C H A P T E R 10 Ion and Water Balance PowerPoint Lecture Slides prepared by Stephen Gehnrich, Salisbury University
Overview Three homeostatic processes Osmotic regulation Osmotic pressure of body fluids Ionic regulation Concentrations of specific ions Nitrogen excretion Excretion of end-products of protein metabolism
Ionic and Osmotic Challenges Marine environments Animals tend to gain salts and lose water Freshwater environments Animals tend to lose salts and gain water Terrestrial environments Animals tend to lose water Many animals move between environments and must be able to alter their homeostatic mechanisms
Ionic Regulation Strategies to meet ionic challenges Ionoconformer Ionoregulator
Osmotic Regulation Strategies to meet osmotic challenges Osmoconformer Internal and external osmolarity similar For example, marine invertebrates Omoregulator Osmolarity constant regardless of external environment For example, most vertebrates Ability to cope with external salinities Stenohaline Can tolerate only narrow range Euryhaline Can tolerate wide range
Osmolarity (mol/l): the osmolar concentration of a solution (commonly used in biology Osmolality (mol/kg): the osmolal concentration of a solution Osmotic concentration (Osm) Salinity (%0, ppt, parts per thousand) NaCl 150 mmol Water 1L Dissolve to Na+ 150 mmol, Cl- 150 mmol Osmolarity = 300 mmol/l Osmotic concentration = 300 mosm NaCl 500 mmol Osmolarity = 1000 mmol/l Osmotic concentration = 1000 mosm Salinity = 500*58 =29 g/l (ppt)
Osmotic properties of cells extracellular ion concentration = intracellular ion concentation? Body fluid (extracellular) osmolarity = intracellular osmolarity?
Osmoregulation in fishes (cyclostome) (freshwater fishes) (Euryhaline fishes) (seawater fishes) freshwater seawater
Ionic and Osmotic Regulation Table 10.1
4 types of strategies for facing salt and water problems Osmoconformer Ionconformer Osmoconformer Ionregulator Osmoregulator Ionregulator Osmoregulator Ionregulator
Classification of Solutes Figure 10.4
Osmoconformers (but not ion-conformer) The cells of osmoconformers are able to cope with high extracellar Osmolarity by increasing their intracellular osmolarity. Organic osmolytes: urea, trimethylamine oxide (TMAO) Elasmobranch: shark, ray coelacanth crab-eating frog hagfish
Marine elasmobranch are hyperosmotic but hypoionic to seawater Shark rectal gland
Most types of FW animals share similar regulatory mechanism Teleost Ray (elasmobranches) Lamprey (cyclostomes) Frog Soft-shell turtle Mussel Crayfish Leech Mosquito larvae
Strategies of FW fishes: Possessing an integument with a low permeability to salts and water Do not drink water Production of dilute urine Reabsorption of salts from kidney Ingesting salts from food Active absorption of salts from skin (amphibian) or gills (fish)
Osmoregulation in marine animals Marine teleosts Strategies: Possessing an integument with a low permeability to salts and water Drink seawater Production of isotonic urine Excretion of salts from kidney (Mg 2+, SO 4 2- ) Active secretion of salts from gills (Na +, Cl - )
Air-breathing animals: sea bird, sea turtle, iguanas, Osmoregulatory problems: Dehydration through their respiratory epithelia Strategies: Drink seawater Production of isotonic urine Active secretion of salts from salt glands (Na +, Cl - )
Salt gland in sea birds (nasal gland)
Shark rectal gland Salt gland of Sea turtle Salt gland of marine Iguanas
Ion- and osmo-regulation of animals: from molecular to cellular function
Relative permeability of phosolipid bilayer (cell membrane) to molecules and ions
Mechanisms for trans-membrane movement of ions Diffusion (move down electrochemical gradient) Passive transport (move down electrochemical gradient) Active transport (move against electrochemical gradient) Passive transport: Ion channels Facilitated Carrier proteins diffusion Active transport: Primary active transport Secondary active transport
5 types of ion transporters Primary active transport Passive transport Passive transport secondary active transport cotransporter exchanger
Passive transport through passive transporter Facilitated diffusion
Primary active transport through Na + /K + -ATPase (Na + pump)
3 major types of active transporters: Na pump (Na/K-ATPase) Ca pump (Ca-ATPase) H pump (H-ATPase, V-ATPase)
Secondary active transportrs Na/H exchanger Na/Ca exchanger Na/K/Cl cotransporter Cl/HCO 3 exchanger
Kinetics of various ion transport proteins
Epithelial Tissue Epithelial tissues form boundary between animal and environment External surfaces For example, skin Internalized surfaces For example, lumen of digestive and excretory systems Epithelial tissues have physiological functions in respiration, digestion, and ion and water regulation
Epithelial Tissue Properties for Ion Movement Four features of transport epithelia Asymmetrical distribution of membrane transporters Solutes selectively transported across membrane Cells interconnected to form impermeable sheet of tissue Little leakage between cells High cell diversity within tissue Abundant mitochondria Large energy (ATP) supply
Epithelial Tissue Properties for Ion Movement Figure 10.8
Solute Movement Epithelial cells use two main routes of transport Transcellular transport Movement through the cell across membranes Paracellular transport Movement between cells Leaky vs. tight epithelia Types of transporters Na + /K + ATPase Ion channels (Cl, K +, Na + ) Electroneutral cotransporters Electroneutral exchangers
Transcellular and Paracellular Transport Figure 10.9
Measurement of trans-epithelial ion transport Ussing chamber Voltage/ Ion clamp technique (Short Circuit Current Vs = 0) Cl - Transepithelial potential Vout = 0 Vin = -30 mv
Epithelial Cells in Fish Gills Fish gill lamellae composed of Mitochondria-rich chloride cells Pavement cells Some mitochondria-rich?? Some mitochondria-poor Transport likely carried out by mitochondria-rich cells
Salt secretion in marine teleosts Salt-secreting cells (chloride cells) Gill filament
Branchial chloride cells in gill filament of marine teleosts lamellae filament
Salt-secreting cells present in : rectal gland of shark, sea bird, sea turtle gills of marine teleosts K channel Epithelial Cl channel (CFTR) Na/K ATPase Na/K/2Cl cotransporter Paracellular pathway Leaky junction
Epithelial Cells in Fish Gills (FW fish) Figure 10.10
Ion Transport by Fish Gills Direction of ion transport depends on water salinity Figure 10.11
Hypothesis of Na, Cl uptake in freshwater fish Gas exchange Ion exchange ph regulation Ammonia excretion
Renal physiology
The Kidney Vertebrate kidneys have six roles in homeostasis Ion balance Osmotic balance Blood pressure ph balance Excretion of metabolic wastes and toxins Hormone production
Kidney Structure and Function Figure 10.19
The Nephron Functional unit of the kidney Composed of Renal tubule Lined with transport epithelium Various segments with specific transport functions Vasculature Glomerulus Ball of capillaries Surrounded by Bowman s capsule Capillary beds surrounding renal tubule
Structure of nephron 15% Renal corpuscle Proximal tubule Henle s loop Distal tubule Collecting duct Glomerulus Bowman s capsule Proximal convoluted tubule Proximal straight tubule Descending thin limb of Henle s loop Ascending thin limb of Henle s loop Thick ascending limb of Henle s loop Distal convoluted tubule Cortical collecting duct Medullary collecting duct Renal pelvis Ureter Bladder Urethra
Urine Production Four processes Filtration Filtrate of blood formed at glomerulus Reabsorption Specific molecules in the filtrate removed Secretion Specific molecules added to the filtrate Excretion Urine is excreted from the body
Filtration Liquid components of the blood are filtered into Bowman s capsule Water and small solutes cross glomerular wall Blood cells and large macromolecules are not filtered Glomerular capillaries are very leaky Podocytes with foot processes form filtration structure Mesangial cells control blood pressure and filtration within glomerulus Filtrate flows from Bowman s capsule into proximal tubule
Blood pressure in the renal glomerulus
GFR Net filtration pressure Permeability of bowman s capsule blood A B C urine blood protein A: endothelium (pores) B: basement membrane C: podocyte (filtration slit)
Intrinsic control of GFR: 1. Autoregulation of blood pressure of afferent arteriole 2. Renal blood flow regulated by juxtaglomerular apparatus (macula densa, juxtaglomerular cells) 3. Sympathetic activation causes vasocontriction of afferent arteriole and reduces hydraulic permeability (podocytes) of Bowman s capsule
Extrinsic Regulators of GFR Hormones Vasopressin (antidiuretic hormone, ADH) Renin-Angiotensin-Aldosterone (RAA) pathway Atrial natriuretic peptide (ANP)
Reabsorption Primary urine Initial filtrate filtered in Bowman s capsule that is isosmotic to blood Most water and salt in primary urine reabsorbed using transport proteins and energy Rate of reabsorption limited by number of transporters Renal threshold Concentration of a specific solute that will overwhelm reabsorptive capacity Each zone of the nephron has transporters for specific solutes
Reabsorption of Glucose Glucose is reabsorbed by secondary active transport Reabsorbed molecules taken up by the blood Figure 10.23
Renal clearance of substance = Amount of substance in urine Amount of substance filtered Glucose clearance= 0 (100% reabsorption) Inulin clearance=1 (no reabsorption, secretion) GFR (L/h) = Inulin in urine (mg/h) Inulin in blood (mg/l)
Transport in Tubule Regions Figure 10.25
Transport in the Proximal Tubule Most reabsorption of solutes and water takes place in proximal tubule Many solutes reabsorbed by Na + cotransport Water follows by osmosis Proximal tubule also carries out secretion Figure 10.27
Secretion Similar to reabsorption, but in reverse Molecules removed from blood and transported into the filtrate Molecules secreted include K +, NH 4+, H +, pharmaceuticals, and water-soluble vitamins Requires transport proteins and energy
Ion and Water Transport in the Loop of Henle Descending limb is permeable to water Water is reabsorbed Volume of primary urine decreases Primary urine becomes more concentrated Ascending limb is impermeable to water Ions are reabsorbed Primary urine becomes dilute Reabsorbed ions accumulate in interstitial fluid An osmotic gradient created in the medulla
Urine concentration: Henle s loop Reabsorption of water 70% Reabsorption of water 30% (regulatory region)
Primary active Na reabsorption Coupling of water reabsorption to Na reabsorption Osmolarity increase water
Proximal tubule: Na reabsorption Na/glucose cotransporter Na/K/Cl cotransporter Ascending limb of Henle s loop: Na, reabsorption Na/H exchanger
Urine concentration: countercurrent multiplier system Renal cortex low Interstitial osmolarity high Renal medulla
Countercurrent flow of circulation (vasa recta)
Renal Na and water regulation: Control of GFR (short-term) Control of Na reabsorption (long-term) Renin-angiotensin Aldosterone baroreceptor hormones Atrial natriuretic peptide (ANP) Vasopressin
Renin-angiotensin system and aldosterone Activity of renal sympathetic nerve Intrarenal Baroreceptor (JGA)
Renin-angiotensin system Angiotensin-converting enzyme (ACE) Aldosterone (mineral corticoid)
Atrial natriuretic peptide (ANP) stimulates Na excretion
Vasopressin Also called antidiuretic hormone (ADH) Peptide hormone Produced in hypothalamus and released by posterior pituitary gland Increases water reabsorption from the collecting duct by increasing number of aquaporins Release stimulated by increasing plasma osmolarity detected by osmoreceptors in the hypothalamus Release is inhibited by increasing blood pressure detected by stretch receptors in atria and baroreceptors in carotid and aortic bodies
Regulation of water reabsorption Collecting duct Aquaporin (water channel) Vasopressin regulated
Vasopressin Increases Cell Permeability Figure 10.34a
Renal (metabolic) acid-base regulation in distal tubule and collecting duct A-type intercalated cell Acid secretion Apical H-ATPase (proton-atpase) B-type intercalated cell Base (bicarbonate) secretion Apical Cl/bicarbonate exchanger (anion exchanger)