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Chapter 26: The Urinary System

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1 Chapter 26: The Urinary System

2 Kidneys (2), ureters (2), urinary bladder (1) & urethra (1)
Kidneys filter blood plasma and return water and solutes back to blood What remains is urine Urine flows from each kidney, down its ureter to the bladder and to the outside via the urethra

3 Overview of Kidney Functions
Regulation of blood ions Na+, K+, Ca+2, Cl- and phosphate ions Regulation of blood pH & glucose Regulation of blood volume conserving or eliminating water Excretion of wastes Regulation of blood pressure Production of hormones (erythropoietin & calcitriol)

4 External Anatomy of the Kidneys
Paired kidneys - kidney-bean-shaped Located just above the waist between the peritoneum & posterior wall of abdomen retroperitoneal along with adrenal glands & ureters Protected by 11th & 12th ribs Right kidney lower because of liver Renal hilus – where ureters and blood vessels enter/exit

5 Layers of the Kidney Renal Capsule – Deepest
smooth transparent sheet of dense irregular connective tissue–helps maintain the shape of the kidney Adipose Capsule – Intermediate protects the kidney from trauma Renal Fascia – Superficial anchors kidney to the surrounding structures and to the abdominal wall


7 Internal Anatomy of the Kidneys
Two Distinct Regions: Renal Cortex: Superficial light red region Renal Medulla Deep, dark reddish-brown region consists of 8-18 cone-shaped renal pyramids separated by renal columns renal papilla point toward the Renal hilum Together the Renal Cortex and the Renal Medulla constitute the parenchyma – functional portion of kidney


9 Number of Nephrons Remains constant from birth – one million
any increase in size of kidney results in an increase in size of individual nephrons If injured, no replacement occurs Dysfunction is not evident until function declines by 25% of normal (other nephrons handle the extra work) Removal of one kidney causes enlargement of the remaining until it can filter at 80% of normal rate of 2 kidneys

10 The Nephron – functional unit of kidney
Renal Corpuscle: filtration Gomerulus Glomerular capsule (Bowman’s capsule) Renal Tubules: reabsorption & secretion Proximal convoluted tubule (PCT) Nephron loop (loop of Henle) Distal convoluted tubule (DCT) DCTs of several nephrons empty into a single Collecting Duct

11 Juxtamedullary Nephrons – 15%
Two Types of Nephrons Cortical Nephrons – 85% Juxtamedullary Nephrons – 15%

12 Glomerular (Bowman’s) Capsule
Bowman’s capsule surrounds glomerulus forming capsular space Two layers of capsule: Visceral layer – Podocytes – “foot cells” modified simple squamous Parietal layer – Simple squamous epithelium Think of it as a fist punched into an inflated bag

13 Histology of Renal Tubule & Collecting Duct
Proximal convoluted tubule (PCT) simple cuboidal with brush border of microvilli - increase surface area for absorption Descending limb of loop of Henle simple squamous Ascending limb of loop of Henle simple cuboidal forms juxtaglomerular apparatus (JGA) where makes contact with afferent arteriole Distal convoluted & collecting ducts simple cuboidal composed of principal & intercalated cells which have microvilli

14 Macula densa is thickened part of ascending limb of Loop of Henle
Juxtaglomerular cells are modified muscle cells in afferent arteriole Together form Juxtaglomerular apparatus (JGA) – helps regulate blood pressure Mesangial cells – specialized smooth muscle Relaxed – increases surface area – increases filtration Contracted – decreases surface area – decreases filtration

15 Blood Supply of Kidney Abundantly supplied with blood vessels
receive 25% of resting cardiac output via renal arteries Blood enters kidney via Renal Artery Radiate outward into renal cortex - branch off into Afferent arterioles (one per nephron) Enters nephron and becomes a ball-shaped capillary network called Glomerulus (filtration) Leaves nephron as Efferent arteriole Efferent arteriole divides to either Peritubular capillaries (Cortical nephron) or Vasa recta (Juxtamedullary nephron) Blood leaves kidney via Renal Vein


17 From Renal Artery Renal Vein

18 Nephrons and collecting ducts perform 3 basic processes:
1. Glomerular filtration – water and most solutes 2. Tubular reabsorption – 99% of water and many useful solutes back into blood 3. Tubular secretion – waste, drugs, excess ions from blood to tubules

19 Glomerular Filtration
Blood pressure produces glomerular filtrate 180 litres/day filtrate produced 99% reabsorbed to blood 1-2 L/day urine Substances filtered out must cross 3 filtration barriers of the endothelial-capsular membrane: Glomerular endothelial cell fenestrations Basal lamina Pedicels of podocytes – filtration slits

20 Filtration Membrane #1 Endothelial fenestrations - Stops all cells and platelets #2 Basal Lamina - Stops large plasma proteins #3 Podocytes - Pedicels (filtration slits) - Stops medium-sized proteins


22 3 Reasons why the volume of fluid filtered by the renal corpuscle is much larger than in other blood capillaries: Large surface area regulated by Mesangial cells. When cells are relaxed, surface area is maximal and filtration rates are high. Filtration membrane is thin and porous – 50x leakier than other blood capillaries Glomerular capillary blood pressure is high – efferent arteriole diameter is smaller than afferent arteriole.

23 Glomerular filtration depends on 3 main pressures:
One pressure promotes filtration Two pressures oppose filtration Glomerular blood hydrostatic pressure (GBHP) – 55 mmHg Promotes filtration by forcing water and solutes through filtration membrane 2. Capsular hydrostatic pressure (CHP) – 15 mmHg Opposes filtration by exerting back pressure against filtration membrane 3. Blood colloid osmotic pressure (BCOP) – 30 mmHg Opposes filtration by the presence of proteins in blood plasma which slows down osmosis

24 Net Filtration Pressure (NFP)
GBHP = 55 mmHg CHP = 15 mmHg BCOP = 30 mmHg Net Filtration Pressure (NFP) = GBHP – CHP – BCOP = 55 – = 10 mmHg Therefore, a pressure of only 10 mmHg causes a normal amount of blood plasma to filter from the glomerulus into the capsular (Bowman’s) space. Changes to either of these three pressures will affect the Glomerular Filtration Rate (GFR)

25 Glomerular Filtration Rate (GFR)
Amount of filtrate formed in all renal corpuscles of both kidneys - average adult male rate is 125 mL/min Homeostasis requires GFR to be constant too high & useful substances are lost due to the speed of fluid passage through nephron too low and sufficient waste products may not be removed from the body Three mechanisms control GFR by either adjusting blood flow in/out of glomerulus (JGA) or altering capillary surface area for filtrations (mesangial cells). (Table 26.2 pg. 996) Renal autoregulation Neural regulation Hormonal regulation

26 1. Renal Autoregulation of GFR
Kidneys themselves help maintain a constant GFR despite normal, everyday changes in blood pressure (BP) Myogenic mechanism systemic increases in BP also increase GFR Increases to BP stretch the afferent arteriole smooth muscle fibers contract, reducing the diameter of the arteriole – decreasing the GFR Tubuloglomerular feedback elevated systemic BP raises the GFR - fluid flows too rapidly through the renal tubules & Na+, Cl- and water are not reabsorbed quickly enough macula densa detects that difference & inhibit release of NO (vasodilator) from the Juxtaglomerular cells afferent arterioles constrict & reduce GFR

27 Tubuloglomerular feedback:
Myogenic mechanism: Increase in BP stretches walls of Afferent arteriole Smooth muscles in vessel walls contract Blood flow decreases which lowers GFR Tubuloglomerular feedback: Macula densa detect changes to filtrate Inhibit release of NO by Juxtaglomerular cells Afferent arteriole contracts

28 2. Neural Regulation of GFR
Kidneys are supplied by sympathetic ANS fibers that cause vasoconstriction of afferent arterioles At rest, renal blood vessels are maximally dilated because sympathetic activity is minimal renal autoregulation predominates With moderate sympathetic stimulation, both afferent & efferent arterioles constrict equally Blood flow decreases - decreasing GFR slightly With extreme sympathetic stimulation (exercise or hemorrhage), vasoconstriction of afferent arterioles greatly reduces blood flow – dramatically reducing GFR lowers urine output & permits blood flow to other tissues

29 3. Hormonal Regulation of GFR
Two hormones contribute to regulation of GFR: Atrial natriuretic peptide (ANP) – increases Angiotensin II – decreases Atrial natriuretic peptide (ANP): stretching of the atria that occurs with an increase in blood volume causes cells of atria to release ANP relaxes glomerular mesangial cells increasing capillary surface area and increasing GFR Angiotensin II: Release stimulated by decrease in blood volume/pressure Enzyme Renin secreted by juxtaglomerular cells potent vasoconstrictor that narrows both afferent & efferent arterioles decreasing GFR

30 Tubular reabsorption Paracellular reabsorption
Epithelial cells all along the renal tubules and collection duct carry out reabsorption Proximal convoluted tubules (PCT) make largest contribution More distal tubule cells fine-tune reabsorption to maintain homeostasis Paracellular reabsorption 50% of reabsorbed material moves between cells by diffusion in some parts of tubule Transcellular reabsorption material moves through both the apical and basal membranes of the tubule cell

31 Reabsorption in the PCT
Largest amount of solute and water reabsorption occurs in PCT 65% of filtered water, Na+ and K+ 100% of glucose and amino acids 80-90% of HCO3 Most solute reabsorption in PCT involves Na+ Water is only reabsorbed by osmosis: obligatory water reabsorption occurs when water is “obliged” to follow the solutes being reabsorbed facultative water reabsorption occurs in collecting duct under the control of antidiuretic hormone

32 Reabsorption of Glucose in PCT
Intracellular sodium levels are kept low due to Na+/K+ pump on basolateral membrane Low intracellular Na+ creates concentration gradient high in filtrate – low in cell Na+ symporters on apical membrane use energy from gradient to bring in glucose Secondary active transport 2 Na+ and 1 glucose attach to symporter and enter cell together Glucose then diffuse out of cell and into peritubular capillaries

33 Reabsorption of Bicarbonate & H+ ion secretion in PCT

34 Reabsorption within Loop of Henle
Osmolarity of interstitial fluid progressively increases deeper into the medulla Descending loop is highly permeable to water. Water moves by osmosis out of the tubule due to increased osmolarity of interstitial fluid (15% of total reabsorption) Thick ascending loop has many Na+, K+ and Cl- transporters. Ions pass out of filtrate and into intersitital fluid (20-30% totol reabsorption) These cells are also impermeable to water, so water does not follow osmotically. Therefore decreasing osmolarity of filtrate This osmotic gradient is how the kidneys are able to produce concentrated or dilute urine (juxtamedullary nephrons)

35 Reabsorption & Secretion in the Collecting Duct
By end of DCT, 95% of solutes & water have been reabsorbed and returned to the bloodstream Cells in the collecting duct make the final adjustments principal cells reabsorb Na+ and secrete K+; facultative reabsorption of H2O (ADH) intercalated cells reabsorb K+ & bicarbonate ions and secrete H+ Hormonally controlled by Angiotensin II, Aldosterone, ANP and Antidiuretic hormone (ADH)

36 Angiotensin II and Aldosterone
decreases GFR by vasoconstricting afferent arteriole enhances absorption of Na + promotes aldosterone production (adrenal gland) which causes principal cells to reabsorb more Na +,Cl-, and water increases blood volume by increasing water reabsorption Atrial Natriuretic Peptide (ANP) inhibits reabsorption of Na + and water in PCT & suppresses secretion of aldosterone & ADH increase excretion of Na + into tubules (obligated water) which increases urine output and decreases blood volume

37 Antidiuretic Hormone (ADH)
Released by posterior pituitary Increased plasma osmolarity or decrease in blood volume (hemorrhage) stimulate ADH secretion Stimulates the insertion of aquaporin-2 channels into the membrane of principal cells of DCT– greatly increases permeability of cells and reabsorption of water from filtrate (concentrated urine) In absences of ADH, principal cells of DCT are impermeable to water – no water reabsorbed increasing urine output (dilute urine) Diabetes insipidus – lack of ADH or insensitive to it excrete 20L urine/day

38 Breaking The Seal Alcohol inhibits release of ADH
Without ADH, DCT are impermeable to water Kidneys can’t reabsorb needed water – instead produce copious amounts of dilute urine Effects of alcohol linger long after you stop drinking – leaving you feeling dehydrated

39 Summary – Absorption & Secretion

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