Q41

Describe the structure of the Loop of Henle.

Thin descending, thin ascending, thick ascending limbs. Cortical nephrons with short loops (85%) & juxtamedullary nephrons with long loops into medullary pyramids (15%).

Describe the function of the Loop of Henle.

Counter current multiplier: maintains gradient of osmolality; requires vasa recta as countercurrent exchangers Thin descending: high permeability to water; it moves out of tubule into interstitium Thin ascending: high permeability to NaCI; it moves out of tubule into interstitium Thick ascending: active transport Na, K, CI, from tubule to interstitium; impermeable to H2O


Q42

Describe the control renal blood flow.

Chemical: Noradrenaline constricts interlobular and afferent arterioles. Angiotensin 11 constricts efferent arterioles > afferent arterioles. Dopamine (made in kidney) vasodilates. Acetylcholine vasodilates. Prostaglandins inc. bl flow in cortex, dec. bl flow in medulla.

Neural: SNS -> dec bl flow. Fall of BP, vasoconstrictor response includes renal bl flow.

– Autoregulation: contractile response of smooth muscle of afferent arteriole to stretch (BP). NO may be involved. Angiotensin 11 plays a role in constricting efferent arterioles, maintaining GFR,


Q43

What are the major buffers of blood? How do they work?

Proteins, albumin; Haemoglobin; histidine residues = x6 proteins; Deoxygenated Hb better than Hb02; Carbonic Acid-Bicarb system, fast with carbonic anhydrase; lib deoxygenated; Hb histidine residues; proteins anions
What are the major buffers in cells?

Hproteins = H+ and Protein- ; H2PO4 = 2H+ and HPO4 –
Describe the Henderson-Hasselbalch equation.

pH = .pK + log [A-] / [HA]. Most effective when [A-] / [HA] = 1, so pH — pK


Q44

Explain how hypotension activates the renin-angiotensin system.

  • Hypotension leads to reduced perfusion pressure of the afferent glomerular arteriole, stimulating release of renin by the juxtaglomerular cells

Pass Criteria:

  • Bold to pass

How does the renin-angiotensin system contribute to the restoration of the blood volume?

  • Renin converts angiotensinogen to angiotensin I
  • Angiotensin converting enzyme converts angiotensin I to angiotensin II
  • Angiotensin II acts on the adrenal cortex’s zona glomerulosa cells to release aldosterone
  • Aldosterone acts on the renal distal tubules to retain Na and water, thus increases intravascular volume. Angiotensin II also a potent arteriolar constrictor and contribuets to a rise in blood pressure.

Pass Criteria:

  • 4/5 bold to pass

What other factors increase renin secretion?

  • Renin (protease) release is stimulated by increases in:
    • Catecholamines
    • Sympathetic activity through renal nerves
    • Prostaglandins
    • Low sodium states – cardiac failure, liver failure, sodium depletion

Pass Criteria:

  • 1/3 bold to pass

Q45

Describe the renal response to acidosis.

PROMPT – Describe the role of buffers in the kidney.

  • Aims to return serum pH to normal by increasing H+ excretion
  • Kidney retains HCO3 by actively secreting H+
  • Renal tubule cells excrete carbonic anhydrase converting CO2 to H+ and HCO3, then tubule cells secrete H+ in exchange for Na+
  • Amount of secreted H+ limited by urinary pH >4.5 (limiting pH)
  • Buffering in tubular fluid pH with HCO2, HPO4 and NH3 allows greater H+ secretion

Pass Criteria:

  • Must know that H+ actively secreted into tubular fluid in exchange for Na
  • Must know about buffering and name 2 buffers

Q46

Describe how the kidney responds to metabolic acidosis.

  • Renal tubule cells secrete H+ into tubular fluid in exchange for Na+
  • HCO3- is actively reabsorbed into the peritubular capillary (for each H+ secreted, 1 Na+ and 1 HCO3- are added into blood).

Pass Criteria:

  • Bold to pass

What substances act as urinary buffers for the excretion of H+?

  • NH3 forms NH4+
  • HCO3 forms CO2 and H2O
  • HPO42- forms H2PO4

Pass Criteria:

  • 2 of 3

How else can the body compensate for a metabolic acidosis?

PROMPT – What other major system is involved in acidosis compensation?

  • The respiratory system responds by increasing ventilation which results in a decrease in PCO2 which causes increase in pH (this is a rapid response)

Pass Criteria:

  • Bold to pass

Q47

What is normal Glomerular Filtration Rate (GFR)?

  • 125mL/min in normal adult 180L/24h/10% lower in women

Pass Criteria:

  • Approximate value

What factors control GFR?

PROMPT – What agents, mediators and clinical factors affect GFR?

  • Hydrostatic pressure/osmotic pressure gradient
  • Size & permeability of capillary bed (mesangial cell contraction/relaxation & loss of renal tissue)
  • K in Starling Forces = GF coefficient = mesangial cell
    • Increase – ANP, Dopamine, PGE2, cAMP
    • Decrease – Endothelins, AGII, vasopressin, norepinephrine, PAD, PGF2, leukotrienes Ca/D4, histamine TxA2
  • Clinical
    • Systemic BP
    • Parenchymal oedema
    • Ureteric obstruction
    • After-efferent arteriolar constriction
    • Plasma proteins

Pass Criteria:

  • 2 of 4 bold
  • Role of mesangial cells
  • 2 vaso active agents
  • 2 clinical examples

Q48

What is the renal response to dehydration?

  • Renin release, converts angiotensinogen to AT1
  • ACE converts AT1 to AT2
  • AT2 increases aldosterone synthesis, vasoconstriction of afferent arteriole
  • Aldosterone – sodium and water retention

Pass Criteria:

  • Need details re secretion i.e. reduced pressure at JG cells of renin and actions of A-2

What is the role of vasopressin in dehydration?

  • Promotes water resorption in collecting duct via aquaporins insertion.
  • Vasoconstriction

Pass Criteria:

  • Bold to pass

Q49

What is normal Glomerular Filtration Rate (GFR) and what factors regulate it?

PROMPT  – How does it change?

PROMPT – Identify two clinical factors that alter Starling Forces

  • Normal GFR = 125mLs/min (180L/24hours). 10% lower in females.
  • Controlled by Starling Forces i.e. GFR = K(PGC-PT)-(?GC-?T)
    • PGC = mean hydrostatic pressure in glomerular capillaries
    • PT = mean hydrostatic pressure in tubule
    • ?GC = osmotic pressure of plasma in glomerular capillaries
    • ?T = osmotic pressure of filtrate in tubule
    • K = GF coefficient; altered by mesangial cell contraction (-> dec area for filtration)
      • Contraction = Angio II, ADH, NA, PAF, TxA2, hsitamine
      • Relaxation – ANP, dopamine, cAMP, PgE2
  • GFR changes along glomerular capillaries with Starling forces dropping from 15 mmHg to 0.
  • Clinical factors altering Starling Forces
    • Alterations in renal blood flow
    • Systemic BP
    • Ureteric obstruction
    • Renal parenchymal oedema
    • Changes in plasma protein concentration
    • Changes in K as above

Pass Criteria:

  • a) approximate value for GFR
  • b) Identify Starling Forces involved
  • c) Identify central role of mesangial cells and two factors which change their degree of contraction
  • d) Identify two clinical factors that alter Starling Forces

How do the kidneys deal with Potassium?

  • Freely filtered at glomerulus (600 mmol/d)
  • Actively reabsorbed in proximal convoluted tubule (560 mmol/d)
  • Secreted in distal tubule – rate proportional to flow
  • Secreted in collecting ducts – aldosterone exreted = 90 mmol/d
  • Total secreted load average 50mmol/d but varies with renal tubular flow and aldosterone level

Pass Criteria:

  • a) freely filtered at glomerulus
  • b) largely reabsorbed in PCT
  • c) Sites of distal secretion plus influence of aldosterone

Q50

What percentage of cardiac output goes to the kidneys?

  • Renal blood flow = 1.2-1.3L/min or approx 25% cardiac output (adult)

Pass Criteria:

  • Renal blood flow 

How is renal blood flow regulated?

PROMPT – What other mechanisms are there?

  • Substances/Chemicals
    • Norepinephrine (noradrenaline)
      • constricts renal vessels
      • Stimulates renal nerves to increase renin secretion
    • Dopamine – renal vasodilatation
    • Angiotensin II – arteriolar constrictor
    • PG – increased cortex flow, decreased medulla flow
    • Acetylcholine – vasodilatation
    • High protein – increased b/flow
  • Renal Nerves
    • Stimulation nerves = increased renin secretion, increased JG sensitivity, increased sodium resorption, renal vasoconstriction
    • Strong stimulation sympathetic – decreased flow
    • Fall in BP = vasoconstriction
  • Autoregulation
    • Renal vascular resistance varies with pressure to keep renal blood flow fairly constant
    • Present in denervated kidney, but not if drugs that paralyse vascular smooth muscle
    • Factors = direct contractile response, NO, angiotensin II

Pass Criteria:

  • 3/6 substances plus nerve or auto – with example 

How can renal blood flow be measured?

  • Fick principle – subs taken up/unit time
  • PAH used to measure renal plasma flow
  • Renal blood flow using plasma flow and haematocrit

Pass Criteria:

  • One example 

Describe the differences in regional blood flow within the kidney.

  • AV O2 difference for kidney = 14mL
  • Cortical blood flow = 5mL/g/min
    • Little O2 consumption
  • Medulla blood flow low (outer = 2.5mL, inner = 0.6mL)
    • Maintenance of osmotic gradient

Pass Criteria:

  • One aspect of regional blood flow to pass