case 7 Flashcards
function of the nephron
The nephron is the functional unit of the kidney. It has three roles: 1.Filtration 2.Selective Reabsorption 3.Secretion
functions of the kidney
Maintenance of Extracellular Fluid Volume (ECFV) – sodium and water (therefore maintaining blood pressure)
Acid-base balance regulation - therefore normally preventing acidosis/alkalosis
Excretion of metabolic waste – urea and creatinine
Endocrine secretion
Renin-angiotensin system (for sodium regulation of blood pressure)
Erythropoietin (for RBC production and regulation)
Vitamin D (for calcium regulation)
the nephron is divided into
Glomerulus - filtration
Proximal Convoluted Tubule – selective reabsorption of water, ions, and all organic nutrients
Descending Limb of Loop of Henle – selective reabsorption of water
Ascending Limb of Loop of Henle – selective reabsorption of sodium and chloride ions
Distal Convoluted Tubule – secretion of ions, acids, drugs, toxins/ variable reabsorption of water sodium and calcium ions
Collecting Tubule – variable reabsorption of water and reabsorption/secretion of sodium, potassium, hydrogen and bicarbonate ions
blood supply of the kidney
- The average cardiac output is 5 litres/min. The kidneys receive 20% of this (1 litre/min).
- The renal blood flow (RBF) is about 10-50 times greater than other the blood supply of other organs.
kidney main function
- The kidneys are the primary means for eliminating waste products of metabolism that are no longer needed by the body.
- These products include urea (from the metabolism of amino acids), creatinine (from muscle creatine), uric acid (from nucleic acids), bilirubin (from Hb breakdown), and metabolites of various hormones.
- The kidneys also eliminate most toxins and other foreign substances that are either produced by the body or ingested, such as pesticides, drugs, and food additives.
glomerulus
• The glomerulus allows for filtration of contents of the blood into the proximal convoluted tubule (PCT).
• Proteins larger than the size of albumin can’t pass into the PCT.
• The fluid must cross:
Wall of glomerular capillary
Basement membrane
Inner layer of Bowman’s capsule
(Podocytes, Pedicels, Filtration slits)
• The glomerulus provides a size and a charge barrier.
• It allows small positive molecules through.
• Large or negatively charged molecules are repelled.
glomerular filtration rate
GFR= K_f ∙[P_GC-(P_BC+π_GC )]
There are certain factors that affect the GFR:
Kf = filtration coefficient
PGC = glomerular capillary hydrostatic pressure (favours filtration)
π GC = glomerular capillary oncotic pressure (opposes filtration)
Oncotic pressure is a form of osmotic pressure exerted by proteins, notably albumin, in a blood vessel that pulls water into the circulatory system.
PBC = Bowman’s capsule hydrostatic pressure (opposes filtration)
autoregulation-maintaining GFR
• Autoregulation is the process by which the RBF and GFR are maintained despite changes in systemic pressure.
• WITHOUT PATHOLOGY, GFR DOES NOT CHANGE!!!
• This graph shows that when the blood pressure increases, the vascular resistance of the afferent arteriole increases too.
• This maintains the RBF and the GFR.
• Autoregulation (the increased vascular resistance) occurs in two ways:
Myogenic – vascular smooth muscle responds to stretch by vasoconstricting.
Tubuloglomerular feedback – distal tubular flow regulates vasoconstriction.
tubuloglomerular feedback
- This process involves the macula densa.
- The macula densa is a collection of densely packed epithelial cells at the junction of the thick ascending limb (TAL) and distal convoluted tubule (DCT).
- As the TAL ascends through the renal cortex, it encounters its own glomerulus, bringing the macula densa to rest at the angle between the afferent and efferent arterioles.
- The macula densa’s position enables it to rapidly alter glomerular resistance in response to changes in the flow rate through the distal nephron.
- The macula densa uses the composition of the tubular fluid as an indicator of GFR.
- A large sodium chloride concentration is indicative of an elevated GFR.
- A low sodium chloride concentration indicates a depressed GFR.
machanism of tubuloglomerular feedback e.g inc GFR
• Increased arterial pressure causes increased glomerular pressure and plasma flow.
• This increases the GFR.
The plasma colloid osmotic pressure increases to limit the increased GFR.
• The increased GFR increases the tubular flow to the proximal convoluted tubule
This leads to increased reabsorption of water and ions in the proximal convoluted tubule and the loop of Henle.
• The increased GFR increases the tubular flow to the early distal convoluted tubule.
There is increased osmolarity of the tubular fluid (i.e. increased NaCl).
• This is sensed by the macula densa by an apical Na-K-2Cl cotransporter (NKCC2).
• The juxtaglomerular cells in the macula densa secrete renin, which results in afferent arteriole constriction.
• This increases the preglomerular resistance, thus decreasing the GFR and keeping it maintained at a steady level.
• This is known as TUBULOGLOMERULAR FEEDBACK
measurement of GFR
‘Renal Clearance’ – volume of plasma which is cleared of substance x per unit time Renal Clearance= (Ux V)/Px Ux = urinary concentration of ‘x’ V = urine volume per unit time Px = plasma concentration of ‘x’ Markers of GFR: Features of a good marker:
Freely filtered Not reabsorbed Not secreted Excreted in urine Creatinine is the marker used in clinical practice. It is a by-product of muscle breakdown. It is affected by: age, gender and ethnicity.
selective reabsorption-sodium regulation
- We reabsorb about 1.5kg of Na+ ions a day.
- We excrete about 9g of Na+ ions a day.
• Plasma [Na+] determines
Extracellular fluid volume
Arterial blood pressure
• Less “expensive” than active water transport. This is because it is easier to transport Na+ ions and allow other things (like water and glucose) to follow. This way we don’t expend excess amounts of ATP.
• Linked to most other renal transport processes e.g. glucose reabsorption.
bulk reabsorption vs fine tuning
• Proximal convoluted tubule – 67% Na+ reabsorbed
• Loop of Henlé – 25% Na+ reabsorbed
This occurs via the Na+-K+-Cl- Cotransporter (NKCC2) in the ascending loop of Henle.
• Distal convoluted tubule & collecting duct – 8% Na+ reabsorbed
- The Na+-K+ pump on the basolateral membrane pumps Na+ ions into the blood, thus lowering the Na+ concentration in the cell.
- This allows the Na+-H+ exchanger on the apical membrane to take up Na+ ions from the urine.
- The anion-Cl- exchanger allows the uptake of HCO3- ions in exchange for Cl- ions.
Therefore, both Na+ ions and HCO3- ions are reabsorbed. - The Na+-K+ pump on the basolateral membrane pumps Na+ ions into the blood, thus lowering the Na+ concentration in the cell.
- Aldosterone combines with a cytoplasmic receptor.
- Hormone-receptor complex initiates transcription in the nucleus.
- New protein channels (ENaC – epithelium sodium channel) and pumps are made.
- Aldosterone-induced proteins modify existing proteins.
- Result is increased Na+ reabsorption and K+ secretion.
glucose transport
• There are three transport protein families that are involved in glucose transport: 1. SLC – solute carrier family SLC5: sodium-linked cotransporters 2. SGLT SGLT1 - transports 1 glucose: 2 Na SGLT2 – transports 1 glucose: 1 Na 3. GLUT GLUT1 and GLUT2
early proximal convoluted tubule
The early proximal convoluted tubule is involved in the mass reabsorption of glucose.
The Na+-K+ pump on the basolateral membrane pumps Na+ ions into the blood, thus lowering the Na+ concentration in the cell.
This allows the Na+-glucose SGLT2 cotransporter on the apical membrane to take up Na+ and sodium from the urine.
Next, the GLUT2 protein allows passage of glucose from inside the cell into the blood.
These have a low-affinity but high capacity because there is a lot of glucose available and these transport proteins allow for mass reabsorption of glucose
late proximal convoluted tubule
The late proximal convoluted tubule is involved in the fine reabsorption of glucose.
The Na+-K+ pump on the basolateral membrane pumps Na+ ions into the blood, thus lowering the Na+ concentration in the cell.
This allows the Na+-glucose SGLT1 cotransporter on the apical membrane to take up Na+ and sodium from the urine.
Next, the GLUT1 protein allows passage of glucose from inside the cell into the blood.
These have a high-affinity but low capacity because glucose has already been mass absorbed and so there is less glucose left in the tubular fluid. These transport proteins allow for fine-tuning reabsorption of glucose.
Glucose excretion: Tm
- Fasting glucose ~ 5 mmol/L and GFR = 125 ml/min
- Filtered glucose = 5 x 0.125 = 0.63 mmol/min
- Transport maximum (Tm) ~ 1.25 mmol/min Plasma glucose ~ 10 mmol/L
- This graph shows that once the Tm is reached, no more glucose can be reabsorbed.
- The excess glucose must be excreted (the point of intersection of the lines).
gluconeogenesis
- The kidney itself is a source of glucose via gluconeogenesis.
- The kidney makes around 20% of all glucose in the body, but it then breaks it back down.
water balance-concentrating urine
• Water reabsorption occurs in the descending loop of Henle.
• The longer the loop of Henle, the greater the amount of water that is reabsorbed.
• If the need to reabsorb more water occurs, this is mediated by the effect of ADH/ Vasopressin.
• ADH inserts aquaporins (AQP2) in the apical membrane of the cells in the late distal convoluted tubule and the cells of the collecting tubule.
This allows water to be reabsorbed from the cells back into the body.
The water will only flow through these channels in the presence of an osmotic gradient caused by Na+ ions.
• AQP3/4 are already present on the basolateral membrane of these cells.
• Aldosterone (via ENaC) also allows reabsorption of water.
calcium ions and magnesium ions
• PCT & Loop of Henlé
91% Ca2+ reabsorbed – paracellular route (passive reabsorption)
89% Mg2+ reabsorbed – paracellular route (passive reabsorption)
• DCT
3-7% Ca2+ reabsorption
5-6% Mg2+ reabsorption
calcium reabsoption
- In the PCT and Loop of Henle, Ca2+ ions are passively reabsorbed.
- In the DCT, Ca2+ ions require transport proteins to be reabsorbed.
- Ca2+ ions enter the cell via TRPV5 transport protein channels.
- Because Ca2+ is an intracellular signalling molecule, we can’t have free Ca2+ in the cell as it would trigger other signalling pathways.
- Therefore, Ca2+ needs to be chaperoned from the apical membrane to the basolateral membrane where they would then enter blood.
- Ca2+ ions bind to an intracellular protein called Calbindin-D28K.
- This allows Ca2+ ions to move to the basolateral membrane.
- Once at the basolateral membrane, these ions can exit the cell into the bloodstream via two transport proteins: (1) Na+/Ca2+ exchanger [NCX1] and (2) plasma-membrane-calcium-ATPase-pump [PMCA1b].
TRPV5 can be regulated by
Parathyroid hormone
Vitamin D – the kidneys activate vitamin D which stimulates TRPV5 in the DCT
Sex hormones
Klotho – this is a protein that’s associated with longevity.
magnesium reabsorption
• Mg2+ reabsorption is less known.
• There is a ROMK potassium channel on the apical membrane of these cells.
• This causes the movement of K+ ions out of the cell and into the tubular fluid.
• This makes the tubular fluid positively charged.
• This favours the movement of Mg2+ ions into the cell via TRPM6 transport protein channels.
This is activated by epidermal growth factor.
• There is a Mg2+ exchanger on the apical membrane but we don’t know what Mg2+ is exchanged for.
secretion-potassium ions
- We absorb potassium from our diet.
- It enters the ECF.
- It is taken up by cells by a Na+-K+ pump. This pump is activated by insulin.
- 98% of the potassium ions is taken up by the cells and 2% remains in the ECF.
- Proximal Convoluted Tubule takes up 65% of K+ ions.
- Loop of Henlé takes up 25% of K+ ions.
- Distal Convoluted Tubule & collecting duct have variable K+ reabsorption and secretion. Here, you either get net reabsorption or net secretion.
- 92% of the potassium is excreted by the kidneys.
- 8% of the potassium is excreted by the colon.
- Potassium needs to be carefully regulated because both hypokalaemia and hyperkalaemia can be fatal.
- Hypolkalaemia causes excess hyperpolarisation which leads to paralysis and so death.
- Hyperkalaemia causes excess depolarisation which also leads to paralysis and so death
proximal convoluted tubule
- On the basolateral membrane, the Na+/K+ pump allows intake of K+ ions.
- These can leave via a K+ channel on the basolateral membrane.
- The net effect is the recycling of K+ ions.
- On the apical membrane there is a K+ channel.
- This allows the outflow of K+ ions, into the urine.
- As the tubular fluid continues through the PCT, there is a gain in the charge of the fluid because of the secretion of positively charged ions into the tubular fluid.
- This causes K+ ions to diffuse through the tight junctions between cells, back into the blood (down an electrochemical gradient).
- This is unregulated.
- In this way, most of the K+ ions are secreted into the tubular fluid, however, some diffuse back into blood.
ascending limb of loop of henle
- On the basolateral membrane, the Na+/K+ pump allows intake of K+ ions.
- These can leave via a K+ channel on the basolateral membrane.
- The net effect is the recycling of K+ ions.
- On the apical membrane there are two K+ transport proteins.
- The NKCC2 is a sodium-potassium- 2 chlorine cotransporter.
- It allows the entry of potassium ions into the cell.
- The ROMK2 potassium channel causes an outflow of K+ ions.
- This causes recycling of K+ ions on the apical membrane, similar to that on the basolateral membrane.
- The net movement of potassium ions favours the influx via NKCC2.
collecting duct
- The collecting duct has two populations of cells: (1) Principal Cells (2) Intercalated Cells.
- Principal cells are involved with the secretion of K+ ions into urine.
- Intercalated cells are involved with the reabsorption of K+ ions from the urine
principle cells
On the basolateral membrane, the Na+/K+ pump allows intake of K+ ions.
There is another K+ channel that allows the uptake of K+ ions form the blood into the cell.
On the apical membrane, there is a K+/Cl- cotransporter which causes the secretion of both K+ and Cl- ions into the urine.
There is also a K+ channel (ROMK1) that allows secretion of K+ ions.
The ROMK1 channel can be stimulated by aldosterone or high plasma K+ (hyperkalaemia).
intercalated cells
On the basolateral membrane, the Na+/K+ pump allows intake of K+ ions.
On the apical membrane, there is a K+/H+ exchanger which causes the reabsorption of K+ ions.
There is also another H+ channel which causes the outflow of H+ ions.
Acidosis or hypokalaemia activate the intercalated cells.
acid base balance in the kidneys
• Normally, there is around 15,000mmol of CO2 produced per day. This is ‘potential acid’, but it usually isn’t a problem as it is efficiently excreted by the lungs.
• Metabolism also produces ~40 mmol H+ per day (‘non-volatile acids’: sulphuric, phosphoric, organic acids).
• There is also a net uptake of ~30 mmol H+ per day by GI tract
• So the kidney has to:
Excrete ~70 mmol H+ per day
Reabsorb all the filtered HCO3 -
- Excess H+ ions in the urine can cause the urine to become very acidic (pH = 1.3), thus painful.
- Therefore, in the urine, the H+ ions needs to be buffered.
concept of carbonic anhydrase enzyme
• These are enzymes that contain Zn.
• There are at least 16 isoforms, but two important isoforms reside in the kidneys:
CA II – soluble cytoplasmic (found freely dispersed in the cytoplasm)
CA IV – extracellular, linked to cell membrane (by a GPI anchor)
• They catalyse the hydration of CO2. But, this is what actually happens…
• Carbonic anhydrase (CA) catalyses the second reaction (CO2 + OH-…)
reabsorption of HCO3
- HCO3- in the tubular fluid is converted to CO2 and OH- ions as a result of CA IV.
- The CO2 diffuses into the cell.
- The OH- ions combine with the H+ ions in the tubular fluid to form water.
- This water diffuses into the cell via osmosis.
- Once in the cell, the water breaks down again into H+ and OH- ions.
- The CO2 combines with the OH- (via CA II) ions to from HCO3- ions.
- At the basolateral membrane, the HCO3- ions exit the cell and enter the blood.
- The net movement of HCO3- ions is from the tubular fluid into the blood.
- This process uses a lot of ATP.
sites of HCO3 reabsorption
• HCO3- ions are reabsorbed throughout the nephron:
PCT = 80%
Thick ascending loop of Henle (TAL) = 10%
DCT = 6%
Collecting duct = 4%
• Less than 0.01% is excreted
proximal convoluted tubule of HCO3
- HCO3- in the tubular fluid is converted to CO2 and OH- ions as a result of CA IV.
- The CO2 diffuses into the cell.
- The OH- ions combine with the H+ ions in the tubular fluid to form water.
- This water diffuses into the cell via osmosis.
- Once in the cell, the water breaks down again into H+ and OH- ions.
- The CO2 combines with the OH- (via CA II) ions to from HCO3- ions.
- At the basolateral membrane, the HCO3- ions exit the cell via a ‘kidney variant of Na+/ HCO3- cotransporter’ (kNBCe1) and enter the blood.
- The H+ ions are secreted from the cell via a H+-ATPase pump and via a Na+/H+ exchanger (NHE3).
- The net movement of HCO3- ions is from the tubular fluid into the blood.
- This process uses a lot of ATP.
- The NHE3 is dominant in proximal tubule. It has a large capacity but limited gradient generation (from 7.35 to 6 pH). This is because the Na+ ions move over a small gradient and so only small amounts of H+ are exchanged (secreted), therefore the pH only decreases by a small amount.
- V-type (vacuolar) H+-ATPase can generate a bigger gradient (from 7.35 to 4/5 pH).
• 1:3 stoichiometry of kNBCe1 makes it electrogenic.
It allows HCO3 - efflux from the cell because of extra drive from membrane potential.
This is because there is a net efflux of 2- charge. This gives the protein the extra drive.
proximal renal tubular acidosis
Proximal Renal Tubular ACIDOSIS (RTA) • Rare autosomal-recessive disease • Impaired HCO3- reabsorption in PCT • Not treatable by HCO3- supplementation • Attributed to mutations in kNBCe1 • Ocular abnormalities too because of kNBCe1 and pBNCe1 expression there too
thick ascending loop of henle and distal convoluted tubule in hco3
- The transport protein for HCO3- ions at the basolateral membrane differs to that in the PCT.
- Here, it is an anion-exchanger (AE2).
- It exchanges HCO3- ions for Cl- ions.
α – Intercalated Cells of the Collecting Tubule and Duct Cells
- The H+ pump at the apical membrane is electrogenic.
- The H+/K+ -ATPase pump at the apical membrane is not electrogenic, therefore allowing transport of H+ ions up a steep gradient.
- The kAE1 exchanger protein at the basolateral membrane exchanges HCO3- ions for Cl- ions.
Distal Renal Tubular ACIDOSIS (dRTA)
- Inability to acidify urine (acid not secreted) – serious systemic consequences
- Treatable with HCO3- supplementation
- Dominant and recessive patterns
- Several transporter mutations – mainly affecting the α – intercalated cells: kAE1, V-type H+ -ATPase, CA II (proximal effects too)
excretion of H+ as titratable acid TA
- The water that diffused into the cell during reabsorption of HCO3- ions split into H+ and OH- ions.
- These H+ is secreted back out of the apical membrane into the tubular fluid.
- Once in the tubular fluid, the H+ ions are buffered by filtered phosphate (HPO42-) into H2PO4-.
- At the same time, HCO3- enters the tubular fluid after being filtered by the glomerulus, which further neutralises it.
excretion of H+ as NH4+
- An alternate way of secreting H+ ions is to utilise the ammonium (NH4+) ions synthesised in the cell during glutamine metabolism.
- Glutamine metabolism produces NH4+ ions and OH- ions.
- The OH- ions can combine with CO2 (via CA II) to form HCO3- ions that can enter the blood.
- The NH4+ ions split into ammonia (NH3) and H+ ions.
- NH3 and H+ ions are secreted into the tubular fluid where they both combine again to form NH4+ for excretion in urine.
sites of net acid secretion
• Acid (H+ ions) is secreted throughout the nephron:
PCT = 40mmol NH4+ and 15mmol titratable acid (TA)
DCT = 5mmol TA
Collecting duct = 10mmol TA
Small amounts of this acid is recycled in the medulla of the kidneys.
Excreted = 40mmol NH4+ and 30mmol TA.
NH4 reabsorption in the thick ascending loop of henle
- On the apical membrane, there is a Na+/K+/Cl- cotransporter (NKCC1).
- There is a lack of selectivity for the K+ ion and instead NH4+ can be selected for.
- Also, there is a ROMK2 channel on the apical membrane.
- This is a K+ channel but can also select for NH4+ ions.
- Once in the cell, NH4+ splits into H+ ions and NH3.
- NH3 can now diffuse into the blood.