Role of the Kidney in Fluid Balance

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Lab 7: The Kidney’s Role in Fluid Balance

Introduction
The renal system performs a vital role in homeostasis. The kidneys’ ability to retain valuable constituents and expel metabolic wastes from the body enables this system to regulate the volume, osmolarity, and pH of body’s internal fluid environment (Sherwood, 2007, p. 511). The functional unit of the kidney, referred to as the nephron, is composed of both tubular components—Bowman’s capsule proximal tubule, loop of Henle, the distal tubule, and the collecting duct—and vascular components—afferent arteriole, glomerulus, efferent arteriole, and peritubular capillaries (Sherwood, 2007, p. 514). Through its vascular and tubular components, the nephron performs three basic functions in order to carry out its regulatory role in the kidney: glomerular filtration, tubular reabsorption, and tubular secretion. In the initial phase of urine creation, otherwise known as glomerular filtration, roughly 20% of plasma from the afferent arteriole flows through the glomerular capillaries and into the Bowman’s capsule. As this newly formed filtrate travels through the nephron, it is subject to proximal and distal tubular reabsorption, where substances from the filtrate return to circulation by moving into the peritubular capillaries. On the other hand, substances can move in the opposite direction, from the peritubular capillaries into the filtrate, referred to as proximal and distal tubular secretion (Sherwood, 2007, p. 515). A critical determinant of urine composition lies is the differences between the proximal and distal tubule. The proximal tubule is specialized for bulk reabsorption (67-80% of filtrate) to balance the glomerular filtration rate without hormones or sympathetic nervous system innervation. Whereas the distal tubule reabsorption is hormonally regulated, allowing the nephron to form dilute or concentrated urine to maintain internal fluid homeostasis (Sherwood, 2007, p. 548). The aim of this study is to investigate kidneys’ responses to changes in fluid volume, osmolarity, and pH in the body’s internal environment. In order to do so, flow rate, specific gravity, sodium concentration, the amount of water and sodium, creatinine and sodium clearance, and pH of urine for four different conditions (hypotonic, isotonic, alkalosis, and control) are examined. The main mechanisms that respond to fluid volume changes—caused by volumetric load consumption of hypotonic and isotonic solutions—are the baroreceptor reflex, atrial natriuretic peptide (ANP), and volume receptors. While the most important hormonal system responsible for osmolarity fluctuations is the rennin-angiotensin-aldosterone system (RAAS). Baroreceptors triggered by changes in mean arterial blood pressure are strategically located in the carotid sinus and the aortic arch, both of which are arteries that supply blood to the brain and the rest of the body, respectively. The baroreceptor reflex plays an important role in regulating blood pressure through adjustments in cardiac output and total peripheral resistance via negative innervation to the sympathetic nervous system (SNS) (Sherwood, 2007, p. 378-380). In addition, ANP is produced and released by the heart’s right atria upon its walls being mechanically stretched from increased amounts of extracellular fluid (ECF). Also located in the right atria are volume receptors, of which stretch as well in the presence of elevated levels of ECF volume. Together, the volume receptors and ANP act to inhibit both sodium and water reabsorption in the distal tubule and collecting duct of the nephron in order to increase the amount excreted in the urine (Sherwood, 2007, p. 529). The two volume regulatory systems work to increase glomerular filtration rate (GFR) in the presence of excess fluid, in order to filter and excrete more fluid from the body per unit time. GFR is the rate at which plasma filters from the glomerular capillaries to the Bowman’s capsule...
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