Chapter 15 The Urinary System

Biology 112Tri-County Technical College

Pendleton, SC

Functions of Urinary System

Regulating blood volume and blood pressure

adjusting volume of water lost in urine, releasing erythropoietin and renin

Regulating plasma [ ]s of Na+, K+, Cl-, and other ions by controlling quantities lost in urine and controlling Ca+2 ion levels by synthesis of calcitriolStabilizing blood pH by controlling loss of hydrogen ions and bicarbonate ions in urine

Functions, cont.Conserving valuable nutrients while eliminating organic waste products, especially nitrogenous products like urea and uric acidAssisting liver in detoxifying poisons and during starvation, deaminating amino acids so can broken down by other tissuePERFORMS vital excretory functions and eliminates organic wastes generated by cells throughout the body

Location is everything…almost

Kidneys, ureters, urinary bladder, and urethraPair of kidneys located retroperitoneal (beneath parietal peritoneum) in superior lumbar region (T12-L3 vertebra)Each kidney drained by ureter into urinary bladder located in lower pelvic regionUrinary bladder is drained by the urethra

Organs of the System, Visual

Kidney StructureMedial indention called HILUS where ureters, renal blood vessels, and nerves enter/exit the kidneyRenal cortex=outer layer (light colored)Renal medulla=beneath cortex with many triangular regions (striped) which are the renal pyramids

Medulla is darker, reddish brown areaBroader base of each pyramid faces cortex with tip facing inner region of kidneyPyramids separated by renal columns

Structure, cont.Medial to hilus is flat, basinlike cavity called renal pelvis

Pelvis continuous with ureter leaving hilus

Extensions of the pelvis, the calyces (minor and major) form cup-shaped areas that enclose tips of the pyramidsCalyces collect urine with drains from tips of pyramids into renal pelvisUrine flows from pelvis into ureterbladder for temporary storage

Kidney Structure, Visual

Nephron StructureAre structural and functional unit of kidneyResponsible for urine productionEach nephron has 2 main structures—Glomerulus and Renal TubuleGlomerulus is knot of capillariesRenal tubule is cup-shaped and closed end completely surrounds glomerulusEnlarged portion of renal tubule called GLOMERULAR (Bowman’s) CAPSULE

Nephron Structure, cont.Inner layer of capsule composed of highly modified cells called podocytes

Podocytes have long branching processes that intertwine with each other and cling to glomerulusOpenings (slits) exist between extensions to form porous membrane around glomerulus

Rest of tubule coils and twists (Proximal) before forming hairpin curve (Loop of Henle) and then Distal Convoluted tubule before entering Collecting Duct

Nephron Structure, cont.Glomerular capsuleproximal convoluted tubuleLoop of Henledistal convoluted tubulecollecting ductAll tubules cells have microvilli (proximal has the most)Cortical nephrons most numerous—located almost entirely within cortexJuxtamedullary nephrons-situated close to cortex-medulla junction and their loops of Henle extend deep into the medulla

Nephron Structure, cont.

Collecting ducts receive urine from many nephronsDucts run downward through medullary pyramids giving them a striped appearanceCollecting ducts deliver final urine product into the calyces and renal pelvis

Nephron Visual

Nephron Blood FlowEvery nephron associated with two capillary beds: glomerulus and peritubular capillary bedGlomerulus fed and drained by arteriolesAfferent (from interlobular artery) is feeder vessel and efferent arteriole receives blood that has passed through the glomerulusGlomerulus specialized for FILTRATION

Nephron Blood Flow, cont.Afferent arteriole has larger diameter than efferent so BP in glomerulus very HIGHThis high pressure forces fluid and solutes (smaller than proteins) OUT of blood into glomerular capsulePeritubular capillaries arise from efferent arteriole that drains glomerulusThese capillaires are LOW pressure porous vessels adapted for ABSORPTION rather than filtration

Nephron Blood Flow, cont.Peritubular capillaries cling closely to whole length of renal tubuleIdeal position to receive solutes and water from tubule cells as these substances are “reabsorbed from filtrate moving through tubulePeritubular capillaries drain into interlobular veins leaving the cortex

Blood Flow Visual

Processes of Urine Formation

Filtration, reabsorption, and secretionFILTRATION is nonselective, passive processGlomerulus acts as filter and filtrate formed essentially blood plasma w/o proteinsBlood proteins and blood cells too large to pass through filtration membraneWhen either appear in urine=problem with glomerular filters

Urine Formation, cont.Systemic BP normal, filtrate WILL be formedArterial BP < too low, glomerular pressure becomes inadequate to force substances out of blood into tubules and filtrate formation STOPSOliguria=abnormally low urinary output (100-400 ml/day)Anuria=urine output < 100 ml/dayLow urine output=low blood pressure, transfusion reactions, acute inflammation, or crush injuries of kidneys

Reabsorption

Designed to reclaim useful substancesFiltrate contains wastes/excess ions that must be removed but also contains water, glucose, amino acids, and ions that must be reclaimed and returned to bloodTubular reabsorption begins as soon as filtrate enters proximal convoluted tubuleWater by osmosis, most others by active transport

Uses membrane carriers and is VERY selective

Reabsorption, cont.

Carrier numbers are the KEYGlucose & AAs completely retained and nitrogenous wastes almost completely excretedVarious ions reabsorbed/excreted to maintain proper pH/electrolyte balance of bloodMost reabsorption occurs in proximal tubules, but under certain conditions, distal CT/collecting duct also active

SecretionTubular secretion essentially reabsorption in reverseH+; K+; and creatinine removed from blood and moved through tubule cells into filtrateAbsolutely essential for ridding body of substances NOT in filtrate (certain drugs) and as additional means of controlling pH

Processes Visual

More Processes Visual

Nitrogenous WastesMost important are urea, uric acid, and creatinineUrea product of deamination of AAsUric acid = nucleic acid metabolismTubule cells have few membrane carriers to reabsorb these substances and usually found in high [ ]s in urineCreatinine actively secreted into filtrate

Water and Electrolyte Balance

Water occupies 3 main locations in body (fluid compartments)

Intracellular fluid (about 2/3 of body fluid)Extracellular fluid (80% interstitial or tissue fluid and 20% is blood plasma)

More to fluid balance than just water-types and amounts of electrolytes (Na+, Ca++, K+) also very important to body homeostasis

Body Fluids Visual

Balance, cont.Water and electrolyte balance linked as kidneys process bloodBody cannot afford to lose more water than it takes inMost water intake is from fluids/foods consumed-about 10% comes from metabolismWater leaves via vaporization from lungs, perspiration, and defecation

More Balancing…Reabsorption of water/electrolytes by kidneys regulated primarily by hormonesBlood volume dropsBP dropsdecreases filtrate formedosmoreceptors in hypothalamus direct posterior pituitary to release ADH (kidney tubule cells) reabsorb more waterblood volume & BP increaseOnly small amount of very [ ]ed urine produced

Balancing the balancingADH not released (injury/destruction of hypothalamus/posterior pituitary) huge amts of very dilute urine flush from body dailyDiabetes insipidus can lead to severe dehydration and electrolyte imbalanceAldosterone (adrenal cortex) major factor regulating Na+ content of ECF and helps regulate [ ] of other ions (Cl-, K+, Mg++)Secretion of aldosterone influenced by falling BP or low levels of Na+ in blood

OMG, more balancing…Na+ MOST responsible for water flowW/WO aldosterone, about 80% of Na+ in filtrate reabsorbed in proximal CTIf aldosterone [ ]s high, most of remaining Na+ reabsorbed in distal CTSodium chloride actually reabsorbed because Cl- follows Na+For each Na+ reabsorbed, a K+ is secreted into filtrate = back to normal balance in bloodAs Na+ reclaimed, water follows passively

Please, no more balancingMost important trigger for aldosterone release is renin-angiotensin mechanismJuxtaglomerular apparatus (consists of modified smooth muscle cells in afferent arteriole + modified epithelial cells in part of DCT) stimulated by low BP or changes in solute content in filtraterelease renin into bloodcatalyzes production of antiotensin IIvasoconstriction of blood vessels and release of aldosterone by adrenal cortex cellsblood volume and BP >

The end of balancing..for now!!

Renin-angiotensin mechanism EXTREMELY important for regulating BPIndividuals with Addison’s disease (hypoaldosteronism) have polypuria

Excrete large volumes of urine and lose tremendous amounts of salt and water in urine

Blood Pressure Homeostasis

Acid-Base Blood Balance

Blood pH must be maintained ~ 7.35-7.45Alkalosis=pH above 7.45; acidosis= <7.357.35 represents higher than optimal H+ [ ] for functioning of most body cellsAny arterial pH between 7.35 and 7.00 called physiological acidosisSmall amounts of acidic substances in ingested foods but most H+s originate as by-products of cellular metabolismadds substances to blood that tend to disturb its acid-base balance

Not the same balancing…Metabolism produces many acids (phosphoric, lactic, types of fatty acids)Carbon dioxide released by metabolism forms carbonic acidCells also release ammonia and other basic substances as they go about their businessBlood buffers can tie up excess acids and bases (temporarily) and lungs have chief responsibility of eliminating carbon dioxide

Who is on first?Kidneys assume most of load for acid-base balance of bloodBuffers first line of defense in resisting pH changesBicarbonate, phosphate, and protein buffer systems are 3 major buffer systems of bodyAll work essentially in the same way….

Bicarbonate BufferingMixture of carbonic acid (H2CO3) and its salt, sodium bicarbonate (NaHCO3)

Bicarbonate ion (HCO3-) act as base to tie up H+s if blood becoming acidicCarbonic acid (H2CO3) dissociates in presence of rising OH- (blood becoming more basic) and releases H+s to bind with OH-sBuffers can tie up excess acids/bases temporarily but CANNOT eliminate them from the bodyLungs can dispose of carbonic acid by eliminating carbon dioxide

Buffer me up…Scotty!!!ONLY kidneys can rid body of other acids (from metabolism) and ONLY kidneys have power to regulate blood levels of alkaline substancesMost IMPORTANT for kidneys are excreting bicarbonate ions and by conserving (reabsorbing) or generating new bicarbonate ions

I Can’t Do It, Captain….Losing a HCO3- from body has same effect of gaining a H+ since it leaves a free hydrogen ionReabsorbing or generating an new HCO3- is same a losing a H+ because it tens to combine with a H+Urine pH ranges from 4.0 to 6.5 reflecting ability of renal tubules to excrete basic or acid ions to maintain blood pH homeostasis

The Urinary BladderSmooth, collapsible, muscular sacThree openings: 2 ureter and 1 urethra (drains the bladder)Smooth triangular region of bladder base outlined by three openings called the trigone (infections tend to persist in this region)Bladder wall contains 3 layers of smooth muscle (detrusor muscle) and its mucosa is special type of epithelium called transitional epithelium

Bladder Visual

But Mommy, I really have to go..

When empty, bladder is collapsed and its walls are thick and foldedUrine accumulates, its muscular walls stretch and transitional epithelium thins allowing bladder to store more urine w/o increasing its internal pressureUrine formed continually by kidneys and usually stored in bladder until its release is convenient (or NOT)

MicturitionAct of emptying the bladderTwo sphincters (valves) control flow from bladder (internal and external urethral sphincter)About 200 mls of urinestretch receptors activatedimpulses to sacral region of spinal cordback to bladder via pelvic splanchnic nervesbladder goes into reflex contractions

Micturition, cont.Contractions force stored urine past internal sphincter (smooth muscle-involuntary) into upper part of urethraPerson feels urge to voidLower external sphincter is skeletal muscle and subject to voluntary control so “going” can be delayedEventually, micturition occurs whether one wills it or not!!!

Some Key TermsIncontinence occurs when one is unable to voluntarily control external sphincter

Normal in children less than 2 YOA; those who sleep too soundly, & emotional problems, pressure, and/or nervous system problems (stroke/spinal cord injury)

Retention is condition in which bladder is unable to expel contained urine

General anesthesia surgery; hypertrophy

Key Terms, cont.Hypertrophy: enlargement of prostate gland which surrounds neck of bladderUrethritis: inflammation of urethra

more common in females

Cystitis: inflammation of urinary bladderSymptoms include dysuria, urinary urgency and frequency, fever, cloudy/blood-tinged urineIf kidneys involved; back pain and severe headache common

Key Terms, IIIEscherichia coli normal flora of intestinal tract

pathogenic in sterile urinary tract

Glomerulonephritis: glomerular filters become clogged with antigen-antibody complexes resulting from streptococcal infections

Occurs most often in children with strep throat or scarlet fever that was not treated promptly or properly

Key Terms IV

Urgency describes feeling that it is necessary to voidFrequency describes frequent voiding of small amounts of urineBoth are generally consequences of aging process which causes bladder shrinkage and loss of bladder tone