The Urinary System Chapter 26.

The Urinary System Chapter 26

Introduction to the Urinary System
Figure 26–1 An Introduction to the Urinary System. The urinary system removes most physiological wastes. The organs of the urinary system are included in this slide. Copyright © 2009 Pearson Education, Inc., publishing as Pearson Benjamin Cummings

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 Copyright © 2009 Pearson Education, Inc., publishing as Pearson Benjamin Cummings

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 Urine is a fluid containing water, ions, and small soluble compounds. Urine leaving the kidneys flows along the urinary tract. Copyright © 2009 Pearson Education, Inc., publishing as Pearson Benjamin Cummings

Five 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) The urinary system removes waste products generated by cells throughout the body, but it has several other essential homeostatic functions that are often overlooked. Copyright © 2009 Pearson Education, Inc., publishing as Pearson Benjamin Cummings

Five 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 5. Assists liver in detoxifying poisons Copyright © 2009 Pearson Education, Inc., publishing as Pearson Benjamin Cummings

The Male Urinary System
Figure 26–18a Organs for the Conduction and Storage of Urine.

The Female Urinary System
Figure 26–18b Organs for the Conduction and Storage of Urine. Copyright © 2009 Pearson Education, Inc., publishing as Pearson Benjamin Cummings

The Kidneys Figure 26–2a The Position of the Kidneys.
The kidneys are located on either side of the vertebral column, between vertebrae T12 and L3. The left kidney lies slightly superior to the right kidney. The right kidney is slightly inferior due to the position of the right lobe of the liver. The superior surface of each kidney is capped by an adrenal gland. The kidney and adrenal glands lie between the muscles of the posterior body wall and the parietal peritonium. Copyright © 2009 Pearson Education, Inc., publishing as Pearson Benjamin Cummings

The Kidneys Figure 26–3 The Gross Anatomy of the Urinary System.
The hilum is a prominent medial indentation which serves as the point of entry for the renal artery which branches off the abominal aorta and renal nerves. It is also the point of exit for the renal vein which branches off the inferior vena cava and ureter. Copyright © 2009 Pearson Education, Inc., publishing as Pearson Benjamin Cummings

The Kidneys Figure 26–4 The Structure of the Kidney.
The fibrous capsule is a layer of collagen fibers that covers the outer surface of the entire kidney. The fibrous capsule also lines the renal sinus, an internal cavity within the kidney. The fibrous capsule is bound to the outer surfaces of the structures within the renal sinus. In this way, it stabilizes the positions of the ureter and of the renal blood vessels and nerves. The kidney itself has an outer cortex and an inner medulla. The renal cortex is the superficial portion of the kidney, in contact with the fibrous capsule. The renal medulla consists of 6 to 18 distinct triangular structures called renal pyramids. The base of each pyramid is in direct contact with the cortex. The tip of each pyramid, a region known as the renal papilla, projects into the renal sinus. Adjacent renal pyramids are separated by bands of cortical tissue called renal columns, which extend into the medulla. Ducts within each renal papilla discharge urine into a cup-shaped drain called a minor calyx. Four or five minor calyces merge to form a major calyx, and two or three major calyces combine to form the renal pelvis, a large funnel-shaped chamber. The renal pelvis fills most of the renal sinus and is connected to the ureter, which drains the kidney. Copyright © 2009 Pearson Education, Inc., publishing as Pearson Benjamin Cummings

The Kidneys Blood Supply to Kidneys
Kidneys receive 20–25% of total cardiac output 1200 mL of blood flows through kidneys each minute Kidney receives blood through renal artery Copyright © 2009 Pearson Education, Inc., publishing as Pearson Benjamin Cummings

The Kidneys Figure 26–5b The Blood Supply to the Kidneys: Circulation in the Renal Cortex. As the renal artery enters the renal sinus, they provide blood to the segmental arteries which further divide into a series of interlobar arteries. These arteries radiate outward through the renal columns between the renal pyramids. The interlobar arteries supply blood to the arcuate arteries, which arch along the boundary between the cortex and medulla of the kidney. Each arturate artery gives rise to a number of cortical radiate arteries. They supply the cortical portions of the adjacent renal lobes. Branching from each cortical radiate artery are numerous afferent arterioles which deliver blood to the capillaries supplying individual nephrons. After passing through the capillaries of the nephrons, blood enters a network of venules and small veins that converge on the cortical radiate veins. The cortical radiate veins in turn empty into interlobar veins, which drain directly into the renal vein. The functional units that carry out the function of the kidneys are the nephrons. There are two different classes of nephrons: cortical nephrons and juxtamedullary nephrons. They are classified based on the length of their associated Loop of Henle (which will be discussed later). Copyright © 2009 Pearson Education, Inc., publishing as Pearson Benjamin Cummings

The Kidneys As the renal artery enters the renal sinus, they provide blood to the segmental arteries which further divide into a series of interlobar arteries. These arteries radiate outward through the renal columns between the renal pyramids. The interlobar arteries supply blood to the arcuate arteries, which arch along the boundary between the cortex and medulla of the kidney. Each arturate artery gives rise to a number of cortical radiate arteries. They supply the cortical portions of the adjacent renal lobes. Branching from each cortical radiate artery are numerous afferent arterioles which deliver blood to the capillaries supplying individual nephrons. After passing through the capillaries of the nephrons, blood enters a network of venules and small veins that converge on the cortical radiate veins. The cortical radiate veins in turn empty into interlobar veins, which drain directly into the renal vein. The functional units that carry out the function of the kidneys are the nephrons. There are two different classes of nephrons: cortical nephrons and juxtamedullary nephrons. They are classified based on the length of their associated Loop of Henle (which will be discussed later). Figure 26–7 The Locations and Structures of Cortical and Juxtamedullary Nephrons.

The Kidneys Figure 26–8 The Renal Corpuscle
As the renal artery enters the renal sinus, they provide blood to the segmental arteries which further divide into a series of interlobar arteries. These arteries radiate outward through the renal columns between the renal pyramids. The interlobar arteries supply blood to the arcuate arteries, which arch along the boundary between the cortex and medulla of the kidney. Each arturate artery gives rise to a number of cortical radiate arteries. They supply the cortical portions of the adjacent renal lobes. Branching from each cortical radiate artery are numerous afferent arterioles which deliver blood to the capillaries supplying individual nephrons. After passing through the capillaries of the nephrons, blood enters a network of venules and small veins that converge on the cortical radiate veins. The cortical radiate veins in turn empty into interlobar veins, which drain directly into the renal vein. The functional units that carry out the function of the kidneys are the nephrons. There are two different classes of nephrons: cortical nephrons and juxtamedullary nephrons. They are classified based on the length of their associated Loop of Henle (which will be discussed later). Figure 26–8 The Renal Corpuscle

The Kidneys Figure 26–5c The Blood Supply to the Kidneys: Flowchart of Renal Circulation. This figure diagrams the flow of blood through the kidneys, with red arrows indicating arteries and blue arrows indicating veins. Copyright © 2009 Pearson Education, Inc., publishing as Pearson Benjamin Cummings

The Kidneys The Nephron Consists of renal tubule and renal corpuscle
Long tubular passageway Begins at renal corpuscle Renal corpuscle Spherical structure consisting of: glomerular capsule (Bowman’s capsule) cup-shaped chamber capillary network (glomerulus) Copyright © 2009 Pearson Education, Inc., publishing as Pearson Benjamin Cummings

The Kidneys 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 Blood arrives at the renal corpuscle by way of an afferent arteriole. This arteriole delivers blood to the glomerulus, which consists of about 50 intertwining capillaries. The glomerulus projects into the glomerular capsule much as the heart projects into the pericardial cavity. Blood leaves the glomerulus in an efferent arteriole. It flows into a network of capillaries called the peritubular capillaries, which surround the renal tubule. These capillaries in turn drain into small venules that return the blood to the venous system. Copyright © 2009 Pearson Education, Inc., publishing as Pearson Benjamin Cummings

The Kidneys Filtration Occurs in renal corpuscle Blood pressure
Forces water and dissolved solutes out of glomerular capillaries into capsular space Produces protein-free solution (filtrate) similar to blood plasma The process of filtration takes place in the renal corpiscule. In this process, blood pressure forces water and dissolved solutes out of the glomerular capillaries and into a chamber, called the capsular space, that is continuous with the lumen of the renal tubule. Filtration produces an essentially protein-free solution, known as filtrate, that is otherwise similar to blood plasma. Copyright © 2009 Pearson Education, Inc., publishing as Pearson Benjamin Cummings

The Kidneys Three regions 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 From the renal corpuscule, filtrate enters the renal tubule, which has three crucial functions listed on this slide. Copyright © 2009 Pearson Education, Inc., publishing as Pearson Benjamin Cummings

The Kidneys 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 renal tubule has two coiled or twisted segments (pct and dct) In the nephron, the terms proximal tubule and distal tubule refer to how far along the renal tubule these structures are situated from the renal corpiscule. Think of proximal (nearer the renal corpiscule) as being first, and distal (farther from the renal corpiscule) as being last. Copyright © 2009 Pearson Education, Inc., publishing as Pearson Benjamin Cummings

The Kidneys Organization of the Nephron Each Nephron
Traveling along tubule, filtrate (tubular fluid) gradually changes composition Changes vary with activities in each segment of nephron Each Nephron Empties into the collecting system: A series of tubes that carries tubular fluid away from nephron The regions of the nephron vary by structure and function. As it travels along the tubule, the filtrate, now called tubular fluid, gradually changes in composistion. The changes that take place and the characteristics of the urine that result are due to the activities underway in each segment of the nephron. This is due in large part to the cellular structure as well as the presence of certain ion channels in the membrane and will be discussed in later slides. Copyright © 2009 Pearson Education, Inc., publishing as Pearson Benjamin Cummings

The Kidneys 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 Copyright © 2009 Pearson Education, Inc., publishing as Pearson Benjamin Cummings

The Kidneys Figure 26–6 The Functional Anatomy of a Representative Nephron and the Collecting System. Since you have already gone over these in detail, run briefly through theses structures. Copyright © 2009 Pearson Education, Inc., publishing as Pearson Benjamin Cummings

The Kidneys Cortical Nephrons Juxtamedullary 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 In the cortical nephrons, the peritubular capillaries surround the entire renal tubule. These capillaries drain into small venules that carry blood to the cortical radiate veins. Cortical nephrons perform most of the reabsorptive and secretory functions of the kidneys because they are more numerous than juxtamedullary nephons. The vasa recta are long, straight capillaries that parallel the nephron loop. Juxtamedullary nephons enable the kidneys to produce concentrated urine. Copyright © 2009 Pearson Education, Inc., publishing as Pearson Benjamin Cummings

The Kidneys 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 outer wall of the capsule is lined by a simple squamous capsular epithelium. This layer is continuous with the visceral epithelium, which covers the glomerular capillaries. The capsular space separates the capsular and visceral epithelia. The two epithelial layers are continuous where the glomerular capillaries are connected to the afferent arteriole and efferent arteriole. Copyright © 2009 Pearson Education, Inc., publishing as Pearson Benjamin Cummings

The Kidneys The Visceral Epithelium Filtration Slits
Consists of large cells (podocytes) With complex processes or “feet” (pedicels) that wrap around specialized lamina densa of glomerular capillaries Filtration Slits Are narrow gaps between adjacent pedicels Copyright © 2009 Pearson Education, Inc., publishing as Pearson Benjamin Cummings

The Kidneys Filtration Filtration at Renal Corpuscle
Blood pressure forces water and small solutes across membrane into capsular space Larger solutes, such as plasma proteins, are excluded Filtration at Renal Corpuscle Is passive Solutes enter capsular space Metabolic wastes and excess ions Glucose, free fatty acids, amino acids, and vitamins In addition to metabolic wastes and excess ions, useful compound such as glucose, free fatty acids, vitamins and other solutes also enter the capsular space. These substances are recaptured before filtrate leaves the kidneys. Copyright © 2009 Pearson Education, Inc., publishing as Pearson Benjamin Cummings

The Kidneys Reabsorption
Useful materials are recaptured before filtrate leaves kidneys Reabsorption occurs in proximal convoluted tubule Copyright © 2009 Pearson Education, Inc., publishing as Pearson Benjamin Cummings

The Kidneys 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 Epithelial Lining of PCT Is simple cuboidal Has microvilli on apical surfaces Functions in reabsorption Secretes substances into lumen Copyright © 2009 Pearson Education, Inc., publishing as Pearson Benjamin Cummings

The Kidneys Tubular Cells
Absorb organic nutrients, ions, water, and plasma proteins from tubular fluid Release them into peritubular fluid (interstitial fluid around renal tubule) Reabsorption is the primary function of the PCT, but the epithelial cells can also secrete substances into the lumen of the renal tubule. Copyright © 2009 Pearson Education, Inc., publishing as Pearson Benjamin Cummings

The Kidneys 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 terms thick and thin refer to the height of the epithelium, not to the diameter of the lumen: Thick segments have a cuboidal epithelium whereas a squamous epithelium lines the thin segments. Copyright © 2009 Pearson Education, Inc., publishing as Pearson Benjamin Cummings

The Kidneys The Thick Descending Limb Ascending Limbs
Has functions similar to PCT Pumps sodium and chloride ions out of tubular fluid Ascending Limbs Of juxtamedullary nephrons in medulla Create high solute concentrations in peritubular fluid The effect of the pumping in the thick descending limb is most noticeable in the medulla, where the long ascending limbs of the juxtamedullary nephrons create unusually high solute concentrations in peritubular fluid. Copyright © 2009 Pearson Education, Inc., publishing as Pearson Benjamin Cummings

The Kidneys The Thin Segments The Thick Ascending Limb
Are freely permeable to water Not to solutes Water movement helps concentrate tubular fluid The Thick Ascending Limb Ends at a sharp angle near the renal corpuscle Where DCT begins Copyright © 2009 Pearson Education, Inc., publishing as Pearson Benjamin Cummings

The Kidneys 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 Copyright © 2009 Pearson Education, Inc., publishing as Pearson Benjamin Cummings

The Kidneys 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 Copyright © 2009 Pearson Education, Inc., publishing as Pearson Benjamin Cummings

The Kidneys Juxtaglomerular Complex
An endocrine structure that secretes Hormone erythropoietin Enzyme renin Formed by Macula densa Juxtaglomerular cells Erythropoietin regulates blood cell production and renin is involved in blood pressure regulation and will be discussed later. The epithelial cells of the dct near the renal corpuscule are taller than those elsewhere along the dct, and their nuclei are clustered together in the region known as the macula densa. The cells of the macula densa are closely associated with unusual smooth muscle fibers in the wall of the afferent arteriole. These fibers are known as juxtaglomerular cells. Copyright © 2009 Pearson Education, Inc., publishing as Pearson Benjamin Cummings

The Kidneys 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 Copyright © 2009 Pearson Education, Inc., publishing as Pearson Benjamin Cummings

The Kidneys Briefly run through the tables in the next few slides.
Copyright © 2009 Pearson Education, Inc., publishing as Pearson Benjamin Cummings

Renal Physiology The goal of urine production The Kidneys
Is to maintain homeostasis By regulating volume and composition of blood Including excretion of metabolic waste products The Kidneys Usually produce concentrated urine 1200–1400 mOsm/L (four times plasma concentration) Copyright © 2009 Pearson Education, Inc., publishing as Pearson Benjamin Cummings

Renal Physiology Three Organic Waste Products Urea Creatinine
Uric acid Are dissolved in bloodstream Are eliminated only while dissolved in urine Removal is accompanied by water loss Urea is the most abundant organic waste. You generate approximately 21g of urea each day, most of it through the breakdown of amino acids. Skeletal muscle generates creatine through the breakdown of creatine phosphate, a high energy compound that plays an important role in muscle contraction. Your body generates roughly 1.8g of creatine each day and virtually all of it is excreted in urine Uric acid is a waste product formed during the recycling of the nitrogenous bases from RNA molecules. You produce approximately 480mg of uric acid each day. Copyright © 2009 Pearson Education, Inc., publishing as Pearson Benjamin Cummings

Renal Physiology Kidney Functions
To concentrate filtrate by glomerular filtration Failure leads to fatal dehydration Absorbs and retains valuable materials for use by other tissues Sugars and amino acids If the kidneys were unable to concentrate the filtrate produced by glomerular filtration, fluid losses would lead to fatal dehydration in a matter of hours. The kidneys also ensure that the fluid that is lost does not contain potentially useful organic substrates that are present in blood plasma, such as sugars or amino acids. These valuable materials must be reabsorbed and retained for use by other tissues. Copyright © 2009 Pearson Education, Inc., publishing as Pearson Benjamin Cummings

Renal Physiology Renal Threshold Is the plasma concentration at which
A specific compound or ion begins to appear in urine Varies with the substance involved Copyright © 2009 Pearson Education, Inc., publishing as Pearson Benjamin Cummings

Renal Physiology Renal Threshold for Glucose
Is approximately 180 mg/dL If plasma glucose is greater than 180 mg/dL Tm of tubular cells is exceeded Glucose appears in urine: glycosuria Copyright © 2009 Pearson Education, Inc., publishing as Pearson Benjamin Cummings

Renal Physiology Renal Threshold for Amino Acids
Is lower than glucose (65 mg/dL) Amino acids commonly appear in urine After a protein-rich meal Aminoaciduria Copyright © 2009 Pearson Education, Inc., publishing as Pearson Benjamin Cummings

Renal Physiology Normal kidney function can only continue as long as filtration, reabsorption and secretion function within relatively narrow limits. A disruption in kidney function has immediate effects on the composition of circulating blood. If both kidneys are affected, death follows within a few days unless medical assistance is provided. Copyright © 2009 Pearson Education, Inc., publishing as Pearson Benjamin Cummings

Renal Physiology Figure 26–9 An Overview of Urine Formation.
Filtration occurs exclusively in the renal corpuscule, rross the filtration membrane. Water reabsorption occurs primarily along the pct and the descending limb of the nephron loop, but also to a variable degree in the dct and collecting system. Variable water reabsorbtion occurs in the dct and collecting system. Solute reabsorption occurs along the pct, that ascending limb of the nephron loop, the dct and the collecting system. Variable solute reabsoption or secretion occurs at the pct, the dct and the collecting system. Copyright © 2009 Pearson Education, Inc., publishing as Pearson Benjamin Cummings

Glomerular Filtration
Glomerular Filtration Rate (GFR) Is the amount of filtrate kidneys produce each minute Averages 125 mL/min About 10% of fluid delivered to kidneys Leaves bloodstream Enters capsular spaces In the course of a single day, the glomeruli generate about 180 liters of filtrate, roughly 70 times the total plasma volume. But as filtrate passes through the renal tubules, about 99% of it is reabsorbed. Copyright © 2009 Pearson Education, Inc., publishing as Pearson Benjamin Cummings

Control of the GFR Autoregulation (local level) Hormonal regulation (initiated by kidneys) Autonomic regulation (by sympathetic division of ANS) Autoregulation (local blood flow regulation) maintains an adequate GFR despite changed in local blood pressure and flow. Changes to the diameters of afferent arterioles, efferent arterioles, and glomerular capillaries maintain GFR. The GFR is regulated by hormones of the renin-angiotensin system and natriuretic peptides which will be discussed in later slides. Most autonomic innervation of the kidneys consists of sympathetic postganglionic fibers. Sympathetic activation has a direct effect of the GFR by producing powerful vasocontriction of afferent areterioles, which decreases the GFR and slows the production of filtrate. In this way, the sympathetic activation triggered by an acute fall in blood pressure or a heart attack overrides the local regulatory mechanisms that act to stabilize the GFR. Copyright © 2009 Pearson Education, Inc., publishing as Pearson Benjamin Cummings

Hormonal Regulation of the GFR By hormones of the Renin–angiotensin system Natriuretic peptides (ANP and BNP) The Renin–Angiotensin System Three stimuli cause the juxtaglomerular complex (JGA) to release renin Decline in blood pressure at glomerulus Fall in systemic pressures Stimulation of juxtaglomerular cells The three triggers for the release of renin by the juxtoglomerular complex: 1. a decline in blood pressure at the glomerulus as a result of a decrease in blood volume, a fall in systemic pressures, or a blockage in the renal artery or its branches; 2. stimulation of juxtaglomerular cells by sympathetic innervation or 3. a decline in the osmotic concentration of the tubular fluid at the macula densa. These triggers are often interrelated, for example, a decline in systemic blood pressure reduces the glomerular filtration rate, while baroreceptor reflexes cause sympathetic activation. Meanwhile, a reduction in the GFR slows the movement of tubular fluid along the nephron. As a result, the tubular fluid is in the ascending limb of the nephron loop longer, and the concentration of sodium and chloride ions in the tubular fluid reaching the macula densa and DCT becomes abnormally low. Copyright © 2009 Pearson Education, Inc., publishing as Pearson Benjamin Cummings

Figure 26–11a The Response to a Reduction in the GFR. Here is a general overview of the response of the renin-angiotensin system to a decline in GFR. A drop in GFR leads to the release of renin by the juxtaglomerular complex. Renin converts the inactive protein antiogensinogen to angiotensin I. Angiotensin I is also inactive but is then converted to angiotensin II by angiotensin-converting enzyme (ACE). This conversion takes place primarily in the capillaries of the lungs. Angiotensin II acts at the nephron, adrenal glands, and in the CNS. In peripheral capillary beds, angiotensin II causes a brief but powerful vasoconstriction of arterioles and precapillary sphincters, raising arterial pressures throughout the body. The combined effect is an increase in systemic blood volume and blood pressure and the restoration of GFR. If blood volume rises, the GFR increases automatically to promote fluid losses that help return blood volume to normal levels. Copyright © 2009 Pearson Education, Inc., publishing as Pearson Benjamin Cummings

Reabsorption and Secretion
Countercurrent Multiplication Is exchange that occurs between two parallel segments of loop of Henle The thin, descending limb The thick, ascending limb The nephron loop reabsorbs about half of the remaining water and two-thirds of the remaining sodium and chloride ions. This reabsorption takes place efficiently according to the principle of countercurrent exchange. The thin descending limb and the thick ascending limb of the nephron loop lie very close together. They are separated only by peritubular fluid. The exchange between these segments is called countercurrent multiplication. Countercurrent refers to the fact that the exchange takes place between fluids moving in opposite directions. Tubular fluid in the descending limb flows toward the renal pelvis, while tubular fluid in the ascending limb flows towards the cortex. Multiplication refers to the fact that the effect of the exchange increases as movement of the fluid continues. The thin descending limb is permeable to water but relatively impermeable to solutes. The thick ascending limb is relatively impermeable to both water and solutes, but it contains active transport mechanisms that pump sodium and chloride ions from the tubular fluid into the peritubular fluid of the medulla. Copyright © 2009 Pearson Education, Inc., publishing as Pearson Benjamin Cummings

Benefits of Countercurrent Multiplication
Efficiently reabsorbs solutes and water before the tubular fluid reaches the DCT and collecting system It establishes a concentration gradient that permits the passive reabsorption of water from the tubular fluid in the collecting system.

Figure 26–13b Countercurrent Multiplication and Concentration of Urine. Reabsorption recovers useful materials that have entered the filtrate. Secretion ejects waste products, toxins, or other undesirable solutes that didn’t leave the bloodstream at the glomerulus. Sodium and chloride are pumped out of the thick ascending limb and into the peritubular fluid. This pumping action raises the osmotic concentration in the peritubular fluid around the thin descending limb. The result is an osmotic flow of water out of the thin descending limb and into the peritubular fluid. This loss of water increases the solute concentration in the thin descending limb. The arrival of the highly concentrated solution in the thick ascending limb speeds up the transport of sodium and chloride ions into the peritubular fluid of the medulla. This process is a simple feedback loop. Solute pumping at the ascending limb leads to higher solute concentrations in the descending limb, which then bring about increased solute pumping in the ascending limb. Copyright © 2009 Pearson Education, Inc., publishing as Pearson Benjamin Cummings

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 Throughout most of the DCT, the tubular cells actively transport Na+ and Cl- out of the tubular fluid. Tubular cells along the distal portions of the DCT also contain ion pumps that reabsorb tubular Na+ in exchange for another cation (usually K+). Copyright © 2009 Pearson Education, Inc., publishing as Pearson Benjamin Cummings

Hypokalemia Produced by prolonged aldosterone stimulation Dangerously reduces plasma concentration Sodium ion conservation is associated with potassium ion loss. Prolonged aldosterone stimulation can therefore produce hyperkalemia, which results in the reduction in plasma potassium concentration. The secretion of aldosterone and its action on the DCT and collecting system are opposed by the natriuretic peptides (ANP and BNP). Copyright © 2009 Pearson Education, Inc., publishing as Pearson Benjamin Cummings

Natriuretic Peptides (ANP and BNP) Oppose secretion of aldosterone And its actions on DCT and collecting system Parathyroid Hormone and Calcitriol Circulating levels regulate reabsorption at the DCT The heart releases natriuretic peptides when increased blood volume or blood pressure stretches the walls of the heart. The atria release atrial natriuretic peptide (ANP), and the ventricles release brain natriuretic peptide (BNP). Among other effects, these hormones trigger the dilation of afferent arterioles and the constriction of efferent arterioles. This mechanism raises glomerular pressures and increases the GFR. The natriuretic peptides also decrease sodium reabsorption at the renal tubules. The net result is increased urine production and reduced blood volume and pressure. The DCT is also the primary site of Ca2+ reabsorption. Cirulating levels of parathyroid hormone and calcitrol regulate this process. Copyright © 2009 Pearson Education, Inc., publishing as Pearson Benjamin Cummings

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 The volume of water lost in urine depends on how much of the remaining water in the tubular fluid is reabsorbed along the DCT and collecting system. Precise control of water reabsorption is possible because these segments are relatively impermeable to water except in the presence of ADH. The higher the circulating levels of ADH, the greater the number of water channels, and the greater of the water permeability of these segments. Copyright © 2009 Pearson Education, Inc., publishing as Pearson Benjamin Cummings

Figure 26–15 The Effects of ADH on the DCT and Collecting Duct. In the absence of ADH, water is not reabsorbed in these segments, so all the fluid reaching the DCT is lost in the urine. The individual then produces large amounts of very dilute urine. As ADH levels rise, the DCT and collecting system become more permeable to water. As a result, the amount of water reabsorbed increases. At the same time, the osmotic concentration of the urine climbs. Under maximum ADH stimulation, the DCT and collecting system become so permeable to water that the osmotic concentration of the urine equals that of the deepest portion of the medulla. Note that the concentration of urine can never exceed that of the medulla, because the concentrating mechanism relies on osmosis. Copyright © 2009 Pearson Education, Inc., publishing as Pearson Benjamin Cummings

Urine Production A healthy adult produces 1200 mL per day (0.6% of filtrate) With osmotic concentration of 800–1000 mOsm/L Copyright © 2009 Pearson Education, Inc., publishing as Pearson Benjamin Cummings

The Vasa Recta Carries water and solutes out of medulla Balances solute reabsorption and osmosis in medulla Recall the that the vasa recta are long, straight capillaries that parellel the long nephron loop of the juxtamedullary nephrons. The solutes and water reasorbed in the renal medulla must be returned to the bloodstream without disrupting the concentration gradient. This return is the function of the vasa recta. Copyright © 2009 Pearson Education, Inc., publishing as Pearson Benjamin Cummings

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 the urine produced each day varies with the metabolic and hormonal events under way. Copyright © 2009 Pearson Education, Inc., publishing as Pearson Benjamin Cummings

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 The composition and concentration of urine are two related, but distinct properties. The composition of urine reflects the filtration, reabsorption and secretion activities of the nephrons Urobilinogens are produced by intestinal bacteria and are absorbed in the colon. Because the composition and concentration of urine vary independently, you can produce a small volume of concentrated urine or a large volume of dilute urine and still excrete the same amount of dissolved materials. For this reason, physicians who are interested in a detailed assessment of renal function commonly analyze the urine produced over a 24-hour period rather than a single urine sample. A standard urinalysis includes an assessment of the color an appearance of urine. Copyright © 2009 Pearson Education, Inc., publishing as Pearson Benjamin Cummings

Urine Transport, Storage, and Elimination
Figure 26–17 A Pyelogram. A pyelogram is an image of the urinary system obtained by taking an x-ray of the kidneys after a radiopaque dye has been administered intravenously. Such an image provides an orientation to the relative sizes and positions of the main structures. Copyright © 2009 Pearson Education, Inc., publishing as Pearson Benjamin Cummings

Diseases Urinary Tract Infection (UTI) Incontinence Dialysis
Why is there blood in urine (hematuria)? Ketoacidosis Glycosuria Proteinuria Polyuria Phenylketonuria Urinary tract infections occur when bacteria gets into the urinary system through the urethra and leads an infection in the bladder, kidneys and tubes that connect them. Most urinary tract infections are bladder infections, which if not treated, may spread to the kidneys which can result in permanent damage. Symptoms include: pain or burning during urination, frequent urges to urinate with little urine leaving the bladder, tenderness in the abdomen, cloudy or foul smelling urine, pain in the lower back, fever and chills and nausea and vomiting. Prescription antibiotics will usually cure a bladder infection. There are multiple types of incontinence, the most common being stress incontinence, all of which are characterized by accidental loss of urine. Stress incontinence may happen when there is an increase in abdominal pressure, such as when you exercise, laugh, sneeze, or cough. Increased pressure on the top of the bladder created by these actions forces urine past the valve that normally keeps urine in the bladder, resulting in urine leakage. Causes of stress incontinence include pregnancy and childbirth, which cause stretching and weakening of the pelvic floor muscles. Other factors may also increase the risk for stress incontinence, such as being overweight, obesity, prostate surgery and certain medications. Stress incontinence often responds well to home treatments, such as kegel exercises. Kegal exercises are basically repeated squeezing of the muscles used to stop urinating. Medications, pessaries (a rubber device placed in the vagina to help support the uterus, which may be pressing on the bladder) or surgery are sometimes required. Chronic kidney disease and acute renal failure causes the kidneys to lose their ability to filter and remove waste and extra fluid from the body. Dialysis is a life-support treatment that uses a special machine to filter harmful wastes, salt, and excess fluid from the blood. For hemodialysis, tubes are attached to the blood vessels and blood is slowly pumped from the body to the dialyzer, where waste products and extra fluid are removed. The filtered blood is then pumped back into the body. Peritoneal dialysis uses a membrane inside the body as a filter and a solution to clear wastes and extra fluid to return electrolyte levels to normal. Both types of dialysis may improve quality of life and increase life expectancy, but it only provides about 10% of normal kidney function and cannot improve chronic kidney disease or kidney failure. Blood in the urine, or hematuria, is a symptom of multiple conditions affecting the urinary tract. Some of these include bladder or kidney infections, bladder or kidney stones, certain kidney diseases (glomerulonephritis- inflammation of nephons), enlarged prostate or prostate cancer, inherited diseases such as sickle cell anemia and cystic kidney disease, certain medications such as aspirin, penicillin, heparin, cyclophosphamide and phenazopyridine, a tumor in the bladder, kidney or prostate, a kidney injury or vigorous exercise. Treatment of the underlying cause will stop hematuria. Ketoacidosis is a diabetes-related condition caused by untreated hyperglycemia and can be life-threatening if untreated. When the lack of insulin prevents the body from using glucose for energy, the body switches to starvation mode and releases fat to use for energy. However, the fat that gets released is converted to ketones, which are utilized by the body at a slower pace. As a result, the ketone levels in blood rise and spill over into urine. Additionally, the high sugar that is also in the urine pulls water out of the body causing dehydration. Symptoms are dehydration and excess thirst, excess urination, vomiting, abdominal pain, drowsiness, difficulty breathing and ketones in the urine. Treatment of ketoacidosis requires insulin and fluids. Ketoacidosis can result in a coma and possibly death if left untreated. Glycosuria is a rare condition in which glucose is excreted in the urine despite normal or low blood glucose levels. This is the result of improper functioning of the renal tubules. In most affected individuals, the condition causes no apparent symptoms or serious affects. When renal glycosuria occurs as an isolated finding with otherwise normal kidney function, the condition is thought to be inherited as an autosomal recessive trait. There are no specific treatments for glycosuria, but dietary regulations will ensure the body maintains an adequate blood glucose level. Proteinuria is the presence of protein in the urine, which may indicate kidney damage. There are three main causes of proteinuria: damage to glomerulus due to disease, increased quantity of proteins in serum (proteinemia) or low reabsorption at proximal tubule. Essentially, excess protein in the urine indicates either an insufficiency of absorption or impaired filtration. Diabetics may suffer from damaged nephrons and develop proteinuria. Treatment relies heavily on the diagnosis of the cause. Medical management consists of angiotensin converting enzyme (ACE) inhibitors as the first treatment. If ACE inhibitors don’t work, aldosterone antagonists or angiotensin receptor blockers can be used to reduce protein loss. Polyuria is excessive or abnormally large production or passage of urine. Polyuria often appears in conjunction with polydipsia (increased thirst). The most common cause is uncontrolled diabetes, causing osmotic diuresis. Other causes include: increased fluid intake, diuretic drugs, diuretic foods (caffeine, alchohol), diabetes, pregnancy, an enlarged prostate, high doses of vitamin B12 and C, hyperthyroidism, etc. To prevent dehydration, antidiruetics such as ADH, angiotensin II and aldosterone produced by the body will slow water loss. However, if the underlying cause is not treated, the body will be unable to keep up with the water loss without more fluid consumption. Phenylketonuria is a rare genetic disorder (autosomal recessive) a in which the body cannot break down the amino acid phenylalanine. Proteins containing phenylalanine are present in breast milk, many types of baby formula, and most foods, especially those with a lot of protein, such as meat, eggs, and diary products. If phenylketonuria isn’t treated, phenylalanine can build up in the blood and lead to intellectual disability, seizures and problems with the brain and spinal cord. Early symptoms in a baby include: a musty odor to the skin, hair and urine, skin problems, losing weight from vomiting and diarrhea, acting fussy and being sensitive to light. Infants in the US and Canada are tested for this disease within a few days after birth. The main treatment is a lifelong reduced-protein diet and medication to lower phenylalanine levels.

Dialysis-Artificial Kidney
This picture is an example of hemodyalsis.

Phenylketonuria Phenylalanine is present in many foods and beverages, diet coke is an example, and warnings are often posted on the ingredient labels. This picture is of two siblings. The boy on the right has phenylketonuria that was not diagnosed at an early stage, leading to severe developmental defects.

The Urinary System Chapter 26.

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The Urinary System Chapter 26.

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