Integrative Physiology II: Fluid and Electrolyte Balance Chapter 20a Integrative Physiology II: Fluid and Electrolyte Balance

Fluid and electrolyte homeostasis Water balance About this Chapter Fluid and electrolyte homeostasis Water balance Sodium balance and ECF volume Potassium balance Behavioral mechanism in salt and water balance Integrated control of volume and osmolarity Acid-base balance

Fluid and Electrolyte Homeostasis Na+ and water ECF volume and osmolarity K+ Cardiac and muscle function Ca2+ Exocytosis, muscle contractions, and other functions H+ and HCO3– pH balance Body must maintain mass balance Excretion routes: kidney and lungs

Fluid and Electrolyte Homeostasis The body’s integrated responses to changes in blood volume and blood pressure Blood volume Blood pressure Volume receptors in atria and carotid and aortic baroreceptors Cardiovascular system Cardiac output, vasoconstriction Thirst causes water intake ECF and ICF volume Behavior Kidneys Conserve H2O to minimize further volume loss trigger homeostatic reflexes Stimulus Receptor Effector Tissue response Systemic response (a) KEY Figure 20-1a

Fluid and Electrolyte Homeostasis Blood volume Blood pressure Volume receptors in atria, endocrine cells in atria, and carotid and aortic baroreceptors trigger homeostatic reflexes Cardiovascular system Kidneys Cardiac output, vasodilation Excrete salts and H2O in urine ECF and ICF volume KEY Stimulus Receptor Effector Blood pressure Tissue response Systemic response (b) Figure 20-1b

Metabolic production 0.3 L/day Water Balance Water gain Water loss 2.2 L/day Food and drink Insensible water loss 0.9 L/day Skin Lungs 0.3 L/day Metabolism Urine 1.5 L/day Feces 0.1 L/day 2.5 L/day Totals 2.5 L/day Intake 2.2 L/day Metabolic production 0.3 L/day Output 2.5 L/day + – = 0 Figure 20-2

The kidneys conserve volume but cannot replace lost volume Water Balance The kidneys conserve volume but cannot replace lost volume Volume gain Volume loss GFR can be adjusted. Glomerular filtration rate (GFR) If volume falls too low, GFR stops. Body fluid volume Kidneys recycle fluid. Kidneys conserve volume. Volume loss in the urine Regulated H2O reabsorption Figure 20-3

Osmolarity changes as filtrate flows through the nephron Urine Concentration Osmolarity changes as filtrate flows through the nephron 50–1200 mOsM urine excreted 300 mOsM 600 mOsM 900 mOsM 1200 mOsM 1200 300 100 Distal tubule Proximal tubule Collecting duct Loop of Henle CORTEX MEDULLA Permeability to water and solutes is regulated by hormones. Variable reabsorption of water and solutes Ions reabsorbed but no water Only water reabsorbed Isosmotic fluid leaving the proximal tubule becomes progressively more concentrated in the descending limb. Removal of solute in the thick ascending limb creates hyposmotic fluid. Hormones control distal nephron permeability to water and solutes. Urine osmolarity depends on reabsorption in the collecting duct. 1 2 3 4 Figure 20-4

Urine Concentration Figure 20-4, step 1 Proximal tubule Distal tubule Isosmotic fluid leaving the proximal tubule becomes progressively more concentrated in the descending limb. 300 mOsM CORTEX 300 100 300 mOsM MEDULLA 1 Permeability to water and solutes is regulated by hormones. Ions reabsorbed but no water 600 mOsM Only water reabsorbed Variable reabsorption of water and solutes 900 mOsM Loop of Henle Collecting duct 1200 1200 mOsM 50–1200 mOsM urine excreted Figure 20-4, step 1

Vasopressin makes the collecting duct permeable to water Water Reabsorption Vasopressin makes the collecting duct permeable to water Figure 20-5a

Water Reabsorption Figure 20-5b

Medullary interstitial fluid Water Reabsorption Vasopressin causes insertion of water pores into the apical membrane Cross section of kidney tubule Collecting duct lumen Medullary interstitial fluid Vasa recta Collecting duct cell 1 Vasopressin binds to membrane receptor. Filtrate 300 mOsM 600 mOsM H2O 600 mOsM 2 Receptor activates cAMP second messenger system. H2O H2O H2O 4 700 mOsM Storage vesicles 3 Cell inserts AQP2 water pores into apical membrane. Second messenger signal 2 Exocytosis of vesicles 1 Aquaporin-2 water pores Vasopressin 4 Water is absorbed by osmosis into the blood. 3 cAMP Vasopressin receptor Figure 20-6

Medullary interstitial fluid Water Reabsorption Cross section of kidney tubule Collecting duct lumen Medullary interstitial fluid Vasa recta Collecting duct cell 1 Vasopressin binds to membrane receptor. Filtrate 300 mOsM 600 mOsM H2O 600 mOsM 2 Receptor activates cAMP second messenger system. H2O H2O H2O 4 700 mOsM Storage vesicles 3 Cell inserts AQP2 water pores into apical membrane. Second messenger signal 2 Exocytosis of vesicles 1 3 Aquaporin-2 water pores cAMP Vasopressin 4 Water is absorbed by osmosis into the blood. Vasopressin receptor Figure 20-6, steps 1–4

Factors Affecting Vasopressin Release Figure 20-7

Water Balance The effect of plasma osmolarity on vasopressin secretion by the posterior pituitary Figure 20-8

Countercurrent Heat Exchanger Warm blood Cold blood Warm blood Warm blood Heat lost to external environment Limb (a) (b) Figure 20-9

Countercurrent exchange in the medulla of the kidney Water Balance Filtrate entering the descending limb Blood in the vasa recta The ascending limb pumps out Na+, K+, and Cl– Countercurrent exchange in the medulla of the kidney 300 mOsM 300 mOsM 300 mOsM 100 mOsM 500 500 600 600 600 600 900 900 900 1200 Vasa recta 900 1200 mOsM 1200 mOsM KEY H2O = K+ = Loop of Henle Cl– = Na+ = (a) Figure 20-10a

Cells of ascending loop of Henle Ion reabsorption (b) KEY 1200 mOsm entering ascending loop of Henle Salt reabsorption Water cannot follow solute Cells of ascending loop of Henle Interstitial fluid 100 mOsm leaving the loop H2O = Cl– = Na+ = K+ = 1 2 3 4 Active reabsorption of ions in the thick ascending limb creates a dilute filtrate in the lumen Figure 20-10b

Fluid and Electrolyte Balance Vasa recta removes water Close anatomical association of the loop of Henle and the vasa recta Urea increases the osmolarity of the medullary interstitium

Homeostatic responses to salt ingestion Sodium Balance Homeostatic responses to salt ingestion Figure 20-11

Sodium Balance Figure 20-12 P cell of distal nephron Interstitial fluid Blood 1 Aldosterone combines with a cytoplasmic receptor. Lumen of distal tubule 2 Hormone-receptor complex initiates transcription in the nucleus. 2 1 Aldosterone 3 Translation and protein synthesis Aldosterone receptor 3 New protein channels and pumps are made. New channels New pumps ATP 4 Proteins modulate existing channels and pumps 4 Aldosterone-induced proteins modify existing proteins. K+ secreted K+ 5 K+ ATP K+ Na+ reabsorbed Na+ 5 Result is increased Na+ reabsorption and K+ secretion. Na+ Na+ Figure 20-12

The renin-angiotensin-aldosterone system (RAAS) Sodium Balance The renin-angiotensin-aldosterone system (RAAS) constantly produces Liver Angiotensinogen in the plasma Blood pressure Granular cells (kidney) produce Renin ANG I in plasma Blood vessel endothelium contains ACE (enzyme) ANG II in plasma Adrenal cortex Arterioles Cardiovascular control center in medulla oblongata Hypothalamus Aldosterone Vasoconstrict Cardiovascular response Vasopressin Thirst Na+ reabsorption Volume and maintain osmolarity Blood pressure Figure 20-13

Decreased blood pressure stimulates renin secretion Sodium Balance Decreased blood pressure stimulates renin secretion Blood pressure Cardio- vascular control center GFR direct effect NaCl transport Sympathetic activity across Macula densa of distal tubule Granular cells of afferent arteriole Paracrines Renin secretion Figure 20-14

Natriuretic peptides promote salt and water excretion Sodium Balance Natriuretic peptides promote salt and water excretion Increased blood volume causes increased atrial stretch Myocardial cells stretch and release Natriuretic peptides Adrenal cortex Medulla oblongata Hypothalamus Kidney Less vasopressin Increased GFR Decreased renin Decreased blood pressure Less aldosterone NaCl and H2O excretion Figure 20-15