Urinary System Anatomy
What are the functions of the kidney?
Bonus: Broadly describe what the ureter, bladder and urethra do.
1. Regulates fluid balance of the body
2. Maintains electrolyte balance
3. Long-term acid-base balance
4. Removes metabolic waste
5. Maintains blood pressure
6. Regulation of erythropoiesis
7. Activates Vitamin D
8. Other metabolic functions
Bonus:
Ureter: carries urine from kidney to bladder
Bladder: temporarily stores urine
Urethra: carries urine from bladder to outside of the body
Describe the three tissue layers of the filtration membrane from the blood to the filtrate.
1st Layer: Endothelial cells of capillary with some fenestrations.
2nd Layer: Basal Lamina (connective tissue) with smaller holes.
3rd Layer: Podocytes with filtration slits (smallest gaps).
This 3 layer membrane ensures that no formed blood elements, proteins, or albumin will enter the filtrate.
True or false: most reabsorption occurs in the proximal tubule.
True!
There is NO secretion occurring in the nephron loop.
The nephron loop has the following reabsorbed:
- Thin descending limb: water (obligatory)
- Thick ascending limb: Na+ & Cl- (secondary active transport)
On a basic level, how does production of dilute urine differ from production of concentrated urine?
Production of dilute urine:
- Easier for the body to make due to good hydration.
- ADH decreases due to its function not being needed here. Aquaporins are closed so more water is excreted in urine.
Production of concentrated urine:
- More complex overall.
- Needs more ADH and aquaporins are open to help retain water.
- Needs an osmotic gradient to help retain water.
Where are the different parts of the renal tubules located in the nephron in relation to the kidneys?
Proximal and Distal Tubule in the renal cortex
Nephron Loop is differentiated by the type of nephron:
For cortical nephrons they are mainly in the renal cortex
For juxtamedullary nephrons they are mainly in the renal medulla
Collecting Duct is in the renal cortex and the pyramid is in the renal medulla
Does the myogenic mechanism affect the efferent arteriole, afferent arteriole, or both arterioles? How does this mechanism work?
The myogenic mechanism will only affect the afferent arteriole!
The stimulus here is a change in glomerular blood pressure:
High BP -> High GFR -> Afferent Arteriole Vasoconstricts -> GHP & GFR Decrease
Low BP -> Low GFR -> Afferent Arteriole Vasodilates -> GHP & GFR Increase
What types of homeostases (3) will be maintained via the reabsorption of the proximal tubule?
- Fluid (as seen with H2O reabsorption)
- Electrolyte (as seen with different ion reabsorption)
- Acid-Base (as seen with bicarbonate ion reabsorption)
How does the solute concentration change as filtrate moves down the thin descending limb? How does the solute concentration change as filtrate moves up the thick ascending limb?
As filtrate moves down the thin descending limb, the osmolarity increases and hence the solute concentration is higher. The filtrate becomes more concentrated as more water moves out of the limb by osmosis, leaving few solutes to follow it.
As filtrate moves up the thick ascending limb, the osmolarity decreases and hence the solute concentration is lower. The filtrate becomes less concentrated as ions are pumped into the interstitial fluid and out of the filtrate.
What are the characteristics and composition of healthy urine?
Bonus: What is specific gravity? How does it relate to urine?
- Specific gravity of urine should be around 1.005 - 1.03.
- Color should be slightly yellow from urochrome (breakdown product of bilirubin) and transparent.
- No formed elements or proteins should be present in urine.
- pH is slightly acidic, average about 6 but ranging from 4.5-8.
Bonus: Specific gravity (SG) is how concentrated something is in comparison to water. A higher SG indicates a higher concentrated substance.
What are the two arterioles that are connected to the glomerular capillaries? And in which direction does each one take blood?
Efferent - away from the glomerular capillary
Afferent - towards the glomerular capillary
How does angiotensin-II help regulate GFR?
Angiotensin-II will help maintain a higher BP.
Blood vessel diameter will be affected:
- increasing GFR to normal range by vasoconstricting efferent arteriole.
- vasoconstricting blood vessels which will increase peripheral resistance & increases systemic BP.
Blood volume will be affected:
- increases reabsorption of Na2+ and water from proximal tubule, which increases BP
- stimulates release of aldosterone & increases reabsorption of Na2+ and water from distal tubule, which will increase BP
- stimulates feeling of thirst in hypothalamus so more water is introduced to the body and BP increases
What is secreted in the proximal tubule?
Nitrogenous waste products:
- Uric acid
- NH4+ (ammonium)
- Creatinine
- Urea
Drugs (e.g. penicillin, morphine)
What is secreted in the distal tubule? What is reabsorbed in the distal tubule?
What is secreted in the collecting duct? What is reabsorbed in the collecting duct?
Distal Tubule:
Secreted: Potassium (K+); Hydrogen Ions (H+)
Reabsorbed: Water; Sodium (Na+); Calcium (Ca2+)
Collecting Duct:
Secreted: Hydrogen Ions (H+)
Reabsorbed: Water; Sodium (Na+); Chlorine (Cl-)
How is micturition regulated?
By the parasympathetic nervous system.
Stretch receptors are present in the urinary bladder & as urine fills the bladder, these receptors inform the spinal cord. Involuntary reflex is initiated. Message travels to the pons of the brain then goes from the cerebral cortex to the external urethral sphincter to allow urination to occur voluntarily.
What type of tissue makes up the proximal, distal tubules, and the thin and thick parts of the nephron loop?
Simple cuboidal epithelium with microvilli in the proximal tubule
Simple cuboidal epithelium with less microvilli in the distal tubule
Simple squamous in the thin descending limp of nephron loop
Simple cuboidal with no microvilli in the ascending limb of the nephron loop
Describe how GHP & GFR are affected by changes in the diameter of the afferent and the efferent arterioles.
If the afferent arteriole vasoconstricts, GHP & GFR decrease.
If the efferent arteriole vasoconstricts, GHP & GFR increase.
If the afferent arteriole vasodilates, GHP & GFR increase.
If the efferent arteriole vasodilates, GHP & GFR decrease.
What is the importance of sodium in reabsorption? What is the primary transport protein seen in this process? What are the secondary transport proteins seen in this process?
Sodium concentration gradients are necessary for the reabsorption of most other substances. This favorable concentration gradient for sodium also helps create an osmotic gradient for water reabsorption by powering secondary active transport.
The primary antiporter (the one using the ATP directly) is the Na+/K+ antiporter.
The secondary active transport proteins are seen in any channels or proteins that will move Na+ into the tubule cell from the lumen. This is secondary because it is indirectly driven by the presence of ATP in the primary antiporter.
How is pH regulated here?
If pH is too low/too acidic:
- Too many H+ ions are present.
- The secretion of H+ ions is increased.
- The reabsorption of HCO3- is increased.
If pH is too high/too basic:
- Too little H+ ions are present.
- The secretion of H+ ions is decreased.
- The reabsorption of HCO3- is decreased.
What two factors are absolutely necessary for the production of concentrated urine?
High amounts of ADH and a favorable osmotic gradient, referred to as the medullary osmotic gradient (MOG).
As filtrate moves through the collecting system with more ADH and a higher MOG present, water is reabsorbed. There will be a higher solute concentration in the interstitial fluid to allow for this osmotic gradient, which allows the body to retain more water.
What does the differing amount of microvilli in the different parts of the tubule tell us about the amount of reabsorption and secretion that happen in each part?
Simple cuboidal epithelium:
With lots of microvilli - as seen in the proximal tubule. Increases the surface area as to allow for lots of absorption.
With some microvilli - as seen in the distal tubule. Same as above, with just a smaller amount of absorption allowed.
With no microvilli - as seen in the thin descending limb of the loop. Allows for permeability of different substances.
Simple squamous epithelium:
As seen in the thick ascending limb of the loop. Allows for permeability of different substances.
What would happen if the sympathetic nervous system stimulation was at a high level?
1. JG cells increase their production of renin
2. Extremely high levels of RAAS are produced
3a. Afferent arteriole vasoconstricts, leading to decreased GHP & GFR
3b. Efferent arteriole vasoconstricts, leading to increased GHP & GFR
4. Afferent arteriole effect is more dominant, and thus net GFR is decreased.
5. More water is retained, hence BP is increased as to maintain a high BP.
Describe the movement of bicarbonate ions in the proximal tubule.
The goal with this process is to maintain a fairly constant pH.
Remember that H+ and HCO3- ions can be converted into H2CO3, which can tend be converted into CO2 & H2O. This reaction is reversible.
Bicarbonate from the filtrate is converted into CO2 form to enter the cell, then converted back to bicarbonate in the cell before it goes to the blood.
Using the antiporter between the cell and the lumen, the pH of the filtrate and of the blood can be regulated. The analogy to think about with this is H+ ions as a bounty hunter and HCO3- as the bounty.
Describe what the following hormones do and how they relate to one another:
- Aldosterone
- Antidiuretic Hormone (ADH)
- Atrial Natriuretic Peptide (ANP)
Aldosterone:
increase reabsorption of Na+ -> increases water reabsorption (if aquaporins are present)
K+ secretion is also increased
ADH:
causes aquaporins to be present -> increases water reabsorption
ANP:
decreases reabsorption of Na+ and inhibits release of ADH & aldosterone -> decreases water reabsorption
Aldosterone and ADH are synergistic, while ANP is antagonistic to both hormones.
What 3 components make up the countercurrent mechanism of the medullary osmotic gradient?
1. Countercurrent Multiplier - This sets up the medullary osmotic gradient. Relates to the permeability of each limb. Located in the nephron loops of the juxtamedullary nephrons.
2. Urea Recycling - This adds solutes as to increase the gradient difference. Located between the nephron loop and collecting system.
3. Countercurrent Exchange - This maintains solute concentration. As blood flows through the vasa recta, it will pick up solutes and drop them off to maintain this concentration. Located from the vasa recta to the nephron loop.