Control of Water Intake and Body Fluids Lecture 4 PDF

Control of water Intake and body fluids Fernando Gomez-Pinilla, Ph.D. Lecture 4; 10/16/23; C144/244 F. Gomez-Pinilla, C144/244; UCLA F. Gomez-Pinilla, C144/244; UCLA Water contents vary across organs F. Gomez-Pinilla, C144/244; UCLA Fluids moves across compartments (cell, extracellular fluid...

Control of water Intake and body fluids Fernando Gomez-Pinilla, Ph.D. Lecture 4; 10/16/23; C144/244 F. Gomez-Pinilla, C144/244; UCLA F. Gomez-Pinilla, C144/244; UCLA Water contents vary across organs F. Gomez-Pinilla, C144/244; UCLA Fluids moves across compartments (cell, extracellular fluid, blood) F. Gomez-Pinilla, C144/244; UCLA Interstitial fluid acts as a reservoir for plasma F. Gomez-Pinilla, C144/244; UCLA Fluids moves across tissues according to hydrostatic force from heart and osmotic pressure at the capillary bed • Intracellular fluid (ICF) • Extracellular fluid (ECF) • ICF and ECF differ in solute contents but the osmolality remains in equilibrium across the two compartments • Hydrostatic pressure and osmotic pressure of blood determine direction of flux F. Gomez-Pinilla, C144/244; UCLA Fluid moves from capillaries to tissue and back to blood. Fluid exchange occurs in the capillary bed Hydrostatic pressure of blood depends on cardiac output, arteriolar vasoconstriction, and the opposing oncotic pressure F. Gomez-Pinilla, C144/244; UCLA Main forces affecting distribution of fluids across capillary and tissue Hydrostatic pressure and osmotic pressure of blood determine direction of flux F. Gomez-Pinilla, C144/244; UCLA Fluid moves from capillary to tissue and then back again Hydrostatic pressure of blood depends on cardiac output, arteriolar vasoconstriction, and the opposing oncotic pressure F. Gomez-Pinilla, C144/244; UCLA Colloidal osmotic (oncotic) pressure produced by proteins moves fluid back to circulation F. Gomez-Pinilla, C144/244; UCLA Arteries have thick walls while veins are distensible F. Gomez-Pinilla, C144/244; UCLA Balance between solutes and water determines fluid homeostasis as regulated by ANS and hormones Osmolality= solute (osmoles)/water (Kg) Solutes: osmotic homeostasis Water: Volume homeostasis Hormonal, behavior The body control fluids by regulating solutes and amount of water in blood Fernando Gomez-Pinilla, UCLA Control of osmotic homeostasis via AVP • Osmolality= solute (osmoles)/water (Kg) • Osmoreceptors in vascular organ of lamina terminalis (OVLT)detects plasma hyperosmolarity and inform magnocellular AVP-secreting cells in hypothalamic SON and PVN. • Arginine Vasopressin (AVP or ADH) is produced in neurohypophysis. AVP acts on the kidney to increase water permeability (aquaporin channels) in the collecting ducts of nephrons, resulting in water reabsorption F. Gomez-Pinilla, C144/244; UCLA When osmolality is too high kidney can secrete salt • Natriuresis: increase in intravascular volume leads to excretion of NaCl by kidney through the hormone atrial natriuretic peptide (ANP, synthesized in heart atrial) and oxytocin • Thirst helps to restore osmolality but secondary to internal mechanisms (i.e., AVP secretion) • What happens to your body weight if you reduce salt intake? F. Gomez-Pinilla, C144/244; UCLA Circulating levels of AVP change according to blood pressure (volume), and plasma osmolality Circulating AVP is proportional to plasma osmolality at low plasma AVP, small changes in plasma AVP lead to large changes in urine volume Diabetes insipidus: impaired secretion of AVP leads to large loss of water by urination. Extreme thirst is typical F. Gomez-Pinilla, C144/244; UCLA Plasma osmolality and blood volume are co-regulated: plasma AVP production varies upon blood volume and pressure Effects of hypovolemia and hyperosmolality are additive F. Gomez-Pinilla, C144/244; UCLA Control of volume homeostasis • Loss of blood volume (hypovolemia, loss of blood pressure) stimulates water retention for restoring volume and blood pressure • Hypovolemia is first detected by stretch receptors in heart atrium. Also stretch receptors in carotid sinus and aortic arch. NSTbrainstem. -- baroreceptor reflex. • The main reaction is to conserve water via urine retention F. Gomez-Pinilla, C144/244; UCLA Maintenance of blood volume Hypovolemia is first detected by stretch receptors in heart atrium, carotid sinus, and aortic arch, and kidney afferent arterioles. NSTbrainstem. -baroreceptor reflex. • Kidney secretes renin resulting in angiotensin II (AII) • AII stimulates AVP secretion by acting on SFO • Osmotic dilution (loss of NaCl) inhibits thirst and AVP secretion • Angiotensin II (and ACTH) stimulates aldosterone in adrenal cortex which helps retain NaCl by kidney F. Gomez-Pinilla, C144/244; UCLA A drop in blood pressure stimulate reninangiotensin system Main effects are vasoconstriction and Na retention to increase blood pressure (Leading to Na retention) F. Gomez-Pinilla, C144/244; UCLA CNS control of drinking • Thirst response: initiated by forebrain osmoreceptors in OVLT responding to increases in osmolality of circulating plasma. Thirst is triggered when dehydration is getting severe • Water retention caused by release of antidiuretic hormone arginine vasopressin (AVP) by post pituitary • Inhibitory signals from oropharinx, stomach, memory drinking may modulate drinking.. • Gastric distention may also play a role F. Gomez-Pinilla, C144/244; UCLA Signals controlling Thirst Response F. Gomez-Pinilla, C144/244; UCLA Plasma membrane has channels that can regulate movement of water: aquaporin channels F. Gomez-Pinilla, C144/244; UCLA How to lose body water (and weight) • Practice regular exercise • Reduce stress, cortisol stimulates the antidiuretic hormone ADH (stress may increase water retention) • Reduce salt intake • Reduce carbohydrate intake, 3-4 grams of water are needed for every gram of glycogen • Drink coffee, caffeine acts a diuretic • Stay hydrated by drinking sufficient water – what happens if we drink TOO MUCH water? F. Gomez-Pinilla, C144/244; UCLA Water contents decline with age F. Gomez-Pinilla, C144/244; UCLA Reduction of body water may affect many functions F. Gomez-Pinilla, C144/244; UCLA Take home question: How to reduce water retention? F. Gomez-Pinilla, C144/244; UCLA Take Home questions: Why women have less water than men (as a percentage)? Why people with more fatty tissue have less water (as a percentage)? https://www.youtube.com/watch?v=mj3c5NOJ2xk F. Gomez-Pinilla, C144/244; UCLA