PHYSIOLOGY: RENAL SYSTEM Flashcards
What are the functions of the kidney?
•Excretory, regulatory, & endocrine organ
Responsible for:
a) Producing urine (1 L/day excreted): Maintains proper balance between water, salts, acids and bases
b) Controlling blood pressure, blood volume, blood constituents
c) Filtering and cleaning blood: Allows toxins, metabolic wastes, and excess ions to be excreted in the urine à Body’s “washing machine”
d) Synthesizing and secreting hormones: Renin to regulate blood pressure; erythropoietin to stimulate red blood cell production, activates vitamin- D -1,25-dihydroxycholecalciferol)
- Total renal blood flow ~25% of cardiac output (1800 L/day or 1.25 L/min)
- From 1800 L/day à filters about 180 L/day; reabsorbs >99% of plasma ultra-filtrate to make urine
- Nephron: functional unit of kidney
- millions per kidney
Which nephron is most responsible for urine?
the Juxtamedullary
How does blood supply get to the nephrons?
Vasculature on top of tubules – arterioles on either side of glomerular capillaries
Afferent artery - glomerular capillaries (filtration here) (Not all blood is filtered)
Efferent artery via peritubular capillaries (wrap around epithelium portion of nephron (proximal/distal tubules))
- 99% reabsorption - proximal tubules into peritubular capillaries into vasculature
Vasa Recta - capillaries close to juxtamedullary nephrons only
- Specialized peritubular caps
- Hairpins close to thin/thick ascending and thin descending limbs of loop of Henle
What two components determine flux across the glomeruls during glomerular filtration?
2 components to determine flux across glomerulus:
1) Permeability
2) Glomerular filtration pressure
How does size and charge effect permeability with GFR?
a. Size:
- Small molecules radii
- 15 - 35 Å : inverse relationship with size and filterability
- Large molecules > 35 Å : no filterability at all
Freely filtered molecules: [plasma] = [Bowman’s space]
Non-freely filtered molecules: [plasma] > [Bowman’s space]
b. Charge: 15 - 35 Å : Cations > neutral > anions
What are the “Starling Forces”?
“Starling Forces”: 4 pressures affecting fluid movement across capillary wall
–2 hydrostatic pressures: push H2O away (out capillary/interstitium)
–2 oncotic pressures: proteins drawing H2O towards them (into the capillary/interstitium)
- Hydrostatic pressure of capillary (PG) – favours filtration
- Hydrostatic pressure of interstitium (PB) – opposes filtration/ favours reabsorption
- Oncotic pressure of blood ( plasma) (pG) – opposes filtration/ favours reabsorption
- Oncotic pressure of interstitial fluid/Bowman’s space (pB) – favours filtration (+) – no proteins in Bowman’s space (0 mmHg)
If a kidney stone blocked a renal calyx, how would this affect filtration pressure in the nephrons emptying into it?
It would increase hydrostaic pressure in the bowman’s space fluid would move back int the capillary. It may oppose filtration where ther’s no movement of fluid at all. Net pressure = 0
Nephrotic Syndrom results in an increased permeability of glomerular capillaries to plasma proteins. What Starling Force will be affected and why?
This results in increased oncontic pressure of intersititial fluid/Bowman’s space (πB) (favours filtration (+)- no protiens in Bowman’s space (0 mmHg). It should normally be zero.
Protein is being filtered out and water is collecting in places it’s not suppose to.
Urinary track obstruction (obstructive uropahty) results in back up tubular flow. What does this result in?
Increased hydrostaic pressure PB (opposes filtration/favors reabsorption)
Backs up tubular flow
Kidney stones cause increased movement of reabsorption force.
Glomerular blood hydrostatic pressure varies with constriction of afferent and efferent arterioles.
What would be the effect of afferent arteriole constriction and dilation?
Constrict afferent:
Decreased Renal plasma flow (RPF)
Decreased GFR
Deceased Hydrostatic Pressure
Dilate afferent:
Increased RPF
Increased GFR
Increased Hydrostatic Pressure
Glomerular blood hydrostatic pressure varies with constriction of afferent and efferent arterioles.
What would be the effect of efferent arteriole constriction and dilation?
Constrict afferent: (Blood backs up & pools)
Decreased Renal plasma flow (RPF)
Increased GFR
Increased Hydrostatic Pressure
Dilate afferent:
Increased RPF
Decreased GFR
Decreased Hydrostatic Pressure
What’s the difference between filtration and reabsorption?
What’s the difference between secretion and excretion?
Filtration: Movement of solutes from glomerular capillaries to Bowman’s Space
Reabsorption: Returns most filtered solutes to circulation
Secretion: Transports solutes from peritubular capillaries and vasa recta into the tubular lumen
Excretion: Solute in urine due to filtration, secretion, reabsorption (sum of 3 processes)
Where does most of the reabsorption of glucose, bicarbonate, and amino acids occur?
In the proximal convoluted tubule (PCT), almost all glucose, bicarbonate, and amino acids are reabsorbed.
2/3 of sodium is reabsorbed in the PCT, while chloride and water are reabsorbed passively.
In the PCT, organic acids and bases are actively secreted into the tubule.
The PCT is the site of action for acetazolamide (inhibition of carbonic anhydrase) and parathyroid hormone (promotes excretion of phosphate).
NOTE: Parathyroid hormone also acts at the distal convoluted tubule.
How much of renal blood flow gets filtered through Bowman’s capsule?
20% of all renal plasma flow enters Bowman’s capsule to begin the filtering process.
What are the mechanisms by which the kidneys regulate serum acid-base content?
The kidneys regulate serum acid-base content through reabsorption of filtered HCO3- and excretion of fixed H+ as either H2PO4- (titratable acid) or NH4+.
In the proximal convoluted tubule cells, H+ and HCO3- are produced from CO2 and H2O in a reaction catalyzed by carbonic anhydrase: CO2 + H2O ↔ H2CO3 ↔ H+ + HCO3-
The H+ is secreted into the tubular lumen by an Na+/H+ exchanger, while the HCO3- is reabsorbed into the interstitium.
NOTE: Here, there is NO net excretion of H+.
Rather, the secreted H+ is paired with another luminal HCO3- to reform CO2 and H2O bybrush border carbonic anhydrase, and brought back into the proximal tubule cells to restart the cycle.
Fixed H+, also generated by mechanism outlined above, is secreted into the tubular lumen by an H+-ATPase, to be excreted either by combining with filtered HPO42- to form H2PO4-or with NH3, generated from glutamine in proximal tubule cells, to form NH4(ammonium). This process may be enhanced by the action of aldosterone on the H+-ATPase.
NOTE: In this process there IS net excretion of H+.
What is reabsorbed in the descending loop of henle?
The descending loop of henle enters the deeper portions of the kidney, where the interstitial osmotic gradient increases, causing the reabsorption of water and the concentration of fluid in the tubule.
The unfiltered blood that exits the glomerulus drains into what blood vessel?
Unlike most other capillary beds, the glomerulus drains into an efferent arteriole rather than a venule.
Name some of the endocrine functions of the kidney (try to name four).
The kidney also has several endocrine functions including the production of:
- EPO - involved in increasing red blood cell production
- 1-α-hydrolyase - stimulated by PTH, catalyzes reaction forming active Vitamin D
- Prostaglandins - local vasodilatory function
- Renin - involved in triggering the renin-angiotensin-aldosterone system
What percentage of cardiac output supplies renal blood flow?
Which part of the kidney receives most of the total renal blood flow – the cortex or the medulla?
RBF (renal blood flow) is normally ~20% of cardiac output.
The renal cortex receives ~90-95% of total RBF.
The renal medulla receives ~5-10% of total RBF.
To reach the kidney, arterial blood leaves the descending (abdominal) aorta to enter the renal artery. Note: the renal artery emerges from the descending aorta at the level of L2(second lumbar vertebra).
From the renal artery, blood travels through a series of named vessels, in sequence:
Renal artery → segmental artery → interlobar artery → arcuate artery → interlobular artery → afferent arteriole → glomerular capillaries → efferent arteriole → peritubular capillaries → interlobular vein → arcuate vein → interlobar vein → renal vein → IVC (inferior vena cava) → right atrium
To what extent is water reabsorbed in the distal convoluted tubule?
The cells of the distal convoluted tubule (DCT) actively reabsorb NaCl, however they areimpermeable to water, making the tubular fluid hypotonic.
Increased sodium reabsorption in the DCT is mediated by aldosterone.
Calcium is reabsorbed in the DCT under the influence of parathyroid hormone.
The NaCl channels in the DCT are the targets for thiazide diuretics.
What is the physiological function of buffers?
Buffers minimize changes in pH upon the addition or removal of H+.
Buffers consist of a solution containing:
Either a weak acid and its conjugate base
Or a weak base and its conjugate acid
The major extracellular buffer is HCO3- (pKa 6.1).
Carbonic anhydrase mediates the following reaction, converting CO2 and H2O to H+ and HCO3- (bicarbonate):
CO2 + H2O ↔ H2CO3 ↔ H+ + HCO3-
The minor extracellular buffer is phosphate. The concentration of phosphate in the blood is so low that it is relatively unimportant. But, because the concentrations are higher in the urine, this is the most important urinary buffer, aiding in the excretion of H+ as H2PO4-(titratable acid).
Hemoglobin, specifically deoxyhemoglobin, is the major intracellular buffer. The greater ability of deoxyhemoglobin to form carbaminohemoglobin explains why deoxygenated blood is better than oxygenated blood at carrying CO2, which is the Haldane effect.