Fundamentals of Anatomy & Physiology

Fundamentals of Anatomy & Physiology
Eleventh Edition Chapter 26 The Urinary System Lecture Presentation by Deborah A. Hutchinson Seattle University

Learning Outcomes 26-1 Identify the organs of the urinary system, and describe the functions of the system Describe the location and structures of the kidneys, identify major blood vessels associated with each kidney, trace the path of blood flow through a kidney, and describe the structure of a nephron Describe the basic processes that form urine Describe the factors that influence glomerular filtration pressure and the rate of filtrate formation Identify the types and functions of transport mechanisms found along each segment of the nephron and collecting system.

Learning Outcomes 26-6 Explain the role of countercurrent multiplication, describe hormonal influence on the volume and concentration of urine, and describe the characteristics of a normal urine sample Describe the structures and functions of the ureters, urinary bladder, and urethra, discuss the voluntary and involuntary regulation of urination, and describe the urinary reflexes Describe the effects of aging on the urinary system Give examples of interactions between the urinary system and other organ systems studied so far.

Introduction to the Urinary System
Removes most metabolic wastes produced by body’s cells Kidneys remove metabolic wastes from circulation Produce urine

26-1 Organs and Functions of Urinary System
Organs of urinary system Kidneys—paired organs that produce urine Urinary tract—eliminates 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

Figure 26–1 An Overview of the Urinary System.
Organs of the Urinary System Kidneys Produce urine Ureters Transport urine toward the urinary bladder Urinary bladder Temporarily stores urine prior to urination Urethra Conducts urine to exterior; in males, it also transports semen Anterior view

Three functions of urinary system Excretion Removal of metabolic wastes from body fluids Elimination Discharge of wastes from body Homeostatic regulation Of volume and solute concentration of blood

Homeostatic functions of urinary system Regulating blood volume and blood pressure By adjusting volume of water lost in urine Releasing erythropoietin and renin Regulating plasma ion concentrations By controlling quantities of sodium, potassium, chloride, and other ions lost in urine Calcium ion level controlled through synthesis of calcitriol

Homeostatic functions of urinary system Helping to stabilize blood pH By controlling loss of hydrogen and bicarbonate ions in urine Conserving valuable nutrients By preventing their loss while removing metabolic wastes (e.g., urea and uric acid) Assisting liver In detoxification of poisons, and deamination of amino acids during starvation

26-2 The Kidneys and Nephrons
Located on either side of vertebral column Left kidney is slightly superior to right kidney Superior surface is capped by adrenal gland Position is maintained by Overlying peritoneum Contact with adjacent visceral organs Supporting connective tissues

Figure 26–2a The Position and Associated Structures of the Kidneys.
Adrenal gland Diaphragm 11th and 12th ribs T12 vertebra Left kidney L3 vertebra Right kidney Ureter Renal artery and vein Inferior vena cava Iliac crest Aorta Urinary bladder Urethra a A posterior view of the trunk

Kidneys are protected and stabilized by 3 layers Fibrous capsule A layer of collagen fibers Covers outer surface of entire organ Perinephric fat A thick layer of adipose tissue Surrounds fibrous capsule Renal fascia A dense, fibrous outer layer Anchors kidney to surrounding structures

Figure 26–2b The Position and Associated Structures of the Kidneys.
External oblique Parietal peritoneum Renal vein Renal artery Stomach Aorta Pancreas Ureter Spleen Left kidney Vertebra Connective Tissue Layers Fibrous capsule Perinephric fat Renal fascia Quadratus lumborum Psoas major Inferior vena cava b A superior view of a transverse section at the level indicated in part a

Typical adult kidney About 10 cm long, 5.5 cm wide, and 3 cm thick Weighs about 150 g Hilum Prominent medial indentation Point of entry for renal artery and renal nerves Point of exit for renal vein and ureter

Figure 26–3 The Gross Anatomy of the Urinary System.
Esophagus (cut) Diaphragm Left adrenal gland Inferior vena cava Celiac trunk Left kidney Right adrenal gland Left renal artery Right kidney Left renal vein Hilum Superior mesenteric artery Left ureter Quadratus lumborum Abdominal aorta Iliacus Left common iliac artery Psoas major Gonadal artery and vein Peritoneum (cut) Rectum (cut) Urinary bladder Anterior view

Renal sinus Internal cavity within kidney Lined by fibrous capsule Bound to outer surfaces of structures in renal sinus Stabilizes positions of ureter, renal blood vessels, and nerves

Renal cortex Superficial region of kidney in contact with fibrous capsule Reddish-brown and granular Renal pyramids 6 to 18 triangular structures in renal medulla Base of each pyramid abuts cortex Tip (renal papilla) projects into renal sinus

Renal columns Bands of cortical tissue that separate adjacent renal pyramids Extend into medulla Have distinct granular texture

Kidney lobe Consists of A renal pyramid Overlying area of renal cortex Adjacent tissues of renal columns Produces urine

Ducts within each renal papilla Discharge urine into a minor calyx, a cup-shaped drain Major calyx Formed by four or five minor calyces Renal pelvis Large, funnel-shaped chamber Formed by two or three major calyces Fills most of renal sinus Connected to ureter, which drains kidney

Figure 26–4a The Internal Anatomy of the Kidney.
Renal cortex Renal medulla Renal pyramid Inner layer of fibrous capsule Renal sinus Connection to minor calyx Adipose tissue in renal sinus Minor calyx Major calyx Renal pelvis Hilum Kidney lobe Renal papilla Renal columns Ureter Fibrous capsule a A diagrammatic view of a frontal section through the left kidney

Figure 26–4b The Internal Anatomy of the Kidney.
Renal cortex Renal medulla Renal pyramids Renal sinus Renal pelvis Hilum Major calyx Minor calyx Ureter Renal papilla Renal columns Kidney lobe Fibrous capsule b A frontal section of the left cadaver kidney

Blood supply to kidneys Kidneys receive 20–25 percent of total cardiac output About 1200 mL of blood flow through kidneys each minute Each kidney receives blood through a renal artery

Segmental arteries Receive blood from renal artery Divide into interlobar arteries Radiate outward through renal columns between renal pyramids Interlobar arteries supply blood to arcuate arteries Arch along boundary between cortex and medulla of kidney

Afferent arterioles Branch from each cortical radiate artery (interlobular artery) Deliver blood to capillaries supplying individual nephrons Cortical radiate veins (interlobular veins) Deliver blood to arcuate veins Arcuate veins empty into interlobar veins Drain directly into renal vein

Figure 26–5a The Blood Supply to the Kidneys.
Cortical radiate veins Cortical radiate arteries Interlobar arteries Cortex Segmental artery Adrenal artery Renal artery Renal vein Interlobar veins Arcuate veins Medulla Arcuate arteries a A sectional view, showing major arteries and veins

Figure 26–5b The Blood Supply to the Kidneys.
Glomerulus Cortical radiate vein Afferent arterioles Cortical radiate artery Arcuate artery Arcuate vein Cortical nephron Juxtamedullary nephron Renal pyramid Renal pyramid Interlobar vein Interlobar artery Minor calyx b Circulation in a single kidney lobe

Figure 26–5c The Blood Supply to the Kidneys.
Renal vein Renal artery Segmental artery Interlobar vein Interlobar artery Arcuate vein Arcuate artery Cortical radiate vein Cortical radiate artery Venule Afferent arteriole NEPHRON Peritubular capillaries Glomerulus Efferent arteriole c A flowchart of renal circulation

Renal nerves Innervate kidneys and ureters Enter each kidney at hilum Follow branches of renal arteries to individual nephrons Sympathetic innervation Adjusts rate of urine formation By changing blood flow at nephron Influences urine composition By stimulating release of renin

Microscopic functional units of kidneys Each consists of renal corpuscle and renal tubule Each renal tubule empties into collecting system Renal corpuscle Spherical structure consisting of Glomerular (Bowman’s) capsule Glomerulus (capillary network)

Figure 26–6a The Anatomy of a Representative Nephron and the Collecting System.
Proximal Convoluted Tubule Distal Convoluted Tubule Cuboidal cells with abundant microvilli Cuboidal cells with few microvilli Mitochondria Renal Corpuscle Renal tubule Squamous cells Efferent arteriole Afferent arteriole Glomerulus Glomerular capsule Descending limb of loop begins Ascending limb of loop ends Capsular space Nephron Loop Descending thin limb (DTL) Squamous cells Thick ascending limb (TAL) Low cuboidal cells Ascending thin limb (ATL) Squamous cells

Glomerular capsule Forms outer wall of renal corpuscle Encapsulates glomerular capillaries Continuous with initial segment of renal tubule Glomerulus Consists of about 50 intertwined capillaries Blood is delivered by afferent arteriole Blood leaves through efferent arteriole

Glomerular capsule Capsular outer layer Simple squamous epithelium Visceral layer Covers glomerular capillaries Capsular space Separates two layers

Podocytes Large cells of visceral layer Have complex foot processes (pedicels) that wrap around glomerular capillaries Filtration slits Narrow (6–9 nm wide) gaps between adjacent foot processes

Figure 26–7a The Renal Corpuscle.
Glomerular Capsule Capsular space Glomerular capillary Capsular outer layer Visceral layer Proximal convoluted tubule Efferent arteriole Distal convoluted tubule Juxtaglomerular Complex Macula densa Extraglomerular mesangial cells Juxtaglomerular cells Afferent arteriole a Important structural features of a renal corpuscle.

Glomerular capillaries Fenestrated capillaries Endothelium contains large-diameter pores Intraglomerular mesangial cells Located among glomerular capillaries Specialized cells derived from smooth muscle Provide support, filtration, and phagocytosis Control diameter of capillaries

Filtration membrane Consists of Fenestrated endothelium Basement membrane Foot processes of podocytes During filtration in renal corpuscle, Blood pressure forces water and small solutes across membrane into capsular space Larger solutes (e.g., plasma proteins) do not pass Solution produced is essentially protein-free filtrate

Figure 26–7b The Renal Corpuscle.
Filtration Membrane Podocyte nucleus Fenestrated endothelium Basement membrane Foot processes of podocytes Intra- glomerular mesangial cell Capillary endothelial cell Pores RBC RBC Foot processes Podocyte Capsular space Capsular outer layer b This cross section through a portion of the glomerulus shows the components of the filtration membrane of the nephron.

Renal tubule Two convoluted segments Proximal convoluted tubule (PCT) Distal convoluted tubule (DCT) Segments are separated by nephron loop (loop of Henle) U-shaped tube Extends at least partially into medulla

Figure 26–6a The Anatomy of a Representative Nephron and the Collecting System.
Proximal Convoluted Tubule Distal Convoluted Tubule Cuboidal cells with abundant microvilli Cuboidal cells with few microvilli Mitochondria Renal Corpuscle Renal tubule Squamous cells Efferent arteriole Afferent arteriole Glomerulus Glomerular capsule Descending limb of loop begins Ascending limb of loop ends Capsular space Nephron Loop Descending thin limb (DTL) Squamous cells Thick ascending limb (TAL) Low cuboidal cells Ascending thin limb (ATL) Squamous cells

Renal tubule While traveling along tubule, the tubular fluid (filtrate) gradually changes in composition Due to substances being reabsorbed or secreted in various segments of nephron

Proximal convoluted tubule (PCT) First segment of renal tubule Entrance to PCT lies opposite of connection of afferent and efferent arterioles with glomerulus Simple cuboidal epithelium Microvilli on apical surfaces Primary function is reabsorption of ions

Nephron loop Descending limb Fluid flows toward renal pelvis Ascending limb Fluid flows toward renal cortex Segments of limbs have thick or thin epithelia Descending thin limb (DTL) Ascending thin limb (ATL) Thick ascending limb (TAL)

Distal convoluted tubule (DCT) Third segment of renal tubule Initial portion passes between afferent and efferent arterioles Smaller luminal diameter than PCT Epithelial cells lack microvilli Primary function is to reabsorb water and selected ions Actively secretes undesirable substances

Juxtaglomerular complex (JGC) Helps regulate blood pressure and filtrate formation Consists of Macula densa Juxtaglomerular cells Extraglomerular mesangial cells

Juxtaglomerular complex Macula densa Epithelial cells of DCT, near renal corpuscle Function as chemoreceptors or baroreceptors Juxtaglomerular cells Smooth muscle cells in wall of afferent arteriole Function as baroreceptors and secrete renin Extraglomerular mesangial cells Located between afferent and efferent arterioles Provide feedback control

Figure 26–7a The Renal Corpuscle.
Glomerular Capsule Capsular space Glomerular capillary Capsular outer layer Visceral layer Proximal convoluted tubule Efferent arteriole Distal convoluted tubule Juxtaglomerular Complex Macula densa Extraglomerular mesangial cells Juxtaglomerular cells Afferent arteriole a Important structural features of a renal corpuscle.

Collecting system A series of tubes that carries tubular fluid away from nephrons Collecting ducts Receive fluid from many nephrons Each collecting duct Begins in cortex Descends into medulla Carries fluid to papillary duct, which drains into a minor calyx

Collecting system Transports tubular fluid from nephrons to renal pelvis Adjusts fluid composition Determines final osmotic concentration and volume of urine

b COLLECTING SYSTEM Collecting Duct Intercalated cell Principal cell
Figure 26–6b The Anatomy of a Representative Nephron and the Collecting System. b COLLECTING SYSTEM Collecting Duct Intercalated cell Principal cell Papillary Duct Columnar cells Minor calyx

Cortical nephrons 85 percent of all nephrons Located mostly within superficial cortex of kidney Nephron loop is relatively short Efferent arterioles deliver blood to a network of peritubular capillaries Juxtamedullary nephrons 15 percent of nephrons Nephron loop extends deep into medulla Efferent arterioles connect to vasa recta

The general appearance and location of nephrons in the kidneys
Figure 26–8a The Locations and Structures of Cortical and Juxtamedullary Nephrons. Cortical nephron Juxtamedullary nephron Cortex Medulla Collecting duct Papillary duct Renal papilla Minor calyx a The general appearance and location of nephrons in the kidneys

b The circulation to a cortical nephron Peritubular capillaries Distal
Figure 26–8b The Locations and Structures of Cortical and Juxtamedullary Nephrons. Peritubular capillaries Distal convoluted tubule Efferent arteriole Afferent arteriole Renal corpuscle Collecting duct Peritubular capillaries Nephron loop b The circulation to a cortical nephron

juxtamedullary nephron
Figure 26–8c The Locations and Structures of Cortical and Juxtamedullary Nephrons. Peritubular capillaries Distal convoluted tubule (DCT) Proximal convoluted tubule (PCT) Renal corpuscle Collecting duct Vasa recta Nephron loop c The circulation to a juxtamedullary nephron

26-3 Renal Physiology Goal of urine production is to maintain homeostasis By regulating volume and composition of blood Involves excretion of metabolic wastes

26-3 Renal Physiology Three metabolic wastes
Urea (most abundant organic waste) Creatinine (from breakdown of creatine phosphate) Uric acid (from recycling of nitrogenous bases) Organic wastes Dissolved in bloodstream Eliminated only when dissolved in urine Removal is accompanied by water loss

26-3 Renal Physiology Kidneys Usually produce concentrated urine
1200 mOsm/L (four times the osmotic concentration of plasma) Kidney functions Concentrating filtrate Failure leads to fatal dehydration within hours Reabsorption and retention of valuable materials Example: sugars and amino acids

26-3 Renal Physiology Basic processes of urine formation Filtration
Blood pressure forces water and solutes across walls of glomerular capillaries Reabsorption Movement of water and solutes from filtrate to peritubular fluid Secretion Transport of solutes from peritubular fluid to tubular fluid

Figure 26–9 An Overview of Urine Formation.
Proximal Convoluted Tubule Distal Convoluted Tubule • Reabsorption of water, ions, and all organic nutrients • Secretion of ions, acids, drugs, toxins • Variable reabsorption of water, sodium ions, and calcium ions (under hormonal control) Renal Corpuscle • Production of filtrate Glomerulus Glomerular capsule Collecting Duct Collecting duct • Variable reabsorption of water and reabsorption or secretion of sodium, potassium, hydrogen, and bicarbonate ions Nephron Loop Descending thin limb • Further reabsorption of water Thick ascending limb • Reabsorption of sodium and chloride ions KEY Papillary Duct Filtration occurs exclusively in the renal corpuscle, across the filtration membrane. • Delivery of urine to minor calyx Water reabsorption occurs primarily along the PCT and the descending thin limb of the nephron loop, but also to a variable degree in the DCT and collecting system. Urine storage and elimination Variable water reabsorption occurs in the DCT and collecting system. Solute reabsorption occurs along the PCT, the thick ascending limb of the nephron loop, the DCT, and the collecting system. Variable solute reabsorption or secretion occurs at the PCT, the DCT, and the collecting system.

Table 26–2 Normal Laboratory Values for Solutes in Plasma and Urine

26-4 Glomerular Filtration
Driven by hydrostatic pressure Involves passage across a filtration membrane Small solute molecules pass through Larger solutes and materials are restricted Three components of membrane Fenestrated endothelium Basement membrane Foot processes of podocytes

Figure 26–10a Glomerular Filtration.
Glomerulus Basement membrane Efferent arteriole Capillary lumen Afferent arteriole Filtration slit Podocyte Foot processes of podocytes Pore Capsular space Filtration membrane a The glomerular filtration membrane

Glomerular capillaries Fenestrated endothelium with small pores Prevent passage of blood cells Allow diffusion of solutes, including plasma proteins Basement membrane More selective Allows passage of only small plasma proteins, nutrients, and ions

Filtration pressures Glomerular filtration is governed by balance between Hydrostatic pressure (fluid pressure) Colloid osmotic pressure (of materials in solution) on each side of capillary walls

Glomerular hydrostatic pressure (GHP) Blood pressure in glomerular capillaries Blood leaving glomerular capillaries Flows into efferent arteriole with luminal diameter smaller than that of afferent arteriole GHP is higher than hydrostatic pressure in peripheral capillaries Tends to push water and solutes Out of bloodstream Into filtrate

Capsular hydrostatic pressure (CsHP) Opposes glomerular hydrostatic pressure Pushes water and solutes Out of filtrate Into bloodstream Results from resistance to flow along nephron and conducting system

Net hydrostatic pressure (NHP) Difference between glomerular hydrostatic pressure and capsular hydrostatic pressure GHP – CsHP = NHP 50 mm Hg –15 mm Hg = 35 mm Hg

Colloid osmotic pressure Pressure due to materials in solution Blood colloid osmotic pressure (BCOP) Osmotic pressure resulting from suspended proteins in blood

Net filtration pressure (NFP) Average pressure forcing water and dissolved substances Out of glomerular capillaries Into capsular spaces Difference between net hydrostatic pressure and blood colloid osmotic pressure in glomerulus NHP – BCOP = NFP 35 mm Hg – 25 mm Hg = 10 mm Hg

Figure 26–10b Glomerular Filtration.
Factors Controlling Glomerular Filtration The glomerular hydrostatic pressure (GHP) is the blood pressure in the glomerular capillaries. This pressure tends to push water and solute molecules out of the plasma and into the filtrate. The GHP, which averages 50 mm Hg, is significantly higher than capillary pressures elsewhere in the systemic circuit, because the efferent arteriole is smaller in diameter than the afferent arteriole. The blood colloid osmotic pressure (BCOP) tends to draw water out of the filtrate and into the plasma; it thus opposes filtration. Over the entire length of the glomerular capillary bed, the BCOP averages about 25 mm Hg. Filtrate in capsular space The net filtration pressure (NFP) is the net pressure acting across the glomerular capillaries. It represents the sum of the hydrostatic pressures and the colloid osmotic pressures. Under normal circumstances, the net filtration pressure is approximately 10 mm Hg. This is the average pressure forcing water and dissolved substances out of the glomerular capillaries and into the capsular space. Plasma proteins 50 10 mm Hg 25 15 Solutes Capsular hydrostatic pressure (CsHP) opposes GHP. CsHP, which tends to push water and solutes out of the filtrate and into the plasma, results from the resistance of filtrate already present in the nephron that must be pushed toward the renal pelvis. The difference between GHP and CsHP is the net hydrostatic pressure (NHP). The capsular colloid osmotic pressure is usually zero because few, if any, plasma proteins enter the capsular space. b Net filtration pressure

Glomerular filtration rate (GFR) Amount of filtrate kidneys produce each minute Averages 125 mL/min About 10 percent of fluid delivered to kidneys Leaves bloodstream Enters capsular spaces Glomeruli generate about 180 liters of filtrate per day Approximately 70 times total plasma volume Net filtration pressure determines GFR

Regulation of GFR Three interacting levels of control Autoregulation (local level) Hormonal regulation (initiated by kidneys) Autonomic regulation (by sympathetic division of ANS)

Autoregulation of GFR Maintains adequate GFR despite changes in local blood pressure and blood flow Involves changing luminal diameters of Afferent arterioles Efferent arterioles Glomerular capillaries

Hormonal regulation of GFR Renin-angiotensin-aldosterone system (RAAS) Natriuretic peptides

Renin-angiotensin-aldosterone system (RAAS) Three stimuli cause juxtaglomerular complex (JGC) to release renin Decrease in blood pressure at glomerulus due to decrease in blood volume, decrease in systemic pressures, or blockage in renal artery or its branches Stimulation of juxtaglomerular cells by sympathetic innervation Decrease in osmotic concentration of tubular fluid at macula densa

RAAS Renin converts inactive angiotensinogen to inactive angiotensin I Angiotensin I is converted to angiotensin II by angiotensin-converting enzyme (ACE) Primarily in capillaries of lungs

Effects of angiotensin II Contraction of vascular smooth muscle in peripheral capillary beds Increased aldosterone secretion by adrenal glands Increases Na+ retention Increased arterial pressures Stimulation of thirst centers Increased production of ADH Overall effect: increase in systemic blood pressure and blood volume and restoration of normal GFR

Figure 26–11 The Response to a Decrease in the GFR.
Renin–Angiotensin–Aldosterone System Endocrine response Neural responses started by angiotensin II Effector Decreased filtration pressure; decreased filtrate and urine production Renin triggers the formation of angiotensin I. Angiotensin-converting enzyme (ACE) in the capillaries of the lungs converts angiotensin I to angiotensin II. Juxtaglomerular complex Angiotensin II increases aldosterone secretion by the adrenal glands. releases renin into the bloodstream Homeostasis DISTURBED BY DECREASING Effector Angiotensin II constricts peripheral arterioles and further constricts the efferent arterioles. Nervous system blood flow to kidneys STIMULUS Aldosterone increases Na+ retention. HOMEOSTASIS NORMAL GLOMERULAR FILTRATION RATE (GFR) Increased stimulation of thirst centers Increased fluid consumption RESTORED Increased fluid retention Increased ADH production Homeostasis RESTORED BY INCREASING Increased blood pressure Increased blood volume Constriction of systemic veins blood flow to kidneys Increased cardiac output Increased sympathetic activation Together, angiotensin II and sympathetic activation stimulate peripheral vasoconstriction.

Hormonal regulation of GFR Increased blood volume Automatically increases GFR to promote fluid loss If increase in blood volume is severe, hormonal factors further increase GFR

Natriuretic peptides Released by heart in response to stretched walls due to increased blood volume or pressure Atrial natriuretic peptide (ANP) is released by atria Trigger dilation of afferent glomerular arterioles and constriction of efferent glomerular arterioles Increase glomerular pressures and increase GFR ANP also decreases sodium reabsorption Net result is increased urine production and decreased blood volume and pressure

Autonomic regulation of GFR Mostly consists of sympathetic postganglionic fibers Sympathetic activation Constricts afferent glomerular arterioles Decreases GFR Slows filtrate production Sympathetic activation can override local regulatory mechanisms that act to stabilize GFR

26-5 Reabsorption and Secretion
Filtration at renal corpuscle Passive Solutes entering capsular space include Metabolic wastes and excess ions Glucose, free fatty acids, amino acids, and vitamins Useful materials are reabsorbed In renal tubules and collecting system

Three functions of renal tubule Reabsorbing useful organic nutrients in filtrate Reabsorbing more than 90 percent of water in filtrate Secreting any wastes that did not enter filtrate at glomerulus

Principles of reabsorption and secretion Reabsorption Recovers useful materials from tubular fluid and returns them to blood 99 percent of filtrate is reabsorbed in renal tubules Secretion Adds substances from blood to tubular fluid Reabsorption and secretion Occur in every segment of renal tubule and in collecting system

Reabsorption and secretion by kidneys involve Diffusion Osmosis Leak channels (channel-mediated diffusion) Carrier-mediated transport

Types of carrier-mediated transport Facilitated diffusion Active transport Cotransport Countertransport

Characteristics of carrier-mediated transport A specific substrate binds to a carrier protein that facilitates movement across membrane A given carrier protein typically works in one direction only Distribution of carrier proteins can vary in different regions of cell surface The membrane of a single tubular cell contains many types of carrier proteins

Transport maximum (Tm) If nutrient concentrations rise in tubular fluid, Reabsorption rates increase until carrier proteins are saturated (Tm is met) A concentration higher than transport maximum exceeds reabsorptive abilities of nephron Some material will remain in tubular fluid and appear in urine Tm determines renal threshold

Renal threshold Plasma concentration at which a specific compound or ion begins to appear in urine Varies with substance involved If plasma glucose is higher than 180 mg/dL, glucose appears in urine (glucosuria) Renal threshold for amino acids is 65 mg/dL After a protein-rich meal, amino acids commonly appear in urine (aminoaciduria)

Osmotic concentration (osmolarity) Total number of solute particles per liter Expressed in osmoles per liter (Osm/L) or milliosmoles per liter (mOsm/L) Body fluids have an osmotic concentration of about 300 mOsm/L Ion concentrations are reported in milliequivalents per liter (mEq/L) Concentrations of large organic molecules are reported in mass per unit volume (e.g., mg/dL)

Table 26–3 Tubular Reabsorption and Secretion

Reabsorption and secretion at PCT PCT cells normally reabsorb 60–70 percent of filtrate volume produced in renal corpuscle Reabsorbed materials enter peritubular fluid And diffuse into peritubular capillaries

Functions of PCT Reabsorption of organic nutrients Active reabsorption of ions Reabsorption of water Passive reabsorption of ions Secretion

Sodium ion reabsorption Important in every PCT process Sodium ions enter tubular cells by Diffusion through sodium ion leak channels Sodium ion–linked cotransport of organic solutes Countertransport for hydrogen ions

Figure 26–12 Transport Activities at the PCT (Part 1 of 2).
Lumen containing tubular fluid Cuboidal epithelial cells Proximal convoluted tubule Distal convoluted tubule Glomerulus Glomerular capsule Collecting duct Nephron loop Urine storage and elimination KEY Water reabsorption Solute reabsorption Variable solute reabsorption or secretion

Figure 26–12 Transport Activities at the PCT (Part 2 of 2).
H+ + HCO3– H2CO3 CO2 + H2O Tubular fluid Cells of proximal convoluted tubule Na+ Na+ Na+ CO2 H2O Glucose and other organic solutes H2CO3 H+ + HCO3– Osmotic water flow 2 K+ Peritubular fluid Na+ HCO3– 3 Na+ Peritubular capillary Na+ H2O H+ HCO3– 3 Na+ KEY Leak channel Diffusion Countertransport Reabsorption Exchange pump Cotransport Secretion

Reabsorption and secretion along nephron loop 60–70 percent of filtrate volume is reabsorbed before tubular fluid reaches nephron loop Nephron loop reabsorbs About 1/2 of remaining water 2/3 of remaining sodium and chloride ions

Descending limb of nephron loop Freely permeable to water but not to solutes Descending thin limb reabsorbs sodium and chloride ions from tubular fluid Ascending limb of nephron loop Impermeable to water Passively and actively removes sodium and chloride ions from tubular fluid Very long in juxtamedullary nephrons, creating high solute concentrations in peritubular fluid

Reabsorption and secretion at DCT Only 15–20 percent of initial filtrate volume reaches DCT Concentrations of electrolytes and metabolic wastes in arriving tubular fluid no longer resemble those in blood plasma

Three processes at DCT Active secretion of ions, acids, drugs, and toxins into tubule Selective reabsorption of sodium and calcium ions from tubular fluid Selective reabsorption of water Assists in concentrating tubular fluid

Reabsorption at DCT Tubular cells actively transport Na+ and Cl– out of tubular fluid Along distal regions, Tubular cells contain ion pumps that reabsorb Na+ in exchange for another cation (usually K+)

Figure 26–13a Tubular Secretion and Solute Reabsorption by the DCT.
Sodium and chloride ion reabsorption along entire length of DCT Distal convoluted tubule Tubular fluid Glomerulus K+ Glomerular capsule Cells of distal convoluted tubule Cl– Na+ Proximal convoluted tubule Collecting duct Nephron loop Urine storage and elimination Peritubular fluid Cl– Na+ K+ Peritubular capillary KEY Leak channel Cotransport Countertransport Cl– Na+ Diffusion Exchange pump Reabsorption A Aldosterone- regulated pump Secretion a The basic pattern of the reabsorption of sodium and chloride ions and the secretion of potassium ions.

Aldosterone Hormone produced by adrenal cortex Stimulates synthesis and incorporation of Na+ pumps and channels In plasma membranes along DCT and collecting duct Reduces Na+ lost in urine

Figure 26–13b Tubular Secretion and Solute Reabsorption by the DCT.
Sodium–potassium exchange in aldosterone- sensitive segment of DCT and collecting duct Tubular fluid K+ A Na+ Sodium ions are reabsorbed in exchange for potassium ions when these ion pumps are stimulated by aldosterone (A). K+ A Na+ KEY Leak channel Cotransport Countertransport K+ Na+ Diffusion Exchange pump Reabsorption A Aldosterone- regulated pump Secretion b Aldosterone-regulated reabsorption of sodium ions, linked to the passive loss of potassium ions.

Hypokalemia Dangerous reduction in plasma K+ concentration Produced by prolonged aldosterone stimulation Atrial natriuretic peptide (ANP) Opposes secretion of aldosterone And its actions on DCT and collecting system Parathyroid hormone Regulates calcium ion reabsorption at DCT

Secretion at 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 concentrations lead to higher rates of secretion

Potassium ion secretion Tubular cells exchange Na+ in tubular fluid For excess K+ in body fluids Potassium ions diffuse into lumen of DCT through K+ leak channels At apical surfaces of tubular cells

Hydrogen ion secretion Hydrogen ions generated by dissociation of carbonic acid Are secreted by sodium ion–linked countertransport In exchange for Na+ in tubular fluid Bicarbonate ions diffuse into bloodstream Help prevent changes in plasma pH

Hydrogen ion secretion Acidifies tubular fluid Elevates blood pH Accelerates when blood pH falls pH of blood decreases in Metabolic acidosis (lactic acidosis) Can develop after exhaustive muscle activity Ketoacidosis Can develop in starvation or diabetes mellitus

Figure 26–13c Tubular Secretion and Solute Reabsorption by the DCT.
Tubular fluid CO2 Cl– H+ NH4+ Cl– Cl– Na+ Na+ NH4+ CO2 H2CO3 H + Amino acid deamination H2O HCO3 – HCO3– Cl– Cl– H+ HCO3– HCO3– Na+ KEY Leak channel Cotransport Countertransport HCO3– Na+ HCO3– Diffusion H+ Exchange pump Reabsorption A Aldosterone- regulated pump Secretion c Hydrogen ion secretion and the acidification of urine occur by two routes. The central theme is the exchange of hydrogen ions in the cytosol for sodium ions in the tubular fluid, and the reabsorption of the bicarbonate ions generated in the process.

Control of blood pH Accomplished by H+ removal and bicarbonate ion production at kidneys Aldosterone stimulates H+ secretion Prolonged aldosterone stimulation can cause alkalosis Abnormally high blood pH

In response to acidosis, PCT and DCT deaminate amino acids Ties up H+ and yields ammonium ions (NH4+) and bicarbonate ions (HCO3–) Ammonium ions are pumped into tubular fluid Bicarbonate ions enter bloodstream via peritubular fluid Benefits of tubular deamination Provides carbon chains for catabolism Generates bicarbonate ions to buffer plasma

Reabsorption and secretion in collecting system Collecting ducts Receive tubular fluid from many nephrons Carry it toward renal sinus Hormones regulate water and solute loss Aldosterone controls sodium ion pumps Actions are opposed by ANP ADH controls permeability to water Secretion is suppressed by ANP

Reabsorption in collecting system Sodium ion reabsorption Na+ exchanged for K+ Bicarbonate ion reabsorption HCO3– exchanged for Cl– Urea reabsorption By diffusion

Secretion in collecting system Controls pH of body fluids Low pH in peritubular fluid Carrier proteins pump H+ into tubular fluid and reabsorb bicarbonate ions High pH in peritubular fluid (uncommon) Collecting system secretes bicarbonate ions and pumps H+ into peritubular fluid

26-6 Countercurrent Multiplication
Exchange between fluids moving in opposite directions Multiplication Effect of exchange increases as movement of fluid continues

Occurs between two parallel limbs of nephron loop Descending thin limb Thick ascending limb These segments Are very close together Are separated only by peritubular fluid Have very different permeability characteristics

Descending thin limb Permeable to water Relatively impermeable to solutes Thick ascending limb Relatively impermeable to water and solutes Contains active transport mechanisms Pump Na+ and Cl− from tubular fluid into peritubular fluid of medulla

Overview of countercurrent multiplication Na+ and Cl− are pumped out of thick ascending limb Elevates osmotic concentration in peritubular fluid around descending thin limb Water flows out of descending thin limb into peritubular fluid Increasing solute concentration of tubular fluid in descending thin limb

Overview of countercurrent multiplication A concentrated solution arrives in thick ascending limb Accelerates transport of Na+ and Cl− into peritubular fluid Solute pumping out of thick ascending limb Increases solute concentration in tubular fluid in descending thin limb Accelerates solute pumping in thick ascending limb

Medullary osmotic gradient Concentration gradient created in peritubular fluid of medulla Active transport at apical surface of thick ascending limb Moves Na+, K+, and Cl– out of tubular fluid Uses Na+–K+/2 Cl− transporter Each cycle of pump carries 1 sodium, 1 potassium, and 2 chloride ions into tubular cell

Medullary osmotic gradient Cotransport carriers pump K+ and Cl– into peritubular fluid K+ is removed by sodium–potassium exchange pump And diffuses back into lumen of tubule through K+ leak channels Net result is that Na+ and Cl– enter peritubular fluid of medulla, raising osmotic concentration Water moves out of descending thin limb Osmotic concentration of tubular fluid increases

Figure 26–14a Countercurrent Multiplication and Urine Concentration.
Proximal convoluted tubule Distal convoluted tubule Tubular fluid Urea Cells of thick ascending limb 2 Cl– Glomerulus K+ This plasma membrane is impermeable to water Na+ 2 Cl– K+ Glomerular capsule KEY Cotransport Collecting duct Exchange pump Reabsorption Nephron loop Peritubular fluid Secretion Cl– K+ Na+ K+ Diffusion KEY a The mechanism of sodium and chloride ion transport involves the Na+– K+/2 Cl– carrier at the apical surface and two carriers at the basal surface of the tubular cell: a potassium–chloride cotransport pump and a sodium–potassium exchange pump. The net result is the transport of sodium and chloride ions into the peritubular fluid. Water reabsorption Solute reabsorption Urine storage and elimination

Figure 26–14b Countercurrent Multiplication and Urine Concentration.
300 300 100 mOsm/L + – Na Cl + – Na Cl Descending thin limb (permeable to water; impermeable to solutes) Na Cl + – Na Cl + – 300 600 600 Na Cl + – Na Cl + – H2O 900 900 KEY Thick ascending limb (impermeable to water; active solute transport) Impermeable to water Impermeable to solutes H2O 900 Impermeable to urea; variable permeability to water H2O Permeable to urea 1200 1200 Renal medulla b Transport of NaCl along the thick ascending limb results in the movement of water from the descending thin limb.

Thick ascending limb Has a highly effective pumping mechanism Almost 2/3 of Na+ and Cl− are pumped out of tubular fluid Fluid arriving at DCT has an osmotic concentration of 100 mOsm/L 1/3 the concentration of peritubular fluid of renal cortex

Thick ascending limb Rate of ion transport is proportional to an ion’s concentration in tubular fluid More Na+ and Cl− are pumped into medulla at start of thick ascending limb Where their concentrations are highest Regional difference in rate of ion transport Creates osmotic gradient in medulla

Medullary osmotic gradient Maximum solute concentration of peritubular fluid near turn of nephron loop is about 1200 mOsm/L Na+ and Cl− pumped out of thick ascending limb make up about 2/3 of that (750 mOsm/L) Remainder from urea

Role of urea Thick ascending limb, DCT, and collecting ducts are impermeable to urea As water is reabsorbed, concentration of urea in tubular fluid rises Tubular fluid reaching papillary duct contains urea at about 450 mOsm/L Papillary ducts are permeable to urea Thus, urea concentration in deepest parts of medulla averages 450 mOsm/L

Figure 26–14c Countercurrent Multiplication and Urine Concentration.
Renal cortex 100 DCT and collecting ducts (impermeable to urea; variable permeability to water) 300 300 300 Descending thin limb (permeable to water; impermeable to urea) 300 H2O 600 H2O H2O H2O H2O 900 900 KEY Impermeable to water Papillary duct (permeable to urea) Impermeable to solutes H2O Impermeable to urea; variable permeability to water 1200 1200 1200 Permeable to urea KEY Na+ – Cl Renal medulla Urea c The permeability characteristics of both the loop and the collecting duct tend to concentrate urea in the tubular fluid and in the medulla. The nephron loop, DCT, and collecting duct are impermeable to urea. As water reabsorption occurs, the urea concentration increases. Papillary duct permeability to urea results in urea making up nearly one-third of the solutes in the deepest portions of the medulla.

Regulation of urine volume and concentration Through control of water reabsorption Water is reabsorbed by osmosis in Proximal convoluted tubule Descending limb of nephron loop Water permeabilities of these regions cannot be adjusted

Water reabsorption Occurs when osmotic concentration of peritubular fluid exceeds that of tubular fluid Ascending limb of nephron loop is impermeable to water In distal convoluted tubule and collecting system, 1–2 percent of water volume in original filtrate is recovered during sodium ion reabsorption

Obligatory water reabsorption Water movement that cannot be prevented Usually recovers 85 percent of filtrate volume Facultative water reabsorption Controls volume of water reabsorbed along DCT and collecting system 15 percent of filtrate volume (27 liters/day) These segments are relatively impermeable to water except in presence of ADH

ADH Hormone that causes insertion of aquaporins (special water channels) 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 Without ADH All fluid reaching DCT is lost in urine Large amounts of dilute urine are produced

Osmotic concentration Of tubular fluid arriving at DCT 100 mOsm/L In presence of ADH, Water moves out of DCT Tubular fluid reaches 300 mOsm/L In minor calyx Approaches 1200 mOsml/L depending on how much ADH is present

Figure 26–15a The Effects of ADH on the DCT and Collecting Duct.
Renal cortex PCT DCT Glomerulus + Obligatory Water Reabsorption Facultative Water Reabsorption Na+Cl– Na+Cl– KEY Distal convoluted tubule = Na+/Cl– transport Glomerulus H2O Na+Cl– Na+Cl– ADH = Antidiuretic hormone H2O H2O Na+Cl– Na+Cl– = Water reabsorption H2O Na+Cl– Na+Cl– = Variable water reabsorption Glomerular capsule H2O Collecting duct Proximal convoluted tubule Solutes = Impermeable to solutes H2O = Impermeable to water Renal medulla Collecting duct = Variable permeability to water Nephron loop a Tubule permeabilities and the osmotic concentration of urine without ADH Large volume of dilute urine Urine storage and elimination

Figure 26–15b The Effects of ADH on the DCT and Collecting Duct.
Renal cortex ADH H2O H2O H2O KEY Na+Cl– Na+Cl– = Na+/Cl– transport ADH H2O Na+Cl– Na+Cl– ADH H2O = Antidiuretic hormone H2O H2O H2O ADH Na+Cl– Na+Cl– = Water reabsorption H2O H2O Na+Cl– Na+Cl– = Variable water reabsorption H2O ADH H2O H2O = Impermeable to solutes H2O H2O H2O = Impermeable to water H2O = Variable permeability to water Renal medulla b Tubule permeabilities and the osmotic concentration of urine with ADH Small volume of concentrated urine

Hypothalamus Continuously secretes low levels of ADH Thus, DCT and collecting system are always permeable to water A healthy adult produces 1200 mL of urine per day (about 0.6 percent of filtrate volume) With osmotic concentration of 855–1335 mOsm/L

Vasa recta returns reabsorbed solutes and water to general circulation Without disrupting medullary osmotic gradient Process is called countercurrent exchange As blood in vasa recta descends into medulla, it increases in osmotic concentration Some solutes absorbed in descending portion do not diffuse out in ascending portion More water moves into ascending portion than is moved out of descending portion

Summary of renal function Step 1: glomerulus Filtrate produced at renal corpuscle has same osmotic concentration as plasma (300 mOsm/L) Lacks plasma proteins Step 2: proximal convoluted tubule (PCT) Active removal of ions and organic nutrients Produces osmotic water flow out of tubular fluid Keeps solutions inside and outside tubule isotonic 60–70 percent of filtrate volume is absorbed here

Summary of renal function Step 3: PCT and descending limb Water moves into peritubular fluid Compulsory reabsorption of water results in small volume of highly concentrated tubular fluid Step 4: thick ascending limb Tubular cells actively transport Na+ and Cl– out Urea accounts for higher proportion of total solute concentration at end of nephron loop

Summary of renal function Step 5: DCT and collecting system Further adjustments in composition of tubular fluid Solute concentration is adjusted through active transport (reabsorption or secretion) Step 6: DCT and collecting system Final adjustments in volume and solute concentration of tubular fluid ADH levels determine final urine volume and concentration

Summary of renal function Step 7: vasa recta Absorbs solutes and water reabsorbed by nephron loop and collecting ducts Maintains concentration gradient of medulla Step 8: papillary duct Permeable to urea, where its concentration is about 450 mOsm/L Same value as in deepest parts of medulla

Figure 26–16 Summary of Renal Function (Part 1 of 2).

Figure 26–16 Summary of Renal Function (Part 2 of 2).

Composition of urine Reflects filtration, reabsorption, and secretion activities of nephrons Some substances (such as urea) are neither actively excreted nor reabsorbed Organic nutrients are completely reabsorbed Other compounds missed by filtration (example: creatinine) are actively secreted into tubular fluid

Normal urine Clear, sterile liquid Concentration of substances Depends on osmotic movement of water across walls of tubules and collecting ducts Yellow color is due to urobilin (pigment) Generated in kidneys from urobilinogens Urinalysis (analysis of a urine sample) Important diagnostic tool

Table 26–5 General Characteristics of Normal Urine

Creatinine clearance Compares creatinine level in urine with that in blood Used to estimate GFR A more accurate GFR can be obtained using inulin A carbohydrate that is not metabolized BUN (blood urea nitrogen) test Measures amount of urea in blood Should be low

26-7 Urine Transport, Storage, Elimination
Filtrate modification and urine production End when fluid enters renal pelvis Urine transport, storage, and elimination Take place in urinary tract Ureters, urinary bladder, urethra Pyelogram Image of urinary system Obtained by taking an X-ray after radiopaque dye has been administered intravenously

Minor calyx Major calyx
Figure 26–17 A Pyelogram. 11th and 12th ribs Minor calyx Major calyx Ureter Urinary bladder Renal pelvis Kidney

Minor and major calyces, renal pelvis, ureters, urinary bladder, and proximal portion of urethra Lined by transitional epithelium Undergo cycles of distention and relaxation without damage

Ureters Pair of muscular tubes Extend from kidneys to urinary bladder Begin at renal pelvis and pass over psoas major Retroperitoneal Penetrate posterior wall of urinary bladder at oblique angle Ureteric orifices are slit-like rather than rounded Helps prevent backflow of urine when urinary bladder contracts

Wall of each ureter has three layers Inner mucosa Transitional epithelium and lamina propria Middle muscular layer Longitudinal and circular bands of smooth muscle Outer connective tissue layer Continuous with fibrous capsule and peritoneum

A transverse section through the ureter.
Figure 26–19a The Histology of the Ureter, Urinary Bladder, and Urethra. Mucosa Transitional epithelium Lamina propria Smooth muscle Outer connective tissue layer Ureter LM × 70 a A transverse section through the ureter.

Peristaltic contractions Occur about every 30 seconds Begin at renal pelvis Sweep along ureter Forcing urine toward urinary bladder Urinary bladder Hollow, muscular organ Serves as temporary storage for urine Full bladder can contain 1 liter of urine

Bladder position Stabilized by several peritoneal folds Posterior, inferior, and anterior surfaces Lie outside peritoneal cavity Ligamentous bands Anchor urinary bladder to pelvic and pubic bones

Figure 26–18a Organs for Conducting and Storing Urine.
Left ureter Rectum Peritoneum Urinary bladder Pubic symphysis Prostate External urethral sphincter Spongy urethra Urethra [see part c] External urethral orifice a Male

Figure 26–18b Organs for Conducting and Storing Urine.
Rectum Right ureter Uterus Peritoneum Urinary bladder Pubic symphysis Internal urethral sphincter Urethra External urethral sphincter Vestibule Vagina b Female

Umbilical ligaments of bladder Median umbilical ligament Extends from anterior, superior border Toward umbilicus (navel) Lateral umbilical ligaments Pass along sides of bladder to umbilicus Vestiges of two umbilical arteries

Figure 26–18c Organs for Conducting and Storing Urine.
Median umbilical ligament Ureter Lateral umbilical ligament Detrusor Rugae Ureteric orifices Center of trigone Neck of urinary bladder Internal urethral sphincter Prostate External urethral sphincter Prostatic urethra Membranous urethra c Urinary bladder in male

Mucosa Lines urinary bladder Has folds (rugae) that disappear as bladder fills Trigone of urinary bladder Triangular area bounded by Openings of ureters Entrance to urethra Acts as a funnel Channels urine into urethra when bladder contracts

Urethral entrance Lies at apex of trigone At most inferior point in urinary bladder Neck of urinary bladder Region surrounding urethral opening Contains a muscular internal urethral sphincter Provides involuntary control of urine discharge

Urinary bladder innervation Postganglionic fibers From ganglia in hypogastric plexus Parasympathetic fibers From intramural ganglia controlled by pelvic nerves

Wall of urinary bladder contains mucosa, submucosa, and muscular layers Muscular layer consists of detrusor muscle Internal and external layers of longitudinal smooth muscle separated by a layer of circular muscle

The wall of the urinary bladder.
Figure 26–19b The Histology of the Ureter, Urinary Bladder, and Urethra. Mucosa Transitional epithelium Lamina propria Submucosa Detrusor Visceral peritoneum Urinary bladder LM × 36 b The wall of the urinary bladder.

Urethra Transports urine from neck of urinary bladder To exterior of body Lamina propria is thick and elastic Mucosa has longitudinal folds Mucus-secreting cells lie in epithelial pockets

Male urethra Extends from neck of urinary bladder to tip of penis (18–20 cm) Prostatic urethra passes through center of prostate Membranous urethra includes short segment that penetrates deep transverse perineal muscle Spongy urethra extends from distal border of deep transverse perineal muscle To external urethral orifice at tip of penis

Figure 26–18a Organs for Conducting and Storing Urine.
Left ureter Rectum Peritoneum Urinary bladder Pubic symphysis Prostate External urethral sphincter Spongy urethra Urethra [see part c] External urethral orifice a Male

Female urethra Very short (3–5 cm) Extends from bladder to vestibule External urethral orifice is near anterior wall of vagina

Figure 26–18b Organs for Conducting and Storing Urine.
Rectum Right ureter Uterus Peritoneum Urinary bladder Pubic symphysis Internal urethral sphincter Urethra External urethral sphincter Vestibule Vagina b Female

External urethral sphincter In both sexes Circular band of skeletal muscle Acts as a valve Under voluntary control Via perineal branch of pudendal nerve Has resting muscle tone Voluntary relaxation permits urination

Male urethra Epithelial mucous glands May form tubules that extend into lamina propria Connective tissues of lamina propria Anchor urethra to surrounding structures Female urethra Lamina propria contains extensive network of veins Entire complex is surrounded by concentric layers of smooth muscle

A transverse section through the female
Figure 26–19c The Histology of the Ureter, Urinary Bladder, and Urethra. Lumen of urethra Smooth muscle Stratified squamous epithelium of mucosa Lamina propria containing mucous epithelial glands Female urethra LM × 50 c A transverse section through the female urethra. A thick layer of smooth muscle surrounds the lumen.

Urinary reflexes Urine storage reflex Involves spinal reflexes and pontine storage center Afferent impulses from stretch receptors in urinary bladder stimulate sympathetic outflow Inhibiting contraction of detrusor Stimulating contraction of internal urethral sphincter Pontine storage center inhibits urination by Decreasing parasympathetic activity Contracting external urethral sphincter

Figure 26–20a The Control of Urination.
Pontine storage center Pons Autonomic ganglion Lower thoracic and upper lumbar spinal cord Detrusor contraction inhibited Stretch receptors in bladder Sacral spinal cord Internal urethral sphincter contracts KEY Afferent External urethral sphincter contracts Stimulated Efferent Inhibited a Urine storage reflex Pontine efferent

Urinary reflexes Urine voiding reflex Involves spinal reflexes and pontine micturition center When bladder contains about 200 mL of urine, urge to urinate appears Stretch receptors send impulses to pontine micturition center, initiating sacral spinal reflexes Detrusor contracts Internal and external urethral sphincters relax

Figure 26–20b The Control of Urination.
Pontine micturition center Detrusor contraction less inhibited Detrusor contracts Internal urethral sphincter relaxes KEY External urethral sphincter relaxes Afferent Stimulated Urination occurs Efferent Inhibited b Urine voiding reflex Pontine efferent

Infants Lack voluntary control over urination Necessary corticospinal connections are not yet established Incontinence Inability to control urination voluntarily May be caused by trauma to internal or external urethral sphincter

26-8 Effects of Aging on the Urinary System
Age-related changes Nephrolithiasis (formation of calculi) Decrease in number of functional nephrons Reduction in GFR Reduced sensitivity to ADH Problems with urinary reflexes External sphincter loses muscle tone Control of urination can be lost due to a stroke, Alzheimer’s disease, and other CNS problems In males, urinary retention may develop

26-9 Waste Excretion Excretory system
Includes systems with excretory functions that affect composition of body fluids Urinary system Integumentary system Respiratory system Digestive system

Figure 26–21 Integration of the URINARY system with the other body systems presented so far.
Integumentary System Nervous System • The Integumentary System prevents excessive fluid loss through skin surface; produces vitamin D3, important for the renal production of calcitriol; sweat glands assist in elimination of water and solutes • The Nervous System adjusts renal blood pressure; monitors distension of urinary bladder and controls urination • The urinary system eliminates nitrogenous wastes; maintains fluid, electrolyte, and acid–base balance of blood, which is critical for neural function • The urinary system excretes nitrogenous wastes; maintains fluid, electrolyte, and acid–base balance of blood that nourishes the skin Endocrine System Respiratory System • The Endocrine System produces aldosterone and antidiuretic hormone (ADH), which adjust rates of fluid and electrolyte reabsorption by kidneys • The Respiratory System assists in the regulation of pH by eliminating carbon dioxide • The urinary system releases renin when local blood pressure drops and erythropoietin (EPO) when renal oxygen levels fall • The urinary system assists in the excretion of carbon dioxide; provides bicarbonate buffers that assist in pH regulation Lymphatic System Cardiovascular System • The Lymphatic System provides adaptive (specific) defense against urinary tract infections • The Cardiovascular System delivers blood to glomerular capillaries, where filtration occurs; accepts fluids and solutes reabsorbed during urine production • The urinary system excretes toxins and wastes generated by cellular activities; acid pH of urine provides innate (nonspecific) defense against urinary tract infections • The urinary system releases renin to elevate blood pressure and erythropoietin to accelerate red blood cell production Digestive System Skeletal 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 Skeletal System provides some protection for kidneys and ureters with its axial divison; pelvis protects urinary bladder and proximal portion of urethra • The urinary system excretes toxins absorbed by the digestive epithelium; excretes bilirubin and nitrogenous wastes from the liver; calcitriol production by kidneys aids calcium and phosphate absorption • The urinary system conserves calcium and phosphate needed for bone growth Muscular System • The Muscular System controls urination by closing urethral sphincters. Muscle layers of trunk provide some protection for urinary organs Urinary System The urinary system excretes metabolic wastes and maintains normal body fluid pH and ion composition. It: • The urinary system excretes waste products of muscle and protein metabolism; assists in regulation of calcium and phosphate concentrations • regulates blood volume and blood pressure • regulates plasma concentrations of sodium, potassium, chloride, and other ions • helps to stabilize blood pH • conserves valuable nutrients

Fundamentals of Anatomy & Physiology

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