The UrinarySystemPart 1Dr. Tamara Alqudah

tamara.alqudah@ju.edu.jo 1

Introduction• The urinary system consists of two kidneys, two ureters,

one urinary bladder, and one urethra.

• The kidneys filter blood plasma, they return most of thewater and solutes to the bloodstream.

• The remaining water and solutes constitute urine

• The urine passes through the ureters and is stored in theurinary bladder until it is excreted from the bodythrough the urethra.

• The kidneys do the major work of the urinary system.The other parts of the system are mainly passagewaysand storage areas.

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Overview of Kidney Functions• The kidneys have several functions:

1. Regulating blood volume, pressure, pH, and

concentration of electrolytes

2. Preserving glucose, amino acids & other nutrients

3. Releasing certain hormones (erythropoietin) and

enzymes (renin)

4. Excreting waste products:1. Ammonia and urea (formed by breaking down

amino acids)2. Uric acid (formed by breaking down nucleic

acids)3. Creatinine (released from muscles).

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The kidneys• The kidney has two distinct regions:1. Renal cortex: the superficial region.

• Main function is filtration to form urine• Renal columns: portions of the renal cortex that extend

between renal pyramids.

2. Renal medulla: the deep inner region.• Main function is to collect and excrete urine.• Made of several cone-shaped renal pyramids

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The nephron• The functional unit of the kidney. 1 million nephrons/

kidney• Each nephron consists of two parts:1. Renal corpuscle: where blood plasma is filtered. Made

of:1. Glomerulus (capillary network)2. Glomerular capsule a double-walled epithelial cup that

surrounds the glomerular capillaries.

2. Renal tubule: into which the filtered fluid passes. Madeof:

1. Proximal convoluted tubule (PCT)2. Nephron loop (loop of Henle)

1. Descending limb2. Ascending limb

3. Distal convoluted tubule (DCT).

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Histology of Glomerular capsule• Glomerular capsule is

made of:• Parietal layer: forms

outer wall of capsule

• Capsular (Bowman’s)space: where fluidfiltered fromglomerularcapillaries enters

• Visceral layer haspodocytes that wrapprojections aroundsingle layer ofendothelial cells ofglomerularcapillaries and forminner wall of capsule

• The distal convoluted tubules of several nephrons emptyinto a single collecting duct (CD).

• Collecting ducts then unite and converge into severalhundred large papillary ducts, which drain into theminor calyces.

• Minor calyces drain into major calyces then into therenal pelvis and finally into the ureter which delivers theurine to the bladder.

•• Urine is stored in the bladder until it is excreted through

the urethra.

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Types of Nephrons• Cortical nephrons

• Corpuscles lie in the outer portion of the renal cortex• Short nephron loops that penetrate only into the outer

region of the renal medulla.• They make 80–85% of all nephrons

• Juxtamedullary nephrons:• Corpuscles lie deep in the cortex• Long nephron loop that extends into the deepest region

of the medulla.• They enable the kidneys to excrete very dilute or very

concentrated urine12

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Blood supply to the kidney• Renal blood flow, the blood flow through both kidneys,

is about 1140 mL per minute. Provided by the renalartery.

• This extensive blood supply is essential for kidneyfunction.

• Each nephron receives one afferent arteriole, whichdivides into a tangled, ball-shaped capillary networkcalled the glomerulus.

• The glomerular capillaries then reunite to form anefferent arteriole that carries blood out of theglomerulus.

• The efferent arterioles divide to form the peritubularcapillaries which surround tubular parts of the nephronin the renal cortex.

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The juxtaglomerularapparatus

• In each nephron, the final part of the ascending limb ofthe nephron loop makes contact with the afferentarteriole serving that renal corpuscle.

• Macula densa are the cells of the nephron in this region.• Juxtaglomerular cells are modified smooth muscle fibers

in the wall of the afferent arteriole in the area in contactwith the macula densa.

• The macula densa and the Juxtaglomerular cellsconstitute the juxtaglomerular apparatus (JGA).

• The JGA helps regulate blood pressure by secreting renin17

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Renal Physiology• To produce urine, nephrons and collecting ducts perform three

basic processes:

1. Glomerular filtration• Water and most solutes in blood plasma move across the wall of the

glomerular capillaries into glomerular capsule and then renal tubule

2. Tubular reabsorption• As filtered fluid moves along tubule and through collecting duct, about 99%

of water and many useful solutes reabsorbed – returned to blood

3. Tubular secretion• As filtered fluid moves along tubule and through collecting duct, other

material secreted into fluid such as wastes, drugs, and excess ions – removessubstances from blood

• Solutes and the fluid that drain into the minor and major calycesand renal pelvis constitute urine and are excreted.

• Excretion of any solute = glomerular filtration + secretion -reabsorption

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Urinary excretion = Glomerular filtration –Tubular reabsorption+Tubular secretion

Renal handling ofdifferent substancesSubstance A, like creatinine

Substance B, many of theelectrolytes.

Substance C, Amino Acids &glucose.

Substance D, organic acids andbases

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Glomerular filtration• Blood enters the glomerular capillaries then water and

most solutes in blood plasma move across the wall ofglomerular capillaries into the glomerular capsule.

• Glomerular filtrate is the fluid that enters theglomerular capsule. It is a protein-free filtrate(ultrafiltrate)

• The daily volume of glomerular filtrate is 180 liters.

• More than 99% of the glomerular filtrate returns to thebloodstream via tubular reabsorption

• Only 1–2 liters is excreted as urine.22

• Glomerular filtration rate (GFR) is the amount of filtrateformed in all renal corpuscles of both kidneys eachminute.

• GFR = 125 ml/min = 180 liters/day

• Filtration fraction: The fraction of blood plasma in theafferent arterioles that becomes glomerular filtrate.

• Filtration fraction (FF)= GFR / Renal Plasma Flow• Renal blood flow = 1140ml/min• Remember Plasma = 55% of blood• Renal plasma flow = 1140 x 0.55= 625ml/min• SO…• FF= 125/625= 0.2• So 20% of renal plasma flow is filtered

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Filtration membrane• Filtration membrane is made of the endothelial

cells of glomerular capillaries and the podocytesencircling the capillaries

• Made of three layers:1. Glomerular endothelial cells fenestrations2. Basal lamina between endothelium and podocytes3. Filtration slits created by the pedicels of podocytes

• Permits filtration of water and small solutes

• Prevents filtration of most plasma proteins & bloodcells

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The Filtration Membrane

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Net Filtration Pressure(NFP)

• The total pressure that promotes filtration

• Glomerular blood hydrostatic pressure (GBHP)is theblood pressure of the glomerular capillaries. It promotesfiltration by forcing water and solutes in blood plasmathrough the filtration membrane.~ 55mmHg

• Capsular hydrostatic pressure (CHP) is the hydrostaticpressure exerted against the filtration membrane byfluid already in the capsular space. It opposes filtration.~ 15 mmHg

• Blood colloid osmotic pressure (BCOP) due to presence ofproteins in blood plasma and it also opposes filtration.30 mmHg

• NFP = GBHP – CHP – BCOP• NFP= 55 - 15- 30 = 10 mmHg

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Net FiltrationPressure

REMEMBERCapsular colloidpressure = 0

(0)

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• The volume of fluid filtered by the renal corpuscle ismuch larger than in other blood capillaries of the bodyfor three reasons:

1. Glomerular capillaries present a large surface area forfiltration because they are long and extensive.

2. The filtration membrane is thin and porous. 50 timesleakier than other capillaries.

3. Glomerular capillary blood pressure is high. Becausethe efferent arteriole is smaller in diameter than theafferent arteriole, resistance to the outflow of bloodfrom the glomerulus is high.

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