Renal Physiology Flashcards
What percentage of cardiac output does each kidney receive?
20%
What is the total renal blood flow of both kidneys (L per min)
1L/min
9 divisions of renal artery
- Renal artery
- Segmental artery
- Interlobar artery
- Arcuate artery
- Interlobular artery
- Afferent arteriole
- Glomerular capillary (nephron)
- Efferent arteriole
- Peritubular capillary (nephron)
How many capillary beds does each nephron have and where are they?
One at the glomerulus and one at the peritubular area
Within each nephron what connects the two sets of capillaries?
efferent arteriole
Within each nephron what connects the two sets of capillaries?
efferent arteriole
Which comes before the other - afferent or efferent arteriole?
afferent (a before e)
What kind of cells is the entire capillary covered by?
podocytes
What is the name for the entire unit of the glomerular tuft and bowman capsule?
renal corpuscle
What is filtrate free of?
cells, larger polypeptides and proteins
Each renal corpuscle contains a compact tuft of interconnected capillary loops called the …
glomerulus
What is the blood supply to the glomerulus from?
afferent arteriole
As blood flows through the glomerulus, how much of the plasma is filtered into Bowman’s capsule? Where does the remaining blood leave the glomerulus by?
- 20%
- The remaining blood then leaves the glomerulus by the efferent arteriole
What is Bowman’s capsule covered by? (histology)
parietal epithelium
What is ‘Bowman’s space’?
The part of Bowman’s capsule that comes in contact with the glomerulus becomes pushed inward slightly but does not make contact with the opposite side of the capsule because a fluid-filled space called Bowman’s space exists within the capsule. The filtrate from the glomerulus collects in Bowman’s space before flowing into the proximal convoluted tubule.
Blood in the glomerulus is separated from the fluid in Bowman’s space by a filtration barrier consisting of 3 layers:
- Single-celled capillary epithelium
- Basement membrane (basal lamina)
- Single celled epithelial lining of Bowman’s capsule
What epithelial cells are in the filtration barrier?
podocytes
How are podocytes different from the rest of the cells lining the rest of Bowman’s capsule?
They have an octopus-like structure in that they possess a large number of foot processes which act as glomerular filtration barrier
Fluid filtration (which layers does it pass through) (3)
- Across endothelial cells
- Across basement membrane
- Between the foot processes of the podocytes
Efferent arterioles carry blood away from the glomerulus and then supply the X capillaries which supply A and B.
Efferent arterioles carry blood away from the glomerulus and then supply the peritubular capillaries which supply the proximal and distal convoluted tubules
Efferent arterioles also supply the — — which supply blood to the …
- vasa recti
- loop of Henle
Both peritubular and vasa recti supply:
- water and solutes to be secreted into the filtrate
- blood to carry away water and solutes reabsorbed by the kidneys
Which of the convoluted tubules is longest and most coiled?
proximal
What is the histology of the brush border of the proximal convoluted tubule?
simple cuboidal
What is the portion of the tubule after the PCT?
loop of Henle
What is the histology of the distal convoluted tubule?
cuboidal epithelium with minimal microvilli
Where does fluid flow to from the distal convoluted tubule?
collecting duct system
What is the collecting duct system comprised of?
cortical collecting duct
medullary collecting duct
From Bowman’s capsule to the collecting-duct system, each nephron is completely separate from the others. This separation ends when …
multiple cortical collecting ducts MERGE
What is the result of additional merging after multiple cortical collecting ducts merge ?
urine drains into the kidney’s central cavity - the RENAL PELVIS - via several hundred large medullary collecting ducts
What is the renal pelvis continuous with?
the ureter draining that kidney
Outer and inner portion of kidney
Outer portion = renal cortex
Inner portion = renal medulla
What does the renal cortex contain?
all the renal corpuscles
What extends from the cortex for varying distances down into the medulla?
the loop of Henle
Types of nephron (+%)
Juxtamedullary - 15%
Cortical - 85%
What is the juxtamedullary part of the nephron?
The renal corpuscle lies in the part of the cortex closes to the cortical-medullary junction
What is the part of the loop of Henle in the juxtamedullary part of the nephron responsible for?
The loop of Henle of these nephrons plunge deep into the medulla and are responsible for generating an osmotic gradient in the medulla that is responsible for the REABSORPTION OF WATER
Which vasculature is in close proximity to the juxtamedullary nephrons?
- vasa recti
- They also loop deeply into the medulla and then return to the cortico-medullary junction
What is the cortical part of the nephron?
Their renal corpuscles lie in the outer cortex and their loop of Henle do not penetrate deep into the medulla
What do some cortical nephrons not contain? What are they involved and not involved in?
- Some cortical nephrons do not have a loop of Henle at all
- They are involved in reabsorption and secretion but do not contribute to the hypertonic medullary interstitium
Near its end, the ascending limb of each loop of Henle passes between X and Y of its own nephron.
afferent and efferent arterioles
There is a path of cells in the wall of the ascending limb of the LOH as it becomes the distal convoluted tubule called the — — and the walls of the afferent arteriole contain — cells known as — cells.
- Macula densa
- Granular cells known as juxtaglomerular cells
The combination of macula densa and juxtaglomerular cells is known as the…?
juxtaglomerular apparatus
What do granular cells in the juxtaglomerular cells secrete into the blood?
renin enzyme
What do the macula densa cells detect? What is their response?
how much NaCl is passing through the distal convoluted tubule and sends signals to the granular cells to produce renin
Flow of glomerular filtrate (order of 12 sites)
- Glomerular capsule
- Proximal convoluted tubule
- Loop of Henle
- Distal convoluted tubule
- Collecting duct
- Papillary duct
- Minor calyx
- Major calyx
- Renal pelvis
- Ureter
- Urinary bladder
- Urethra
What are the similarities and differences between glomerular filtrate and blood plasma?
Glomerular filtrate is cell-free and has no large proteins, but other than that contains virtually all substances in virtually the same concentrations as in plasma.
What is the only exception to the rule that all non-protein plasma substances have the same concentrations in the glomerular filtrate as in the plasma?
Certain low-molecular-weight substances that would otherwise be filterable are BOUND to plasma proteins and are thus not filtered e.g. half the plasma calcium and virtually all of the plasma fatty acids are bound to plasma protein and thus are not filtered.
What can freely pass through the filtration barrier? Give 4 examples
- small molecules and ions up to 10kDa can pass freely
- glucose, uric acid, potassium, creatinine
What can’t pass through the glomerular basement membrane and why?
Fixed negative charge in the glomerular basement membrane repels negatively charged anions such as albumin
Albumin and its clinical relevance
Albumin has a molecular weight of 66kDa (>10kDa) and is negatively charged. This means it CANNOT pass through into the tubule.
Small amounts of albumin in urine is the first sign of diabetic nephropathy (damage to filtration barrier due to diabetes)
Damage to the filtration barrier can lead to protein leak and a condition known as…
nephrotic syndrome
Causes of nephrotic syndrome
- immune conditions
- genetic abnormalities
- proteins involved in podocytes/slit diaphragm
What else can damage the filtration barrier?
- Diabetes
- Early sign of diabetic nephropathy = microalbuminuria (low levels of albumin in the urine)
What determines GFR?
- pressure gradients
- size of molecule
- charge of molecule
- rate of blood flow
- surface area; GFR is directly proportional to membrane permeability and surface area
- binding to plasma proteins e.g. Ca2+, hormones, fatty acids
Measuring GFR: what is the equation?
GFR = (Urine(M) x urine flow rate)/Plasma(m)
Normal GFR
125ml/min
How many litres filtered in 24 hours?
180L
How much is reabsorbed?
99%
Total plasma volume?
3L
How many times is plasma volume filtered in a day?
~60 times
Measuring GFR: when does it fall?
disease causing loss of nephrons
What does GFR not detect?
problems in tubule function e.g. nephrotic syndrome
What is the only protein normally found in urine?
Tamm-Horsfall protein (uromodulin) which is produced by the thick ascending limb
2 mechanisms of auto-regulation
Myogenic
Tubuloglomerular feedback
What is myogenic autoregulation?
Pressure within the afferent arteriole rises, causing stretching of the smooth muscle wall which triggers the contraction of smooth muscle → arteriolar constriction
PASSIVE mechanism
Where does myogenic autoregulation occur?
capillary walls
What does the myogenic autoregulation mechanism prevent?
an increase in systemic arterial pressure from reaching and damaging the capillaries
What is the range of the myogenic autoregulation mechanism?
90-200mmHg
Tubuloglomerular feedback
The cells of the macula densa in the distal tubule detect NaCl arrival. Macula densa cells release prostaglandins in response to a reduction in NaCl. This in turn acts on granular cells, triggering renin release, thereby activating the RAAS system.
Tubuloglomerular feedback - constriction of afferent arteriole
Response to increased sodium chloride concentration.
Causes decreased GFR because hydrostatic pressure in the glomerular capillaries is decreases
Tubuloglomerular feedback - constriction of efferent arteriole
increases hydrostatic pressure in the glomerular capillaries and thereby increases GFR.
Tubuloglomerular feedback - dilation of afferent arteriole
increases P(GC) and thus GFR
Tubuloglomerular feedback - dilation of efferent arteriole
Respond to increased sodium chloride concentration
Decreases the PGC and thus the GFR
Fast autoregulation response is via … and slow response is via …
- fast → GFR
- slow → RAAS
What is the filtration fraction equation? What does it calculate?
- Filtration fraction = GFR/Renal plasma flow
- the proportion that gets filtered
Although renal blood flow is 1L/min because X% of it is plasma, what is renal plasma flow?
60% of renal blood flow is plasma, so renal plasma flow is actually 600ml/minute
What is GFR value?
125mL/min
normal values for filtration fraction …
125/600 = ~0.2%
What is the rate of urine flow and what does it show when compared to the rate of renal blood flow?
Since the rate of urine flow is 1mL/minute compared to renal blood flow of 1L/minute, it shows that most is reabsorbed
What is renal clearance?
The volume of plasma from which a substance is completely removed from the kidney per unit time
Creatinine and renal clearance:
Creatinine is freely filtered at the glomerulus and is neither absorbed or secreted in the tubule, thus all of it ends up in the urine.
What is the value for renal clearance of creatinine?
125mL/min (equal to GFR)
Renal clearance equation
(urine conc. x urine vol.) / plasma concentration
Hydrostatic pressure values for glomerular capillary and Bowman’s space
- P(GC) =
- 45mmHg
- P(BS) =
- 10mmHg
Net osmotic pressure and oncotic pressure values for GC and BS
- πGC
- 25mmHg and rising
- πBS
- zero
Explain net oncotic/osmotic pressure for bowman’s space
no proteins
Hydrostatic and osmotic pressure gradients across the length of the glomerular capillary
- Hydrostatic pressure is constant across the length of the glomerular capillary
- Oncotic pressure increases along the length of the glomerular capillary as the proteins become more concentrated (not filtered): roughly 20% higher concentration
- By the end of the glomerular capillaries the forces have reached an equilibrium
When in the peritubular capillaries, what changes? (hydrostatic and osmotic pressure)
- there is a lot of resistance and so the pressure drops further
- there is a high oncotic pressure and this aids fluid reabsorption
GFR & pressure equations
GFR = (PGC - PBS) - (πGC-πBS)
= Hydrostatic pressure - Oncotic pressure
= Kf(PGC-PBS-πGC)
What is Kf?
the filtration coefficient; the product of the permeability of the filtration barrier and surface area for filtration (size and number of nephrons)
Which pressures oppose and which favour filtration?
- Glomerular capillary hydrostatic pressure → FAVOURS
- Bowman’s space hydrostatic pressure → OPPOSES
- Glomerular capillary osmotic/oncotic pressure → OPPOSES
- Bowman’s space osmotic/oncotic pressure → since there are no proteins in the filtrate due to filtration the osmotic force is zero
What is retained very efficiently in the tubules?
glucose
amino acids
Function of proximal tubule
- bulk reabsorption: Na, Cl, glucose, amino acids, HCO3 (bicarbonate)
- secretion of organic ions
Function of Loop of Henle
- more Na reabsorption
- urinary dilution
- generation of medullary hypertonicity (where medulla is made very concentrated meaning water wants to flow into it)
Function of distal tubule
- fine regulation of Na, K, Ca, Pi
- the separation of Na from H2O (essentially where separation of salt from water occurs)
Function of collecting duct
- similar to distal tubule
- acid secretion
- regulated H2O reabsorption thereby concentrating the urine
Proximal tubule - Primary active Na+ transport and Secondary active transport for glucose, amino acids and lactate:
- The primary active transport of sodium ions occurs via a basolateral sodium-potassium ATPase pump and transport is from proximal tubule cells into interstitial fluid.
- 3 sodium ions are exchanged for 2 potassium ions, which keeps the intercellular concentration of sodium low compared to the tubular lumen, so sodium moves down a concentration gradient from the lumen of the tubule to the epithelial cells.
- As sodium moves into the proximal tubule epithelial cells, other substances such as glucose and phosphate are co-transported (secondary active transport).
- As sodium moves into the epithelial cells, H+ moves out INTO the lumen.
- Thus, in the proximal tubule, the reabsorption of sodium drives the reabsorption of co-transported sodium, and the secretion of H+.
Proximal tubule - Water transport
- As Na+ and other ions are reabsorbed, water follows passively by osmosis
- Glucose and phosphate reabsorption also contribute to this osmosis since the removal of solutes from the tubular lumen decrease the local osmolarity of the tubular fluid adjacent to the cell
- Solute presence in interstitial fluid increases its osmolarity
- The difference in water concentration between the lumen and interstitial fluid results in net diffusion of water from the lumen across the tubule’s cell plasma membrane and tight junctions INTO the interstitial fluid
Proximal tubule bicarbonate reabsorption
- Proximal bicarbonate reabsorption is an active process that depends on the tubular secretion of H+ which then combines in the lumen with filtered HCO3-
- Inside the tubular cells, CO2 and H2O combine to form carbonic acid (H2CO3) under the action of carbonic anhydrase
- The newly formed H2CO3 rapidly dissociates to form H+ and HCO3-
- The HCO3- moves down its concentration gradient via facilitated diffusion across the basolateral membrane into the interstitial fluid and then into the blood
- At the same time, the H+ is secreted into the lumen via the Na/H+ co-transporter
- The secreted H+ is not excreted but instead combines in the lumen with the filtered HCO3- to generate H2CO3 which then is converted to CO2 + H2O under the action of carbonic anhydrase in the lumen
- The CO2 and H2O then diffuse into the tubular cells and can then be available for another cycle of hydrogen ion generation
Proximal tubule amino acid reabsorption
- Similar to that of glucose and phosphate i.e. co-transported with Na+
- Various cotransporters responsible for the reabsorption of different amino acids
Features of proximal tubule pathology
- Aminoaciduria
- Glycosuria or glucosuria
- Bicarbonate wasting
- Falcon syndrome
What is aminoaciduria?
amino acid in urine
What is glycosuria/glucosuria?
glucose in urine
What is Falcon Syndrome?
amino acids, glucose and bicarbonates in urine
Why are the transcellular and paracellular membranes of the proximal convoluted tubule somewhat leaky?
weak tight junctions at the borders of membrane cells meaning that Na+ and Cl- can freely flow in or out
What is the ‘transport maximum (Tm)’?
Many of the mediated-transport-reabsorptive systems in the renal tubule have a limit to the amounts of material they can transport per unit time
Why is there a transport maximum?
This is because binding sites on the membrane transport proteins become saturated when the concentration of a transported substance increases to a certain level
Important example of transport maximum: secondary active transport of glucose:
- Plasma glucose concentration in a healthy person normally does not exceed 150mg/100ml even after a sugar meal
- When plasma glucose concentration exceeds the transport maximum for a significant number of nephrons, glucose started to appear in the urine
The relationship between filtered load and proximal tubule reabsorption:
- A greater filtration fraction (due to increased load) will increase the osmotic pressure in the downstream peritubular capillaries resulting in more reabsorption
- Efferent arteriolar constriction reduces peritubular capillary hydrostatic pressure
Water permeability of Loop of Henle limbs
- The descending limb is water permeable
- The ascending limb is water impermeable
Where does solute reabsorption in the Loop of Henle take place?
The thick ascending limb
Why is a hyperosmotic interstitial fluid required?
To enable water to be drawn out from collecting ducts under the action of ADH thereby concentrating the urine
How does the Loop of Henle achieve hyperosmotic interstitial fluid?
- The fluid entering the loop from the PCT flows down the descending limb then turns a corner and then flows up the ascending limb in the countercurrent multiplier system.
- Along the entire length of the ascending limb, sodium and chloride ions are reabsorbed from the lumen into the medullary interstitial fluid via many channels and pumps.
- NKCC2 pump
- transports 1 Na+, 1 K+ and 2 Cl- into the ascending limb
- In the upper thick part of the ascending limb, this reabsorption is achieved by transporters that actively cotransport Na+ and Cl-. Those transporters are not present in the thin portion, so the process occurs via simple diffusion instead.
- Since the ascending limb is impermeable to water, very little water follows the salt. This results in the interstitial fluid of the medulla being very hyperosmotic compared to the fluid in the ascending limb due to the fact that solutes are reabsorbed without water
- The descending limb is permeable to water and does not reabsorb salts, thus meaning that a net diffusion of water occurs OUT of the descending limb into the concentrated interstitial fluid, until the osmolarities inside the limb and IF are again equal.
Effect of diuretic:
A diuretic called Furosemide can inhibit the NKCC2 pump on the thick ascending part of the loop of Henle thereby reducing the amount of Na+, Cl- & K+ ions able to enter the medullary interstitium thereby reducing hyperosmolarity meaning that less water will diffuse out of the collecting ducts into the blood resulting in more water loss in the urine and thus dehydration
Medullary circulation: What is the action of vasa recta in the medulla?
- The vasa recta form hairpin loops that run parallel to the loops of Henle and collecting ducts
- Blood enters the top of the vessel loop and as the blood flows down the loop deeper and deeper into the medulla - Na+ and Cl- diffuse into the vessel and water diffuses out of the vessel
- However, after the bend in the loop is reached, the blood then flows up the ascending vessel loop where the process is almost completely reversed.
- Thus, the hairpin-loop structure of the vasal recta minimise excessive loss of solute from the interstitium by diffusion.
Urea involvement in counter-current medullary interstitium
- As urea passes through the remainder of the nephron, it is reabsorbed, secrete into the tubule, and then reabsorbed again.
- This traps urea, which is an osmotically active molecule, in the medullary interstitium, thus increasing its osmolarity.
What feature of the distal convoluted tubule helps with its role? What can inhibit this?
- The DCT continues the active dilution of urine by reabsorption of Na+ in water-impermeable setting
- It has NCC (sodium chloride cotransporter) in the plasma membrane which help in the reabsorption of Na+ and Cl-
this cotransporter can be inhibited by the drug thiazide resulting in less Na+ and Cl- reabsorption.
Permeability of the collecting duct
highly water impermeable
2 cell types in the collecting duct
Principal cells - have epithelial sodium channels
Intercalated cells - secrete acid into collecting duct
Aldosterone action on principal cells
- Aldosterone increases the transcription (via a steroid receptor) of sodium channels and Na/K/ATPase which increases apical Na+ INFLUX and facilitates K+ EFFLUX.
- Therefore, aldosterone drives Na+ reabsorption and K+ secretion
ADH action on principal cells
- ADH binds to adenyl-cyclase coupled vasopressin receptors on principal cell membranes
- Kinase action results in the insertion of vesicles containing aquaporin-2 into apical membrane
- These increase water permeability and thus reabsorption of water
How much of the adult male body weight does water comprise?
60%
Main cation in extracellular space
sodium
Main cation in intracellular space
potassium
Intracellular pH
7.0
Extracellular pH
7.4
Fluid movement is regulated by controlling — movement
sodium
Tonicity is regulated by controlling — movement
H2O
Urine limits per 24h (volume and osmolarity?)
- Volume: 400ml - 20L a day
- 50-1200mglOsm/kg
What is ADH structurally?
9 amino acid peptide hormone
Where is ADH produced?
hypothalamus
Where is ADH secreted?
posterior pituitary gland
ADH release is controlled by…
hypothalamic osmoreceptors which detect changes in osmolarity
What is the half-life of ADH and why is this significant?
- 15 minutes
- This is very short - meaning we are able to adapt quickly to osmolarity changes
How sensitive are osmoreceptors?
Can detect a 1-2% change in osmolarity
As well as affecting the collecting ducts, what else does ADH do?
ADH (like angiotensin II) causes widespread arteriolar constriction which helps restore arterial blood pressure to normal
Effect of MDMA on ADH secretion
increases secretion - resulting in dilute blood - seizures and fits
Effect of alcohol on ADH secretion
decreases secretion - resulting in dehydration
Effect of nicotine on ADH secretion
increases secretion
Baroreceptor control of ADH secretion
- Baroreceptors in the aortic arch and carotid body, upon detecting a decrease in cardiovascular pressure due to a decrease extracellular fluid pressure, will decrease their rate of firing
- This means that baroreceptors transmit fewer impulses via afferent neurones and ascending pathways to the hypothalamus resulting in ADH secretion
Which is more sensitive - baroreceptor reflex or osmoreceptor reflex?
- Osmoreceptor
- Baroreceptor has a high threshold, meaning that there must be a sizeable reduction in cardiovascular pressure to trigger it
How else does ADH help maintain hyperosmolarity of the medulla?
Increases urea permeability of the collecting duct
How is the feeling of thirst stimulated and what does it result in?
Increases urea permeability of the collecting duct
% reabsorption of sodium in proximal tubule
60%
% reabsorption of sodium in Loop of Henle
25%
% reabsorption of sodium in distal tubule
10%
% reabsorption of sodium in collecting duct
4%
What is the major agent determining the rate of tubular Na+ reabsorption?
aldosterone
3 things which initiate release of aldosterone
- The cells of the macula densa in the distal convoluted tubule detect LESS NaCl in the tubule
- Sympathetic stimulation
- Little or no arteriolar stretch (i.e low blood volume due to lack of Na+ and therefore H2O)
Process of aldosterone action
- The juxtaglomerular cells located in the afferent arterioles are stimulated to release the enzyme renin
- Renin then enters the blood, where is cleaves angiotensinogen (produced in the liver) to a smaller polypeptide; angiotensin I.
- Angiotensin I is a biologically inactive peptide which undergoes further cleave under the angiotensin-converting-enzyme (ACE) which is produced in the lungs, to form the active agent angiotensin II.
What is the action of angiotensin II?
- Angiotensin II stimulates the cells of the zona glomerulosa in the adrenal cortex of the adrenal glands to secrete the steroid hormone aldosterone.
- Aldosterone is a vasoconstrictor, so secretion results in vasoconstriction especially at the efferent arteriole; this in turn results in the increase in pressure resulting in an increased GFR
- This increases Na+ reabsorption in the proximal convoluted tubule
- It also stimulates thirst and ADH release
What is the action of aldosterone?
It acts on the principal cells of the collecting duct where it stimulates the transcription of epithelial sodium channels thereby resulting in increased Na+ reabsorption and H2O reabsorption
What is another result of aldosterone?
The ENac enables more reabsorption of Na+ but as Na comes into the principal cells, potassium is exchanged so if you reabsorb more Na+ then you will leak more K+
Speed of ADH vs aldosterone?
Aldosterone acts more slowly than vasopressin since it induces changes in gene expression and protein synthesis
What is another controller of sodium reabsorption?
ANP atrial natriuretic peptide
Which cells synthesis and secrete ANP?
cells in the cardiac area
What does ANP act on? To do what?
several tubular segments to INHIBIT Na+ reabsorption by blocking the epithelial sodium channels in the collecting ducts
What is another role of ANP?
Natriuresis - Renal vasodilator resulting in afferent arteriole dilation which increases GFR which further contributes to increased Na+ excretion
What does ANP directly inhibit?
aldosterone secretion → leads to increased Na+ excretion
When does the secretion of ANP happen?
The secretion of ANP increases when there is an excess of Na+ in the body due to the fact that, as a result of excess Na+ there is no more H2O in the vessels meaning blood volume increases meaning the atria become more stretched which in turn stimulate ANP secretion
What is negative effect of hypokalaemia or hyperkalaemia
- abnormal heart rhythms (arrhythmias)
- abnormalities of skeletal muscle contraction and neuronal action potential conduction
How does a healthy person remain in potassium balance?
by daily excreting an amount of potassium in the urine equal to the amount ingested minus the amount eliminated in faeces and sweat
Where is 90% of the filtered K+ reabsorbed?
proximal tubule
Which part of nephron can secrete K+?
- cortical collecting duct
- When there is potassium depletion (and thus where the aim is to minimise K+ loss) there is no secretion by the cortical collecting ducts
Factors affecting K+ secretion
(most important) When a high K+ diet is ingested, plasma potassium concentration increases resulting in an enhanced basolateral uptake of K+ via the Na/K/ATPase pump - resulting in enhanced K+ secretion. When a low K+ diet is ingested (e.g. during diarrhoea) vice versa happens.
Aldosterone + K+ secretion
- Aldosterone enhances K+ secretion because more K+ has to be exchanged for Na+ and the cells of the zona glomerulosa in the adrenal cortex of the adrenal glands are sensitive to K+ conc. and therefore secrete aldosterone
- Thus an increased intake of K+ will lead to an increased extracellular K+ concentration which in turn directly stimulates the adrenal cortex to produce aldosterone → increased K+ secretion and thereby eliminates excess potassium from body
Parathyroid hormone effect on kidneys
- Parathyroid hormone is released by the parathyroid glands in response to Ca2+ plasma concentrations. It directly increases Ca2+ reabsorption in the kidneys thereby decreasing urinary Ca2+ excretion.
- It also stimulates the formation of the active form of vitamin D: the horone 1,25-dihydroxyvitamin D (calcitriol) → parathyroid stimulates part of the hydrolysis reaction in the kidney which activates vitamin D
