BI0004 - Lecture 11 - Gas exchange

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  1. What does gas exchange involve?
    Ventilation, circulation, and respiration.

    Ventilation occurs when air or water moves through a specialized gas-exchange organ.

    Gas exchange takes place by diffusion between air/water and blood at the respiratory surface. Molecular oxygen is taken up from the environment and molecular CO2 is discharged

    Circulation transports O2, CO2, nutrients and wastes throughout the body.

    Gas exchange takes place between blood and cells in tissues where cellular respiration has led to low [O2] and high [CO2]
  2. How do O2 and CO2 diffuse?
    Comment on partial pressure and give examples.
    O2 and CO2 diffuse down their partial pressure gradients.

    Partial pressure = Pressure of a particular gas in a mixture.

    Partial pressure = fractional composition of a particular gas in the mixture x the total pressure exerted by the mixture.

    • Air pressure at sea level = 760mm Hg
    • PO2 = 0.21 x 760 = 160 mm Hg
    • PCO2 = 0.0003 x 760 = 0.29 mm Hg

    Gases in the air dissolve in water until they reach equilibrium, at which their partial pressure in the water is the same as their partial pressure in the air (e.g. water at sea level has a PO2 of 160 mm Hg).

    However, the concentration of O2 is much lower in water than in air, because O2 is much less soluble in water than it is in air.
  3. What factors influence the concentration of dissolved gases?
    Solubility: O2 is poorly soluble in water. Blood contains a carrier molecule that binds O2 and carries it to the tissues.

    Temperature: as temperature increases, solubility decreases.

    Presence of other solutes: as the concentration of other solutes increases, less gas can dissolve in the water.

    Partial pressure of the gas in contact with water: as altitude increases, atmospheric PO2 decreases so PO2 in water decreases.

    Warm, salty, deep, or stagnant aquatic environments contain less oxygen than cold, fresh, shallow, or turbulent environments.

    The concentration of O2 in water is around 40x less than in air. Water is also more dense and more viscous.

    Water breathers face more challenges than air breathers.
  4. How are gases exchanged on the respiratory surface?
    Cells must be bathed in liquid in order to maintain the plasma membrane, therefore the respiratory surface must be moist, so gases can dissolve then diffuse across the membrane.

    Movement across the respiratory surface is entirely by diffusion.

    Rate of diffusion is proportional to the surface area and inversely proportional to the square of the distance moved.

    Consequently, respiratory surfaces tend to be large and thin.

    • The structure of the respiratory surface depends on the organism's:
    • size (SA:V)
    • habitat (Terrestrial or aquatic)
    • metabolic demands (ectothermic or endothermic)
  5. How do gases exchange in protists, sponges, cnidarians and flatworms?
    Gases exchange directly with the environment in protists, sponges, cnidarians and flatworms.
  6. How does respiration work for earthworms and some amphibians?
    The skin functions as the respiratory organ in earthworms and some amphibians.

    • A dense capillary network just beneath the skin facilitates gas exchange between the circulatory system and the environment.
    • Skin breathers can only live in water or damp environments.

    For most animals, the body surface is insufficient for gas exchange, and/or the environment is not moist enough. A respiratory organ is needed, which is extensively folded or branched to maximise the surface area for gas exchange.
  7. How does respiration work for aquatic animals?
    Aquatic animals have gills.

    Gills are outfoldings of the body surface or throat that are suspended in water and present a large surface area for gas exchange. They may be internal or external.
  8. Comment on invertebrate gills.
    Invertebrate gills are diverse.

    Sea star (echinoderm) gills are simple projections of skin extending from the coelom (body cavity). Fluid in the coelom circulates in and out of the gills, aiding gas transport. Tube feet surfaces also function in gas exchange.

    Scallop (mollusc) gills are flattened plates projecting from the body inside the shell. Cilia circulate water over the gill surfaces.

    Marine polychaete (annelid) worms have paired flattened appendages (parapodia) that function as gills for crawling and swimming.

    Crayfish and other crustaceans have long feathery gills within the exoskeleton. Specialised paddle like appendages drive water over the gill surfaces.
  9. Comment on the structure of Bony fish gills?
    • Bony fish gills are all similar structures.
    • Fish gills are located on both sides of the head and consist of four arches.

    Coordinated opening and closing of the mouth and operculum creates a pressure gradient that moves water over the gills. During swimming with the mouth open, water can be forced through the gills (ram ventilation)

    Water flows in only one direction

    Each arch holds two rows of gill filaments, each consisting of flattened plates (lamellae). Blood flowing through the lamellae picks up O2 from the water.
  10. What does it mean to say that fish gills are a counter current exchange system?
    Countercurrent exchange: The exchange of a substance or heat between two fluids (blood and water) flowing in opposite directions.

    • O2-poor blood entering the gill capillary meets O2-depleted water, but the water PO2 is still greater than the blood PO2.
    • As the blood passes down the capillary, its PO2 increases and so does that of the water.
    • A PO2 gradient favours diffusion of O2 from water to blood along the whole length of the capillary
  11. Where are gills suited to?
    Gills can be extremely efficient, but are only suited to aquatic animals.

    Fish gills extract more than 80% of the dissolved O2

    The low O2 content, density and viscosity of water means water-breathers expend considerable energy in carrying out gas exchange

    Gills are only suitable for aquatic animals because, in air, the wet membrane surface would lose too much water through evaporation, and the fine gill filaments would collapse without water to support them.

    In terrestrial animals the respiratory surfaces are enclosed within the body, exposed to the air only though narrow tubes.
  12. How does respiration work in insects?
    Insect tracheae transport air close enough to cells to allow gas exchange across their membranes.

    • An extensive system of tubes (tracheae) within the body connect to the outside via openings (spiracles) which can be closed to limit water loss.
    • The ends of the tracheae are highly branched tracheoles, filled with fluid.
    • During activity, the fluid can be withdrawn into the body to increase the SA of air-filled tracheoles in contact with cells.
  13. In insects, what happens when demand for oxygen is high?
    Muscle activity ventilates the trachae when demands for oxygen is high.

    Alternating contraction and relaxation of the flight muscles pumps air through the tracheal system, bringing O2 close to the numerous mitochondria needed to support their high metabolic rate during flight (10-200X more O2 consumed than at rest)

    The insect open circulatory system is not involved in gas transport, so no need for respiratory pigments in haemolymph.
  14. What are lungs?
    Lungs are localised respiratory organs, infoldings of the throat used for gas exchange

    Lungs are found in terestrail animals, and some aquatic animals that live in O2-poor water or that spend time exposed to air

    Most reptiles and all birds are wholly dependent on lungs.

    Amphibians may have small lungs, but rely primarily on diffusion across the skin for gas exchange

    Turtles have lungs but also use gas exchange across moist epithelial surfaces continuous with their mouth or anus.

    Some fish, including lungfish have lungs.

    Invertebrates including spiders and land snails have lungs.

    Lung respiratory surface is not in direct contact with the rest of the body, so the circulatory system bridges the gap.
  15. How does air movement take place in spiders and snails?
    In spiders and snails, air movement takes place by diffusion only.

    The snail's mantle cavity is highly vascularised, and provides a surface for gas exchange. Air enters through a single opening, the pneumostome.

    An air filled cavity (book lung) in the spider's abdomen consists of stacks of haemolymph-filled plates. The plates maximise the surface area and so maximise gas exchange with the atmosphere. A Tracheal system may also be present.
  16. What ventilation do frogs and other amphibians use?
    Frogs and other amphibians have simple blood vessel-lined sacs, and use positive pressure ventilation

    The frog lowers the floor of this throat, increasing its volume and drawing air through the nasal passages and into the oral cavity.

    It then closes its nasal passages and contracts its throat muscles, increasing the pressure on the air in the oral cavity, and forcing it into the lungs.

    Elastic recoil of lungs and compression by body wall force air out of lungs during exhalation.
  17. What ventilation method is used in mammals?
    In mammals, negative pressure ventilation is used.

    Contraction of the diaphragm and rib muscles increases the lung volume, lowering pressure and causing the lungs to expand and draw air in.

    Pressure within the cavity changes from -5 mm Hg to -8 mm Hg relative to the atmosphere.

    Air passes through progressively smaller passages to the alveoli.

    Relaxation of the diaphragm and rib muscles causes the volume of the lung cavity to decrease, so internal pressure increases, lung volume decreases, and air flows out.
  18. Where does gas exchange take place in mammals?
    In mammals, lungs are finely divided into alveoli, where gas exchange takes place

    Alveoli are lined by a thin, moist epithelium across which gases can diffuse into or out of the network of capillaries.

    Branches of the pulmonary arteries carry O2-poor blood to the alveoli; branches of the pulmonary veins transport O2-rich blood to the heart.
  19. In mammals, what does each inhalation result in?
    In mammals, each inhalation mixes fresh air with oxygen-depleted air.

    The residual volume (air remaining after forced exhalation) and dead space (air moving in and out that occupies areas without a respiratory surface) means that maximum pO2 in alveoli is much less than in air. Humans extract ~25% of the O2.

    A giraffe has smaller lungs than predicted from its body size, and the long trachea means increased dead space.

    Limited space in the abdominothoracic cavity has constrained lung size. It compensates with increased tidal volume to give a dead space:tidal volume ratio similar to other mammals.

    Despite the slow breathing needed to avoid wind damage to the narrow trachea, sufficient air is exchanged per breath to meet its gas exchange requirements
  20. How are birds adapted of ventilation (steps)?
    Birds are adapted for very efficient ventilation.

    8-9 air sacs are present in addition to lungs, and function to keep air flowing over lungs. Gas exchange takes place in tiny channels (parabronchi) rather than alveoli (blind sacs).

    Passage of air through the entire system (lungs and air sacs) requires two cycles of inhalation and exhalation.

    1. Inhalation: air enters posterior air sacs

    2. Exhalation: posterior air sacs contract, air enters parabronchi in posterior of lung.

    3. Inhalation: air moves anteriorly through parabronchi and into anterior air sacs.

    4. Exhalation: anterior air sacs contract, air is expelled from body
  21. How are birds adapted for very efficient ventilation (description)?
    Airflow within the lung parabronchi is unidirectional (contrasts with alveoli)

    The entire system is ventilated - dead space is limited to just the distance between the mouth and the anterior air sacs - so inhaled air is used more efficiently than in mammals.

    Gas exchange occurs during both inhalation and exhalation (compared with only during inhalation in mammals).

    Blood circulates through the lung in capillaries that are perpendicular to the parabronchi - this crosscurrent circulation - is much more efficient than the weblike capillaries surrounding mammalian alveoli (but less so than fish countercurrent gill circulation).

    Maximum pO2 in bird lungs is greater than in mammal lungs - function better at high altitudes.
  22. What can bar-headed geese do?
    They can migrate over the himalayas at an altitude where humans struggle to obtain enough O2.
  23. What has evolution of respiratory pigments greatly increased?
    The O2-carrying capacity of the circulatory fluid.

    Only 4.5 ml O2 dissolves in 1L fluid at normal body temperature and air pressure. Even assuming a high rate of O2 transfer to tissues, many hundreds of litres of blood would have to be pumped every minute to supply the 2L/min O2 needed during intense excercise.

    Haemoglobin increases the amount of O2 that can be transferred in mammalian blood to 200 ml/L

    Evolution of pigments such as haemoglobin allowed cellular respiration rates to increase, and so contributed to the diversification of animals.

    High rates of ATP production allowed increased rates of growth, movement, digestion etc.
  24. What is haemoglobin?
    Haemoglobin is found in almost all vertebrates (within erthrocytes) and in some invertebrates.

    Each subunit of a haemoglobin tetramer has a haem group with an iron atom at its centre, which can bind one molecule of O2

    Oxygenated haemoglobin is bright red, deoxygenated haemoglobin is dark red.
  25. What is Haemocyanin?
    Haemocyanin is found in arthropods and many molluscs, suspended directly in the haemolymph.

    Haemocyanin has copper as its O2-binding component

    Oxygenated haemocyanin is blue, deoxygenated haemocyanin is colourless.
  26. What is myoglobin?
    Myoglobin is an oxygen-storage protein found in muscles, especially abundant in marine mammals.

    Animals that stay submerged for extended periods and/or dive to great depths have additional adaptations.

    A seal can store 2x O2/kg body mass than a human, achieved by a large spleen and a high concentration of myoglobin in the muscles. Other adaptations conserve O2 when underwater.
  27. What does the pronghorn show?
    The pronghorn shows adaptations at every stage of oxygen metabolism: uptake, delivery and use.

    Its maximal rate of O2 consumption (VO2max) is 5x that of a domestic goat. Its physiology allows it to maintain a speed averaging 65 km/h over long distances.
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BI0004 - Lecture 11 - Gas exchange
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