The Kidneys
Adult kidneys, of which there are two, are approximately 3 centimeters thick, 6 centimeters wide, and 12 centimeters long 1 1. The kidneys lie in the back abdominal wall. Their major function is to maintain homeostasis within body and they accomplish this by filtering the blood 1 2. The end process of kidney function is urine, which is produced to maintain the internal body environment through the regulation of certain solutes, such as potassium and sodium ions 1 3.
The kidneys are arguably one of the most complex of all the organs of the human body. There is an overwhelming amount of information regarding the many different mechanisms of kidney function, including how it regulates solute concentration in urine, how it adjusts itself according to changes in the internal environment, and the physiology that explains how it all works. However, not all of this information is necessary for the proposed curriculum unit. In order to help focus on only what is necessary to teach osmosis and diffusion, only urine formation and the transport of salt, water and glucose will be discussed in detail.
The Nephron and Urine Production
The basic functional unit of the kidney is the nephron. The nephron is a tubular structure that is lined with one layer of epithelial cells. On average, each kidney is comprised of approximately one million nephrons. Blood first enters the kidneys by way of the renal artery. A large volume (1300 mL) of blood enters the kidney every minute. Upon entering the renal artery, blood then travels through the afferent arteriole, which leads into an individual nephron 1 4.
The Glomerulus
The afferent arteriole directs the blood into a capillary structure called the glomerulus. The glomerulus functions as a high pressure filter. As blood passes through the glomerulus, proteins and blood cells are separated from the plasma. The filtrate, a protein-free plasma solution, ends up in the Bowman's capsule of the nephron. The Bowman's capsule is a membrane bound sac that surrounds the glomerulus. The blood cells and proteins that were not filtered into the Bowman's capsule flow out of the nephron through the efferent arteriole, which carries blood out of the nephron. At this point, the fluid that has been collected in the nephron is referred to as filtrate and is the precursor to what will eventually become urine 1 5.
The glomerulus is essentially a capillary bed, but it has unique properties that make it possible for simple filtration to occur without the use of any energy (other than the energy that is consumed in the heart to create pressure in the blood). The first is that the pressure in the glomerular capillaries is much higher than an average capillary bed and the second is that the capillaries are much more permeable than most capillary beds 1 6.
The Proximal Tubule
The filtrate that is produced in the glomerulus is protein-free plasma that contains glucose, amino acids, vitamins, minerals and any other solutes that are contained in the blood. If the nephron's structure allowed for only simple filtration, as occurs in the glomerulus, all of these important chemicals necessary in the body for healthy functioning would be excreted in the urine and would not be available for use. If these molecules were lost, they would have to be continually replaced in the diet. Fortunately, the nephron is able to recover most of these useful, filtered molecules. As the filtrate moves through the remainder of the nephron, these materials are reabsorbed into the body, since a healthy kidney does not produce urine that contains glucose or many of the other solutes found in the filtrate at this early stage of urine production.
The next region of the nephron is the proximal tubule, which receives about 120 mL of filtrate per minute. The proximal tubule is the region of the nephron responsible for the reabsorption of certain solutes, especially glucose. To understand how glucose is moved out of the fluid in the tubule, and eventually reabsorbed into the blood stream by way of the peritubular capillary bed that surrounds the nephrons, it is necessary to follow the movement of sodium ions that are also present at high concentration in the filtrate. There is a high concentration of sodium ions inside of the proximal tubule because of the initial filtration process. But sodium concentrations are kept relatively low in the epithelial cells that line the proximal tubules. Because of the concentration gradient, sodium ions will move from inside the tubule, through the cells and into the interstitial space. Of course, this requires a protein to transport them out of the cell, since ions cannot pass through the lipid bilayer. Sodium ions are transported into the cells and then pumped from the cells to the interstitial space outside of the cells. The peritubular capillary bed that surrounds the nephron has a low salt concentration as well, since salt was originally removed in the glomerulus. Therefore, the sodium ions diffuse into the capillary bed and are reabsorbed. As salt concentrations increase in the interstitial space outside of the proximal tubule, osmotic pressure increases and water moves outside of the proximal tubule as well. Ultimately, this water is also reabsorbed by the peritubular capillaries 1 7.
Reabsorption of Glucose
Glucose, like most molecules, is not capable of simply diffusing across the cell membrane when there is a concentration gradient. In order for glucose to be reabsorbed by the body after being filtered out of the blood, glucose transport is actually coupled with the sodium ion transport, as described above. Transport proteins in the cell membrane are activated when both glucose and Na + are available. The transporters require both substances to be present and they operate without the use of any energy. As previously stated, Na + concentrations are kept low in the cells that line the proximal tubules. Therefore, sodium will continue to move down its gradient, so long as there is glucose available in order to activate the transport protein. In this mechanism, sodium is transported from the lumen of the proximal tubule into the cells that line the tubule. Once inside, the sodium and the glucose molecules are still at a higher concentration than they are in the surrounding interstitial space and the peritubular capillaries. There are specialized transport proteins on the side of the nephron cells closest to the peritubular capillaries that are capable of transporting glucose separately from hydrogen. Since glucose is moving down its concentration gradient, as is sodium, this movement of molecules is passive and requires no energy 1 8.
Figure 2. A. Movement of solutes and water from the tubular fluid to the blood is regulated by tubular epithelial cells. B. Tubular epithelial cells transport sodium ions from the luminal fluid to the interstitium. C. Tubular cells have co-transporters that allow glucose to be reabsorbed together with sodium. Reproduced with permission from ref. 2 (Saltzman, 2009).
The Loop of Henle and Reabsorption of Water
At this point in the journey to create urine from blood filtrate, the glucose absorbed in the filtrate has been reabsorbed by the body, as has some salt and some water. However, it is in the body's best interest for the kidney to recover as much water as possible from the filtrate, creating the most concentrated urine. In order to get water to leave the filtrate through diffusion, the area surrounding the nephron must have a high salt concentration. A high salt concentration in the interstitial fluid outside of the nephron will provide a driving force for osmosis, allowing water to be recovered from the filtrate.
The reabsorption of water occurs in many places in the nephron, but especially in the collecting duct, which is the final segment of tubule in the nephron. To allow for the reabsorption of water, the nephron needs a mechanism for creating high solute concentrations (i.e. high osmotic forces) in the fluid outside the collecting duct. This is accomplished by the Loop of Henle. If you could imagine a tube shaped like a U, this is how the Loop of Henle looks. In the descending side of the loop, the cells are water permeable. In the ascending portion of the loop, the cells are not permeable to water. The cells in the ascending tubule also contain pumps that use energy to transport salt into the interstitial fluid surrounding the Loop and collecting duct. These pumps that remove salt from the ascending loop are important because they pump NaCl into the interstitial fluid that surrounds the Loop of Henle. This helps keep the concentration of salt higher in the interstitial fluid outside of the descending loop 1 9. Because of the presence of high salt concentrations in the interstitial fluid, as the filtrate travels down the collecting duct—on the way out of the nephron—there is a strong driving force for water to diffuse through the cells that line the collecting duct. In these last stages of flow through the tubule, the filtrate can become extremely concentrated 2 0.
As osmosis continues and more water moves into the interstitial fluid surrounding the loop, the filtrate becomes much more concentrated. This is the instance of osmosis that the curriculum unit will focus on, and rather than get into specifics of osmotic pressure and the role of the ascending loop, the "story" will be simplified and students will simply be informed that the body uses energy—within the Loop of Henle—to keep the area surrounding the Loop of Henle and collecting duct high in salt concentration, in order to encourage osmosis and the reabsorption of water by the body. Of course, this eliminates much of the more subtle detail involved in this process. If this were to be taught in a high school class, the role of the ascending loop would be appropriate to include and could also be used to further explain active transport, as opposed to the passive transport that happens more frequently in the kidney.
Tubular Secretions
The final stage of urine formation removes certain waste products from the blood that could be toxic if they were allowed to accumulate. Tubular secretions are how metabolic wastes, certain drugs, hydrogen and potassium ions and other materials end up in the urine 2 1. The collection of these materials occurs in the distal tubule. The filtrate now flows into the collecting duct of the nephron. As the filtrate travels through the collecting duct, certain materials may once again be reabsorbed, depending on the current conditions of the body. The mechanism for this response to the internal environment is key to understanding how the kidney maintains homeostasis 2 2. It is not important to this unit however, and will not be described in detail.
The Collecting Duct and Excretion of Urine
The collecting duct contains the filtrate, which is now referred to as urine. The process described above is occurring in every one of the one million nephrons in each kidney. Each collecting duct at the end of each nephron will funnel into a larger tubule. This happens repeatedly until all of the urine is funneled into the ureter 2 3. The ureter is a tubular structure that carries the urine from the renal pelvis to the bladder. Once urine volume in the bladder reaches a specific volume, approximately 200- 400 mL, a receptor in the wall lining of the bladder is stimulated, creating the feeling that the bladder needs to be emptied. From the bladder, urine leaves the body through the urethra 2 4.
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