Urinary System: Urine Flashcards
Kidneys maintain the body’s
Fluid osmotic concentration (amount of solute in body fluids)
Dehydration (elevated blood osmolarity) results in
A small volume of concentrated urine
◦ conserves water to lower blood osmolarity
Overhydration (lower blood osmolarity) results in
A large volume of dilute urine
◦ gets rid of water to bring blood osmolarity up
In the juxtamedullary nephrons & associated vasa recta counter current is created by
◦ fluid flowing in opposite directions in two adjacent segments of same tube with hairpin turn
◦ establishes & maintains a medullary osmotic gradient from cortex to depths of medulla – allows kidneys to vary urine concentration
The juxtamedullary nephron loop is a
Countercurrent multiplier
The vasa recta is a
Countercurrent exchanger
Regarding the countercurrent mechanism: the descending limb is permeable to
H2O (NaCl cannot leave)
Regarding the countercurrent mechanism: the ascending limb is permeable to
NaCl (H2O cannot leave)
NaCl reabsorption in the ascending limb creates a positive feedback loop through establishing
An osmotic gradient in the surrounding interstitial fluid to favour H2O reabsorption from adjacent descending
◦ As filtrate moves through loop, H2O leaves in descending = concentrated filtrate = increased osmolarity
◦ Concentrated filtrate moves into ascending removes NaCl = decreased osmolarity
Compared to the ascending and descending limbs, blood within the vasa recta
remains
Isosmotic to surrounding fluid
The vasa recta maintains the countercurrent multiplier’s osmotic gradient due to
H2O & solutes being able to move freely in & out of it
To concentrate urine:
◦ Juxtamedullary nephron loop creates the medullary osmotic gradient
◦ Vasa recta maintains the medullary osmotic gradient
Hormone that increases the number of aquaporins in collecting tubule (duct) cells
Antidiuretic hormone (ADH)
◦ more aquaporins = more water reabsorption into bloodstream
◦ results in more concentrated urine
To conserve water during dehydration urine concentration is controlled by
The medullary osmotic gradient
◦ allows for regulation of urine osmolarity to exceed 300 mOsm
◦ w/o you could only produce urine that is the same concentration as your body fluids
Dehydration results in
↑ ADH (adds aquaporins) → ↑ H2O reabsorption → small volume of concentrated urine
Overhydration results in
↓ ADH (removes aquaporins) → ↓H2O reabsorption → large volume of dilute urine
Diuretics increase urine output by
◦ Inhibiting ADH release to ↓ H2O reabsorption (ex: alcohol)
◦ Inhibiting Na+ & obligatory water reabsorption in PCT (ex: caffeine, hypertension drugs)
An osmotic diuretic is
A substance that is not reabsorbed & carries H2O with it (↓ H2O reabsorption)
◦ Diabetes mellitus – unusually high glucose concentration pulls H2O from the body
Urine is composed of
◦ 95% H2O
◦ 5% solutes (nitrogenous wastes urea, uric acid
& creatinine)
pH of urine is
6 (slightly acidic); varies from about 4.5–8.0 based on metabolism or diet
Pigment from hemoglobin breakdown that gives urine yellow colour
Urobilin (urochrome)
Ammonia scent of urine results from
Bacterial metabolism of urea
Tubes that actively convey urine from the kidneys to the bladder are
Ureters
◦ inner mucosa is continuous with the kidney pelvis & the bladder
◦ urine propelled via peristalsis from double-layered muscularis
Urinary bladder is
A muscular sac that expands to store urine until voiding
◦ Capacity of ~ 500-800 ml
◦ expansion permitted by layer of detrusor muscle & highly folded inner mucosa of transitional epithelium (rugae)
Urethra is
A muscular tube that drains urine from the body
◦ 3–4 cm long in females
◦ ~ 20 cm in males
Involuntary urethral sphincter
◦ detrusor smooth muscle
Internal urethral sphincter
Voluntary urethral sphincter
◦ skeletal muscle
External urethral sphincter
Micturition (urination) requires simultaneous:
◦ Contraction of detrusor muscle
◦ Relaxation of internal & external urethral sphincters
Afferent impulses from bladder’s stretch receptors relayed to pons & cerebral cortex provide
Conscious awareness of bladder fullness
◦ induces conscious urination via relaxing external urethral
sphincter
Micturition (urination) is stimulated by
Parasympathetic pathways
Micturition (urination) is inhibited by
Sympathetic pathways
