Chapter 18 - The Urinary System

C h a p t e r 18 The Urinary System Copyright © 2010 Education, Inc. Introduction to the Urinary System Figure 18-1 The urinary system, consisting of the kidneys, ureters, urinary bladder, and urethra, has three primary functions Introduction to the Urinary System Three Functions of the Urinary System Excretion: Removal of organic wastes from body fluids Elimination: Discharge of waste products Homeostatic regulation: Of blood plasma volume and solute concentration Introduction to the Urinary System Kidneys — organs that produce urine Urinary tract — organs that eliminate urine Ureters (paired tubes) Urinary bladder (muscular sac) Urethra (exit tube) Urination or micturition — process of eliminating urine Contraction of muscular urinary bladder forces urine through urethra and out of body Introduction to the Urinary System Four Homeostatic Functions of Urinary System Regulates blood volume and blood pressure: By adjusting volume of water lost in urine Releasing erythropoietin and renin Regulates plasma ion concentrations: Sodium, potassium, and chloride ions (by controlling quantities lost in urine) Calcium ion levels (through synthesis of calcitriol) Introduction to the Urinary System Four Homeostatic Functions of Urinary System 3. Helps stabilize blood pH: By controlling loss of hydrogen ions and bicarbonate ions in urine 4. Conserves valuable nutrients: By preventing excretion while excreting organic waste products The highly vascular kidneys contain functional units called nephrons,which perform filtration, reabsorption, and secretion The Kidneys Are located on either side of the vertebral column Left kidney lies superior to right kidney Superior surface capped by suprarenal (adrenal) gland Position is maintained by: Overlying peritoneum Contact with adjacent visceral organs Supporting connective tissues Kidney Function: Urinary System Structure The Kidneys Figure 18-2a The Kidneys Figure 18-2b The Kidneys Typical Adult Kidney Is about 10 cm long, 5.5 cm wide, and 3 cm thick (4 in. ? 2.2 in. ?1.2 in.) Weighs about 150 g (5.25 oz) The Kidneys Hilum Point of entry for renal artery and renal nerves Point of exit for renal vein and ureter The Kidneys Sectional Anatomy of the Kidneys Renal sinus: Internal cavity within kidney Lined by fibrous renal capsule: bound to outer surfaces of structures in renal sinus stabilizes positions of ureter, renal blood vessels, and nerves The Kidneys Renal Cortex Superficial portion of kidney in contact with renal capsule Reddish brown and granular The Kidneys Renal Pyramids 6 to 18 distinct conical or triangular structures in renal medulla Tip (renal papilla) projects into renal sinus The Kidneys Renal Columns Bands of cortical tissue separate adjacent renal pyramids Extend into medulla Have distinct granular texture The Kidneys Renal Papilla Ducts discharge urine into minor calyx, a cup-shaped drain Major Calyx Formed by four or five minor calyces The Kidneys Renal Pelvis Large, funnel-shaped chamber Consists of two or three major calyces Fills most of renal sinus Connected to ureter, which drains kidney The Kidneys Figure 18-3a The Kidneys Figure 18-3b The Kidneys Nephrons Microscopic, tubular structures in cortex of each renal lobe Where urine production begins The Kidneys Figure 18-3c Blood Supply to Kidneys Kidneys receive 20% to 25% of total cardiac output 1200 mL of blood flows through kidneys each minute Kidney receives blood through renal artery Blood Supply to Kidneys Figure 18-4a Blood Supply to Kidneys Figure 18-4b Blood Supply to Kidneys Figure 18-4 c,d The Nephron Consists of renal tubule and renal corpuscle Renal tubule Long tubular passageway Begins at renal corpuscle Renal corpuscle Spherical structure consisting of: glomerular capsule (Bowman’s capsule) cup-shaped chamber capillary network (glomerulus) The Nephron Glomerulus Consists of 50 intertwining capillaries Blood delivered via afferent arteriole Blood leaves in efferent arteriole: Flows into peritubular capillaries Which drain into small venules And return blood to venous system The Nephron Three Functions of Renal Tubule Reabsorb useful organic nutrients that enter filtrate Reabsorb more than 90% of water in filtrate Secrete waste products that failed to enter renal corpuscle through filtration at glomerulus The Nephron Segments of Renal Tubule Located in cortex: Proximal convoluted tubule (PCT) Distal convoluted tubule (DCT) Separated by nephron loop (loop of Henle): U-shaped tube Extends partially into medulla The Nephron Organization of the Nephron Traveling along tubule, filtrate (tubular fluid) gradually changes composition Changes vary with activities in each segment of nephron The Nephron Each Nephron Empties into the collecting system: A series of tubes that carries tubular fluid away from nephron The Nephron Collecting Ducts Receive fluid from many nephrons Each collecting duct: Begins in cortex Descends into medulla Carries fluid to papillary duct that drains into a minor calyx Figure 18-5 The Nephron Cortical Nephrons 85% of all nephrons Located mostly within superficial cortex of kidney Nephron loop (Loop of Henle) is relatively short Efferent arteriole delivers blood to a network of peritubular capillaries Juxtamedullary Nephrons 15% of nephrons Nephron loops extend deep into medulla Peritubular capillaries connect to vasa recta The Nephron The Renal Corpuscle Each renal corpuscle: Is 150–250 µm in diameter Glomerular capsule: is connected to initial segment of renal tubule forms outer wall of renal corpuscle encapsulates glomerular capillaries Glomerulus: knot of capillaries The Nephron The Glomerular Capsule Outer wall is lined by simple squamous capsular epithelium: Continuous with visceral epithelium that covers glomerular capillaries: separated by capsular space The Nephron The Visceral Epithelium Consists of large cells (podocytes): With complex processes or “feet” (pedicels) that wrap around specialized lamina densa of glomerular capillaries The Nephron Filtration Slits Are narrow gaps between adjacent pedicels Materials passing out of blood at glomerulus: Must be small enough to pass between filtration slits The Renal Corpuscle Figure 18-6a The Renal Corpuscle Figure 18-6b The Renal Corpuscle Figure 18-6c The Nephron The Proximal Convoluted Tubule (PCT) Is the first segment of renal tubule Entrance to PCT lies opposite point of connection of afferent and efferent arterioles with glomerulus The Nephron Tubular Cells Absorb organic nutrients, ions, water, and plasma proteins from tubular fluid Release them into peritubular fluid (interstitial fluid around renal tubule) The Nephron Nephron loop (also called loop of Henle) Renal tubule turns toward renal medulla: Leads to nephron loop Descending limb: Fluid flows toward renal pelvis Ascending limb: Fluid flows toward renal cortex Each limb contains: Thick segment Thin segment The Nephron The Distal Convoluted Tubule (DCT) The third segment of the renal tubule Initial portion passes between afferent and efferent arterioles Has a smaller diameter than PCT Epithelial cells lack microvilli The Nephron Three Processes at the DCT Active secretion of ions, acids, drugs, and toxins Selective reabsorption of sodium and calcium ions from tubular fluid Selective reabsorption of water: Concentrates tubular fluid The Nephron Juxtaglomerular Complex An endocrine structure that secretes: Hormone erythropoietin Enzyme renin Formed by: Macula densa Juxtaglomerular cells The Nephron Macula Densa Epithelial cells of DCT, near renal corpuscle Tall cells with densely clustered nuclei Juxtaglomerular Cells Smooth muscle fibers in wall of afferent arteriole: Associated with cells of macula densa Together with macula densa forms juxtaglomerular complex (JGC) The Nephron The Collecting System The distal convoluted tubule opens into the collecting system Individual nephrons drain into a nearby collecting duct Several collecting ducts: Converge into a larger papillary duct Which empties into a minor calyx Transports tubular fluid from nephron to renal pelvis Adjusts fluid composition Determines final osmotic concentration and volume of urine Different portions of the nephron form urine by filtration, reabsorption, and secretion Renal Physiology The goal of urine production Is to maintain homeostasis By regulating volume and composition of blood Including excretion of metabolic waste products Renal Physiology Three Organic Waste Products Urea Creatinine Uric acid Renal Physiology Organic Waste Products Are dissolved in the bloodstream Are eliminated only while dissolved in urine Removal is accompanied by water loss Basic Processes of Urine Formation Filtration Reabsorption Secretion Filtration at the Glomerulus Filtration Hydrostatic pressure forces water through membrane pores: Small solute molecules pass through pores Larger solutes and suspended materials are retained Occurs across capillary walls: As water and dissolved materials are pushed into interstitial fluids Filtration at the Glomerulus Filtration In some sites, such as the liver, pores are large: Plasma proteins can enter interstitial fluids At the renal corpuscle: Specialized membrane restricts all circulating proteins Reabsorption and Secretion at the Renal Tubule Reabsorption and Secretion At the kidneys, they involve: Diffusion Osmosis Channel-mediated diffusion Carrier-mediated transport Reabsorption and Secretion at the Renal Tubule An Overview of Renal Function Water and solute reabsorption: Primarily along proximal convoluted tubules Active secretion: Primarily at proximal and distal convoluted tubules Long loops of juxtamedullary nephrons and collecting system: Regulate final volume and solute concentration of urine Reabsorption and Secretion at the Renal Tubule Reabsorption and Secretion at the PCT PCT cells normally reabsorb 60% to 70% of filtrate produced in renal corpuscle Reabsorbed materials enter peritubular fluid: And diffuse into peritubular capillaries Reabsorption and Secretion at the Renal Tubule Five Functions of the PCT Reabsorption of organic nutrients Active reabsorption of ions Reabsorption of water Passive reabsorption of ions Secretion Reabsorption and Secretion at the Renal Tubule The Nephron Loop Nephron loop reabsorbs about 1/2 of water and 2/3 of sodium and chloride ions remaining in tubular fluid Concentrates the medulla Reabsorption and Secretion at the Renal Tubule Tubular Fluid at DCT Arrives with osmotic concentration of 100 mOsm/L: 1/3 concentration of peritubular fluid of renal cortex Rate of ion transport across thick ascending limb is proportional to ion’s concentration in tubular fluid Reabsorption and Secretion at the Renal Tubule Reabsorption and Secretion at the DCT Composition and volume of tubular fluid: Changes from capsular space to distal convoluted tubule: only 15% to 20% of initial filtrate volume reaches DCT concentrations of electrolytes and organic wastes in arriving tubular fluid no longer resemble blood plasma Reabsorption and Secretion at the Renal Tubule Reabsorption at the DCT Selective reabsorption or secretion, primarily along DCT, makes final adjustments in solute composition and volume of tubular fluid Tubular Cells at the DCT Actively transport Na+ and Cl– out of tubular fluid Along distal portions: contain ion pumps reabsorb tubular Na+ in exchange for K+ Reabsorption and Secretion at the Renal Tubule Aldosterone Is a hormone produced by suprarenal cortex Controls ion pump and channels Stimulates synthesis and incorporation of Na+ pumps and channels In plasma membranes along DCT and collecting duct Reduces Na+ lost in urine Reabsorption and Secretion at the Renal Tubule Secretion at the DCT Blood entering peritubular capillaries: Contains undesirable substances that did not cross filtration membrane at glomerulus Rate of K+ and H+ secretion rises or falls: According to concentrations in peritubular fluid Higher concentration and higher rate of secretion Reabsorption and Secretion at the Renal Tubule Potassium Ion Secretion Ions diffuse into lumen through potassium channels: At apical surfaces of tubular cells Tubular cells exchange Na+ in tubular fluid: For excess K+ in body fluids Reabsorption and Secretion at the Renal Tubule Hydrogen Ion Secretion Hydrogen ions are generated by dissociation of carbonic acid by enzyme carbonic anhydrase Secretion is associated with reabsorption of sodium: Secreted by sodium-linked countertransport In exchange for Na+ in tubular fluid Bicarbonate ions diffuse into bloodstream: Buffer changes in plasma pH Reabsorption and Secretion at the Renal Tubule Reabsorption and Secretion along the Collecting System Collecting ducts: Receive tubular fluid from nephrons Carry it toward renal sinus Reabsorption and Secretion at the Renal Tubule Regulating Water and Solute Loss in the Collecting System By aldosterone: Controls sodium ion pumps Actions are opposed by natriuretic peptides By ADH: Controls permeability to water Is suppressed by natriuretic peptides Reabsorption and Secretion at the Renal Tubule ADH Hormone that causes special water channels to appear in apical cell membranes Increases rate of osmotic water movement Higher levels of ADH increase: Number of water channels Water permeability of DCT and collecting system Reabsorption and Secretion at the Renal Tubule Osmotic Concentration Of tubular fluid arriving at DCT: 100 mOsm/L In the presence of ADH (in cortex): 300 mOsm/L In minor calyx: 1200 mOsml/L Reabsorption and Secretion at the Renal Tubule Without ADH Water is not reabsorbed All fluid reaching DCT is lost in urine: Producing large amounts of dilute urine The Effects of ADH on the DCT and Collecting Duct Figure 18-7 The Composition of Normal Urine Results from filtration, absorption, and secretion activities of nephrons Some compounds (such as urea) are neither excreted nor reabsorbed Organic nutrients are completely reabsorbed Other compounds missed by filtration process (e.g., creatinine) are actively secreted into tubular fluid The Composition of Normal Urine A urine sample depends on osmotic movement of water across walls of tubules and collecting ducts Is a clear, sterile solution Yellow color (pigment urobilin): Generated in kidneys from urobilinogens Urinalysis, the analysis of a urine sample, is an important diagnostic tool Summary: Renal Function Step 1: Glomerulus Filtrate produced at renal corpuscle has the same composition as blood plasma (minus plasma proteins) Step 2: Proximal Convoluted Tubule (PCT) Active removal of ions and organic substrates: Produces osmotic water flow out of tubular fluid Reduces volume of filtrate Keeps solutions inside and outside tubule isotonic Summary: Renal Function Step 3: PCT and Descending Limb Water moves into peritubular fluids, leaving highly concentrated tubular fluid Reduction in volume occurs by obligatory water reabsorption Step 4: Thick Ascending Limb Tubular cells actively transport Na+ and Cl– out of tubule Urea accounts for higher proportion of total osmotic concentration Summary: Renal Function Step 5: DCT and Collecting Ducts Final adjustments in composition of tubular fluid Osmotic concentration is adjusted through active transport (reabsorption or secretion) Step 6: DCT and Collecting Ducts Final adjustments in volume and osmotic concentration of tubular fluid Exposure to ADH determines final urine concentration Summary: Renal Function Step 7: Vasa Recta Absorbs solutes and water reabsorbed by nephron loop and the ducts Maintains concentration gradient of medulla Urine Production Ends when fluid enters the renal pelvis Figure 18-8 Normal kidney function depends on a stable GFR Control of the GFR Autoregulation (local level) Autonomic regulation (by sympathetic division of ANS) Hormonal regulation (initiated by kidneys) Local Regulation of Kidney Function Autoregulation of the GFR Maintains GFR despite changes in local blood pressure and blood flow By changing diameters of afferent arterioles, efferent arterioles, and glomerular capillaries Local Regulation of Kidney Function Autoregulation of the GFR Reduced blood flow or glomerular blood pressure triggers: Dilation of afferent arteriole Dilation of glomerular capillaries Constriction of efferent arterioles Rise in renal blood pressure: Stretches walls of afferent arterioles Causes smooth muscle cells to contract Constricts afferent arterioles Decreases glomerular blood flow Sympathetic Activation and Kidney Function Autonomic Regulation of the GFR Mostly consists of sympathetic postganglionic fibers Sympathetic activation: Constricts afferent arterioles Decreases GFR Slows filtrate production Changes in blood flow to kidneys due to sympathetic stimulation: May be opposed by autoregulation at local level Hormonal Regulation of the GFR By hormones of the Renin–angiotensin system Natriuretic peptides (ANP and BNP) Hormonal Regulation of the GFR The Renin–Angiotensin System Three stimuli cause the juxtaglomerular complex (JGA) to release renin: Decline in blood pressure at glomerulus due to decrease in blood volume Fall in systemic pressures due to blockage in renal artery or tributaries Stimulation of juxtaglomerular cells by sympathetic innervation due to decline in osmotic concentration of tubular fluid at macula densa Hormonal Regulation of the GFR The Renin–Angiotensin System: Angiotensin II Activation Constricts efferent arterioles of nephron: Elevating glomerular pressures and filtration rates Stimulates reabsorption of sodium ions and water at PCT Stimulates secretion of aldosterone by suprarenal (adrenal) cortex Stimulates thirst Triggers release of antidiuretic hormone (ADH): Stimulates reabsorption of water in distal portion of DCT and collecting system Hormonal Regulation of the GFR The Renin–Angiotensin System: Angiotensin II Increases sympathetic motor tone: Mobilizing the venous reserve Increasing cardiac output Stimulating peripheral vasoconstriction Causes brief, powerful vasoconstriction: Of arterioles and precapillary sphincters Elevating arterial pressures throughout the body Hormonal Regulation of the GFR The Renin–Angiotensin System Aldosterone: Accelerates sodium reabsorption: in DCT and cortical portion of collecting system Hormonal Regulation of the GFR Figure 18-9 Hormonal Regulation of the GFR Figure 18-9 Hormonal Regulation of the GFR Atrial Natriuretic Peptides Are released by the heart in response to stretching walls due to increased blood volume or pressure Trigger dilation of afferent arterioles and constriction of efferent arterioles Elevates glomerular pressures and increases GFR Urine is transported by the ureters, stored in the bladder, and eliminated through the urethra, aided by the micturition reflex Urine Transport, Storage, and Elimination Takes place in the urinary tract Ureters Urinary bladder Urethra The Ureters Are a pair of muscular tubes Extend from kidneys to urinary bladder Begin at renal pelvis Pass over psoas major muscles Are retroperitoneal, attached to posterior abdominal wall Penetrate posterior wall of the urinary bladder Pass through bladder wall at oblique angle Ureteral openings are slitlike rather than rounded Shape helps prevent backflow of urine when urinary bladder contracts The Urinary Bladder Is a hollow, muscular organ Functions as a temporary reservoir for urine storage Full bladder can contain 1 liter of urine The Urinary Bladder Bladder Position Is stabilized by several peritoneal folds Posterior, inferior, and anterior surfaces: Lie outside peritoneal cavity Ligamentous bands: Anchor urinary bladder to pelvic and pubic bones Organs for the Conduction and Storage of Urine Figure 18-10a Organs for the Conduction and Storage of Urine Figure 18-10b Organs for the Conduction and Storage of Urine Figure 18-10c The Urethra Extends from neck of urinary bladder To the exterior of the body The Urethra The Male Urethra Extends from neck of urinary bladder to tip of penis (18–20 cm; 7–8 in.) Prostatic urethra passes through center of prostate gland Membranous urethra includes short segment that penetrates the urogenital diaphragm Spongy urethra (penile urethra) extends from urogenital diaphragm to external urethral orifice The Urethra The Female Urethra Is very short (3–5 cm; 1–2 in.) Extends from bladder to vestibule External urethral orifice is near anterior wall of vagina The Urethra The External Urethral Sphincter In both sexes: Is a circular band of skeletal muscle Where urethra passes through urogenital diaphragm Acts as a valve Is under voluntary control: Via perineal branch of pudendal nerve Has resting muscle tone Voluntarily relaxation permits micturition The Micturition Reflex and Urination As the bladder fills with urine Stretch receptors in urinary bladder stimulate sensory fibers in pelvic nerve Stimulus travels from afferent fibers in pelvic nerves to sacral spinal cord Efferent fibers in pelvic nerves Stimulate ganglionic neurons in wall of bladder The Micturition Reflex and Urination Postganglionic neuron in intramural ganglion stimulates detrusor muscle contraction Interneuron relays sensation to thalamus Projection fibers from thalamus deliver sensation to cerebral cortex Voluntary relaxation of external urethral sphincter causes relaxation of internal urethral sphincter The Micturition Reflex and Urination Begins when stretch receptors stimulate parasympathetic preganglionic motor neurons Volume >500 mL triggers micturition reflex The Micturition Reflex Figure 18-11 Fluid balance, electrolyte balance, and acid–base balance are interrelated and essential to homeostasis Introduction Water Is 99% of fluid outside cells (extracellular fluid) Is an essential ingredient of cytosol (intracellular fluid) All cellular operations rely on water: As a diffusion medium for gases, nutrients, and waste products Fluid, Electrolyte, and Acid–Base Balance Fluid Balance Is a daily balance between: Amount of water gained Amount of water lost to environment Involves regulating content and distribution of body water in ECF and ICF Fluid, Electrolyte, and Acid–Base Balance The Digestive System Is the primary source of water gains: Plus a small amount from metabolic activity The Urinary System Is the primary route of water loss Fluid, Electrolyte, and Acid–Base Balance Electrolytes Are ions released through dissociation of inorganic compounds Can conduct electrical current in solution Electrolyte balance: When the gains and losses of all electrolytes are equal Primarily involves balancing rates of absorption across digestive tract with rates of loss at kidneys and sweat glands Fluid, Electrolyte, and Acid–Base Balance Acid–Base Balance Precisely balances production and loss of hydrogen ions (pH) The body generates acids during normal metabolism: Tends to reduce pH Fluid, Electrolyte, and Acid–Base Balance The Kidneys Secrete hydrogen ions into urine Generate buffers that enter bloodstream: In distal segments of distal convoluted tubule (DCT) and collecting system The Lungs Affect pH balance through elimination of carbon dioxide Fluid Compartments Figure 18-12 Ions in Body Fluids Figure 18-13 Blood pressure and osmosis are involved in maintaining fluid and electrolyte balance Fluid Balance When the body loses water Plasma volume decreases Electrolyte concentrations rise When the body loses electrolytes Water is lost by osmosis Regulatory mechanisms are different Fluid Balance Water circulates freely in ECF compartment At capillary beds, hydrostatic pressure forces water out of plasma and into interstitial spaces Water is reabsorbed along distal portion of capillary bed when it enters lymphatic vessels ECF and ICF are normally in osmotic equilibrium: No large-scale circulation between compartments Fluid Balance Fluid Gains and Losses Water losses: Body loses about 2500 mL of water each day through urine, feces, and insensible perspiration Fever can also increase water loss Sensible perspiration (sweat) varies with activities and can cause significant water loss (4 L/hr) Fluid Balance Fluid Gains and Losses Water gains: About 2500 mL/day Required to balance water loss Through: eating (1000 mL) drinking (1200 mL) metabolic generation (300 mL) Fluid Shifts Are rapid water movements between ECF and ICF In response to an osmotic gradient If ECF osmotic concentration increases: Fluid becomes hypertonic to ICF Water moves from cells to ECF If ECF osmotic concentration decreases: Fluid becomes hypotonic to ICF Water moves from ECF to cells ICF volume is much greater than ECF volume: ICF acts as water reserve Prevents large osmotic changes in ECF Electrolyte Balance Requires rates of gain and loss of each electrolyte in the body to be equal Electrolyte concentration directly affects water balance Concentrations of individual electrolytes affect cell functions Electrolyte Balance Rules of Electrolyte Balance Most common problems with electrolyte balance are caused by imbalance between gains and losses of sodium ions Problems with potassium balance are less common but more dangerous than sodium imbalance Electrolyte Balance Sodium Balance Sodium ion uptake across digestive epithelium Sodium ion excretion in urine and perspiration Electrolyte Balance Sodium Balance Typical Na+ gain and loss: Is 48–144 mEq (1.1–3.3 g) per day If gains exceed losses: Total ECF content rises If losses exceed gains: ECF content declines Electrolyte Balance Sodium Balance and ECF Volume Changes in ECF Na+ content: Do not produce change in concentration Corresponding water gain or loss keeps concentration constant Electrolyte Balance Sodium Balance and ECF Volume Na+ regulatory mechanism changes ECF volume: Keeps concentration stable When Na+ losses exceed gains: ECF volume decreases (increased water loss) Maintaining osmotic concentration Electrolyte Balance Potassium Balance 98% of potassium in the human body is in ICF Cells expend energy to recover potassium ions diffused from cytoplasm into ECF Electrolyte Balance Processes of Potassium Balance Rate of gain across digestive epithelium Rate of loss into urine Electrolyte Balance Potassium Loss in Urine Is regulated by activities of ion pumps: Along distal portions of nephron and collecting system Na+ from tubular fluid is exchanged for K+ in peritubular fluid Is limited to amount gained by absorption across digestive epithelium (about 50–150 mEq or 1.9–5.8 g/day) In acid–base balance, buffer systems and respiratory and renal compensation mechanisms regulate hydrogen ions in body fluids Acid–Base Balance pH of body fluids is altered by Introduction of acids or bases Acids and bases may be strong or weak Acid–Base Balance Carbonic Acid Is a weak acid In ECF at normal pH: Equilibrium state exists Is diagrammed H2CO3 ? H+ + HCO3– Acid–Base Balance The Importance of pH Control pH of body fluids depends on dissolved: Acids Bases Salts pH of ECF: Is narrowly limited, usually 7.35 to 7.45 Acid–Base Balance Acidosis Physiological state resulting from abnormally low plasma pH Acidemia: plasma pH <7.35 Alkalosis Physiological state resulting from abnormally high plasma pH Alkalemia: plasma pH >7.45 Acid–Base Balance Acidosis and Alkalosis Affect all body systems: Particularly nervous and cardiovascular systems Both are dangerous: But acidosis is more common Because normal cellular activities generate acids Acid–Base Balance Carbon Dioxide In solution in peripheral tissues: Interacts with water to form carbonic acid Carbonic acid dissociates to release: Hydrogen ions Bicarbonate ions Acid–Base Balance Carbonic Anhydrase (CA) Enzyme that catalyzes dissociation of carbonic acid Found in: Cytoplasm of red blood cells Liver and kidney cells Parietal cells of stomach Other cells Acid–Base Balance CO2 and pH Most CO2 in solution converts to carbonic acid: Most carbonic acid dissociates PCO2 is the most important factor affecting pH in body tissues: PCO2 and pH are inversely related Acid–Base Balance CO2 and pH When CO2 levels rise: H+ and bicarbonate ions are released pH goes down At alveoli: CO2 diffuses into atmosphere H+ and bicarbonate ions in alveolar capillaries drop Blood pH rises PCO2 and Plasma pH Figure 18-14 Buffers and Buffer Systems Buffers Are dissolved compounds that stabilize pH: By providing or removing H+ Weak acids: Can donate H+ Weak bases: Can absorb H+ Buffers and Buffer Systems Buffer System Consists of a combination of: A weak acid And the anion released by its dissociation The anion functions as a weak base In solution, molecules of weak acid exist in equilibrium with its dissociation products Buffers and Buffer Systems Three Major Buffer Systems Protein buffer systems: Help regulate pH in ECF and ICF Interact extensively with other buffer systems Carbonic acid–bicarbonate buffer system: Most important in ECF Phosphate buffer system: Buffers pH of ICF and urine Buffers and Buffer Systems Protein Buffer Systems Depend on amino acids Respond to pH changes by accepting or releasing H+ If pH rises: Carboxyl group of amino acid dissociates Acting as weak acid, releasing a hydrogen ion Carboxyl group becomes carboxylate ion Buffers and Buffer Systems The Hemoglobin Buffer System CO2 diffuses across RBC membrane: No transport mechanism required As carbonic acid dissociates: Bicarbonate ions diffuse into plasma In exchange for chloride ions (chloride shift) Hydrogen ions are buffered by hemoglobin molecules Buffers and Buffer Systems The Hemoglobin Buffer System Is the only intracellular buffer system with an immediate effect on ECF pH Helps prevent major changes in pH when plasma PCO2 is rising or falling Buffers and Buffer Systems Carbonic Acid–Bicarbonate Buffer System Carbon dioxide: Most body cells constantly generate carbon dioxide Most carbon dioxide is converted to carbonic acid, which dissociates into H+ and a bicarbonate ion Is formed by carbonic acid and its dissociation products Prevents changes in pH caused by organic acids and fixed acids in ECF Buffers and Buffer Systems Carbonic Acid–Bicarbonate Buffer System Cannot protect ECF from changes in pH that result from elevated or depressed levels of CO2 Functions only when respiratory system and respiratory control centers are working normally Ability to buffer acids is limited by availability of bicarbonate ions Buffers and Buffer Systems Phosphate Buffer System Consists of anion H2PO4– (a weak acid) Works like the carbonic acid–bicarbonate buffer system Is important in buffering pH of ICF Buffers and Buffer Systems Limitations of Buffer Systems Provide only temporary solution to acid–base imbalance Do not eliminate H+ ions Have only a limited supply of buffer molecules Maintaining Acid–Base Balance For homeostasis to be preserved, captured H+ must Be permanently tied up in water molecules: Through CO2 removal at lungs Be removed from body fluids: Through secretion at kidney Maintaining Acid–Base Balance Requires balancing H+ gains and losses Coordinates actions of buffer systems with Respiratory mechanisms Renal mechanisms Maintaining Acid–Base Balance Respiratory and Renal Mechanisms Support buffer systems by: Secreting or absorbing H+ Controlling excretion of acids and bases Generating additional buffers Maintaining Acid–Base Balance Respiratory Compensation Is a change in respiratory rate: That helps stabilize pH of ECF Occurs whenever body pH moves outside normal limits Directly affects carbonic acid–bicarbonate buffer system Maintaining Acid–Base Balance Respiratory Compensation Increasing or decreasing the rate of respiration alters pH by lowering or raising the PCO2 When PCO2 rises: pH falls Addition of CO2 drives buffer system to the right When PCO2 falls: pH rises Removal of CO2 drives buffer system to the left Maintaining Acid–Base Balance Renal Compensation Is a change in rates of H+ and HCO3– secretion or reabsorption by kidneys in response to changes in plasma pH The body normally generates enough organic and fixed acids each day to add 100 mEq of H+ to ECF Kidneys assist lungs by eliminating any CO2 that: Enters renal tubules during filtration Diffuses into tubular fluid en route to renal pelvis Acid–Base Disorders Disorders Circulating buffers Respiratory performance Renal function Cardiovascular conditions Heart failure Hypotension Conditions affecting the CNS Neural damage or disease that affects respiratory and cardiovascular reflexes Acid–Base Disorders Respiratory Acid–Base Disorders Result from imbalance between: CO2 generation in peripheral tissues CO2 excretion at lungs Cause abnormal CO2 levels in ECF Acid–Base Disorders Metabolic Acid–Base Disorders Result from: Generation of organic or fixed acids Conditions affecting HCO3– concentration in ECF Age-related changes affect kidney function and the micturition reflex Age-Related Changes Decline in number of functional nephrons Reduction in GFR Reduced sensitivity to ADH Problems with micturition reflex Sphincter muscles lose tone leading to incontinence Control of micturition can be lost due to a stroke, Alzheimer disease, and other CNS problems In males, urinary retention may develop if enlarged prostate gland compresses the urethra and restricts urine flow Age-Related Changes A gradual decrease of total body water content with age A net loss in body mineral content in many people over age 60 as muscle mass and skeletal mass decrease Increased incidence of disorders affecting major systems with increasing age The urinary system is one of several body systems involved in waste excretion Excretion Integument Sweat Respiration Carbon dioxide Digestive Feces The Urinary System in Perspective FIGURE 18-15 Functional Relationships Between the Urinary System and Other Systems Copyright © 2010 Education, Inc. The Integumentary System assists in elimination of water and solutes, especially sodium and chloride ions, with sweat glands; keratinized epidermis prevents excessive fluid loss through skin surface; epidermis produces vitamin D3, important for the renal production of calcitriol The Integumentary System The Skeletal System The Skeletal System provides some protection for kidneys and ureters; pelvis protects urinary bladder and proximal portion of urethra The Urinary System conserves calcium and phosphate needed for bone growth The Nervous System The Nervous System adjusts renal blood pressure; monitors distension of urinary bladder and controls urination The Endocrine System The Endocrine System’s hormones aldosterone and ADH adjust rates of fluid and electrolyte reabsorption in kidneys The Urinary System releases renin when local blood pressure declines, and erythropoietin (EPO) when renal oxygen levels decline The Cardiovascular System The Cardiovascular System delivers blood to capillaries, where filtration occurs; accepts fluids and solutes reabsorbed during urine production The Urinary System releases renin to elevate blood pressure and erythropoietin (EPO) to accelerate red blood cell production The Lymphatic System The Lymphoid System provides specific defenses against urinary tract infections The Urinary System eliminates toxins and wastes generated by cellular activities; acid pH of urine provides nonspecific defense against urinary tract infections The Respiratory System The Respiratory System assists in the regulation of pH by eliminating carbon dioxide The Urinary System assists in the elimination of carbon dioxide; provides bicarbonate buffers that assist in pH regulation The Digestive System The Digestive System absorbs water needed to excrete wastes at kidneys; absorbs ions needed to maintain normal body fluid concentrations; liver removes bilirubin The Urinary System excretes toxins absorbed by the digestive tract; excretes bilirubin and nitrogenous wastes produced by the liver; calcitrol production by kidneys aids calcium and phosphate absorption along digestive tract The Muscular System The Muscular System provides some protection for urinary organs with muscle layers of the trunk; sphincter muscles close the urethral opening The Urinary System removes waste products of protein metabolism; assists in regulation of calcium and phosphate The Reproductive System The Reproductive System’s secretions by male accessory organs may have antibacterial action that prevents urethral infections The Urinary System carries semen in the male urethra