Renal System

Pressure, Glucose Carriers, Solute Concentration, and Hormone Effects on Urine in Nephrons Mengjie Mellisa Wu 3/2/2009

Colorado School of Mines

AP Renal System Lab

H. King

Introduction The urinary system consists of kidneys, urinary tract, urinary bladder, and the urethra. The purpose of this system is excretion, elimination of the organic waste products, and homeostatic regulation of the blood plasma in terms of volume and concentration. Urination, or micturition, is the process of elimination of the urine from the urinary bladder via the urethra caused by the contraction of the muscular urinary bladder. Also the system helps regulate the blood volume, blood pressure, plasma concentrations of sodium, potassium, chloride, blood pH, and nutrients in the blood plasma. These controls are processed in the kidneys, which are located both sides of the vertebral column, between T12 and L4. The left kidney is superior to the right kidney. There are three layers of connective tissues in the kidney: renal capsule, adipose capsule, and renal fascia. The renal artery, vein, and nerves enter the kidney through the hilus, a medial indentation. Then there are two layers of the kidney including an outer cortex and an inner medulla. These two components hold the 1.2 million of nephrons, microscopic tubular structures, in each kidney. Each nephron is comprised of a renal tubule and a renal corpuscle. The renal corpuscle is a spherical structure that consists of the Bowman’s capsule. This capsule is about 200 um in diameter and contains the glomerulus, a capillary network. The entrance to glomerulus is the afferent arteriole and leaves by the efferent arteriole. Here, the glomerular filtration will allow protein-free plasma, the filtrate, to college in the Bowman’s capsule, which then is carried throughout the rest of the nephron.

Figure 1 – Nephron with the exchanges of solute and solvent with its anatomy. The renal tubule is the relatively long (about 50 mm in length) tubular pathway, and it consists of the proximal convoluted tubule (PCT), distal convoluted tubule (DCT) and the loop of Henle. This path transport the filtrate, now called the tubular fluid, and allows for changes in concentration of water and

ions. Tubular fluid then passes the connecting tubule and the collecting duct, which leads to the minor calyx. Urine is then collected into the major calyx and transported to the bladder through the ureter, exiting the urine through the hilus. There are two types of nephrons, cortical nephrons and juxtamedullary nephrons. The cortical nephrons comprise about 85% of the nephrons, and located mostly within the cortex of the kidney. Its loop of Henle is relatively short and enters the medulla. Peritubular capillaries supply blood to nephron by surrounding the entire renal tubule. The juxtamedulary nephrons have a relatively long loop of Henle that is located deep within the medulla. The peritubular capillaries are connected to the vasa recta, which forms long capillaries parallel to the loop of Henle. Water exchanges occur in the PCT, descending limb of the loop of Henle, and the collecting duct of the nephron. In the PCT and DCT, solutes leave can leave the nephrons. In the collecting duct, an active transport takes place, exchanging the sodium and potassium ions. Experimental To see the pressure effect on the glomerular filtration, a change in blood pressure was applied. Starting with a base setting of pressure gauge of 70 mmHg, an afferent radius of 0.50 mm, and an efferent radius of 0.45 mm, the blood pressure was steadily increased by increments of 5 mmHg until reaching 100 mmHg with each trial. During each run, the Glomerular Filtration Rate was recorded. Within the renal tubules, ion exchanges occur through the membrane of the nephron through both active and passive transport. The interstitial fluid, which surrounds the renal tubule, drives the passive transport of solutes down their concentration gradient. The base used for comparison used a 300 mosm concentration, this amount was increased by 300 mosm with each run until it reaches a maximum of 1200 mosm, and this is the normal value for interstitial solute concentration within the kidney. Antidiuretic hormone was added in each run as well. Glucose enters into the Bowman’s capsule from the plasma as part of the filtrate, but is mostly reabsorbed. This is done by secondary active transport, which is driven by the gradient transport of sodium ions. The amount of glucose carriers was set as 100 as base, then increased 100 each run until reading the maximum of 500 glucose carriers. Hormones were tested for their effect on reabsorption. Antidiuretic hormone (ADH) and aldosterone were tested by adding them to the collecting ducts of the nephron. At first the control (baseline urine volume) was ran with no hormones or glucose carriers, and the concentration gradient was set as 1200. On the second run, aldosterone was added onto the collecting duct, and the urine volume was measured again. During the third run, the only hormone used was ADH. In the final run, both aldosterone and ADH were added onto the collecting duct. Data Table 1 – Effects of Pressure on Glomerular Filtration

The glomerular pressure and glomerular filtration rate increased linearly with blood pressure. However, the urine volume increased non-linearly (table A.1). Table 2- Effect of Solute Gradient on Urine Concentration

The potassium concentration increases and the urine volume decreases with increasing concentration gradient (Fig A.2). The volume of urine is seen to not decrease linearly. Table 3 – Reabsorption of Glucose

The glucose concentration stopped downing effect once the glucose carriers passed 400 glucose carriers. Each run was made with the following number of glucose carriers: 100, 200, 300, 400, 500. Table 4 – Effect of Hormones on Reabsorption

Discussion As the blood pressure increased, the glomerulus pressure increased linearly (Fig A.1). This is because as blood pressure increases the amount of blood enters the afferent arteriole of the glomerulus faster,

thus causing a greater glomerulus pressure. A greater pressure in the glomerulus would cause the glomerular filtration rate would increase proportionally. Since the urine volume depends on various factors, it did not change linearly with pressure. The urine volume produced increased because more filtrate was being pushed into the collecting tubules per unit of time. This would be beneficial if the body had drank an excess amount of water, because filtering and allowing the body to excrete the fluid faster would prevent cells from bursting. Also the increase urine volume will help excrete more toxins from the body. This is especially useful in filtering out not only high concentration of solutes but also any drugs that has hazardous effect if they remain in the body too long. Solute concentration in the urine increased as the interstitial fluid’s concentration increased. The solute difference changes linearly (Fig A.2). This is because the concentration gradient dictates the flow of ions across the membrane. Since the concentration of the fluid around the nephron increases, water molecules move out of the nephrons because of osmosis. Water will diffuse from the less concentrated solution , the tubular fluid, into the more concentrated solution, the interstitial fluid. Increasing the concentration gradient by increasing the concentration of the interstitial fluid will cause a more concentrated urine to form. This is because water is leaving in the convoluted tubules and solutes are entering the nephrons. If ADH was not added to the collecting duct, water would be unable to leave the collecting tubule. This would cause more of less concentrated urine produced. As more glucose carriers were added, the glucose concentration in the urine was decreased. This is because as more carriers are added, the faster glucose can be reabsorbed before the tubular fluid passes the collecting tubule. However, after a certain amount of carriers added, all the glucose would be reabsorbed, so any additional carriers would make no difference. The cut-off limit was between 300 and 400 glucose carriers. A diabetic person would have a large amount of glucose in the urine. This is because the glucose is unable to be reabsorbed and excreted with the rest of the tubular fluid. The baseline urine volume was 201 mL. When aldosterone hormone was added, the urine volume was 180.9 mL. This indicates that aldosterone did promote the uptake of sodium from the filtrate into the body and release potassium from the body. This increases the potassium concentration in the urine. Since this is a shift of electrolyte, it also causes more water to be reabsorbed into the body. Aldosterone is an adrenal cortical hormone, made by the adrenal gland superior in the kidneys. When the blood pressure drops, the cells in the arterioles initiates the release of renin. It converts angiotensinogen into angiotensin I by acting as an proteolytic enzyme. The endothelial cells then releases converting enzyme to convert angiotensin I into angiotensin II. Angiotensin II reaches the adrenal cortex and stimulates the release of aldosterone. As of the urine volume, ADH has a greater effect. This is because ADH can bind to receptors in the principal cells and open aquaporins, which are water channels. In particular, ADH acts on the distal tubule and the collecting duct to increase the uptake of water from the tubular fluid in the nephrons. This prevents the quick dehydration of the body. It is driven by osmolality, the amount of solute particles per kilogram of solvent, and permits osmosis appropriately to maintain homeostasis. ADH

changes the concentration of potassium greatly. Unlike aldosterone, it does not change the amount of solute, but rather the relative concentration by decreasing the solvent. When both hormones were present, the potassium concentration increases even more, and the total urine volume increases slight. The urine volume increases because of the concentration gradient will cause some water to flow back into the collecting duct. The potassium large increase in concentration is due to the duo effect of both ADH lowering the amount of solvent and aldosterone increasing the potassium solute with the sodium potassium active transport. Conclusion Increasing blood pressure causes an increase in both the glomerulus pressure and glomerular filtration rate. Increasing the solute concentration within the interstitial fluid causes an increase of diffusion into the nephron because of the change in concentration gradient and water leaves in the collecting tubules. Glucose carriers are responsible for the reabsorption of the glucose from the tubule fluid, however pass a certain amount of carriers, any additional carriers would make no difference. Diabetics who lack sufficient glucose carriers would lose much of their glucose in the urine, and the urine would have a detectable amount of glucose. Two hormones involved in maintaining homeostasis in the body are aldosterone and ADH. These two increase the solute concentration in the urine by either adding more solute in through active transport, or decreasing the volume of solvent. ADH is stimulated by osmolality and aldosterone is stimulated through the renin-angiotensin system, which ultimately makes angiotensin II to trigger the adrenal cortex in releasing aldosterone. Reference [1] Klabunde, Richard. Diuretics. http://www.cvpharmacology.com/diuretic/diuretics.htm [2] Martini. 2004. Fundamentals of Anatomy and Physiology. 6ed. [3] Stabler. PhysioEx. 8.0 for Human Physiology Laboratory simulations in physiology. [4] King, Hugh. Professor. Colorado School of Mines. 2009

Appendix

Figure A.1 – Effect of blood pressure. Green line (top) is the urine volume, blue line (middle) glomerular filtration rate, and red line (bottom) is the glomerular pressure.

Figure A.2 – Effect of the solute gradient on potassium concentration and urine volume.