Exchange Surfaces

4 Key excerpts on "Exchange Surfaces"

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  eBook - ePub

  The Respiratory System

  • Britannica Educational Publishing, Kara Rogers(Authors)
  • 2010(Publication Date)
  • Britannica Educational Publishing

    (Publisher)

  AS EXCHANGE AND RESPIRATORY ADAPTATION

  I nhaled air is rich in oxygen, which is needed to support the functions of the body’s various tissues. For inhaled oxygen to reach these tissues, however, it must first undergo a process of gas exchange that occurs at the level of the alveoli in the lungs. Blood vessels that pass alongside the alveoli membranes absorb the oxygen and, in exchange, transfer carbon dioxide to the alveoli. The oxygen is then distributed by the blood to the tissues, whereas the carbon dioxide is expelled from the alveoli during exhalation. At high altitudes or during activities such as deep-sea diving, the respiratory system, as well as other organ systems, adapt to variations in atmospheric pressure. This process of adaptation is necessary to maintain normal physiological function.

  GAS EXCHANGE

  Respiratory gases—oxygen and carbon dioxide—move between the air and the blood across the respiratory Exchange Surfaces in the lungs. The structure of the human lung provides an immense internal surface that facilitates gas exchange between the alveoli and the blood in the pulmonary capillaries. The area of the alveolar surface in the adult human is about 160 square metres (1,722 square feet). Gas exchange across the membranous barrier between the alveoli and capillaries is enhanced by the thin nature of the membrane, about 0.5 micrometre, or 1/100 of the diameter of a human hair.

  Changes in the atmosphere’s pressure occur when deep-sea diving and require the respiratory system to adapt . Shutterstock.com

  Respiratory gases move between the environment and the respiring tissues by two principal mechanisms, convection and diffusion. Convection, or mass flow, is responsible for movement of air from the environment into the lungs and for movement of blood between the lungs and the tissues. Respiratory gases also move by diffusion across tissue barriers such as membranes. Diffusion is the primary mode of transport of gases between air and blood in the lungs and between blood and respiring tissues in the body. The process of diffusion is driven by the difference in partial pressures of a gas between two locales. In a mixture of gases, the partial pressure of each gas is directly proportional to its concentration. The partial pressure of a gas in fluid is a measure of its tendency to leave the fluid when exposed to a gas or fluid that does not contain that gas. A gas will diffuse from an area of greater partial pressure to an area of lower partial pressure regardless of the distribution of the partial pressures of other gases. There are large changes in the partial pressures of oxygen and carbon dioxide as these gases move between air and the respiring tissues. The partial pressure of carbon dioxide in this pathway is lower than the partial pressure of oxygen, caused by differing modes of transport in the blood, but almost equal quantities of the two gases are involved in metabolism and gas exchange.

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  Exercise Physiology

  for Health and Sports Performance

  • Nick Draper, Helen Marshall(Authors)
  • 2014(Publication Date)
  • Routledge

    (Publisher)

  surfactant. Surfactant is chiefly composed of lipoproteins and phospholipids and is essential to gaseous exchange.

  The secreted alveolar fluid lines the inner surface of the alveolar walls. If composed solely of water, alveolar fluid would result in the collapse of alveoli between breaths due to its high surface tension. The surfactant within the alveolar fluid, however, reduces the surface tension, as lipids and water do not mix well, maintaining the alveolar structure during pulmonary ventilation. Macrophages are a third type of alveolar cell; they serve an immune function and maintain the gaseous Exchange Surfaces by removing debris from the alveoli.

  Gas exchange

  As can be seen from Figure 6.4 , alveoli are surrounded by an intricate network of capillaries. Gaseous exchange takes place across the respiratory membrane formed between the capillaries and the alveoli. This consists of the cellular walls of the alveoli and capillaries bound together by their fused protective basement membranes. Despite the complexity of the respiratory membrane, which is required to protect and maintain the integrity of the alveoli and capillaries, the structure is very thin. The membrane is 15 times thinner than a sheet of tissue paper, enabling gaseous exchange to proceed very rapidly. The unique structure of the lungs, which are thought to be comprised of over 300 million alveoli, means there is a vast surface area available for gaseous exchange between the alveoli and the millions of capillaries that surround them. This surface area is around 75 m2 , almost the equivalent of a singles badminton court.

  Gaseous exchange takes place by diffusion, the flow of gases across the respiratory membrane from high to low concentration. Through this process, carbon dioxide which is in higher concentration in the capillaries, diffuses into the alveoli while oxygen, in higher concentration in the alveoli, diffuses into the capillaries. The concentration of a gas is measured by its partial pressure, first described by the English scientist John Dalton and subsequently known as Dalton’s law. Table 6.1 shows the percentage and partial pressure of each of the gases and water vapour in atmospheric air. Dalton’s law states that the total pressure of a mixture of gases (such as that found in air) is equivalent to the sum of the partial pressures of each of the gases. The partial pressure of oxygen (PO2 ) needs to be higher in the alveolar air than in the blood for oxygen to diffuse into the bloodstream. Similarly, if the partial pressure of carbon dioxide (PCO2

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  Physiology for Dental Students

  • D. B. Ferguson(Author)
  • 2014(Publication Date)
  • Butterworth-Heinemann

    (Publisher)

  Although the word respiration is derived from the Latin for breathing, the term has been widened to include those biological oxidations necessary for energy production. Respiration, then, includes inspiration and expiration (the movement of air in and out of the lungs), the exchange of oxygen and carbon dioxide at the lung membranes, the transport of oxygen and carbon dioxide to and from the tissues of the body, and the chemical processes of oxidation of foodstuffs. The last of these topics is usually dealt with by biochemists rather than physiologists.

  The apparatus of breathing

  The lungs and respiratory passages, or airways, are an invagination of the body surface specifically modified for the easy and rapid transfer of the respiratory gases, oxygen and carbon dioxide, to and from the blood. Air is moved actively through the nose, passes the oro-nasal cavity, and then reaches the trachea which divides into a right and a left bronchus both of which subdivide into bronchioles. These divide into smaller and smaller branches until they terminate in bubble-like sacs, the alveoli. These are formed of a single layer of epithelial cells and profusely covered with capillaries. Capillary blood is separated from the air in the alveoli by two cell layers only - the alveolar epithelium and the capillary endothelium. Special secretory cells in the lungs, pneumocytes, produce a phospholipid-lecithin material which forms a layer over the inner surface (the air surface) of the alveoli and acts as a surfactant, lowering the surface tension of the fluid lining the walls. This is important because the extremely thin walls of the alveoli are very responsive to changes in pressure. The alveoli behave rather as if they were bubbles of varying sizes on the ends of the bronchiolar tubes. The forces exerted on the walls of bubbles at equilibrium are described by the Law of Laplace

  where P is tThe vascular tubing he pressure inside the bubble tending to expand it, T is the tension in the bubble wall tending to contract it, and r is the radius of the bubble. This Law (already mentioned in Chap. 7 to explain some of the characteristics of the heart and blood vessels) predicts that if the surface tension of a bubble remained constant the pressure needed to maintain the bubble would increase as the radius decreased. Thus small bubbles collapse more readily than large bubbles. Collapse of the alveoli is prevented because the surface tension of the surfactant decreases as its film thickness increases. As the alveoli are deflated and decrease in radius, the surfactant lining spread over a smaller surface increases in thickness and lowers the surface tension to equilibrate the forces on the alveolar walls. The total surface area of the alveoli is of the order of 60 m2

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  Back to Basics in Physiology

  O2 and CO2 in the Respiratory and Cardiovascular Systems

  • Juan Pablo Arroyo, Adam J. Schweickert(Authors)
  • 2015(Publication Date)
  • Academic Press

    (Publisher)

  But most vitally important (pun intended) would be the cells whose job is to make sure that all of these specialized cells have enough energy to do all of these things; cells that help supply the rest of the cells in our body with O 2 and help rid the body of excess CO 2. And these are the cells that make up the respiratory system, cardiovascular system, and blood. In the next chapter, we’re going to look at how the body engineered a solution to this problem: how to deliver oxygen from the atmosphere to the cell, and how to deliver carbon dioxide from the cell to the atmosphere. As we’ll see, rather than trying to reinvent the wheel, the body relies on diffusion to do most of the work. At every level of the body, diffusion is what drives gas exchange. Whether it’s through creating a larger gradient, maximizing surface area, or minimizing the distance oxygen and carbon dioxide need to travel, the body engineered a system where diffusion does most of the work. It will become apparent in subsequent chapters that the lungs, heart, blood vessels, and blood all work in symphony to make sure that a healthy gradient from the atmosphere to the cell is always maintained. They make sure that there is as much surface area as possible through which oxygen and carbon dioxide can diffuse, and they make sure that there is as little a distance over which it needs to take place. They simply serve to get the gases close to where they need to go, and they let diffusion do the rest. This is a recurrent theme throughout the body, and thus will be a recurrent theme throughout this book. Key Diffusion is what drives gas exchange to the trillions of cells within the body

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