What if gravity is reversed

Free University of Berlin

by Karl Kirsch and Hanns-Christian Gunga

Just as water has shaped fish, gravity has shaped the structure and function of the human body - not excluding the circulatory system and the salt water population of humans. It was the scientific efforts of gravitational physiologists in the last century that examined the effects of gravity on the human body with the help of centrifuges and other aids, as well as the various effects of gravity in the weightlessness of space. The upright man was an excellent subject for study because he has to constantly assert himself in the fight against gravity.

Depending on age, gender and level of training, the human body consists of around 60 percent water. In women and the elderly, this percentage is somewhat lower because of the higher proportion of fat in their body mass, while in children and well-trained athletes, the water content of the body is higher.

The amount of water in the human body

With a body weight of 70 kilograms, you have to expect about 42 liters of water. This would be the distribution space of a readily water-soluble substance that would cross all cell boundaries, such as alcohol. The water in the human body is distributed over two separate fluid spaces (compartments). Of the above-mentioned 42 liters of water, there are around 25 to 28 liters in the cells. This water is the solution space for the numerous biochemical reactions that constantly take place in the body. Its amount and composition change only slowly compared to the volume of fluid that is in the blood vessels and in the intercellular spaces. In this case, one speaks of extracellular water, which makes up about twelve to 15 liters and is divided into an intravascular and interstitial volume. The intravascular volume (blood volume) is moved by the heart through the vascular system in order to transport nutrients and oxygen to the cells and to remove carbon dioxide, the breakdown products of cell metabolism and heat. While the intracellular volume is hardly influenced by gravity in the short term, the extracellular volume is strongly subject to the effects of gravity. This becomes all the more clear when one first looks at the circulatory system through the eyes of a gravitational physiologist.

The blood volume makes up about six to eight percent of the body weight, making it one of the largest organs in the human body. When standing upright, the vascular system can be understood as a long, vertically positioned tube system that extends from the sole of the foot to the top of the head.

About 80 percent of the blood volume is in the slightly stretchable veins, in which the blood collects just below the heart when standing, indicated by the increasingly black hatching in Figure 1. The venous pressures increase continuously from top to bottom. The expansion pressures occurring in the venous sections of the circuit are indicated on the right-hand side of Figure 1 in mm Hg. However, these values ​​only apply if a continuous column of fluid would develop from the heart to the leg veins, which is prevented by venous valves. If the venous valves leak, the veins expand greatly and can take up more volume. Varicose veins develop. Patients with varicose veins therefore have an increased blood volume of around 10 to 15 percent in order to adequately fill the dilated venous system.

Gravity must therefore have a significant influence on all mechanisms that regulate the volume of fluid in the circulatory system, because it influences the distribution of fluid along the body axis.

Figure 1: The blood volume distribution along the body axis in humans compared to four-legged friends; Illustration: Kirsch / Gunga

Blood volume, blood volume distribution and gravity

One difficulty for the human body is to keep the volume of blood as close as possible to the heart so that it can be ejected when necessary. It can be seen from Figure 1 that about 70 percent of the volume in humans is stored below the heart, from which it can be concluded that considerable effort is required to fill the heart in a standing person. In this regard, it is easier for your four-legged friend, because 70 percent of the blood volume is at heart level and is easily available when needed. It is therefore to be understood that compared to four-legged friends, in relation to their body mass, humans have a blood volume that is ten to 15 percent higher. The blood volume distribution and the absolute size of the blood volume are closely linked. This can also be illustrated using another example. If a person has to remain in a horizontal body position for several days, as if they were bedridden, their blood volume decreases very quickly, because in a horizontal body position almost 15 percent of the blood volume, which is normally extrathoracic, i.e. outside the chest, is shifted intrathoracically near the heart . In the long run, the body registers this as too much volume and eliminates it. In the veins near the heart and in the atria of the heart there are stretch receptors that send signals to the center when there is too much volume and, among other things, inhibit the secretion of antidiuretic hormone (ADH). As a result, less water is retained in the kidney and consequently excreted. This reflex is called the Gauer-Henry reflex after its discoverers. Gauer was director at the Physiological Institute of the Free University of Berlin from 1962 to 1979 and was involved in various research activities in this important field of medicine. In the meantime, a number of other hormonal mechanisms have been uncovered that are related to the volume filling of the veins near the heart and the atria of the heart. Another circulatory problem linked to gravity should be pointed out here.

For a person who is 180 cm tall, the heart is about 150 cm above the floor, but is still 30 cm from the brain. The heart has to pump the blood to the brain against gravity. This can sometimes cause considerable difficulties if, for example, the ejection volume of the heart is reduced due to a lack of volume - as in bed rest and high sweat loss.

Figure 2: Heart and circulation in humans, giraffes and dinosaurs. Note the distance between the heart and the brain; Illustration: Kirsch / Gunga

The second figure shows the problems that gravity poses to the circulatory system, more precisely the transport of blood volume. If the human heart has to transport the blood volume about 30 centimeters high to the brain, this is already two meters and more for the giraffe, and almost eight meters for the dinosaur shown. These animals create columns of fluid in the arterial system that are meter-long, some of them vertically positioned, in which blood pressures of several hundred millimeters of mercury arise, which then have to be overcome in order to convey the blood towards the brain.

Figure 3 shows arterial blood pressures at heart level, measured in humans and in a giraffe. The systolic pressure, which is normally around 120 mm Hg in healthy people, reaches up to 370 mm Hg in the giraffe. This pressure work places a considerable strain on the heart of these animals. These hearts are correspondingly big. Heart weights of over 300 kilograms could be calculated for the dinosaurs. In comparison, the weight of a human heart is only about 300 grams.

Conversely, the dynamic pressures exerted by the heart add to the hydrostatic pressures below the heart. Even in humans, depending on body size, we find pressures of 150 to 180 mm Hg in the arteries of the back of the foot. It is easy to calculate how high the pressures in the ankle arteries of giraffes and dinosaurs should be. The question arises as to why these vessels do not burst or why these creatures do not exhibit edema. It would be expected that the high pressures would force fluid into the tissue.

Fig. 3: Arterial blood pressure in humans and in the giraffe

The cycle in space

Certainly in humans more fluid is pressed into the tissues of the legs, especially in the ankle area, which leads to swelling, as you can see in the evening when you take off your socks. This fluid is transported back into the circulation via the lymphatic system. Raising the legs supports this return transport.

If gravity has such an influence on the distribution of fluid along the human body axis as well as on the regulation of fluid volume, the question arises how this behaves in weightlessness and what happens to astronauts. One would expect a redistribution of fluids in space. Indeed it is. Just a few minutes after entering weightlessness, the astronauts notice that their legs are thinning and their faces swell: they develop stork legs and swollen heads. The latter is felt to be extremely uncomfortable because the mucous membranes of the mouth, throat and nose swell and are heavily filled with blood. The astronauts feel as if they are standing upside down on earth. The volume evacuated from the legs collects in the vessels near the heart and expands the heart chambers and the pulmonary vessels. The astronauts feel uncomfortable and limit their fluid intake. The high intrathoracic volume signals a high blood volume to the body anyway, which reduces the feeling of thirst. This is also part of the Gauer-Henry reflex mentioned above. Ultimately, the astronauts have a reduced blood volume, whereby not only the liquid components are reduced, but also cellular components such as the red blood cells.

Due to the shift in blood volume - from the periphery of the legs to the center of the heart and lungs - one should have assumed that the pressures there would have to be higher than the values ​​found on earth. However, our measurements on the Spacelab mission in 1983 and later on the D 1 mission in 1985 showed the opposite. The venous pressures in the veins near the heart had dropped dramatically. American colleagues later confirmed these findings by sending astronauts into space equipped with a cardiac catheter. We had not thought on earth that the heart is normally surrounded by the blood-filled lungs, which surround the heart like a fluid-filled sponge and which exert pressure due to the force of gravity. When entering weightlessness, this externally exerted pressure on the vessels near the heart disappears and as a result the pressures drop suddenly.

This was an example of how the mechanics of the heart is decisively influenced by gravity, the consequences of which could only be considered when the experiment in space had actually been carried out.

The changes in the volume distribution along the body axis that were predicted and actually found by the astronauts have led to numerous considerations as to how this could be achieved in the long term in the earth's gravitational field in order to be able to study the volume-regulating mechanisms in detail. This has led to numerous investigation models, which have subsequently enriched the experimental circulatory physiology considerably.

Two of these models are shown in Figure 4. On the one hand you can - as shown in the upper part - put the person head-deep at an angle of six degrees. This is well tolerated and can be sustained for days and weeks. It moves fluid from the legs into the chest and head. As a result, urine excretion increases and the plasma volume, i.e. the blood fluid, decreases by around 15 percent. At the same time, the blood circulation in the head area increases. After six to eight hours, a test person would have great difficulty standing upright after getting up, he would feel uncomfortable and his eyes would go black. The blood would collect in the legs and the heart would have less volume available. A circulatory collapse would be the result. These are exactly the symptoms astronauts experience when they return from space.

The application of this head-down model brought further insights to light. If test subjects are left in the head-down position for several weeks, there is an increase in thickness of the skull bones, probably due to the increased blood circulation in the head, while the weight-bearing bones in the lower half of the body lose substance. Previously it was thought that immobilization of the body alone was responsible for bone loss. Today we know that the distribution of blood flow along the body axis is also an essential factor for bone development.

Figure 4: Blood volume distribution in humans in the head-down position (above) and standing in the water; Illustration: Kirsch / Gunga

Below that, Figure 4 shows another model for the redistribution of body fluids, which we are familiar with from everyday life. If you stand in the water up to your chest or neck, the hydrostatic pressure of the water compresses the slightly compressible leg veins and the blood escapes first into the vessels of the abdomen and chest (center B) and later, when the water is up to your neck, as shown on the far right (C), almost exclusively in the thoracic organs. The heart chambers are stretched and this indicates too much volume. In reality it is just too much in an unfamiliar place. This volume distribution also occurs when you lie down horizontally in the bathtub. Snorkeling diving has the same effect. In all of these situations, the lungs are exposed to normal atmospheric pressure, while the hydrostatic pressure underwater in the rest of the body is added.

The body reacts to this, as already described above: there is an increased urine excretion. The Gauer-Henry reflex comes into play and the body's fluid level is reduced. Among other things, this was part of the cure effect that was ascribed to the baths in earlier centuries, when the use of diuretics was not yet available for heart patients. The stay in a bath also led to a drainage of the patient, which meant a relief for the sick circulation. This observation has already been described in the Roman writer Livius. The snorkel divers of the Roman fleet, who would be called combat swimmers today, were called urinatores.

So it was the gravity physiologists who, after 2,000 years in space experiments, uncovered the mechanism of bath diuresis. You can see that science often has to take long detours in order to achieve its goal. Simple predictions are not possible with a system as complex as the human organism.

Needless to say, much of what was described here and worked out by gravitational physiologists is nowadays common textbook knowledge for medical professionals. What needs to be said, however, is where this knowledge comes from.