Regulation of volume & tonicity of ecf

REGULATION OF VOLUME AND TONICITY (OSMOLALITY) OF EXTRACELLULAR FLUID - PHYSIOLOGY AND APPLIED.

1. REGULATION OF VOLUME AND TONICITY OF ECF Dr.AnuPriya J

2. Scheme Introduction Extracellular fluid (ECF) Regulation of Osmolality of ECF -Water balance -Role of ADH & thirst mechanism -Role of osmoreceptors & volume receptors Regulation of volume of ECF -Sodium balance -Effective circulating volume & volume sensors -Regulation of sodium balance Applied aspects

3. It is important to regulate ECF volume to maintain blood pressure, essential for adequate tissue perfusion & function. Changes in extracellular osmolality cause changes in cell volume that seriously compromise cell function especially in the CNS. Introduction

4. Introduction Regulation of body fluid volume and osmolality (water and electrolyte balance) is an integrated function of various organ systems Kidneys play a major role

5. Body composition

6. Constituents and their contribution to plasma osmolality Constituent (Solute - ion) Osmolality ( mOsm /kg H 2 O) Sodium 135-145 Potassium 3.5-5 Chloride 95-110 Calcium 2.2-2.6 Phosphate 4 Proteins 5 Sulphate 1 Bicarbonate 22-26 PLASMA OSMOLALITY 280-295

7. Basic principles Plasma osmolality = 2(Na + ) + 2(K + ) + urea + glucose Simplified as Plasma osmolality ( mmol /kg)= 2 x plasma Na + ( mmol /L) Sodium and its associated anions make the largest contribution to plasma osmolality . Water balance in the body is the most important determinant of the body fluid osmolality .

8. Regulation of Osmolality Water balance INPUT Food 800-1000ml/day Oxidation of food 300-400ml/day Liquid 1-2L/day (highly variable) TOTAL 2100-3400ml/day OUTPUT Insensible loss 800-1000ml/day Sweat 200ml (highly variable) Feces 100-200ml/day Urine 1-2L/day(highly variable) TOTAL 2100-3400ml/day

9. Regulation of Osmolality Increased Osmolality of ECF

10. Na + content in the body is the main determinant of ECF volume. Regulation of volume

11. Oral sodium intake = Renal sodium out put + Extra renal sodium out put . Kidneys excrete or conserve sodium in response to increase or decrease in ECF volume not changes in ECF sodium concentration. It is not ECF volume per se , but effective circulating volume that regulates sodium excretion. Regulation of volume

12. Regulation of volume EFFECTIVE CIRCULATING VOLUME (ECV) The portion of the ECF volume that is contained within the vascular system and is effectively perfusing the tissues. Regulation of ECV is closely related with regulation of sodium balance. ECV reflects the activity of volume sensors located in the vascular system. Kidneys play a major role

13. SODIUM BALANCE Sodium is actively pumped out of the cells by Na + -K + ATPase pump. 65% of total body Na + is extracellular. ECF volume is the reflection of total body Na + content. Normal volume regulatory mechanisms ensure that Na + loss balances Na + gain Regulation of volume

14. Regulation of volume SODIUM BALANCE Input Food & water 100-400 mmol /day Output Urine100-400mmol/day Sweat & feces - negligible

15. For an expansion in ECF volume to stimulate Na + excretion, the expansion must make itself evident in the part of the ECF compartment where the ECF volume sensors are located. The thoracic blood vessels appear to be the site of greatest importance.

16. ECF Volume Receptors “Central” vascular sensors Low pressure sensors Cardiac atria Pulmonary vasculature High pressure sensors Carotid sinus Aortic arch Juxtaglomerular apparatus(renal afferent arteriole) Sensors in the CNS Sensors in the Liver

17. Four parallel effector pathways Renin – angiotensin aldosterone Sympathetic division of ANS Post.pituitary - ↑ se AVP(ADH – Antidiuretic hormone) Atrial Natriuretic Peptide

18. Effects of Angiotensin II Aldosterone release. Vasoconstriction of renal and other systemic blood vessels. Stimulation of thirst and ADH secretion. Increased Tubuloglomerular feedback

19. 5. Enhancement of NaCl reabsorption by the proximal tubule , thick ascending limb of Henle’s loop, the distal tubule and the collecting duct. Directly by stimulating apical Na + -H + exchange in tubule cells. Indirectly by lowering renal plasma flow. Effects of Angiotensin II

20. Effects of Aldosterone Stimulates NaCl reabsorption by the thick ascending limb of loop of Henle , distal tubule and collecting duct ( aldosterone -sensitive distal nephron ) ENaC

21. Renal sympathetic nerve activity Afferent and efferent arteriolar vasoconstriction ( α adrenergic receptors) → →decreased GFR → filtered load of Na + to the nephrons is reduced. Renin secretion stimulated by the cells of the afferent arterioles ( β adrenergic receptors). NaCl reabsorption along the nephrons is directly stimulated ( α adrenergic receptors).

22. Natriuretic peptides Vasodilation of afferent arteriole and Vasoconstriction of efferent arteriole → increases GFR → increased filtered load of Na + Inhibition of renin secretion by the afferent arterioles. Inhibition of aldosterone secretion (directly and indirectly via inhibition of renin secretion).

23. Inhibition of NaCl reabsorption by the collecting duct. Inhibition of ADH secretion and its action on the collecting duct. Natriuretic peptides

24. Salt appetite Some areas in the same region where thirst and ADH osmoreceptors are located. 2 primary stimuli that increase salt appetite Decreased ECF sodium concentration Decreased blood volume or blood pressure associated with circulatory insufficiency

25. Change in osmolality affects cell function Hyperosmolality Hypoosmolality Central pontine myelinolysis

26. Edema Increased ECF volume Decreased effective circulating volume Examples

27. Iso -osmotic volume expansion Causes Infusion of isotonic fluids Causes Diarrhoea Vomiting Haemorrhage Burns Iso -osmotic volume contraction Applied

28. Hyper-osmotic volume expansion Hyper-osmotic volume contraction Causes Excessive amount of hypertonic saline Causes Decreased water intake Diabetes mellitus Diabetes insipidus Excessive sweating Alcoholism In tracheostomy patients, insensible water loss – upto 500ml via lungs Applied

29. Hypo-osmotic volume expansion Hypo-osmotic volume contraction Causes SIADH Ingestion of large volume of water Excessive infusion of hypotonic saline Nephrogenic syndrome of inappropriate antidiuresis Rectocolonic washouts with plain water Causes Adrenocortical insufficiency (renal loss of NaCl ) Vomiting Aspiration of gastric secretions Applied

30. References Berne & Levy - Physiology, 6 th Edition Boron & Boulpaep - Medical Physiology, 2 nd Edition Best & Taylor's Physiological Basis Of Medical Practice, 13/ E. Guyton And Hall Textbook Of Medical Physiology 12 th Edition Ganong’s Review Of Medical Physiology 24 th Edition Harrison's Principles Of Internal Medicine 18 th Edition Textbook of Medical physiology by Prof GK Pal 2 nd edition Internet References