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Regulating the Internal Environment. Campbell 6e Chapter 44 Pages 936-952. Water: A Balancing Act. Protists that live in fresh water environments are subjected to a continuous influx of water.

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regulating the internal environment

Regulating the Internal Environment

Campbell 6e Chapter 44 Pages 936-952

water a balancing act
Water: A Balancing Act
  • Protists that live in fresh water environments are subjected to a continuous influx of water.
  • The solute concentration inside the cell is higher than that in the surrounding water, so water continuously diffuses in.
This inward diffusion of water does not depend on any biological activity of the cell.
  • It is a purely physical phenomenon that depends on only the difference in solute concentration (or more specifically, the osmotic pressure difference) between the inside of the cell and the medium and the permeability of the plasma membrane.
  • Water that diffuses in must be moved out.
  • Here, Paramecium is using the contractile vacuole to pump out excess water.
  • Molecules passively diffuse from regions of high to low concentration.
  • Aquatic animals are generally hyperosmotic to their surroundings: their internal solute concentration is much higher than their surroundings.
Because of this, aquatic animals must develop physiological mechanisms to prevent excess flow of water into their bodies.
  • They must also develop mechanisms to prevent the loss of solutes as excess water is excreted.
The process by which organisms actively maintain their internal solute concentration is called osmoregulation.
  • Animals must actively transport solutes from their surroundings into their blood against the concentration gradient.
osmoregulation mechanisms
Osmoregulation Mechanisms
  • There are 2 main ways osmoregulation is accomplished.
  • Osmoconformers
  • Osmoregulators
  • Animals, such as crabs, have an internal salt concentration very similar to that of the surrounding ocean.
  • Such animals are known as osmoconformers, as there is little water transport between the inside of the animal and the isotonic outside environment.
  • There are three main types of osmoregulatory environments in which animals live: freshwater, marine, and terrestrial.
  • Aquatic animals are either euryhaline or stenohaline, depending on their ability to tolerate different salinities.
tolerance of change in osmolarity
Tolerance of Change in Osmolarity

Both of the following can be osmoconformers or osmoregulators.

  • Stenohaline organisms cannot tolerate large changes in external osmolarity.
  • Euryhaline organisms can survive large changes in external osmolarity.
freshwater osmoregulators
Freshwater Osmoregulators
  • Freshwater animals (all osmoregulators) include invertebrates, fishes, amphibians, reptiles, and mammals. The freshwater animals are generally hyperosmoticto their environment. The problems that they face because of this are that they are subject to swelling by movement of water into their bodies owing to the osmotic gradient, and they are subject to the continual loss of body salts to the surrounding environment (which has a low salt content).
The way these animals deal with these problems is to produce a large volume of dilute urine.
  • The kidney absorbs the salts that are needed, and the rest of the water is excreted.
  • Another way these animals deal with lack of salt is by obtaining it from the food they ingest.
  • A key salt replacement mechanism for freshwater animals is active transportof salt from the external dilute medium across the epithelium into the interstitial fluid and blood.
  • Amphibian’s skin and fish gills are active in this process.
  • Freshwater animals tend to take in water passively and to remove it actively through osmotic work of kidneys (in vertebrates) or kidney-like organs (in invertebrates).
marine osmoregulators
Marine Osmoregulators
  • Among marine animals, most invertebrates are osmoconformers whereas most vertebrates are osmoregulators. 
  • Marine animals do not need to expend as much energy in regulating the osmolarity of their body fluids.
  • Marine vertebrates have internal concentrations of salt that are about one-third of the surrounding seawater.
There is a tendency for marine fishes to lose water to the environment through the gill epithelium.
  • Marine fishes obtain water in food and drink sea water. The talk intake is disposed of through active transport out of the gills.
  • Very little urine is produced, an adaptation that conserves water.
terrestrial osmoregulators
Terrestrial Osmoregulators
  • Air breathing animals are subject to dehydration through their respiratory epithelia.
  • Humans and most other air-breathing animals require a constant source of fresh drinking water to excrete accumulated salts and metabolic waste products.
nitrogenous wastes
Nitrogenous Wastes
  • The breakdown of proteins produces nitrogenous wastes.
  • When macromolecules are broken down for energy or converted into carbs or fats, enzymes remove nitrogen in the form of ammonia.
  • Ammonia is very toxic and must be removed from the body.
There are 3 forms of nitrogenous wastes. The type of organism (habitat, diet, etc.) dictates the form.
  • Ammonia
  • Urea
  • Uric Acid
  • Ammonia is highly toxic and highly soluble in water.
  • If the organism has a sufficient source of water (aquatic), ammonia can simply be excreted in the water.
  • Aquatic animals such as bony fishes, aquatic invertebrates, and amphibians excrete ammonia because it is easily eliminated in the water.
This is the course taken by many (if not most) aquatic organisms, particularly those in freshwater.
  • Ammonia will diffuse passively out of respiratory structures such as gills.
  • It takes a lot of water to dissolve and flush ammonia, however, and each ammonia molecule carries only one nitrogen.
  • Organisms with less fresh water available, such as some marine organisms and all terrestrial organisms, are not as likely to waste water excreting nitrogen one atom at a time.
  • They will often invest some energy to convert the ammonia into urea, which is less toxic, has two nitrogen atoms, and therefore takes less water to excrete.
Because it is less toxic, it can be allowed to accumulate in the blood to some extent, and many organisms have specialized organs to remove urea and other wastes from the blood and excrete them.
uric acid
Uric Acid
  • Some organisms go to greater lengths still to deal with nitrogen. Where water is at a real premium, even the low toxicity and reduced water loss possible with urea excretion is not enough.
  • Uric acid is not very toxic and is not very soluble in water. Excretion of wastes in the form of uric acid conserves water because it can be produced in a concentrated form due to its low toxicity.
Uric acid has 4 nitrogen atoms per molecule and is excreted with just enough water so that the crystals don't scratch on the way out.
  • It has evolved in two groups with major water loss problems - terrestrial invertebrates and egg-laying vertebrates (obviously an embryo can't just step out for a drink, and whatever it excretes is going to be very close by until hatching).
excretory systems
Excretory Systems
  • Excretory systems regulate the chemical composition of body fluids by removing metabolic wastes and retaining the proper amounts of water, salts, and nutrients.
  • Components of this system in vertebrates include the kidneys, liver, lungs, and skin.
excretory system functions
Excretory System Functions
  • Collect water and filter body fluids.
  • Remove and concentrate waste products from body fluids and return other substances to body fluids as necessary for homeostasis.
  • Eliminate excretory products from the body.
flame bulb system
Flame-Bulb System
  • Many invertebrates such as flatworms use a protonephridium as their excretory organ.
  • At the end of each blind tubule of the protonephridium is a ciliated flame cell.
  • As fluid passes down the tubule, solutes are reabsorbed and returned to the body fluids.
Planarians have two protonephridia composed of branched tubules that empty wastes through excretory pores on their surface.
  • Earthworms have two metanephridia in almost all of the body segments.
  • Each metanephridium consists of a tubule with ciliated opening on one end and an excretory pore that opens to the outside of the body at the other end.
  • Fluid is moved in by cilia. Some substances and water are reabsorbed in a network of capillaries that surround the tubule.
  • This system produces large amount of urine (60% of body wt./day).
malpighian tubules
Malpighian Tubules
  • The excretory organs of insects are malpighian tubules.
  • They collect water and uric acid from surrounding hemolymph (blood) and empty it into the gut.
  • Water and useful materials are reabsorbed by the intestine but wastes remain in the intestine.
  • ALL vertebrates have paired kidneys.
  • Excretion is not the primary function of kidneys.
  • Kidneys regulate body fluid levels as a primary duty, and remove wastes as a secondary one.
kidney functions
Kidney Functions
  • Maintain volume of extracellular fluid
  • Maintain ionic balance in extracellular fluid
  • Maintain pH and osmotic concentration of the extracellular fluid.
  • Excrete toxic metabolic by-products such as urea, ammonia, and uric acid.
The human kidneys:
  • are two bean-shaped organs, one on each side of the backbone.
  • Represent about 0.5% of the total weight of the body,
  • but receive 20–25% of the total arterial blood pumped by the heart.
  • Each contains from one to two million nephrons.
human excretory
Human Excretory
  • The urinary system is made-up of the kidneys, ureters, bladder, and urethra.
  • The nephron, an evolutionary modification of the nephridium, is the kidney's functional unit.
  • Waste is filtered from the blood and collected as urine in each kidney.
Urine leaves the kidneys by ureters, and collects in the bladder.
  • The bladder can distend to store urine that eventually leaves through the urethra.
nephron function
Nephron Function
  • Glomerular filtration of water and solutes from the blood.
  • Tubular reabsorption of water and conserved molecules back into the blood.
  • Tubular secretion of ions and other waste products from surrounding capillaries into the distal tubule.
The nephron is a tube; closed at one end, open at the other. It consists of a:
  • Bowman's capsule.  Located at the closed end, the wall of the nephron is pushed in forming a double-walled chamber.
  • Glomerulus.  A capillary network within the Bowman's capsule. Blood leaving the glomerulus passes into a second capillary network surrounding the
  • Proximal convoluted tubule. Coiled and lined with cells carpeted with microvilli and stuffed with mitochondria.
Loop of Henle.  It makes a hairpin turn and returns to the
  • Distal convoluted tubule, which is also highly coiled and surrounded by capillaries.
  • Collecting tubule. It leads to the pelvis of the kidney from where urine flows to the bladder and, periodically, out to the outside world.
formation of urine
Formation of Urine
  • Blood enters the glomerulus under pressure.
  • This causes water, small molecules (but not macromolecules like proteins) and ions to filter through the capillary walls into the Bowman's capsule. This fluid is called nephric filtrate.
  • It is simply blood plasma minus almost all of the plasma proteins. Essentially it is no different from interstitial fluid.
Nephric filtrate collects within the Bowman's capsule and then flows into the proximal tubule.
  • Here all of the glucose, and amino acids, >90% of the uric acid, and ~60% of inorganic salts are reabsorbed by active transport.
  • The active transport of Na+ out of the proximal tubule is controlled by angiotensin II.
The active transport of phosphate (PO43-) is regulated (suppressed by) the parathyroid hormone.
  • As these solutes are removed from the nephric filtrate, a large volume of the water follows them by osmosis (80–85% of the 180 liters deposited in the Bowman's capsules in 24 hours).
As the fluid flows into the descending segment of the loop of Henle, water continues to leave by osmosis because the interstitial fluid is very hypertonic. This is caused by the active transport of Na+ out of the tubular fluid as it moves up the ascending segment of the loop of Henle.
  • In the distal tubules, more sodium is reclaimed by active transport, and still more water follows by osmosis.
hormone control
Hormone Control
  • Water reabsorption is controlled by the antidiuretic hormone (ADH) in negative feedback.
  • ADH is released from the pituitary gland in the brain.
  • Dropping levels of fluid in the blood signal the hypothalamus to cause the pituitary to release ADH into the blood.
ADH acts to increase water absorption in the kidneys. This puts more water back in the blood, increasing the concentration of the urine.
  • When too much fluid is present in the blood, sensors in the heart signal the hypothalamus to cause a reduction of the amounts of ADH in the blood.
  • This increases the amount of water absorbed by the kidneys, producing large quantities of a more dilute urine.
  • Aldosterone, a hormone secreted by the kidneys, regulates the transfer of sodium from the nephron to the blood.
  • When sodium levels in the blood fall, aldosterone is released into the blood, causing more sodium to pass from the nephron to the blood.
  • This causes water to flow into the blood by osmosis. Renin is released into the blood to control aldosterone.