Lecture #11 – Animal Osmoregulation and Excretion

Key Concepts Water and metabolic waste The osmotic challenges of different environments The sodium/potassium pump and ion channels Nitrogenous waste Osmoregulation and excretion in invertebrates Osmoregulation and excretion in vertebrates

Water and Metabolic Waste Osmoregulation ≠ Excretion All organismal systems exist within a water based environment The cell solution is water based Interstitial fluid is water based Blood and hemolymph are water based All metabolic processes produce waste Metabolic processes that produce nitrogen typically produce very toxic ammonia

Critical Thinking The cellular metabolism of _____________ will produce nitrogenous waste.

Critical Thinking The cellular metabolism of ___________ will produce nitrogenous waste.

Water and Metabolic Waste All animals have some mechanism to regulate water balance All animals have some mechanism to regulate solute concentration All animals have some mechanism to excrete nitrogenous waste products Osmoregulation and excretion systems vary by habitat and phylogeny (evolutionary history)

Animals live in different environments Marine….Freshwater….Terrestrial All animals must balance water uptake vs. water loss and regulate solute concentration within cells and tissues

The osmotic challenges of different environments – water balance Water regulation strategies vary by environment Body fluids range from 2-3 orders of magnitude more concentrated than freshwater Body fluids are about one order of magnitude less concentrated than seawater for osmoregulators Body fluids are isotonic to seawater for osmoconformers Terrestrial animals face the challenge of extreme dehydration

The osmotic challenges of different environments – solute balance All animals regulate solute content, regardless of their water regulation strategy Osmoregulation always requires metabolic energy expenditure

The osmotic challenges of different environments – solute balance In most environments, ~5% of basal metabolic rate is used for osmoregulation More in extreme environments Less for osmoconformers Strategies involve active transport of solutes and adaptations that adjust tissue solute concentrations

Water Balance in a Marine Environment Marine animals that regulate water balance are hypotonic relative to salt water (less salty) Where does water go???

Critical Thinking Marine animals that regulate water balance are hypotonic relative to salt water – where does water go???

Critical Thinking Marine animals that regulate water balance are hypotonic relative to salt water – where does water go???

Critical Thinking Marine animals that regulate water balance are hypotonic relative to salt water – where does water go???

Water Balance in a Marine Environment Marine animals that regulate water balance are hypotonic relative to salt water They dehydrate and must drink lots of water Marine bony fish excrete very little urine Most marine invertebrates are osmoconformers that are isotonic to seawater Water balance is in dynamic equilibrium with surrounding seawater

Solute Balance in a Marine Environment Marine osmoregulators Gain solutes because of diffusion gradient Excess sodium and chloride transported back to seawater using metabolic energy, a set of linked transport proteins, and a leaky epithelium Kidneys filter out excess calcium, magnesium and sulfates Marine osmoconformers Actively regulate solute concentrations to maintain homeostasis

Specialized chloride cells in the gills actively accumulate chloride, resulting in removal of both Cl- and Na+ Figure showing how chloride cells in fish gills regulate salts From Gorka, by email 13 November 2008

Solute Balance in a Marine Environment Marine osmoregulators Gain solutes because of diffusion gradient Excess sodium and chloride transported back to seawater using metabolic energy, a set of linked transport proteins, and a leaky epithelium Kidneys filter out excess calcium, magnesium and sulfates Marine osmoconformers Actively regulate solute concentrations to maintain homeostasis

Water Balance in a Freshwater Environment All freshwater animals are regulators and hypertonic relative to their environment (more salty) Where does water go???

Critical Thinking All freshwater animals are regulators and hypertonic relative to freshwater – where does water go???

Critical Thinking All freshwater animals are regulators and hypertonic relative to freshwater – where does water go???

Water Balance in a Freshwater Environment All freshwater animals are regulators They are constantly taking in water and must excrete large volumes of urine Most maintain lower cytoplasm solute concentrations than marine regulators – helps reduce the solute gradient and thus limits water uptake Some animals can switch environments and strategies (salmon)

Some animals have the ability to go dormant by extreme dehydration http://www.youtube.com/watch?v=CKamWp610ng The waterbear song by MalWebb

Solute Balance in a Freshwater Environment Large volume of urine depletes solutes Urine is dilute, but there are still losses Active transport at gills replenishes some solutes Additional solutes acquired in food

Marine osmoregulators dehydrate and drink to maintain water balance; regulate solutes by active transport Freshwater animals gain water, pee alot to maintain water balance; regulate solutes by active transport Figure showing a comparison between osmoregulation strategies of marine and freshwater fish

Water Balance in a Terrestrial Environment Dehydration is a serious threat Most animals die if they lose more than 10-12% of their body water Animals that live on land have adaptations to reduce water loss

Critical Thinking Animals that live on land have adaptations to reduce water loss – such as???

Critical Thinking Animals that live on land have adaptations to reduce water loss – such as???

Solute Balance in a Terrestrial Environment Solutes are regulated primarily by the excretory system More later

The sodium/potassium pump and ion channels in transport epithelia ATP powered Na+/Cl- pumps regulate solute concentration in most animals First modeled in sharks, later found in other animals Position of membrane proteins and the direction of transport determines regulatory function Varies between different groups of animals Figure showing the Na/K pump and membrane ion channels. This figure is used in the next 9 slides.

The Pump Metabolic energy is used to transport K+ into the cell and Na+ out This produces an electrochemical gradient

Critical Thinking What kind of electrochemical gradient???

Critical Thinking What kind of electrochemical gradient???

Critical Thinking What kind of electrochemical gradient???

The Na+/Cl-/K+ Cotransporter A cotransporter protein uses this gradient to move sodium, chloride and potassium into the cell

The Na+/Cl-/K+ Cotransporter Sodium is cycled back out Potassium and chloride accumulate inside the cell

Selective Ion Channels Ion channels allow passive diffusion of chloride and potassium out of the cell Placement of these channels determines direction of transport – varies by animal

Additional Ion Channels In some cases sodium also diffuses between the epithelial cells Shark rectal glands Marine bony fish gills

Additional Ion Channels In other animals, chloride pumps, additional cotransporters and aquaporins are important Membrane structure reflects function

Nitrogenous Waste Metabolism of proteins and nucleic acids releases nitrogen in the form of ammonia Ammonia is toxic because it raises pH Different groups of animals have evolved different strategies for dealing with ammonia, based on environment Figure showing different forms of nitrogenous waste in different groups of animals

Critical Thinking Why does ammonia raise pH??? Remember chemistry……

Critical Thinking Why does ammonia raise pH???

Critical Thinking Why does ammonia raise pH??? Base – a substance that accepts protons

Nitrogenous Waste Metabolism of proteins and nucleic acids releases nitrogen in the form of ammonia Ammonia is toxic because it raises pH Different groups of animals have evolved different strategies for dealing with ammonia, based on environment

Nitrogenous Waste Most aquatic animals excrete ammonia or ammonium directly across the skin or gills Plenty of water available to dilute the toxic effects Freshwater fish also lose ammonia in their very dilute urine

Nitrogenous Waste Most terrestrial animals cannot tolerate the water loss inherent in ammonia excretion They use metabolic energy to convert ammonia to urea Urea is 100,000 times less toxic than ammonia and can be safely excreted in urine Urea is produced in the liver, carried in blood to the kidneys

Nitrogenous Waste Insects, birds, many reptiles and some other land animals use even more metabolic energy to convert ammonia to uric acid Uric acid is excreted as a paste with little water loss Energy expensive

Osmoregulation and excretion in invertebrates Earliest inverts still rely on diffusion Sponges, jellies Most inverts have some variation on a tubular filtration system Three basic processes occur in a tubular system that penetrates into the tissues and opens to the outside environment Filtration Selective reabsorption and secretion Excretion

Protonephridia in flatworms, rotifers, and a few other inverts System of tubules is diffusely spread throughout the body Beating cilia at the closed end of the tube draw interstitial fluid into the tubule Solutes are reabsorbed before dilute urine is excreted Figure showing flatworm protonephridia

Protonephridia in flatworms, rotifers, and a few other inverts In freshwater flatworms most N waste diffuses across the skin or into the gastrovascular cavity Excretion 1o maintains water and solute balance In other flatworms, the protonephridia excrete nitrogenous waste

Metanephridia in the earthworms Tubules collect body fluid through a ciliated opening from one segment and excrete urine from the adjacent segment Hydrostatic pressure facilitates collection Figure showing annelid metanephridia

Metanephridia in the earthworms Vascularized tubules reabsorb solutes and maintain water balance N waste is excreted in dilute urine

Critical Thinking Earthworms are terrestrial – why would they have to get rid of excess water by producing dilute urine???

Critical Thinking Earthworms are terrestrial – why would they have to get rid of excess water by producing dilute urine???

Malphigian tubules in insects and other terrestrial arthropods System of closed tubules uses ATP-powered pumps to transport solutes from the hemolymph Water follows ψ gradient into the tubules Figure showing arthropod malphigian tubules. Same or similar figure is used in the next 3 slides.

Malphigian tubules in insects and other terrestrial arthropods Nitrogenous wastes and other solutes diffuse into the tubules on their gradients Dilute filtrate passes into the digestive tract

Malphigian tubules in insects and other terrestrial arthropods Solutes and water are reabsorbed in the rectum Again, using ATP-powered pumps

Malphigian tubules in insects and other terrestrial arthropods Uric acid is excreted from same opening as digestive wastes Mixed wastes are very dry Effective water conservation has helped this group become so successful on land

Osmoregulation and excretion in vertebrates Almost all vertebrates have a system of tubules (nephrons) in a pair of compact organs – the kidneys Each nephron is vascularized Each nephron drains into a series of coalescing ducts that drain urine to the external environment Many adaptations to different environments Most adaptations alter the concentration and volume of excreted urine

Critical Thinking Which of the world’s environments has produced the most concentrated urine???

Critical Thinking Which of the world’s environments has produced the most concentrated urine???

The Human Excretory System Kidneys filter blood and concentrate the urine Ureter drains to bladder Bladder stores Urethra drains urine to the external environment Diagram of the human excretory system

The Human Excretory System Each kidney is composed of nephrons These are the functional sub-units of the kidney Each nephron is vascularized Diagram of the human excretory system showing closeup of nephron

Critical Thinking Each nephron is vascularized….. What exactly does that mean???

Critical Thinking Each nephron is vascularized….. What exactly does that mean???

Nephron Structure Each nephron starts at a cup-shaped closed end Corpuscle Site of filtration Next is the proximal convoluted tubule in the outer region of the kidney (cortex) Diagram of nephron structure

Nephron Structure The Loop of Henle descends into the inner region of the kidney (medulla) The distal tubule drains into the collecting duct All these tubules are involved with secretion, reabsorption and the concentration of urine

Remember the 2 major steps to urine formation: Filtration and reabsorption/secretion Enormous quantities of blood are filtered daily 1,100 – 2,000 liters of blood filtered daily ~180 liters of filtrate produced daily Most water and many solutes are reabsorbed; some solutes are secreted ~1.5 liters of urine produced daily Water conservation!!!

Filtration in the Corpuscle Occurs as arterial blood enters the glomerulus A capillary bed with unusually porous epithelia Diagram showing the interior epithelia of the glomerulus surrounding the capillaries in the corpuscle

Filtration in the Corpuscle Blood enters AND LEAVES the glomerulus under pressure Glomerulus is surrounded by Bowman’s Capsule The invaginated but closed end of the nephron The enclosed space maintains pressure

Filtration in the Corpuscle Diagram of renal corpuscle

Filtration in the Corpuscle The interior epithelium of Bowman’s Capsule has special cells with finger-like processes that produce slits The slits allow the passage of water, nitrogenous wastes, many solutes Large proteins and red blood cells are too large to be filtered out and remain in the arteriole

Epithelial cells lining Bowman’s Capsule have extensions that make filtration slits – podocytes! Diagram of podocytes and porous capillary The bulk is the cell body

Materials are filtered through pores in the capillary epithelium, across the basement membrane and through filtration slits into the lumen of Bowman’s Capsule, passing then into the tubule

Filtration in the Corpuscle Anything small enough to pass makes up the initial filtrate Water Urea Solutes Glucose Amino acids Vitamins… Filtration forced by blood pressure Large volume of filtrate produced (180l/day)

Stepwise – From Filtrate to Urine Diagram showing overview of urine production

The Proximal Tubule Secretion – substances are transported from the blood into the tubule Reabsorption – substances are transported from the filtrate back into the blood

The Proximal Tubule – Secretion Body pH is partly maintained by secretion of excess H+ Proximal tubule epithelia cells also make and secrete ammonia (NH3) which neutralizes the filtrate pH by bonding to the secreted protons Drugs and other toxins processed by the liver are secreted into the filtrate

The Proximal Tubule – Reabsorption Tubule epithelium is very selective Waste products remain in the filtrate Valuable resources are transported back to the blood Water (99%) NaCl, K+ Glucose, amino acids Bicarbonate Vitamins…

The Proximal Tubule – Reabsorption ATP powered Na+/Cl- pump builds gradient Transport molecules speed passage Note increased surface area facing tubule lumen Diagram of tubule membrane proteins including Na/K pump

Critical Thinking What’s driving water transport???

Critical Thinking What’s driving water transport???

The Loop of Henle Differences in membrane permeability set up osmotic gradients that recover water and salts and concentrate the urine

Three Regions Diagram of Loop of Henle. This diagram is used in the next 3 slides

The Descending Limb Permeable to water Impermeable to solutes Water is recovered because of the increase in solutes in the surrounding interstitial fluids from the cortex to the inner medulla

The Thin Ascending Limb Not permeable to water Very permeable to Na+ and Cl- These solutes are recovered through passive transport Solutes help maintain the interstitial fluid gradient

The Thick Ascending Limb Na+ and Cl- continued to be recovered by active transport High metabolic cost, but helps to maintain the gradient that concentrates urea in the urine

The Distal Tubule Filtrate entering the distal tubule contains mostly urea and other wastes Na+, Cl- and water continue to be reabsorbed The amount depends on body condition Hormone activity maintains Na+ homeostasis Some secretion also occurs Diagram of the distal tubule and collecting duct. This diagram is used in the next 2 slides.

The Collecting Duct The final concentration of urine occurs as the filtrate passes down the collecting duct and back through the concentration gradient in the interstitial fluid of the kidney Water reabsorption is regulated by hormones to maintain homeostatis Dehydrated individuals produce more concentrated urine

The Collecting Duct Some salt is actively transported The far end of the collecting duct is permeable to urea Urea trickles out into the inner medulla Helps establish and maintain the concentration gradient

The Big Picture Blood is effectively filtered to remove nitrogenous waste Filtrate is effectively treated to isolate urea and return the good stuff to the blood Water is conserved – an important adaptation to terrestrial conditions

REVIEW – Key Concepts Water and metabolic waste The osmotic challenges of different environments The sodium/potassium pump and ion channels Nitrogenous waste Osmoregulation and excretion in invertebrates Osmoregulation and excretion in vertebrates

Hands On Back to the rat Dissect out the excretory system Snip away the membranes to reveal the kidneys, ureters, bladder and urethra If you have demolished your rat, work with a team member