renal physiology

Renal Physiology Intensive Care Training Program Radboud University Medical Centre Nijmegen Salt and water balance • E...

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Renal Physiology Intensive Care Training Program Radboud University Medical Centre Nijmegen

Salt and water balance • ECF volume regulated by total body

content of NaCl (sodium is the main osmotic constituent of the ECF)

• Water content of the body is the main

determinant of the osmolality (total body osmoles largely a function of intracellular milieu)

EXTRACELLULAR 40%

♂ 70 kg - TOTAL BODY WATER = 42 liters

INTRACELLULAR 60%

What happens with + increased Na intake?

Compare hypertonic saline for ICP↑ ECF expansion increases amount of urinary sodium

Effective circulating volume • Most probably related to arterial filling • Both low- and high pressure sensors of

which the low pressure sensors are the most important

Effective Circulating Volume

A

B

Renin-Angiotensin-Aldosteron axis

Renin release • Decreased systemic blood pressure

(increased sympathetic outflow to JGA)

• Decreased NaCl concentration at the macula densa

• Decreased renal perfusion pressure sensed in granular cells afferent arterioles

Aldosterone effect

Role of Angiotensin II

Pressure diuresis Independent from 4 pathways

Osmolality control Vasopressin

• Water retention/excretion by the kidneys • Stimulation/inhibition of thirst mechanism

Circumventricular organs = breeching of BBB Subfornical organ

Organum Vasculosum Lamina Terminalis

Location of osmoreceptors

Atrial low-pressure receptors Vagus nerve

•Breakdown of AVP by liver and kidney •Plasma half-life 18 minutes •Liver and kidney failure increases AVP levels

Plasma AVP secretion in response to plasma osmolality

Nerve IX

Vagus nerve -

Non-osmotic stimuli also enhance AVP secretion

AVP and the kidney

200 L 15 L

Non osmotic stimuli of AVP secretion • Reduction in effective circulating volume (Both hypovolemia and CHF)

• Low arterial pressure • Pregnancy • Pain, nausea and medication (morphine) e.g. postoperative

GFR =

[Ux] × V [Px]

Ideal substance

(ml/min)

Glomerular Filtration Rate

•Freely filterable •Neither reabsorbed nor secreted •Not synthesised, broken down or accumulated •Physiologically inert

Creatinine clearance

• Secreted by the tubules • Stems from normal metabolism of creatine phosphate in muscle (relatively constant)

Effect of sudden decrease in GFR

Glomerular ultrafiltration

Glomerular hemodynamics

Decreased GFR in AKI

Pressure profile along the renal vasculature

Renal blood flow Approximately 20% of CO ≈ 1 L Renal plasma flow ≈ 600 mL GFR dependent on RPF

Renal blood flow

Prowle JR. Blood Purif 2009;28:216-225

Effect of afferent and efferent arteriolar constriction

RPF = Clearance of PAH

Peritubular capillaries: deliver oxygen to epithelial cells and fluid uptake

Autoregulation

Both renal myogenic and tubuloglomerular feedback mechanism

Tubuloglomerular feedback mechanism

Volatile acids

[H+] = 40 nmol/l

Total body acid-base balance

Role of kidney in acidbase balance • Reabsorb 4320 mmol of filtered HCO • Excreting 30 mmol H /day as titratable acid 3-

+

(mostly phosphate buffer)



Excreting 40 mmol H+/day as NH4+

4320 mEq/day

Proximal tubule

Symporter NBCe I

Type IV

HCO3- reabsorption

Type II

Combination of H+ with urinary buffers HPO42- + H+ ➙ H2PO4-

Quantification by urine titration with alkali to raise pH to that of blood ⅓ of RNAE - mostly phosphate

Titratable acid

Preferable - 2/3 of RNAE

Ammoniagenesis and NH4+ secretion

Urinary Net Charge •

During acidosis UNC (Na+ + K+ - Cl-) should be < 0 reflecting excretion of NH4+

• An value of 0 or positive value is indicative of a defect in NH4+ secretion

Regulation • Respiratory and metabolic acidosis increase proximal H+ secretion and NH3 synthesis

• ECV contraction stimulates renal H

+

secretion bij increasing levels of AG II, aldosterone and sympathetic activity

• • Gluco- and mineralocorticoids stimulate Hypokalemia increases H+ secretion acid secretion