There have been alternative suggestions made to explain the space-flight reduction of RBCs. These include the possibilities that the "space anemia" is due to the loss of muscle mass which occurs in space flight. Because muscles are used less in microgravity (there isn't even any "walking around" to do), the muscles lose mass and require less oxygen. This results in a lowered oxygen-carrying capacity requirement for the blood. The body responds to this by reducing the number of RBCs produced. (Or, it may be that the body responds to this by increasing the destruction rate of RBCs.)

Another theory for the reduction in RBC mass has to do with the well-documented fact that astronauts lose calcium under conditions of microgravity. The loss of body calcium could disrupt the bone architecture, which would result in the loss of bone strength. This alteration of bone metabolism may also affect the bone marrow, and, therefore, affect normal RBC production in the marrow.


  1. What does blood do?
  2. How many times total does a single RBC travel around the body in its entire lifetime?
  3. What do blood vessels do?
  4. What are the two distinct components of blood?
  5. Would a centrifuge work in space?
  6. What is "space anemia?"
  7. In science, how do we prove or disproved competing theories?

You can go back to where you came from, or jump back to the beginning.

The next section is about the fluid regulation system.

Last modified: Oct 8, 1994

Author: Ken Jenks


Contact Info:
Raymond J. Noonan, Ph.D.
Health and Physical Education Department
Fashion Institute of Technology of the
State University of New York (FIT-SUNY);
SexQuest/The Sex Institute, NYC
P.O. Box 20166, New York, NY 10014
(212) 217-7460

Author of:

R. J. Noonan. (1998). A Philosophical Inquiry into the Role of Sexology in
Space Life Sciences Research and Human Factors
Considerations for Extended Spaceflight
Dr. Ray Noonan’s Dissertation Information Pages:
[Abstract] [Table of Contents] [Preface] [AsMA 2000 Presentation Abstract]


First published on the Web on June 14, 1998
This page was last changed on March 25, 2002; Ver. 3a
Copyright © 1998-2002 Raymond J. Noonan, Ph.D.

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