Osmoregulation

Osmoregulation

• Osmoregulation – the active regulation of the osmotic pressure of an organism’s fluids to maintain homeostasis of the organism’s water content– Osmotic pressure (of a solution) – a measure of its tendency to take in water by osmosis

– Osmosis – the diffusion of water across a semi-permeable membrane

• Always occurs from a more dilute solution to a less dilute solution

• Higher concentration of solutes more osmotic pressure

Osmoregulation

• To maintain osmotic balance, the extracellular compartment of an animal’s body must be able to take water from, and excrete excess water, into the environment

• Exchanges of water and inorganic ions (electrolytes) between the body and the external environment occurs across specialized epithelial cells, and, in most vertebrates, through a filtration process in the kidneys

Osmoregulation

kidney

Osmoregulation

• Homeostasis maintains a constant level of ions in extracellular fluids– Na+ - major cation– Cl- – major anion– Ca+2, Mg+2, and K+ – also very important

Osmoregulation– Hypertonic solution – cell loses water (and shrinks);

more solutes in solution relative to cell– Isotonic solution - no net water movement; equal

number of solutes relative to cell– Hypotonic solution - cell gains water (expands);

fewer solutes relative to cell

REMEMBER, HIGHER CONCENTRATION OF SOLUTES INCREASES OSMOTIC PRESSURE AND

THE TENDENCY TO TAKE IN WATER BY OSMOSIS

Osmoregulation

Turgor pressure in plants!

Osmoconformers

• In most marine invertebrates, the osmolarity of their blood fluids is the same as that of their external environment, seawater– That is, their extra-cellular fluids are isotonic to

their external environment– NO tendency for water to leave or enter the body– An organism in osmotic equilibrium with its

environment is called an osmoconformer

Osmoconformers  Sodium Magnesium Calcium Potassium Chloride

Seawater 478.3 54.5 10.5 10.1 558.4

Jellyfish 474 53 10 10.7 580

Polychaete 476 54.6 10.5 10.5 557

Sea urchin 474 53.5 10.6 10.1 557

Mussel 474 52.6 11.9 12 553

Squid 456 55.4 10.6 22.2 578

Isopod 566 20.2 34.9 13.3 629

Crab 488 44.1 13.6 12.4 554

Lobster 541 9.3 11.9 7.8 552

Hagfish 537 18 5.9 9.1 542Adapted from Potts and Parry 1964 as cited by J. Levinton 2009

Osmoregulators

• An organism that actively discharges water (in a hypotonic environment) or takes in water (in a hypertonic environment) is called an osmoregulator

• Osmoregulators can maintain a relatively constant blood osmolarity despite differences in concentration of solutes in the surrounding environment

Osmoregulators

• Examples…– Freshwater vertebrates have much higher solute

concentration in their body fluids than in the surrounding water (increase solutes increases osmotic pressure; increased tendency to take in water)

– Freshwater vertebrates must prevent water from entering their bodies as much as possible and actively transport ions back into their body

Osmoregulators

• More examples…– Most marine vertebrates are hypotonic to their

environment; these animals are in danger of losing water by osmosis

– Marine vertebrates must retain water and eliminate excess ions via their kidneys and gills

– They retain water by drinking seawater (yum!)

Osmoregulators

• Even more examples…– Terrestrial vertebrates have a higher

concentration of water than in the air in their surrounding environment; these animals tend to lose water to the air by evaporation from the skin and lungs

– These animals evolved osmoregulatory/urinary systems to retain water

Nitrogenous Wastes

• In many animals, the removal of water and ions is coupled with the removal of wastes generated by metabolism

• Amino acids (proteins) and nucleic acids are nitrogen-containing molecules

• When proteins and nucleic acids are metabolized (catabolized for energy or converted into carbohydrates or lipids), they become nitrogenous wastes, that must be eliminated from the body

Nitrogenous Wastes: Ammonia

• Nitrogenous wastes are toxic to the body and must be removed

• Ammonia is formed in the liver by the metabolism of amino and nucleic acids

• Ammonia is very toxic to cells and must be present only in very dilute concentrations

• Bony fish drink seawater (marine) or have excess water to remove (freshwater) and therefore have very dilute urine, and so excrete ammonia safely

Nitrogenous Wastes: Urea

• Urea and uric acid are less toxic to the organism, and are eliminated by cartilagenous fish, adult amphibians and mammals

• Urea is water-soluable and so can be excreted in large amounts in the urine

• Urea is also synthesized in the liver; takes energy to convert from ammonia

Nitrogenous Wastes: Uric Acid

• Uric acid is also less toxic, but is NOT water-soluable

• Birds, reptiles and insects excrete uric acid• Uric acid does not require large amounts of

water to be excreted• Uric acid is that “special something” you find on

your windshield; forms a pasty, white material• Costs more to generate uric acid than urea, but

conserves water!

Nitrogeous Wastes: Uric Acid

• The insoluability of uric acid is very important to reptiles and birds, which lay eggs– Uric acid accumulates inside egg with developing

embryo, but precipitates and crystalizes– Does not interfere with developing embryo; does

not dissolve in yolk– Worth the extra cost of production– Gout in humans – deposits of uric acid crystals in

joints ; kidney stones; inability to properly make uric acid water soluable

Nitrogenous Wastes

Osmoregulatory Organs

• Single-celled organisms use contractile vacuoles to rid wastes (freshwater protozoans are constantly gaining water by osmosis)

• Invertebrates use specialized cells and tubules– Flatworms (Platyhelminthes) use protonephridia

which branch into bulblike flame cells throughout the body

• Cilia inside the flame cells draw in fluids from the body; substances then excreted through pores

Osmoregulatory Organs

• Earthworms have nephridia - a system of tubules that open to the inside and outside of the body

• Fluid enters internal openings called nephrostomes by filtration

• Tubules are lined with epithelial cells that reabsorb essential salts

Osmoregulatory Organs

• Insects use malpighian tubules– Extensions of the digestive tract– Waste products are transferred into the tubules

by active transport• Secretion of K+ creates an osmotic gradient that causes

water to enter the tubules by osmosis from the body’s open circulatory system

• Most of the water and K is then reabsorbed into the circulatory system via the epithelium of the hindgut, leaving behind only small molecules and waste products to be excreted from the rectum

Mammalian Kidneys

• The vertebrate kidney consists of many nephrons, the filtering unit of the kidney

• In humans, the kidneys consist of a pair of fist-sized organs located in the lower back

• The kidney receives bloom from a renal artery and produces urine

• Urine drains from each kidney through a ureter towards the urinary bladder (and into the urethra)

Mammalian Kidneys• The kidney has three basic functions:

– Filtration – fluid in the blood is filtered into the tubule system, leaving behind cells and large proteins in the blood and a filtrate composed of water and all of the solutes from the blood

– Reabsorption – important solutes, such as glucose, amino acids and inorganic ions, and water are selectively moved out of the filtrate back to the blood (reabsorbed)

– Secretion – substances from the blood are moved into the tubule system ‘permanently’ (e.g., toxins)

The Human Kidney

Mammalian Kidneys: Filtration

• Each nephron consists of a long tubule and associated blood vessels

• Blood is carried by an affarent arteriole to a tuft of capillaries in the renal cortex, the glomerulus

• Blood is filtered in the glomerulus as the blood pressure forces fluid through porous capillary walls– Blood cells and large proteins are too large to

enter

Mammalian Kidneys: Filtration

• Large amounts of blood plasma, containing water and dissolved molecules enter the nephron tubules in a region known as Bowman’s capsule

• Bowman’s capsule envelopes the glomerulus• Blood components that were not filtered drain

into an efferent arteriole, which empties into a second bed of capillaries

• Filtrate then flows through the loop of Henle (birds and mammals), which dips into the medulla before ascending back into the cortex

Mammalian Kidneys: Filtration

• After leaving the loop of Henle, the fluid drains into a collecting duct– Merges with other collecting ducts to empty its

contents, now called urine, into the renal pelvis– Filtration is a nonselective process with regard to

small molecules– Filtrate initially consists of water, urea, salts,

glucose, vitamins, etc.

Mammalian Kidneys: Reabsorption and Secretion

• Most of the water and dissolved solutes that enter the glomerular filtrate must be returned to the blood by reabsorption

• Reabsorption is the selective transport from the filtrate back to the blood

• Sugars, vitamins, organic nutrients, and water are reabsorbed (don’t want to lose!)

• Secretion is a very selective process

Mammalian Kidneys: Reabsorption and Secretion

• Reabsorption of glucose and amino acids is driven by active transport carriers

• LOTS of capillaries surround the loop of Henle to readjust composition of filtrate

• Approximately 25x the volume of your body is filtered each day

• Aquatic animals have short loops of Henle; desert organisms have longer loops (conserve water!)

Mammalian Kidneys: Reabsorption and Secretion

• The function of the loop of Henle is to create a gradient of increasing osmolarity from the cortex to the medulla; allows water to be reabsorbed via osmosis in the collecting duct as it runs down into the medulla past the loop of Henle

• The descending and ascending limbs of the loop of Henle differ structurally in regards to their permeability to water– Ascending limb is impermeable to water; descending

limb is thin and permeable

Kidneys: Hormonal Control

• Kidneys maintain relatively constant levels of blood volume, pressure, and osmolarity

• These homeostatic functions of kidneys are coordinated primarily by hormones

• Antidiuretic hormone (ADH) is released by the pituitary gland when the solute concentration of the blood rises– Makes the epithelium of the distal tubules and the

collecting ducts more permeable to water; stimulates water reabsorption from the collecting duct (no ADH, no reabsorption in collecting duct)

Kidneys: Hormonal Control

• Alcohol inhibits ADH, dehydrates the body!• … what a hangover is; the headache results

from the dehydration of your organ systems• …why they say to drink water before you go to

bed after drinking

Osmotic balance

• The kidney appeared first in freshwater fish• Fish, amphibian and reptile kidneys can only

produce a hypotonic (or isotonic) urine (no loops of Henle)

• Marine bony fish must excrete electolytes and retain water; most monovalent ions are actively transported across gill surfaces; divalent ions excreted in urine (isotonic urine)

Osmoregulation in marine fish

Osmotic balance

• Freshwater fish must retain electrolytes and keep water out; freshwater fish do NOT drink water and excrete a large volume of dilute urine (hypotonic urine); reabsorb ions across the nephron tubules and actively transport ions across gill surfaces from the water into their blood

Osmoregulation in freshwater fish

Osmotic balance

• Marine reptiles eliminate excess salt through salt glands located near the nose or the eye– “crocodile tears”

• Marine birds drink seawater and then excrete excess salt through salt glands, which dribble down the beak

Marine Bird Salt Gland

Salt glands in action (Southern fulmar)!

Marine Mammal Osmoregulation

• Marine mammals osmoregulate by not drinking seawater and excreting highly concentrated urine

Harbor seal kidney