Chapter 24: The Urinary System

Chapter 24: The Urinary System

Overview of kidney functions n Regulation of blood ionic composition n Regulation of blood ph n Regulation of blood volume n Regulation of blood pressure n Maintenance of blood osmolarity n Production of hormones (calcitrol and erythropoitin) n Regulation of blood glucose level n Excretion of wastes from metabolic reactions and foreign substances (drugs or toxins)

Anatomy and histology of the kidneys n External anatomy Renal hilium indent where ureter emerges along with blood vessels, lymphatic vessels and nerves 3 layers of tissue n n n Renal capsule deep layer continuous with outer coat of ureter, barrier against trauma, maintains kidney shape Adipose capsule mass of fatty tissue that protects kidney from trauma and holds it in place Renal fascia superficial layer thin layer of connective tissue that anchors kidney to surrounding structures and abdominal wall

Organs of the urinary system in a female

Position and coverings of the kidneys

Internal anatomy Renal cortex superficial n n n Outer cortical zone Inner juxtamedullary zone Renal columns portions of cortex that extend between renal pyramids Renal medulla inner region n Several cone shaped renal pyramids base faces cortex and renal papilla points toward hilium Renal lobe renal pyramid, overlying cortex area, and ½ of each adjacent renal column

Anatomy of the kidneys n Parenchyma (functional portion) of kidney Renal cortex and renal pyramids of medulla n Nephron microscopic functional units of kidney n Urine formed by nephron drains into Papillary ducts Minor and major calyces Renal pelvis Ureter Urinary bladder

Internal anatomy of the kidneys

Blood and nerve supply of the kidneys n n Blood supply Although kidneys constitute less than 0.5% of total body mass, they receive 20-25% of resting cardiac output Left and right renal artery enters kidney Branches into segmental, interlobar, arcuate, interlobular arteries Each nephron receives one afferent arteriole Divides into glomerulus capillary ball Reunite to form efferent arteriole (uniue) Divide to form peritubular capillaries or some have vasa recta Peritubular venule, interlobar vein and renal vein exits kidney Renal nerves are part of the sympathetic autonomic nervous system Most are vasomotor nerves regulating blood flow

Blood supply of the kidneys

The nephron functional units of kidney 2 parts n Renal corpuscle filters blood plasma Glomerulus capillary network Glomerular (Bowman s) capsule double-walled cup surrounding glomerulus n Renal tubule filtered fluid passes into Proximal convoluted tubule Descending and ascending loop of Henle (nephron loop) Distal convoluted tubule

Nephrons Renal corpuscle and both convoluted tubules in cortex, loop of Henle extend into medulla Distal convoluted tubule of several nephrons empty into single collecting duct Cortical nephrons 80-85% of nephrons n Renal corpuscle in outer portion of cortex and short loops of Henle extend only into outer region of medulla Juxtamedullary nephrons other 25-20% n n n n Renal corpuscle deep in cortex and long loops of Henle extend deep into medulla Receive blood from peritubular capillaries and vasa recta Ascending limb has thick and thin regions Enable kidney to secrete very dilute or very concentrated urine

The structure of nephrons and associated blood vessels

The structure of nephrons and associated blood vessels

Histology of nephron and collecting duct Glomerular capsule n n n Visceral layer has podocytes that wrap projections around single layer of endothelial cells of glomerular capillaries and form inner wall of capsule Parietal layer forms outer wall of capsule Fluid filtered from glomerular capillaries enters capsular (Bowman s) space

Histology of a renal corpuscle

Renal tubule and collecting duct n n n Proximal convoluted tubule cells have microvilli with brush border increases surface area Juxtaglomerular appraratus helps regulate blood pressure in kidney Macula densa cells in final part of ascending loop of Henle Juxtaglomerular cells cells of afferent and efferent arterioles contain modified smooth muscle fibers Last part of distal convoluted tubule and collecting duct Principal cells receptors for antidiuretic hormone (ADH) and aldosterone Intercalated cells role in blood ph homeostasis

Overview of renal physiology 1. Glomerular filtration Water and most solutes in blood plasma move across the wall of the glomerular capillaries into glomerular capsule and then renal tubule 2. Tubular reabsorption As filtered fluid moves along tubule and through collecting duct, about 99% of water and many useful solutes reabsorbed returned to blood 3. Tubular secretion As filtered fluid moves along tubule and through collecting duct, other material secreted into fluid such as wastes, drugs, and excess ions removes substances from blood n Solutes in the fluid that drains into the renal pelvis remain in the fluid and are excreted n Excretion of any solute = glomerular filtration + secretion - reabsorption

Structures and functions of a nephron Renal corpuscle Renal tubule and collecting duct Afferent arteriole Glomerular capsule 1 Filtration from blood plasma into nephron Efferent arteriole Fluid in renal tubule 2 Tubular reabsorption 3 from fluid into blood Tubular secretion from blood into fluid Urine (contains excreted substances) Peritubular capillaries Blood (contains reabsorbed substances)

Glomerular filtration n Glomerular filtrate fluid that enters capsular space Daily volume 150-180 liters more than 99% returned to blood plasma via tubular reabsorption n Filtration membrane endothelial cells of glomerular capillaries and podocytes encircling capillaries Permits filtration of water and small solutes Prevents filtration of most plasma proteins, blood cells and platelets 3 barriers to cross glomerular endothelial cells fenestrations, basal lamina between endothelium and podocytes and pedicels of podocytes create filtration slits Volume of fluid filtered is large because of large surface area, thin and porous membrane, and high glomerular capillary blood pressure

The filtration membrane

Podocyte of visceral layer of glomerular (Bowman s) capsule Filtration slit Pedicel 1 2 3 Fenestration (pore) of glomerular endothelial cell: prevents filtration of blood cells but allows all components of blood plasma to pass through Basal lamina of glomerulus: prevents filtration of larger proteins Slit membrane between pedicels: prevents filtration of medium-sized proteins (a) Details of filtration membrane Pedicel of podocyte Filtration slit Basal lamina Lumen of glomerulus Fenestration (pore) of glomerular endothelial cell TEM 78,000x (b) Filtration membrane

Net filtration pressure n Net filtration pressure (NFP) is the total pressure that promotes filtration NFP = GBHP CHP BCOP Glomerular blood hydrostatic pressure is the blood pressure of the glomerular capillaries forcing water and solutes through filtration slits Capsular hydrostatic pressure is the hydrostatic pressure exerted against the filtration membrane by fluid already in the capsular space and represents back pressure Blood colloid osmotic pressure due to presence of proteins in blood plasma and also opposes filtration

The pressures that drive glomerular filtration

1 GLOMERULAR BLOOD HYDROSTATIC PRESSURE (GBHP) = 55 mmhg 2 CAPSULAR HYDROSTATIC PRESSURE (CHP) = 15 mmhg Afferent arteriole 3 BLOOD COLLOID OSMOTIC PRESSURE (BCOP) = 30 mmhg Proximal convoluted tubule Efferent arteriole Glomerular (Bowman's) capsule Capsular space NET FILTRATION PRESSURE (NFP) =GBHP CHP BCOP = 55 mmhg 15 mmhg 30 mmhg = 10 mmhg

Glomerular filtration n Glomerular filtration rate amount of filtrate formed in all the renal corpuscles of both kidneys each minute Homeostasis reuires kidneys maintain a relatively constant GFR n n Too high substances pass too uickly and are not reabsorbed Too low nearly all reabsorbed and some waste products not adeuately excreted GFR directly related to pressures that determine net filtration pressure

3 Mechanisms regulating GFR 1. Renal autoregulation n Kidneys themselves maintain constant renal blood flow and GFR using Myogenic mechanism occurs when stretching triggers contraction of smooth muscle cells in afferent arterioles reduces GFR Tubuloglomerular mechanism macula densa provides feedback to glomerulus, inhibits release of NO causing afferent arterioles to constrict and decreasing GFR

Tuboglomerular feedback

Mechanisms regulating GFR 2. Neural regulation Kidney blood vessels supplied by sympathetic ANS fibers that release norepinephrine causing vasoconstriction Moderate stimulation both afferent and efferent arterioles constrict to same degree and GFR decreases Greater stimulation constricts afferent arterioles more and GFR drops 3. Hormonal regulation Angiotensin II reduces GFR potent vasoconstrictor of both afferent and efferent arterioles Atrial natriuretic peptide increases GFR stretching of atria causes release, increases capillary surface area for filtration

Tubular reabsorption and tubular secretion n Reabsorption return of most of the filtered water and many solutes to the bloodstream About 99% of filtered water reabsorbed Proximal convoluted tubule cells make largest contribution Both active and passive processes n Secretion transfer of material from blood into tubular fluid Helps control blood ph Helps eliminate substances from the body

Reabsorption routes and transport mechanisms n n Reabsorption routes Paracellular reabsorption n Between adjacent tubule cells n Tight junction do not completely seal off interstitial fluid from tubule fluid n Passive Transcellular reabsorption through an individual cell Transport mechanisms Reabsorption of Na + especially important Primary active transport n Sodium-potassium pumps in basolateral membrane only Secondary active transport n Symporters, antiporters Transport maximum (T m ) n Upper limit to how fast it can work Obligatory vs. facultative water reabsorption

Reabsorption routes: paracellular reabsorption and transcellular reabsorption

Reabsorption and secretion in proximal convoluted tubule (PCT) Largest amount of solute and water reabsorption Secretes variable amounts of H +, NH 4 + and urea Most solute reabsorption involves Na + n Symporters for glucose, amino acids, lactic acid, watersoluble vitamins, phosphate and sulfate n Na + / H + antiporter causes Na + to be reabsorbed and H + to be secreted Solute reabsorption promotes osmosis creates osmotic gradient n Auaporin-1 in cells lining PCT and descending limb of loop of Henle n As water leaves tubular fluid, solute concentration increases Urea and ammonia in blood are filtered at glomerulus and secreted by proximal convoluted tubule cells

Reabsorption and secretion in the proximal convoluted tubule

Reabsorption in the loop of Henle Chemical composition of tubular fluid uite different from filtrate n Glucose, amino acids and other nutrients reabsorbed Osmolarity still close to that of blood n Reabsorption of water and solutes balanced For the first time reabsorption of water is NOT automatically coupled to reabsorption of solutes n Independent regulation of both volume and osmolarity of body fluids Na + -K + -2Cl - symporters function in Na + and Cl - reabsorption promotes reabsorption of cations Little or no water is reabsorbed in ascending limb osmolarity decreases

Na + K + -2Cl - symporter in the thick ascending limb of the loop of Henle

Reabsorption and secretion in the late distale convoluted tubule and collecting duct n Reabsorption on the early distal convoluted tubule Na + -Cl - symporters reabsorb Na + and Cl - Major site where parathyroid hormone stimulates reabsorption of Ca + depending on body s needs n Reabsorption and secretion in the late distal convoluted tubule and collecting duct 90-95% of filtered solutes and fluid have been returned by now Principal cells reabsorb Na + and secrete K + Intercalated cells reabsorb K + and HCO - 3 and secrete H + Amount of water reabsorption and solute reabsorption and secretion depends on body s needs

Hormonal regulation of tubular reabsorption and secretion Angiotensin II - when blood volume and blood pressure decrease n Decreases GFR, enhances reabsorption of Na +, Cl - and water in PCT Aldosterone - when blood volume and blood pressure decrease n Stimulates principal cells in collecting duct to reabsorb more Na + and Cl - and secrete more K + Parathyroid hormone n Stimulates cells in DCT to reabsorb more Ca 2+

Regulation of facultative water reabsorption by ADH Antidiuretic hormone (ADH or vasopressin) n Increases water permeability of cells by inserting auaporin-2 in last part of DCT and collecting duct Atrial natriuretic peptide (ANP) n Large increase in blood volume promotes release of ANP n Decreases blood volume and pressure by inhibiting reabsorption of Na + and water in PCT and collecting duct, suppress secretion of ADH and aldosterone

Production of dilute and concentrated urine n Even though your fluid intake can be highly variable, total fluid volume in your body remains stable n Depends in large part on the kidneys to regulate the rate of water loss in urine n ADH controls whether dilute or concentrated urine is formed Absent or low ADH = dilute urine Higher levels = more concentrated urine through increased water reabsorption

Formation of dilute urine Glomerular filtrate has same osmolarity as blood 300 mosm/liter Fluid leaving PCT is isotonic to plasma When dilute urine is being formed: the osmolarity of fluid increases as it goes down the descending loop of Henle the osmolarity of fluid decreases as it goes up the ascending limb, and decreases still more as it flows through the rest of the nephron and collecting duct

Formation of dilute urine n n n n Osmolarity of interstitial fluid of renal medulla becomes greater, more water is reabsorbed from tubular fluid so fluid become more concentrated Water cannot leave in thick portion of ascending limb but solutes leave making fluid more dilute than blood plasma Additional solutes but not much water leaves in DCT Low ADH makes late DCT and collecting duct have low water permeability

Mechanism of urine concentration in longloop juxtamedullary nephrons

Vasa recta Loop of Henle Juxtamedullary nephron and its blood supply together Osmotic gradient 300 400 600 800 1000 1200 Afferent arteriole Efferent arteriole Proximal convoluted tubule Interstitial fluid in renal medulla 1 2 Symporters in thick ascending limb cause buildup of Na + and Cl Countercurrent flow through loop of Henle establishes an osmotic gradient Glomerular (Bowman s) capsule Glomerulus H 2 O H 2 O H 2 O Distal convoluted tubule 300 380 580 780 980 Na + CI 1200 200 H 2 O H 2 O H 2 O 100 200 400 600 800 Loop of Henle Urea H 2 O H 2 O H 2 O H 2 O 300 300 320 400 600 800 1000 1200 Collecting duct 3 4 Papillary duct Interstitial fluid in renal cortex 1200 Concentrated urine 300 Principal cells in collecting duct reabsorb more water when ADH is present Urea recycling causes buildup of urea in the renal medulla H 2 O Na + CI 500 700 900 H 2 O 1100 H 2 O Na + CI Blood flow Presense of Na + -K + -2CI symporters Flow of tubular fluid 320 H 2 O Na + CI 400 600 800 H 2 O Na + CI 1000 1200 (a) Reabsorption of Na + CI and water in a long-loop juxtamedullary nephron (b) Recycling of salts and urea in the vasa recta

Evaluation of kidney function n Urinalysis Analysis of the volume and physical, chemical and microscopic properties of urine Water accounts for 95% of total urine volume Typical solutes are filtered and secreted substances that are not reabsorbed If disease alters metabolism or kidney function, traces if substances normally not present or normal constituents in abnormal amounts may appear

Evaluation of kidney function n Blood tests Blood urea nitrogen (BUN) measures blood nitrogen that is part of the urea resulting from catabolism and deamination of amino acids Plasma creatinine results from catabolism of creatine phosphate in skeletal muscle measure of renal function n Renal plasma clearance More useful in diagnosis of kidney problems than above Volume of blood cleared of a substance per unit time High renal plasma clearance indicates efficient excretion of a substance into urine PAH administered to measure renal plasma flow

Urine transportation, storage, and elimination n Ureters Each of 2 ureters transports urine from renal pelvis of one kidney to the bladder Peristaltic waves, hydrostatic pressure and gravity move urine No anatomical valve at the opening of the ureter into bladder when bladder fills it compresses the opening and prevents backflow

Ureters, urinary bladder, and urethra in a female

Urinary bladder and urethra n Urinary bladder Hollow, distensible muscular organ Capacity averages 700-800mL Micturition discharge of urine from bladder n Combination of voluntary and involuntary muscle contractions n When volume increases stretch receptors send signals to micturition center in spinal cord triggering spinal reflex micturition reflex n In early childhood we learn to initiate and stop it voluntarily n Urethra Small tube leading from internal urethral orifice in floor of bladder to exterior of the body In males discharges semen as well as urine

Comparison between female and male urethras