Osmoregulation and Fluid Balance

Osmoregulation

• Osmoregulation maintains water and salt balance through selective retention and secretion
• Maintaining this balance is important for organic molecule function and optimal protein function based on inorganic ion concentration

Fluid Compartments

• Intracellular fluids (ICF) exist inside animal cells
• Extracellular fluids (ECF) exist outside animal cells, such as interstitial fluid and blood plasma
• Each fluid contains different ions, electrolytes, and organic compounds
• Fluids account for over half of an animal's body weight
• Osmoregulation occurs by regulating water in the blood and interstitial fluid, which spares individual cells the cost of disruption and allows cells to exist in a more stable environment
• ATP regulates intracellular ion composition
• Most cells are water permeable, allowing them to maintain ionic differences without osmotic differences

Transport

• Transcellular transport involves movement through cells across membranes
• Paracellular transport involves movement between cells, categorized by "leaky" or "tight" epithelia
• Some transporter types include Na-K+ATPase, and Ca2+-ATPase, as well as ion channels and electroneutral co-transporters/exchangers
• Water moves from low to high solute concentrations (high to low water potential) through osmotic gradients
• Osmotic pressure differences are caused by ion concentration differences
• Water cannot be actively pumped

Osmolarity & Volume Regulation

• Osmolarity measures solute concentration in osmols; 1 mol glucose = 1 osmol, and 1 mol NaCl = 2 osmol
• Osmolarity changes cause transmembrane osmotic gradients and affect cell volume, leading to swelling or shrinking
• Cells maintain constant volume against osmotic disturbances
• Cells activate membrane transport and/or metabolic processes to return to normal volume upon swelling or shrinking
• Volume sensing mechanisms can detect changes as small as 3%

Regulatory Mechanisms

• Regulatory volume increase (RVI) increases the osmotic gradient of solute into the cell
• Regulatory volume decrease (RVD) increases the osmotic gradient of solutes out of the cell

Homeostatic Processes

• Ionic regulation maintains concentrations of specific ions, critical for polarized animal cells
• Volume regulation maintains the total amount of water
• Cell volume is regulated by maintaining the water level inside cells and solutes
• Osmotic regulation controls osmotic pressure of circulatory bodily fluids
• Animals developed different strategies to deal with osmotic stress

Osmoconformers

• Osmoconformers' body fluids and cells match the environment's osmotic pressure
• They are mainly found in aquatic animals in the ocean averaging 1000 mOsm/L
• These animals don't actively control extracellular osmotic conditions but may control extracellular osmolytes
• They exhibit high cellular osmotic tolerance
• Cells and tissues cope with high extracellular osmolarities by increasing intracellular osmolarities with compatible osmolytes

Osmoregulators

• Osmoregulators homeostatically regulate bodily fluids' osmotic pressure
• Bodily fluids are usually different from the external environment
• They maintain constant extracellular osmolarity and ion composition through strict extracellular osmotic homeostasis
• Cells and tissues struggle to cope with large changes in extracellular osmolarity and ion concentration

Osmoregulation and Excretion in Freshwater Fish

• Freshwater fish are strong osmoregulators who maintain a steady internal osmotic condition relative to their surroundings
• Fish blood has a higher osmolarity
• They absorb more water through osmosis and lose salt
• Important to have physiological processes to compensate for ion loss
• Gills containing chloride or ionosites actively uptake salt and absorb salts from their food
• Fish produce large volumes of dilute urine and don't drink water
• Urine and gut help take up salts through gills

Osmoregulation in Marine Bony Fish

• Marine bony fish have much lower osmolarity than freshwater fish
• Marine fish lose water (osmosis) and gain salt (passive diffusion)
• Marine fish are susceptible to dehydration
• Osmoregulators have strategies that compensate to lose salt and gain water
• They have chloride cells that excrete ions
• They drink seawater and absorb water
• Their kidneys produce small volumes of isotonic urine
• Kidneys are less significant than the gills

Osmoconformers and Solutes

• Osmoconforming is a common strategy among marine invertebrates and is energetically less expensive than osmoregulation
• Their ECF is similar to seawater (1000 mOsm) and dominated by NaCl
• There's they do not lose water because there it is not a net osmotic gradient
• ICF has the same osmotic pressure as ECF with universal solutes such as K+ (400 mOsm) and organic osmolytes (600 mOsm)
• Common organic osmolytes are carbohydrates, free amino acids, methylamines, urea, and methylsulfonium solutes
• Organic osmolytes generally don't disturb macromolecules and may stabilize them against denaturing forces

Solute Classifications

• Solutes are distinguished by their effects on macromolecules
• Perturbing solutes disrupt macromolecular function and include Na+, K+, Cl-, SO4-, and charged amino acids
• Compatible solutes have little effect on macromolecular functions and include polyols like glycerol and glucose, and uncharged amino acids
• Counteracting solutes disrupt macromolecular functions on their own but counteract disruptive effects of other solutes when used in combination

Urea & Osmoconformer Types

• Urea is an osmolyte cartilaginous fish that increase tissue osmolarity
• Urea reduces water loss in a marine environment
• Methylamines counteract urea's perturbing effects
• Stenohaline osmoconformers are restricted to a narrow salinity range and cannot regulate osmolytes
• Euryhaline osmoconformers are tolerant to external salinity changes, successful in intertidal zones, and regulate organic osmolytes

Nitrogen Waste

• Nitrogen waste problems are associated with toxic ammonia production
• Ammonia production is a function of protein and nucleic acid metabolism
• Accumulation of ammonia results in death
• Ammonia soluble exertion requires large amounts of water
• Animals evolved different strategies for nitrogen waste

Nitrogen Waste Strategies

• Most aquatic species excrete N-waste as ammonia, which is energetically least expensive but requires lots of water
• Ammoniotelic animals excrete N-waste this way
• Ammonia is highly soluble in water and toxic at low concentrations
• It easily permeates membranes and diffuses out of the body into the water for invertebrates, and is excreted through the gills and from kidneys in a minor degree for fish
• Mammals, sharks, and amphibians excrete N-waste through urea and uric acid, which is energetically expensive but requires very little water

Uerotelic and Uricotelic Animals

• Uerotelic animals are less toxic than ammonia with nitrogen waste, tolerate more concentrated forms of exertion, sacrifices less water, and uses energy to create exertion
• Uricotelic animals have limited availability for water by adapting, the nitrogen waste exertion is 1000x less soluble than ammonia, less toxic so can be stored longest inside organism
• Exertion method is most energetically expensive and used for shelled eggs in vertebrates

Osmoregulation in Terrestrial Animals

• Primary organs are external surfaces like gills or skin, guts, salt glands and kidneys
• Salt glands are typically found in elasmobranchs, birds, and reptiles in seawater or deserts to get rid of excess salt or conserve water by secreting hyperosmotic NaCl solution through active NaCl transport

Kidney Function

• It is an internal organ concerned with osmoregulation with common design and physiological principles
• Nephrons in the kidney remove N-waste and regulates water to produces urine
• The nephron is the main functional unit with a collecting area, proximal/distal tubule, storage bladder, and final duct Kidney functions include regulation of water and solute levels and the removal of N-waste/metabolic wastes
• Functions are a result of extensive interaction between blood and tubules and membrane exchange mechanisms

Mammalian Kidneys

• Mammals have a pair of kidneys that the renal artery brings blood into
• Blood is taken out by the renal vein
• There are two main kidney structures: renal cortex and renal medulla and the renal medulla is composed of renal pyramids

Nephron & Kidney Physiology

• Renal pyramids converge into structures called renal pelvis, which connects to the ureter
• The nephron is the main functional unit in the kidney
• In human nephrons, the proximal and distal tubules connect via the loop of Henle
• Juxtamedullary nephrons have long loops that go into the renal medulla and produce concentrated hyperosmotic urine
• Cortical nephrons have short loops in the cortex and primarily reabsorb solutes from the urine into the blood

The Nephron (anatomy)

• The glomerulus, encapsulated by the glomerular capsule, constitute the renal corpuscle
• the afferent arteriole brings blood into the glomerulus for filtering, the filtered blood then exits the glomerulus through the efferent arteriole
• The proximal convoluted tubule connects the renal corpuscle
• The loop of Henle has two regions called descending and ascending
• The peritubular capillaries surrounding the loop of Henle help concentrate urine and maintain an osmotic gradient in the renal medulla
• The distal convoluted tubule opens into the collecting duct

Kidney Physiology

• Ultrafiltration of the glomerulus which receives blood through afferent arteriole, then undergoes filtration
• Filtering of blood into the tubule creates urine
• Reabsorption process absorbs needed substances, where they move from tubular fluid across the tubular wall into the vasa recta
• Secretion is the secretion of substances (for elimination) that move from the vasa recta across the tubular wall into the tubular fluid
• Excretion is filtration-reabsorption+secretion; excretion is urination

Glomerular Filtration

• The first step in urine formation separates the plasma fraction of the blood
• Glomerular filtration is driven by blood hydrostatic pressure and filtrate (small solutes and water) has waste, and essential molecules but filtrate has no cells
• There is very little protein in filtrate

Renal Corpuscle

• Blood flows into the glomerulus through the afferent arteriole and out through the efferent arteriole
• The afferent arteriole diameter is greater than the efferent, which creates hydrostatic pressure for ultrafiltration
• The glomerular capillaries are leaky
• Blood in the glomerular capillaries is separated from the Bowman's capsule
• The layers includes a single layer of endothelial cells that contain gaps called finestra as well as a basement membrane,
• The layer of epithelial cells called podocytes forms filtration structure

Glomerular Filtration Forces

• Capillary blood pressure favors filtration
• Plasma-colloid osmotic pressure opposes filtration
• Bowman's capsule hydrostatic pressure opposes filtration
• Net filtration pressure=the difference between force favoring filtration and force opposing filtration

Glomerular Filtration Rate

• Glomerular filtration rate (GFR) is ~125 ml/min, 7.5 liters per hour, 180 liters per day
• The entire plasma volume is filtered every 45 minutes
• ~99% of the filtrate is reabsorbed by the nephron tubules

Osmoregulation in Terrestrial Animals & Proximal Convoluted Tubules

• Transports water and it's solutes
• Solute and water reabsorption is proximal convoluted tubule
• Reabsorption includes 65% of filtered water
• Reabsorption includes 65-70% of filtered Na+ and Cl-, and also glucose and amino acids
• Secretion has variable proton secretion for acid/base regulation
• Secretion also has organic molecule excretion
• All tubular exchange with blood occurs via a single layer of renal epithelial cells, where they have different function properties, performing exchange either transcellularly or paracellularly

Loop of Henle

• The loop of Henle consists of ascending and descending limbs and changes that occur in tubular fluid in descending limb are distinct from ascending limb
• Establishes an osmotic gradient for water reabsorption
• Reabsorbs 20% of filtered water, Na+, and Cl-
• In descending limbs, water can permeate, but not NaCl; water moves out, concentrating the filtrate in the medulla
• In ascending limbs, NaCl is actively reabsorbed with active Na+ and Cl- reabsorption and the limb is impermeable to water
• The process concentrates interstitial fluid in the medulla and dilutes ultrafiltrate and water stays in
• Water is staying in however salts are actively moved out
• Because of this the filtrate becomes more dilute because there is no net movement of water yet salts are actively moved out and osmotic pressure of filtrate decreases

Urea

• Urea contributes to the osmotic gradient and accumulates in the renal medulla

Distal Convoluted Tubules

• The fluid volume is only about 20% of original filtered volume
• This fluid is hypotonic to plasma
• ~7.5% of filtered sodium is reabsorbed
• Distal Convoluted Tubules is not permeable to water, so water remains as salts are actively transported

Collecting Ducts

• The main function is to concentrate urine by using the osmotic gradient in the medulla and hormone regulation
• Water permeability is regulated by antidiuretic hormone (vasopressin)
• The ADH regulatory system in the hypothalamus and posterior pituitary regulates urine concentration by dehydration activation increases in water
• This causes permeability to water in the collecting duct and DCT , increasing absorption of water and when water balance is restored, negative feedback occurs to reduce ADH secretion
• Alcohol suppresses ADH synthesis and dehydration happens

Osmoregulation and Excretion in Terrestrial Invertebrates

• Nephridia
• Protonephridia (flatworms)
• Metanephridia (mollusks)
• Excretory tubules (insects)

Protonephridia

• These structures consist of a network of closed tubules throughout the body Interstitial fluid filters into lumen at flame bulb

Freshwater Flatworms and Parasitic Flatworms

• Freshwater Flatworms primarily operate with osmoregulation and most waste diffuses out of the body or in the gastrovascular cavity

• Parasitic Flatworms live in an isotonic environment and therefore do mainly exertion of nitrogenous waste

Metanephridia

• These structures filter coelomic fluid, taking them to reabsorb solutes and waste

Malpighian Tubules

• Part of the hindgut
• K+ is secreted into the tubule lumen, drawing Cl- ions/water in, resulting in the secretion of nitrogenous wastes

Respiration

• Exchange occurs between the respiratory gases O2 and CO2 and meets a constant and urgent necessity due to its role as a terminal electron acceptor and is more efficient in aerobic respiration
• Organisms require oxygen to create energy for bodily functions, otherwise, they will die due to their dependence on aerobic respiration for energy
• External respiration transports O2 into the body and CO2 out of the body through a gas exchange membrane (thin layer of 1-2 epithelia)
• Separates the internal tissues from the environmental medium