urinary System : 1 urinary System SMS 1113
Dr. Mohanad R. Alwan Types of Nephron : 2 Types of Nephron (1) Cortical (sub-capsular) nephrons are the most common type of nephron. Their renal corpuscles are found in the outer cortex and their short loops are formed by distal straight tubules in the outer medulla.
(2) Juxtamedullary nephrons account for approximately 20% of all nephrons; their corpuscles are found adjacent to the medulla. They are long looped nephrons whose loop is formed by thin limbs. These nephrons are responsible for producing the urine-concentrating mechanism of the kidney.
(3) Intermediate nephrons have their renal corpuscles in the mid-region of the cortex and their loops are intermediate in length. Glomerulus : 3 Glomerulus Structure of a Nephron:
2 main structures
Glomerulus – a knot of capillaries
Renal tubule (about 2 inches long).
The endothelium of the glomerular capillaries is fenestrated Glomerular capsule : 4 Glomerular capsule a double-walled epithelial cup capillaries
External parietal layer: (simple sequamous epithelium)
Visceral layer (Podocysts) Visceral layer of Bowman’s capsule : 5 Visceral layer of Bowman’s capsule The podocytes possess processes called pedicles that wrap around the capillaries and interdigitate to form filtration slits approximately 25nm wide.
The mesangial cells serve a phagocytic function and keep the basement membrane clear of debris. All 3 components combine to retains cells and macromolecules (proteins > 70 kd) but permit the passage of water and small solutes (salts, glucose, amino acids, nitrogenous wastes) into the urinary space as filtrate. Filtrate membrane : 6 Filtrate membrane Consist of 3 layers
Basement membrane (intervening basement membrane)
Foot processes of podocytes of glomerular capsule. Slide 7: 7 Urinary Filtration Membrane : 8 Urinary Filtration Membrane Endothelial cell
70-90 nm fenestra restrict proteins > 70kd
heparan sulfate is negatively charged
produced by endothelial cells & podocytes
phagocytosed by mesangial cells
pedicels 20-40 nm apart
diaphragm 6 nm thick with 3-5 nm slits
podocalyxin in glycocalyx is negatively charged Slide 9: 9 Renal tubule – site of selective re-absorption / secretion of solutes : 10 Renal tubule – site of selective re-absorption / secretion of solutes The renal tubule serves to recover water and other desirable solutes (sugar, ions, small proteins) from the filtrate.
The proximal convoluted tubule receives filtrate from the urinary space and is the site of the selective re-absorption of most solutes including all the glucose and amino acids and most of the water and salts. Slide 11: 11 Renal tubule : 12 Renal tubule Proteins are absorbed by pinocytosis, broken down by lysosomal degradation and released as amino acids to the peritubular capillary network. This is also the site of pH balancing and elimination (active transport) of creatine. Slide 13: 13 Histology of tubules : 14 Histology of tubules Structurally the proximal convoluted tubule is formed by a simple cuboidal to low columnar epithelium.
The apical surface is covered with microvilli creating a light microscopic brush border that increases the surface area for ion absorption.
The cells are tightly bound to one another to seal off the intercellular spaces from the lumen using junctional processes apically and interdigitating plicae (folds) laterally.
Basally, interdigitating processes contain numerous mitochondria which create light microscopic basal striations that are associated with ion transport. Histology… : 15 Histology… Proximal convoluted tubules are the most abundant tubule in the cortex.
They have a eosinophilic cytoplasm with a basal nucleus.
The brush border is rarely preserved and the indistinct cells margins are due to basal and lateral border interdigitations.
The proximal straight tubules are located within or near the medulla, depending upon the type of nephron.
They are formed by lower cuboidal epithelium and their microvilli and basal and lateral interdigitations are less well developed. Histologically they are similar to proximal convoluted tubules Histology…. : 16 Histology…. Descending thin tubules are located within the medulla and are formed by low cuboidal to squamous epithelium.
The microvilli are poorly developed as are the basal and lateral interdigitations creating a very leak cell that serves as the site of passive transport of ions (inward) and water (outward) between the lumen and interstitium.
The ascending thin tubules are also located within the medulla and are similar in appearance to descending thin tubules.
However, these tubes are impermeable to water and permit passive transport of NaCl into the interstitium. Histology…. : 17 Histology…. Distal straight tubules are located within both the medulla and cortex .
Histologically these appear as a simple cuboidal epithelium with sparse microvilli and lacking lateral interdigitations.
The nucleus is apical and basal interdigitations with abundant mitochondria are present.
These tubes are impermeable to water and are the site of ion transport from the lumen to the interstitium that establishes the ion gradient of medullary interstitium. Histology…. : 18 Histology…. Distal convoluted tubules are located within the cortex. They are approximately 1/3rd as long as their proximal counterparts. They contact the renal corpuscle forming a macula densa which is part of the juxtaglomerular apparatus (see below). Histologically they are similar to the distal straight tubules and also function in ion exchange. Collecting tubules…. : 19 Collecting tubules…. The collecting system starts in the cortex as a continuation of the distal convoluted tubules and descend through the medulla.
As the ducts coalesce and increase in size, the cells of the tubes change from somewhat squamous to cuboidal to columnar and similarly become increasing stratified.
They terminate at the tip of the renal pyramid as the papillary ducts . Collecting tubules…. : 20 Collecting tubules…. Histologically they appear as tubes with distinguishable cells margins, central nuclei and poorly staining cytoplasm.
At the EM level many posses a single cilium and sparse microvilli.
The collecting system functions to concentrate urine through ADH-regulated and ADH-independent water channels PCT : 21 PCT the walls formed by cuboidal epithelial cells
Their Luminal surfaces have dense microvilli. Renal (Uriniferous) Tubule : 22 Renal (Uriniferous) Tubule Proximal convoluted tubule (PCT)
longest, most coiled, simple cuboidal with brush border
Nephron loop - U shaped; descending + ascending limbs
thick segment (simple cuboidal) initial part of descending limb and part or all of ascending limb, active transport of salts
thin segment (simple squamous) very water permeable
Distal convoluted tubule (DCT)
cuboidal, minimal microvilli Slide 23: 23 Kidney Cortex – PCT (P) & DCT (D) P P D D Slide 24: 24 Long section of Collect tubules (CT) and PCT (P) CT CT P Slide 25: 25 Proximal Convoluted Tubules (PCT) : 26 Proximal Convoluted Tubules (PCT) Reabsorbs 65% of GF to peritubular capillaries
Great length, prominent microvilli and abundant mitochondria for active transport
Reabsorbs greater variety of chemicals than other parts of nephron
transcellular route - through epithelial cells of PCT
paracellular route - between epithelial cells of PCT
Transport maximum: when transport proteins of plasma membrane are saturated; glucose > 220 mg/dL remains in urine (glycosuria) Slide 27: 27 Loop of Henle : 28 Loop of Henle Descending limb
Proximal part: similar cells of the PCT
Thin segment: simple sequamous epithelium
Cuboidal epithelium or low columnar? thick segment Thin loop of Henle : 29 Thin loop of Henle Thick loop of Henle : 30 Thick loop of Henle DCT : 31 DCT The wall: cuboidal epithelium (like PCT)
Lack micrivilli Collecting ducts : 32 Collecting ducts Have 2 types of cells
cuboidal cells with extensive microvilli
Maintain the acid –base balance (blood pH)
maintain water-sodium (Na+) balance
Have receptors to antidiuretic hormone & aldosterone DCT and Collecting Duct : 33 DCT and Collecting Duct Effect of aldosterone
? BP causes angiotensin II formation
angiotensin II stimulates adrenal cortex
adrenal cortex secretes aldosterone
aldosterone promotes Na+ reabsorption
Na+ reabsorption promotes water reabsorption
water reabsorption ? urine volume
BP drops less rapidly DCT and Collecting Duct 2 : 34 DCT and Collecting Duct 2 Effect of atrial natriuretic factor (ANF)
? BP stimulates right atrium
atrium secretes ANF
ANF promotes Na+ and water excretion
Effect of ADH
dehydration stimulates hypothalamus
hypothalamus stimulates posterior pituitary
posterior pituitary releases ADH
ADH ? water reabsorption
urine volume ? Collecting Duct Concentrates Urine : 35 Collecting Duct Concentrates Urine Osmolarity 4x as high deep in medulla
Medullary portion of CD is permeable to water but not to NaCl Cortical Collecting Tubule : 36 Cortical Collecting Tubule Medullary Collecting Tubule : 37 Medullary Collecting Tubule Control of Water Loss : 38 Control of Water Loss Producing hypotonic urine
NaCl reabsorbed by cortical CD
water remains in urine
Producing hypertonic urine
tubular reabsorption ?
less NaCl remains in CD
ADH ? CD’s water permeability
more water is reabsorbed
urine is more concentrated Countercurrent Multiplier : 39 Countercurrent Multiplier Recaptures NaCl and returns it to renal medulla
reabsorbs water but not salt
concentrates tubular fluid
reabsorbs Na+, K+, and Cl-
maintains high osmolarity of renal medulla
impermeable to water
tubular fluid becomes hypotonic
Recycling of urea: collecting duct-medulla
urea accounts for 40% of high osmolarity of medulla Countercurrent Multiplier of Nephron Loop Diagram : 40 Countercurrent Multiplier of Nephron Loop Diagram Countercurrent Exchange System : 41 Countercurrent Exchange System Formed by vasa recta
provide blood supply to medulla
do not remove NaCl from medulla
water diffuses out of blood
NaCl diffuses into blood
water diffuses into blood
NaCl diffuses out of blood Maintenance of Osmolarity in Renal Medulla : 42 Maintenance of Osmolarity in Renal Medulla Summary of Tubular Reabsorption and Secretion : 43 Summary of Tubular Reabsorption and Secretion Interstitium….. : 44 Interstitium….. The interstitium is the connective tissue matrix of the kidney. [Yes. it is the stroma of the kidney, but remember not all stromas are connective tissue.] It is sparse in the cortex and most abundant the deep medulla. The extracellular matrix consists of collagen fibers and glycosamionglycans (GAGs) and the cells are fibroblasts and macrophages. Slide 45: 45 Slide 46: 46 Slide 47: 47 Slide 48: 48 Slide 49: 49 Slide 50: 50 Slide 51: 51 Slide 52: 52 Ureters : 53 Ureters Ureters:The ureters are muscular tubes connecting the renal pelvis to the urinary bladder.
25 cm long
Enters on the floor of bladder Slide 54: 54 Slide 55: 55 Slide 56: 56 Ureter Slide 57: 57 Ureter – folded mucus membrane Transitional Epithelium Urinary Bladder : 58 Urinary Bladder Urinary Bladder
Muscular sac on floor of pelvic cavity.
Located in pelvic cavity, posterior to pubic symphysis
Average bladder volume is 500 ml
Max capacity is 700-800 ml Urinary Bladder : 59 Urinary Bladder Consists from:
parietal peritoneum, superiorly; fibrous adventitia rest
muscularis: detrusor muscle, 3 layers of smooth muscle
mucosa: transitional epithelium
trigone: openings of ureters and urethra, triangular
rugae: relaxed bladder wrinkled, highly distensible
The outer layer may contain Pacinian corpuscles Urinary Bladder and Urethra - Female : 60 Urinary Bladder and Urethra - Female Slide 61: 61 Bladder Urinary Bladder : 62 Urinary Bladder Slide 63: 63 Slide 64: 64 Urethra : 65 Urethra Urethra
Conveys urine out of body
Female urethra – 3 - 4 cm
Opens into external urethral oriface
Lies between vaginal oriface and clitoris
Male urethra – 18 cm
Prostatic urethra – 2.5 cm
Membranous urethra – 0.5 cm
Penile urethra – 15 cm Urethra : 66 Urethra Blood supply… : 67 Blood supply… Arterial: renal artery > inter-lobar arteries > arcuate arteries > inter-lobular arteries > afferent arterioles > renal corpuscle (capillaries!) > efferent arteriole > peritubular capillary network (for cortical nephrons) --or-- > vasa recta (for juxtamedullary nephrons).
Venous:: Peritubular capillary network > interlobular veins > arcuate veins > interlobar veins > renal vein.
The vasa recta (L. straight vessels) are comprised of arteriolae rectae and venulae rectae and their associated capillaries. Blood Supply Diagram : 68 Blood Supply Diagram Nerve Supply : 69 Nerve Supply The smooth muscle of the glomerular arterioles receives sympathetic innervation.
Vasoconstriction of afferent arteriole decreases filtration rate (lowers pressure) whereas vasoconstriction of the efferent arterioles increases filtration rate.
Neither innervation is neither for normal function. Urine flow : 70 Urine flow urinary space of Bowman's capsule ---?
> convoluted proximal tubule ---?
> descending, thick, straight, proximal tubule ---?
> descending, thin, straight, proximal tubule --?
> ascending, thin, straight proximal tubule ---?
> ascending, thick, straight, distal tubule ----?
> convoluted, distal tubule ---?
> collecting tubule ----?
> collecting ducts ----?
> papillary ducts of Bellini -----?
> minor calyx ------?
> major calyx ----?
> renal pelvis -----?
> ureter -----?
> bladder -----?
> urethra -----?
> outside Slide 71: 71 Thank you