REGULATION OF FLUID & ELECTROLYTE BALANCE

Transcription

1 REGULATION OF FLUID & ELECTROLYTE BALANCE 1

2 REGULATION OF FLUID & ELECTROLYTE BALANCE The kidney is the primary organ that maintains the total volume, ph, and osmolarity of the extracellular fluid within narrow limits. The kidney accomplishes this by altering urine volume and osmolarity. The kidney, in turn, is regulated by neural, hormonal, and local factors. In today s lab we will study how the kidney responds to changes in the composition of the extracellular fluid. OBJECTIVES: After completing this activity, students will be able to: 1. define fluid and electrolyte balance and to discuss the kidney s role in its regulation. 2. understand the role of antidiuretic hormone (ADH), aldosterone, renin, and atrial natriuretic peptide (ANP) in regulating fluid and electrolyte balance and in maintaining homeostasis. 3. predict how changes in blood volume and osmolarity will alter urine composition (color, transparency, volume, specific gravity, and chloride concentration.) 4. analyze the results of case studies and explain the hormonal regulation occurring in each condition. 5. prepare and interpret graphs 6. use flow charts to model physiological changes that occur in response to disturbances in osmolarity or plasma volume. 7. use principles from chemistry (concentration and osmolarity), physics (specific gravity, density, transparency), mathematics (graphs, means), and physiology to explain the body s responses to perturbations in fluid and electrolyte balance. 8. collaborate with team members to evaluate case studies. 9. communicate conclusions generated from the case studies during a class presentation 10. relate the changes in plasma osmolarity and volume that occur in these case studies to real world situations that may occur. REGULATION OF FLUID AND ELECTROLYTE BALANCE Have you ever noticed the need for a drink after eating that large bucket of popcorn at the movies? Or on television, patients entering the ER with substantial blood loss are immediately given intravenous fluids (an IV)? Both scenarios relate to fluid and electrolyte balance. What do these terms mean? Fluid refers to water. For water balance to occur, water intake through 2

3 ingested liquids and foods and cellular metabolism must equal water output via sweating, urine, feces, and breathing. Water balance is essential for the body to be properly hydrated, avoiding both dehydration and over-hydration. Electrolytes are inorganic compounds that dissociate in water to form ions. They get their name because they can conduct an electrical current in solution. Sodium is the most abundant ion of the extracellular fluid and is the main contributor to the osmolarity or solute concentration of blood. One of the key tasks of the kidneys is to regulate fluid and electrolyte balance by controlling the volume and composition of the urine. These adjustments are essential because the osmolarity of body fluids must be around 300 milliosmols/liter. There are three hormones that play key roles in regulating fluid and electrolyte balance: 1) antidiuretic hormone, released from the posterior pituitary; 2) aldosterone, secreted from the adrenal cortex; and 3) atrial natriuretic peptide, produced by the heart. We will consider the role of each in turn. Antidiuretic Hormone (ADH) is a hormone that prevents fluid loss and promotes the conservation of body water. The term antidiuretic is derived from anti, meaning against, and diuresis, which refers to fluid loss. The primary stimulus for ADH release from the posterior pituitary gland is an increase in blood osmolarity (that is, increased solute concentration and decreased water concentration). The elevation in blood osmolarity is detected in the hypothalamus by specialized neurons called osmoreceptors. ADH acts by increasing the reabsorption of water in the distal convoluted tubules and collecting ducts of the nephrons in the kidney. The net result of this mechanism is that water is conserved. Under these conditions a small volume of highly concentrated (hypertonic) urine is excreted. Another action of ADH is to stimulate thirst. This results in an increase in water intake, which lowers blood osmolarity and helps to restore homeostasis. If ADH is absent, as occurs in the disorder called diabetes insipidus, water reabsorption in the kidney is decreased dramatically and large volumes of dilute urine are excreted, up to 25 liters per day! Aldosterone is a hormone that regulates blood sodium levels. Aldosterone specifically increases sodium reabsorption in the distal convoluted tubule and collecting duct of the nephrons in the kidneys. The result of this mechanism is to conserve sodium. Because water follows salt, this may also lead to water retention when ADH is present. Another action of aldosterone is to increase the secretion of potassium by the kidney resulting in its decrease in the blood and increase in the urine. Aldosterone release from the adrenal cortex is triggered directly by an increase in potassium (primarily) or a decrease in sodium in the blood reaching the adrenal cortex. Aldosterone release is also stimulated by the activation of the renin-angiotensin system. In this mechanism, the juxtaglomerular cells of the kidneys release renin in response to a decrease in blood volume, a reduction in blood pressure, or stimulation by the sympathetic nervous system. Renin is an enzyme that converts a plasma protein called angiotensinogen to angiotensin I. Angiotensin I is in turn acted upon by angiotensin converting enzyme (ACE) to form Angiotensin II. Angiotensin II has two major actions: 1) it stimulates aldosterone release from the adrenal cortex, which increases sodium reabsorption and results in sodium conservation; and 2) it causes vasoconstriction, which elevates blood pressure. As a result of 3

4 these mechanisms, homeostasis is restored. Atrial natriuretic peptide (ANP) is a hormone that promotes both fluid and sodium loss by the kidneys. The name natriuretic actually means salt excreting. ANP release from the atria is stimulated when blood volume and pressure are elevated. ANP has three major effects: 1) it decreases aldosterone release, resulting in a decrease in sodium reabsorption and increased sodium loss in the urine; 2) it decreases ADH release, which decreases water reabsorption and increases water loss to lower blood volume and pressure; and 3) it decreases thirst. A practice table to help you summarize these hormones and their actions is located on page 15. BLOOD VOLUME AND OSMOLARITY AFFECT THE VOLUME AND COMPOSITION OF URINE The volume and composition of urine reflect one's state of hydration. For example, if Phil Physiology has a low fluid intake and becomes dehydrated, he will excrete a small volume of concentrated urine. His body is trying to conserve water. Concentrated urine has a high specific gravity. Specific gravity is the ratio of the weight of a substance to the weight of an equal volume of water. Water has a specific gravity of 1.000, since equal volumes of water have equal weights at equal temperatures. The specific gravity of normal urine ranges from to and depends on the amount of solutes. The greater the concentration of solutes, the higher the specific gravity will be. At the other extreme, Anna Anatomy has a high fluid intake and is over-hydrated. She will excrete a large volume of dilute urine having a low specific gravity. Let s explore four specific cases in which blood volume and/or blood osmolarity has been perturbed. Use your understanding of the factors that regulate hormone release and the subsequent actions of the hormones to predict the effects of these perturbations on urine volume and osmolarity. ACTIVITY: For each case, work with your team to predict how the parameters listed would be altered (increased, decreased, or not changed.) Put an,, or nc (no change) on each line. The information in the introduction to this lab will be helpful! 4

5 CASE I: BLOOD VOLUME EXPANSION / NO CHANGE IN BLOOD OSMOLARITY Can you think of a situation in which blood volume would be increased and blood osmolarity would be unchanged? SITUATION: Blood volume: Ý Blood osmolarity: no change Ý Blood volume Blood pressure Y Atrial naturetic peptide (ANP) ADH Thirst Aldosterone water reabsorption in DCT and CD blood volume and pressure water in urine blood volume after drinking water sodium reabsorption in DCT and CD sodium excretion in the urine CASE II: Blood Volume Expansion and Decrease in Osmolarity Can you think of a situation in which blood volume would be increased and blood osmolarity would be decreased? 5

6 SITUATION: Blood volume: ñ Blood osmolarity: ò ò Blood volume ò Blood sodium ADH Aldosterone release water reabsorption in DCT and CD urine volume and pressure (dilute or concentrated?) sodium reabsorption in DCT and CD sodium excretion in the urine CASE III: Blood Volume Unchanged and Increase in Osmolarity Can you think of a situation in which blood volume is unchanged and blood osmolarity would be increased? SITUATION: Blood volume: no change Blood osmolarity: ñ ñ Blood osmolarity (hi blood sodium) Aldosterone release Osmoreceptors in hypothalamus ADH release from posterior pituitary thirst water reabsorption in DCT and CD urine volume and pressure (dilute or concentrated?) 6

7 CASE IV: Blood Volume Depletion and No change in Blood Osmolarity Can you think of a situation in which blood volume is decreased and blood osmolarity would be unchanged? SITUATION: Blood volume: decrease Blood osmolarity: no change ò Blood osmolarity Blood pressure Angiotensionogen Renin renin release from juxtaglomerular apparatus Angiotensin I Angiotensin converting enzyme (ACE) Angiotensin II blood vessel diameter aldosterone BP sodium reabsorption in DCT and CD Water follows sodium in urine 7

8 ACTIVITY: Group Reports. 1. Present your cases to the class. Justify your predictions and discuss them with your classmates and instructor. Notes: 2. How will urine volume, specific gravity, and urine NaCl be affected in each case? Summarize your predictions in Table 9-1 below. After you have done this review this with your classmates. Table 9-1 Case Blood Volume Expansion/ No change in Osmolariy Blood Volume Expansion/ Decrease in Osmolarity Blood Volume Unchanged/ Osmolarity increased Blood Volume Depletion/ No change in osmolarity Urine Volume Increase, Decrease or No Change Specific Gravity Increase, Decrease or No Change Urine NaCl Increase, Decrease or No Change DRY LAB EXPERIMENT In a lab experiment, a class of A & P students divided into one of the four groups below to drink the following solutions: GROUP NORMAL FLUID INTAKE ISOTONIC SOLUTION WATER LOADED SALT LOADED SOLUTION DRUNK 25 ml of distilled water 1 liter of 0.9% saline 1 liter of distilled water ml of 2.0% saline Please note: None of the groups were asked to significantly decrease blood volume as in one of the cases presented earlier. Why? Before the experiment began each group voided their urine into a container labeled Time Zero. Each group then drank their assigned solution. 8

9 ACTIVITY: For each group, indicate in Table 9-2 if the blood volume and osmolarity were increased, decreased, or unaffected by drinking the assigned solution. Table 9-2. Solution Consumed Blood Volume (Increase, Decrease, No Change) Blood Osmolarity (Increase, Decrease, No Change) Normal Intake 25 ml water Isotonic Solution 1000 ml 0.9% NaCl Water Loaded 1000 ml water Salt Loaded 150 ml 2% NaCl Urine samples were collected at 30-minute intervals for a period of 90 min. The following tests were conducted on each sample: a. TRANSPARENCY Recorded as clear, slightly cloudy or very cloudy. b. COLOR Recorded as colorless, pale, medium, or deep yellow. c. URINE VOLUME Measured in milliliters. d. URINE SPECIFIC GRAVITY the ratio of the weight of a substance to the weight of an equal volume of water. The specific gravity of water is e. URINE NaCl Recorded as mg/ml. FLOW CHART OF EXPERIMENT VOID at 0 MIN VOID at 30 MIN VOID at 60 MIN VOID at 90 MIN Drink assigned solution Analyze Sample Analyze sample Analyze sample Analyze sample Each of the four groups analyzed their samples, averaged their data, and compiled it in the 9

10 Specific Gravity table below. However, the groups did not identify the solutions they drank! The data appears on the table as Unknowns A to D in no particular order. EXPERIMENTAL DATA (Table 9-3): Mystery Results. Which of the four solutions did each of the unknown groups drink? Table 9-3. Group Color Specific Time 0 Time 30 minutes Time 60 minutes Time 90 minutes Gravity Volume NaCl Color Specific Gravity Volume NaCl Color Specific Gravity Volume NaCl Color Specific Gravity Volume NaCl ml mg/ml ml mg/ml ml mg/ml ml mg/ml Unknown A yellow pale yellow no color no color Unknown B yellow yellow dark yellow dark yellow Unknown C yellow yellow pale yellow pale yellow Unknown D yellow yellow yellow yellow ACTIVITY: DATA ANALYSIS. 1. GRAPHING: Prepare three graphs using the data from the table on the previous page: a. specific gravity vs. time b. volume (ml/min) vs. time c. NaCl (mg/ml) vs. time On each graph, plot the data for each of the four unknown groups. You will need to choose axes that encompass the data for all of the unknowns. After plotting, be sure to label each line. Alternately, your instructor may direct you to prepare graphs using Excel. Graph A. Changes in Urine Specific Gravity Time (Min)

11 Chloride (mg/ml) Volume (mls) Graph B. Changes in Urine Volume Time (Min) Graph C. Changes in Urine NaCl Time (Min)

12 ACTIVITY: INTERPRETING DATA 1. Using your graphs, summarize how the urine volume, specific gravity, and chloride content changed over the course of the experiment for each Unknown by completing Table 9.4: Table 9-4 Group Unknown A Unknown B Unknown C Unknown D Urine Volume (Increase, Decrease or No Change) Urine Specific Gravity (Increase, Decrease or No Change) Urine Chloride (Increase, Decrease or No Change) ACTIVITY: DRAWING CONCLUSIONS 1. Solve the mystery! Review your data in the table above. Which of the unknown groups, A, B, C, or D, drank each of the solutions listed below? Justify your conclusion by stating your reasoning in Table 9-5. Table 9-5 Solution Consumed Normal Fluid Intake 25 ml water Isotonic Solution 1000 ml 0.9% NaCl Unknown (A, B, C, or D?) Your reasoning: Water Loaded 1000 ml water Salt Loaded 150 ml 2% NaCl 12

13 ACTIVITY: EXPLAINING THE MECHANISMS 1. Which group produced the greatest urine volume during the 90-minute period? Explain why. 2. Which group produced urine with the lowest specific gravity and chloride content? Explain why. 3. Explain the differences in color between the urine of the water loaded group and the urine of the salt-loaded group. 4. Which group produced a small volume of concentrated urine? Explain why. 5. Which group was trying to conserve water? Conserve salt? 6. How did the specific gravity of the urine produced by the salt-loaded group compare with that of the isotonic loaded group? Explain the differences. 13

14 7. How did the volume of urine output compare to the volume drank in each group? Explain the differences. 8. What hormone was inhibited in the salt loaded group? What hormone was stimulated? 9. Which hormone was inhibited in the water loaded group? What hormone was stimulated? 10. Which group was thirsty? What is the mechanism responsible? 11. Which group had elevated levels of Atrial Natriuretic Peptide? What are the benefits of high ANP levels under these conditions? 14

15 ACTIVITY: Review the hormones and their actions by completing the table below: Hormone Released from: Released stimulated by: ACTION Anti-diuretic Hormone (ADH) Effects on BLOOD Volume & Osmolarity Effects on URINE Volume & Osmolarity Other Effects Renin Aldosterone Atrial Natriuretic Peptide (ANP) 15

16 Check your understanding! 1. Define fluid and electrolyte. 2. What is osmolarity and what is the normal value for blood osmolarity? 3. What is the major ion that contributes to blood osmolarity? 4. Describe how the body regulates osmolarity and blood volume. 5. What is ADH? Where is it produced? What causes it to be secreted? What is its action? What affect does it have on blood osmolarity, blood volume, urine osmolarity, and urine volume? 6. What is aldosterone? Where is it produced? Name three factors that lead to aldosterone release. What is its action? What effect does aldosterone have on blood osmolarity and urine osmolarity? 7. What is renin? Where is it produced? What factors cause renin to be released? Describe in detail how renin leads to the production of angiotensin II? What are the 2 major actions of angiotensin II? 8. What is atrial natriuretic peptide? Where is it produced? What factors lead to its release? What is its action? What effect does it have on blood volume and urine volume? 9. In the dry lab, students divided into four groups that drank 4 different fluids: normal fluid intake (25 ml water), isotonic solution (1000 ml 0.9% sodium chloride), water loaded (1000 ml water) and salt loaded (150 ml 2% sodium chloride). For each of the four groups, how did the solution drunk affect blood osmolarity and blood volume? Predict how drinking these four solutions would affect urine color, urine volume, specific gravity, and chloride content. What hormones are stimulated and inhibited in each of these conditions? What events cause the observed changes? Which group was thirsty and why? Be able to examine experimental data in both numerical and graphical form and predict what group produced it and what mechanisms were working to produce this data. 16

17 [NaCl] (mg/ml) Sample Graphs Specific gravity Volume [Cloride] time (min) 17

18 References: Fluid and Electrolyte Balance. Germann, W.J. and Stanfield, C.L. Principles of Human Physiology. San Francisco, CA: Pearson Education Martini. F.H., Nath, J.L.and Bartholomew, E.F Fundamentals of Anatomy and Physiology. Boston, MA: Pearson Education. Sherwood, L Human Physiology, From Cells to Systems. Belmont, CA: Brooks/Cole, Cengage Learning Silverthorne, D.U Human Physiology, An Integrated Approach. Boston, MA: Pearson Education, Inc. Acknowledgements: I am grateful to the students in the Anatomy and Physiology 2 classes at Bloomsburg University for their feedback and enthusiasm. I also appreciate the contributions and helpful suggestions of my colleagues in Anatomy and Physiology: Drs. Margaret Till, Angela Hess, Carl Hansen, Mark Melnychuk, Jennifer Venditti, William Coleman, and William Schwindinger. 18