Urinary System | Chapter 24 | BIOL117

what are the different organs that make up of the urinary system and their PRIMARy functions?

1. kidneys - filter blood

2. ureters - transport urine

3. bladder - holds urine

4. urethra - eliminates urine

what are the functions of the urinary system?

1. elimination of metabolic wastes

2. regulation of ions level (e.g., Na+, K+ and Ca2+)

3. regulation of acd-base balance (e.g., H+ and HCO3-)

4. regulation of blood pressure

5. regulation of water levels

6. elimination of biologically active molecules (e.g., hormones, drugs)

7. formation of calcitriol (vitamin D metabolism)

8. production and release of erythropoietin

9. potential engagement in gluconeogenesis

____________ are two symmetrical bean-shaped organs with a concave medial border, ileum, and a lateral convex border

kidneys

__________ rests on the superior aspect of the kidney

adrenal glands

where is the kidneys located?

1. the left and right upper quadrant

2. on posterior abdominal wall

3. lateral to the vertebral column

why is the right kidney lower than the left kidney?

to accommodate the position of the liver

what are the different layers of the kidneys? (inner most to outermost)

1. fibrous capsule (dense irregular CT that helps maintains kidney’s shape

2. perinephric fat (dense irregular connective CT to help anchor the kidney to surrounding structure)

   → help cushion and support the kidneys)

3. renal fascia - external to perinephric fat made up of dense irregular CT to anchor the kidney to surrounding structures

4. paranephric fat (adipose CT that help cushion and support the kidneys)

what are the two region of functional tisue in the kidneys?

1. renal cortex

2. renal medulla

renal columns

extension of the cortex projecting into the medulla

renal pyramids

portion of the medulla divided by renal columns with wide base ar the external edge of the medulla and the medial aspect ar the renal papilla

corticomedullary junction

located at the medial apex where the renal papilla is

renal sinus

1. medially located urine drainage area

2. organized into minor calyces, major calyces, and the renal pelvis

minor calyces

1. unflle-shaped structures of renal pyramids

2. merge to form major calyz

renal pelvis

formed from merged major calyces

nephron

microscopic functional filtration unit of the kidney that consist of the renal corpuscle and renal tubules

renal corpuscle

enlarged bulbous region of nephron within the renal cortex that composed of two structures of glomerulus and Bowman’s (glomerular) capsule

glomerulus

tangle of capillary loops (glomerular capillaries) that allow blood to enter via the afferent arteriole and leaves via the efferent arteriole

in the bowman’s capsule, blood enters via the _________ arteriole and exits via the _________ arteriole

afferent; efferent

what are the three layers of the Bowman’s capsule?

1. visceral

   → directly overlies glomerular capillaries

2. parietal

   → simple squamous epithelium

3. capsular space

   → receive filtrate and modified to form urine

vascular pole

afferent and efferent arterioles attach to glomerulus

tubular pole

origin of renal tubule

what are the three sections of the renal tubules? (in order)

1. proximal convoluted tubules (PCT)

2. nephron loop

3. distal convoluted tubule (DCT)

proximal convoluted tubule (PCT)

1. first region of the renal tubule

2. originates at tubular pole of renal corpuscle

3. made up of cuboidal epithelium

4. lined with microvilli to increase surface area for more efficient reabsorption capacity

nephron loop (AKA hairpin turn)

1. originates at sharp bend in PCT

2. made up of te descending limb and the ascending limp

   → descending - extends medially from the PCT

   → ascending - return to the renal cortex and ends at DCT

3. thin segment lined with simple squamous epithelium

4. thick segment lined with simple cuboidal epithelium

Distal Convoluted Tubule (DCT)

1. originates in the renal cortex at the end of the ascending limb

2. extends to collecting tubule

3. lined by simple cuboidal epithelium without microvilli

4. appears clear when viewed under a microscope

what are the two types of nephron?

1. cortical

2. juxtamedullary

3. classified based on two factors: relative position of renal corpuscles in the cortex and the length of the nephron loop

cortical nephrons

1. oriented with renal corpuscles near peripheral cortex

2. short nephron loop barely penetrate the medulla

3. made up of 85% of nephrons

juxtamedullary nephrons

1. oriented at the renal corpuscles adjacent to the corticomedullary junctions

2. long nephron loops extend deep into the medulla

3. help establish salt concentration gradent in the interstitial space and allow foe regulation of urine concentration by ADH

4. made up of about 25% of nephrons

__________ nephrons help establish salt concentration gradient in interstitial space by allow for regulation of urine concentration by ADH

juxtamedullary

what are the specialized cells making up the collecting tubules and the collecting ducts and their PRIMARY functions?

1. principal cells - responsive to hormones aldosterone and antidiuretic hormone (ADH)

2. intercalated cells (A & B)

   → help regulate urine pH and blood pH

multiple collecting tubules empty into larger ______________

collecting ducts

numerous collecting ducts empty into ____________ located within the renal papilla

papillary duct

juxtaglomerular (JG) apparatus

1. regions made up of the granular cells, macula dense, and extraglomerular mesangial cells at the glomerular hilum

2. helps regulate filtrate formation and systemic blood pressure

granular cells (compositions, location, and functions)

1. made up of the juxtaglomerular (JG) apparatus

2. modified smooth muscles cells of afferent arteriole

3. located near entrance to renal corpuscle

4. contract when simulated by stretch or sympathetic stimulation

5. synthesize, store, and release renin

macula densa (compositions, location, and functions)

1. made up of the juxtaglomerular (JG) apparatus

2. modified epithelial cells in the wall of DCT

3. detect changes in NaCl concentration of fluid in the lumen of DCT

4. signal granular cells to release renin through paracrine simulation

exraglomerular mesangial cells

1. smooth muscle cells between arterioles

2. helps with vasoconstriction to regulate flow

blood are delivered to each kidney by __________

renal arteries arisen from the abdominal aorta

segmental arteries

branch from renal arteries within the renal sinus

interlobar arteries

1. branch from segmental arteries

2. travel through renal columns toward corticomedullary junction

arcuate arteries

1. branch from interlobar arteries at corticomedullary junction

2. branch parallel to base of medullary pyramid

interlobar arteries

1. branch from arcute arteries

2. project peripherally into cortex

3. afferent arterioles branching off

afferent artioles

1. ones that enters the renal corpuscle forming the glomerulus

2. some blood plasma filtered here

efferent arteriole

1. ones that leave the corpuscle

2. branches into the peritubular capillaries or vasa recta

peritubular capillaries

1. intertwined around PCT and DCT

2. primarily reside in cortex of kidney

vasa recta

1. straight capillaries bed associated with the nephron loop

2. primarily reside in medulla of kidney

blood move through the ____________ capillaries to be filtered and then it passes through the ____________ capillaries or ____________ for gas exchange, nutrients, waste and then finally drain into network of veins

glomerular; peritubular; vasa recta

filtrate

water and solutes filtered from blood plasma at the glomerulus that is able to move across the wall of glomerular capillaries into the capsular space

trace the movement of filtrate → tubular fluid → urine starting from the capsular space…

1. capsular space

2. proximal convoluted tubule

3. descending limb of nephron loop

4. ascending limb of nephron loop

5. distal convoluted tubule

6. collecting tubules

7. collecting ducts

8. papillary duct

9. minor calyx

10. major calyx

11. renal pelvis

12. ureter

13. urinary bladder

14. urethra

glomerular filtration

the movement of subtances from the blood within the glomeruls into the capsular space

tubular reabsorption

the movement of sustbances from the tubular fluid back into the blood

tubular secretion

the movement of susbtances from the lood into the tubular fluid

what are the different steps of urine formaton?

1. glomerular filtration (happens in the glomerular capillaries that separate some water and dissolved solutes from blood) —> water and solutes enter capsular space of renal corpuscle (filtrate)

2. tubular reabsorption (happens in the tubules where the water and solutes would move and get reabsorbed back into the BV to maintain balance of fluid electrolytes)

3. tubular secretion (happens in the, movement of solute by active transport out of the blood into tubular fluid to maintain fluid and electrolytes balance

what are the characteristics of the filtration membrane of the glomerulus

1. porous, thin, negatively charged structure

2. formed by glomerulus and visceral layer of the glomerular capsule

3. made up of different layers

what are the different layer of the filtration membrane and their unique functions?

1. endothelium of glomerulus - fenestrated to allow plasma and dissolved substances to pass and restrict passage for larger substances

2. basement membrane of glomerulus - restrict passage of large plasma proteins (glycoproteins and proteoglycan)

3. visceral layer of glomerular capsule - composed of podocytes and have long processes called pedicels that support capillary wall and restrict passage of most small proteins

4. mesangial cells - positions between glomerular capillary loosp that have phagocytic, contractile, and signalling properoties

true or false: 180 L of urine is produced daily

true; filtered plasma with certain solutes and minimal amounts of proteins

some filtered material that are trapped within the basement membrane are phagocytized by ________cells

mesangial

what are the three categories of substances in the blood?

1. freely filtered (small substances that pass easily through filtration membrane)

2. not filtered (form elements an large proteins that cannot pass thru)

3. limited filtration (proteins of intermediate size tha usually blocked from filtration but occasionally go through)

glomerular hydrostatic (blood) pressure (HPg)

1. “pushes” water and solutes out of the glomerulus into the capsular space of renal corpuscle

   → allow filtration to occur

2. higher than blood pressure of other systemic capillaries

3. dependent on the diameter of the afferent and efferent arteriole

blood colloid ostomic pressure (OPg)

1. osmotic pressure exerted by dissolved solutes

2. opposes filtration

   → draws fluid back into glomerulus

capsular hydrostatic pressure (HPc)

1. pressure in the glomerular capsule due to filtrate

2. impede movement of additional fluid

net filtration pressure (NFP) (slide 46)

HPg - (OPg - HPc) = NFP

blood going out through the efferent is _________ because most of the water leaked out of it

viscous

which of the filtration pressure is directly related to blood pressure?

glomerular hydrostatic pressure (HPg)

glomerular filtration rate (GFR)

rate at which the volume of filtrate ie formed (volume per unit of time - usually per 1 min)

what does increased net filtration pressure cause?

1. increases GFR

2. increases solutes and water remaining in tubular fluid

3. increases substances in urine

4. decreases filtrate reabsorption

glomerular filtration rate (GFR) helps kidney _____________ based on physiologic condition

control urine production

what are the factors that influence glomerular filtration rate?

1. changing the luminal diameter of afferent arteriole

2. altering surface areas of filtration membrane

intrinsic controls of regulation of glomerular filtration rate

processes of regulation that happens within the kidney itself through myogenic response and tubuloglomerular feedback mechanism

extrinsic controls of regulation of glomerular filtration rate

proccesses of regulation that happens external to the kidney

myogenic response

contraction or relaxation of smooth muscle of afferent arteriole in response to stretch

• causes smooth muscle cells to relax/contract and the vessel to dilate/constrict

• allows more/less blood into the glomerulus

  → increased BP = more stretched

• compensate for lower/greater system pressure

• GFR remains normal

what would the myogenic response be if there is there decrease in systemic blood pressure?

afferent arteriole vasodilates to allow more blood into the glomerulus to offset a decrease in systemic blood pressure

what would the myogenic response be if there is there increase in systemic blood pressure?

afferent arteriole constrict to allow less blood into the glomerulus to offset an inrease in systemic blood pressure

tubuloglomerular feedback mechanism of the regulation glomerular filtration rate

1. back up system second to the myogenic response to increased/decrease blood pressure - also control the contraction or relaxation of smooth muscle of afferent arteriole

2. detected by macula densa cells in juxtaglomerular apparatus

3. result in further vasoconstriction/vasodilation of afferent arteriole

what is the purpose of renal autoregulation?

• maintains stable GFR despite BP fluctuations

• effective range: 80–180 mmHg

• uses myogenic mechanism (afferent arteriole) + tubuloglomerular feedback

how does the kidney respond when BP drops below 80 mmHg?

• Arterioles maximally dilated (can’t compensate further)

• because ↓ Glomerular hydrostatic pressure → ↓ GFR

• Critical consequence:

  • Waste accumulation (azotemia)

  • Oliguria/anuria if severe

how does the kidney respond to BP >180 mmHg?

• Arterioles maximally constricted (can’t compensate further)

• ↑ Glomerular pressure → ↑ GFR

• Consequences:

  • Hyperfiltration injury (glomerular damage)

  • Proteinuria (loss of filtration barrier integrity)

what happens when autoregulation of GFR and BP fails?

• <80 mmHg: Acute kidney injury (prerenal azotemia)

• >180 mmHg: Glomerulosclerosis (e.g., in hypertension)

• Key sign: GFR becomes BP-dependent

why is autoregulation limited to 80–180 mmHg?

• lower limit: Arterioles physically can’t dilate further

• upper limit: Smooth muscle can’t constrict enough to counteract higher pressure

how does the kidney response to BP <50 mmHg?

• GFR → near zero

• Cessation of urine output

• Life-threatening:

  • Electrolyte imbalances (hyperkalemia)

  • Metabolic acidosis

how do extrinsic controls differ from renal autoregulation?

• Autoregulation: Maintains constant GFR (BP 80-180 mmHg)

• Extrinsic controls: Adjust GFR for body needs (e.g., stress, hydration)

• Primary mechanisms:

  • Sympathetic NS

  • RAAS (renin-angiotensin-aldosterone system)

how does sympathetic activation reduce GFR?

1. Vasoconstriction of afferent > efferent arterioles (via α₁-adrenergic receptors)

2. ↓ Blood flow → ↓ glomerular pressure → ↓ GFR

3. JG cells release renin → activates RAAS

   • Angiotensin II further constricts arterioles

   • Mesangial cell contraction ↓ filtration surface area

what stimulates JG cells to release renin?

1. ↓ BP (renal baroreceptor response)

2. Sympathetic stimulation (β₁-adrenergic)

3. ↓ NaCl at macula densa (tubuloglomerular feedback)

how does angiotensin II decrease GFR?

• Constricts afferent & efferent arterioles (efferent > afferent)

• Contracts mesangial cells → ↓ glomerular SA

• Net effect: ↓ GFR, ↑ filtration fraction (conserves water)

Why does the body reduce GFR during stress?

• Prioritizes blood flow to heart/brain (shunts from kidneys)

• Conserves water/salt for BP maintenance

• Trade-off: Temporary ↓ waste excretion

how can chronic sympathetic activation harm the kidneys?

1. prolonged vasoconstriction can lead to ischemic injury

2. RAAS overactivation → hypertension and glomerular damage

3. e.g., seen in heart failure, chronic stress, hemorrhagic shock

what is ANP and what triggers its release?

1. atrial natriuretic peptide (ANP)

2. released by cardiac atrial myocytes in response to atrial stretch, hypervolemia

3. opposes RAAS

ANP’s actions in one phrase?

"Dump the Volume!"

• Dilates arterioles

• Urine output ↑

• Mesangial relaxation

• Pressures drop

how does ANP (atrial natriuretic peptide) increases GFR?

1. Dilates afferent arteriole → ↑ glomerular pressure

2. Inhibits renin release → ↓ angiotensin II → relaxes mesangial cells → ↑ filtration surface area

3. Net effect: ↑ GFR, ↑ urine output (diuresis)

what are ANP’s overall effects on blood volume and pressure?

• ↓ Blood volume: ↑ Na⁺/water excretion (natriuresis)

• ↓ Blood pressure:

  • Vasodilation (systemic)

  • ↓ Aldosterone (reduces Na⁺ reabsorption)

contrast ANP and sympathetic effects on GFR.

• ANP: ↑ GFR (afferent dilation, ↓ RAAS) → lowers BP

• Sympathetic: ↓ GFR (afferent constriction, ↑ RAAS) → raises BP

• Balanced during homeostasis

When is ANP most active?

• Volume overload (e.g., heart failure)

• High-salt diet

• Therapeutic target: Synthetic ANP (nesiritide) for acute HF

what is the functional unit for reabsorption/secretion in nephrons?

• Simple epithelium of tubule wall

• Forms transport barrier with:

  • Luminal membrane (faces filtrate)

  • Basolateral membrane (faces blood)

paracellular transport

movement of susbtances between epithelial cells

• Driven by:

  • Concentration gradients

  • Solvent drag (water flow)

describe transcellular transport steps.

1. Luminal membrane: Substance enters cell

2. Cytoplasm: May be modified (e.g., metabolized)

3. Basolateral membrane: Exits to interstitial fluid

how does transport differ for reabsorption vs. secretion?

• Reabsorption:
  Luminal → Basolateral (filtrate → blood)
  Example: Glucose, Na⁺ in PCT

• Secretion:
  Basolateral → Luminal (blood → filtrate)
  Example: K⁺, H⁺ in DCT

transport maximum (Tm) (what it is and what factors control it)

1. maximum rate of substances that can be reabsorbed (or secreted) across tubule epithelium per a certain time

2. depend on the number of transport proteins in the membrane

   → if no more than 375 mg/min, glucose in tubule all reabsorbed

   → if greater than 375 mg/min, excess glucose excreted in urine

renal threshold

max plasma concentration of a substance that can be transported in the blood without appearing in the urine

• if Tm is exceeded, substance excreted in urine

how are nutrients typically handled by the kidney?

• 100% reabsorbed in healthy individuals (PCT)

• Each nutrient has specific transport proteins

• Occurs primarily via secondary active transport

how is glucose transported into tubule cells?

• Via Na+/glucose symporter proteins

• Energy from Na+ moving down its gradient

• Moves glucose up its gradient into tubule

• Secondary active transport

how does glucose exit tubule cells?

• moved by uniporters across basolateral membrane

• returned to blood in peritubular capillaries