Urinary System Flashcards
Excretion
removal of organic waste products from
body fluids
Regulation
maintenance of homeostasis in volume and solute concentration of plasma
Elimination
discharge of waste products into environment
The Kidneys
• Paired, retroperitoneal organs (behind abdominal wall)
• Surrounded by:
- Fibrous capsule
- Fat capsule
- Renal fascia
The Nephron
• Functional unit of the kidney
• ~1.25 million per kidney
Juxtamedullary nephron
• Consists of a renal corpuscle and a renal tubule
• Surrounded by capillary network
Renal corpuscle
•Site where filtration of
plasma occurs
•Blood pressure pushes fluid
out of capillaries into
capsular space
- Continuous with renal tubule
Glomerulus
- Capillary network
- Blood arrives via afferent arteriole (in)
- Blood leaves via efferent arteriole (out)
Glomerular capsule has 2
layers
- Outer capsular epithelium
- Inner visceral epithelium
- Capsular space between
- Podocytes: large cells in visceral epithelium
• Wrap around capillaries
. “Feet” called pedicels
Renal tubule
•Proximal convoluted tubule (PCT)
- Simple cuboidal epithelium w/microvilli
• Nephron loop
- Thin segment: simple squamous; thick:
simple cuboidal
• Distal convoluted tubule (DCT)
- Simple cuboidal epithelium
•Empties into collecting duct
Nephron structure
• 85% are cortical nephrons
- Almost entirely within cortex
- Nephron loops surrounded by peritubular capillaries
• 15% are juxtamedullary nephrons
- Nephron loops extend into medulla
- Surrounded by vasa recta
Filtration
- BP forces H2O and small solutes from plasma into
nephron
Reabsorption
- Removal of water & solutes from filtrate into blood
Secretion
Addition of solutes to filtrate from blood
Nitrogenous wastes
• Urea: breakdown of amino acids
• Creatinine: breakdown of creatine phosphate
• Uric acid: breakdown of RNA nitrogenous bases
• Others: ammonia, urobilin, bilirubin…
Glomerular Filtration
• Filtration membrane
- Capillary membrane
• fenestrated (has holes)
• Prevents movement of blood cells
- Dense layer
• Only allows small proteins, nutrients and ions through
- Filtration slits
• Prevent passage of most proteins
Filtration Pressures
• Glomerular Hydrostatic Pressure (GHP)
- Blood pressure in capillaries
- 50 mmHg
• Capsular Hydrostatic Pressure (CsHP)
- Resistance in nephron
- 15 mmHg
• Net Hydrostatic Pressure (NHP)
- Difference between GHP and CsHP
- 50 mm Hg - 15 mm Hg = 35 mm Hg
Colloid osmotic pressure (COP)
-Osmotic pressure resulting from suspended proteins
- Tends to draw water back into plasma
- 25 mm Hg
Filtration pressure
- Difference between NHP and COP
- 35 mm Hg - 25 mm Hg = 10 mm Hg
If blood pressure goes up then filtration pressure increase which
Results in more filtrate produce
Glomerular Filtration Rate (GFR)
• Amount of filtrate produced by the kidneys each minute
• Averages 125 mL/min
• Glomeruli generate 180 L filtrate per day
- 99% reabsorbed in renal tubules
Autoregulation
• Decline in glomerular blood pressure triggers:
- Dilation of afferent arteriole and glomerular capillaries
- Constriction of efferent arteriole (increases pressure)
Hormonal Regulation
• Renin-angiotensin system
• Renin produced in nephrons at the juxtaglomerular complex (JGC)
Juxtaglomerular complex (JGG)
- Epithelial cells of DCT near renal corpuscle (macula densa)
- Smooth muscle fibers in afferent arteriole
Renin released due to:
- Decline in BP at glomerulus
- Decline in osmotic concentration (ie. solute concentration) of tubular fluid at macula densa
•no enough solutes, not enough filtration so increases solutes for more filtration
Release of renin results in production of angiotensin |I in the blood
- Causes constriction of efferent arteriole
- Secretion of aldosterone by adrenal glands
• Increases Na+ reabsorption in DCT & CD - Triggers sensation of thirst
- Release of antidiuretic hormone (ADH)
• Increases water reabsorption in DCT & CD - Widespread peripheral vasoconstriction
Reabsorption & Secretion at PCT
• Reabsorbs 60-70% of filtrate
- “bulk reabsorber”
- Organic nutrients: sugar, amino acids
- Ions: Sodium
- Water
- Urea
- Requires channels or carrier proteins
Transport Maximum
• Carrier proteins can be saturated
• Determines the renal threshold
- Plasma concentration at which substance of interest begins to appear in urine
- Renal threshold for glucose: 180 mg/dL
• Glycosuria: presence of glucose in urine
The Nephron Loop
Reabsorbs ½ of remaining water, 2/3 remaining sodium & chloride
Thin segment of descending limb
impermeable to solutes, permeable to water
Thick segment of ascending limb
- impermeable to solutes and water
- Has proteins that pump sodium & chloride into peritubular fluid
Countercurrent multiplication
• Na+ & CI- pumped out of thick ascending limb into peritubular fluid, increasing osmotic concentration
• Water flows out of thin descending limb into peritubular fluid, increasing osmolarity in thin descending limb
The Nephron Loop
Fluid arrives at DCT with concentration of 100 mOsm/L
- Waste products make up more of the concentration at this point
Reabsorption at DCT
• Calcium reabsorption is regulated by parathyroid hormone
• Na+ reabsorption is regulated by aldosterone
• Stimulates synthesis of ion pumps and channels
• Water reabsorption is regulated by antidiuretic hormone
(ADH)
- Stimulates synthesis of aquaporins
- Also occurs in collecting duct
Secretion at DCT
• Potassium
• Hydrogen lons
• Both are “traded” for sodium ions
- Sodium ions are reabsorbed from the DCT
Regulation of urine volume & concentration
• Obligatory water reabsorption
- Occurs in PCT & nephron loop
- Recovers 85% of filtrate volume
• Facultative water reabsorption
- Occurs in DCT & collecting duct
- ADH & aldosterone allow for precise control of urine volume & concentration
(Can be fine tuned, adjusted)
No antidieuric
Not retaining water
With antidiueric
• able to reabsorb water
•More concentrated urine, less volume
Ureters
• Transport urine from kidneys to bladder
• Three layers
- Transitional epithelium
- Smooth muscle
- Outer connective tissue layer
(No modification of urine)
Urinary bladder
• Urine storage
• Lined with transitional epithelium
- Folds called rugae
• Three layers of smooth muscle
- Detrusor muscle
After Kidneys:
- Ureters
- Bladder
- Urethra
Urethra
• Transports urine from bladder to outside
• Lined with stratified squamous epithelium
• The Male Urethra
- from neck of urinary bladder to tip of penis
(18-20 cm; 7-8 in.)
• The Female Urethra
- Is very short (3-5 cm; 1-2 in.)> reason why women get more UTIs than men
- External urethral orifice is near anterior wall of vagina
Urination
• Micturition reflex
- Stretch receptors in wall of bladder detect increased pressure caused by urine volume
(200mL or more)
• Info sent to spinal cord, brain (awareness)
- Wall of bladder contracts, further increasing
pressure
- Brain sends signal to external urethral sphincter to relax (voluntary)
• internal urethral sphincter then relaxes (involuntary)
