13: Gas Exchange + Circulation Pt. 1

• Surface area determines the rate at which nutrients/gases diffuse across a membrane and the rate at which waste products diffuse out
• As an object gets larger, its volume increases much faster than its surface area does
• Partial Pressure: the pressure of a particular gas in a mixture of gases
• Oxygen and CO2 diffuse between the environment and cells along their respective partial-pressure gradients
• In both air and water, oxygen and CO2 move from regions of high partial pressure to regions of low partial pressure
• At the top of Mt. Everest, the partial pressure of O2 is low and thus, it is harder to breathe and take in the needed oxygen

  • Because the partial pressure gradient between the atmosphere and your lung tissues is small
• Fick's Law of Diffusion state that the rate of diffusion of a gas depends on 5 parameters:

  1. Solubility of the gas
  2. Temperature
  3. Surface area available for diffusion
  4. Differences in partial pressures of the gas across the gas-exchange surface
  5. Thickness of the barrier in diffusion
• Fick's law identified traits that allow animals to maximize the rate at which oxygen and CO2 diffuse across surfaces

  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
• Many small animals exchange gases by direct diffusion across the body surface

  • This is possible due to high surface-area-to-volume ratio
• Respiratory organs provide a greater surface are for gas exchange
• Gas exchange involves 5 major steps:

  1. Ventilation: the movement of air or water through a specialized gas exchange organ
  2. Diffusion at the respiratory surface: O2 moves from air or water -> the blood and CO2 moves from blood -> air or water
  3. Circulation: transport of dissolved O2 and CO2 throughout the body via the circulatory system
  4. Diffusion at the tissues: O2 from blood -> tissues, CO2 from tissues -> blood
  5. Cellular respiration: cell's use of O2 + production of CO2
  6. Leads to low [O2] and high [CO2] in tissues
• Ventilation and diffusion at the respiratory surface are accomplished by the respiratory system

  • In some animals, this is the skin, but in most species, it involves a specialized organ like lungs, gills, or trachea
• Respiratory system: the collection of cells, tissues, and organs responsible for gas exchange
• Circulatory system: responsible for moving O2, CO2, and other materials around the body

  • A muscular heart propels a special liquid transport tissue through the body
• Aquatic animals live in an environment that contains much less oxygen than the environment inhabited by terrestrial animals

  • To extract a given amount of oxygen, an aquatic animal has to process 30 times for water than the amount of air a terrestrial animal breathes
• Water is about 1000x times denser than air and much more viscous

  • Water breathers have to expend more energy to ventilate their respiratory surfaces than air breathers do
• Gills:

  • Outgrowths of the body surface or throat, used for gas exchange in aquatic animals
  • Present an extremely large surface are for O2 to diffuse across an extremely thin epithelium
  • Gills can be external or internal
• Most fishes ventilate their gills by opening and closing their mouths and the operculum, creating a pressure gradient that moves water over the gills
• Operculum: a stiff flap over the gills
• Particularly fast swimmers, like tuna, force water through their gills by swimming with their mouth open, through a process called ram ventilation
• Movement of water through gills s in one direction over long, thin structures called gill filaments that extend from each gill arch
• Each gill filament is composed of hundreds or thousands of sheetlike gill lamellae

  • A bed of small blood vessels called capillaries runs through each lamella
• The flow of blood through the capillaries is in the opposite direction to the flow of water over the gill surface

  • This sets up a countercurrent exchange system in each lamella
• The countercurren exchange system in each lamella results in 2 advantages:

  1. A slight gradient in partial pressure of O2 b/w the water and bood exists along the entire length of the lamella
  2. A large difference in O2 partial pressure exists b/w the start and end of the system
• The trachea carries inhaled air to narrow tubes called bronchi

  • The bronchi branch off into even narrower tubes called bronchioles
• The organ of ventilation is the lung

  • The lung encloses the bronchioles and portions of the bronchi
• Mammalian lungs are divided into tiny sacs called alveoli, which greatly increase the surface area for gas exchange
• Humans have approximately 150 million alveoli per lung
• Alveoli provide an interface between air and blood that consists of:

  • A thin aqueous film
  • A layer of epithelial cells
  • Some extracellular matrix (ECM) material
  • The wall of a capillary
• Negative pressure ventilation is used by humans and other mammals

  • Ventilate their lungs by changing pressure withing their chest cavity
  • The change in volume is caused by a downward motion of a thin muscular sheet called the diaphragm
• Each lung is surrounded by a pleural sac

  • 2 layers of cells with small space between them
  • Pleural cavity
  • Pleural cavity contains a small volume of pleural fluid
  • Intrapleural pressure is subatmospheric
  • Keeps lungs inflated
• 2 steps are involved - inhalation and exhalation

  1. During inhalation, the diaphragm moves down and the pressure in the chest cavity is lowered, causing the lungs to expand and air to move in
  2. During exhalation, as the diaphragm relaxes, the chest cavity volume decreases, and air is exhaled
  3. This is a passive process driven by the elastic recoil of the lungs and chest wall as the diaphragm and rib muscles relax
  4. It can become energy demanding during exercise
• At rest, the mammalian rate of breathing is established by the medullary respiratory centre, an area at the base of the brain
• The medullary respiratory centre stimulates the rib and diaphragm muscle to expand and contract
• During exercise, muscles take up more oxygen from the blood

  • The partial pressure of oxygen in the blood (PO2) decreases
  • At the same time, the muscles release larger amounts of CO2, increasing its partial pressure (PCO2) in the blood
• Increased CO2 reacts with water in the blood and cerebrospinal fluid (CSF) to form carbonic acid, H2CO3, which quickly dissociates into a hydrogen ion and a bicarbonate ion, HCO3-
• The release of hydrogen ions lowers the blood and CSF pH, which is sensed by specialized neurons, leading to the medullary respiratory centre and increasing the breathing rate
• Increased breathing rate:

  • Increases the rate of oxygen delivery to tissues and muscles
  • Increases the rate of elimination of CO2
  • Maintains relatively constant partial pressure of O2 and CO2, even during intense exercise
• Blood is a connective tissue consisting of cells in a watery extracellular matrix called plasma
• Blood has many functions:

  • Transport O2 and CO2
  • Transport nutrients of cells from the digestive system
  • Move waste products to the kidney and liver
  • Convey hormones to target tissues and organs
  • Deliver immune system cells to the site of infection
  • Distribute heat throughout the body