Animal Osmoregulation and Excretion

Water and Salt Concentration Regulation in Animals

Osmosis and Water Movement

• Animals regulate water and salt concentrations by transporting salts, which directs water movement via osmosis.

• Osmosis involves the net diffusion of water from an area of higher free water concentration (lower solute concentration) to an area of lower free water concentration (higher solute concentration).

• Cell Lysis: Occurs when a cell is hyperosmotic to its surroundings, leading to a net inflow of water into the cell.

• Osmotic Balance: Achieved when a cell is isoosmotic to its surroundings, resulting in no net water movement.

• Cell Shriveling: Happens when a cell is hypoosmotic to its surroundings, causing a net outflow of water from the cell.

• Excretory systems maintain salt and water balance and remove nitrogenous wastes.

Osmoregulation

• Maintaining fluid balance requires keeping solute and water concentrations within narrow limits.

• Osmoregulation controls solute concentrations and balances water gain and loss.

• Excretion is the process of ridding the body of nitrogenous metabolites and other metabolic waste products.

• Osmoregulation requires controlled movement of water and solutes across plasma membranes.

• The driving force for this movement is the concentration gradient of solutes across the membrane.

Osmolarity and Osmotic Balance

• Water enters and leaves cells by osmosis.

• Osmolarity is the solute concentration of a solution, determining water movement across a selectively permeable membrane.

• Isoosmotic Solutions: Water molecules cross the membrane at equal rates in both directions, resulting in no net movement of water.

• Hypoosmotic to Hyperosmotic Solutions: Net water flow is from the hypoosmotic (less concentrated) solution to the hyperosmotic (more concentrated) solution.

Osmoregulatory Strategies

• Animals maintain water balance in two ways:

  • Osmoconformers: Are isoosmotic with their surroundings and do not regulate their osmolarity.

  • Osmoregulators: Expend energy to control water uptake and loss in hyperosmotic or hypoosmotic environments.

• Osmoregulation enables animals to inhabit environments unsuitable for osmoconformers.

• Osmoconformers are in osmotic equilibrium with their environment, including most marine invertebrates and cartilaginous fish (sharks and relatives).

• Vertebrates are osmoregulators, maintaining constant blood osmolarity despite environmental differences.

• Freshwater vertebrates are hypertonic to their environment, adapting to prevent water entry and actively transport ions back into their bodies.

• Marine vertebrates are hypotonic to their environment, retaining water by drinking seawater and eliminating excess ions through kidneys and gills.

Osmoregulatory Organs

• Removal of water or salts is often coupled with metabolic waste removal via the excretory system.

• Single-celled protists use contractile vacuoles.

• Invertebrates use specialized cells and tubules:

  • Flatworms use protonephridia with flame cells, open to the outside but not inside.

  • Earthworms use nephridia, open to both the inside and outside of the body.

• Insects use Malpighian tubules, extensions of the digestive tract.

  • Waste and $K^+$ are secreted into tubules by active transport.

  • This creates an osmotic gradient, drawing water into tubules by osmosis.

  • Most water and $K^+$ are reabsorbed into the open circulatory system through the hindgut epithelium.

Vertebrate Kidneys

• Vertebrate kidneys consist of thousands of nephrons.

  • They create tubular fluid by filtering blood under pressure through the glomerulus.

  • The filtrate contains small molecules, water, and waste products.

  • Most molecules and water are reabsorbed into the blood.

  • Waste products are eliminated in urine.

• Kidneys evolved among freshwater teleosts (bony fishes).

  • Body fluids are hypertonic, causing water influx and solute loss.

    1. Fishes do not drink water and excrete large amounts of dilute urine.

    2. Ions are reabsorbed across nephrons.

• Marine bony fishes have hypotonic body fluids relative to seawater.

  • Water tends to leave their bodies by osmosis across the gills.

  • They drink large amounts of seawater.

  • Monovalent ions are actively transported out of the blood across gill surfaces.

  • They excrete urine isotonic to body fluids, containing divalent cations.

• Cartilaginous fish reabsorb urea from nephron tubules.

  • They maintain high blood urea concentrations, about 100 times higher than mammals.

  • Their blood is isotonic to the surrounding sea, so they do not need to drink seawater or remove large amounts of ions.

• Amphibian kidneys are similar to those of freshwater fish.

• Reptile kidneys are diverse.

  • Marine reptiles drink seawater and excrete isotonic urine, eliminating excess salt via salt glands.

  • Terrestrial reptiles reabsorb much salt and water in their nephron tubules and empty urine into the cloaca.

• Mammals and birds produce urine hypertonic to body fluids using the loop of Henle.

  • Birds have fewer nephrons with long loops and cannot concentrate urine as much as mammals.

  • Marine birds excrete excess salt from salt glands near the eyes.

Nitrogenous Wastes

• Catabolism of amino acids and nucleic acids produces nitrogenous wastes that must be eliminated.

  • The first step is removing the amino group, combining it with $H^+$ to form ammonia ($NH_3$) in the liver.

  • Ammonia is toxic and must be kept in dilute concentrations.

• Bony fishes and amphibian tadpoles eliminate most ammonia by diffusion via gills.

• Elasmobranchs, adult amphibians, and mammals convert ammonia into urea, which is soluble in water.

• Birds, terrestrial reptiles, and insects convert ammonia into insoluble uric acid, which saves water but costs more energy.

• Mammals also produce uric acid from purine degradation.

  • Most mammals have uricase, converting uric acid into soluble allantoin.

  • Humans lack this enzyme, and excessive uric acid accumulation in joints causes gout.

Mammalian Kidney Anatomy

• Each kidney receives blood from a renal artery and produces urine.

  • Urine drains through a ureter into the urinary bladder.

• The ureter flares open to form the renal pelvis within the kidney.

  • The renal pelvis receives urine from the renal tissue.

• The kidney is divided into an outer renal cortex and inner renal medulla.

• The kidney has three basic functions:

  1. Filtration: Fluid in the blood is filtered out of the glomerulus into the tubule system.

  2. Reabsorption: Selective movement of solutes out of the filtrate back into the blood via peritubular capillaries.

  3. Secretion: Movement of substances from the blood into the extracellular fluid, then into the filtrate in the tubular system.

• Each kidney contains about 1 million nephrons.

  • Juxtamedullary nephrons: Have long loops that dip deeply into the medulla.

  • Cortical nephrons: Have shorter loops.

• Blood is carried by an afferent arteriole to the glomerulus, where it is filtered through porous capillary walls.

• Unfiltered blood components drain into an efferent arteriole, which empties into peritubular capillaries.

• Glomerular filtrate enters Bowman’s capsule, then the proximal convoluted tubule.

  • It moves down the medulla and back up into the cortex via the loop of Henle.

• Fluid is delivered to the distal convoluted tubule in the cortex after leaving the loop of Henle.

  • It then drains into a collecting duct.

  • Collecting ducts merge to empty urine into the renal pelvis.

Reabsorption and Secretion

• Most water and dissolved solutes in the glomerular filtrate are returned to the blood by reabsorption.

  • Water is reabsorbed in the proximal convoluted tubule.

  • Glucose and amino acids are reabsorbed via active transport carriers.

• Secretion involves transport across capillary membranes and kidney tubules into the filtrate.

• A major kidney function is eliminating harmful substances that animals ingest.

• Urine contains nitrogenous wastes and may contain excess $K^+$, $H^+$, and other ions.

• Kidneys are critically involved in maintaining homeostasis.

• A mechanism is needed to create an osmotic gradient between the glomerular filtrate and the blood.

• Virtually all nutrient molecules and two-thirds of the $NaCl$ and water are reabsorbed by the proximal convoluted tubule.

  • Active transport of $Na^+$ out of the proximal tubule is followed by passive movement of $Cl^-$ and water.

• The loop of Henle creates an osmolarity gradient from the cortex to the medulla.

  • Active extrusion of $NaCl$ from the ascending loop creates an osmotic gradient.

  • This allows reabsorption of water from the descending loop and collecting duct.

  • The two limbs of the loop form a countercurrent multiplier system, creating a hypertonic renal medulla.

• Filtrate reaching the distal convoluted tubule and entering the collecting duct is hypotonic.

  • The hypertonic interstitial fluid of the renal medulla pulls water out of the collecting duct and into surrounding blood vessels.

Electrolyte Balance

• Kidneys regulate electrolyte balance by reabsorption and secretion of $K^+$, $H^+$, and $HCO_3^-$.

Urinary System

• The urinary system includes:

  • Paired kidneys

  • Paired ureters (kidneys to bladder)

  • Urinary bladder

  • Urethra (bladder to exterior)

• Functions of the Kidneys:

  1. Retrieve essential materials and dispose of wastes: They conserve water, essential electrolytes, and metabolites, and they remove certain waste products of metabolism from the body

  2. Regulate and maintain the composition and volume of extracellular fluid

  3. Maintain acid–base balance by excreting hydrogen ions when bodily fluids become too acidic or excreting bicarbonates when bodily fluids become too basic.

• The primary organs of the excretory system are the kidneys.

• Kidneys process 423 gallons (1601 L) of blood per day and produce about 1.5 quarts (1.42 L) of urine.

• The renal artery transports blood to the kidney for filtering, and the renal vein transports filtered blood away.

• The ureter conducts urine from the kidney to the urinary bladder.

• The base of the ureter expands in the kidney to form the renal pelvis, which collects urine.

• The outer area of the kidney is the cortex, and the inner area is the medulla.

• The urethra carries urine from the bladder to the urethral orifice for expulsion.

• The adrenal glands are small, yellowish glands atop the kidneys.

• The medial border of the kidney is concave and contains the hilum, through which renal vessels and nerves pass and the renal pelvis exits.

• The kidney surface is covered by a connective tissue capsule.

• Urine flows from the collecting duct to minor calyx, then major calyx, before entering the renal pelvis.

• Cortical nephrons are mainly in the renal cortex; juxtamedullary nephrons have long loops extending deep into the renal medulla.

• The nephron tubule is surrounded by peritubular capillaries and vasa recta, which carry away reabsorbed molecules and ions.

The kidney is related as follows:

• Collecting duct -> Renal Pelvis -> Ureter

Hormonal Control of Osmoregulation

• Kidneys maintain constant blood volume, pressure, and osmolarity and regulate $K^+$ and $Na^+$ concentrations and blood pH.

• These functions are coordinated primarily by hormones.

Key Hormones Involved

• Antidiuretic hormone (ADH):

  • Produced by the hypothalamus and secreted by the posterior pituitary gland.

  • Stimulated by increased blood osmolarity.

  • Increases water reabsorption by making distal tubule and collecting duct walls more permeable to water (negative feedback).

• Aldosterone:

  • Secreted by the adrenal cortex.

  • Stimulated by low $Na^+$ levels in the blood.

  • Causes distal tubule and collecting ducts to reabsorb $Na^+$, followed by $Cl^-$ and water.

  • Low $Na^+$ levels are accompanied by decreased blood volume, activating the renin-angiotensin-aldosterone system.

• Atrial natriuretic hormone:

  • Opposes aldosterone’s action.

  • Secreted by the right atrium in response to increased blood volume.

  • Promotes salt and water excretion, lowering blood volume.