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REN > Week 2 > Flashcards

Week 2 Flashcards

1
Q

Body water balance

A

Intake: highly variable but typical values /24 hours are:
Drinking: 1500ml very variable
Food: 500ml
Metabolism: 400ml
Total: 2400ml
Output:
Lungs: 400ml variable with exercise
Skin: 400ml variable with temp
Faeces: 100ml variable with disease
Urine: 1500ml
Of the outputs urine is under the greatest independent control in order to maintain water body balance. The others can be highly variable depending on temp, metabolic rate and diet

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2
Q

Body fluid compartments

A

A fluid is a substance that deforms under a shear stress. In physiology the important fluids are those in which water, or a fat/lipid are the solvent
Key compartments:
- intracellular water
- interstitial water: filling the space between cells, amongst the extracellular matrix
-fat
- (blood) plasma
- transcellular fluid: separated from the extracellular fluid be a membrane (eg CSF, peritoneal fluid, aqueous humor) . Can expand lots

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3
Q

Body water content

A

Total body water is approx. 42L for a 70kg person
Proportionally greater in men than in women
Proportionally reduces with age
Volumes of body fluid compartments in a 70kg person
Total body fluid: 42L
Extracellular:
—Plasma- 3L
—Interstitial fluid- 10L
Intracellular- 28L
Transcellular fluid: 1L

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4
Q

Transcellular compartments: examples

A

Peritoneal space: can expand greatly (used therapeutically during peritoneal dialysis)
CSF: protected by the blood-brain barrier (endothelial cells joined by tight junctions; with a role for glia)
Pleural cavity
Synovial fluid

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5
Q

Measuring body fluid compartments

A

Destructive methods of measuring fluid compartments
- plasma volume: by exsanguination (draining someone of blood) and centrifugation
-total body water: weigh a body, then desiccate it (by heating) then re weigh

One approach is to inject a substance which is known to distribute in a given compartment and then calculate the ‘volume of distribution’ Vd which is:
“ the volume of fluid required to contain the total amount of drug in the body at the same concentration as that present in the plasma”
Vd= Q (amount of drug) / Cp (plasma concentration of drug
Sometimes expressed in units of Vd/body mass ie L/kg

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6
Q

Measuring total body water

A

In order to find a substance that mixes uniformly with water throughout the body the best option is to use water by using deuterium H^2 or tritium H^3 to replace normal isotope of hydrogen 1^H in the water.
Calculation of the volume then proceeds in the same way as for the other ‘volume of distribution’ calculations

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7
Q

Markers for other volumes

A

Plasma volume: labelled proteins injected intravascularly; Evan’s blue (which binds to plasma proteins)
Extracellular fluid: 36Cl- (although some passes intracellularly), thiosulfate/thiocyanate, inulin (polysaccharide). None of these are perfect, although thiosulfate/thiocyanate seem to be most accurately. Note that this will not measure transcellular fluid.

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8
Q

Plasma

A

Plasma: this is the fluid component of the blood, and usually represents about 55% of the blood by volume. The rest of the volume is occupied by cells
Haematocrit: a measure of the proportion of the blood occupied by cells (usually around 45%)

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9
Q

Constituents of body fluids

A

Only about one half of the extracellular calcium is present as free Ca2+ ions
The plasma also contains about 60g.l-1 of protein of which albumin is most prevalent (40g.l-1)

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10
Q

Calcium

A

About half of the Calcium in the circulation is bound to albumin which means that any change in the albumin concentration will change the total Ca2+ concentration without changing the free calcium concentration
It’s the free calcium concentration which is biologically active but more difficult to measure
So body regulates the free calcium concentration rather than total calcium concentration

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11
Q

Correcting calcium for hypoalbuminaemia

A

Because its the free calcium thats more physiologically important but ‘total’ calcium is measured clinicians need to correct the calcium to account for changes in the concentration of binding sites
One approach might be to calculate the free calcium concentration but this is rarely done
A ‘corrected’ total calcium is calculated doesn’t correspond to real physical quantity but helps to interpret result based on normal ranges of free total calcium with ca2+ in mM and albumin in g/l
Ca2+ (corrected)= ca2+ total + 0.020 (40-albumin)
-the total Ca2+ conc that would occur if albumin was corrected at normal level

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12
Q

Osmole (osm or osmol)

A

This is a measure of the number of molecules that a compound dissociates into when dissolved in solution
Useful for measuring osmotic forces

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13
Q

Osmolality v osmolarity

A

Osmolality: number of osmoles per unit mass of the solvent (osm.kg-1)
Osmolarity: number of osmoles per unit volume of the solution (osm.L-1)
In physiological ranges the difference is very small as the density of water is close to 1kg.L-1 and the difference between the volume of the solvent and the volume of the solution is very small

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14
Q

Osmotic pressure

A

At the interface between 2 solution molecules exchange because of diffusion
If the concentration of any species is different on either side of the interface, there will be a net movement of molecules from one side of the membrane to the other
In the case of water the force (per unit area) required to oppose such a new movement is called the osmotic pressure
In the context of biological tissues the interface between solution is at a semi-permeable membrane usually the plasma membrane

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15
Q

Osmotic pressure alternative definition

A

The amount of pressure required to oppose osmosis
In this configuration the osmotic pressure is equal to the hydrostatic pressure generated which is pgh (p= density of solution, g =acceleration due to gravity, h=height)

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16
Q

Isotonic v isosmotic

A

If two solutions are isosmotic they share the same Osmolality
If a solution is isotonic then applying the solution to cells (traditionally red blood cells) will not cause net fluid movement
It’s possible for a solution to isosmotic with respect to intracellular fluid yet not isotonic an example is urea
For isotonic solutions across a semipermeable membrane there will be no net movement of water across the membrane; just like a chemical equilibrium water moves in both directions but at equal rates

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17
Q

Example urea solution

A

Urea crosses relatively freely through plasma membranes in cells that express a urea transporter. Hence an isosmolar solution of urea if applied to cells is not isotonic because urea will enter the cells, increasing the intracellular osmotic pressure and hence encouraging the water to enter the cells. This causes cell swelling and ultimately rupture

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18
Q

Movements across capillaries

A

Components of capillary walls create a barrier to diffusion in the same way that our plasma membrane does: hence it too can be considered a semipermeable membrane
The net movement of water will be a balance between hydrostatic force and osmotic pressures. Across capillary membranes the ions (which are small) are in equilibrium so the main osmotic forces are due to the proteins; the protein-mediated force is sometimes called the oncotic pressure
(Oncotic pressure- osmotic pressure due to proteins)

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19
Q

An example of the importance of osmotic pressure changes:

A

Fall in plasma albumin:
Causes- liver failure (decreased production), renal failure (increased loss).
Effects: pulmonary, peripheral, oedema, ascites
Oedema is a critical problem in the brain
Corrected by mannitol

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20
Q

mannitol

A

A stable sugar alcohol that when injected intravascularly increases the plasma and extracellular space Osmolality
By doing so it ‘pulls’ water from the intracellular and transcellular spaces (such as CSF)
Hence one of its uses is to decrease intracranial pressure such as that following intracranial haemorrhage
It is also an osmotic diuretic
Modern use: inhaled for cystic fibrosis management

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21
Q

The nephron

A

Afferent (in) and efferent (out) arterioles - capillary tuft
Glomerulus
Bowman’s space
Proximal tubule
Loop of Henle
Distal tubule
Collecting duct

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22
Q

blood flow to the kidney

A

The combined blood flow to the 2 kidneys is about 1.1L.min-1 which means a renal plasma flow rate of 600ml.min-1 ( allowing for the volume of cells in the blood)
This large flow hints at the importance of the kidney

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23
Q

Glomerular ultrafiltration

A

Filtration in the kidney occurs in the glomerulus where a ‘semipermeable membrane’ separates the cells and plasma in the capillaries from the ‘filtrate’ which forms in the Bowman’s space. This filtration involves bulk flow rather than strictly due to diffusion
The 2 forces which drive the flow are identical to those determining movement in all capillary beds:
-hydrostatic: higher hydrostatic pressure (50mmHg) in the capillaries drives fluid out; much higher than in most capillaries
-osmotic/oncotic: higher osmotic pressure in the capillaries (particularly due to plasma proteins) impedes the flow
— the osmotic force increases along the length of the capillaries but equilibrium is not normally achieved (in people)

24
Q

Osmotic and oncotic pressure

A

The total osmotic pressure of plasma is very high; that due to the proteins is very low. However most of the constituents of plasma are of low molecular mass and size and hence of equal distribution across the glomerular capillaries its the osmotic pressure of the proteins (the oncotic pressure) that affects the driving force
Osmotic pressure can be estimated by one form of the Morse equation (derived from the van’t Hoff equation):
Osmotic pressure= nCRT
Where nC is the Osmolality (0.28osm.kg-1), R= ideal gas constant (0.082 L.atm.mol-1.K-1), T=temperature (310K) given total osmotic pressure as 7.1atm
Very high pressure indicates how difficult it is to prevent flow across a semipermeable membrane using hydrostatic pressure
However total protein Osmolality is only 0.9mOsm.kg-1 yielding predicted pressure of about 17mmHg measured values are slightly higher at 25mmHg

25
Q

Filtration pressure along peritubular capillaries

A

Once the efferent capillary leaves the glomerulus it enters a portal vein and travels to a second capillary bed surrounding the Loop of Henle; here the hydrostatic pressure is much more similar to a systemic capillary, while the osmotic (oncotic) pressure is much higher

26
Q

Filtration pressure along the glomerular capillary

A

The oncotic pressure will initially be equal to that found in plasma it then increases along the length of the capillary but will never exceed the hydrostatic pressure
It increases because as blood flows along capillary water tends to be lost but proteins are retained
So protein concentration increases and so oncotic pressure increases

Bowman’s space hydrostatic pressure remains constant, the force that drives fluid out from the capillary into Bowman’s space is the capillary hydrostatic pressure. The forces that oppose this is the capillary oncotic pressure and Bowman’s space hydrostatic pressure.
Net perfusion pressure= difference between capillary hydrostatic pressure and sum of oncotic and bowman’s hydrostatic pressure. There’s always a net outward driving force from capillaries into bowman’s space

27
Q

Glomerular Barriers to diffusion

A

There are 3 layers separating the blood from the lumen of the Bowman’s capsule:
1. Endothelial cells of glomerular capillaries. Tight junctions
2. Glomerular basement membrane
3. Epithelial cells of Bowman’s capsule- these are generated by specialised epithelial cells called podocytes. Podocytes form filtration slits between their process through which filtrates can pass
The endothelial cells have small (60nm) holes (fenestrations) between them
The endothelial cells have a negatively charged glycocalyx which creates a charge barrier that is particularly effective for proteins
The basement membrane also consists of fixed, negatively charged proteins (eg collagen) helps control movement of substances across it

28
Q

Podocytes

A

Epithelial cells of Bowman’s capsule
From their cells, small processes (pedicels: foot processes) project and interdigitate with their neighbours to form another barrier to the movement of fluid

29
Q

Fraction filtered: size and charge

A

Dextrins: a group of low molecular weight carbohydrates produced by hydrolysis of starch and glycogen
Cationic dextran: + charged molecule, most easily filtered, if molecule has small size it’ll be freely filtered. As size increases it begins to be excluded so fraction thats filtered decreases
Neutral dextran: second most easily filtered if small enough will be freely filtered
Anionic dextran: least easily filtered

30
Q

Bulk flow v diffusion

A

Most of the movement of substances through the filtration barrier is by bulk flow rather than by diffusion
Bulk flow or bulk transport or advection is the movement of solutions from an area of high pressure to an area of low pressure. As the solvent (water) moves it carries any solutes dissolved in it, this process is sometimes described as solvent drag
Diffusion: the movement of a substance from an area of higher concentration to lower concentration. As a result of Brownian motion the glomerular barrier is sufficiently tortuous and distance sufficiently great that diffusion alone cant explain glomerular filtration

31
Q

The filtrate

A

Those molecules filtered are therefore:
-most molecules less the 10kDa in size (so Na+, K+, Mg2+/Ca2+, Cl-. HCO3-, glucose and urea)
-larger molecular may be found particularly if positively charges and in the case of damage to the glomeruli making them ‘leaky’. Molecules with mass>100kDa dont usually pass into filtrate
The glomerular filtration rate GFR is about 120ml.min-1 or 180l.day-1
600ml.min-1 plasma reach kidney so only 1/5 plasma ends in filtrate so 1/5 sodium filtered etc
Each nephron filters about 30-50 nL.min-1
The hydrostatic pressure in Bowman’s space (about 10mmHg) helps to drive the filtrate along the rest of the nephron

32
Q

Filtration fraction

A

This is the proportion of the plasma flow thats filtered by the glomerulus
Glomerular flow rate/ renal plasma flow
120ml.min-1/ 600ml.min-1 =0.2 1/5

33
Q

Control of glomerular hydrostatic pressure

A

There are 2 main ways to locally increase the glomerular capillary pressure: dilate the afferent arteriole, contract the efferent arteriole
Proximal afferent constriction: the pressure downstream will fall so decrease in GFR and decrease plasma Flow rate
Distal efferent constriction: the pressure behind constriction increase, this results in an increase GFR but decrease in plasma flow rate
Proximal/afferent dilation: pressure in glomerulus not protected by resistance in afferent arteriole so GMR increases and so does plasma flow rate
Distal/efferent dilation: decrease in resistance downstream results in fall in hydrostatic pressure in capillaries decrease GFR and increase plasma flow rate

34
Q

Measuring GFR

A

GFR can be measured using substances that are freely filtered but the neither secreted nor reabsorbed over the length of the tubules
Such substance can either be injected intravenous (such as inulin) or be produced by body at a steady rate (creatinine)

35
Q

Measuring creatinine in the urine

A

The amount of creatinine in the urine= Ccru * V
Where Ccru is the concentration of creatinine in the urine and V is the volume of urine produced
To calculate a rate of creatinine excretion divide by collection time:
Rate of creatinine excretion= Ccru * V/t (V dot) (rate of production or urine)

36
Q

Rates of flow

A

In the glomerulus we have fluid flowing in via the afferent arteriole at the plasma flow rate (PFR)
Some then flows out the other side at rate of PRF-GFR
About 20% will flow into filtrate at GFR
How to measure flow of substance:
-multiply flow rate by concentration of substance present in solution
-because creatinine is freely filtered the concentration in filtrate is equal to concentration in plasma

37
Q

Calculating GFR

A

GFR * Ccrp= rate at which creatinine is filtered in glomerulus
If there’s no secretion or reabsorption along the length of the tubule system then rate at which creatinine is filtered is equal to rate at which it’s excreted
CcruV(dot) =GFR * Ccrp
GFR= Ccru* V(dot)/ Ccrp
So the GFR can be calculated by measuring 3 parameters: 2 from urine and plasma creatinine concentration

38
Q

Empirical estimates of GFR in humans

A

For routine measurement in humans taking a 24 hour urine collection and blood sample is cumbersome so we estimate GFR using a single blood sample
Relies on the fact the rate of production of creatinine is fairly Constant correlated with biological sex, mass and age
At equilibrium the rate of creatinine production= rate of creatinine loss in kidney GFR * Ccrp so for a given person their GFR will vary inversely proportional to their Ccrp
Estimated GFR (eGFR):
eGFR= k/ Ccrp
Constant k that’s correlated with age, mass and sex
Cockcroft-Gault formula: eGFR= (140-age)massC/Ccrp
Age in years, mass in kg, C=1.23 in men and 1.04 women

39
Q

GFR with age

A

Formula only appropriate for adults
As we age we progressively lose nephrons and GFR falls; this by itself causes the Cr to rise. The age-dependent term in the numerator really implies that the rate of creatinine production falls with age
The normal loss of renal function with age will place a limit of longevity should other causes of death (particularly cardiovascular and oncological) be significantly reduced

40
Q

Proteinuria

A

As the glomerulus is the first and key barrier to protein loss, protein in the urine suggest glomerular dysfunction. This is a key feature of a key set of renal failures, which when sufficiently severe constitute nephrotic syndrome
The set of conditions were there is a predominant dysfunction of the glomerulus are called glomerulonephritis

41
Q

Congenital nephrotic syndrome

A

This is a rare genetic disorder involving a component of the glomerular barrier between podocytes either: nephrin or podocin
In this condition the glomerulus is more permeable to plasma proteins and so more protein is found in the urine
However the more common cause of protein in urine is glomerulonephritis or diabetes

42
Q

Proximal tubule division

A

The early part 60% is called the proximal convoluted tubule
The rest is the proximal straight tubule
It’s present in the renal cortex
Function is reabsorption- lots of water and other substances are reabsorbed
The surface area of the proximal tubules is greatly enhanced by the presence of microvilli forming a brush border

43
Q

Transport in the proximal convoluted tubule

A

The selective distribution of ion channels, exchangers and cotransporters (secondary active transport) and pumps (primary active transport) on the apical and basolateral membranes in key to directional ion movement
There’s movement of ions through cells (transcellular) and between cells (paracellular)
The movement of Na+ creates an osmotic gradient for the movement of water transcellulary and paracellularly
This segment of the tubule is quite water permeable implying that the filtrate is (almost) isotonic with the interstitial space which in the cortex means that it’s effectively isotonic with plasma by end of tubule about 70% water is reabsorbed
Uses the movement of Na+ down its electrochemical gradient into the epithelial cells to drive the movement of other substances (eg glucose and amino acids)
Uses the Na+/K+ ATPase (and other mechanisms) to move Na+ out of the cell on the basolateral membrane

44
Q

Water movement in the proximal tubule

A

Occurs via both the paracellular route and transcellular route through aquaporin 1 (AQP1)
Water flows through the paracellular route because of the net outward hydrostatic and osmotic forces
The proximal convoluted tube is quite water permeable implying that the filtrate is (almost) isotonic with the interstitial space which in the cortex means that its effectively isotonic with plasma
Tight junctions between epithelial cells

45
Q

Fates of molecular along the proximal tubule

A

Along the length of the proximal tubule many substances are absorbed except inulin
This changes their concentration by time they reach end of proximal tubule
Urea and chloride Both are absorbed weakly into proximal tubule but because of their rate of absorption is slower then water they end up at a higher concentration than they started with
Na and K reabsorbed in proportion to water so concentration remain constant
Amino acids and glucose: more strongly reabsorbed so their concentration at end of proximal tubule much lower

46
Q

Glucose absorption in the proximal tubule

A

On the apical membrane on the epithelial cells that make up the proximal convoluted tube there are SGLT transporters SGLT1 and SGLT2
SLGTs use the electrochemical gradient of Na to help drive reabsorption of glucose, early in proximal tubule we have more SGLT 2 (only needs one Na+)
Under normal conditions 90% of the glucose is transported by the low affinity/ high capacity sodium glucose cotransporter 2 SGLT2
But as glucose concentration begins to fall in the proximal tubule this is transporter no longer had enough energy so 2 Na molecules are required to drive the low glucose concentrations remaining
On the basolateral membrane there are also channels for glucose- GLUT 1 and 2 they allow glucose to move down passively along its chemical gradient into cortical interstitial space
The movement of glucose is rate limited theres a maximum rate at which transport by secondary active transport mechanisms and channels can operate. This is called the maximal tubular load of glucose (about 380mg.min-1, 2.1mmol.min-1) this is tubular maximum Tm transport

47
Q

Quantifying glucose uptake/excretion

A

Filtration: glucose is a small molecule so is readily/freely filtered. If GFR is constant then the only thing that will change the rate at which glucose is filtered is glucose concentration so we get a linear relationship between filtered glucose concentration and rate at which glucose is filtered
Reabsorption: keeps pace with glucose filtration until it reaches a maximum level of absorption- 380mg/min then it continues to work at max rate. If plasma glucose too high then not all is reabsorbed so ends up in urine. This is one way how body copes with excess glucose- why people with DM end up with glucose in urine

48
Q

SGLT2 inhibition

A

SGLT2 inhibitors have recently been used for the treatment of diabetes
By inhibiting SGLT2 we decrease max transport rate and therefore lower glucose concentration as more is lost in urine
Eg. Canagliflozin, dapagliflozin
The concept is that by inhibiting the transporters you cause glucosuria, and hence drop blood glucose

49
Q

Amino acids in proximal tubule

A

The plasma amino acid concentration is 2.5-3.5mM total
Transport is Tm limited
Given the diversity of amino acids it’s not surprising that many different transporters that contribute but most are cotransporters that use the Na+ gradient (analogous to SGLTs for glucose)
Few use H+ gradient

50
Q

HCO3- absorption in the proximal tubule

A

Regulation of HCO3- is important for pH regulation in body
So almost all bicarbonate being filtered need to be reabsorbed- most done in proximal tubule doesn’t happen directly via bicarbonate transporters but indirectly via CO2
Main mechanism:
-in the filtrate HCO3- (25mM in filtrate) reacts with excess H+ (entering through Na+/H+ exchanger) to produce CO2
-this is an equilibrium reaction which involved CA (carbonic anhydrase)
-CO2 enters the cell where it is also in the same equilibrium
-on the basolateral membrane there is a cotransporter which transports 3 bicarbonates ions into the cortical interstitial space in along with 1 Na+, this uses electrochemical gradient of bicarbonate ions to drive Na reabsorption
- the H+ ion is exchanged via Na/H exchanger on apical surface, this is important mechanism for Na+ reabsorption

51
Q

Acetazolamide

A

Acts mainly in the proximal tubule
Blocks carbonic anhydrase
Weak diuretic
Uses: glaucoma, mountain sickness prophylaxis
How might acetazolamide affect pH= urine become alkaline, causes metabolic acidosis

52
Q

Caffeine

A

Commonly consumed drug with a diuretic effect
Action of diuresis depends of A1 receptors (adenosine receptor) antagonism and hence:
-increases glomerular filtration rate by opposing the vasoconstriction of renal afferent arteriole mediated by adenosine during tubuloglomerular feedback
-inhibits Na+ reabsorption at the level of the renal proximal tubules
So by increasing GFR and inhibiting Na+ reabsorption water Reabsorption is inhibited leading to diuresis

53
Q

Cl- in the proximal tubule

A

Chloride is reabsorbed transcellularly by a complex mechanism
Transcellular mechanism:
-chloride ions are exchanged for other anions across the apical surface via antiporters.
-eg movement of chloride in exchange for carbonate ions
-eg movement of chloride in exchange for methanoate
-methanoate is an equilibrium with methanoic acid which is not charged so can cross back into cells more easily
-this creates a circular loop which drive reabsorption of chloride (secondary active transport)
-on the basolateral surface the chloride ions either:
-move down chloride channels
-move through co transporters along with K+

Chloride ions also move paracellularly:
-given the absorption of HCO3- with the charge difference balanced by Na+ absorption less Cl- is moved than Na+ in the early proximal tubule
-given that water is reabsorbed with the Na+ and HCO3- this means that the Cl- concentration modestly increases along the proximal tubule
-as this Cl- concentration increases (towards the end of the proximal tubule) it drives (passive) paracellular Cl- movement down its concentration gradient

54
Q

Albumin in the proximal tubule

A

Very little albumin enters the filtrate, but that which does binds to the plasma membrane (onto microvilli) of the tubule cells, is endocytosed, then catabolised to its amino acids for subsequent recycling in the body

55
Q

Organic anion secretion in the proximal tubule (PAH as example)

A

There are other substances that need to be cleared from the proximal tubule
Sometimes the body wants to get rid of substances at a rate faster than filtration alone can mange
In such circumstances theres a mechanism the kidneys can use:
-fluid flowing into efferent arterioles travel along through portal system to reach second capillary network in and around the tubular networks, including the proximal tubule
-some substances from that fluid can be actively transported from the cortical interstitial space into the filtrate, this is called secretion.
-many organic anions are actively secreted in the proximal tubule (eg.. penicillin, p-aminohippuric acid (PAH), furosemide) the negative charge often comes from carboxylates of sulfonates
-this especially happens with drugs that body does not want to have or molecules tagged by liver for secretion by kidney
-these organic anions compete with one another for excretion
-substances can be transported by family of exchangers across basolateral membrane then apical membrane in order to dump unwanted drugs into a filtrate
Basolateral membranes:
-organic anion transporters (OAT)
Luminal membrane:
-multidrug resistance-associated protein (MRP)
They are important for secretion of unwanted substances
AKG is important for generating secondary active transport mechanism on basolateral membrane

REN (11 decks)

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