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Regulation of Fluid & Electrolyte Balance

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Regulation of Fluid & Electrolyte Balance REGULATION OF FLUID & ELECTROLYTE BALANCE 1 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 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 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 1.001 to 1.035 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 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 ___ sodium reabsorption DCT and CD ___ blood volume after in DCT and CD drinking water ___ blood volume and ___ sodium excretion in pressure the urine ___ water in 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 SITUATION: Blood volume: ñ ò Blood volume Blood osmolarity: ò ò Blood sodium ___ ADH ___ Aldosterone release ___ water reabsorption in DCT and CD ___ sodium reabsorption in DCT and CD ___ urine volume and pressure ___ sodium excretion in (dilute or concentrated?) 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 ñ Blood osmolarity Blood osmolarity: ñ (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?)
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