The Effects of Chronic Dehydration on the Kidneys
Chronic dehydration causes severe and some time irreversible damage to the kidneys. It is a condition where a significient volume of fluid is loss over time without replacement. This dehydrated state overwhelmed the kidneys compensenatory systems’ ability to reverse it. Dehydration speaks primarily of water loss. Volume loss, chiefly blood volume loss or hypovolemia describes the lost of water and salt. Our kidneys manipulate both, through reabsorption and excretion to maintain homeostasis in our body. Homeostasis is the ability of an organ to maintain internal stability and to resist change. Hypovolemia an abnormally low circulating blood volume is perceived by the kidneys as hypoperfusion. Hypoperfusion, a decreased blood flow through the kidneys leads to acute renal failure (ARF)[1,2].
Let’s do first things first. Briefly, let’s review the kidneys and how they normally function within our body. Our kidneys develop early in the 5th week of pregnancy and begin functioning approximately 3 weeks, after they develop. Urine formation is believe to start approximately during the 12-19 weeks of pregnancy. Amniotic fluid which surrounds the fetus during pregnancy is composed mostly of urine. The amount produced throughout pregnancy can provide information regarding fetal kidney function status. The nephron is the structural and functional unit of the kidneys where filtration, reabsorption and elimination of waste occur. Each kidney has about 400,000- 800,000 nephrons. The total number of nephrons falls as we age. From approximately the 12-19 week of pregnancy onward, we rely on our kidneys to carry out their basic function, mostly without thought or assistance from us.
More often than not, we are born with two kidneys. They are on average a little smaller than the size of a fist. They are located under our ribs, above the dimples created by the posterior superior iliac spine. The position of the kidneys in our body are closer to our back than to our navel, one on each side of the vertebral column or spine. The right kidney is positioned lower then the left kidney due to the location of the liver on the right side of the body. Our kidneys physically resemble the kidney bean used for cooking. The kidneys efficiently perform five essential functions that we are aware of and probably contribute to many others. They maintain a normal fluid volume and consistency of each cell and its surrounding within our body; they secrete hormones; they remove unwanted fluid and waste from metabolism and drugs, as urine; they selectively reabsorb substances that we need; and they regulate our blood volume and pressure [3].
Now let’s have a review of dehydration. Dehydration is a condition that occurs when more water leaves our body than what we take in. There are many ways for water to leave our body. We lose water when we breathe, sweat, urinate and during bowel movements. The water that enters our body comes from what we drink and eat. In order to have a balanced water volume in our body we are expected to take in approximately the same amount of water as the amount, which leaves our body. The issue for our kidneys is balancing the amount that leaves and the rate that it leaves.
Dehydration is an abnormal condition of the body. Dehydration can be caused by diarrhea, vomiting, sweating, inability to drink water, medication such as diuretics (“water pills”), burns or diseases such as diabetes. Also, under certain conditions fluid can be trapped in spaces within our body and not available for circulation. This creates problems for cells and organs because there is no exchange of nutrient and/or removal of waste.
Diarrhea is the most common cause of dehydration. A significant amount of water can be lost rapidly during this process. Diarrhea consists of very frequent, loose or watery, large bowel movements. Unrelenting diarrhea is dangerous, as substantial amounts of water can be lost with each bowel movement, without adequate rapid replacement. Persistant vomiting can be another serious cause of fluid loss and it is difficult for a person to replace water if they cannot stop vomiting or if they are unable to tolerate liquids. Large amounts of water can be lost when our body tries to cool itself by sweating. For example we can loose about a pound of water, while taking a brisk walk during certain weather conditions. We also lose water through sweat when we exercise and when we have a fever. Although, sweating during these situations is normal, prolong loss of water through sweating or rapid sweating without replacement of fluids can be very unsafe. Uncontrolled glucose that’s a common feature in the blood of diabetics causes the body to lose striking amounts of water. The presence of high glucose in the blood, induces rapid removal by the kidneys and water follows the glucose that the kidneys remove. People who have severe burns lose water through the skin. Normally our skin acts as a barrier and keeps fluid in but when it is destroyed by burns it can no longer fulfill this role. Sometimes people take medication such as diuretics to lose weight. Diuretics are also given to treat hypertension. A side effect of this medication is that it can also cause a state of dehydration, from excessive water loss. Ascities is an example where fluid is trapped in spaces and not available for circulation [4]. Any or all of these processes leads to a state where more fluid leaves our body than we put in it or fluid is not available to mantain the volume balance needs of the body. Chronic dehydration indicates a loss of water volume for an extended period of time, in the absence of ample replacement.
Our kidneys are able to monitor the amount of body fluid, the concentrations of electrolytes like sodium and potassium, and the acid-base balance of the body. Acid-base balance is a normal balance between production and excretion of acid or alkali by the body, resulting in a stable concentration of hydrogen (H+) in body fluids. Our kidneys also filter waste products of body metabolism, like urea from protein metabolism and uric acid from DNA breakdown. During metabolism some substances are broken down to yield energy for vital processes while other substances, necessary for life, are produced. Blood urea nitrogen (BUN) and creatinine (Cr) are examples of two waste products which can be measured, in the blood. Levels of BUN and CR increase in the blood when our kidneys are not working properly.
Regulation of fluid volume also involves regulate of blood volume and pressure. Blood is composed of blood cells suspended in a liquid called blood plasma. Plasma, which constitutes 55% of blood fluid, is mostly water (92% by volume). Would you be surprise to know that water contributes approximately 60% to our body weight? Total Water Volume (TWV) of a man of average build (70 kg) has a volume of approximately 42 liters and represents approximately 60% of the body weight. Would you be surprise to know that on average, our kidneys get rid of approximately 1.5 liters of urine daily? In order to remove all of that fluid and waste our kidneys must process 1700 liters of blood a day. Approximately 20% of the blood received from cardiac output of the heart is processed through our kidneys, every minute of the day. Cardiac output is the volume of blood pumped from the right or left ventricle in one minute. As blood flows through the kidneys, they make rapid adjustment for variation between water intake and removal every day, all day, even when we are asleep.
Let’s have a look at what happens in the kidneys when we are dehydrated and how they attempt to compensate or restore fluid balance. The kidneys need an adequate amount of blood flowing through them constantly in order to maintain homeostasis in our body. Hypovolemia is detected in the body by a fall in pressure within the arterial system, specifically mean arterial pressure (MAP). MAP is the average pressure within an artery over a complete cycle of one heartbeat. In the presence of adequate blood volume, the arterial system is stretched and pressure is measurable from the pressure of blood against the walls of any blood vessel. Sensors that detect changes in arterial pressure are called baroreceptors. Hypovolemia creates a condition where these arteries are not stretched as much, due to a decrease in the amount of blood volume flowing through them, thus causing a corresponding decrease in pressure inside the vessels [5,6].
Pre-renal acute renal failure (ARF) is the most common form of ARF. The term “renal” is derived from the Latin name for kidney. Prerenal ARF is caused by decreased blood supply to the kidneys. This low blood flow to the kidneys causes them to rapidly lose most of their ability to filter out waste from the food we eat and from muscle metabolism. Normally as blood flows through the kidney, specific sensors within the kidney decide how much water to excrete as urine, along with concentration of electrolytes. When the kidney senses an imbalance, it attempts to correct it. The presence of volume loss, by any means, in significant amounts, directs the kidneys to reabsorb more sodium and water and excrete less urine. The kidneys will hold on to as much water as possible and the urine becomes very concentrated and dark in color. When adequate water is present in the body, the urine is much more dilute, and the urine becomes clear in color [7].
Restoration of volume to the kidneys is managed in several ways. This system is controlled by renin, a hormone produced in the kidney. Renin is also a part of the fluid and blood pressure regulation systems of the body. Renin does not work by itself, it recruits angiotensin and aldosterone and is known as the renin-angiotensin-aldosterone system. Angiotensin is a substance with vasoconstrictive (constrict blood vessels) activity that function physiologically in controlling arterial pressure. Aldosterone is a hormone secreted by the adrenal cortex. It promotes the retention of sodium and bicarbonate, the excretion of potassium and hydrogen ions, and the secondary retention of water. As well, our sympathetic nervous system is activated when blood volume is low and there is a release of arginine vasopressin (AVP), from the hypothalamus. AVP is a hormone which stimulates contraction of the muscular tissues of the capillaries and small arteries, raising blood pressure, and increases peristalsis in the intestines and influences resorption of water by the kidney tubules, resulting in concentration of urine. Our sympathetic nervous system enables the other systems to act together in a common purpose in order to produce results impossible by the actions of one system alone. All of these systems act in concert initiating a response to the volume depletion by causing the major vessels of the body to constrict and propel blood to the heart, then through the heart and then out to the organs of the body. The results of the actions of these systems are that there is more blood made available from the muscles and gut, through constriction of the vessels to these organs; less salt is lost through sweating; thirst and desire for salt is increased; salt and water is retained by the kidneys and less urine is produced and excreted. Also there is an increase of blood pressure within the blood vessel and more blood volume flows through the kidneys [8].
Adequate hydration of the kidneys is vital to their proper functioning. Under conditions where there is severe imbalance of fluid volume in the body, the kidneys utilize all of their compensatory mechanisms to restore and correct this imbalance. Compensatory mechanisms are meant for ‘in case of’ events. . ‘In case of’ events are designed to be temporary. They are not designed to completely take the place of the normal functions of an organ. There is always a point where the ability to compensate is lost. Even prolong use of compensatory mechanisms opens the door to the possibility of damage and destruction to other organs. In the case of the kidneys’ response to hypovolemia, blood supply is redirected from the muscles and gut in order to restore balance of perfusion to the kidneys, thus temporarily depriving these organs of vital nutrients, oxygen and removal of waste [9,10]. History has taught us that we can’t keep robbing one organ at the expense of another.
Our kidneys are truly remarkable organs. They have the ability to recover from almost complete loss of function, under certain conditions. Favorable recovery results are seen with prompt and adequate restoration of blood flow to the kidneys when there is no damage to the tissue itself due to the loss of perfusion. Early correction of volume balance is vital to correcting and/or maintaining kidney function. Prolonged periods of hypovolemia leads to damage of renal tissue. Renal tissue is damaged by hypoperfusion in another specific way by preventing the delivery oxygen to the kidneys. Our kidneys are very dependant on oxygen and secretes a hormone, erythoprotien to stimulate new production of red blood cells, from our bone marrow, to correct oxygen shortage. Once renal tissue damage develops, it is difficult and sometimes impossible ,to reverse. In the absence of chronic kidney disease, consistent, ample hydration is both a preventative measure and essential to our kidneys’ health.
References:
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