Biology Chapter 44: Osmoregulation and Excretion

Osmoregulation and Excretion

• Osmoregulation and excretion are crucial processes for maintaining fluid and solute balance in animals.
• Osmoregulation controls solute concentrations and water balance.
• Excretion rids the body of nitrogenous metabolites and waste products.

Osmoregulation Basics

• Osmoregulation involves the controlled movement of water and solutes across plasma membranes.
• Concentration gradients of solutes drive this movement.
• Osmolarity refers to the solute concentration of a solution.
• Water moves by osmosis from hypoosmotic (less concentrated) to hyperosmotic (more concentrated) solutions.

Osmoregulatory Strategies

• Animals maintain water balance as either osmoconformers or osmoregulators.
• Osmoconformers are isoosmotic with their surroundings and do not regulate osmolarity.
• Osmoregulators expend energy to control water uptake and loss in hyperosmotic or hypoosmotic environments.
• Stenohaline animals cannot tolerate substantial changes in external osmolarity
• Euryhaline animals can survive large fluctuations in external osmolarity

Osmoregulation in Different Environments

• Marine invertebrates are mostly osmoconformers.
• Marine vertebrates and some invertebrates are osmoregulators.
• Marine bony fish are hypoosmotic to seawater, balancing water loss by drinking large amounts of it.
• Marine Bony fish eliminate ingested salts through their gills and kidneys.
• Freshwater animals take in water by osmosis from their hypoosmotic environment.
• Freshwater animals lose salts by diffusion.
• Salts lost by diffusion are replaced in foods and by uptake across the gills
• The adaptation called anhydrobiosis: tardigrades (water bears) can dehydrate from about 85% water to 2% water in the dehydrated, inactive state, easily reversible when water is added
• Terrestrial animals prevent dehydration with body coverings and behaviors to balance water loss

Energetics of Osmoregulation

• Osmoregulators expend energy to maintain osmotic gradients.
• The amount of energy required depends on how different an animal's osmolarity is from its surroundings, how easily water and solutes move across its surface, and the work required to pump solutes.

Transport Epithelia

• Animals regulate the solute content of body fluid that bathes their cells.
• Transport epithelia are specialized cells for controlled solute movement with extensive surface areas.
• Marine birds and iguanas use nasal glands to remove excess sodium chloride from the blood.

Nitrogenous Wastes

• Animals must eliminate ammonia, a toxic metabolite produced by breakdown of nitrogenous molecules.
• Ammonia is removed from the body by excretion.
• Metabolic wastes must be dissolved in water for excretion, impacting water balance.
• Animals excrete nitrogenous wastes as ammonia, urea, or uric acid, varying in toxicity and energy cost.

Forms of Nitrogenous Waste

• Ammonia is most toxic and has the least energy cost.
• Animals excreting waste as ammonia need access to lots of water.
• In invertebrates ammonia release occurs across the whole body surface
• Urea is less toxic than ammonia, has high water solubility, and is excreted via kidneys
• Urea is produced in the liver and requires less water than ammonia
• Uric acid is the least toxic, does not dissolve readily in water and can be secreted as a paste with little water loss
• Uric acid is even more energetically expensive to produce than Urea
• The kind of nitrogenous wastes excreted depends on an animal's habitat, particularly water availability.
• Nitrogenous waste is also affected by the immediate environment of the animal egg, energy budget, and diet.

Excretory Systems

• Excretory systems dispose of metabolic wastes and control body fluid composition.
• They play a central role in homeostasis.

Excretory Processes

• Excretory systems produce urine by refining a filtrate derived from body fluids.
• Filtration, reabsorption, secretion, and excretion are key functions of most excretory systems.

Survey of Excretory Systems

• Excretory systems vary widely among animal groups but are generally built on a network of tubules.
• Protonephridia, metanephridia, Malpighian tubules, and kidneys/nephron all function as excretory systems.

Kidneys and Nephrons

• Kidneys are the excretory organs of vertebrates functioning in excretion and osmoregulation.
• Excretion filters waste products and then expels them from the organism whereas osmoregulation is about controlling water and solute concentrations so that osmotic pressure is maintained.
• They consist of organized tubules.
• The body includes ducts and other structures that carry urine from the tubules out of the kidney and out of the body
• Urine produced by each kidney exits the renal pelvis through a duct called the ureter.
• The ureters drain into a common sac called the urinary bladder.
• Urine is expelled from the bladder through the urethra.
• The kidneys have an outer renal cortex and an inner renal medulla.
• Closely packed excretory tubules and associated blood vessels lie tightly within these regions
• Tubules process a filtrate produced from blood entering the kidney.
• The remaining fluid exits as urine and collects in the renal pelvis.
• Nephrons are the functional units of the vertebrate kidney.
• 85% of nephrons in the human kidney are cortical nephrons which extend only a short distance into the medulla
• Juxtamedullary nephrons extend deep into the medulla.
• Juxtamedullary nephrons are essential for production of urine that is hyperosmotic to body fluids and are essential for water conservation.
• Nephrons consist of a single long tubule and a ball of capillaries called the glomerulus.
• Bowman's capsule surrounds the glomerulus
• Processing of the filtrate occurs as it passes through three major regions of the nephron as well as the collecting duct: the proximal tubule, the loop of Henle, and the distal tubule
• A collecting duct receives the processed filtrate from many nephrons and transports it to the renal pelvis.
• Peritubular capillaries surround the proximal and distal tubules, while other branches form the vasa recta, serving the renal medulla.

Nephron Filtrate Processing

• The filtrate produced in Bowman's capsule contains salts, glucose, amino acids, vitamins, nitrogenous wastes, and other small molecules.
• Under normal conditions, about 1,600 L of blood flows through a pair of human kidneys each day.
• Only about 1.5 L of urine is transported to the bladder for excretion.

From Blood Filtrate to Urine: A Closer Look

• The proximal tubule is responsible for the reabsorption of water, essential ions, and nutrients.
• Molecules are either actively or passively transported from the filtrate to the interstitial fluid and capillaries surrounding it.
• As the filtrate passes through the proximal tubule, it becomes more concentrated.
• Some toxic materials are actively secreted into the filtrate.
• The descending limb of the loop of Henle continues reabsorption of water through the use of aquaporin proteins.
• The high osmolarity of the surrounding interstitial fluid drives the movement of water from the filtrate.
• The filtrate becomes increasingly concentrated as it moves down the tubule.
• In the ascending limb of the loop of Henle, salt, but not water, is able to diffuse from the tubule to the interstitial fluid.
• The filtrate becomes increasingly dilute.
• The distal tubule regulates the K+ and NaCl concentrations of body fluids.
• Movement of H+ and HCO3- contributes to pH regulation.
• The collecting duct carries the filtrate through the medulla to the renal pelvis.
• One of the most important tasks is the reabsorption of water.
• Urine is hyperosmotic to body fluids.

Solute Gradients and Water Conservation

• The mammalian kidney's ability to conserve water is a key terrestrial adaptation.
• Hyperosmotic urine can be produced only because considerable energy is expended to transport solutes against concentration gradients.
• The two primary solutes affecting osmolarity are NaCl and urea

Urine Concentration in the Mammalian Kidney

• (Proximal tubule) Filtrate volume decreases and water and salt are reabsorbed
• (Descending limb of the loop of Henle) Water exits the filtrate, increasing solute concentration.
• (Ascending limb of the loop of Henle) NaCl diffuses out to maintain a high osmolarity in the interstitial fluid.
• Energy is expended to actively transport NaCl from the filtrate in the upper part of the ascending limb.

Countercurrent Multiplier System

• The countercurrent multiplier system in the loop of Henle maintains a high salt concentration in the kidney.
• The system allows the vasa recta to supply the kidney with nutrients without interfering with the osmolarity gradient.
• (Collecting Duct) Osmosis extracts water from the filtrate to encounters interstitial fluid of increasing osmolarity
• Urine produced is isoosmotic to the interstitial fluid of the inner medulla but hyperosmotic to blood and interstitial fluids elsewhere in the body.

Adaptations of the Vertebrate Kidney

• Variations in nephron structure and function equip the kidneys of different vertebrates for osmoregulation.
• The juxtamedullary nephron is key to water conservation in terrestrial animals.
• Dry environments = long loops of Henle.
• Fresh water = relatively short loops.
• The South American vampire bat feeds on blood.
• The vampire bat alternates between producing large amounts of dilute urine as it feeds and small amounts of very hyperosmotic urine when roosting.
• This adaptation is essential to an unusual food source.
• Birds have shorter loops of Henle than mammals but conserve water by excreting uric acid instead of urea.
• Other reptiles have only cortical nephrons but reabsorb water from wastes in the cloaca.
• Reptiles also excrete nitrogenous waste as uric acid.
• Freshwater fishes excrete large volumes of very dilute urine.
• Salt concentration relies on reabsorption of ions from filtrate in the distal tubules.
• Amphibians conserve water on land by reabsorbing it from the urinary bladder.
• Marine Bony Fishes marine bony fishes have fewer and smaller nephrons than freshwater fishes, and their nephrons lack a distal tubule
• Marine fish: kidneys have small glomeruli; some lack glomeruli entirely
• Marine bony fishes: Filtration rates are low, and very little urine is excreted
• Marine fishes: osmoregulation relies on specialized chloride cells in the gills.

Hormonal Control of Kidney Function

• Mammals can control the volume and osmolarity of urine in response to changes in salt intake and water availability.
• Two major control circuits respond to different stimuli together restore and maintain normal water and salt balance.

Homeostatic Regulation of the Kidney

• A combination of nervous and hormonal controls manges the osmoregulatory functions of the mammalian kidney.
• These controls contribute to homeostasis for blood pressure and blood volume.
• Antidiuretic hormone (ADH; vasopressin) increases the production of urine.
• When osmolarity rises above its set point, ADH release into the bloodstream increases.
• ADH molecules released from the posterior pituitary bind to and activate membrane receptors on collecting duct cells.
• This initiates a signal cascade leading to insertion of aquaporin proteins into the membrane lining the collection duct.
• The increase in water recapture reduces urine volume; blood osmolarity decreases.
• Alcohol inhibits the release of ADH.
• Mutation in ADH production causes severe dehydration and results in diabetes.
• Mutations in an aquaporin gene have a similar effect resulting Lots of dilute urine, Dehydration, and Solute imbalance

The Renin-Angiotensin-Aldosterone System (RAAS)

• This is part of a complex feedback circuit that functions in homeostasis.
• A drop in blood pressure near the glomerulus causes the juxtaglomerular apparatus (JGA) to release the enzyme renin.
• Renin triggers the formation of the peptide angiotensin II.
• Angiotensin II raises blood pressure by triggering vasoconstriction and decreasing blood flow to the kidneys.
• Angiotensin II stimulates the release of the hormone aldosterone.
• This increases absorption of sodium and water in the distal tubule of kidney.
• Overall, increases blood volume and pressure.

Coordinated Salt and Water Balance

• ADH and RAAS both increase water reabsorption, but only RAAS maintains body fluid osmolarity by stimulating Sodium reabsorption.
• Atrial natriuretic peptide (ANP) opposes the RAAS.
• ANP is released in response to an increase in blood volume and pressure and inhibits the release of renin.
• Its action lowers blood pressure and volume.
• The use of Angiotensin converting enzyme (ACE) inhibitors or Diuretics (Lasix) aids in reducing blood volume and pressure.

Disorders of the Kidneys and Urinary System

• Chronic Kidney Disease/Kidney Failure
• Hypertension damages blood vessels
• Glomerulonephritis; Acute or chronic inflammation of glomerulus
• Infections of the urinary tract; usually starts in bladder and moves to ureters and kidneys
• Kidney stones; Hard granules that can form in renal pelvis
• Polycystic Kidney Disease Genetic disease with numerous cysts on the kidney

Control of Micturition

• Micturition (urination) is the process of urine excretion from the urinary bladder
• As the bladder fills, the smooth muscle in the walls of the bladder stretch and trigger the micturition reflex.
• At 200ml of urine, the detrusor muscle, begins to contract and the internal sphincter begins to relax, resulting in the "urge” to urinate.
• Unless the external muscle is contracted, at 500ml the detrusor muscle will force open the internal shut and urinate without assistance.

Bladder Control Problems

• Urinary incontinence is a loss of control over urination causing, sudden urge
• Overactive bladder is when the brain thinks bladder is full but not
• Disorders and damage can lead to bladder control problems.