Water Management

Solute Management

Water management is closely tied to the management of your solutes, especially sodium(Na⁺): Major ECF cation; important for fluid balance, nerve function.. Your body doesn’t control waterThe universal solvent essential for life. directly compared to how it uses solute concentrations to control water. When thinking of water management in a nephronThe functional unit of the kidney that filters blood and produces urine., you first need to recognize that the PCT is tasked with reabsorbing a lot of substances. The DCT also reabsorbs but does so more when it is under the control of certain hormones. As solutes are reabsorbed, the water follows by passive transport. The medulla of the pyramids increase in osmolarityA measure of solute concentration in fluid; affects fluid movement between compartments. toward the renal basin. This increase subjects the loops to a tonicityThe ability of a solution to affect the water balance in a cell.. This tonicity pulls water from the descending loopThe portion of the nephron loop that is permeable to water but not solutes, leading to water reabsor arm. Looking at this diagram here, water is never, ever, ever, transported actively. That is not something that these cellsThe basic structural and functional units of life. of the nephron are set up to do. They don’t have active transporters for water. But, they do have active transporters for solutes. By making areas of high tonicity, we can control the direction of the flow of water.

Control of Water Loss

The nephron employs two different methods of controlling water loss or reabsorbing water. Obligatory water reabsorptionThe process of fluid moving back into capillaries from surrounding tissues due to colloid osmotic p occurs when water moves from the filtrateThe fluid that is filtered from the blood into the nephron and will eventually become urine. to the peritubular capillary. This happens in the vasa rectaCapillaries surrounding the loop of Henle in juxtamedullary nephrons that help maintain the medullar as well as in the PCT or the descending nephron loop. If the loop dips deeper into the medula, it will reabsorb more water. This is like the loops of the juxtamedullaryRefers to nephrons or structures located near the boundary between the renal cortex and medulla. nephrons.

Cortical nephrons don’t dip as deeply into the medulla. Therefore, they have a lower capacity for obligatory water reabsorption than juxtamedullary nephrons. We know that the ascending arm of the nephron loop cannot move water. It cannot do so in any direction. The DCT and the collecting ductA duct in the nephron that collects urine from multiple nephrons and adjusts water reabsorption. can both reabsorb water. However, they can only perform this action under the influence of hormones. This is what is known as facultative water reabsorption.

 Obligatory water reabsorption happens all the time facultative water reabsorption happens only under the influence of hormones. For example, low blood pressureThe force exerted by gases in the respiratory system, affecting airflow and gas exchange. can trigger the release of the anti diuretic hormone. This hormone, also known as ADH, is released from the posterior pituitary gland(PPG): Stores and releases oxytocin and ADH produced by the hypothalamus.. It will float in systemic circulation. It moves to the kidney where it attaches to simple cuboidal cells of the DCT and the collecting duct. Upon attachment, it helps create a protein. This protein will be inserted in the cell membrane of these cells. It allows water to be reabsorbed. Without these aquaporins water loses the opportunity to follow all of the solutes that the DCT is reabsorbing.

When ADHD is turned on you have an increased capacity to reabsorb water or an increased capacity to conserve water. This would reduce your osmolarity but increase the osmolarity of your urineThe liquid waste excreted by the kidneys.. It would increase your water content. However, it would decrease the water content of your urine, making it more concentrated. r

Countercurrent Multiplier &  Exchange

Obligatory water reabsorption is never enough to satisfy the needs of an enormous Organism like a human on land. Kidneys in general, whether human or otherwise, are excellent adaptations for conserving water for a life on land. The presence of juxtamedullary nephrons in mammals shows that obligatory water reabsorption wouldn’t be enough without these nephrons. Recall that the medullary pyramids converge at the renal papillaThe tip of a renal pyramid where urine drains into the minor calyx.. This papilla weeps urine into the minor calluses. As you approach the point of the pyramid, the interstitial fluids become more concentrated with solutes. The concentration of solutes increases as you move closer to the apex.

This creates a situation in which there is incredibly high tonicity outside of the nephron. This high tonicity sucks water from the filtrate moving through the descending arm of the nephron loop. As previously mentioned, if the loop is longer, there will be more obligatory water reabsorption. This is the case in juxtamedullary nephrons. Another key player in obligatory water reabsorption is the length of the nephron loop. Additionally, stub medullary nephrons are surrounded by a vasa recta capillary bed. Vasa recta capillary beds are highly organized. They have an arterial side of the bed that hugs the ascending nephron loop arm. The eventual side of the bed hugs the descending nephron loop arm. You might think that I made a mistake. The venule side of the bed must be closer to the end of the nephron. But I did not.  The flow direction of the filtrate in the descending nephron loop is completely opposite. It is opposite to the capillary with which it exchanges. The same applies to the ascending nephron loop. The associated capillary has blood flowing in the opposite direction as the filtrate in the nephron. This is what is called a counter currentThe flow of electrical charge, as in ions moving across a neuron’s membrane. multiple wear mechanism.

The sheer factA statement based on direct observation that is repeatedly confirmed. that the fluids are flowing in opposite directions increases the exchange rate between them. This is the same concept used in dialysis where a patient’s blood is circulated countercurrent to diastolic. This countercurrent mechanism again allows the greatest exchange of both solutes and water. Countercurrent mechanisms can be for more than just solids and liquids. They can also be for temperature, which we will see in the reproductive systemThe organ system responsible for producing gametes and offspring..

Properties of Urine

Obligatory and facultative water management influences urine properties. It relates to the proportional amounts of water and solutes. The easiest way to determine solute to solvent proportion is to observe the color. A darker color indicates a higher solute to solvent proportion. A lighter color indicates a lower solute to solvent proportion.

Specific gravity compares the weight of your urine to one unit of water. Therefore if your urine is heavily laden with solutes your specific gravityA measure of urine concentration that compares the density of urine to water. will increase. A high specific gravity indicates a very concentrated urine. It could possibly be from somebody under the influence of ADH.  There is another measurement we use for urine. It also indicates the proportion of solutes to solvents in our urine. This measurement is osmolarity. I find that students often get confused when considering urine. They struggle to assess the osmolarity of blood versus the osmolarity of urine. For example urine that has a high osmolarity is very concentrated.

This would be urine from someone who is under the influence of ADH and is desperately trying to conserve water.  Try to think of urine as a solutionA homogeneous mixture of two or more substances. to problems present in the blood. This is a poor example. However, if you have a high blood volume, you might be getting rid of water in your urine. In that way, urine helps with the osmolarity problem in your blood. It does so by decreasing the osmolarity of your urine. Let’s say for an example that you are horrendously dehydrated because you drink too much coffee. The osmolarity of your blood is quite high. Your body could be trying to get rid of solutes to decrease your osmolarity. It may also aim to decrease the solute load in your blood.  Yes, you could possibly drink water. However, thirst is a very complicated mechanism. It’s not considered until the next chapter.  let’s just focus on outputs here not inputs .