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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.


One of the wonders of the human body is its adaptability. We adapt to our surroundings by internally changing the way our body functions, and we do it automatically. Remember, as we enter a new environment like the one we encounter during space flight, our bodily condition changes from "Earth-normal" to "space-normal." We don't even have to be aware that these changes are occurring. And, as far as we know, the changes which do occur are appropriate as long as we remain in space. The real challenge to the body comes when an astronaut returns to the gravity field of Earth. In fact, some of the most difficult problems that an astronaut must cope with appear during and just after re-entry and landing.

What is the "space-normal" condition for humans? As of yet, we have no final and complete answer to this question. We are only now scratching the surface of knowledge in this area of physiology. The complex interactive nature of the body itself becomes very clear as we learn more and more about how inter-dependent all of the parts of the body are. In other words, the body is a very sophisticated system made up of many functioning parts and mechanisms, all of which depend on other parts and mechanisms. For instance, the cardiovascular system cannot carry out its prescribed duties for the body unless the pulmonary (lungs) system contributes its share faithfully. The combined system with the heart and lungs working together is often referred to as the cardiopulmonary system.

As long as we have an atmosphere that provides the needed gases at the right pressure for proper breathing, a normal human cardiopulmonary system here on Earth can perform the essential function of respiration, which is the overall process of exchanging oxygen and carbon dioxide between the environment and the blood. Respiration may be broken into three stages. The first is the process of breathing that involves the movement of air into and out of the lungs. The second stage is the exchange of gases between the internal surface of the lungs and the blood. The third is the exchange of gases between the blood and the tissue cells.

Movement of the air from the external environment into the lungs is accomplished by the action of two groups of muscles. The first is the diaphragm, a muscular wall that divides the body cavity into two parts. The second are the rib muscles (the intercostal muscles). These muscles act together to change the size of the chest cavity. The rib muscles are attached to your ribs which, in turn, encircle the pulmonary organs and chest cavity. Together, this system is often referred to as the rib cage.

The process of respiration involves many interactions. Let us begin with inspiration (breathing in). Air is drawn into the lungs as a result of the combined expansion of the rib cage and the lowering of the diaphragm (in normal breathing it is lowered about 1 cm.; in heavy breathing it can be lowered up to 10 cm.). When the lungs are expanded in this state, atmospheric pressure (the pressure outside of the body) is higher than the pressure in the lungs. Air is pushed from the higher to the lower pressure area into the lungs through the trachea. In the lungs, air is conducted through the bronchi and their branches (the bronchioles) to about 300 million small air sacs or alveoli, which are honeycomb-like structures, each about 200 to 250 microns in diameter. (A micron is a millionth of a meter.) It is from the alveoli that the blood receives its oxygen. The red blood cells (RBCs) flow through the pulmonary capillaries, where they pick up the oxygen from the alveoli. Oxygenated blood flows back to the heart.

Figure 1. The respiratory tract. When breathing in (inspiration), air enters through the nasal cavities or oral cavity into the pharynx and into the trachea. This process allows the air to be warmed and humidified before entering the delicate tissue bed of ihe lungs. The trachea branches into the bronchi (singular: bronchus), which eventually branches into microscopic sacs called alveolae. When breathing out (expiration), the flow is reversed.

In expiration (breathing out), the rib cage and the diaphragm relax and the lungs contract. Now the air pressure inside the lungs is higher than the atmospheric pressure and air is forced out. However, the lungs never completely deflate, that is, some air always stays in the lungs. This volume of air that always stays in the lungs is called residual volume. The volume of air that moves in or out of the lungs in one normal breath is called the tidal volume. Definitions for other important respiratory measurements are found in Table 1.

Table 1. Definitions of respiratory measurements.

Tidal volume                 The amount of air that moves in or
                             out in one normal breath (~ 500 ml.).

Inspiratory reserve volume   The amount of air that can be inhaled
			     beyond the normal indrawn breath 
                             (~ 2900 ml.).

Expiratory reserve volume    The amount of air that can be exhaled
			     beyond the normal exhaled breath
			     (~ 1100 ml.).

Vital capacity               The amount of air that can be inhaled
			     in the deepest breath and exhaled
			     completely (~ 4500 ml.).  Vital
			     capacity = tidal volume + inspiratory
			     reserve volume + expiratory reserve

Residual volume              The amount of air that cannot be
			     expelled from the lungs no matter how
			     hard one tries (~ 1200 ml.).

Total lung capacity          The amount of air that can be accommodated
			     by the lungs. Total lung capacity =
			     vital capacity + residual volume.
Proper functioning of the cardiopulmonary system is essential for a human being to survive. In fact, the lungs work so closely with the heart and blood vessels that certain heart function measurements can actually be obtained from measurements of pulmonary function. That is, measurements of the gas exchange processes in the lungs yield information on the blood flow through the heart; flow rates such as venous return (cardiac input) and cardiac output can be obtained from pulmonary function measurements. This works because the amount of blood flowing into the right side of the heart equals the amount of blood flowing out of the right side of the heart and into the lung's capillary beds. Blood flow through the lungs is called pulmonary blood flow (PBF). Thus,
   cardiac input = cardiac output = PBF.
The measurement of cardiac output is a very important one to make when investigating cardiovascular fitness. The direct measurement of cardiac output is extremely difficult, especially for space-flight investigations, because it requires a technique that is invasive and overly risky for healthy subjects. In fact, up until now, the absence of reliable, non-invasive techniques for obtaining a measurement of cardiac output while in space has limited us to data obtained before and after missions. The crucial information about the inflight cardiac output changes is still lacking.

More About Respiration

All human tissues and activity are supported by energy obtained from complex "burning" of carbon compounds with oxygen from air to form carbon dioxide. Oxygen uptake depends on your level of activity, and ranges from 4 to 80 ml/Kg/minute (milliliters of oxygen per kilogram of body mass per minute). Carbon dioxide production is typically 0.75 to 1.1 times oxygen usage. All of the living cells in the human body consume oxygen and give off carbon dioxide as a waste product.

Respiration maintains a positive pressure gradient from the atmosphere to the cells for oxygen (higher partial pressure of oxygen outside the cell than inside), and a negative pressure gradient for carbon dioxide from the cells to the atmosphere (lower partial pressure of carbon dioxide outside the cell than inside). In this way, oxygen can travel into the cells, and carbon dioxide can travel out of them.

Ventilation (breathing) is the mechanical process by which lung tissue is exposed to fresh atmospheric gas. Your breathing controls both the rate and volume of air.

Hemoglobin in red blood cells absorbs large quantities of oxygen and smaller quantities of carbon dioxide and transports them to and from the cells, exchanging these gases in the lungs during ventilation.

The Effects of Microgravity on Breathing

On Earth, gravity affects the way the lungs operate, and may even exaggerate and contribute to the medical problems associated with such lung disorders as emphysema and tuberculosis. For example, because of the upright posture in humans, gravity causes the distribution of ventilation (the rate at which air is renewed each minute in the gas exchange area of the lungs) to be greatest near the bottom of each lung and become progressively smaller toward the top.

Gravity has a similar effect on perfusion (blood flow) through the lung, but the two (ventilation and perfusion) do not balance out at the ideal value for gas exchange (oxygen uptake by the blood and release of carbon dioxide from the blood). In microgravity, these gravitational effects should disappear, and lung function should change. This is because of the direct effect of gravity removal on both ventilation and blood flow, and because of the indirect effect of removing the weight of the rib cage and the lungs themselves. On Earth, the weights of the rib cage and lungs tend to distort the anatomy of the lung, and these distortions may affect lung performance.

Select here for a brief discussion about air in spacecraft.


  1. Why is blood flow important to proper function of the lungs?
  2. What do the lungs do with carbon dioxide?
  3. What's the difference between ventilation and respiration?

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

The next section is about the immune system.

Last modified: Feb 27, 1995

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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