Chapter 14 The Urinary System Human Physiology by Lauralee Sherwood ©2007 Brooks/Cole-Thomson Learning

Kidneys Overview of kidney functions Maintain H2O balance in the body Maintain proper osmolarity of body fluids, primarily through regulating H2O balance Regulate the quantity and concentration of most ECF ions Maintain proper plasma volume Help maintain proper acid-base balance in the body Excreting (eliminating) the end products (wastes) of bodily metabolism Excreting many foreign compounds Producing erythropoietin Producing renin Converting vitamin D into its active form Chapter 14 The Urinary System Human Physiology by Lauralee Sherwood ©2007 Brooks/Cole-Thomson Learning

Urinary System Know organs and functions Chapter 14 The Urinary System Human Physiology by Lauralee Sherwood ©2007 Brooks/Cole-Thomson Learning

Nephron Functional unit of the kidney Smallest unit that can perform all the functions of the kidney Approximately 1 million nephrons/kidney Each nephron has two components Vascular component Tubular component Arrangement of nephrons within kidney gives rise to two distinct regions Outer region Renal cortex (granular in appearance) Inner region Renal medulla Made up of striated triangles called renal pyramids Chapter 14 The Urinary System Human Physiology by Lauralee Sherwood ©2007 Brooks/Cole-Thomson Learning

Nephron Two types of nephrons Distinguished by location and length of their structures Juxtamedullary nephrons Cortical nephrons Chapter 14 The Urinary System Human Physiology by Lauralee Sherwood ©2007 Brooks/Cole-Thomson Learning

Nephron Chapter 14 The Urinary System Human Physiology by Lauralee Sherwood ©2007 Brooks/Cole-Thomson Learning

Glomerular filtration Basic Renal Processes Glomerular filtration Tubular reabsorption Tubular secretion Urine results from these three processes. Chapter 14 The Urinary System Human Physiology by Lauralee Sherwood ©2007 Brooks/Cole-Thomson Learning

Glomerular Filtration Fluid filtered from the glomerulus into Bowman’s capsule pass through three layers of the glomerular membrane Glomerular capillary wall Single layer of endothelial cells More permeable to water and solutes than capillaries elsewhere in the body Basement membrane Acellular gelatinous layer Composed of collagen and glycoproteins Inner layer of Bowman’s capsule Consists of podocytes that encircle the glomerulus tuft Chapter 14 The Urinary System Human Physiology by Lauralee Sherwood ©2007 Brooks/Cole-Thomson Learning

Glomerular Filtration and Layers of Glomerular Membrane Chapter 14 The Urinary System Human Physiology by Lauralee Sherwood ©2007 Brooks/Cole-Thomson Learning

Forces Involved in Glomerular Filtration Three physical forces involved Glomerular capillary blood pressure Plasma-colloid osmotic pressure Bowman’s capsule hydrostatic pressure Chapter 14 The Urinary System Human Physiology by Lauralee Sherwood ©2007 Brooks/Cole-Thomson Learning

Glomerular Filtration Rate Unregulated influences on the GFR Pathologically plasma-colloid osmotic pressure and Bowman’s capsule hydrostatic pressure can change Plasma-colloid osmotic pressure Severely burned patient ↑ GFR Dehydrating diarrhea ↓ GFR Bowman’s capsule hydrostatic pressure Obstructions such as kidney stone of enlarged prostate can decrease filtration and elevate capsular hydrostatic pressure Controlled adjustments in GFR Glomerular capillary blood pressure can be controlled to adjust GFR to suit the body’s needs Two major control mechanisms Autoregulation (aimed at preventing spontaneous changes in GFR) Myogenic mechanism Tubuloglomerular feedback (TGF) Extrinsic sympathetic control (aimed at long-term regulation of arterial blood pressure) Mediated by sympathetic nervous system input to afferent arterioles Baroreceptor reflex Chapter 14 The Urinary System Human Physiology by Lauralee Sherwood ©2007 Brooks/Cole-Thomson Learning

Adjustments of Afferent Arteriole Caliber to Alter The GFR Chapter 14 The Urinary System Human Physiology by Lauralee Sherwood ©2007 Brooks/Cole-Thomson Learning

Glomerular Filtration Rate Influenced by changes in filtration coefficient This coefficient is not constant but is subject to physiological control. The coefficient depends on surface area and the permeability of the glomerular membranes. Both can be modified by contractile activity within the membrane. The kidneys receive 20 to 25 percent of the cardiac output The total blood flow through the kidneys average 1,140 ml per minute. If the cardiac output is 5 liters, this figure is 22 % of the cardiac output. This kidneys need to receive this large blood flow to monitor and control the ECF Chapter 14 The Urinary System Human Physiology by Lauralee Sherwood ©2007 Brooks/Cole-Thomson Learning

Basic Renal Processes Tubular reabsorption Glomerular filtration Tubular reabsorption Tubular secretion Urine results from these three processes. Chapter 14 The Urinary System Human Physiology by Lauralee Sherwood ©2007 Brooks/Cole-Thomson Learning

Reabsorption Transepithelial Transport Passive reabsorption No energy is required for the substance’s net movement Occurs down electrochemical or osmotic gradients Active reabsorption Occurs if any one of the steps in transepithelial transport of a substance requires energy Movement occurs against electrochemical gradient Chapter 14 The Urinary System Human Physiology by Lauralee Sherwood ©2007 Brooks/Cole-Thomson Learning

Na+ Reabsorption Tubule area % of Na+ reabsorbed An active Na+ - K+ ATPase pump in basolateral membrane is essential for Na+ reabsorption Of total energy spent by kidneys, 80% is used for Na+ transport Na+ is not reabsorbed in the descending limb of the loop of Henle Water follows reabsorbed sodium by osmosis which has a main effect on blood volume and blood pressure Na+ Reabsorption Tubule area % of Na+ reabsorbed Role of Na+ reabsorption Proximal tubule 67% Plays role in reabsorbing glucose, amino acids, H2O, Cl-, and urea Ascending limb of the loop of Henle 25% Plays critical role in kidneys’ ability to produce urine of varying concentrations Distal and collecting tubules 8% Variable and subject to hormonal control; plays role in regulating ECF volume Chapter 14 The Urinary System Human Physiology by Lauralee Sherwood ©2007 Brooks/Cole-Thomson Learning

Sodium Reabsorption Chapter 14 The Urinary System Human Physiology by Lauralee Sherwood ©2007 Brooks/Cole-Thomson Learning

RAAS Renin-angiotensin-aldosterone system Most import and best known hormonal system involved in regulating Na+ Chapter 14 The Urinary System Human Physiology by Lauralee Sherwood ©2007 Brooks/Cole-Thomson Learning

Passive Reabsorption of Urea at the End of the Proximal Tubule Glucose and amino acids are reabsorbed by sodium-dependent, secondary active transport. Electrolytes other than Na+ that are reabsorbed by the tubules have their own independently functioning carrier systems within the proximal tubule The reabsorption of water in the proximal tubule increases the concentration of urea in the tubule, as water is lost from the tubule. This produces a concentration gradient for urea from the tubule into the interstitial fluid. Generally, unwanted waste products are not reabsorbed Passive Reabsorption of Urea at the End of the Proximal Tubule Chapter 14 The Urinary System Human Physiology by Lauralee Sherwood ©2007 Brooks/Cole-Thomson Learning

Basic Renal Processes Tubular secretion Glomerular filtration Tubular reabsorption Tubular secretion Transfer of substances from peritubular capillaries into the tubular lumen Involves transepithelial transport (steps are reversed) Kidney tubules can selectively add some substances to the substances already filtered Chapter 14 The Urinary System Human Physiology by Lauralee Sherwood ©2007 Brooks/Cole-Thomson Learning

Tubular Secretion Most important secretory systems are for H+ K+ Important in regulating acid-base balance Secreted in proximal, distal, and collecting tubules K+ Keeps plasma K+ concentration at appropriate level to maintain normal membrane excitability in muscles and nerves Secreted only in the distal and collecting tubules under control of aldosterone Organic ions Accomplish more efficient elimination of foreign organic compounds from the body Secreted only in the proximal tubule Chapter 14 The Urinary System Human Physiology by Lauralee Sherwood ©2007 Brooks/Cole-Thomson Learning

Plasma Clearance Volume of plasma cleared of a particular substance per minute (not the amount of the substance removed) Varies for different substances, depending on how the kidneys handle each substance Chapter 14 The Urinary System Human Physiology by Lauralee Sherwood ©2007 Brooks/Cole-Thomson Learning

Urine Excretion Depending on the body’s state of hydration, the kidneys secrete urine of varying concentrations. Too much water in the ECF establishes a hypotonic ECF (result = dilute urine) A water deficit establishes a hypertonic ECF (result = conc. urine) A large, vertical osmotic gradient is established in the interstitial fluid of the medulla (from 100 to 1200 mosm/liter to 1200 mosm/liter). This increase follows the hairpin loop of Henle deeper and deeper into the medulla. This osmotic gradient exists between the tubular lumen and the surrounding interstitial fluid. Chapter 14 The Urinary System Human Physiology by Lauralee Sherwood ©2007 Brooks/Cole-Thomson Learning

Countercurrent Multiplication The medullary vertical osmotic gradient is established by countercurrent multiplication Comparing the descending and ascending limbs of the loop of Henle: The descending ling is highly permeable to water but does not extrude sodium for reabsorption. The ascending limb actively transports NaCl out of the tubular lumen into the surrounding interstitial fluid. It is impermeable to water. Therefore, water does not follow the salt by osmosis. There is a countercurrent flow produced by the close proximity of the two limbs. The ascending limb produces an interstitial fluid that becomes hypertonic to the ascending limb. It does this by pumping out sodium ions. Water does not follow. This interstitial fluid faces against the flow of fluid (countercurrent) in the descending limb, attracting the water by osmosis for reabsorption Chapter 14 The Urinary System Human Physiology by Lauralee Sherwood ©2007 Brooks/Cole-Thomson Learning

Role of Vasopressin Vasopressin-controlled, variable water reabsorption occurs in the final tubular segments. 65 percent of water reabsorption is obligatory in the proximal tubule. In the distal tubule and collecting duct it is variable, based on the secretion of ADH. The secretion of vasopressin increases the permeability of the tubule cells to water. An osmotic gradient exists outside the tubules for the transport of water by osmosis. Vasopressin is produced in the hypothalamus and stored in the posterior pituitary. The release of this substance signals the distal tubule and collecting duct, facilitating the reabsorption of water. Vasopressin works on tubule cells through a cyclic AMP mechanism. During a water deficit, the secretion of vasopressin increases. This increases water reabsorption. During an excess of water, the secretion of vasopressin decreases. Less water is reabsorbed. More is eliminated. Chapter 14 The Urinary System Human Physiology by Lauralee Sherwood ©2007 Brooks/Cole-Thomson Learning

Nephron Vascular component Dominant part is the glomerulus Ball-like tuft of capillaries Water and solutes are filtered through glomerulus as blood passes through it Filtered fluid then passes through nephron’s tubular component From renal artery, inflowing blood passes through afferent arterioles which deliver blood to glomerulus Efferent arteriole transports blood from glomerulus Efferent arteriole breaks down into peritubular capillaries which surround tubular part of nephron Peritubular capillaries join into venules which transport blood into the renal vein Tubular component Hollow, fluid-filled tube formed by a single layer of epithelial cells Components Bowman’s capsule Proximal tubule Loop of Henle Descending limb Ascending limb Juxtaglomerular apparatus Distal tubule Collecting duct or tubule Chapter 14 The Urinary System Human Physiology by Lauralee Sherwood ©2007 Brooks/Cole-Thomson Learning

Countercurrent

Renal Anatomy

Renal Processes 1

Renal Processes 2

Urine Concentration

Remaining class periods 4/13, 4/16, 4/20, 4/23 and 5/2 for students not graduating. Exam 4 Exam IV (9, 12 14) will be available 4/17 at 12:00 PM until 4/18 at 5:00  PM.  I will not reset this exam after 4/17 at 5:00 PM Lab exam 3 ECG and Endocrinology 4/20 Endocrinology lectures 4/20 and 4/23 Last Exam (18,19) cumulative material will be available 4/26 at 12:00 PM until 4/27 at 5:00 PM.  I will not reset this exam after 4/26 at 12:00 PM 5/2 review final exam and determine final grades.