Renal Regulation of Blood Osmolarity

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Abstract: The purpose of the experiment was to demonstrate how ADH affects urine volume and concentration, highlighting its influence on blood plasma osmolarity. Instead of using invasive methods like drawing blood, urine volume and concentration were measured as indicators. It is anticipated that the group who consumed 6gm of NaCl will experience a gradual increase in ADH levels due to the higher osmolarity caused by the salt.

The experiment aims to demonstrate renal regulation of osmolarity through urinalysis. Two groups were formed for the experiment. The first group consumed 800ml of distilled water while the second group consumed a solution of 6mg NaCl dissolved in a small amount of water. Both groups were instructed to record the time of each voiding from waking up until the control urine sample obtained just before the start of the experiment.

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In addition, the participants were instructed not to change their usual food and drink consumption on the specified day. The control urine, which was collected in a large specimen cup, represents the last urine sample prior to the experiment. The osmolarity of blood plasma typically falls within the range of 275-295 mosmol/L. Since urine is a byproduct of filtered plasma, it serves as a suitable and noninvasive method to illustrate the role of ADH secretion in regulating osmolarity. As the experiment commenced, the groups consumed the entire solutions provided, and the time of consumption was recorded.

During the experiment, the groups were told not to consume anything until the lab was finished. In the 30-minute intervals between voiding, three tests were conducted on the urine samples. The first test used the control urine and measured the time since the last void. The experiment took a total of 120 minutes, resulting in four test urines and five sets to analyze and record. The first test involved measuring the urinary output by pouring it into a beaker and measuring it in milliliters.

The measurement was calculated using the formula x ml/y min. This measurement was taken from the time of the previous void until the current void. Another test was conducted to determine the specific gravity, which indicates the number of solutes present in the solution. The normal specific gravity range is 1.003-1.030. An increase in solutes leads to an increase in urine concentration.The specific gravity was obtained by placing a drop of each test urine, including control urine, on the refractometer tip and noting down the corresponding number.Lastly, as part of the final test, the sodium ion concentration in urine was measured.

In the experiment, ten drops of urine were mixed with one drop of 20% potassium chromate in a test tube, resulting in a bright yellow color. The mixture was then shaken while adding 2.9% silver nitrate drop by drop. The number of drops needed for the solution to turn orange brown was noted. In our specific experiment, each drop of 2.9% silver nitrate represented one mg NaCl/ml. Measuring the sodium levels in plasma is crucial because it impacts neuronal irritability and action potentials in cell membranes.

Every participant in the study recorded their data and inputted it into a spreadsheet. The data from the four separate laboratories were merged and averaged by groups, in order to enhance the sample size. The results section of this report presents the collected data, averages, and graphs. The experiment’s underlying theory primarily revolved around the effective operation of hypothalamic osmoreceptors located in the supra-optic nucleus of the hypothalamus. These osmoreceptors are responsible for detecting alterations in blood osmolarity.

The objective of the experiment was to illustrate the role of hormone regulation in maintaining homeostasis within our bodies. When osmolarity decreases, the secretion of Antidiuretic Hormone (ADH) should decrease as well. Conversely, when osmolarity increases, ADH secretion should increase. It is essential for bodily fluids to remain within their normal ranges in order for the body to function properly. Failure of compensatory mechanisms can result in acute or chronic diseases. The kidneys, along with other hormones like aldosterone and those involved in the renin-angiotensin system, play a significant role in regulating osmolarity. ADH is produced in the cell bodies of the supra-optic nucleus of the hypothalamus and acts as a hormone released from the posterior pituitary that controls blood osmolarity.

The osmoreceptors detect changes in osmolarity, resulting in corresponding adjustments to the firing of action potentials. An increase in osmolarity leads to increased action potential firing from the supra-optic nucleus through the infundibulum to the posterior pituitary. Conversely, a decrease in osmolarity decreases action potential firing. When there is an increase in firing, synaptic vesicles in the posterior pituitary release ADH into nearby capillaries, which is then transported to the kidneys via the bloodstream.

The primary function of kidneys relies on nephrons (Martini, 959). Nephrons are responsible for reabsorbing water, organic molecules, and ions, as well as serving as sites for drug secretion, toxin elimination, acid secretion, and ammonia excretion. Nephrons consist of two main parts: the renal corpuscle and renal tubule. Filtration begins at the renal corpuscle where water and solutes are forced out from capillaries into capsular space under blood pressure. The filtrate then continues through renal tubules where reabsorption occurs.

The distal convoluted tubule (DCT) is the site for reabsorption of water. It is also the site where the ADH receptors are located. When the osmolarity of blood changes, the osmoreceptors of the hypothalamus detect it and send ADH to the posterior pituitary via an action potential. The ADH is then released into the bloodstream at the synaptic vesicle. It travels through the blood to the DCT, where it tells the nephron whether to retain more or less water.

The kidneys can return water to the bloodstream, boosting blood volume and reducing osmolarity. In contrast, if the kidneys eliminate water in urine, it leads to a reduction in blood volume and an increase in osmolarity. The tubular fluid goes through the DCT prior to entering a collecting duct that acts as the end point of the nephron. Ultimately, all individual collecting ducts combine to form a larger papillary duct that guides fluid towards the ureters for removal via bladder.

Aldosterone, similar to ADH, targets cells in the kidneys. Specifically, it acts on sodium ion pumps and sodium channels in cell membranes along the DCT and collecting duct (Martini, 974). It is produced and secreted from the zona glomerulosa of the adrenal cortex. The main role of aldosterone is to retain sodium ions in the bloodstream while eliminating potassium ions. Unlike ADH, aldosterone’s target cells also exist in salivary glands, sweat glands, and the pancreas, making it an important hormone for regulating blood osmolarity.

The blood’s sodium ion levels play a crucial role in osmolarity. If aldosterone regulates sodium retention, it is essential to consider this hormone in connection with ADH. The statement “water follows salt” accurately describes how aldosterone increases blood volume. When the kidney’s target cells are stimulated, sodium is reabsorbed into the plasma, and as a secondary effect, more water is retained. Aldosterone’s effects are most pronounced when ADH levels are normal.

According to Martini (p615,999), the taste buds become more sensitive to salty foods, leading to an increase in salt intake. The renin-angiotensin system is responsible for maintaining normal blood volume and pressure by controlling the secretion of ADH and aldosterone. This system involves multiple target sites and starts with the formation and secretion of renin at the juxtaglomular apparatus of the kidney. Renin is produced when there is reduced blood flow to the kidneys or sympathetic stimulation from vasomotor nerves that regulate blood flow and renal resistance through arteriole constriction or dilation.

Renin is an enzyme that converts angiotensinogen produced in the liver to angiotensin I. This compound then travels through the bloodstream to the lungs, where it undergoes changes and forms Angiotensin II. Angiotensin II then goes back to the adrenal cortex, causing aldosterone release. When it reaches the supra-optic nucleus in the hypothalamus, it stimulates ADH secretion and triggers thirst, leading to increased fluid consumption. In the presence of aldosterone, sodium is retained and potassium is excreted by target cells such as DCT, resulting in fluid retention.

The antidiuretic hormone (ADH) is responsible for fluid retention and regulating blood volume and pressure. Increased fluid retention leads to increased blood volume and pressure. Natriuretic Peptide Hormones, released by cardiac muscle cells of the atria when high blood pressure and volume are detected, counteract ADH, aldosterone, and angiotensin II secretion.

This will result in decreased thirst, sodium and water reabsorption by the kidneys, as well as reduced plasma sodium content, plasma volume, and blood pressure (Kirkpatrick, 2010). All the hormones mentioned in this description are regulated by interconnected negative feedback mechanisms. The body is a complex integration of systems that collaborate to uphold homeostasis. The results include spreadsheets containing raw data for each of the four lab classes, comprising six distinct tables for the average data.

There are three tables presenting averages for each specific test for the water drinkers and three tables for the salt drinkers. Additionally, the averages are plotted against time to illustrate the differences between the two groups. The average urinary output for the water drinkers showed a significant increase compared to the control urine. Conversely, in the salt consumption group, the urinary output decreased over the course of 120 minutes. Examining the averages and graphs for specific gravity, it was observed that the specific gravity of the water group initially decreased by the 60 minute mark and then began to slightly increase.

The salt group numbers slightly decreased for the 30-minute test and then remained elevated throughout the experiment. Similarly, the urine sodium content yielded similar outcomes as the specific gravity test. The water group’s values sharply declined during the 60-minute test and then began to gradually increase. On the other hand, the salt group’s values progressively rose throughout the entire duration. In conclusion, these findings support the discussed theory on the influence of ADH on osmolarity and the role of kidneys in regulating this phenomenon.

The study confirmed that the group who drank 800ml of distilled water experienced an increase in urinary output, while the group who consumed a shot of water with 6mg NaCl saw a decrease. This was supported by the observed decrease in specific gravity and urine sodium content values in the water drinking subjects, indicating a more diluted urine. Conversely, the salt consuming group showed an increase in specific gravity and urine sodium content, indicating a more concentrated urine.

Flushing the body with water lowers osmolarity, decreases ADH secretion, and decreases urine sodium content and solute concentration. Consequently, urine output increases because ADH receptors in the kidneys are not activated. The kidneys do not retain water, and it is eliminated through the bladder. After the 60 minute test, average water levels begin to decrease again. This prompts ADH secretion to prevent excessive water loss and maintain normal blood volume and pressure.

The activation of aldosterone in response to a decrease in blood sodium content will also happen. It is important to remember that “water follows salt” and that aldosterone functions best when ADH is present. So, we should observe a change in sodium content at the same time. This can be seen in the graphs showing an increase in urine sodium content and specific gravity, indicating an inverse relationship with urinary output. The tests conducted on the group that consumed 6mg NaCl also demonstrate the effects of ADH, aldosterone, and renal regulation of osmolarity.

The initial urinary output increased for 30 minutes but subsequently decreased, suggesting an increase in ADH secretion due to higher osmolarity. Consequently, aldosterone inhibition would release some sodium content into the urine. Both urine sodium content and specific gravity exhibit a similar pattern of slight decrease followed by an increase compared to urinary output. To fully understand the body’s compensation process, extending the experiment’s duration is necessary.

The values at the 60, 90, and 120 minute mark remained steady, indicating that homeostasis had not yet been achieved by the body. By analyzing the graphs, we can see that as time goes on, urine sodium content and specific gravity would eventually decrease while urinary output would increase. This suggests that plasma osmolarity would eventually return to normal levels. The regulation of osmolarity by the kidneys is a critical compensatory mechanism our bodies possess. As mentioned earlier, it is crucial to maintain a blood osmolarity range of 275-295 mosmol/L for proper functioning. Prolonged deviation from this range can lead to cell damage and potentially even death.

The disruption of the balance in our bodies can lead to disease when our systems, which were designed to work together, are overloaded. In these cases, our compensatory mechanisms may not function optimally, causing imbalances. Kirkpatrick (2010) and Martini et al. (2006) have discussed the role of natriuretic peptide hormones and the urinary system in maintaining this balance.

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Renal Regulation of Blood Osmolarity. (2018, Jun 15). Retrieved from

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