In a normal human body, the maintenance of a relatively constant and unchanging volume and composition of fluids is very much important for homeostasis (constant internal environment). This is very essential because clinical problems arise from abnormalities in the systems that maintain a constant internal environment.
The relative constancy of the body fluids is remarkable because there is continuous exchange of fluid and solutes with the external environment as well as within the different compartments of the body (Guyton & Hall, 2006). In order to prevent body fluid volumes from increasing or decreasing, there must be a variable intake which must be matched carefully by equal output from the body.
Usually, the two major sources of water in the body are from ingested water or liquids and synthesized water. Water ingested in the form of fluids normally adds about 2100ml/day to the body fluids while water synthesized in the body as a result of oxidation of carbohydrates adds about 2300ml/day (Guyton & Hall, 2006).
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The body has different compartments, majorly, the extracellular and intracellular compartments. The distribution of fluid between these two compartments is determined mainly by the osmotic effect of the solutes present in each of the fluids. The cell membrane is a semi-permeable membrane that allows selective diffusion of some solutes and free movement of water. In order to maintain a constant environment for the cell, the concentration of solutes inside and outside the cell has to be held constant.
When large volumes of hypotonic solutions (e.g. pure water) are ingested the extracellular compartment becomes diluted and its osmolarity drops. If something is not quickly done about this, there would be a net difference between the osmolarity of the intracellular and extracellular compartment, leading to a net diffusion of water into the cell. Ultimately, this causes the cell to swell up.
This experiment examines the short term regulation of osmolarity by the kidneys. It tries to verify the hypothesis that the kidney controls the extracellular fluid volume and plasma osmolarity. In this report, the terms "osmolarity" and "osmolality" will be used interchangeably because water is the fluid involved and 1 liter of water is the same as 1kg of water.
Materials and Methods
Five subjects were used for this experiment and each subject had his/her height and weight measured using a nomogram. Also, the computation of results was done with the aid of an Advancedâ„¢ Micro-Osmometer (MODEL - 3300).
Prior to the commencement of the experiment, each of the five subjects emptied their bladders. Fifteen minutes after emptying their bladders, the first urine sample was collected. After another fifteen minutes, a second urine sample was collected, and the average of the two urine samples recorded. This was used for the base urine production rate (15 minutes).
Shortly after the second urine sample was collected, each of the five subjects was subjected to a fluid load of water. The volume of the water used was 600cm3/m2 of body surface area. Fifteen minutes after loading, Sample 1 of the second set of samples was gotten. This was continued subsequently with urine samples collected after 15 minute intervals.
Table 1. Mean values for all subjects.
Mean Volume excreted (cm3)
Mean Osmolarity (mOsmol/l)
Figure 1. Graph showing the changes in both volume excreted and osmolarity with respect to time.
Mean for Total Volume Excreted (cm3) - 616.35
Mean for Volume of water Load (cm3) - 1221.2
Mean of Volume Retained (cm3) - 604.85
The data which was used to draw the graph in Fig. 1 was gotten from the mean results for both the volume excreted and the osmolarity. Also, the mean total volume excreted, the mean volume of water load, and the mean of volume retained were calculated.
The results of the experiment, as stated in the results section, verified the hypothesis that the short term regulation of osmolarity is mediated by the kidneys. This is illustrated by the trend shown by the graph in Fig. 1. As diuresis increased, osmolarity decreased. After the water load, it was noticed that the volume excreted began to increase. After some minutes (90 minutes), it reached a peak, and then started to descend. On the other hand, the osmolarity began to fall after loading each subject with water. After about 75 minutes, it began to rise. This further confirmed the fact that the kidneys had to increase the volume of water excreted in response to increased water intake, so as to maintain a constant osmolarity of the body fluids.
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The mean volume excreted was calculated by subtracting the mean base production at rest from the mean urine volume. This was necessary in assessing how much of the water load was excreted with respect to time.
The initial hydration status of each of the subjects was one of the parameters measured by this experiment. The base resting production rate of urine production reflects the initial hydration status, and also assesses the level of hydration and can help in deciphering who is well hydrated and who is dehydrated. According to Guyton & Hall (2006), the normal urine osmolarity is between 50 and 1400 mOsm/l. A low urine osmolarity implies that there is more water in the body, compared to when there is a high urine osmolarity, indicating that there is water deficit. In most of the subjects being examined, it appears that most of them have mean resting osmolarities above 800mOsm/l. Results such as these implies that the subjects were relatively dehydrated. The relative dehydration may occur when the subjects have not taken enough water for some time, have ingested solutes, or have been exposed to water loss through evaporation.
Also, it was noted that the mean total volume excreted was about 50% of the mean water load. This means that a large percentage of the water loaded was retained in the body. This is a likely consequence of the relative dehydration which occurred earlier. This confirms the fact that diuresis in a relatively dehydrated subject will not occur as early as a normal or well hydrated subject. In a hypothetical situation where the body fluid volume is normal and the subject is well hydrated, the volume of water loaded should be equal to the volume of urine excreted. However, because the body needs to replenish some of its fluid volume, some of the water loaded is used to do this, before the rest of it is let out in order to maintain a constant plasma (extracellular) osmolarity.
Some of the results in the data sets for the third and fourth subjects show negative values in the volume of urine excreted. These negative values arise from the fact that the subjects did not produce any urine sample for the specified period. The subjects cannot be said to have gone into renal shutdown due to the fact that after some minutes, the volume of urine excreted began to increase.
The osmolarity of the extracellular fluid is the most important parameter being held constant by the kidneys. Table 1 shows that at the end of the experiment, the mean osmolarity levels out at 272.4mOsm/l, which is very much close to the normal plasma osmolarity. This is the whole essence of the renal function. The normal plasma osmolarity is between 275 and 290 mOsm/l. Alterations of these values will have serious consequences on the cells. If the osmolarity is increased above normal, water will flow out of the cell, causing it to shrink. On the other hand, if the osmolarity is reduced, water will tend to flow into the cell, making it to swell up. If this process is not tightly regulated, cells in closed compartments like the brain might swell, disrupting their normal functions.
The results of the experiment are reliable. This is because they are strong enough to verify the hypothesis. However, it was noticed that the mean osmolarity dropped a bit after the 90th minute, before it began to rise again after about 120 minutes. Normally, as the kidneys try to increase the volume of water excreted in response to intake of large volumes of water, the osmolality increases without falling until an optimal level is reached. This is likely due to an error of measurement. The error may also be due to differences in the kidney functions of each individual.
The normal functioning kidney has a remarkable potential to alter the relative proportions of both solutes and water in the urine in response to various challenges. In a situation where there is excess water in the body system and the plasma osmolarity is reduced, the kidney corrects this by inducing the excretion of large volumes of low concentrated urine. Conversely, in cases where there is water deficit and plasma osmolarity is high, a mechanism is activated by the kidney to cause the excretion of small amounts of highly concentrated urine. The kidneys can excrete urine with an osmolarity as low as 50 mOsm/L, a concentration which is about one-sixth the osmolarity of normal extracellular fluid, or urine with an osmolarity as high as 1200 to 1400 mOsm/L (Guyton & Hall, 2006). This is important for survival and adaptation when there are harsh conditions and water intake is limited.
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The regulation of diuresis by the kidney is mediated by a powerful hormone, Antidiuretic hormone (ADH), also referred to as vasopressin or arginine vasopressin. ADH acts on the distal tubules and the collecting ducts of the kidneys to regulate their permeability to water. When there is water deficit in the body system, there is increased secretion of ADH by the posterior pituitary gland. This is possible due to the presence of osmoreceptors in the hypothalamus which monitor plasma osmolarity (normal 280 mOsmols/l) and are able to detect at least +/- 1% changes (Lobban & Schefter, 1993). ADH is transported to the kidneys where it has its effect by increasing the permeability of the distal and collecting tubules to water. This then allows the reabsorption of large amounts of water, thereby decreasing urine volume.
Conversely, when there is an excess volume of water in the body, the osmoreceptors detect this and signal the hypothalamus to stimulate the posterior pituitary to reduce the secretion of ADH. This reduces the permeability of the distal tubules and collecting ducts to water, causing the excretion of large amounts of low concentrated urine. Thus, it can be said that ADH directly mediates the influence of the kidneys on the regulation of diuresis.
The process of diuresis is controlled by the kidneys in response to changes in the osmolarity of the extracellular fluids. This experiment has shown that the kidney is involved in the immediate, short term, regulation of plasma osmolarity. Conditions that affect the ability to function in this regard will have serious consequences on the cells, tissues and organs of the body.