Urinary physiology Flashcards
What are the functions of the kidneys?
Removes metabolic waste from blood by filtration and excretion
Regulates plasma electrolytes and blood pressure (by renin-angiotensin mechanism)
Help to stabilize the PH
Reabsorption of small molecules (amino acids. Glucose, and peptides)
Produces erythropoietin (a stimulant of RBC production by bone marrow)
What is the vertebral levels of the kidneys?
Retroperitoneal position at T12-L3
Partly peritonised
What does the urinary system develop from?
Intermediate mesoderm + cloaca
Intermediate Mesoderm (nephrogenic cord/urogenital ridge) gives rise to kidney & ureter
Cloaca gives rise to urinary bladder & urethra
What do the kidneys develop from?
Arises from intermediate mesoderm forming the urogenital ridge either side of the aorta
Develops into three sets of tubular nephric structures
Phronephros
>Mostly progresses, cranial most tube
Mesonephros
Meranephros
What happens to the mesonephros?
Located along the midsection of the embryo and develops into mesonephric tubules + duct
Tubules regress, however the duct persists opening into the cloaca
What happens to the metanephros?
Gives rise to the definitive adult kidney.
Develops from an outgrowth of the caudal mesonephric duct, theureteric bud, and from a condensation of nearby intermediate mesoderm, themetanephric blastema.
What does the metanephric blastema go on to create?
excretory portion of kidney (nephron): Excretory tubules (Nephrons) -Bowman’s Capsule -PCT -Loop of Henle -DCT
What does the uteric bud go on to form?
collecting portion of kidney:
Collecting ducts
Calyces
Ureters
How does the nephron develop?
Metanephric vesicle turns into the metanephric tubule
This merges with the collecting duct
The glomerulus then invaginates the nephron
The uriniferous tubule lengthens
Why and how do the kidneys ascend?
Mainly due to differential growth of lower body inferiorly away from kidneys (appear to “ascend”)
Kidneys come to lie retroperitoneally in upper abdomen (T12-L3)
Hilum rotates from ventral to medial (~90degrees)
Ascent stops when contact with adrenal (suprarenal) glands
Segmental breakdown and reform of vascular supply during ascent
Accessory renal arteries (always at inferior pole)
How does the cloaca divide?
Cloaca divided by urorectal septum into urogenital sinus and rectum. Parts of UG sinus: Cranial Pelvic Caudal
How does the trigone develop?
Trigone derived from absorption of mesonephric ducts
Mesodermal lining replaced with endodermal epithelium
What does the urogenital sinus become?
Urogenital sinus gives rise to: Cranial part - forms bladder Middle part - forms prostatic and membranous urethra in males, and entire urethra in females Caudal part - forms penile urethra in males
What is the process of filtration?
Receives ~20-25% of CO
No red blood cells and only a fraction of the plasma is filtered through bowman’s capsule
Remainder passes via efferent arterioles into peritubular capillaires and then renal veins
What is filtration dependant on?
Filtration dependent on hydrostatic forces + oncotic pressure
Glomerular capillary pressure higher than other capillaries because afferent arterioles short + wide so high hydrostatic pressure
Combined with efferent being thin builds up pressure to be able to filter
What affects GFR?
Dependent on afferent/efferent diameter controlled by
Sympathetic VC nerves –> afferent and efferent constriction, greater sensitivity of afferent arteriole.
b) Circulating catecholamines –> constriction 1°ily afferent
c) Angiotensin II –> constriction, of efferent at [low], both afferent and efferent at [high].
What is autoregulation?
Renal vasculature has intrinsic ability to adjust resistance in response to arterial BP changes
BF/GFR kept almost constant (autoregulation)
When MBP is 50 or less filtration would stop
Independent of nerves or hormones
Dilation of afferent arterioles if mean arterial pressure decreases, constriction if increases
What happens in the kidneys when bloodpressure is compirmised?
I.e in haemorrhage
Activation of sympathetic VC nerves and AII can override autoregulation liberating blood to immediately more important organs
However, prolonged exposure to a reduction on renal BP can lead to irreparable damage and potential death
What is responsible for reabsorption?
Peritubular vessels
How does reabsorption happen in the kidneys?
Offers resistance along whole length of peritubuluar vessels, so large pressure drop allowing hydrostatic pressure to fall as well
PPC very low because hydrostatic P overcoming frictional resistancein efferent arteriols
PP high compared to normal, loss of 20% plasma concentrates plasma protein
PP»_space; PPC only reabsorption
What are the mechanisms of reabsorption?
Many substances reabsorbed by carrier proteins
Have maximum transport capacity due to saturation of carriers
If maximum exceeded, rest is excreted
Glucose is one such molecule, freely filtered. Up to 10mmoles will be reabsorbed
How does the kidney regulate substances?
Sulphate and phosphate ions for example
Their Tm set at a level where normal plasma causes saturation
Causes excretion of anything above normal
How is sodium reabsorbed?
Reabsorbed via active transport in proximal tubule
Chlorine follows the electrical chemical gradient
Causes osmotic force that brings water with it
What is the process of reabsorption for the main ions?
Sodium by active transport
Electrochemical gradient drives anion reabsorption
Water moves by osmoles
Concenterations of other solutes increases as other fluid in lumen decreases
>Non actively reabsorbed solutes depends on the amount of water removed, and permeability of membrane to any solute
Why is sodium reabsorption so important for the kidneys?
Concentrates the tubule creating greater concentration gradient for passive molecules
High sodium in tubule facilitates glucose transport, low sodium inhibits as they share the same molecule from the lumen side
What is tubular secretion>
Transport substances from peritubular capillaries into the lumen
Important for protein bound molecules due to restricted filtration
Carrier proteins not very specific so that organic acids can be used for other molecules like drugs
How do the kidneys handle potassium?
Balance essential for life (based on Tm)
Reabsorbed at proximal tubule
Any increase in renal tubule cell potassium will lead to postassium secretion. And reduction in intracellular potassium leads to decreased secretion.
How does aldosterone affect potassium secretion?
Secretion also regulated by aldosterone
Increase in potassium in ECF stimulates aldosterone release
Aldosterone stimulates kidneys to increase tubule cell potassium secretion
Also stimulates sodium reabsorption at distal tubule
Hydrogen ions actively secreted from tubule cells to maintain acid/base balance
What is the maximum concentration of urine?
4x concentration of plasma
What is obligatory water loss?
Waste products + excess ions that must be excreted every day ~ 600mosmoles
Requires at least 500mls of water to be excreted
Must ingest water otherwise will urinate to death
If excess water intake can have 10x dilution
What mechanism is behind water being extracted form urine?
In loop of henle, the ascending limb actively co-transports sodium/chlorine out of tubule lumen into intersitium. Impermeable to water
Descending limb is freely permeable to water, buy relativelt impermeable to sodium chloride
What is the process of the loop of henle?
Descending limb exposed to greater osmolarity in interstitium, water moves out to equate it
Water reabsorbed into the vasa recta (no longer in interstitium)
Concentrated fluid in descending limb delivers high concentration to ascending limb
Here sodium chloride Is actively transported out, further concentrating interstitium
Net effect, progressively concentrated as it moves down descending limb
Progressively diluted as it moves up ascending limb
Never more than 200osmole gradient at any horizontal level
What effect does furosemide have on the loop of henle?
Stops use of active transporter of NaCl from ascending limb so all concentration differences lost - isotonic urine
What does the counter current in the loop of henle accomplish?
Significance - increasingly concentrated gradient in interstitium
Functions of vasa recta
1. Provide O2 for medulla.
2. In providing O2 must not disturb gradient.
3. Removes volume from the interstitium
What are the vasa recta?
Specialised arrancement of peritubular capillaries
Arranged in hair-pin loops so they do not interfere with the gradient
Freely permeable to water and solutes
flow rate through the vasa recta is very low so that there is plenty of time for equilibration to occur with the interstitium, further ensuring that the medullary gradient is not disturbed
What controls the permability of the collecting duct?
ADH
What is ADH?
A polypeptide synthesises in hypothalamus
Posterior pituirary hormone
Half life ~10 minutes so can be rapidly adjusted to body’s need
What is the primary function of ADH?
Plasma osmolarity control
When plasma osmolarity pressure increases, ADH secretion increases
Changes in neuronal discharge mediated by osmoreceptors in anterior hypothalamus
Other receptors mediate thirst
What is the mechanism of osmoreceptors?
When high osmolarity, water leaves cell shrinking it
Stretch sensitive ion channels activated leading to more ADH secretion
When low osmolarity water enters cell swelling it
>Decrease in ADH
Small changes in either direction lead to rapid secretion/ inhibition of ADH
Osmolarity and not tonicity
How does ADH achieve its effect?
Concentrates urine in the collecting duct via controlling its permeability
Due to water channels in luminal membrane
