 Cells require O 2 for aerobic respiration and expel CO 2 as a waste product

 Gas exchange involves the diffusion of gases across a membrane  Rate of diffusion (R) is governed by Fick’s Law:  R = DA p d D= diffusion constant (size of molecule, membrane permeability, etc) A= area over which diffusion occurs p = pressure difference between sides of the membrane d = distance across which diffusion must occur

R = DA p d To maximize diffusion, R can be increased by: Increasing A ( area over which diffusion occurs ) Increasing p ( pressure difference between sides of the membrane ) Decreasing d ( distance across which diffusion must occur ) Evolutionary changes have occurred to maximize R

 The part of the organism across which gases are exchanged with the environment is the respiratory surface

 Must be moist ◦ plasma membranes must be surrounded by water to be stable  Must be sufficiently large ◦ maximize A in Fick’s Law

 Cell Membranes ◦ in unicellular organisms ◦ some simpler animals (sponges, cnidarians, flatworms)

 Respiratory surface = a single layer of epithelial cells ◦ separates outer respiratory medium (air or water) from the organism’s transport system (blood)

 Skin (cutaneous respiration) ◦ Skin must be moist ◦ organisms with flat or wormlike bodies so skin in sufficient surface area ◦ or in frogs and some turtles to supplement respiration using lungs

 Specialized region of body is folded and branched to provide large surface area  This maximizes A in Fick’s Law  Also decrease d by bringing the respiratory medium close to the internal fluid  Three such systems: ◦ Gills (Aquatic organisms) ◦ Trachea (insects) ◦ Lungs (terrestrial vertebrates)

 most aquatic organisms  outfoldings of the body surface specialized for gas exchange  Water is the respiratory medium

 Respiratory surface always moist  Oxygen content of water is much less than that of air  denser medium so harder to ventilate

 requires a lot of energy to ventilate gills b/c water is denser than air  pumping operculum, ram ventilation

Water flow Water flow Gill arch Operculum Gills Gill filaments Gill filament Oxygen- deficient blood Oxygen- deficient blood Oxygen- rich blood Oxygen- rich blood Gill raker Blood flow Lamellae with capillary networks Water flow Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.

 Enhances gas exchange in the gills of fish  blood is continually loaded with O 2 b/c it meets water with increasing O 2 concentration

Blood (0% O 2 saturation) Water (15% O 2 saturation) Blood (85% O 2 saturation) Water (100% O 2 saturation) Blood (50% O 2 saturation) 60% Blood (0% O 2 saturation) Water (100% O 2 saturation) Water (50% O 2 saturation) No further net diffusion 40% 30%70% 20%80% 10%90% Countercurrent ExchangeConcurrent Exchange 15% 30% 40% 50% 60% 70% 80% 90% 100% 10% 20% 30% 40% 50% 60% 70% 80% 85% a.a.b. Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. 50%

 Higher oxygen concentration  ventilation is easier b/c air is less dense  respiratory surface loses water to air by evaporation

 Air tubes that branch throughout the body  finest tubes (tracheoles) extend to nearly every cell in the body  gas diffuses across moist epithelium that lines the terminal ends

 Found in insects  Open Circulatory system of insects is NOT involved in transporting gases  Ventilation ◦ diffusion ◦ body movements

 Localized in one area of body ◦ circulatory system must transport gases  Ventilation ◦ Negative pressure breathing- mammals

Expiration Inspiration Muscles contract Lungs Air Sternocleido- mastoid muscles contract (for forced inspiration) Diaphragm contracts Muscles relax Diaphragm relaxes a.a. b. Abdominal muscles contract (for forced expiration) Air Negative pressure breathing

Nasal cavity Nostril Larynx Trachea Right lung Left lung Pharynx Left bronchus Glottis Diaphragm Pulmonary venule Pulmonary arteriole Blood flow Bronchiole Alveolar sac Alveoli Capillary network on surface of alveoli Smooth muscle Lungs

 Ventilation ◦ air sacs act as bellows in birds  air flows in one direction during both inhalation & exhalation

Anterior air sacs Posterior air sacs Parabronchi of lung Trachea Inspiration Expiration InspirationExpiration Cycle 2 Cycle 1 Trachea Anterior air sacs Lung Posterior air sacs a.a.b. Lungs of Birds

 Occurs in the circulatory system when needed  O 2 is transported by respiratory pigments ◦ hemoglobin on red blood cells or hemocyanin in the plasma

 CO 2 is transported by respiratory pigments and dissolved in the plasma and in red blood cells as bicarbonate ion (HCO 3 - )