1. Anatomy and Physiology, Seventh Edition
Rod R. Seeley Idaho State University
Trent D. Stephens Idaho State University
Philip Tate Phoenix College
Chapter 26
Lecture Outline*
*See PowerPoint Image Slides for all figures and tables pre-inserted into PowerPoint without notes.
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.

2. Chapter 26
Urinary System

3. Urinary System Functions
Filtering of blood: involves three processes- filtration, reabsorption, secretion.
Regulation of
Blood volume
Concentration of blood solutes: Na+, Cl-, K+, Ca2+, HPO4-2
pH of extracellular fluid: secrete H+
Blood cell synthesis
Synthesis of vitamin D

4. Urinary System Anatomy

5. Location and External Anatomy of Kidneys
Renal capsule: fibrous connective tissue. Surrounds each kidney
Perirenal fat
Engulfs renal capsule and acts as cushioning
Renal fascia: thin layer loose connective tissue
Anchors kidneys and surrounding adipose to abdominal wall
Hilum
Renal artery and nerves enter and renal vein and ureter exit kidneys
Opens into renal sinus (cavity filled with fat and loose connective tissue)
Location
Lie behind peritoneum (retroperitoneal) on posterior abdominal wall on either side of vertebral column
Lumbar vertebrae and rib cage partially protect
Right kidney slightly lower than left

6. Internal Anatomy of Kidneys
Cortex: outer area
Renal columns: part of cortical tissue that extends into medulla
Medulla: inner area; surrounds renal sinus
Renal pyramids: cone-shaped. Base is boundary between cortex and medulla. Apex of pyramid is renal papilla, points toward sinus.
Calyces
Minor: papillae extend into funnel of minor calyx
Major: converge to form pelvis
Pelvis: enlarged chamber formed by major calyces
Ureter: exits at the hilum; connects to urinary bladder

7. The Nephron Functional and histological unit of the kidney
Parts of the nephron: Bowman’s capsule, proximal tubule, loop of Henle (nephronic loop), distal tubule
Urine continues from the nephron to collecting ducts, papillary ducts, minor calyses, major calyses, and the renal pelvis
Collecting ducts, parts of the loops of Henle, and papillary ducts are in the renal medulla

8. Types of Nephrons
Juxtamedullary nephrons. Renal corpuscle near the cortical medullary border. Loops of Henle extend deep into the medulla.
Cortical nephrons. Renal corpuscle nearer to the periphery of the cortex. Loops of Henle do not extend deep into the medulla.
Renal corpuscle. Bowman’s capsule plus a capillary bed called the glomerulus.

9. Renal Corpuscle
Bowman’s capsule: outer parietal (simple squamous epithelium) and visceral (cells called podocytes) layers.
Glomerulus: network of capillaries. Blood enters through afferent arteriole, exits through efferent arteriole.

10. Bowman’s Capsule
Parietal layer: outer. Simple squamous epithelium that becomes cube-shaped where Bowman’s capsule ends and proximal tubule begins
Visceral layer: inner. Specialized podocytes that wrap around the glomerular capillaries

11. Fenestrae: window-like openings in the endothelial cells of the glomerular capillaries.
Filtrations slits: gaps between the cell processes of the podocytes. Basement membrane sandwiched between the endothelial cells of the glomerular capillaries and the podocytes.
Filtration membrane: capillary endothelium, basement membrane and podocytes. First stage of urine formation occurs here when fluid from blood in capillaries moves across filtration membrane into the lumen inside Bowman’s capsule.
Filtration Membrane

12. Circulation in the Glomerulus
Afferent arteriole: supplies blood to glomerulus
Efferent arteriole: drains glomerulus
Both vessels have a layer of smooth muscle
Juxtaglomerular apparatus: sight of renin production
Juxtaglomerular cells- ring of smooth muscle in the afferent arteriole where the latter enters Bowman’s capsule
Macula densa- Specialized tubule cells of the distal tubule. The distal tubule lies between the afferent and efferent arterioles.

13. Histology of the Nephron
Proximal tubule: simple cuboidal epithelium with many microvilli
Loops of Henle
Descending limb: first part similar to proximal tubule. Latter part simple squamous epithelium and thinner
Ascending limb: first part simple squamous epithelium and thin, distal part thicker and simple cuboidal
Distal tubule: shorter than proximal tubule. Simple cuboidal, but smaller cells and very few microvilli
Collecting ducts: form where many distal tubules come together. Larger in diameter, simple cuboidal epithelium. Form medullary rays and lead to papillary ducts

14. Circulation Through the Kidney
Arterial supply:
Renal arteries branch from abdominal aorta
Segmental arteries branch from renal
Interlobar arteries ascend within renal columns toward cortex
Arcuate arteries branch and arch over the base of the pyramids
Interlobular arteries project into cortex and give rise to afferent arterioles

15. Circulation Through the Kidney
The part of the circulation involved with urine formation
Afferent arterioles supply blood to glomerulus
Glomerulus
Efferent arterioles exit the renal corpuscle
Peritubular capillaries form a plexus around the proximal and distal tubules
Vasa recta: specialized parts of peritubular capillaries that course into medulla along with loops of Henle, then back toward cortex
Circulation Through the Kidney

16. Circulation Through the Kidney
Venous drainage
Peritubular capillaries drain into interlobular veins and lead to
Arcuates
Interlobar veins
Renal veins

17. Anatomy and Histology of Ureters and Bladder
Ureters: bring urine from renal pelvis to urinary bladder. Lined by transitional epithelium
Urinary bladder: hollow muscular container. In pelvic cavity posterior to symphysis pubis. Lined with transitional epithelium; muscle part of wall is detrusor
Trigone: interior of urinary bladder. Triangular area between the entry of the two ureters and the exit of the urethra. Area expands less than rest of bladder during filling

18. Anatomy and Histology of Urethra
Male: extends from the inferior part of the urinary bladder through the penis
Female: shorter; opens into vestibule anterior to vaginal opening
Internal urinary sphincter: in males, elastic connective tissue and smooth muscle keep semen from entering urinary bladder during ejaculation
External urinary sphincter: skeletal muscle surrounds urethra as it extends through pelvic floor. Acts as a valve

19. Urine Formation
Nephrons considered functional units the kidney: smallest structural component capable of producing urine

20. Filtration
Movement of fluid, derived from blood flowing through the glomerulus, across filtration membrane
Filtrate: water, small molecules, ions that can pass through membrane
Pressure difference forces filtrate across filtration membrane
Renal fraction: part of total cardiac output that passes through the kidneys. Varies from 12-30%; averages 21%
Renal blood flow rate: 1176 mL/min
Renal plasma flow rate: renal blood flow rate X fraction of blood that is plasma: 650 mL/min
Filtration fraction: part of plasma that is filtered into lumen of Bowman’s capsules; average 19%
Glomerular filtration rate (GFR): amount of filtrate produced each minute. 180 L/day
Average urine production/day: 1-2 L. Most of filtrate must be reabsorbed

22. Filtration
Filtration membrane: filtration barrier. It prevents blood cells and proteins from entering lumen of Bowman’s capsule, but is many times more permeable than a typical capillary
Fenestrated endothelium, basement membrane and pores formed by podocytes
Some albumin and small hormonal proteins enter the filtrate, but these are reabsorbed and metabolized by the cells of the proximal tubule. Very little protein normally found in urine
Filtration pressure: pressure gradient responsible for filtration; forces fluid from glomerular capillary across membrane into lumen of Bowman’s capsules
Forces that affect movement of fluid into or out of the lumen of Bowman’s capsule
Glomerular capillary pressure (GCP): blood pressure inside capillary tends to move fluid out of capillary into Bowman’s capsule
Capsule pressure (CP): pressure of filtrate already in the lumen
Blood colloid osmotic pressure (BCOP): osmotic pressure caused by proteins in blood. Favors fluid movement into the capillary from the lumen. BCOP greater at end of glomerular capillary than at beginning because of fluid leaving capillary and entering lumen
Filtration pressure (10 mm Hg) = GCP (50 mm Hg) – CP (10 mm Hg) – BCOP (30 mm Hg)

23. Filtration Pressure

24. Filtration
Colloid osmotic pressure in Bowman’s capsule normally close to zero. During diseases like glomerular nephritis, proteins enter the filtrate and filtrate exerts an osmotic pressure, increasing volume of filtrate
High glomerular capillary pressure results from
Low resistance to blood flow in afferent arterioles
Low resistance to blood flow in glomerular capillaries
High resistance to blood flow in efferent arterioles: small diameter vessels
Pressure lower in peritubular capillaries downstream from efferent arterioles
Filtrate is forced across filtration membrane; fluid moves into peritubular capillaries from interstitial fluid
Changes in afferent and efferent arteriole diameter alter filtration pressure
Dilation of afferent arterioles/constriction efferent arterioles increases glomerular capillary pressure, increasing filtration pressure and thus glomerular filtration

25. Tubular Reabsorption: Overview
Tubular reabsorption: occurs as filtrate flows through the lumens of proximal tubule, loop of Henle, distal tubule, and collecting ducts
Results because of
Diffusion
Facilitated diffusion
Active transport
Cotransport
Osmosis
Substances transported to interstitial fluid and reabsorbed into peritubular capillaries: inorganic salts, organic molecules, 99% of filtrate volume. These substances return to general circulation through venous system

26. Reabsorption in Proximal Tubule
Substances pass through cells of tubule wall. Each cell has
Apical surface: surface that faces filtrate. Apical membrane
Basal surface: faces interstitial fluid. Basal membrane
Lateral surfaces: surfaces between cells
Active transport of Na+ across the basal membrane from cytoplasm to interstitial fluid linked to reabsorption of most solutes
Because of active transport, the concentration of Na+ is low inside the cell and Na+ moves into nephron cell from filtrate through the apical membrane. Other substances moved by cotransport from the filtrate into the nephron cell are substances which should be retained by the body
Substances transported
Through apical membrane: Na+, Cl-, glucose, amino acids, and water.
Through basal membrane: Na+, K+,
Cl-, glucose, amino acids, water
Reabsorption in Proximal Tubule

27. Reabsorption in Proximal Tubule
Number of carrier molecules limits rate of transport
In diabetes mellitus
Concentration of glucose in filtrate exceeds rate of transport
High concentration of glucose in plasma (and thus in filtrate) reflected in glucose in the urine
Diffusion between cells: from lumen of nephron into interstitial fluid
Depends on rate of transport of same solutes through the cells of the tubule
K+, Ca2+, and Mg2+
Filtrate volume reduced by 65% due to osmosis of water

28. Reabsorption in Loop of Henle
Loop of Henle descends into medulla; interstitial fluid is high in solutes.
Descending thin segment is highly permeable to water and moderately permeable to urea, sodium, most other ions (passive).
Water moves out of nephron, solutes in. Volume of filtrate reduced by another 15%.
Ascending thin segment is not permeable to water, but is permeable to solutes. Solutes diffuse out of the tubule and into the more dilute interstitial fluid as the ascending limb projects toward the cortex. Solutes diffuse into the descending vasa recta.

29. Reabsorption in Loop of Henle
The wall of the ascending limb of the loop of Henle is not permeable to water. Na+ moves across the wall of the basal membrane by active transport, establishing a concentration gradient for Na+. K+ and Cl- are cotransported with Na across the apical membrane and ions pass by facilitated diffusion across the basal cell membrane of the tubule cells.
At the end of the loop of Henle, inside of nephron is 100 mOsm/kg. Interstitial fluid in the cortex is 300mOsm/kg. Filtrate within DCT is much more dilute than the interstitial fluid which surrounds it.

30. Reabsorption in Distal Tubule and Collecting Duct
Active transport of Na+ out of tubule cells into interstitial fluid with cotransport of Cl-
Na+ moves from filtrate into tubule cells due to concentration gradient
Collecting ducts extend from cortex (interstitial fluid 300 mOsm/kg) through medulla (interstitial fluid very high)
Water moves by osmosis from distal tubule and collecting duct into more concentrated interstitial fluid
Permeability of wall of distal tubule and collecting ducts have variable permeability to water
Urine can vary in concentration from low volume of high concentration to high volume of low concentration

31. Changes in Concentration of Solutes in the Nephron
Urea: enters glomerular filtrate.
As volume of filtrate decreases, concentration of urea increases
Walls of nephron not very permeable to urea: only 40-60% passively reabsorbed
Urate ions, creatinine, sulfates, phosphates, nitrates partially reabsorbed
Concentration is high in urine
Toxic substances and are eliminated

32. Tubular Secretion
Moves metabolic by-products, drugs, molecules not normally produced by the body into tubule of nephron
Active or passive
Ammonia: produced by epithelial cells of nephron from deamination of amino acids. Diffuses into lumen
H+, K+, penicillin, and substances such as para-aminohippuric acid (PAH): actively secreted into nephron

33. Secretion of Hydrogen and Potassium
Hydrogen ions secreted into filtrate by countertransport in proximal tubule
H+ either diffuse from peritubular capillaries into interstitial fluid and then into epithelial cells of tubule or derived from reaction between carbon dioxide and water in cells of tubule.
Na+ and HCO3- cotransported across basal membrane into interstitial fluid, then diffuse into peritubular capillaries

34. Secretion of Hydrogen and Potassium
H+ and K+ secreted into filtrate by countertransport in distal tubule. Na+ and K+ move by active transport across the basal membrane. Na+ and HCO3- cotransported across basal membrane into interstitial fluid, then diffuse into peritubular capillaries

35. Urine Production In ascending limb of loop of Henle
Na+, Cl-, K+ transported out of filtrate
Water remains
In distal tubules and collecting ducts
Water movement out regulated by ADH
If absent, water not reabsorbed and dilute urine produced
If ADH present, water moves out, concentrated urine produced
In Proximal tubules
Na+ and other substances removed
Water follows passively
Filtrate volume reduced
In descending limb of loop of Henle
Water exits passively, solute enters
Filtrate volume reduced 15%

36. Urine Concentration Mechanism
When large volume of water consumed
Eliminate excess without losing large amounts of electrolytes
Response is that kidneys produce large volume of dilute urine
When drinking water not available
Kidneys produce small volume of concentrated urine
Removes waste and prevents rapid dehydration
Mechanisms that create urine of variable concentration
Maintenance of high concentration of solutes in medulla
Countercurrent functions of loops of Henle
Control of permeability of distal nephron to water

37. Medullary Concentration Gradient
In order to concentrate urine (and prevent a large volume of water from being lost), the kidney must maintain a high concentration of solutes in the medulla
Interstitial fluid concentration (mOsm/kg) is 300 in the cortical region and gradually increases to 1200 at the tip of the pyramids in the medulla
Maintenance of this gradient depends upon
Functions of loops of Henle
Vasa recta flowing countercurrent to filtrate in loops of Henle
Distribution and recycling of urea

38. Creating/Maintaining High Solute Concentration in Medulla
Active transport of Na+ and cotransport of ions such as K+ and Cl- and other ions out of the thick portion of ascending limb into interstitial fluid
Impermeability of thin and thick parts of ascending limb of loop of Henle to water
Vasa recta remove excess water and solutes that enter the medulla without destroying the high concentration of solutes in interstitial fluid of medulla
Active transport of ions from collecting ducts into interstitial fluid of medulla
Passive diffusion of urea from collecting ducts into interstitial fluid of medulla, impermeability of the ascending limb and permeability of the descending limb of the loops of Henle to urea

39. Loops of Henle Juxtamedullary nephrons: long loops.
Walls of descending limbs permeable to water, water moves out into interstitial fluid
Walls of ascending limb impermeable to water
Solute diffuses out of thin segment of ascending limb as it passes though progressively less concentrated interstitial fluid
Na+, K+ and Cl- actively transported out of ascending limb into interstitial fluid
Thus, water enters interstitial fluid from descending limbs and solutes enter interstitial fluid from ascending limbs

40. Vasa Recta
Countercurrent systems that remove excess water and solutes from medulla: parallel tubes in which fluid flows, but in opposite directions
Blood flows through vasa recta to the medulla, vessels turn near tip of renal pyramid, then blood flows in opposite direction
Walls are permeable to water and to solutes: as blood flows toward medulla, water moves out, solutes diffuse in. As blood flows back toward cortex, water moves into vasa recta, some solutes diffuse out
Diffusion is such that slightly more water and slightly more solute are carried from the medulla by the vasa recta than enter it

41. Loops of Henle and vasa recta function together to maintain a high concentration of solutes in the interstitial fluids of the medulla and to carry away the water and solutes that enter the medulla from the loops of Henle and collecting ducts
Water moves out of descending limb and enters vasa recta
Solutes diffuse out of ascending thin segment and enter vasa recta, but water does not
Solutes transported out of thick segment of ascending enter the vasa recta
Excess water and solutes carried away from medulla without reducing high concentration of solutes
Concentration of filtrate reduced to 100 mOsm/kg by the time it reaches distal tubule

42. Water and solutes move out of the collecting duct into the vasa recta

43. Urea Responsible for large part of high osmolality in medulla
Descending limbs of loops of Henle permeable to urea; urea diffuses into interstitial fluid
Ascending limbs and distal tubules impermeable to urea
Collecting ducts permeable to urea; some diffuses out into interstitial fluid
Urea flows in a cycle maintaining high urea concentration in medulla

44. Urine Concentrating Mechanisms

45. ADH and the Nephron

46. Renin/Angiotensin/Aldosterone

47. Other Hormones Atrial natriuretic hormone
Produced by right atrium of heart when blood volume increases stretching cells
Inhibits Na+ reabsorption
Inhibits ADH production
Increases volume of urine produced
Venous return is lowered, volume in right atrium decreases
Prostaglandins and kinins: produced in kidney. Role unclear

48. Autoregulation and Sympathetic Stimulation
Involves changes in degree of constriction in afferent arterioles
As systemic BP increases, afferent arterioles constrict and prevent increase in renal blood flow
Increased rate of blood flow of filtrate past cells of macula densa: signal sent to juxtaglomerular apparatus, afferent arteriole constricts
Sympathetic stimulation: norepinephrine
Constricts small arteries and afferent arterioles
Decreases renal blood flow and thus filtrate formation
During shock or intense exercise: intense sympathetic stimulation, rate of filtrate formation drops to a few mm

49. Clearance and Tubular Load
Plasma clearance: calculated using substances like inulin
Volume of plasma cleared of a specific substance each minute
Used to estimate GFR
Used to calculate renal plasma flow. Calculated using substances like PAH
Used to determine which drugs or other substances excreted by kidney
Tubular load
Total amount of substance that passes through filtration membrane into nephrons each minute

50. Maximum rate at which a substance can be actively absorbed
Each substance has its own tubular maximum
Normally, glucose concentration in the plasma (and thus filtrate) is lower than the tubular maximum and all of it is reabsorbed; none of it is found in the urine
In diabetes mellitus tubular load exceeds tubular maximum and glucose appears in urine. Urine volume increases because glucose in filtrate increases osmolality of filtrate reducing the effectiveness of water reabsorption
Tubular Maximum

51. Urine Movement Hydrostatic pressure forces urine through nephron
Peristalsis moves urine through ureters from region of renal pelvis to urinary bladder. Occur from once every few seconds to once every 2-3 minutes
Parasympathetic stimulation: increase frequency
Sympathetic stimulation: decrease frequency
Ureters enter bladder obliquely through trigone. Pressure in bladder compresses ureter and prevents backflow

52. Micturition Reflex

53. Effects of Aging
Gradual decrease in size of kidneys, but only one-third of one kidney necessary for homeostasis
Amount of blood flowing through gradually decreases
Number of glomeruli decrease and ability to secrete and reabsorb decreases
Ability to concentrate urine declines and kidney becomes less responsive to ADH and aldosterone
Reduced ability to participate in vitamin D synthesis contributing to Ca2+ deficiency, osteoporosis, and bone fractures