The Urinary System - Pearson
24The Urinary SystemAnimals living in an aquatic environment face little risk of becoming dehydrated. However, animals that started to spend more time on dry land millionsof years ago needed mechanisms to conserve water and prevent dehydration.The organ system that performs this function in humans—the urinary system—is thetopic of this chapter. The organs of the urinary system are organs of excretion—theyremove wastes and water from the body. Specifically, the urinary system “cleans theblood” of metabolic wastes, which are substances produced by the body that it cannotuse for any purpose. However, as you will learn in this chapter, the urinary system does24.1Overview of theUrinary System 94124.224.3Anatomy of the Kidneys 943Overview ofRenal Physiology 95124.4Renal Physiology I:Glomerular Filtration 95124.5Renal Physiology II:Tubular Reabsorptionand Secretion 960far more: This system is also essential for removing toxins, maintaining homeostasis ofmany factors (including blood pH and blood pressure), and producing erythrocytes.Read on to discover how the urinary system is vital to your body’s homeostasis.MODULE24.1Overview of the Urinary SystemLearning Outcomes1. List and describe the organs of the urinary system.2. Describe the major functions of the kidneys.24.6Renal Physiology III:Regulation of UrineConcentration and Volume 96824.7Putting It All Together:The Big Picture of RenalPhysiology 97424.824.9Urine and Renal Clearance 974Urine Transport, Storage,and Elimination 976The urinary system is composed of the paired kidneys and the urinary tract. The kidneysfilter the blood to remove metabolic wastes and then modify the resulting fluid, which allows these organs to maintain fluid, electrolyte, acid-base, and blood pressure homeostasis.This process produces urine, a fluid that consists of water, electrolytes, and metabolic wastes.Then the remaining organs of the urinary system—those of the urinary tract—transport,store, and eventually eliminate urine from the body. In this module, we first examine thebasic structures of the urinary system, and then turn to the functional roles of the kidneys.Overview of Urinary System StructuresAs you can see in Figure 24.1, the kidneys resemble their namesake, the kidney bean, in bothshape and color. The kidneys are situated against the posterior abdominal wall and are retroperitoneal (reh′-troh-pair-ih-ton-EE-ul; retro- “behind”) organs, meaning they are locatedposterior to the peritoneal membranes. Note, however, that the two kidneys differ slightlyin position—the left kidney extends from about T12 to L3, whereas the right kidney sitsslightly lower on the abdominal wall because of the position of the liver. The superior portionsPhoto: This scanning electron micrographshows glomeruli, the filtering units of thekidneys.9416th proofM24 AMER2952 01 SE C24 941-984.indd 9419/15/14 6:29 PM
942 Chapter 24 The Urinary SystemVertebralcolumnDiaphragmAdrenalgland11th and12th ribsKidneyPeritoneum(cut)AortaInferiorvena cavaUreterUrinarybladderUrethra(a) Anterior view(b) Posterior viewFigure 24.1 Organs of the urinary system in a female.Practice art labelingof both kidneys are partially protected by the 11th and 12th pairs ofribs. Each kidney is capped by an adrenal gland (ad- “near,” ren “kidney”); these glands perform endocrine functions and secretea variety of hormones (see Chapter 16).The urinary tract is composed of the paired ureters (YOOReh-terz), the urinary bladder, and the urethra (yoo-REE-thrah).Urine leaves each kidney through one of the two ureters, tubesthat run along the posterior body wall, connecting the kidneyswith the hollow urinary bladder. The bladder, which stores theurine, sits on the floor of the pelvic cavity. Urine is expelled fromthe body through the tube called the urethra, which connects theurinary bladder with the outside of the body.The regulation of homeostasis by the urinary system takesplace in the kidneys, so let’s take a quick look at what the kidneys do and how they do it. The kidneys perform the followingfunctions: Quick Check 1. What are the organs of the urinary system? Overview of Kidney Function Flashback1. What are three factors that determine blood pressure?(p. 670)2. What is erythropoietin, and what is its main function? (p. 727)Removal of metabolic wastes. As we have discussed, thekidneys filter the blood, removing metabolic wastes. Thesewastes are eliminated from the body via the urine.Regulation of fluid and electrolyte balance. The kidneysregulate blood solute concentration, or osmolarity, by conserving or eliminating water and electrolytes such as sodium,potassium, and calcium ions.Regulation of acid-base balance. The kidneys assist in thelong-term regulation of blood pH by conserving or eliminating hydrogen (H ) and bicarbonate (HCO3-) ions.Maintenance of blood pressure. The kidneys directly influence systemic blood pressure through their control ofblood volume. Additionally, they secrete an enzyme that influences both blood volume and peripheral resistance.Regulation of erythropoiesis. The kidneys regulate redblood cell production in the bone marrow by releasingthe hormone erythropoietin (eh-rith′-row-POY-eh-tin; seeChapter 19).Performing other metabolic functions. The kidneys playmany important metabolic roles, including detoxifyingsubstances in the blood, activating vitamin D, and making new glucose through the process of gluconeogenesis(glew′-koh-nee-oh-JEN-eh-sis).6th proofM24 AMER2952 01 SE C24 941-984.indd 9429/15/14 6:29 PM
24.2 Anatomy of the KidneysQuick Check 2. List the basic functions of the kidneys.Apply What You Learned 1. Inflammation of the peritoneal membranes, orperitonitis, can cause dysfunction of multiple organsin the abdominal cavity. Would you expect peritonitisto impact the kidneys? Explain. 2. Explain why a patient with long-term renal failure might have adecreased number of erythrocytes in his or her blood.See answers in Appendix A.Let’s now take a closer look at the anatomy of the kidneys. In thismodule, we explore first the external and internal anatomy of thekidneys. We then turn our attention to the structure and basicroles of the kidneys’ functional units: the nephrons (NEF-ronz;nephro- “kidney”). We conclude the module with an examination of the two main types of nephrons.External Anatomy of the KidneysThe kidneys are held in place on the posterior body wall and protected by three external layers of connective tissue (Figure 24.2).From superficial to deep, these layers are as follows: MODULE24.2Anatomy of the Kidneys Learning Outcomes1. Describe the external structure of the kidney, including itslocation, support structures, and covering.2. Trace the path of blood flow through the kidneys.3. Identify the major structures and subdivisions of the renalcorpuscle, renal tubules, and renal capillaries.4. Describe the histological structure of the proximal tubule,nephron loop, distal tubule, and collecting system, and trace thepathway of filtrate flow through these tubules.5. Compare and contrast cortical and juxtamedullary nephrons.943 Renal fascia. The renal fascia (FASH-ee-ah) is a layer ofdense irregular connective tissue that anchors each kidneyto the peritoneum and to the fascia covering the muscles ofthe posterior abdominal wall.Adipose capsule. The middle and thickest layer, called theadipose capsule, consists of adipose tissue that wedges eachkidney in place and shields it from physical shock. During prolonged starvation, the body uses the fatty acids inthe adipose capsule of the kidney for fuel. This causes thekidney to droop, a condition called nephroptosis (nef-ropTOH-sis; -ptosis “drooping”).Renal capsule. The renal capsule is an extremely thin layerof dense irregular connective tissue that covers the exteriorof each kidney like plastic wrap. It protects the kidney frominfection and physical trauma.Without its connective tissue coverings, a typical adult kidneyis about the size of a large bar of soap (11 cm long, 6 cm wide,AnteriorLiverRenal arteryStomachRenal veinPancreasLargeintestinePeritonealcavityRenal fasciaSpleenInferior viewLeftkidneyAdipose capsuleRenal capsuleRight kidneyVertebra (L1)HilumRenal pelvisPosteriorFigure 24.2 Position and external structure of the kidneys. Note that you are looking at an inferior view of this transverse section of the trunk.6th proofM24 AMER2952 01 SE C24 941-984.indd 9439/15/14 6:29 PM
944 Chapter 24 The Urinary Systemand 3 cm thick), and weighs about 150 grams. On the m edialsurface we find an opening called the hilum (HY-lum), throughwhich the renal artery, renal vein, renal nerves, and ureter enter and exit the kidney. The hilum opens to a central cavitycalled the renal sinus, which is lined by the renal capsule andfilled with urine-draining structures and adipose tissue. This connective tissue anchors the ureter, blood vessels, and nervesin place.Quick Check 1. What are the three connective tissue coverings of thekidney?Internal Anatomy of the KidneysA frontal section of the kidney reveals the three distinct regions ofthis organ: the outermost renal cortex, the middle renal medulla,and the inner renal pelvis (Figure 24.3a). Together, the renal cortexand the renal medulla make up the urine-forming portion of thekidney. The renal pelvis and associated structures drain urine thatthe cortex and medulla have formed.Notice in Figure 24.3a that the renal cortex is reddish-brown.This is due to its rich blood supply—it houses 90–95% of thekidney’s blood vessels. At specific points, extensions of the renalcortex called renal columns pass through the renal medulla toward the renal pelvis. The renal columns house blood vesselsthat branch from the renal artery as they travel to the outer portion of the cortex.Within the renal medulla we also find cone-shaped renal pyramids (or medullary pyramids), which are separated fromone another by a renal column on each side. Notice that the renalpyramids are darker in color and appear striped, reflecting thatthey are made up of parallel bundles of small tubes, with fewerblood vessels than in the renal cortex.The renal cortex and renal medulla of each kidney containover one million microscopic filtering structures called nephrons.Nephrons are the functional units of the kidney—each one is capable of filtering the blood and producing urine. Figure 24.3bshows the basic structure of a nephron, which consists of two maincomponents: the globe-shaped renal corpuscle, and a long, snaking tube of epithelium called the renal tubule. Notice how thesecomponents are arranged in the kidney. The renal corpuscle andthe majority of the renal tubule reside in the renal cortex, whereasvarying amounts of the renal tubule dip into the renal medulla.The tip of each renal pyramid tapers into a slender papilla(pah-PIL-ah; papil- “nipple”), which borders on the first urinedraining structure, a cup-shaped tube called a minor calyx (KAYliks; plural calyces, KAL-ih-seez). Urine from three to four minorcalyces drains into a larger major calyx (shown in Figure 24.3a).Two to three major calyces, in turn, drain urine into the largecollecting chamber that is the renal pelvis, which leads into the ureter. Smooth muscle tissue in the walls of the calyces and renalpelvis contracts to help propel urine toward the ureter. Both thecalyces and the renal pelvis reside in the renal l pelvisRenalpyramidin renalmedullaRenalcolumnMajor calyxMinor calyxRenal sinus(with adipose)Practice art labelingPapillaRenalcapsule(a) Kidney, frontal sectionMinorcalyx(b) Section of the renal cortex and renalpyramid showing nephronsFigure 24.3 Internal anatomy of the kidney, including the nephron.6th proofM24 AMER2952 01 SE C24 941-984.indd 9449/15/14 6:29 PM
24.2 Anatomy of the KidneysQuick Check 2. What are the three regions of the kidney, and how do theydiffer structurally? 3. What is the functional unit of the kidney?Blood Supply of the Kidneys Flashback1. How is blood delivered to and drained from a typicalcapillary bed? (p. 670)2. What are the major arteries and veins that supply and drainthe kidneys? (p. 683)The kidneys receive approximately one-fourth of the total cardiacoutput—about 1200 ml per minute—from the right and left r enalarteries, which branch from the abdominal aorta. The r enal arteriesfan out into ever-smaller vessels as they pass through the renal sinus to the renal columns and cortex. Figure 24.4 traces the pathof blood flow through the kidney. From largest to smallest, thearteries shown in Figure 24.4 are as follows: 1 Renal artery 2 Segmental artery 3 Interlobar artery 4 Arcuate artery 5 Interlobular (cortical radiate) artery.You learned in the blood vessels chapter that systemic capillarybeds are fed by arterioles and drained by venules (see Chapter 18).But in the kidneys, we find an unusual capillary bed that is bothfed and drained by arterioles (Figure 24.4b). In the kidney’s renalcortex, the interlobular arteries branch into tiny afferent arterioles(affer- “to carry toward”), which feed a ball-shaped capillarybed called the glomerulus (gloh-MAIR-yoo-lus; glom- “ball”).The glomerulus, which consists of glomerular capillaries and supporting cells, is part of the renal corpuscle of the nephron. Theglomerular capillaries are then drained by a second arteriole calledthe efferent arteriole (effer- “to carry away”). The efferent arteriole feeds a second capillary bed known as the peritubularcapillaries, which together form a plexus or network around therenal tubule of each nephron. The order of this part of the system is as follows: 6 Afferent arteriole 7 Glomerulus 8 Efferent arteriole 9 Peritubular capillaries.The venous route of blood out of the kidney parallels the arterial path. Groups of peritubular venules unite into a series ofprogressively larger venous vessels. The sequence, from smallestto largest (see Figure 24.4), is as follows: 10 Interlobular vein 11 Arcuate vein 12 Interlobar vein 13 Renal vein. Notethat no segmental veins are present in the kidneys; the i nterlobarveins merge in the renal sinus to form the large renal vein. Therenal vein then exits the kidney through the hilum and emptiesinto the inferior vena cava.7 Glomerulus6 Afferentarteriole3 Interlobarartery5 Interlobularartery2 Segmentalartery9458 Efferentarteriole9 Peritubularcapillaries1 Renalartery4 Arcuateartery13 Renalvein11 Arcuatevein(a) Blood flowthrough kidney12 Interlobarvein10 Interlobularvein(b) Blood flow around nephronFigure 24.4 Blood flow through the kidney.Practice art labeling6th proofM24 AMER2952 01 SE C24 941-984.indd 9459/15/14 6:30 PM
946 Chapter 24 The Urinary SystemRenalcorpuscleQuick CheckDistaltubule 4. Trace the sequence of blood flow through the kidneys fromthe renal artery to the renal vein. 5. How does the arrangement of capillary beds in the kidneysdiffer from those elsewhere in the body?Microanatomy of the Kidney:The Nephron and Collecting SystemProximaltubule FlashbackRenalcortexRenalmedulla1. What are fenestrated capillaries? (p. 686)As you learned in this module, most of the functions of the kidneys occur in their tiny nephrons (Figure 24.5). Nephrons filterthe blood and modify the filtered fluid as it passes through the renal tubules. This fluid then leaves the nephron and drains intothe tubules of the collecting system (which are not considered partof the nephron), where it is further modified until it finally becomes urine. Let’s take a closer look at each of these components.NephronloopCollectingductThe NephronAs we introduced earlier, the nephron has two main divisions:the renal corpuscle and the renal tubule. Both structures arecomposed of multiple parts.The Renal Corpuscle The renal corpuscle is responsible for filtering the blood. Each globe-shaped renal corpuscle consists of twoparts: the glomerulus and an outer sheath of epithelial tissue calledthe glomerular capsule (or Bowman’s capsule; Figure 24.6). Theglomerulus is a group of looping fenestrated capillaries. Thesecapillaries are called fenestrated because of the large pores, or fenestrations (fenestre- “window”), present within their plasmamembranes and between their endothelial cells. These fenestrations, which make the capillaries extremely “leaky,” or permeable,form a main part of the filtering structure of the renal corpuscle.Surrounding the glomerulus is the double-layered glomerularcapsule, which consists of an outer parietal layer (pah-RY-eh-tal)and an inner visceral layer. The parietal layer is a globelike extension of the renal tubule consisting of simple squamous epithelium. The visceral layer consists of modified epithelial cells calledpodocytes (POH-doh-sytz; podo- “foot”) that wrap aroundthe glomerular capillaries. Extending from each podocyte are extensions called foot processes, or pedicels (PED-ih-selz). Pedicelsweave together to form filtration slits, which make up anotherpart of the renal corpuscle’s filtering structure. Between the parietal and visceral layers we find a hollow region, or lumen, calledthe capsular space, which is continuous with the beginning ofthe renal tubule lumen.The podocytes and fenestrated glomerular capillaries formpart of a complex membrane (the filtering structure we mentioned earlier) that filters blood flowing through the glomerulus. This structure allows a large volume of fluid to be filteredfrom the blood (which we discuss in the next module). The fluidFigure 24.5 A generalized nephron and collecting system.Practice art labelingthat passes through the filter to leave the glomerular capillaries,which is known as filtrate, first enters the capsular space, thenflows into the renal tubule lumen.The Renal Tubule Newly formed filtrate flows from the capsularspace into the “pipes” of the nephron: the renal tubule. The renaltubule is a winding tube responsible for modifying the filtrate.It has three regions: the proximal tubule, nephron loop, and distal tubule, each of which differs in structure and function(Figure 24.7).1. Proximal tubule. The initial and longest segment of the renaltubule through which filtrate flows is the proximal tubule;it consists of simple cuboidal epithelial cells with prominent microvilli. This part of the tubule has both convoluted(coiled) and straight sections, which is why this text uses thebroader term proximal tubule (rather than proximal convoluted tubule). The many microvilli projecting into the lumenof the proximal tubule form a brush border, so named becausethe fine projections resemble the bristles on a brush. This border greatly increases surface area.6th proofM24 AMER2952 01 SE C24 941-984.indd 9469/15/14 6:30 PM
24.2 Anatomy of the KidneysGlomerular capsule:Glomerularcapillaries (coveredby podocytes)PodocyteFiltration slitsCapillary947PedicelsSquamous epithelium(parietal layer)Capsular spacePodocyte(visceral buleSEM (92,000 )(a) The renal corpuscle(b) SEM of capillary surrounded by podocytesFigure 24.6 The renal corpuscle.2. Nephron loop (loop of Henle). The remaining filtrate flowson to the nephron loop, also known as the loop of Henle,which is the only part of the renal tubule that dips intothe renal medulla. The nephron loop has two limbs: The descending limb travels toward the renal medulla, turns 180 ,and becomes the ascending limb, which climbs back towardthe renal cortex. The descending limb is composed of simple squamous epithelium, and thus is often called the thin descending limb. In some nephrons this thin segment alsoforms the bend region and part of the ascending limb, so thereis also a thin ascending limb. However, the majority of the ascending limb is composed of thicker simple cuboidal epithelium, and is therefore referred to as the thick ascending limb.Brush border3. Distal tubule. The final segment through which the fil
24.1 Overview of the Urinary System 941 24.2 Anatomy of the Kidneys 943 24.3 Overview of Renal Physiology 951 24.4 Renal Physiology I: Glomerular Filtration 951 24.5 Renal Physiology II: Tubular Reabsorption and Secretion 960 24.6 Renal Physiology III: Regulation of Urine
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an indwelling urinary catheter. The indwelling urinary catheter is considered a foreign object in the lower urinary tract, which means a CAUTI differs from an infection occurring in the urinary bladder of a patient who is not catheterized (Leidl 2001). CAUTIs do not produce the
Urinary and Reproductive Systems URINARY SYSTEM TABLE 12-1. Organs of the urinary system STRUCTURE TEXT REFERENCES AND NOTES kidneys DESCRIBED: P. 812 FIG. 27.1 ureters urinary bladder urethra TABLE 12-2. Gross anatomy of the kidney, coronal section STRUCTURE TEXT REFE
