Electrolytes 1

• An electrolyte is a compound that dissociates into ions when dissolved in water.
• Water and electrolyte balance is associated with excretion - animals produce urine to excrete waste and this leads to water loss.
• Electrolytes and water move through organisms by diffusion and osmosis respectively.
• Osmosis

  • Movement of water down its concentration gradient across a semipermeable membrane
  • Water moves quickly across lipid bilayers
  • Special case of diffusion called osmosis
  • Only occurs across selectively permeable membranes
  • A solution’s osmolarity is the concentration of solutes in a solution, measured in osmoses/litre
• Osmotic Stress

  • Occurs when concentration of dissolved substances in a cell or tissue is abnormal
  • Osmoregulation is the process by which organisms control the concentration of water and solutes within their bodies
  • Organisms such as sponges and jellyfish do not osmoregulate
  • Seawater nearly matches the electrolyte concentrations found within these animals - osmoconformers
  • Seawater is isosmotic in comparison to the cells and tissues (concentrations equal)
  • Osmoregulation is required in marine vertebrates because seawater is hyperosmotic to their tissues
• Osmotic Stress in Seawater

  • Water tends to flow by osmosis out of the gill epithelium - marine fishes must replace the water or cells will shrivel and die
  • Marine fishes must drink large amounts of water to replace the loss of water, which also brings more electrolytes
  • To rid themselves of these excess electrolytes, marine bony fishes actively pump ions out into the seawater
  • Membrane proteins in the gill epithelium carry out this process
  • The fishes also lose electrolytes by excreting small quantities of highly concentrated urine
• Osmotic Stress in Fresh Water

  • Freshwater animals are under stress cause they gain water and lose solutes
  • Freshwater is hyposmotic to fish tissues
  • If fish does not get rid of incoming water, its cells will burst and it will die
  • To achieve homeostasis, they excrete large amounts of water in their urine and don’t drink
  • Electrolytes diffuse out of the gill epithelium into the environment
  • The fishes replace electrolytes by eating food or actively transporting them into the body
• Osmotic Stress on Land

  • Terrestrial animals constantly lose water to the environment by evaporation
  • Epithelial cells in respiratory structures have a moist surface to promote gas exchange
  • But this allows for a large amount of water loss through evaporation
  • There is a trade-off between gas exchange and osmoregulation
  • Animals replace loss of water by drinking, ingesting water from food, or by metabolic pathways
• Electrolyte Transport Across Cell Membranes

  • There are no known mechanisms for actively transporting water across membranes
  • Cells use pumps to transport ions to set up osmotic gradients - water follows by osmosis often through aquaporins
• Marine Fish and Osmoregulation

  • Shark rectal gland secretes a concentrated salt solution
  • Early experiments show that normal salt excretion occurred only in the solution in the rectal gland contained ATP
• • Salt excretion is a multistep process:

  1. Na+/K+-ATPase pumps Na+ out of the epithelia cells across the basolateral surface into the interstitial fluid.
  2. Na+, Cl−, and K+ enter the cell, powered by the Na+ gradient.
  3. Chloride channels allow Cl− to diffuse down its concentration gradient into the lumen of the gland located in the apical membrane.
  4. Na+ diffuses into the lumen of the gland, following its electrochemical gradient.
• In many animals, epithelial cells that transport Na+ and Cl− have the same membrane proteins as found in the shark rectal gland

  • These species include:
  • Marine birds and reptiles that drink salt water and excrete NaCl via glands in their nostrils
  • Marine fish that excrete salt from their gills – Mammals that transport salt in their kidneys
• Salt Excretion

  • Research on the shark rectal gland also had an unforeseen benefit for biomedical research
  • A human protein called cystic fibrosis transmembrane regulator (CFTR) was identified and found to be 80% identical to the shark chloride channel
  • Subsequent studies supported the hypothesis that cystic fibrosis results from a defect in a chloride channel
• Kidney

  • Osmoregulation occurs primarily through events that take place in the kidney (in land-dwelling vertebrates)
  • Kidney is responsible for water and electrolyte balance as well as the excretion of nitrogenous wastes
• • General principles of kidney function:
  • Water is not pumped directly - it moves via osmotic gradients set up by active transport of ions
  • The formation of the filtrate is not particularly selective
  • In contrast to filtrate formation, reabsorption is highly selective for certain molecules and ions
  • Any remaining waste products are then eliminated with the feces
  • In contrast to filtrate formation, reabsorption is tightly regulated in response to osmotic stress
• • General principles of kidney function:
  • Water is not pumped directly - it moves via osmotic gradients set up by active transport of ions
  • The formation of the filtrate is not particularly selective
  • In contrast to filtrate formation, reabsorption is highly selective for certain molecules and ions
  • Any remaining waste products are then eliminated with the feces
  • In contrast to filtrate formation, reabsorption is tightly regulated in response to osmotic stress
• Kidney Structure

  • Renal artery brings blood containing nitrogenous wastes into the kidney
  • Renal vein carries cleaned blood away
  • Urine formed in kidney is transported via ureter to bladder
  • Most of kidney’s mass is made up of nephrons
  • Nephron is responsible for water and electrolyte balance
  • Most of the nephrons are located in the outer region of the kidney (cortex) - some extend into the kidney’s inner region (medulla)
• Kidney Function

  • Water cannot be transported actively - crosses membrane by osmosis
  • To move water, cells in kidney set up strong osmotic gradients in the interstitial fluid surrounding the nephrons
  • By regulating these gradients and specific channel proteins, kidney cells exert precise control over loss or retention of water and electrolytes
• Nephrons have four major regions and are closely associated with a collecting duct:

  • Renal corpuscle - filters blood, forming a filtrate consisting of ions, nutrients, wastes, and water
  • Proximal tubule has epithelial cells that reabsorb nutrients, ions, and water from the filtrate into the blood
  • The loop of Henle establishes a strong osmotic gradient in the interstitial fluid surrounding the loop
  • The distal tubule reabsorbs ions and water in a way that helps maintain water and electrolyte balance
  • The collecting duct, may reabsorb more water to maintain homeostasis
• Urea moves from the urine to the interstitial fluid at the base of the collecting duct contributing to the osmotic gradient set up by the loop of Henle.
• Renal Corpuscle

  • Urine formation begins here, which is made up of the glomerulus and Bowman’s capsule
  • The glomerulus is a cluster of capillaries that bring blood to the nephron from the renal artery
  • The Bowman’s capsule is the region of the nephron that surrounds the glomerulus
  • Glomerular capillaries have large pores surrounded by cells whose membranes fold into a series of slits and ridges
• Renal Corpuscle (Part 2)

  • Filtration is based on size
  • Larger molecules remain in the blood and cannot enter the nephron
  • Blood pressure supplies the force to perform filtration
  • Forcing water and small solutes through the pores
  • This allows the renal corpuscle to strain large volumes of fluid without expending energy
• Renal Corpuscle (Part 3)

  • The renal corpuscles of a human kidney are capable of producing about 180 litres of filtrate per day
  • About 99% of the filtrate is reabsorbed—only a tiny fraction of the original volume is actually excreted
  • Filtering large volumes from the blood allows wastes to be removed effectively
  • Pairing this process with reabsorption allows waste excretion to occur with a minimum of water and nutrient loss