Key Concepts Animals have to take in oxygen and expel carbon dioxide to sustain cellular respiration and stay alive Terrestrial animals and aquatic animals face different challenges in performing gas exchange Gas exchange organs maximize the rate of O2 and CO2 diffusion by 1 presenting a large thin surface area to the environment and 2 maintaining a steep partial pressure gradient that favors entry of O2 and elimination of CO2 2011 Pearson Education Inc Key Concepts Blood is a specialized tissue that transports gases along with nutrients and wastes in some animals Hemoglobin is an oxygencarrying protein that is extremely efficient at taking up oxygen in the lungs and delivering it to tissues Circulatory systems use the pressure generated by one or more hearts to transport blood and other substances throughout the body 2011 Pearson Education Inc Introduction Oxygen required for and carbon dioxide produced by cellular respiration must be continuously exchanged with the environment In addition these gases along with wastes nutrients and other types of molecules must be transported throughout the body 2011 Pearson Education Inc The Respiratory and Circulatory Systems Cells must obtain oxygen and expel carbon dioxide continuously to support ATP production by mitochondria Gas exchange involves four steps ventilation gas exchange circulation and cellular respiration 2011 Pearson Education Inc The Four Steps of Gas Exchange 1 Ventilation occurs when air or water moves through a specialized gas exchange organ such as lungs or gills 2 Gas exchange takes place as CO2 and O2 diffuse between air or water and the blood at the ventilatory surface 3 Through circulation the dissolved O2 and CO2 are transported throughout the body 4 Gas exchange between blood and cells occurs in tissues where cellular respiration has led to low O2 levels and high CO2 levels O2 and CO2 diffuse between blood and cells 2011 Pearson Education Inc 2011 Pearson Education Inc The Respiratory and Circulatory Systems Ventilation and gas exchange are accomplished by the respiratory system The respiratory system is comprised of the structures responsible for gas exchange between the individual and its environment In some animals the gas exchange surface is the skin but in most species it is located in a specialized organ like the lungs of tetrapods the tracheae of insects or the gills found in mollusks arthropods and fish 2011 Pearson Education Inc The Respiratory and Circulatory Systems The circulatory system is responsible for moving O2 CO2 and other materials around the body In most species a specialized liquid transport tissue is propelled throughout the body by a muscular heart through a system of vessels 2011 Pearson Education Inc Air and Water as Respiratory Media Gas exchange between the environment and cells is based on diffusion Oxygen is high in the environment and low in tissues while carbon dioxide is high in tissues and low in the environment Oxygen thus tends to move from the environment into tissues and carbon dioxide tends to move from tissues to the environment 2011 Pearson Education Inc How Do O2 and CO2 Behave in Air To understand how gases move by diffusion it is important to express their presence in terms of partial pressures instead of percentages Partial pressure is the pressure of a particular gas in a mixture of gases To calculate the partial pressure of a particular gas multiply the fractional composition of that gas by the total pressure exerted by the entire mixture 2011 Pearson Education Inc 2011 Pearson Education Inc How Do O2 and CO2 Behave in Air Oxygen and carbon dioxide diffuse between the environment and cells along their respective partial pressure gradients In both air and water oxygen and carbon dioxide move from regions of high partial pressure to regions of low partial pressure 2011 Pearson Education Inc How Do O2 and CO2 Behave in Water To obtain oxygen water breathers face a much more challenging environment than air breathers do Water contains much less oxygen than air does To extract a given amount of oxygen an aquatic animal has to process 30 times more water than the amount of air a terrestrial animal breathes Water is about a thousand times denser than air and flows less easily so water breathers have to expend more energy to ventilate their respiratory surfaces than do air breathers 2011 Pearson Education Inc What Affects the Amount of Gas in a Solution Oxygen and carbon dioxide diffuse into water from the atmosphere but the amount of gas that dissolves depends on several factors 1 The solubility of the gas in water 2 The temperature of the water 3 The presence of other solutes 4 The partial pressure of the gas in contact with the water 2011 Pearson Education Inc What Affects O2 Availability in Aquatic Habitats The partial pressure of oxygen varies in different types of aquatic habitats Habitats with large numbers of photosynthetic organisms tend to be relatively oxygen rich while habitats where most organisms live off existing organic material tend to be oxygen poor The most important issue in oxygen availability however is often surface area which has a large impact on oxygen s ability to diffuse into water 2011 Pearson Education Inc Organs of Gas Exchange Many small animals exchange gases by direct diffusion across the body surface They mostly must live in wet environments Large animals or those that live in dry environments need a specialized organ for gas exchange Respiratory organs provide a greater surface area for gas exchange enough to meet the demands of a large body filled with cells 2011 Pearson Education Inc Physical Parameters The Law of Diffusion Fick s law identifies traits that allow animals to maximize the rate at which oxygen and carbon dioxide diffuse across surfaces Specifically Fick s law states that all gases including O 2 and CO2 diffuse in the largest amounts when three conditions are met 1 The surface area for gas exchange is large 2 The respiratory surface is extremely thin 3 The partial pressure gradient of the gas across the surface is large 2011 Pearson Education Inc 2011 Pearson Education Inc How Do Fish Gills Work Gills are outgrowths of the body surface or throat used for gas exchange in aquatic animals Gills are efficient solutions to the problems posed by water breathing because they present an extremely large surface area for oxygen to diffuse across an extremely thin epithelium Among invertebrates the structure of gills is extremely