© 2016 Pearson Education, Inc.

Why This Matters Understanding the process of how the body eliminates wastes and toxins via urine will help you better understand what is happening when kidneys fail so you can better advise your patients with renal disease. © 2016 Pearson Education, Inc.

Function of Kidneys Kidneys, a major excretory organ, maintain the body’s internal environment by: Regulating total water volume and total solute concentration in water Regulating ion concentrations in extracellular fluid (ECF) Ensuring long-term acid-base balance Excreting metabolic wastes, toxins, drugs Producing erythropoietin and renin (regulate RBC production and blood pressure respectively) © 2016 Pearson Education, Inc.

Function of Kidneys Activating vitamin D Carrying out gluconeogenesis, if needed Kidneys are part of the urinary system, which also includes: Ureters: transport urine from kidneys to urinary bladder Urinary bladder: temporary storage reservoir for urine Urethra: transports urine out of body © 2016 Pearson Education, Inc.

Figure 25.1 The urinary system.
Hepatic veins (cut) Esophagus (cut) Inferior vena cava Renal artery Adrenal gland Renal hilum Aorta Renal vein Kidney Iliac crest Ureter Rectum (cut) Uterus (part of female reproductive system) Urinary bladder Urethra © 2016 Pearson Education, Inc.

Figure 25.2 Dissection of urinary system organs (male).
Kidney Renal artery Renal hilum Renal vein Ureter Urinary bladder © 2016 Pearson Education, Inc.

25.1 Gross Anatomy of Kidneys
Location and External Anatomy Retroperitoneal, in the superior lumbar region Right kidney is crowded by liver, so is lower than left Adrenal (suprarenal) gland sits atop each kidney Convex lateral surface Concave medial surface with vertical renal hilum leads to internal space, renal sinus Ureters, renal blood vessels, lymphatics, and nerves enter and exit at hilum © 2016 Pearson Education, Inc.

Figure 25.3b Position of the kidneys against the posterior body wall.
12th rib © 2016 Pearson Education, Inc.

Figure 25.3a Position of the kidneys against the posterior body wall.
Anterior Inferior vena cava Aorta Peritoneum Peritoneal cavity (organs removed) Supportive tissue layers Renal vein • Renal fascia anterior Renal artery posterior • Perirenal fat capsule • Fibrous capsule Body of vertebra L2 Body wall Posterior © 2016 Pearson Education, Inc.

Internal Gross Anatomy
Internal kidney has three distinct regions Renal cortex: granular-appearing superficial region Renal medulla: deep to cortex, composed of cone-shaped medullary (renal) pyramids Broad base of pyramid faces cortex Papilla, tip of pyramid, points internally Renal pyramids are separated by renal columns, inward extensions of cortical tissue Lobe: medullary pyramid and its surrounding cortical tissue; about eight lobes per kidney © 2016 Pearson Education, Inc.

Internal Gross Anatomy (cont.)
3. Renal pelvis Funnel-shaped tube continuous with ureter Minor calyces Cup-shaped areas that collect urine draining from pyramidal papillae Major calyces Areas that collect urine from minor calyces Empty urine into renal pelvis Urine flow Renal pyramid  minor calyx  major calyx  renal pelvis  ureter © 2016 Pearson Education, Inc.

Figure 25.4 Internal anatomy of the kidney.
Renal hilum Renal cortex Renal medulla Major calyx Papilla of pyramid Renal pelvis Minor calyx Ureter Renal pyramid in renal medulla Renal column Fibrous capsule Photograph of right kidney, frontal section Diagrammatic view © 2016 Pearson Education, Inc.

Clinical – Homeostatic Imbalance 25.2
Pyelitis Infection of renal pelvis and calyces Pyelonephritis Infection or inflammation of entire kidney Infections in females are usually caused by fecal bacteria entering urinary tract Severe cases can cause swelling of kidney and abscess formation, and pus may fill renal pelvis If left untreated, kidney damage may result Normally is successfully treated with antibiotics © 2016 Pearson Education, Inc.

Blood and Nerve Supply Blood: kidneys cleanse blood and adjust its composition, so it has a rich blood supply Renal arteries deliver about one-fourth (1200 ml) of cardiac output to kidneys each minute Arterial flow: renal  segmental  interlobar  arcuate  cortical radiate (interlobular) Venous flow: cortical radiate  arcuate  interlobar  renal veins No segmental veins Nerve supply: via sympathetic fibers from renal plexus © 2016 Pearson Education, Inc.

Figure 25.5a Blood vessels of the kidney.
Cortical radiate vein Cortical radiate artery Arcuate vein Arcuate artery Interlobar vein Interlobar artery Segmental arteries Renal vein Renal artery Renal pelvis Ureter Renal medulla Renal cortex Frontal section illustrating major blood vessels © 2016 Pearson Education, Inc.

Figure 25.5b Blood vessels of the kidney.
Aorta Inferior vena cava Renal artery Renal vein Peritubular capillaries or vasa recta Afferent arteriole Efferent arteriole Glomerulus (capillaries) Nephron-associated blood vessels (see Figure 25.8) Path of blood flow through renal blood vessels © 2016 Pearson Education, Inc.

Renal Corpuscle Two parts of renal corpuscle 1. Glomerulus
Tuft of capillaries composed of fenestrated endothelium Highly porous capillaries Allows for efficient filtrate formation Filtrate: plasma-derived fluid that renal tubules process to form urine © 2016 Pearson Education, Inc.

Renal Corpuscle (cont.)
2. Glomerular capsule Also called Bowman’s capsule: cup-shaped, hollow structure surrounding glomerulus Consists of two layers Parietal layer : simple squamous epithelium Visceral layer : clings to glomerular capillaries; branching epithelial podocytes Extensions terminate in foot processes that cling to basement membrane Filtration slits between foot processes allow filtrate to pass into capsular space © 2016 Pearson Education, Inc.

Figure 25.6 Location and structure of nephrons.
Renal cortex Renal medulla Renal pelvis Glomerular capsule: parietal layer Ureter Basement membrane Podocyte Kidney Fenestrated endothelium of the glomerulus Renal corpuscle • Glomerular capsule • Glomerulus Glomerular capsule: visceral layer Distal convoluted tubule Apical microvilli Mitochondria Highly Proximal convoluted tubule infolded basolateral membrane Proximal convoluted tubule cells Cortex Apical side Medulla Basolateral side Thick segment Distal convoluted tubule cells Thin segment Nephron loop • Descending limb • Ascending limb Collecting duct Nephron loop (thin-segment) cells Principal cell Intercalated cell Collecting duct cells © 2016 Pearson Education, Inc.

Renal Tubule and Collecting Duct
Renal tubule is about 3 cm (1.2 in.) long Consists of single layer of epithelial cells, but each region has its own unique histology and function Three major parts 1. Proximal convoluted tubule Proximal, closest to renal corpuscle 2. Nephron loop 3. Distal convoluted tubule Distal, farthest from renal corpuscle Distal convoluted tubule drains into collecting duct © 2016 Pearson Education, Inc.

Renal Tubule and Collecting Duct (cont.)
1. Proximal convoluted tubule (PCT) Cuboidal cells with dense microvilli that form brush border Increase surface area Also have large mitochondria Functions in reabsorption and secretion Confined to cortex © 2016 Pearson Education, Inc.

2. Nephron loop Formerly called loop of Henle U-shaped structure consisting of two limbs Descending limb Proximal part of descending limb is continuous with proximal tubule Distal portion also called descending thin limb; simple squamous epithelium Ascending limb Thick ascending limb Thin in some nephrons Cuboidal or columnar cells © 2016 Pearson Education, Inc.

Figure 25.7 Renal cortical tissue.
Renal corpuscle Proximal convoluted tubule (fuzzy lumen due to long microvilli) • Squamous epithelium of parietal layer of glomerular capsule • Glomerular capsular space Distal convoluted tubule (clear lumen) • Glomerulus © 2016 Pearson Education, Inc.

Collecting ducts Collecting ducts receive filtrate from many nephrons Run through medullary pyramids Give pyramids their striped appearance Ducts fuse together to deliver urine through papillae into minor calyces © 2016 Pearson Education, Inc.

Figure 25.6 Location and structure of nephrons.
Renal cortex Renal medulla Renal pelvis Glomerular capsule: parietal layer Ureter Basement membrane Podocyte Kidney Fenestrated endothelium of the glomerulus Renal corpuscle • Glomerular capsule • Glomerulus Glomerular capsule: visceral layer Distal convoluted tubule Apical microvilli Mitochondria Highly Proximal convoluted tubule infolded basolateral membrane Proximal convoluted tubule cells Cortex Apical side Medulla Basolateral side Thick segment Distal convoluted tubule cells Thin segment Nephron loop • Descending limb • Ascending limb Collecting duct Nephron loop (thin-segment) cells Principal cell Intercalated cell Collecting duct cells © 2016 Pearson Education, Inc.

Classes of Nephrons Two major groups of nephrons Cortical nephrons
Make up 85% of nephrons Almost entirely in cortex Juxtamedullary nephrons Long nephron loops deeply invade medulla Ascending limbs have thick and thin segments Important in production of concentrated urine © 2016 Pearson Education, Inc.

Juxtamedullary nephron
Figure 25.8 Cortical and juxtamedullary nephrons, and their blood vessels. Cortical nephron Juxtamedullary nephron • Short nephron loop • Glomerulus further from the cortex-medulla junction • Efferent arteriole supplies peritubular capillaries • Long nephron loop • Glomerulus closer to the cortex-medulla junction • Efferent arteriole supplies vasa recta Glomerulus (capillaries) Efferent arteriole Cortical radiate vein Cortical radiate artery Renal corpuscle Glomerular capsule Afferent arteriole Collecting duct Proximal convoluted tubule Distal convoluted tubule Afferent arteriole Efferent arteriole Peritubular capillaries Ascending limb of nephron loop Cortex-medulla junction Arcuate vein Kidney Vasa recta Arcuate artery Nephron loop Descending limb of nephron loop © 2016 Pearson Education, Inc.

Nephron Capillary Beds
Renal tubules are associated with two capillary beds Glomerulus Peritubular capillaries Juxtamedullary nephrons are associated with Vasa recta © 2016 Pearson Education, Inc.

Nephron Capillary Beds (cont.)
Glomerulus Capillaries are specialized for filtration Different from other capillary beds because they are fed and drained by arteriole Afferent arteriole enters glomerulus and leaves via efferent arteriole Afferent arteriole arises from cortical radiate arteries Efferent feed into either peritubular capillaries or vasa recta Blood pressure in glomerulus high because: Afferent arterioles are larger in diameter than efferent arterioles Arterioles are high-resistance vessels © 2016 Pearson Education, Inc.

Nephron Capillary Beds (cont.)
Peritubular capillaries Around the proximal & distal convoluted tubules Water & many solutes moves from tubules into peritubiular capillaries; some solutes (but not water) move from peritubular caps to tubules Vasa recta Long, thin-walled vessels parallel to long nephron loops of juxtamedullary nephrons Function in formation of concentrated urine © 2016 Pearson Education, Inc.

Juxtamedullary nephron
Figure 25.8 Cortical and juxtamedullary nephrons, and their blood vessels. Cortical nephron Juxtamedullary nephron • Short nephron loop • Glomerulus further from the cortex-medulla junction • Efferent arteriole supplies peritubular capillaries • Long nephron loop • Glomerulus closer to the cortex-medulla junction • Efferent arteriole supplies vasa recta Glomerulus (capillaries) Efferent arteriole Cortical radiate vein Cortical radiate artery Renal corpuscle Glomerular capsule Afferent arteriole Collecting duct Proximal convoluted tubule Distal convoluted tubule Afferent arteriole Efferent arteriole Peritubular capillaries Ascending limb of nephron loop Cortex-medulla junction Arcuate vein Kidney Vasa recta Arcuate artery Nephron loop Descending limb of nephron loop © 2016 Pearson Education, Inc.

Juxtaglomerular Complex (JGC)
Each nephron has one juxtaglomerular complex (JGC) Involves modified portions of: Distal portion of ascending limb of nephron loop Afferent (sometimes efferent) arteriole Important in regulating rate of filtrate formation and blood pressure © 2016 Pearson Education, Inc.

Juxtaglomerular Complex (JGC) (cont.)
JGC includes: Macula densa Cells contain chemoreceptors that sense NaCl content of filtrate Granular cells (juxtaglomerular, or JG cells) Enlarged, smooth muscle cells of arteriole Act as mechanoreceptors to sense blood pressure in afferent arteriole Secrete renin, an enzyme of the RAA system that regulates BP Extraglomerular mesangial cells May pass signals between macula densa and granular cells © 2016 Pearson Education, Inc.

Figure 25.10 Juxtaglomerular complex (JGC) of a nephron.
capsule Efferent arteriole Glomerulus Parietal layer of glomerular capsule Foot processes of podocytes Podocyte cell body (visceral layer) Capsular space Red blood cell Afferent arteriole Efferent arteriole Proximal tubule cell Juxtaglomerular complex • Macula densa cells of the ascending limb of nephron loop • Extraglomerular mesangial cells Lumens of glomerular capillaries • Granular cells Endothelial cell of glomerular capillary Afferent arteriole Glomerular mesangial cells Juxtaglomerular complex Renal corpuscle © 2016 Pearson Education, Inc.

25.3 Physiology of Kidney 180 L of fluid processed daily, but only 1.5 L of urine is formed Kidneys filter body’s entire plasma volume 60 times each day Consume 20–25% of oxygen used by body at rest Filtrate (produced by glomerular filtration) is basically blood plasma minus proteins Urine is produced from filtrate Urine <1% of original filtrate Contains metabolic wastes and unneeded substances © 2016 Pearson Education, Inc.

25.3 Physiology of Kidney Three processes are involved in urine formation and adjustment of blood composition: 1. Glomerular filtration: produces cell- and protein-free filtrate 2. Tubular reabsorption: selectively returns 99% of substances from filtrate to blood in renal tubules and collecting ducts 3. Tubular secretion: selectively moves substances from blood to filtrate in renal tubules and collecting ducts © 2016 Pearson Education, Inc.

Figure 25.11 The three major renal processes.
Afferent arteriole Glomerular capillaries Efferent arteriole Cortical radiate artery Glomerular capsule 1 Renal tubule and collecting duct containing filtrate Peritubular capillary 2 3 Three major renal processes: To cortical radiate vein 1 Glomerular filtration 2 Tubular reabsorption 3 Tubular secretion Urine © 2016 Pearson Education, Inc.

25.4 Step 1: Glomerular Filtration
Glomerular filtration is a passive process No metabolic energy required Hydrostatic pressure forces fluids and solutes through filtration membrane into glomerular capsule No reabsorption into capillaries of glomerulus occurs © 2016 Pearson Education, Inc.

The Filtration Membrane
Porous membrane between blood and interior of glomerular capsule Allows water and solutes smaller than plasma proteins to pass Normally no cells can pass Contains three layers Fenestrated endothelium of glomerular capillaries Basement membrane: fused basal laminae of two other layers Foot processes of podocytes with filtration slits; slit diaphragms repel macromolecules © 2016 Pearson Education, Inc.

Figure 25.12a The filtration membrane.
Glomerular capsular space Efferent arteriole Afferent arteriole Proximal convoluted tubule Glomerular capillary covered by podocytes that form the visceral layer of glomerular capsule Parietal layer of glomerular capsule Renal corpuscle © 2016 Pearson Education, Inc.

Figure 25.12b The filtration membrane.
Cytoplasmic extensions of podocytes Filtration slits Podocyte cell body Fenestrations (pores) Glomerular capillary endothelium (podocyte covering and basement membrane removed) Foot processes of podocyte Glomerular capillary surrounded by podocytes © 2016 Pearson Education, Inc.

Figure 25.12d The filtration membrane.
Capillary Filtration membrane • Capillary endothelium • Basement membrane • Foot processes of podocyte of glomerular capsule Filtration slit Slit diaphragm Plasma Filtrate in capsular space Fenestration (pore) Foot processes of podocyte Three layers of the filtration membrane © 2016 Pearson Education, Inc.

Pressures That Affect Filtration
Outward pressures Forces that promote filtrate formation Hydrostatic pressure in glomerular capillaries (HPgc) is essentially glomerular blood pressure Chief force pushing water, solutes out of blood Quite high: 55 mm Hg Compared to ~ 26 mm Hg seen in most capillary beds Reason is that efferent arteriole is a high-resistance vessel with a diameter smaller than afferent arteriole © 2016 Pearson Education, Inc.

Pressures That Affect Filtration (cont.)
Inward Pressures Forces inhibiting filtrate formation: Hydrostatic pressure in capsular space (HPcs): filtrate pressure in capsule; 15 mm Hg Colloid osmotic pressure in capillaries (OPgc): “pull” of proteins in blood; 30 mm Hg Net filtration pressure (NFP): sum of forces 55 mm Hg forcing out minus 45 mm Hg opposing = net outward force of 10 mm Hg Pressure responsible for filtrate formation Main controllable factor determining glomerular filtration rate (GFR) © 2016 Pearson Education, Inc.

Figure 25.13 Forces determining net filtration pressure (NFP).
Glomerular capsule Efferent arteriole HPgc = 55 mm Hg OPgc = 30 mm Hg Afferent arteriole HPcs = 15 mm Hg NFP = Net filtration pressure = outward pressures – inward pressures = (HPgc) – (HPcs + OPgc) = (55) – ( ) = 10 mm Hg © 2016 Pearson Education, Inc.

Glomerular Filtration Rate (GFR)
GFR = volume of filtrate formed per minute by both kidneys (normal = 120–125 ml/min) GFR is directly proportional to: Net filtration pressure (NFP) Primary pressure is glomerular hydrostatic pressure Total surface area available for filtration Glomerular mesangial cells control by contracting Filtration membrane permeability Much more permeable than other capillaries © 2016 Pearson Education, Inc.

Regulation of Glomerular Filtration
GFR affects systemic blood pressure Increased GFR causes increased urine output, which lowers blood pressure, and vice versa Goal of extrinsic controls: maintain systemic blood pressure Nervous system and endocrine mechanisms are main extrinsic controls © 2016 Pearson Education, Inc.

Regulation of Glomerular Filtration (cont.)
Extrinsic controls: Neural and hormonal mechanisms Purpose of extrinsic controls is to regulate GFR to maintain systemic blood pressure Extrinsic controls will override renal intrinsic controls if blood volume needs to be increased Sympathetic nervous system Under normal conditions at rest Renal blood vessels dilated Renal autoregulation mechanisms prevail © 2016 Pearson Education, Inc.

Sympathetic nervous system (cont.) Under abnormal conditions, such as extremely low ECF volume (low blood pressure) Norepinephrine is released by sympathetic nervous system and epinephrine is released by adrenal medulla, causing: Systemic vasoconstriction, which increases blood pressure Constriction of afferent arterioles, which decreases GFR Blood volume and pressure increases © 2016 Pearson Education, Inc.

Renin-angiotensin-aldosterone mechanism Main mechanism for increasing blood pressure Three pathways to renin release by granular cells Direct stimulation of granular cells by sympathetic nervous system Stimulation by activated macula densa cells when filtrate NaCl concentration is low Reduced stretch of granular cells © 2016 Pearson Education, Inc.

afferent arterioles; GFR Inhibits baroreceptors peripheral resistance
Figure Regulation of glomerular filtration rate (GFR) in the kidneys. SYSTEMIC BLOOD PRESSURE Blood pressure in afferent arterioles; GFR GFR Granular cells of juxtaglomerular complex of kidney Inhibits baroreceptors in blood vessels of systemic circulation Release Stretch of smooth muscle in walls of afferent arterioles Filtrate flow and NaCl in ascending limb of nephron loop Renin Sympathetic nervous system Catalyzes cascade resulting in formation of Acts on Vasodilation of afferent arterioles Angiotensin II Macula densa cells of juxtaglomerular complex of kidney Aldosterone secretion by adrenal cortex Vasoconstriction of systemic arterioles; peripheral resistance Release of vasoactive chemicals inhibited Na+ reabsorption by kidney tubules; water follows Vasodilation of afferent arterioles Blood volume GFR Systemic blood pressure Myogenic mechanism of autoregulation Tubuloglomerular mechanism of autoregulation Hormonal (renin-angiotensin-aldosterone) mechanism Neural controls Intrinsic mechanisms directly regulate GFR despite moderate changes in blood pressure (between 80 and 180 mm Hg mean arterial pressure). Extrinsic mechanisms indirectly regulate GFR by maintaining systemic blood pressure, which drives filtration in the kidneys. © 2016 Pearson Education, Inc.

Anuria: abnormally low urinary output (less than 50 ml/day) May indicate that glomerular blood pressure is too low to cause filtration Renal failure and anuria can also result from situations in which nephrons stop functioning Example: acute nephritis, transfusion reactions, and crush injuries © 2016 Pearson Education, Inc.

25.5 Step 2: Tubular Reabsorption
Tubular reabsorption quickly reclaims most of tubular contents and returns them to blood Selective transepithelial process Almost all organic nutrients are reabsorbed Water and ion reabsorption is hormonally regulated and adjusted Includes active and passive tubular reabsorption Substances can follow two routes: Transcellular Paracellular © 2016 Pearson Education, Inc.

25.5 Step 2: Tubular Reabsorption
Transcellular route Solute enters apical membrane of tubule cells Travels through cytosol of tubule cells Exits basolateral membrane of tubule cells Enters blood through endothelium of peritubular capillaries Paracellular route Between tubule cells Limited by tight junctions, but leaky in proximal nephron Water, Ca2+, Mg2+, K+, and some Na+ in the PCT move via this route © 2016 Pearson Education, Inc.

Peri- tubular capillary
Figure Transcellular and paracellular routes of tubular reabsorption. Slide 1 The transcellular route involves: Filtrate in tubule lumen Tubule cell Interstitial fluid Peri- tubular capillary Transport across the apical membrane. 1 Tight junction Lateral intercellular space 2 Diffusion through the cytosol. Transport across the basolateral membrane. (Often involves the lateral intercellular spaces because membrane transporters transport ions into these spaces.) 3 3 4 H2O and solutes 1 2 3 4 Transcellular route Movement through the interstitial fluid and into the capillary. 4 Apical membrane Capillary endothelial cell The paracellular route involves: • Movement through leaky tight junctions, particularly in the proximal convoluted tubule. • Movement through the interstitial fluid and into the capillary. Paracellular route H2O and solutes Basolateral membranes © 2016 Pearson Education, Inc.

Tubular Reabsorption of Sodium
Sodium transport across the basolateral membrane Na+ is most abundant cation in filtrate Transport of Na+ across basolateral membrane of tubule cell is via primary active transport Na+-K+ ATPase pumps Na+ into interstitial space Na+ is then swept by bulk flow into peritubular capillaries © 2016 Pearson Education, Inc.

Tubular Reabsorption of Sodium (cont.)
Transport across apical membrane Na+ enters tubule cell at apical surface via secondary active transport (cotransport) or via facilitated diffusion through channels Active pumping of Na+ at basolateral membrane results in strong electrochemical gradient within tubule cell Results in low intracellular Na+ levels that facilitates Na+ diffusion K+ leaks out of cell into interstitial fluid, leaving a net negative charge inside cell, which also acts to pull Na+ inward © 2016 Pearson Education, Inc.

Tubular Reabsorption of Nutrients, Water, and Ions
Na+ reabsorption by primary active transport provides energy and means for reabsorbing almost every other substance Secondary active transport Electrochemical gradient created by pumps at basolateral surface give “push” needed for transport of other solutes Organic nutrients reabsorbed by secondary active transport are cotransported with Na+ Glucose, amino acids, some ions, vitamins © 2016 Pearson Education, Inc.

Tubular Reabsorption of Nutrients, Water, and Ions (cont.)
Passive tubular reabsorption of water Movement of Na+ and other solutes creates osmotic gradient for water Water is reabsorbed by osmosis, aided by water-filled pores called aquaporins Obligatory water reabsorption Aquaporins are always present in PCT Facultative water reabsorption Aquaporins are inserted in collecting ducts only if ADH is present © 2016 Pearson Education, Inc.

Tubular Reabsorption of Nutrients, Water, and Ions (cont.)
Passive tubular reabsorption of solutes Solute concentration in filtrate increases as water is reabsorbed Creates concentration gradients for solutes, which drive their entry into tubule cell and peritubular capillaries Fat-soluble substances, some ions, and urea will follow water into peritubular capillaries down their concentration gradients For this reason, lipid-soluble drugs and environmental pollutants are reabsorbed even though it is not desirable © 2016 Pearson Education, Inc.

Figure 25.16 Reabsorption by PCT cells.
Slide 1 Nucleus At the basolateral membrane, Na+ is pumped into the interstitial space by the Na+-K+ ATPase. Active Na+ transport creates concentration gradients that drive: 1 Filtrate in tubule lumen Interstitial fluid Peri- tubular capillary Tubule cell “Downhill” Na+ entry at the apical membrane. 2 Na+ 2 3Na+ 3Na+ 3 1 Reabsorption of organic nutrients and certain ions by cotransport at the apical membrane. 2K+ 2K+ 3 Glucose Amino acids Some ions Vitamins K+ Reabsorption of water by osmosis through aquaporins. Water reabsorption increases the concentration of the solutes that are left behind. These solutes can then be reabsorbed as they move down their gradients: 4 4 H2O 5 Lipid-soluble substances Lipid-soluble substances diffuse by the transcellular route. 5 6 Various ions and urea Various ions (e.g., Cl−, Ca2+, K+) and urea diffuse by the paracellular route. 6 Tight junction Paracellular route Primary active transport Transport protein Secondary active transport Ion channel Passive transport (diffusion) Aquaporin © 2016 Pearson Education, Inc.

Transport Maximum Transcellular transport systems are specific and limited Transport maximum (Tm) exists for almost every reabsorbed substance Reflects number of carriers in renal tubules that are available When carriers for a solute are saturated, excess is excreted in urine Example: hyperglycemia leads to high blood glucose levels that exceed Tm, and glucose spills over into urine © 2016 Pearson Education, Inc.

Reabsorptive Capabilities of Renal Tubules and Collecting Ducts
Proximal convoluted tubule Site of most reabsorption All nutrients, such as glucose and amino acids, are reabsorbed 65% of Na+ and water reabsorbed Many ions Almost all uric acid About half of urea (later secreted back into filtrate) © 2016 Pearson Education, Inc.

Reabsorptive Capabilities of Renal Tubules and Collecting Ducts (cont
Nephron loop Descending limb: H2O can leave, solutes cannot Ascending limb: H2O cannot leave, solutes can Thin segment is passive to Na+ movement Thick segment has Na+-K+-2Cl– symporters and Na+-H+ antiporters that transport Na+ into cell Some Na+ can pass into cell by paracellular route in this area of limb © 2016 Pearson Education, Inc.

Distal convoluted tubule and collecting duct Reabsorption is hormonally regulated in these areas Antidiuretic hormone (ADH) Released by posterior pituitary gland Causes principal cells of collecting ducts to insert aquaporins in apical membranes, increasing water reabsorption Increased ADH levels cause an increase in water reabsorption © 2016 Pearson Education, Inc.

Aldosterone Targets collecting ducts (principal cells) and distal DCT Promotes synthesis of apical Na+ and K+ channels, and basolateral Na+-K+ ATPases for Na+ reabsorption (water follows) As a result, little Na+ leaves body Without aldosterone, daily loss of filtered Na+ would be 2%, which is incompatible with life Functions: increase blood pressure and decrease K+ levels © 2016 Pearson Education, Inc.

Atrial natriuretic peptide Reduces blood Na+, resulting in decreased blood volume and blood pressure Released by cardiac atrial cells if blood volume or pressure elevated Parathyroid hormone Acts on DCT to increase Ca2+ reabsorption © 2016 Pearson Education, Inc.

Figure 25.17 Summary of tubular reabsorption and secretion.
Cortex 65% of filtrate volume reabsorbed • H2O • Na+, HCO3−, and many other ions • Glucose, amino acids, and other nutrients Regulated reabsorption • Na+(by aldosterone; Cl− follows) • Ca2+ (by parathyroid hormone) • H+ and NH4+ • Some drugs Regulated secretion • K+ (by aldosterone) • H2O • Na+, K+, Cl− Outer medulla Regulated reabsorption • H2O (by ADH) • Na+ (by aldosterone; Cl− follows) • Urea (increased by ADH) • Urea Regulated secretion • K+ (by aldosterone) Inner medulla • Reabsorption or secretion to maintain blood pH described in Chapter 26; involves H+, HCO3−, and NH4+ Reabsorption Secretion © 2016 Pearson Education, Inc.

25.6 Step 3: Tubular Secretion
Tubular secretion is reabsorption in reverse Occurs almost completely in PCT Selected substances are moved from peritublar capillaries through tubule cells out into filtrate K+, H+, NH4+, creatinine, organic acids and bases Substances synthesized in tubule cells also are secreted (example: HCO3–) © 2016 Pearson Education, Inc.

25.6 Step 3: Tubular Secretion
Tubular secretion is important for: Disposing of substances, such as drugs or metabolites, that are bound to plasma proteins Eliminating undesirable substances that were passively reabsorbed (example: urea and uric acid) Ridding body of excess K+ (aldosterone effect) Controlling blood pH by altering amounts of H+ or HCO3– in urine © 2016 Pearson Education, Inc.

Figure 25.17 Summary of tubular reabsorption and secretion.
Cortex 65% of filtrate volume reabsorbed • H2O • Na+, HCO3−, and many other ions • Glucose, amino acids, and other nutrients Regulated reabsorption • Na+(by aldosterone; Cl− follows) • Ca2+ (by parathyroid hormone) • H+ and NH4+ • Some drugs Regulated secretion • K+ (by aldosterone) • H2O • Na+, K+, Cl− Outer medulla Regulated reabsorption • H2O (by ADH) • Na+ (by aldosterone; Cl− follows) • Urea (increased by ADH) • Urea Regulated secretion • K+ (by aldosterone) Inner medulla • Reabsorption or secretion to maintain blood pH described in Chapter 26; involves H+, HCO3−, and NH4+ Reabsorption Secretion © 2016 Pearson Education, Inc.

25.7 Regulation of Urine Concentration and Volume
One main function of kidneys is to make any adjustment needed to maintain body fluid osmotic concentration at around 300 mOsm Osmolality: number of solute particles in 1 kg of H2O 1 osmol = 1 mole of particle per kg H2O Body fluids have much smaller amounts, so expressed in milliosmols (mOsm) = osmol © 2016 Pearson Education, Inc.

25.7 Regulation of Urine Concentration and Volume
Kidneys produce only small amounts of urine if the body is dehydrated, or dilute urine if overhydrated © 2016 Pearson Education, Inc.

Formation of Dilute or Concentrated Urine
When body is over-hydrated, kidneys produce large volume of dilute urine ADH production decreases; urine ~100 mOsm, or even as low as 50 mOsm with aldosterone + ADH When body is dehydrated, kidneys produce small volume of concentrated urine Maximal ADH is released; urine ~1200 mOsm In severe dehydration, 99% of water filtered at glomerulus is reabsorbed © 2016 Pearson Education, Inc.

Figure 25.19a Mechanism for forming dilute or concentrated urine.
If we were so overhydrated we had no ADH... Osmolality of extracellular fluids ADH release from posterior pituitary Number of aquaporins (H2O channels) in collecting duct H2O reabsorption from collecting duct Large volume of dilute urine Collecting duct Descending limb of nephron loop 300 100 DCT 100 Cortex 300 100 300 NaCI H2O NaCI 600 400 Osmolality of interstitial fluid (mOsm) 600 100 Outer medulla NaCI 900 700 900 H2O Urea Inner medulla 1200 100 1200 Large volume of dilute urine Active transport Passive transport © 2016 Pearson Education, Inc.

Figure 25.19b Mechanism for forming dilute or concentrated urine.
If we were so dehydrated we had maximal ADH... Osmolality of extracellular fluids ADH release from posterior pituitary Number of aquaporins (H2O channels) in collecting duct H2O reabsorption from collecting duct Small volume of concentrated urine Collecting duct H2O Descending limb of nephron loop 300 100 150 H2O DCT 300 Cortex 300 100 300 300 H2O NaCI 400 H2O H2O NaCI 600 400 600 Osmolality of interstitial fluid (mOsm) 600 Outer medulla H2O NaCI Urea 900 700 900 900 H2O H2O Urea Inner medulla H2O 1200 1200 1200 Urea contributes to the osmotic gradient. ADH increases its recycling. Small volume of concentrated urine © 2016 Pearson Education, Inc.

Diuretics Chemicals that enhance urinary output
ADH inhibitors, such as alcohol Na+ reabsorption inhibitors (and resultant H2O reabsorption), such as caffeine or drugs for hypertension or edema Loop diuretics inhibit medullary gradient formation Osmotic diuretics: substance not reabsorbed, so water remains in urine; for example, in diabetic patient, high glucose concentration pulls water from body © 2016 Pearson Education, Inc.

25.8 Clinical Evaluation of Kidney
Urinalysis: urine is examined for signs of disease Can also be used to test for illegal substances Assessing renal function requires both blood and urine examination Example: renal function can be assessed by measuring nitrogenous wastes in blood only To determine renal clearance, both blood and urine are required © 2016 Pearson Education, Inc.

Chronic renal disease: defined as a GFR < 60 ml/min for 3 months Filtrate formation decreases, nitrogenous wastes accumulate in blood, pH becomes acidic Seen in diabetes mellitus and hypertension © 2016 Pearson Education, Inc.

Renal failure: defined as GFR < 15 ml/min Causes uremia: ionic and hormonal imbalances, metabolic abnormalities, toxic molecule accumulation Symptoms: fatigue, anorexia, nausea, mental changes, cramps Treatment: hemodialysis or transplant © 2016 Pearson Education, Inc.

Urine Chemical composition 95% water and 5% solutes Nitrogenous wastes
Urea (from amino acid breakdown): largest solute component Uric acid (from nucleic acid metabolism) Creatinine (metabolite of creatine phosphate) What should not be in urine Dipstick: no glucose, protein, ketones, blood © 2016 Pearson Education, Inc.

Urine (cont.) Physical characteristics Color and transparency Odor
Clear (Cloudy may indicate urinary tract infection) Pale to deep yellow (abnormal color may be due to certain foods, bile pigments, blood, drugs) Odor Slightly aromatic when fresh Develops ammonia odor upon standing as bacteria metabolize urea May be altered by some drugs or vegetables In diabetes mellitus, urine may have acetone smell due to ketones (ketonuria) © 2016 Pearson Education, Inc.

Urine (cont.) pH Specific gravity
Urine is slightly acidic (~pH 6, with range of 4.5 to 8.0) Acidic diet (protein, whole wheat) can cause drop in pH Alkaline diet (vegetarian), prolonged vomiting, or urinary tract infections can cause an increase in pH Specific gravity Ratio of mass of substance to mass of equal volume of water (specific gravity of water = 1) Ranges from to because urine is made up of water and solutes © 2016 Pearson Education, Inc.

25.9 Transport, Storage, and Elimination of Urine
Ureters Ureters: slender tubes that convey urine from kidneys to bladder Begin at renal pelvis Retroperitoneal Enter base of bladder through posterior wall © 2016 Pearson Education, Inc.

Ureters (cont.) Three layers of ureter wall (from inside out)
Mucosa: consists of transitional epithelium Muscularis: smooth muscle sheets contract in response to stretch Propels urine into bladder: gravity alone is not enough; must also be pushed by peristaltic wave action of smooth muscle Rate of peristalsis adjusted to rate of urine formation Sympathetic and parasympathetic innervate ureters but play insignificant role in peristalsis Adventitia: outer fibrous connective tissue © 2016 Pearson Education, Inc.

Figure 25.20 Cross-sectional view of the ureter wall (10).
Lumen Mucosa • Transitional epithelium • Lamina propria • Muscularis • Longitudinal layer • Circular layer Adventitia © 2016 Pearson Education, Inc.

Renal calculi = “kidney stones” in renal pelvis Crystallized calcium, magnesium, or uric acid salts Large stones block ureter, causing pressure and pain Causes: Chronic bacterial infection; urine retention; elevated blood Ca or elevated urine pH Treatment: shock wave lithotripsy—noninvasive, uses shock waves (acoustic pulses) to shatter stones © 2016 Pearson Education, Inc.

Urinary Bladder Urinary bladder anatomy
Muscular sac for temporary storage of urine Retroperitoneal, on pelvic floor posterior to pubic symphysis Males: prostate inferior to bladder neck Females: anterior to vagina and uterus Has openings for ureters and urethra Trigone Smooth triangular area outlined by openings for ureters and urethra Infections tend to persist in this region © 2016 Pearson Education, Inc.

Urinary Bladder (cont.)
Layers of bladder wall Mucosa: transitional epithelial mucosa Muscular layer: thick detrusor muscle = smooth muscle Fibrous adventitia, except on superior surface where it is covered by peritoneum © 2016 Pearson Education, Inc.

Figure 25.21a Structure of the urinary bladder and urethra.
Peritoneum Ureter Rugae Detrusor Adventitia Ureteric orifices Trigone of bladder Bladder neck Internal urethral sphincter Prostate Prostatic urethra Intermediate part of the urethra External urethral sphincter Urogenital diaphragm Spongy urethra Erectile tissue of penis External urethral orifice Male. The long male urethra has three regions: prostatic, intermediate, and spongy. © 2016 Pearson Education, Inc.

Figure 25.21b Structure of the urinary bladder and urethra.
Peritoneum Ureter Rugae Detrusor Ureteric orifices Bladder neck Internal urethral sphincter Trigone External urethral sphincter Urogenital diaphragm Urethra External urethral orifice Female. © 2016 Pearson Education, Inc.

Urinary Bladder (cont.)
Urine storage capacity Collapses when empty Rugae appear Expands and rises superiorly during filling without significant rise in internal pressure Moderately full bladder is ~12 cm long (5 in.) and can hold ~ 500 ml (1 pint) Can hold twice that amount if necessary but can burst if overdistended © 2016 Pearson Education, Inc.

Urethra (cont.) Muscular tube that drains urinary bladder Sphincters
Internal urethral sphincter Involuntary (smooth muscle) at bladder-urethra junction Contracts to open External urethral sphincter Voluntary (skeletal) muscle surrounding urethra as it passes through pelvic floor © 2016 Pearson Education, Inc.

Urethra (cont.) Female urethra (3–4 cm):
External urethral orifice: anterior to vaginal opening; posterior to clitoris Male urethra carries semen and urine Three named regions Prostatic urethra (2.5 cm): within prostate Intermediate part of the urethra (membranous urethra) (2 cm): passes through urogenital diaphragm from prostate to beginning of penis Spongy urethra (15 cm): passes through penis; opens via external urethral orifice © 2016 Pearson Education, Inc.

Urinary tract infections can be caused by: Improper toilet habits, such as wiping back to front after defecation Short urethra of females can allow fecal bacteria to easily enter urethra Most UTIs occur in sexually active women 40% of women get urinary tract infections Intercourse drives bacteria from vagina and external genital region toward bladder Use of spermicides magnifies problem Treatment: antibiotics can cure most urinary tract infections © 2016 Pearson Education, Inc.

Micturition Micturition, also called urination or voiding
Three simultaneous events must occur Contraction of detrusor by ANS Opening of internal urethral sphincter by ANS Opening of external urethral sphincter by somatic nervous system © 2016 Pearson Education, Inc.

Figure 25.23 Control of micturition.
Brain Higher brain centers Urinary bladder fills, stretching bladder wall Allow or inhibit micturition as appropriate Afferent impulses from stretch receptors Pontine micturition center Pontine storage center Simple spinal reflex Promotes micturition by acting on all three spinal efferents Inhibits micturition by acting on all three spinal efferents Spinal cord Spinal cord Parasympathetic activity Sympathetic activity Somatic motor nerve activity Parasympathetic activity Sympathetic activity Somatic motor nerve activity Detrusor contracts; internal urethral sphincter opens External urethral sphincter opens Inhibits Micturition © 2016 Pearson Education, Inc.

Urinary incontinence: in adults, usually caused by weakened pelvic muscles Stress incontinence Increased intra-abdominal pressure forces urine through external sphincter Laughing, coughing, or sneezing can cause incontinence Overflow incontinence Urine dribbles when bladder overfills © 2016 Pearson Education, Inc.

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