25: 25 The Urinary System Part B


Other Factors Affecting Glomerular Filtration: Other Factors Affecting Glomerular Filtration Prostaglandins (PGE2 and PGI2) Vasodilators produced in response to sympathetic stimulation and angiotensin II Are thought to prevent renal damage when peripheral resistance is increased Nitric oxide – vasodilator produced by the vascular endothelium Adenosine – vasoconstrictor of renal vasculature Endothelin – a powerful vasoconstrictor secreted by tubule cells


Tubular Reabsorption: Tubular Reabsorption A transepithelial process whereby most tubule contents are returned to the blood Transported substances move through three membranes Luminal and basolateral membranes of tubule cells Endothelium of peritubular capillaries Only Ca2+, Mg2+, K+, and some Na+ are reabsorbed via paracellular pathways


Tubular Reabsorption: Tubular Reabsorption All organic nutrients are reabsorbed Water and ion reabsorption is hormonally controlled Reabsorption may be an active (requiring ATP) or passive process


Sodium Reabsorption: Primary Active Transport: Sodium reabsorption is almost always by active transport Na+ enters the tubule cells at the luminal membrane Is actively transported out of the tubules by a Na+-K+ ATPase pump Sodium Reabsorption: Primary Active Transport


Sodium Reabsorption: Primary Active Transport: From there it moves to peritubular capillaries due to: Low hydrostatic pressure High osmotic pressure of the blood Na+ reabsorption provides the energy and the means for reabsorbing most other solutes Sodium Reabsorption: Primary Active Transport


Routes of Water and Solute Reabsorption: Routes of Water and Solute Reabsorption Figure 25.11


Reabsorption by PCT Cells: Reabsorption by PCT Cells Active pumping of Na+ drives reabsorption of: Water by osmosis, aided by water-filled pores called aquaporins Cations and fat-soluble substances by diffusion Organic nutrients and selected cations by secondary active transport


Reabsorption by PCT Cells: Reabsorption by PCT Cells Figure 25.12


Nonreabsorbed Substances: Nonreabsorbed Substances A transport maximum (Tm): Reflects the number of carriers in the renal tubules available Exists for nearly every substance that is actively reabsorbed When the carriers are saturated, excess of that substance is excreted


Nonreabsorbed Substances: Nonreabsorbed Substances Substances are not reabsorbed if they: Lack carriers Are not lipid soluble Are too large to pass through membrane pores Urea, creatinine, and uric acid are the most important nonreabsorbed substances


Absorptive Capabilities of Renal Tubules and Collecting Ducts: Substances reabsorbed in PCT include: Sodium, all nutrients, cations, anions, and water Urea and lipid-soluble solutes Small proteins Loop of Henle reabsorbs: H2O, Na+, Cl, K+ in the descending limb Ca2+, Mg2+, and Na+ in the ascending limb Absorptive Capabilities of Renal Tubules and Collecting Ducts


Absorptive Capabilities of Renal Tubules and Collecting Ducts: DCT absorbs: Ca2+, Na+, H+, K+, and water HCO3 and Cl Collecting duct absorbs: Water and urea Absorptive Capabilities of Renal Tubules and Collecting Ducts


Na+ Entry into Tubule Cells: Na+ Entry into Tubule Cells Passive entry: Na+-K+ ATPase pump In the PCT: facilitated diffusion using symport and antiport carriers In the ascending loop of Henle: facilitated diffusion via Na+-K+-2Cl symport system In the DCT: Na+-Cl– symporter In collecting tubules: diffusion through membrane pores


Atrial Natriuretic Peptide Activity: Atrial Natriuretic Peptide Activity ANP reduces blood Na+ which: Decreases blood volume Lowers blood pressure ANP lowers blood Na+ by: Acting directly on medullary ducts to inhibit Na+ reabsorption Counteracting the effects of angiotensin II Indirectly stimulating an increase in GFR reducing water reabsorption


Tubular Secretion: Tubular Secretion Essentially reabsorption in reverse, where substances move from peritubular capillaries or tubule cells into filtrate Tubular secretion is important for: Disposing of substances not already in the filtrate Eliminating undesirable substances such as urea and uric acid Ridding the body of excess potassium ions Controlling blood pH


Regulation of Urine Concentration and Volume: Regulation of Urine Concentration and Volume Osmolality The number of solute particles dissolved in 1L of water Reflects the solution’s ability to cause osmosis Body fluids are measured in milliosmols (mOsm) The kidneys keep the solute load of body fluids constant at about 300 mOsm This is accomplished by the countercurrent mechanism


Countercurrent Mechanism: Countercurrent Mechanism Interaction between the flow of filtrate through the loop of Henle (countercurrent multiplier) and the flow of blood through the vasa recta blood vessels (countercurrent exchanger) The solute concentration in the loop of Henle ranges from 300 mOsm to 1200 mOsm Dissipation of the medullary osmotic gradient is prevented because the blood in the vasa recta equilibrates with the interstitial fluid


Osmotic Gradient in the Renal Medulla: Osmotic Gradient in the Renal Medulla Figure 25.13


Loop of Henle: Countercurrent Multiplier: Loop of Henle: Countercurrent Multiplier The descending loop of Henle: Is relatively impermeable to solutes Is permeable to water The ascending loop of Henle: Is permeable to solutes Is impermeable to water Collecting ducts in the deep medullary regions are permeable to urea


Loop of Henle: Countercurrent Exchanger: Loop of Henle: Countercurrent Exchanger The vasa recta is a countercurrent exchanger that: Maintains the osmotic gradient Delivers blood to the cells in the area InterActive Physiology®: Urinary System: Early Filtrate Processing PLAY


Loop of Henle: Countercurrent Mechanism: Loop of Henle: Countercurrent Mechanism Figure 25.14


Formation of Dilute Urine: Formation of Dilute Urine Filtrate is diluted in the ascending loop of Henle Dilute urine is created by allowing this filtrate to continue into the renal pelvis This will happen as long as antidiuretic hormone (ADH) is not being secreted


Formation of Dilute Urine: Formation of Dilute Urine Collecting ducts remain impermeable to water; no further water reabsorption occurs Sodium and selected ions can be removed by active and passive mechanisms Urine osmolality can be as low as 50 mOsm (one-sixth that of plasma)


Formation of Concentrated Urine: Formation of Concentrated Urine Antidiuretic hormone (ADH) inhibits diuresis This equalizes the osmolality of the filtrate and the interstitial fluid In the presence of ADH, 99% of the water in filtrate is reabsorbed


Formation of Concentrated Urine: Formation of Concentrated Urine ADH-dependent water reabsorption is called facultative water reabsorption ADH is the signal to produce concentrated urine The kidneys’ ability to respond depends upon the high medullary osmotic gradient InterActive Physiology®: Urinary System: Late Filtrate Processing PLAY


Formation of Dilute and Concentrated Urine: Formation of Dilute and Concentrated Urine Figure 25.15a, b


Diuretics: Diuretics Chemicals that enhance the urinary output include: Any substance not reabsorbed Substances that exceed the ability of the renal tubules to reabsorb it Substances that inhibit Na+ reabsorption


Diuretics: Diuretics Osmotic diuretics include: High glucose levels – carries water out with the glucose Alcohol – inhibits the release of ADH Caffeine and most diuretic drugs – inhibit sodium ion reabsorption Lasix and Diuril – inhibit Na+-associated symporters


Renal Clearance: Renal Clearance The volume of plasma that is cleared of a particular substance in a given time Renal clearance tests are used to: Determine the GFR Detect glomerular damage Follow the progress of diagnosed renal disease


Renal Clearance: Renal Clearance RC = UV/P RC = renal clearance rate U = concentration (mg/ml) of the substance in urine V = flow rate of urine formation (ml/min) P = concentration of the same substance in plasma


Physical Characteristics of Urine: Physical Characteristics of Urine Color and transparency Clear, pale to deep yellow (due to urochrome) Concentrated urine has a deeper yellow color Drugs, vitamin supplements, and diet can change the color of urine Cloudy urine may indicate infection of the urinary tract


Physical Characteristics of Urine: Physical Characteristics of Urine Odor Fresh urine is slightly aromatic Standing urine develops an ammonia odor Some drugs and vegetables (asparagus) alter the usual odor


Physical Characteristics of Urine: Physical Characteristics of Urine pH Slightly acidic (pH 6) with a range of 4.5 to 8.0 Diet can alter pH Specific gravity Ranges from 1.001 to 1.035 Is dependent on solute concentration


Chemical Composition of Urine: Chemical Composition of Urine Urine is 95% water and 5% solutes Nitrogenous wastes include urea, uric acid, and creatinine Other normal solutes include: Sodium, potassium, phosphate, and sulfate ions Calcium, magnesium, and bicarbonate ions Abnormally high concentrations of any urinary constituents may indicate pathology


Ureters: Ureters Slender tubes that convey urine from the kidneys to the bladder Ureters enter the base of the bladder through the posterior wall This closes their distal ends as bladder pressure increases and prevents backflow of urine into the ureters


Ureters: Ureters Ureters have a trilayered wall Transitional epithelial mucosa Smooth muscle muscularis Fibrous connective tissue adventitia Ureters actively propel urine to the bladder via response to smooth muscle stretch


Urinary Bladder: Urinary Bladder Smooth, collapsible, muscular sac that temporarily stores urine It lies retroperitoneally on the pelvic floor posterior to the pubic symphysis Males – prostate gland surrounds the neck inferiorly Females – anterior to the vagina and uterus Trigone – triangular area outlined by the openings for the ureters and the urethra Clinically important because infections tend to persist in this region


Urinary Bladder: Urinary Bladder The bladder wall has three layers Transitional epithelial mucosa A thick muscular layer A fibrous adventitia The bladder is distensible and collapses when empty As urine accumulates, the bladder expands without significant rise in internal pressure


Urinary Bladder: Urinary Bladder Figure 25.18a, b


Urethra: Urethra Muscular tube that: Drains urine from the bladder Conveys it out of the body


Urethra: Urethra Sphincters keep the urethra closed when urine is not being passed Internal urethral sphincter – involuntary sphincter at the bladder-urethra junction External urethral sphincter – voluntary sphincter surrounding the urethra as it passes through the urogenital diaphragm Levator ani muscle – voluntary urethral sphincter


Urethra: Urethra The female urethra is tightly bound to the anterior vaginal wall Its external opening lies anterior to the vaginal opening and posterior to the clitoris The male urethra has three named regions Prostatic urethra – runs within the prostate gland Membranous urethra – runs through the urogenital diaphragm Spongy (penile) urethra – passes through the penis and opens via the external urethral orifice


Urethra: Urethra Figure 25.18a. b


Micturition (Voiding or Urination): Micturition (Voiding or Urination) The act of emptying the bladder Distension of bladder walls initiates spinal reflexes that: Stimulate contraction of the external urethral sphincter Inhibit the detrusor muscle and internal sphincter (temporarily) Voiding reflexes: Stimulate the detrusor muscle to contract Inhibit the internal and external sphincters


Micturition (Voiding or Urination): Micturition (Voiding or Urination) Figure 25.20a, b


Developmental Aspects: Developmental Aspects Three sets of embryonic kidneys develop, with only the last set persisting The pronephros never functions but its pronephric duct persists and connects to the cloaca The mesonephros claims this duct and it becomes the mesonephric duct The final metanephros develop by the fifth week and develop into adult kidneys


Developmental Aspects: Developmental Aspects Figure 25.21a, b


Developmental Aspects: Developmental Aspects Figure 25.21c, d


Developmental Aspects: Developmental Aspects Metanephros develop as ureteric buds that incline mesoderm to form nephrons Distal ends of ureteric tubes form the renal pelves, calyces, and collecting ducts Proximal ends called ureteric ducts become the ureters Metanephric kidneys are excreting urine by the third month The cloaca eventually develops into the rectum and anal canal


Developmental Aspects: Developmental Aspects Infants have small bladders and the kidneys cannot concentrate urine, resulting in frequent micturition Control of the voluntary urethral sphincter develops with the nervous system E. coli bacteria account for 80% of all urinary tract infections Sexually transmitted diseases can also inflame the urinary tract Kidney function declines with age, with many elderly becoming incontinent