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Chapter 26: The Urinary System

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1 Chapter 26: The Urinary System
BIO 211 Lecture Instructor: Dr. Gollwitzer

2 Today in class we will discuss:
The interrelationship between the CVS and urinary system The major functions of the urinary system Excretion Elimination Homeostatic regulation The basic principles of urine formation Major functions of each portion of the nephron and collecting system The 3 basic processes involved in urine formation Glomerular filtration Filtration pressures Tubular reabsorption Tubular secretion

3 CVS and Urinary System CVS delivers nutrients (from digestive tract) and O2 (from lungs) to cells in peripheral tissues CVS carries CO2 and waste products from peripheral tissues to sites of excretion CO2 removed at lungs Most physiological waste products removed by urinary system

4 Major Functions of Urinary System
Excretion Elimination Homeostatic regulation of: Blood plasma volume Solute concentration

5 Major Functions of Urinary System
Excretion Removal of organic wastes (e.g., urea, uric acid, creatinine) from body fluids (= urine formation) Performed by kidneys which act as filtering units Elimination Discharge of waste products into environment (urination) Occurs when urinary bladder contracts and forces urine through urethra and out of body

6 Major Functions of Urinary System: Homeostatic Regulation
Regulation of blood volume (water balance) and BP Adjusts volume of water lost in urine Releases Renin Involved in production of angiotensin II that affects BP, thirst, and other hormones (ADH, aldosterone) that affect water retention by kidneys Erythropoietin Stimulates erythropoiesis in bone marrow, maintains RBC volume

7 Major Functions of Urinary System: Homeostatic Regulation
Regulation of plasma ion concentrations (electrolyte balance) Controls amounts lost in urine (e.g., Na+, K+, Cl-) Controls Ca2+ levels by synthesis of calcitriol Reabsorption (conservation) of valuable nutrients Recycles valuable nutrients e.g., amino acids, glucose Prevents excretion in urine

8 Major Functions of Urinary System: Homeostatic Regulation
Stabilization of blood pH (acid-base balance) Controls loss of H+ and HCO3- in urine Detoxification Of poisons, e.g., drugs Deamination Removes NH2 (amino group) so amino acids can be metabolized

9 Basic Principles of Urine Formation
Urine = fluid containing: Water Ions Soluble compounds Goal of urine production To maintain homeostasis By regulating volume and composition of blood

10 Basic Principles of Urine Formation
Involves excretion of solutes (i.e., metabolic/organic waste products) Urea Most abundant Produced by breakdown of amino acids Creatinine Generated in skeletal muscle by breakdown of creatine phosphate (CP, high energy compound that plays a role as energy source in muscle contraction) Uric acid Formed by recycling nitrogenous bases from RNA

11 Basic Principles of Urine Formation
Waste products dissolved in bloodstream can only be eliminated when dissolved in urine Thus removal accompanied by unavoidable water loss To avoid dehydration, kidneys concentrate filtrate (i.e., reabsorb water) produced by glomerular filtration

12 Functional Anatomy of Nephron and Collecting System
Figure 26–6

13 3 Processes Involved in Urine Formation
Glomerular filtration Forces water and solutes out of blood in glomerulus into capsular space  filtrate Tubular reabsorption Recovers useful materials from filtrate Tubular secretion Ejects waste products, toxins, and other undesirable solutes into tubules

14 Glomerular Filtration
Occurs in renal corpuscle Hydrostatic pressure forces water and solutes: Out of blood in glomerulus Into capsular space  filtrate Occurs solely on basis of size Small solute molecules carried with filtrate

15 Glomerular Filtration
Involves passage across filtration membrane which is composed of 3 cellular units Glomerular capillary endothelium Lamina densa Filtration slits

16 Glomerular Filtration
Glomerular capillary endothelium Filtered through pores in fenestrated capillaries Least selective filter Pores too small for RBCs to pass through Large enough for plasma proteins

17 Renal Corpuscle Figure 26–8

18 Glomerular Filtration
Lamina densa Basement membrane of glomerular capillaries More selective filter Blocks passage of large proteins Only small polypeptides, nutrients, and ions can cross

19 Figure 26–10, 7th edition

20 Glomerular Filtration
Filtration slits Gaps between pedicels of podocytes (visceral epithelium around glomerulus) Finest filter No polypeptides pass through Only nutrients, ions into capsular space Thus, glomerular filtrate: Does not contain plasma proteins or polypeptides Does contain small organic molecules (e.g., nutrients) and ions in same concentration as in plasma

21 Filtration Pressures Filtration pressure = balance between:
Hydrostatic (fluid) pressures Glomerular hydrostatic pressure (GHP) in capillaries (50 mmg Hg) Capsular hydrostatic pressure (CHP) (15 mm Hg) Blood osmotic pressure (BOP) (25 mm Hg)

22 Filtration Pressures Hydrostatic (fluid) pressures
Glomerular hydrostatic pressure (GHP) (50 mm Hg) = BP in glomerular capillaries Higher in glomerulus than in peripheral capillaries (35 mm Hg) Because efferent arteriole smaller in diameter than afferent arteriole, need higher BP to force blood into it Promotes filtration – pushes water and solutes out of plasma in capillaries into filtrate Opposed by…

23 Filtration Pressures Hydrostatic (fluid) pressures
Capsular hydrostatic pressure (CHP) (15 mm Hg) Opposes filtration – pushes water and solutes out of filtrate into plasma in capillaries Results from resistance to flow along nephron and conducting system that causes water to collect in Bowman’s capsule More water in capsule  more pressure

24 Filtration Pressures Blood osmotic pressure (BOP) (25 mm Hg)
Results from presence of suspended proteins in blood Promotes return of water into glomerulus Opposes filtration Tends to draw water out of filtrate and into plasma

25 Figure 26–10, 7th edition

26 Summary of Filtration Pressures
Hydrostatic pressures GHP (pushing out of glomerulus) = 50 mm Hg CHP (pushing into glomerulus) = 15 mm Hg Net = 35 mm Hg (pushing out of glomerulus) Osmotic pressure BOP (draws into glomerulus) = 25 mm Hg Filtration pressure = 10 mm Hg Difference between net hydrostatic pressure and blood osmotic pressure

27 Summary of Filtration Pressures
Problems that affect filtration pressure Can seriously disrupt kidney function Can cause a variety of clinical symptoms, e.g., Drop in systolic pressure from 120 to < 110 mm Hg would eliminate filtration pressure (10 mm Hg)

28 Today in class we will discuss:
The 3 basic processes involved in urine formation Glomerular filtration Glomerular Filtration Rate Renal Failure Tubular reabsorption PCT, Loop of Henle & Countercurrent Exchange,DCT Collecting System Tubular secretion PCT, DCT and Collecting system Urine Compare/contrast to plasma General characteristics Hormone influence of volume and concentration Voluntary & involuntary regulation of urination and the micturition reflex

29 Glomerular Filtration Rate (GFR)
Gomerular filtration Vital first step essential to all other kidney functions Must occur so: Waste products excreted pH controlled Blood volume maintained GFR = amount of filtrate kidneys produce per minute Avg GFR = 125 mL/min or 50 gal/day (out of 480 gallons of blood flow/day) 10% of fluid delivered by renal arteries enters capsular spaces 99% of this reabsorbed so urinate only 0.5 gallons/day

30 Glomerular Filtration Rate (GFR)
Measured using creatinine clearance test (CCT) Breakdown of CP in muscle  creatinine Creatinine enters filtrate at glomerulus and is not reabsorbed so is excreted in urine Can compare amount of creatinine in blood vs. in urine during 24 hour and estimate GFR If glomerulus damaged, GFR will be altered (have more or less creatinine in urine than normal)

31 Glomerular Filtration Rate (GFR)
GFR depends on: Adequate blood flow to glomerulus Maintenance of normal filtration pressures Affected by anything that reduces renal blood flow or BP, e.g., Hypotension, hemorrhage, shock, dehydration Decreased renal blood volume and/or BP  decreased filtration pressure  decreased GFR

32 Control of GFR GFR increased by: EPO (relatively minor)
Renin-angiotensin system Natriuretic peptides (ANP and BNP)

33 Control of GFR Decreased BP and/or blood volume 
Decreased O2  JGA  EPO  Increased RBCs  Increased O2 delivery Increased blood volume  increased BP  Increased filtration pressure Increased GFR Decreased renal blood flow  JGA  renin-angiotensin system  Increased blood volume  increased BP 

34 EPO and Renin Figure 18–19b

35 Renin-Angiotensin System
Renin (enzyme)  (prohormone) angiotensinogen  (hormone) angiotensin I (in liver) Angiotensin I  angiotensin II (in lung capillaries) Angiotensin II  increased blood volume and BP  increased GFR

36 Primary Effects of Angiotensin II
Stimulates constriction of efferent arterioles  increased glomerular pressure Directly stimulates reabsorption of Na+ and H2O in DCT  increased blood volume and BP Stimulates adrenal cortex  aldosterone  reabsorption of Na+ (and H2O)  increased blood volume and BP Stimulates posterior pituitary  ADH  reabsorption of H2O  increased blood volume and BP Stimulates thirst  increased blood volume and BP Stimulates vasoconstriction of arterioles

37 Renin-Angiotensin System: Response to Reduction in GFR
Figure 26–11-0

38 Control of GFR Increased blood volume or BP  stretched cardiac muscle cells  natriuretic peptides ANP = atrial NP BNP = brain NP (produced by ventricles) Natriuretic peptides Increase GFR Decrease blood volume and BP Via 2 mechanisms

39 Natriuretic Peptides Increase GFR
Act opposite to angiotensin II Increase Na+ and H2O loss Inhibit renin release Inhibit secretion of aldosterone and ADH Suppress thirst Prevent increased BP by angiotensin II and NE Increase glomerular pressures Dilate afferent arterioles Constrict efferent arterioles Also increase tubular reabsorption of Na+ Decreases blood volume and BP

40 Renal Failure When filtration (GFR) slows, urine production decreases
Symptoms appear because water, ions, and metabolic wastes retained rather than excreted Almost all systems affected: fluid balance, pH, muscular contraction, neural function, digestive function, metabolism Leads to: Hypertension (due to blood “backing up”) Anemia due to lack of erythropoietin production CNS problems (sleepiness, seizures, delirium, coma, death)

41 Renal Failure Acute renal failure Chronic renal failure
From exposure to toxic drugs, renal ischemia, urinary obstruction, trauma Develops quickly, but usually temporary With supportive treatment can survive Chronic renal failure Condition deteriorates gradually Cannot be reversed Dialysis or kidney transplant may prolong life

42 Reabsorption and Secretion
Occur in all segments of renal tubules Relative importance changes from segment to segment

43 Tubular Reabsorption Molecules move from filtrate  across tubular epithelium into peritubular interstitial fluid and blood Water, valuable solutes (e.g., nutrients, proteins, amino acids, glucose) Occurs through diffusion, osmosis (H2O), active transport by carrier proteins Occurs primarily along PCT (also along renal tubule and collecting system)

44 Tubular Secretion Molecules move from peritubular fluid into tubular fluid Lowers plasma concentration of undesirable materials Necessary because filtration does not force all solutes out of plasma Primary method of excretion for many drugs Occurs primarily at PCT and DCT

45 Reabsorption and Secretion: PCT
Primarily reabsorption 60-70% of filtrate Includes: Organic nutrients (99-100%), e.g., glucose, amino acids, proteins, lipids, vitamins Water (60-70%) Ions (60-70%), e.g., Na+, Cl-; also K+, Ca2+, HCO3- Reabsorbed materials enter peritubular fluid and capillaries Secretion H+, NH4+, creatinine, drugs, toxins

46 Reabsorption: Loop of Henle
Na+, Cl- Water Accomplished by countercurrent exchange Refers to exchange by tubular fluids moving in opposite directions Fluid in descending limb flows toward renal pelvis Fluid in ascending limb flows toward cortex

47 Countercurrent Exchange
Occurs because of different permeabilities of segments of LOH Descending limb (thin) Permeable to water Relatively impermeable to solutes Ascending limb (thick) Relatively impermeable to water and solutes Has active transport mechanisms Pump Na+ and Cl- from tubular fluid into peritubular fluid

48 Countercurrent Exchange
Na+ and Cl- pumped out of thick ascending limb into peritubular fluid Increases osmotic concentration in peritubular fluid around thin descending limb Results in osmotic flow of H2O out of thin descending limb into peritubular fluid  increased solute concentration in thin descending limb Arrival of concentrated solution in thick ascending limb increases transport of Na+ and Cl- into peritubular fluid

49 Overview of Urine Formation
Figure 26–16

50 Reabsorption and Secretion: DCT
Reabsorption (by vasa recta) Na+ (under influence of aldosterone), Cl- Ca2+(under influence of PTH and calcitriol) H2O (under influence of ADH) Secretion K+ (in exchange for Na+), H+ NH4+ (from deamination; produces lactic acid, ketone bodies  acidosis) Creatinine, drugs, toxins

51 Reabsorption and Secretion: Collecting System
Makes final adjustments to ion concentration and urine volume Reabsorption Na+ (under influence of aldosterone) H2O (under influence of ADH) HCO3- Urea (distal portion) Secretion K+, H+

52 Figure 26–15

53 Summary: Urine Formation
Involves all parts of nephron and collecting system Processes occur primarily in certain areas Glomerular filtration at the renal corpuscle Nutrient reabsorption in the PCT Water and salt conservation in loop of Henle Tubular secretion in the DCT Regulation of final volume and solute concentration occurs in loops of Henle and collecting system

54 Normal Kidney Function
Continues as long as filtration, reabsorption, and secretion function within narrow limits Disruption of kidney function has immediate effects on composition of circulating blood If both kidneys affected, death occurs within few days

55 Normal Kidney Function
Glomeruli produce approx 48 gallons (180 L) of filtrate/day 70X plasma volume! Almost all fluid volume must be reabsorbed to avoid fatal dehydration

56 Urine Clear, sterile solution
Yellow (“straw”) color due to pigment (urobilin) Urinalysis = analysis of urine sample Results from filtration, absorption, secretion activities of nephron

57 Table 26–5

58 Urine vs. Plasma Little to no metabolites and nutrients (glucose, lipids, amino acids, proteins) Slightly increased Na+, greatly increased K+ and Cl-, and greatly decreased HCO3- Very high levels of nitrogenous wastes (creatinine, urea, ammonia, uric acid) Lower pH (6.0 vs. 7.4) Much greater water content (95% vs. 50%)

59 Table 26–2

60 Diuresis Elimination of urine
Usually used to indicate production large volumes of urine Diuretics Drugs that promote water loss in urine Reduce Blood volume Blood pressure Extracellular fluid volume

61 Micturition Reflex Coordinates the process of urination
Begins when stretch receptors in bladder stimulate parasympathetic neurons Results in contraction of detrusor muscle contraction Voluntary relaxation of external urethral sphincter causes relaxation of internal urethral sphincter

62 Micturition Reflex Figure 26–20

63 Voluntary Control Infants Incontinence =
Lack voluntary control over urination Corticospinal connections are not established Incontinence = Inability to voluntarily control urination May be caused by trauma to internal or external urethral sphincter

64 Age-Related Changes in Urinary System
Decline in number of functional nephrons Reduction in GFR Reduced sensitivity to ADH

65 Age-Related Changes in Urinary System
Problems with micturition reflex Sphincter muscles lose tone  incontinence Lose control due to: Stroke Alzheimer’s disease CNS problems In males, enlarged prostate compresses urethra, restricts urine flow urinary retention

66 The Excretory System Includes all systems with excretory functions that affect body fluids composition Urinary system Integumentary system Respiratory system Digestive system

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