How Your Body Changes in Pregnancy: The Urinary System

In previous instalments of this series, we have looked at how pregnancy changes the physiology of the respiratory system and the circulatory system. Today, we’ll explore the urinary system and how it changes in pregnancy. Also known as the excretory system and the urinary tract, the urinary system includes the two kidneys, which do the actual filtering, plus bladder, which stores urine, the ureters, which carry urine from the kidneys to the bladder, and the urethra, which carries urine from the bladder to outside the body during micturition (the process of urinating). The role of the kidneys is to filter the blood, removing waste from the body, and in the process also controlling the amount of water in the body and the concentrations of various electrolytes (ions, which are charged atoms and molecules) and the pH of the blood. All of these functions are intimately connected, a concept that is easy to grasp if we consider the example of the electrolyte sodium ion (Na+).

Normally, the concentration of sodium ion in the blood falls within the range of 135 to 145 milliequivalents per liter (mEq/L). When the concentration drops below this range, we call it hyponatremia and when it rises above this range, we call it hypernatremia. But while the kidneys have control over the amount of sodium ion that remains in or exists the blood, in most cases the conditions of hypernatremia and hyponatremia are the results of problems with the amount of water in the blood. When the kidneys retain far too much water in the body, or when a psychiatric conditions leads a person to drink excessively large quantities of water, hyponatremia (low sodium) develops through a dilution effect. Similarly, when a person is dehydrated, either because of high activity or lack of water consumption or because the kidneys release too much water, it can cause hypernatremia (an abnormally high sodium level), because less water in the blood means that sodium becomes more concentrated. The take home message here is that water and sodium are closely connected, and so a sodium problem is usually a water problem.

This is in contrast with certain other ions, such as potassium (K+), whose concentration in the blood the kidneys have a very direct role in helping to control, but this involvement with potassium concentration in the blood also relates in complex ways to the kidneys’ influence over the pH of blood. Speaking of pH, on account of the hormone progesterone, during pregnancy 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 this accelerated removal of carbon dioxide increases the pH of your blood, which is to say makes the blood slightly more alkaline (less acidic) than usual. Known as respiratory alkalosis, this condition is detected by the kidneys, which then seek to reverse the situation by lowering the concentration of another electrolyte called bicarbonate, a negative ion. This partly compensates for the respiratory alkalosis and, actually, it illustrates the almost symphonic interplay between the lungs and the kidneys in regulating the blood’s pH, and with it the pH in most body tissues. Keep this in mind the next time you hear one of those commercials for the so-called “alkalinizing diet”, designed by people who don’t seem to know —or don’t want consumers to know— about how body pH is really controlled.

As we learned in the installment about the circulatory system, during pregnancy the volume of blood in the body increases substantially. As a result, the size of the kidneys and ureters tend to grow as a kind of swelling effect as pregnancy advances. Meanwhile, as pregnancy advances, there is an increase in what doctors call the glomerular filtration rate (GFR). This is a measure of how much volume of blood is filtered through the kidneys over a particular amount of time and rises during pregnancy as a result of the increases in both the volume of blood and in the cardiac output (the volume of blood pumped per minute). Associated with the increase in GFR of pregnancy is an increase in the clearance of a substance called creatinine, and increases in excretion of urea, creatinine, urate and bicarbonate, whose concentrations thus rise in the urine and fall in the blood. Meanwhile, although we noted above that changes in sodium in the blood usually result from changes in water rather than visa versa, pregnancy is one situation in which there the kidneys directly influence sodium. In this case, they increase their retention of sodium, which in turn causes increased retention of water, leading pregnant women to experience some swelling.

Meanwhile, especially by the time you reach the third trimester, the smooth muscle of the bladder wall relaxes more than usual. This causes you to retain urine, increasing your risk of developing a urinary tract infection, but you also feel urgency to urinate, because of increasing pressure on the bladder from the growing uterus. If you experience symptoms of a urinary tract infection, such as burning during urination and a sensation that you always need to urinate more after you urinate, visit your doctor, because infection of the lower urinary tract is easily treatable and if it is ignored it can lead to infection of the upper urinary tract (kidneys), which can be very serious, particularly during pregnancy.

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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