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How Your Body Changes in Pregnancy: Acid-Base Balance

In previous installments of this series, we have looked at how pregnancy changes the physiology of the respiratory system ,the circulatory system, the digestive system, and the urinary system. Today, we’ll explore acid-base physiology and how it changes during pregnancy. Acid base balance in the body is controlled particularly by the lungs and kidneys. This is a very important concept to appreciate, not only because acid-base disturbances relate to numerous different health problems, but also because there are numerous health marketing scams offering what they call an “alkalizing diet”, which is not scientific, but they make sound scientific to consumers by employing some terminology that many people may recall vaguely from high school or college chemistry. This is a common practice of pseudoscience. Whenever somebody tries to sell you a product, or an idea, that is out of tune with what you hear from actual medical doctors, and the individual or company is hitting you with scientific terminology, it is probably not science. It is probably excrement of male cattle (BS).

Often, such marketers will push the product, or idea, with a claim such as “This is what your doctor doesn’t want you to know.” This is particularly common with anything related to chemistry, but what you should keep in mind is that your primary care physician almost certainly knows more chemistry than a person who bought into a fad diet or nutritional supplement franchise. This is certainly true when it comes to the acid-base physiology in the human body, which maintains health by keeping pH of body tissues with in a very narrow range. The normal range depends on which body tissue, of course. Stomach fluid, which receives hydrochloric acid from special cells called parietal cells, has a very acidic pH, typically in the range of 1.5-3.5. That is a wide range, but the acidity is needed for digesting foods, and the stomach lining (normally) protects itself from the acid and it’s not typical of other body tissues.

Most of your body tissues are represented by the pH of the blood, which in a healthy, non-pregnant person is 7.4, although 7.35 to 7.45 is considered the normal range. As you may remember from chemistry, even if high school was your most recent chemistry, pure water has a neutral pH, which means 7.0. This means that your body, overall, is supposed to be about 4 tenths of a pH point into the alkaline direction. Knowing this, marketers of the alkalinizing diet scam have created a narrative saying that alkaline foods are good and acids foods bad, because low pH (high acidity) is associated with disease states, such as cancer and diabetes. But the idea that they have and push is completely out of context. While uncontrolled diabetes can lead to two parallel conditions characterized by the blood and other tissues being too acidic (called diabetic ketoacidosis and lactic acidosis), and while this process is certainly related to diet, the dietary issue is not that the food is too acidic. The issue is that the body cells cannot absorb glucose (blood sugar), so they have to burn something else instead of glucose to generate energy. This, in turn produces certain chemical compounds that are acidic, which travel in the blood, making it acidic. This is called acidemia, if the pH of the blood falls below 7.35, even though, as noted above, anything above 7.0 is technically alkaline. Similarly, while a major risk factor for stomach cancer is an ulcer, caused by the bacterial species Helicobacter pylori, preventing the stomach acid from protecting itself against its acid, the problem here is not the acidity in the stomach, but the bacterial infection. While alkaline food can partly neutralize stomach acid, the effect is modest, and the acidity of the stomach stays in the stomach. It doesn’t transfer to the blood; if it did, the person could not be alive. Whereas normal blood pH is 7.35-7.45, bad things start happening when pH shifts a little bit more, and generally, any blood pH below 6.8 or above 7.8 is considered incompatible with life. Contrasting with this reality is the narrative of the alkaline diet marketers who claim that certain foods, while they may not change the pH much in the stomach, have an effect once they get passed the stomach, through the small intestine (which has an alkaline environment, by the way) and into the blood and cells. The problem with this reasoning is that, as noted above, changes in tenths or even hundreds of a pH point are very noticeable. When things are working normally, even the slightest deviation triggers adjustments involving the lungs, the kidneys, and chemicals in the blood and in red blood cells known as buffers. When these things are not working normally, you are overtly ill, not subtly ill, as the alkaline diet marketers would have you think.

To give you an idea of what a buffer is, imagine that you release water onto a dry sponge, one drop at a time, when monitoring the dryness and wetness of the environment around the sponge. At first, it remains totally dry, but if the sponge absorbs a lot of water, it will saturate. Some water will leak out, but not as much as you have dropped into the sponge. Similarly, the buffer chemicals in the blood hold onto a certain amount of acidity. Add acid to the blood and it is buffered, meaning that most is soaked up by the buffering chemicals, so the pH drops only slightly. The same happens, if something alkaline is added to the blood; it is buffered in the other direction, but in either case even the slightest change is noticed by the body, because one of the systems of buffering chemicals involves includes a chemical —called carbonic acid or bicarbonate, depending on whether it has a certain proton or not— which interconverts with carbon dioxide, a gas that is continuously removed by your lungs. The faster and deeper you breathe, the faster you off-gas carbon dioxide, and the more alkaline your blood becomes. In contrast, if your breathing slows, you retain more carbon dioxide, making your blood more acidic. The respiratory system is very responsive to slight changes in blood pH and there is no substance that you can obtain from a fad diet marketer that can get into your blood and keep up with this normal physiological mechanism.

On the other hand, your kidneys also alter blood pH, working in concert with the lungs, but over a period of hours to days, compared with the lungs working over seconds to minutes. When the pressure of carbon dioxide in the blood increases, pH decreases, but we mentioned above that carbon dioxide is related to bicarbonate by way of carbonic acid. Bicarbonate is a base, whose concentration is very much related to the pressure of carbon dioxide in the blood. More bicarbonate in the blood decreases the amount of acid and visa versa. All of these molecules are part of the sponge and the key here is that concentration of bicarbonate in the blood is also controlled by the kidneys. If the kidneys sense that there is too much acid in the blood, for instance on account of lungs not working well, causing too much carbon dioxide to be retained, the kidneys will retain more than the usual amount of bicarbonate, which will compensate for the acidity, at least mostly compensate. Similarly, if the blood starts becoming too alkaline, the kidneys will accelerate the removal of bicarbonate, thereby compensating over time.

Now during pregnancy, on account of the hormone progesterone, the respiratory center in your brain is stimulated slightly more than it is when you are not pregnant. The resulting increase in breathing removes carbon dioxide from your blood faster than when you are not pregnant, and so in pregnancy, it’s actually normal for your blood pH to be slightly into the alkalemic realm, meaning above 7.4 and even beyond the high end of the normal range that goes to 7.45, meaning that anything up to 7.47 is normal. This is called respiratory alkalemia (commonly, the term respiratory alkalosis is used, but this technically refers to the process causing a rise in pH, in this case in the lungs, whereas alkalemia refers to high pH actually in the blood), and it is kept from getting too extreme by the kidneys, which lower the concentration of bicarbonate by accelerating its removal from the body.

David Warmflash
Dr. David Warmflash is a science communicator and physician with a research background in astrobiology and space medicine. He has completed research fellowships at NASA Johnson Space Center, the University of Pennsylvania, and Brandeis University. Since 2002, he has been collaborating with The Planetary Society on experiments helping us to understand the effects of deep space radiation on life forms, and since 2011 has worked nearly full time in medical writing and science journalism. His focus area includes the emergence of new biotechnologies and their impact on biomedicine, public health, and society.

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