Session 5 Flashcards
Describe the regulation of body fluid osmolarity
[*] Body fluid osmolality is maintained by a process of osmoregulation at about 275-295 mOsm/kg. NB: at 37 degrees C, osmolality (osmoles / kg) and osmolarity (osmoles/L) are virtually identical.
[*] The hormonal regulation of plasma osmolarity occurs in the late distal tubule and collecting duct of the nephron.
[*] Disorders of water balance manifest as changes in body fluid osmolarity.
[*] Symptoms reflect changes in plasma osmolarity.
[*] The major cation of the ECF is sodium, thus changes in sodium ion concentration occur. This is not a problem with sodium balance; changes in sodium balance affect plasma volume but is a problem with osmolarity due to changes in water balance. WATER BALANCE REGULATES OSMOLARITY; SODIUM BALANCE REGULATES PLASMA VOLUME
[*] If water intake < water excretion = Plasma osmolarity increases
[*] If water intake > water excretion = Plasma osmolarity decreases
[*] If plasma osmolarity decreases renal water excretion increases (i.e. there is more water in ECF and leads to water being lost).
[*] If plasma osmolarity increases renal water excretion decreases (i.e. there is less water in ECF and water is conserved).
Describe the variation in urinary output
[*] Body must match ingestion to excretion: most people on average urinate 1-1.5L/day and ingest 600-1000mOsm/day.
[*] Urinary osmolarity therefore ~500-700 mOsm/L but dependent on amount of salt in diet.
- It could be excreted as 100mOsm/L in 10 L
- OR 1000mOsm/L in 1L
- Typically urine output varies between 50-1200mOsm?L
[*] Changes in plasma osmolarity are corrected by altering the total amount of water (solvent) not by changes in the solute concentration.
Explain about Osmoreceptors including their location and what do they do?
[*] Two different Efferent pathways: ADH and thirst
[*] Sensors (Hypothalamic Osmoreceptors) sense changes in plasma osmolality
- Located in the Organum Vasculoum of the Laminae Terminalis (OVLT) of the hypothalamus
- OVLT is anterior and ventral to the third ventricle
- The Osmoreceptors are cells that have fenestrated leaky endothelium so the cells are directly exposed to the systemic circulation.
- They send 2nd degree signal responses which are mediated via two pathways leading to 2 different complimentary outcomes
Concentration of urine
Thirst
Describe the thirst mechanism
[*] The sensation of thirst represents an extremely powerfully homeostatic protective mechanism ensuring that, providing water is available, dehydration is rapidly corrected by water intake.
- Although the sensation of thirst is usually perceived peripherally as in drying of the oral mucosa (dry mouth), the “drinking-centre” is in the lateral pre-optic area of the hypothalamus and it is this that regulates thirst. This drinking centre is response to raised plasma osmotic pressure and reduced ECF volume.
- The thirst mechanism is set so that it is active only when the level of hyperosmotic dehydration begins to surpass the protective capacity of the kidney (change is >10%). If the thirst mechanism is working normally, and access to water is unrestricted, renal conservation of water is not essential for efficient osmoregulation of body fluids.
Describe the ADH mechanism
[*] ADH or vasopressin is a small peptide only 9 amino acids long. It acts in the late distal tubule and collecting duct to allow water reabsorption and regulate the osmolarity of plasma.
- ADH is synthesised in the hypothalamus and stored in the posterior pituitary gland. Under conditions of predominant loss of water osmoreceptors in hypothalamus initiate release of ADH.
- Increase in osmolarity (i.e. loss of water) stimulates ADH (1% change)
- Decreased osmolarity inhibits ADH secretion
- This is done via the insertion of an Aquaporin channel into the tubule wall.
- Water filtered in the glomerulus and contained in the glomerular filtrate is then able to pass out of the tubule through the aquaporin channel back into the plasma
- If plasma ADH levels are low, then there is little water reabsorbed in the late distal tubule or collecting duct. A large volume of dilute urine is produced (water diuresis). If plasma ADH levels are high, a small volume of concentrated urine is produced (anti-diuresis).
- ADH also increases the permeability of the tubules to urea. Urea is an effective osmole and helps with the process that allows the kidney to produce concentrated urine.
[*] Effectors form negative feedback loops that begin within the anterior hypothalamus.
- Increased Osmolarity stimulates osmoreceptors.
- Secretion of ADH to decrease renal water excretion
- Result: feedback loop which stabilizes osmolarity
Describe drinking behaviour
Drinking is induced by increases in plasma osmolarity or by decreases in ECF volume
- Thirst which, if fulfilled, increases intake of free water.
- Stop when sufficient fluid has been consumed. Our body is able to recognise water is coming in even though GI tract absorption hasn’t occurred and water water hasn’t reached the circulation yet (osmolarity hasn’t been corrected yet) – the metering mechanisms are unknown though.
Salt Appetite:
- Salt ingestion is the analogue of thirst – we have a hedonistic appetite (desire to eat salt which is opposite to thirst – eating for pleasure rather than need)
- Regulatory appetite (deficiency drives need)
Describe ADH effect on different regions of the nephron
[*] ADH has no effect on the descending loop of Henle as it has squamous epithelium with loose junctions so is already permeable to water.
[*] In the PCT, 65% of the water is reabsorbed (along with 67% of Na+ accompanied by Cl- reabsorption – isomotic)
[*] In the Loop of Henle, lots of salts have been reabsorbed but volume hasn’t really changed (ascending loop of Henle is known as diluting segment)
[*] In the Cortical Collecting Duct, a tiny amount of background ADH which is always present causes a little bit of water reabsorption and maximum ADH causes slightly more water reabsorption
[*] In the medullary collecting duct, >99% of water ends up being reabsorbed if maximal ADH secretion. The amount of ADH secreted is proportional to change in plasma osmolarity. Urine is extremely concentrated.
Describe the osmolarity of tubular flow along the nephron
Explain about Osmolarity vs Haemodynamic pressures
- Changes in blood volume and pressure have an effect on the response to changes in osmolarity.
- A decrease in extracellular volume causes set point of ADH secretion to lower osomlarity values and the slope of the relationship is steeper
- When faced with circulatory collapse, the kidneys continue to conserve H20 even though this will reduce osmolarity of body fluids (the body chooses to maintain volume and pressure over osmolarity).
- When there is an increase in plasma pressure, the opposite occurs. The set point of ADH secretion is shifted higher and slope decreases. We are prepared to accept an increase in osmolarity rather than an increase in pressure.
- Volume is more important than osmolarity if volume crashes.
Describe the water permeability of the collecting ducts and tubules
[*] Apical membranes do not contain water channels (Aquaporin 2) in the absence of ADH.
[*] When ADH is released, the apical membrane has Aquaporin 2 channels rapidly inserted into it (via a cascade of protein kinase A signalling) and becomes water permeable
[*] Turnover of Aquaporin 2 is < 18 minutes
[*] With the removal of ADH, the aquaporin 2 channel is retrieved from the apical membrane by endocytosis
[*] The basolateral membrane always contains Aquaporin 3 and 4 so is always permeable to water.
[*] Any water which enters across the apical membrane is thus able to pass into the peritubular blood – resulting in a net absorption of water.
Describe the corticopapillary osmotic gradient
[*] Juxtamedullary nephrons make 20-30% of total nephrons and are responsible for the development of the osmotic gradients in the renal medulla, which are used to concentrate urine.
[*] Isosmotic at cortico-medullary border. Then as you go deeper into the renal parenchyma, the osmolarity gradient increases until the medullary interstitium (gap between cells) is hyperosmotic up to 1000mosmole/L at papilla.
[*] Essential mechanism
- Active NaCl transport in thick ascending limb
- Recycling of urea (this is why greater change occurs in the medullary collecting duct compared to the cortical collecting duct as this where urea can be reabsorbed – through the aquaporin channels alongside water if ADH is present)
- Unusual arrangement of blood vessels in medulla descending components in close opposition to ascending components (flow of blood in vasa recta is in opposite direction to flow of fluid in the tubule)
How is the medullary gradient generated? And what is meant by Urea Recycling?
[*] To generate the medullary gradient, the thick ascending limb of the loop of Henle is crucial – diluting action on the filtrate:
- Removes solute without water and therefore increases osmolarity of the interstitium (filtrate in the tubule is now hyposmotic)
- Block NaK2Cl (NaKCC) transporters with a loop diuretic and the medullary interstitium becomes isosmotic and copious dilute urine is produced.
[*] Recycling of Urea:
- In the medullary CD, Urea moves into the intersitium down the concentration gradient and depending on the concentration gradient, may diffuse back into the loop (cycle of movement)
- Under the influence of ADH, fractional excretion of urea decreases and urea re-cycling increases
- The ascending limb is impermeable to H20 but it is permeable to NaCl and Urea. As Urea is high in interstitium and low tubular fluid, it moves in passive (no ATP) processes – down concentration gradient.
- Urea then passes back into the collecting duct where it is reabsorbed in the medullary position and more water follows. Urea is therefore recycled.
Explain about Counter Current Multiplication
- The tubule is initially filled with isotonic fluid
- Na+ is pumped out of the ascending loop, raising the osmotic pressure outside and lowering it inside. This creates the gradient. The maximum gradient (inside to out) is 200 mosm/L
- Round 1: Water flows out of the descending tubule by osmosis, raising the osmotic pressure in the descending tubule to 400 mOsm/L. Fresh fluid enters from the glomerulus, pushing concentrated fluid (400 mOsm/L) into the ascending limb.
- Round 2: the Na+ pump produces another 200 mOsm/L gradient across the membrane, but it is starting from a more concentrated solution so the external (interstitial) osmolarity rises to 500mOsm/L
- Round 3: the third round of Na+ pumping raises interstitial concentration to 700mOsm/L (in the deepest part of the medullary interstitium) and so on.
- The pumping, osmotic flow and filtration flows occur together as a continuous process. The final gradient will be limited by diffusional processes
What maintains the corticopapillary concentration gradient?
[*] Concentration gradient is produced by the loop of Henle acting as a counter current multiplier BUT it is maintained by the vasa recta, acting as a counter-current exchanger (taking water away, preventing water from washing out the concentration gradient).
- Because the flow in vasa recta is in opposite direction to fluid flow in the tubule, the osmotic gradient is maintained.
- Blood flow in renal cortex is high – is one of the highest per gram of any tissue in the body. Descending blood runs alongside the ascending fluid in the tubule of the ascending limb of Loop of Henle.
- Blood flow through renal medulla is low (5-10% of total Renal Plasma Flow)
- Compromise: need to delivery nutrients and need to maintain medullary hyper-tonicity
- Kidney uses a hair-pin configuration for vasa recta with entry and exit through the same region of the kidney, thus creating a counter current exchange mechanism.
- The descending limb of Loop of Henle has a very concentrated filtrate and blood is also very concentrated at this point so it is in a perfect position to reabsorb water therefore water is not available to wash out concentration gradient.
- Vasa recta has no capacity for active transport.
Describe what happens in the limbs of the vasa recta
Start with osmotic stratification in medullary interstitium (everything is driven by concentration gradients) due to presence of ADH
- Counter current multiplier gives NaCl gradient that moves salt into the interstitium.
- Also cortex to papilla gradient of urea
- In presence of ADH
Descending limb of vasa recta:
- Isosmotic blood in vasa recta enters hyperosmotic milieu of the medulla (high concentration Na+ ions, Cl- ions + urea)
- Na+, Cl- + urea diffuse into the lumen of vasa recta
- Osmolarity of blood in vasa recta increases as it reaches tip – bottom – of hairpin loop
Ascending limb of vasa recta:
- Blood ascending towards cortex will have higher solute content than surrounding intersititum
- Water moves in from the descending limb of the Loop of Henle.
Consider an alternative explanation of how the vasa recta acts like a counter-current exchange? (LUSUMA NOTES)
- The concentration gradient that the loop of Henle sets up would not last long though without the Vasa Recta.
- These are blood vessels that run alongside the loops, but with opposite flow direction. This counter-current flow allows for the maintenance of the concentration gradient.
- Isosmotic blood in the descending limb of the vasa recta enters the hyperosmotic milieu of the medulla, where there is a high concentration of ions (Na+, Cl-, Urea). These ions therefore diffuse into the vasa recta and water diffuses out.
- The osmolarity of the blood in the vasa recta increases as it reaches the tip of the hairpin loop, where it is isosmotic with the medullary Intersticium.
- Blood ascending towards the cortex will have a higher solute content than the surrounding Intersticium, so solutes move back out. Water will also move back in from the descending limb of the loop of Henle.
- Therefore, although there is a large amount of fluid and solute exchange across the vasa recta, there is little net dilution of the concentration of the interstitial fluid because of the U shape of the vasa recta allowing it to act as a counter current exchanger.
- The vasa recta therefore do not create the medullary hyperosmolarity, but do prevent it from being dissipated.
Consider the role of ADH in the production of hypo- and hyperosmotic urine
[*] Hypo-osmotic urine
- Reabsorb solute from nephron
- No ADH stimulation means no Aquaporin in the latter DCT and Collecting Ducts
- Limited water reuptake in latter DCT, and limited in collecting duct
- Tubular fluid rich in water passes through the hyperosmotic renal pyramid with no change in water content
- Loss of large amount of dilute urine
- Diuresis
[*] If plasma osmolarity increases, body needs to produce a hyperosmotic urine to lower osmolarity of plasma.
- The kidney must reabsorb as much water as possible from the kidney tubule
- Needs a hypertonic interstitium (which eventually returns to peritubular capillaries => circulation), creating a concentration gradient so water can move out of the tubule into the interstitium passively.
Describe the percentage of unbound, free calcium ions in the blood
[*] Unbound free calcium ions account for 50% of the total body calcium concentration and are freely filtered at the glomerulus. The remaining 45% of the total body calcium concentration is protein bound (80% to albumin) and 5% is complexed to anions e.g. HCO3- and citrate and PO4^3-.
- Plasma pH influences this distribution by altering protein binding to Ca2+
- Free Ca2+ reference range: 1.1-1.3 mmol/L
- Total adjusted calcium reference range: 2.20-2.60 mmol/L
- NB: free calcium cannot be measured easily due to lactate accumulation in test tube sample (anaerobic respiration occurs) which changes free [Ca2+] therefore total adjusted calcium reference range is calculated.
Describe the sources and metabolism of Vitamin D
- The form of Vitamin D present in sunlight is Cholecalciferol (D3)
- The form of Vitamin D present in oily fish, eggs and cereals is Ergocalciferol (D2)
- 25-hydroxyase converts D3 and D2 into 25-(OH) Vitamin D (Calciferol) in the liver
- Then in the kidney, 1 alpha-hydroxylase (PTH stimulated) converts 25-(OH) Vitamin D into 1,25-(OH)2 Vitamin D ( active form, aka calcitriol)
- When levels of 1,25-(OH)2 Vitamin D have reached sufficient levels, the body starts converting 25-(OH) Vitamin D into 24,2-(OH)2 Vitamin D which is inert. This prevents Vitamin D toxicity
What is the action of Calcitriol?
On bone:
- Increases the availability of calcium and phosphate via intestinal uptake (binds to Calcium to increase gut reabsorption)
- Promotes osteoblast activity and maturation of osteoclasts precursor cells
On kidney
- Inhibition of renal 1 alpha-hydroxylase by intestinal absorbed phosphate
- Promotes synthesis of 24,25-(OH)2D
- Small effect on renal calcium and phosphate reabsorption
Other actions: regulates the functions of a wide variety of cells and tissues:
- Cell differentiation and proliferation
- May decrease proliferative activity of some tumour cells
- Inhibition of cellular growth
- Stimulation of insulin secretion
- Modulation of immune and haemopoietic systems
- Inhibitor of renin production
Status of Evidence of Vit D deficiency of Chronic Diseases
Compare hyperparathyroidism and hypercalcaemia due to malignancy
In primary hyperparathyroidism:
- Raised: plasma calcium, PTH, urinary cAMP, plasma calcitriol, bone resorption, bone formation
- Suppressed: plasma phosphate
Humoral Hypercalcaemia of malignancy:
- Raised: plasma calcium, urinary cAMP, bone resorption
- Suppressed: plasma phosphate, PTH, plasma calcitriol, bone formation
What are different types of calcium stones? And what factors contribute to their formation
[*] Calcium stones are the most common (70-80%)
[*] Factors involved in renal stone formation:
- Low urine volume
- Hypercalcuria which could absorptive (increased absorption due to an abnormality in the intestines leading to increased excretion by the kidneys so plasma [Ca2+] is normal) or renal (problem in the kidneys so more Ca2+ goes into the urine – inefficient reabsorption)
- Primary hyperparathyroidism
- Hyperoxaluria
- Hyperuricosuria
- Hypocitraturia
- Hypomagnesuria
- Others
What are the effects of Pharmacologic Therapy?
- Sodium Cellulose Phosphate suppresses urinary calcium
- Thiazide increases calcium reabsorption in kidney leading to less Ca2+ excretion
- Allopurinol inhibits xanthine oxidase => inhibits urinary uric acid formation
- Potassium Citrate increases urinary citrate (hypocitraturia is an important risk factor for kidney stone formation
