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Renal System > Physiology > Flashcards

Physiology Flashcards

1
Q

Define osmolarity

A

Osmolarity is the concentration of osmotically active particles present in a solution

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2
Q

What 2 factors are required to calculate osmolarity?

A
  1. The molar concentration of the solution
  2. the number of osmotically active particles present
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3
Q

What’s the difference between osmolality and osmolarity?

Is this difference relevant in regards to body fluids?

A
  • Osmolality has units of osmol/kg of water
  • Osmolarity has units of osmol/L
  • For weak salt solutions (incl. body fluids) these 2 terms are interchangeable
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4
Q

Define tonicity

A

Tonicity is the effect a solution has on cell volume

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5
Q

What happens to cells (e.g. RBC) when placed in a hypertonic/isotonic/hypotonic solution?

A
  1. Hypertonic
    • concentrated salt solution
    • water moves from cell to solution
    • cell shrinkage
  2. Isotonic
    • normal
    • equal amounts of water move from cell to solution and solution to cell
  3. Hypotonic
    • Water moves from solution to cell
    • cell lysis
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6
Q

When discussing tonicity you have to remember osmolarity but also what?

A

The permeability of the cell membrane to the solution!

Although related to osmolarity tonicity also takes into consideration the ability of a solute to cross the cell membrane.

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7
Q

List the 2 body fluid compartments

A

Total body water exists as 2 major compartments:

  1. ICF
  2. and ECF
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8
Q

What makes up total body water?

A

Exists as Intracellular fluid (67% of TBW) and Extracellular fluid (33% of TBW)

  • extracellular fluid includes:
    • plasma (20% of ECF)
    • interstitial fluid (80% of ECF)
    • lymph (negligible) + transcellular fluid (negligible)
  • total body water:
    • males =60% of body weight
    • females= 50% of body weight (females have more body fat)
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9
Q

How do we measure body fluid compartments?

A

Tracers

obtain the “distribution volume” of a tracer

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10
Q

What tracers are used for what body fluid compartments?

A
  • TBW: 3H2O
  • ECF: Inulin
  • Plasma: labelled albumin

TBW= ECF + ICF

so we can calculate ICF if we know (measure) TBW and ECF

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11
Q

Explain how you’d use the dilution principle to measure volume of distribution

A
  1. Imagine adding a dose of tracer (D= 42mg) to a container holding a large and unknown volume of water (V)
  2. You mix the tracer/allow it to equilibrate with the water
  3. You then take a small sample volume from the container (5ml) and measure the concentration of the tracer (C) in this sample
  4. On analysis C= 0.005mg/5ml= 0.001 mg/ml= 1mg/litre
  5. The volume of the water in the container can be calculated as:

Volume (litres)= Dose (D)/Sample conc. (C)= 42mg/1mg per litre= 42 litres

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12
Q

How is homeostasis maintained?

A

Input has to equal output

BUT kidneys can compensate for water loss by decreasing urine production to a point

some of the waste products excreted in urine can only be released in solution

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13
Q

What are the osmotic concentrations of ECF and ICF?

A

They are both identical at 300mosmol/l

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14
Q

dsecribe the relationship between solute concentrations and water distribution

A

Because changes in solute concentrations lead to immediate changes in water distribution, the regulation of fluid balance and electrolyte balance are tightly intertwined

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15
Q

Define fluid shift

A

Fluid shift is the movement of water between the ICF and the ECF in response to an osmotic gradient

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16
Q

List the potential challenges to fluid homeostasis

A
  1. Gain or loss of water
    • leads to change in fluid osmolarity
    • similar changes in ICF & ECF
    • they’ll both either increase or decrease
  2. Gain or loss of NaCl
    • leads to a change in fluid osmolarity
    • Na “excluded” from ICF (remember ion distributions) leading to osmotic water movements
      • these 2 factors combine to produce opposite changes in ICF and ECF volumes:
        • ECF NaCl gain: increase ECF and decreases ICF and vice versa
  3. Gain or loss of isotonic fluid
    • leads to no changes in fluid osmolarity
    • only changes ECF volume
    • e.g. if 0.9& NaCl sol given to someone after a haemorrhage
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17
Q

Why is electrolyte balance important?

A
  1. Total electrolyte concentrations can directly affect water balance (via changes in osmolarity)
  2. The concentrations of individual electrolytes can affect cell function
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18
Q

Which 2 ions are particularly important in electrolyte balance and why?

A

Na and K are particularly important

  1. they are major contributors to the osmotic concentrations of the ECF and ICF respectively
  2. they directly affect the functioning of all cells
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19
Q

Discuss the importance of sodium balance

A

>90% of the osmotic concentrations of the ECf results from the presence of sodium salts

Na is mainly present in the ECF therefore it is a major determinant of ECF volume

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20
Q

Discuss the importance of potassium balance

A

Minor fluctuations in plasma [K] can have detrimental consequences

K plays a key role in establishing membrane potential

>95% of body K is intracellular: small leakagess or increases in cellular uptake may severely affect [K]plasma leading to:

  1. muscle weakness –> paralysis
  2. cardiac irregularities –> cardiac arrest
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21
Q

How does salt imbalance manifest?

A

Salt imbalance is manifested as changes in extracellular fluid volume

Regulation of ECF is important for long-term regulation of blood pressure

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22
Q

List at least 5 functions of the kidneys

A
  1. Water balance
  2. Salt balance
  3. Maintenance of plasma volume
  4. Maintenane of plasma osmolarity
  5. Acid-base balance
  6. Excretion of metabolic waste products
  7. Excretion of exogenous foreign compounds
  8. Secretion of renin
  9. Secretion or erythropoietin (RBC production)
  10. Conversion of Vitamin D into its active form
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23
Q

How many nephrons does each kidney have?

A

Each kidney is composed of 1 million nephrons

24
Q

What is the fluid that flows through the nephron and collecting duct called?

A

The fluid flowing through the nephron and collecting duct is known as tubular fluid and then when it leaves the end of the collecting duct it is called urine and won’t undergo any more changes

25
Q

How many types of nephron are there? And what are they called?

A

There are two types of nephron called:

  • juxtamedullary
  • cortical
26
Q

Discuss the differences between the juxtamedullary and cortical nephrons

A
  • juxtamedullary nephrons have longer Loops of Henle which go deeper into the medulla
    • responsible for more concentrated urine
  • cortical nephrons mostly sit in cortex
27
Q

Which important part of the juxtaglomerular apparatus monitors salt?

A

Macula densa cells monitor amount of salt that is in tubular fluid as it passes through this region. They are salt-sensitive cells!

28
Q

How much of the plasma that enters the glomerulus is filtered into the tubules?

A

20%

29
Q

What is the equation for rate of filtration (into the Bowman’s capsule) of a substance?

A

Rate of filtration of X= [X]plasma X GFR

GFR = glomerular filtration rate and usually stays constant at about 125ml/min

30
Q

What is the equation for rate of excretion of a substance?

A

Rate of excretion of X= [X]urine x Vu

Vu = urine production rate

31
Q

What is the equation for the rate of reabsorption of a substance?

A

Rate of reabsorption of X = rate of filtration of X- rate of excretion of X

32
Q

What is the equation for the rate of secretion of a substance?

A

Rate of secretion of X= rate of excretion of X - rate of filtration of X

Rate of filtration and rate of excretion are relatively easy to measure hence rates of reabsorption and secretion reflect tubular modification of filtrate: obtained as the difference between filtration and excretion

33
Q

What are the filtration barriers in the glomerulus?

A
  1. glomerular capillary endothelium
    • barrier to RBC
  2. basement membrane (a.k.a. basal lamina)
    • plasma protein barrier
  3. Slit processes of podocytes
    • plasma protein barrier

Collectively, they make the glomerular membrane

34
Q

List the forces that comprise net filtration pressure?

A
  1. Glomerular capillary B.P
  2. Capillary oncotic pressure
  3. Bowman’s Capsule hydrostatic pressure
  4. Bowman’s Capsule oncotic pressure
35
Q

Define Starling forces

A

Starling forces are the balance of hydrostatic pressure and osmotic forces

36
Q

Define glomerular filtration rate

A

GFR= Rate at which protein-free plasma is filtered from the glomeruli into the Bowman’s capsule per unit time

37
Q

What is the normal GFR?

A

125ML/MIN

38
Q

What is the major determinant of GFR?

A

Glomerular capillary fluid (blood) pressure is the major determinant of GFR

39
Q

How is renal blood flow and glomerular filtration rate regulated?

A
  1. Extrinsic regulation of GFR
    • sympathetic control via baroreceptor reflex
  2. Autoregulation of GFR
    • myogenic mechanism
    • tubuloglomerular feedback mechanism
40
Q

What is the direct effect of arterial blood pressure on GFR?

A
41
Q

How is GFR controlled by alterations in arterial B.P?

A
  1. Fall in blood volume (e.g. haemorrhage)
  2. Decreased in arterial B.P
  3. Detected by aortic and carotid-sinus baroreceptors
  4. Increased sympathetic activity
  5. Generalised arteriolar vasoconstriction
  6. Constriction of afferent arterioles
  7. Decreased BP in glomerular capillaries
  8. Decreased GFR
  9. Decreased urine vol whichs helps compensate for the fall in blood volume due to haemorrhage
42
Q

Is GFR affected by short term changes in systemic arterial pressure?

A

Changes in systemic arterial blood pressure do NOT necessarily result in changes in GFR

RBF & GFR protected from changes in MABP over wide ranges of MABP

43
Q

Explain autoregulation of GFR

A

Two types:

  1. Myogenic
    • If vascular smooth muscle is stretched (i.e. arterial pressure is increased), it contracts thus constricting the arteriole
  2. Tubuloglomerular feedback
    • involves the juxtaglomerular apparatus (mechanism remains unclear)
    • if GFR rises, more NaCl flows through the tubule leading to constriction of afferent arterioles
44
Q

Define plasma clearance

A

Plasma clearance is a measure of how effectively the kidneys can “clean” the blood of a substance

  • it equals the volume of plasma completely cleared of a particular substance per minute
  • each substance that is handled by the kidney will have its own specific plasma clearance value
45
Q

How would you calculate the plasma clearance of a substance?

A

Clearance of substance X= [X]urine x Vurine / [X]plasma

[X]urine =urine concentration of substance X

Vurine = urine flow rate

[X]plasma = plasma conc. of substance X

46
Q

Which substance can be used to clinically determine the GFR and why?

A

INULIN

(not insulin!)

  • freely filtered at glomerulus
  • neither absorbed nor secreted
  • not metabolised by kidney
  • not toxic
  • easily measured in urine and blood

INULIN CLEARANCE = GFR

47
Q

Discuss the clearance of glucose and other substances like it

A

For substances which are filtered, completely erabsorbed and not secreted

clearance = 0!

This also applies to a substance that is not filtered and not secreted

48
Q

Discuss the clearance of H+ and other substances like it

A

For substances which are filtered, secreted but not reabsorbed

All of the filtered plasma is cleared of a substance, and the peritubular plasma from which the substance is secreted, is also cleared

49
Q

How is renal plasma flow calculated clinically?

A

Para-amino hippuric acid (exogenous organic anion) is used clinically to measure renal plasma flow

PAH is 1) freely filtered at glomerulus, 2) secreted into the tubule (not reabsorbed) & 3) completely cleared from the plasma

i.e. all the PAH in the plasma that escapes filtration is secreted from the peritubular capillaries

50
Q

What characteristics are needed for a substance to be used as a clearance marker?

A
  1. Non-toxic
  2. Inert (i.e. not metabolised)
  3. Easy to measure
  • a GFR marker should be filtered freely; NOT secreted or reabsorbed
  • a RPF marker should be filtered AND completely secreted
51
Q

Define filtration fraction

A

Filtration fraction is the fraction of plasma flowing through the glomeruli that is filtered into the tubules

20% of the plasma that enters the glomeruli is filtered. The remaining 80% moves on to the peritubular capillaries.

52
Q

Define facilitated diffusion

A

“Facilitated diffusion passive carrier-mediated transport of a substance down its concentration gradient”

53
Q

Define primary active transport

A

“Primary active transport is when energy is directly required to operate the carrier and move the substrate against its concentration gradient.”

54
Q

Define secondary active transport

A

“Secondary active transport describes when a carrier molecule is transported coupled to the concentration gradient of an ion (usually Na+)”

55
Q

Define transport maximum (Tm)

A

the highest rate in miligrams per minute at which the renal tubules can transfer a substance either from tubular luminal fluid to interstitial fluid

56
Q

Define renal threshold

A

the conc. at which a substance is moved from blood to urine to be excreted

57
Q

Define GFR

A

GFR is the rate at which protein-free plasma is filtered from the glomeruli into the Bowman’s capsule per unit time

Renal System (1 decks)

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