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MBBS Year 2 - Physiology > RENAL - Glomerular Filtration and Renal Blood Flow > Flashcards

RENAL - Glomerular Filtration and Renal Blood Flow Flashcards

1
Q

Label the diagram of the nephron on SLIDE 8.

A

COMPLETE

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2
Q

What is the usual GFR per day?

A

180 litres per day

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3
Q

How are the glomerular capillaries specialised for filtration?

A
  • Endothelial cells have fenestrations - free movement of molecules out of blood
  • Underneath is basement membrane - collagenous barrier to large molecules
  • Podocytes - final layers - specialised cells with foot like processes encapsulating capillary.
  • Small gaps between processes prevent escape of larger molecules
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4
Q

Describe the charges of the layers of the glomerular capillaries and what they can and cannot filter.

A
  • NEGATIVELY CHARGED - repel substances in blood - small enough to pass through filter but are negatively charged themselves
  • RBCs not filtered - smaller substances dissolved in plasma (such as water, salts and glucose) are freely filtered
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5
Q

Describe the pressures at work in the glomerulus that facilitate blood filtration.

A
  • HYDROSTATIC - pressure exerted by blood on walls of capillaries.
  • COUNTERING PRESSURE from filtered fluid within Bowman’s capsule
  • ONCOTIC - osmotic pressure exerted by plasma proteins that drive fluid back - high pressures in capillaries drive fluid back across glomerulus into blood
  • ONCOTIC PRESSURE OF CAPSULE negligible - because so few proteins dissolved in filtrate
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6
Q

How can net filtration pressure be calculated?

A

Net Filtration Pressure = HPGC – HPCS - πGC
- Normally 10 mmHg of driving pressure from capillaries into nephron

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7
Q

(a) What does net filtration pressure only take into account?

(b) What is the effect of the capillaries being fenestrated?

A

(a) Driving pressure of individual nephrons

(b) Surface area larger than normal capillaries

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8
Q

What is GFR and what is a formula for it?

A
  • Volume of fluid filtered at all glomeruli in a given timeframe
  • GFR = Vascular permeability coefficient x net filtration pressure
  • Permeability coefficient - accounts for permeability and surface area of capillaries
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9
Q

(a) What makes arterioles special?

(b) What happens when afferent arteriole is constricted?

A

(a) Constrict due to smooth muscle
(b) Reduced volume of blood flowing into glomerulus - reduced hydrostatic pressure within glomerulus - reduced GFR

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10
Q

What happens when efferent arteriole is constricted?

A
  • Reduced volume of blood leaving glomerulus - blood backs up in glomerulus
  • Raised hydrostatic pressure
  • GFR increases
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11
Q

Describe changes to hydrostatic pressure in the Bowman’s capsule.

A
  • Don’t normally occur
  • In pathological states - such as urinary tract obstructions - raised pressure.
  • GFR decreases
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12
Q

Describe renal plasma clearance and how is rate of excretion calculated.

A
  • Rate of removal and excretion of substances from plasma
  • RATE OF EXCRETION = (FILTRATION RATE + SECRETION RATE) - REABSORPTION RATE
  • Account for secretion - because some substances secreted into nephron by peri-tubular vessels
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13
Q

What is the formula for rate of clearance of a substance?

A

(Concentration of substance in urine multiplied by volume of urine) divided by concentration of substance in plasma

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14
Q

What happens if a substance is freely filtered but not secreted or reabsorbed?

A

Clearance = GFR

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15
Q

Describe inulin.

A
  • Dietary fibre
  • Freely filtered at glomerulus
  • Not reabsorbed or secreted
  • Not toxic
  • Clearance = GFR
  • Infusions used to clinically determine GFR (not entirely convenient - infusions have to be constant over given timeframe to maintain constant plasma level)
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16
Q

What happens if a substance is freely filtered but reabsorbed?

A

Clearance < GFR
- Example is glucose - clearance from plasma = 0ml/min

17
Q

What happens if a substance is not reabsorbed but is freely filtered and secreted?

A

Clearance > GFR

18
Q

Describe PAH (para-amino hippuric acid)

A
  • Freely filtered at glomerulus
  • Secreted into tubule and not reabsorbed
  • Completely cleared from plasma
  • Rate of clearance = renal plasma flow
19
Q

Give examples of clearance values for inulin, creatinine and PAH.

A
  • 125 ml/min for both inulin and creatinine.
  • CREATININE - product of muscle breakdown. Produced at constant rate - freely filtered at glomerulus but slightly secreted. GFR overestimated by about 10% - can be used clinically
  • PAH - 600 ml/min
20
Q

What is the formula for eGFR and how can it be determined?

A

eGFR = 175 × [creatinine× 0.011312]-1.154× [age]-0.203× [1.212if afro Caribbean race] × [0.742if female]
- Determined from serum creatinine
- An eGFR >90ml/min/1.73m2 is considered normal, with kidney disease categorised by decreasing levels of eGFR

21
Q

Describe autoregulation in the kidneys

A
  • Volume of blood delivered to kidneys exceeds metabolic demand
  • Maintains constant GFR between systemic arterial BP of 70-180 mmHg
  • If blood pressure were to increase by 25% (from 100mmHg to 125mmHg), GFR would increase similarly (from 180L/day to 225 L/day).
  • If reabsorption stayed the same, then urine output would increase to around 46L/day.
  • Given that plasma volume is around 3L, this would rapidly lead to hypovolaemia
22
Q

(a) How much blood do the kidneys receive?

(b) What is the formula for renal blood flow?

(c) Describe total renal vascular resistance.

A

(a) 1litre per minute (20% of cardiac output)

(b) (RENAL ARTERY PRESSURE - RENAL VEIN PRESSURE) divided by total renal vascular resistance

(c) Determined by resistance in interlobar arteries, afferent/efferent arterioles. Kidneys - strong control over blood flow. Changes in arterial pressure don’t result in large changes to RBF

23
Q

Describe tubuloglomerular feedback. PART 1

A
  • Reduced blood pressure - reduced renal perfusion - decrease in GFR
  • Decrease in flow through Loop of Henle - increase in amount of NaCl that can be reabsorbed (in contact with nephron for longer)
  • Decreased sodium reaches distal tubule
  • Reduced sodium detected by macula densa cells in JGA
24
Q

Describe tubuloglomerular feedback. PART 2

A
  • Macula densa cells send signal to afferent arteriole which dilates
  • Signal sent to juxtaglomerular cells - RAAS activated - increased AngII concentration - constriction of efferent arteriole
  • Increased hydrostatic pressure in glomerular capillaries - increase and normalisation of GFR
25
Q

Describe myogenic control in intrinsic control of RBF.

A
  • Pulse pressure exerted on walls of arteries
  • Calcium enters smooth muscle cells - reflex contraction
  • VASOCONSTRICTION - reduced blood flow, maintains GFR and protects glomerulus from hypertensive damage
  • Prevents kidney damage during hypertension
26
Q

Describe control of RBF. PART 1

A
    • Decreased BP (via decrease in sodium delivery to distal tubule) activates baroreflex - stimulates renal sympathetic nerve activity
      - Nerves stimulate juxtaglomerular cells - increase renin releases and AngII circulation
27
Q

Describe control of RBF. PART 2

A
  • AngII constricts renal vessels e.g afferent arteriole - can be counteracted by locally produced vasodilatory factors e.g NO
  • Causes preferential vasoconstriction of efferent arteriole - increased GFR through increase in glomerular hydrostatic pressure
  • AngII constricts renal artery - decrease in RBF.
  • Decreased RBF and increased hydrostatic pressure - normal GFR maintained
28
Q

Describe the role of renal sympathetic nerves.

A
  • Innervates almost all blood vessels in kidneys
  • SEVERE DROP IN BLOOD VOLUME/PRESSURE - activation of nerves. Constriction of blood vessels and decreased GFR
  • Mild/moderate sympathetic stimulation - little effect on RBF - activates RAAS to increase sodium and water reabsorption
  • Constriction of blood vessels via the renal sympathetic nerves appears to be most important during severe acute disturbances, such as haemorrhage or brain ischaemia
29
Q

How can vascular permeability coefficient be measured?

A

Rearranging the GFR formula

Normally around 12.5 ml/min/mmHg

30
Q

Describe the juxtaglomerular apparatus.

A
  • Consists of the macula densa, which are a group of specialised cells on the wall of the distal tubule, and the juxtaglomerular cells, that line the wall of the afferent and efferent arterioles

MBBS Year 2 - Physiology (22 decks)

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