Renal3 - Renal Physiology Lecture 3 Regulation of sodium...

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Unformatted text preview: Renal Physiology Lecture 3 Regulation of sodium balance Total-body balance of sodium and water Sodium (Na) and water are important components of the body fluids. - salt and water are not separable - their regulation is extremely closely regulated - one cannot refer to their regulation separately - nonetheless they have separate functions Total-body balance of Na and water has to be maintained to sustain normal blood pressure and life. 500 ml the only physiologically regulated water output We are in water and sodium balance water intake = water output sodium intake = sodium output Depending on intake: water output can vary from 0.4 L/day to 25 L/day; sodium chloride output can vary from 0.05 g/day to 25 g/day. Basic renal processes for sodium and water Both sodium and water are freely filtered but ~99% is reabsorbed (no secretion). - never 100% reabsorbed - 1-2% is the regulatable portion - water and sodium is filtered, reabsorbed but never secreted The majority of sodium and water reabsorption (~2/3) occurs in the proximal tubule. But the major hormonal control of reabsorption occurs on the DCT and CD. distal convoluted tube and collecting duct Basic renal processes for sodium and water 1) Sodium reabsorption is an active process occurring in all tubular segments (except the descending thin limb of Henle’s loop) 2) Water reabsorption is by diffusion and is dependent upon sodium reabsorption. follows the movement of sodium comes first Active sodium reabsorption On the basolateral membrane: Active Na+/K+-ATPase pumps transport sodium out of the cells and keep the intracellular concentration of sodium low. On the apical (luminal) membrane: Sodium moves downhill from the tubular lumen into the tubular epithelial cells. Each tubular segment has different mechanisms. e.g. In the proximal tubule: Na+-H+ antiporter (counterporter) Na+-glucose cotransporter In the CCD: diffusion via Na+ channel - sodium reabsorption requires energy, active process Active sodium reabsorption epithelial cell distal part of the tubule where hormonal control is occuring sodium is pumped using ATP against the concentration gradient into the ISF keeping concentrations low in the collecting ducts sodium moves along the concentration gradient from the tubule to the collecting ducts (tubular epithelial cells) Na+/K+-ATPase in basolateral membrane Renal sodium regulation Sodium intake Sodium intake Urinary sodium excretion Urinary sodium excretion We are in sodium balance (keeping total body sodium constant). What is sensing total body sodium? Sodium is the major extracellular solute, thus changes in total body sodium result in similar changes in extracellular fluid volume. Intracellular fluid (40% of BW) 15% Total body water 3/4 Interstitial fluid (60% of BW) Extracellular fluid (20% of BW) 1/4 Plasma 5% Total body sodium is sensed as intravascular filling by baroreceptors in the cardiovascular system. can be in the venous/arterial system - there is no sensor in the body that can measure how much sodium is in the body - sodium is a major extracellular solute (it exists in extremely small concentrations in tissue) - its concentration can be reflected by the volume of ISF Attention!! Plasma concentration of sodium is NOT a marker for total body sodium. PNa only reflects the relative relationship of total body Na and water. - it only reflects the relationship between sodium and water - high concentration --> too much sodium or not enough water, cannot determine which Renal regulation of sodium Sodium excreted = Sodium filtered - Sodium reabsorbed (sodium is NOT secreted in the tubules) Sodium excretion could be regulated by 1. GFR (minor role) 2. Sodium reabsorption (most important) Renal regulation of sodium -control by GFR ⇑ Na+ and H2O loss due to diarrhea ⇓ Plasma volume ⇓ Venous pressure ⇓ Venous return ⇓ Atrial pressure ⇓ Ventricular end-diastolic vol Reflexes mediated by venous, atrial and arterial baroreceptors ⇑ Activity of renal sympathetic nerves ⇓ Stroke volume ⇓ Cardiac output ⇓ Arterial blood pressure Kidneys Vander’s, 10th ed. Fig. 14-18, page 547. (9th ed. Fig. 14-17 page 535) ⇑ Constriction of a.a. ⇓ Net GF pressure ⇓ GFR Direct effect ⇓ Na+ and H2O excreted Renal regulation of sodium -control by reabsorption Key hormone: aldosterone (steroid hormone secreted by - part of the RAA system the adrenal cortex, zona glomerulosa ) -- steroid hormoneare not part of the kidney adrenal glands Aldosterone stimulates sodium reabsorption in the DCT and CCD. No aldosterone: ~2 % of filtered load is excreted (equivalent to 35 g of sodium chloride). High aldosterone: ~0 % of filtered load is excreted. Na reabsorption 2/3 of sodium is absorbed in the proximal tubule DCT and Hormonal control - final fine tuning of sodium reabsorption by hormonal control (aldosterone) - by the time the fluid reaches the regulatory domain the amount of sodium in the fluid is very low, but this is still the most important part of sodium excretion - very vigorous in stimulating sodium reabsorption --> how can the kidneys regulate levels of aldosterone? Aldosterone Also increases H+ secretion. Upregulate expression Regulation of aldosterone secretion: renin-angiotensin system Plasma K ↑ ACTH + ANP - Figure 14.05 Juxtaglomerular apparatus can be stimulated by three things: sympathetic nerves, pressure in the afferent arteriole and signals from macula densa cells (renin secretion) TAL taller, thinner cells ⇓ Plasma volume Regulation of renin secretion by extracellular fluid volume (important mechanisms for Na balance) ⇑ Activity of renal sympathetic nerve ⇓ Arterial pressure (direct effect of less stretch) ⇓ GFR, which causes ⇓ flow to macula densa ⇓ NaCl delivery to macula densa Aldosterone does NOT stimulate H2O reabsorption directly in the CCD. Vander’s 10th ed. Fig. 14-20, page 549 (9th ed. Fig. 14-19 page 537) Renal juxtaglomerular cells ⇑ Renin secretion ⇑ Plasma renin ⇑ Plasma angiotensin II Adrenal cortex ⇑ Aldosterone secretion ⇑ Plasma aldosterone CCD ⇑ Sodium and H2O reabsorption ⇓ Sodium and H2O excretion water reabsorption is not a direct effect Other factors influencing renal sodium excretion Atrial natriuretic peptide (ANP) • ANP is a peptide hormone secreted by cells in the cardiac atria. • ANP acts on the tubules to inhibit sodium reabsorption (opposite actions of aldosterone) and increases GFR. • Increased total body sodium (thus increased extracellular fluid/plasma volume) stimulates ANP secretion. when plasma volumes grow, the atrial cells are stretched and they begin to secrete ANP Blood pressure • Increased blood pressure increases sodium excretion (pressure natriuresis). Action of ANP ...
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This note was uploaded on 04/26/2011 for the course PHGY 210 taught by Professor Trippenbach during the Winter '08 term at McGill.

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