Introduction to Physiological Principles Physiology

Ion and Water Balance Overview Environment has many different meanings External world for the whole animal Extracellular fluid for a cell Cytoplasm for intracellular enzymes Epithelial tissues provide a barrier with environment Maintains a favorable profile of solutes and solutions Overview, Cont. Animals use combinations of tissues to control ion and water balance in their environments These tissues regulate three homeostatic processes Osmotic regulation Driving force for movement of water Movement of solutes across membranes Ionic regulation Those involved in osmoregulatory strategies Nitrogen excretion Toxic end product of protein catabolism Ionic and Osmotic Challenges Marine Animals tend to gain salts and lose water Freshwater Animals tend to lose salts and gain water Terrestrial 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 Exert little control over ion profile within the extracellular space Exclusively found in marine animals For example, many invertebrates Ionoregulator Control ion profile of extracellular space For example, most vertebrates 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 Ionic and Osmotic Regulation Ability to cope with external salinities Stenohaline Can tolerate only narrow range Euryhaline Can tolerate wide range Ionic and Osmotic Regulation Table 10.1 Sources of Water and Solutes Water Dietary water Water preformed in plant and animal tissue Metabolic water Water generated as result of oxidative phosphorylation Drinking Solutes K+: intracellular fluids Na+: extracellular fluids Classification of Solutes Distinguished by their effects on macromolecules Perturbing Disrupt macromolecular function Na+, K+, Cl–, SO4+, charged amino acids Compatible Little affect on macromolecular function Polyols (glycerol, glucose) and uncharged amino acids Counteracting Disrupt function on their own Counteract disruptive effects of other solutes when employed in combination Cell Volume Cells transport solutes in and out of ECF to control cell volume Water follows solutes by osmosis Animal regulates composition of the ECF to maintain appropriate cell volume Interstitial fluid especially RVI RVD How Controlled? 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 Integument Animals change flux of water across body surface by mediating permeability of the integument Aquaporin proteins increase water permeability 100-fold Typically animals need to reduce water flux Cover external surfaces with layer of hydrophobic molecules Mucus Cornified stratum corneum with keratin For example, terrestrial amniotes Cuticle with chitin For example, arthropods Epithelial Tissue Properties for Ion Movement Four features of transport epithelia Asymmetrical distribution of membrane transporters Cells interconnected to form impermeable sheet of tissue High cell diversity within tissue Abundant mitochondria 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 Epithelial Cells in Fish Gills Fish gill lamellae composed of Mitochondria-rich chloride cells (PNA+) Pavement cells Some mitochondria-rich (PNA-) Some mitochondria-poor Transport likely carried out by mitochondria-rich cells Ion Transport by Fish Gills Direction of ion transport depends on water salinity Figure 10.11 H+ and HCO3- out for Na+ and Cl- From 0.01 mM to 100-200 mM!! Saltwater-Freshwater Transitions Some fish migrate between saltwater and freshwater (diadromous fish) Catadromous Live in freshwater, migrate to saltwater to spawn For example, eels Anadromous Live in saltwater; migrate to freshwater to spawn For example, salmon Ion transport functions of epithelia must change during migration Reverse direction of active NaCl transport Controlled by hormones Digestive Epithelia Water and salts from drinking and food transported across digestive epithelium Mediate ion and water transfers Transcellular and paracellular transport involved Driven by osmotic gradients and facilitated by aquaporins Internal and external fluids isosmotic – ions secreted into interstitial space to create a transepithelial osmotic gradient driving movement of water across tight junctions Absorbed water and salts enter blood Salt Glands Reptiles and birds possess salt glands Glands located near the eye, drain into ducts that empty near nostril Excrete hyperosmotic solutions of Na+ and Cl– Large amount of salt excreted in small volume of water Hyperosmotic solutions produced by ion pumps and a countercurrent multiplier Rectal Glands Accessory excretory organ in elasmobranchs Empties into digestive tract Actively transport Na+ and Cl– from blood into lumen of the gland Ion transport similar to chloride cell and salt glands Rate of salt excretion regulated by hormones Nitrogen Excretion Ammonia produced during amino acid breakdown is toxic and must be excreted Ammonia nitrogen excreted in three forms Ammonia (ammonioteles) Uric acid (uricoteles) Urea (ureoteles) Figure 10.15 Ammonia Excretion Advantages Ammonia released by deamination of amino acids Requires little energy to produce Disadvantages Highly toxic Requires large volumes of water to store and excrete Figure 10.16 Uric Acid Excretion Earliest evolutionary solution to permit nitrogen excretion without water Advantages Few toxic effects Can accumulate But, In humans, Gout Can be excreted in small volume of water Disadvantages Expensive to produce Figure 10.17 Figure 10.18 Urea Excretion Advantages Only slightly toxic Relatively inexpensive to produce Disadvantages Urea is a perturbing solute Cartilaginous fish use urea to increase tissue osmolarity Helps prevent water loss in marine environment Urea’s perturbing effects counteracted by methylamines (TMAO, betaine, sarcosine) The Kidney Most animals maintain ion and water balance using some form of internal organ Multiple cell types combine to produce a tubelike structure 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 Mammalian kidney has two layers Outer cortex Inner medulla Urine leaves kidney via ureter Ureters empty into urinary bladder 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 from a single arteriole Surrounded by Bowman’s capsule Capillary beds surrounding renal tubule The Nephron and Its Vasculature Figure 10.20 and Figure 10.21 Efferent always more constricted Increases pressure in the glomerulus 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 Regulated by numerous hormones and neurotransmitters that also affect cardiovascular properties 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 Retrievedfrom http://sitemaker.umich.edu/ransom.lab/files/glomerulus.jpg, 13, 03,09 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 e.g. Reabsorption of Glucose Glucose is reabsorbed by secondary active transport Reabsorbed molecules taken up by the blood Figure 10.23 Renal Threshold Figure 10.24 Secretion Similar to reabsorption, but in reverse Molecules removed from blood and transported into the filtrate Molecules secreted include K+, NH4+, H+, pharmaceuticals, and water-soluble vitamins Requires transport proteins and energy Tubule Regions Different regions of the tubule have different transport functions and permeabilities Proximal tubule Most of solute and water reabsorption Loop of Henle Descending limb Ascending limb Distal tubule Reabsorption completed for most solutes Collecting duct Drains multiple nephrons Carries urine to renal pelvis Transport in Tubule Regions Figure 10.25 Obligatory (must occur because of osmolarity of surrounding interstitial fluid) Facultative (under hormonal control) Transport in Tubule Regions Differences in transport and permeability due to differences in epithelium along the tubule Figure 10.26 Transport in the Proximal Tubule Most reabsorption of solutes and water from glomerular filtrate Active transport of Na+, glucose and AA into cortex Many solutes reabsorbed by Na+ cotransport Water follows by osmosis Taken up by peritubular capillaries and returned to venous blood Proximal tubule also carries out secretion Figure 10.27 Transport in the Loop of Henle Descending limb is permeable to water Water is reabsorbed Volume of primary urine decreases Primary urine concentrated Ascending limb is impermeable to water Ions are actively transported (thick) and reabsorbed Primary urine becomes dilute Reabsorbed ions accumulate in interstitial fluid Osmotic gradient created in medulla Osmotic Gradient Figure 10.28 Countercurrent Multiplier Loop of Henle does not directly concentrate urine, rather it acts as countercurrent multiplier Creates an osmotic gradient in medulla that facilitates reabsorption of water Low osmolarity near cortex High osmolarity deep in medulla Figure 10.28 The ability of the kidney to produce urine hyperosmotic to blood plasma is due to the loop of Henle. Countercurrent Multiplier Animation Thick ascending limb Active Na+ transport, not permeable to water ? solute concentration in surrounding fluid Descending limb Permeable to water, not to Na+ and Cl- Water flows into tubular fluid due to osmosis Thin ascending limb Permeable to ions but not water Ions move down concentration gradient Maintenance of Medullary Osmotic Gradient Vasa recta countercurrent multiplier Transport in the Distal Tubule Distal tubule can reabsorb salts and water Distal tubule can secrete potassium Transport function of distal tubule affected by hormones Parathyroid hormone increases Ca2+ reabsorption Aldosterone increases K+ secretion Figure 10.30 Excretion After urine is produced, it leaves kidney and enters urinary bladder via ureters Urine temporarily stored in bladder Urine leaves bladder via urethra Sphincters of smooth muscle control flow of urine out of bladder Opening and closing of sphincters controlled by a spinal cord reflex arc (micturition reflex) and can be influenced by voluntary controls For any substance, the amount excreted = amount in primary filtrate + amount secreted – amount reabsorbed Regulation of Urinary Function Hormones affect kidney function Steroid hormones For example, aldosterone Slow response Peptide hormones For example, vasopressin Rapid response Dietary factors that affect urine output Diuretics Stimulate excretion of water Antidiuretics Reduce excretion of water Glomerular Filtration Rate (GFR) Is determined by pressure across glomerular wall Three main forces Glomerular capillary hydrostatic pressure Bowman’s capsule hydrostatic pressure Oncotic pressure – osmotic pressure due to protein concentration in blood Intrinsic Regulators of GFR Two intrinsic pathways: both affect vasoconstriction/vasodilation of afferent arteriole Myogenic regulation Arteriolar smooth muscle Tubuloglomerular feedback Juxtaglomerular apparatus Macula densa cells in distal tubule Juxtaglomerular (JG) cells in afferent arteriole Juxtaglomerular Apparatus Figure 10.32 Intrinsic Controls of GFR Figure 10.33 Extrinsic Regulators of GFR Hormones Vasopressin (antidiuretic hormone, ADH) Renin-Angiotensin-Aldosterone (RAA) pathway Atrial natriuretic peptide (ANP) Vasopressin Also called antidiuretic hormone (ADH) Peptide hormone produced in hypothalamus and released by posterior pituitary gland ? water reabsorption from the collecting duct by ? number of aquaporins Release stimulated by ? plasma osmolarity detected by osmoreceptors in the hypothalamus Release is inhibited by ? bp detected by stretch receptors in atria and baroreceptors in carotid and aortic bodies Vasopressin Increases Cell Permeability Figure 10.34a Urine Concentration Osmotic concentration of final urine depends on permeability (aquaporins) of the collecting duct, which can be regulated by vasopressin Impermeable Water not reabsorbed from collecting duct Dilute urine (formed in ascending limb) excreted Permeable Water reabsorbed from collecting duct Concentrated urine (formed in collecting duct) excreted Aldosterone Mineralcorticoids control ion excretion Steroid hormone produced by adrenal cortex in tetrapods Aldosterone is the mineralcorticoid in tetrapods Targets cells in distal tubule and collecting duct Stimulates Na+ reabsorption and enhances K+ excretion Release stimulated by Ag II and increases in circulating K+ Aldosterone Stimulates Na+ Reabsorption Figure 10.34b What transport proteins would you expect and where would they be inserted? Renin-Angiotensin-Aldosterone (RAA) Pathway Juxtaglomerular cells secrete enzyme renin Secretion of renin controlled in three ways: Baroreceptors in JG cells release renin in response to low bp Sympathetic neurons in cardiovascular control center of medulla trigger renin secretion in response to low bp Macula densa cells in distal tubule respond to ? in flow by releasing a paracrine signal that induces JG cells to release renin Renin secreted when blood pressure or GFR lower than normal Renin-Angiotensin-Aldosterone Pathway Angiotensinogen ? renin Angiotensin I ? ACE Angiotensin II Helps regulate MAP Angiotensin II a vasoconstrictor Raises blood pressure by ? resistance Aldosterone increases Na+ (and water) retention Raises blood pressure by ? blood volume Atrial Natriuretic Peptide (ANP) Produced in specialized cells within the atria Secreted in response to stretch associated with increase in blood volume ANP increases urine output and consequently lowers blood volume and pressure Acts as an antagonist with RAA pathway Increases excretion of Na+ in urine Inhibits secretion of vasopressin Thirst Detected and controlled by hypothalamus Osmoreceptors monitor ionic concentration of body fluids Receptors monitor levels of angiotensin II Integrating Systems – Regulation of BP Invertebrates Simple animals like worm taxa have protonephridia Similar to vertebrate tubule Fluids taken from interstitial space Most developed in freshwater organisms Molluscs and annelids have more complex metanephridia Fluid taken from blood or coelom Sponges use simple contractile vacuoles to expel cellular waste, including water Insects Malpighian tubules are the insect equivalent to vertebrate kidney Empties into hindgut Primary urine formed by secretion, not filtration Reabsorption in hindgut modifies primary urine Diuretic hormones increase urine formation CRF-related diuretic hormones Insect (myo)kinins Cardioacceleratory peptides Less is known about antidiuretic hormones Chondrichthian Kidneys Sharks slightly hyperosmotic to seawater due to high urea concentrations Countercurrent arrangement recovers up to 90% of the urea from primary urine Final urine slightly hyposmotic relative to shark tissues and isosmotic to seawater Fish Kidneys Role of the kidney differs in freshwater and seawater Freshwater Ions reabsorbed from primary urine Excretion of very dilute urine Seawater Produce only small amounts of urine Most ion, water, and nitrogen excretion responsibilities met by gills and skin Some marine fish lack glomeruli (aglomerular kidney) All fish nephrons lack a loop of Henle Amphibian Kidney Structure and function of changes with metamorphosis Structure Pronephros in larval forms Tubule opens into coelom More mammal-like nephron in adult Function In aquatic life, little need for water retention Excretion of dilute urine Conserve water on land Reduce the GFR Reabsorb water from bladder Comparison of Vertebrate Nephrons Figure 10.37 Terrestrial Animals Major innovation was loop of Henle, allowing production of concentrated urine Mammals producing more concentrated urine have longer loop of Henle and relatively thicker medulla Birds and reptiles without a loop of Henle conserve water by excreting uric acid