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The purpose of this laboratory is to explore how the kidney reacts to a change in water volume and solute concentration in the blood. This objective will be accomplished by drinking different amounts of water and of NaCl and analyzing the urine. We hypothesized three different scenarios for the three different test groups. The group that consumed only water would have the highest water volume and lowest solute concentration in their urine; the group that consumed only NaCl would have the smallest water volume and highest solute concentration in their urine; and the group that consumed both NaCl and water would have a constant water volume and solute concentration in their urine.
The kidney is a vital organ when it comes to ridding the body of toxins and absorbing valuable molecules, such as water, sodium, and glucose. A method utilized by the kidney is osmoregulation, which is a way to preserve homeostasis of water volume by changing the amount of solute inside the cell (Bourque 2008). An instance of this occurring is within a study that measured how the volume of a kidney cell changed when they put the cell under “hyperosmotic shock,” or made water leave the cell by increasing solute concentration around the cell (Lucio et al. 2003). They noticed that after some time, the cell took back in water in order to reach osmoregulation (Lucio et al. 2003).
The main way the kidney sustains osmoregulation is through the nephron. The surface area in the nephron is needed to concentrate urine because as the fluid travels through the nephron, the kidney uptakes important molecules, such as water. If the nephron has a larger surface area and is longer, then that gives the kidney more time to reabsorb water back into the body (Question 5). This means less water will be excreted, and the urine will be more concentrated. In contrast, short nephrons give the kidney less area to absorb water, making the urine less concentrated. The length of the nephron depends on the organism and available water sources.
A similar study to this laboratory showed that when subjects consumed a saline solution, the amount of sodium excreted increased (Jensen et al. 2013).
In this laboratory, there were three different test groups. Group 1 drank only 1 L of water. Group 2 drank 900 mL of water and 100 mL of water with 6 g of NaCl. Group 3 drank only 100 mL of water with 6 g of NaCl. Three urine samples were taken in this lab over a time frame of 60 minutes. The first urine sample was taken before the groups drank the solutions, the second was taken after 30 minutes, and the final sample was taken after 60 total minutes. After each urine sample was taken, the total volume of urine was written down. Each of the three urine sample was tested to determine NaCl concentration. Ten drops of urine was placed in a test tube. Four drops of 5% potassium chromate (KCr) were placed in the test tube, then it was shaken. 2.9% silver nitrate (AgNO3) was placed one drop at a time into the test tube and shaken, until the liquid turned a dark red color. The number of AgNO3 drops was recorded.
Figure 1 shows the average amount of urine (mL) each group had for three different time intervals. At time 0, Group 1 had the least amount of urine and Group 2 had the most. After 30 minutes, Group 1 had the same amount of urine, Group 2 slightly decreased in volume, and Group 3 decreased the most in volume. After 60 minutes, Group 1 increased and had the highest volume of urine. Group 2 slightly decreased with the second highest volume of urine. Group 3 decreased the most and had the lowest volume of urine (Question 2).
Figure 2 shows the concentration of NaCl (mg/mL) in the urine of different groups at different time intervals. At time 0, Group 1 had the highest concentration of NaCl (7 mg/mL), while Groups 2 and 3 both began with concentrations around 5 mg/mL. After 30 minutes, Group 1 and 3 decreased in concentration (5.5 and 4.5 mg/mL respectively), while Group 2 increased to around 7 mg/mL. After 60 minutes, Group 3 had the highest concentration of NaCl (7.6 mg/mL), Group 1 had the lowest (2.9 mg/mL), and Group 2 had a concentration of 3.1 mg/mL.
The hypothesis stated for the three different groups was supported by the results of this experiment. After 60 minutes, Group 1 experienced an increase in the volume of their urine over time (Figure 1), while they had the smallest concentration of NaCl (Figure 2). Group 3 experienced a decrease in their volume of urine after 60 minutes (Figure 1), and an increase in the amount of NaCl (Figure 2). Group 2 kept a constant volume of urine over 60 minutes (Figure 1), while they experienced a decrease in the amount of NaCl (Figure 2).
The experimental group I participated in was Group 1, which only consumed 1 L of water. The physiology changes of the body from the treatment of only drinking water would include making the body hypotonic. This means there was an excess of water in the body when compared to solute concentration. The body wants to have a balanced ratio and maintain osmoregulation, so it gets rid of water while keeping as much salt as it can. This results in the urine having a higher volume of water and a lower concentration of solute (Question 3).
This experiment dealt with AgNO3 to get rid of the chloride ions in the urine. KCr showed that this occurred by turning the urine red. Our group required less drops of AgNO3, because we had a higher volume of water in our urine than NaCl. Since we only consumed water, the body wanted to get rid of more water, while it wanted to keep salt within the body. This means we had less salt in our urine samples than other groups, so we needed less drops to precipitate the chloride ions (Question 4).
Pitfalls that could have messed up data accuracy include if we put the wrong number of drops of KCr into the urine sample and if we recorded a different total volume of urine that we excreted.
Jensen JM, Mose FH, Bech JN, Nielsen S, Pederson EB. 2013. Effect of volume expansion with hypertonic- and isotonic saline and isotonic glucose on sodium and water transport in the principal cells in the kidney. BMC Nephrology [Internet]. [cited 16 Nov 2015];14:202.
Lucio AD, Santos RAS, Mesquita ON. 2003. Measurements and modeling of water transport and osmoregulation in a single kidney cell using optical tweezers and videomicroscopy. Phys Rev E Stat Nonlin Soft Matter Phys [Internet]. [cited 16 Nov 2015];68(4).
Bourque CW. 2008. Central mechanisms of osmosensation and systemic osmoregulation. Nature Reviews.Neuroscience [Internet]. [cited 16 Nov 2015];9(7):519-31.
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