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Gas Exchange: Animals.

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Presentation on theme: "Gas Exchange: Animals."— Presentation transcript:

1 Gas Exchange: Animals

2 Cellular Respiration All living things obtain the energy they need by metabolizing energy-rich compounds, such as carbohydrates and fats In most organisms, this metabolism takes place by respiration, a process that requires oxygen (and produces carbon dioxide, which must be removed from the body in animals)

3 Cellular Respiration Cellular respiration is the process by which animals and other organisms obtain the energy available in carbohydrates Cells take the carbohydrates into their cytoplasm where, through a series of metabolic reactions, it is broken down into ATP O2 is the oxidizing agent (electron acceptor) in plants and animals (aerobic) Bacteria and Archaea use inorganic molecules such as sulfur, methane, iron, and metal ions (anaerobic)

4 Gas Exchange Across Respiratory Surfaces
Gas exchange involves diffusion across membranes The external environment in gas exchange is always aqeuous Diffusion is passive; driven by the difference in O2 and CO2 on the two sides of the membrane The rate of diffusion is governed by Fick’s law of diffusion

5 Fick’s Law of Diffusion
R = rate of diffusion D = diffusion constant (molecule specific) A = area over which diffusion occurs Dp = pressure difference between two sides d = distance over which diffusion occurs R = D A Dp d

6 Gas Exchange Across Respiratory Surfaces
The rate of diffusion can be optimized by Increasing surface area Decreasing the distance over which diffusion occurs Increasing the concentration difference R = D A Dp d The evolution of respiratory systems has involved changes in all of these factors

7 Maximization of Gas Diffusion
The levels of O2 needed for cellular respiration cannot be obtained by diffusion alone over distances >0.5mm Multicellular organisms require structural adaptations to enhance gas exchange Increasing pressure difference Increasing area and decreasing distance

8 Maximization of Gas Diffusion
Increasing pressure difference, Δp – many organisms create a water current that continuously replaces the water over the respiratory surfaces (the part of an organism over which gases are exchanged with the environment) Cilia often used to produce this current Because of the continuous replenishment of water, the external oxygen concentration does not decrease along the diffusion pathway

9 Maximization of Gas Diffusion
Increasing area and decreasing distance – Vertebrates (and more complex invertebrates) possess respiratory organs that increase the surface area available for diffusion Gills, tracheae, and lungs These adaptations bring the external environment (air or water) close to the internal fluid such as blood or hemolymph, which is circulated throughout the body

10 Maximization of Gas Diffusion

11 Maximization of Gas Diffusion

12 Maximization of Gas Diffusion
H20 Large surface area, high blood flow Countercurrent exchange – deoxygenated blood flows in one direction, while oxygenated blood flows in the other; maintains a concentration gradient

13 Countercurrent gas exchange

14 Gills Gills are specialized extensions of tissue that project into water External gills are not enclosed within body structures; many fish and amphibian larvae External gills require constant movement to ensure contact with fresh (high O2) water axolotl

15 Gills Other types of aquatic animals evolved branchial chambers, which provide a means of pumping water past stationary (internal) gills Mantle cavity of mollusks – contraction of muscular walls of cavity draws water in towards gills (and expels it) Branchial chamber of crustaceans – located between body and exoskeleton, with an opening at a limb; movement of limb draws water through chamber and over gills

16 Gills In bony fishes, the gills are located between the oral cavity and the opercular cavities These two cavities operate as pumps that alternately expand Water is moved into the mouth, through the gills and out of the fish through the open operculum, or gill cover

17 Gills

18 Gas exchange in fish Mobile fish (such as tuna) swim with their mouth open to continuously move water passed the gills (Ram ventilation) Most bony fish use pumping action to ventilate; some can alternate

19 Gas exchange in fish Bony fish have four gill arches on each side of their heads Each gill arch is composed of two rows of gill filaments, which consist of lamellae, thin membranous plates that project out into the flow of water Water flows past the lamellae in one direction only; blood flows opposite to this direction (countercurrent gas exchange)

20 High O2 Low O2


22 Gas exchange in fish Most cartilaginous fish swim constantly
Others must pump H2O across gills Sand tiger sharks and nurse sharks alternate between pumping and RAM - spiracles - 5 gills; 6-7 in more primitive sp.

23 Cutaneous Respiration
O2 and CO2 are able to diffuse across cutaneous (skin) membranes in some vertebrates (amphibians and turtles) Requires constant moisture Supplementary to lung respiration; only a few species rely on cutaneous respiration exclusively Many turtles can respire underwater in this fashion, while some are capable of cloacal respiration, especially during hibernation

24 Tracheal systems Tracheal systems are found in arthropods
No single respiratory organ Respiratory system consists of small, branched trachae, or air ducts, which branch into tracheoles, a series of tubes which transfer gases directly across cellular membranes Air enters into trachea through spiracles In most species, can be open and closed

25 Lungs Lungs replace gills in terrestrial animals
Air is less (structurally) supportive than water Unlike gills, internal air passages such as trachea and lungs can remain open because the body provides the necessary structural support Water evaporates Terrestrial organisms constantly lose water to the atmosphere; gills would provide an enormous surface area for water loss The lung minimizes evaporation by moving air through an internal tubular passage

26 Lungs Air drawn into the respiratory passages becomes saturated with water vapor prior to reaching the inner region of the lungs A thin, wet membrane permits gas exchange A two-way flow system (gases move into and out of lungs through same airway passages)

27 Lungs Air contains a constant proportion of gases
78.09% nitrogen 20.95% oxygen 0.93% argon and other inert gases 0.03% carbon dioxide Because of gravity, air exerts a downward pressure (atmospheric pressure; 760mm Hg) The pressure contributed by each gas is called its partial pressure


29 Lungs of Amphibians The lungs of amphibians are saclike outpouchings of the gut Surface area increased by folds Amphibians breathe by forcing air into their lungs: positive pressure breathing They fill their oral cavity with air, close their mouth and nostrils, and then elevate the floor of their oral cavity; this pushes air into their lungs in the same way that a pressurized tank is used to fill balloons

30 a. b. Nostrils open Air External nostril Buccal cavity Esophagus Lungs
closed a. b. Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.

31 Lungs of Reptiles Terrestrial reptiles have dry, scaly skins which prevent cutaneous respiration Reptiles expand their rib cages by muscular contraction; this creates a lower pressure inside the lungs compared to the atmosphere, which moves air into the lungs: negative pressure breathing Reptilian lungs have more surface area than amphibians

32 Lungs of Mammals Endothermic (“warm-blooded”) animals, such as birds and mammals, require more efficient respiratory systems than ectothermic (“cold-blooded”) animals due to their increased metabolic demands The lungs of mammals are packed with millions of alveoli, sites of gas exchange Provides lung with enormous surface area for gas exchange

33 Lungs of Mammals The alveolus (singular) is composed of epithelium only 1 cell thick, and is surrounded by capillaries with walls that are also only 1 cell layer thick The distance across which gas must diffuse is very small, only μm R = D A Dp d

34 Maximization of Gas Diffusion

35 Lungs of Mammals Inhaled air is taken in through the mouth or nose, past the pharynx to the larynx (the voice box), where it passes through an opening in the vocal cords, the glottis, into the trachea, a tube supported by C-shaped rings of cartilage The trachea bifurcates into right and left bronchi, which each enter into their respective lung, and further divide into bronchioles that deliver air to the alveoli

36 Lungs of Mammals

37 Lungs of Mammals Alveoli are surrounded by an extensive capillary network Gas exchange between the air and blood occurs across the thin walls of the alveoli Red blood cells pass through capillaries in single file; O2 from alveoli enters the red blood cells and binds to hemoglobin Surface area of respiratory system is greatly enhanced; much more than amphibians and reptiles

38 Lungs of Birds Most efficient respiration of all terrestrial vertebrates Gas exchange occurs in parabronchi Air flows through the parabronchi in one direction only In other terrestrial vertebrates, inhaled (fresh) air is mixed with O2-depleted air from the previous breath In birds, the unidirectional flow allows only fresh air to enter the site of gas exchange

39 Lungs of Birds Respiration in birds occurs in 2 cycles:
1. Inhaled air is drawn in from the trachea into posterior air sacs, and exhaled into lungs 2. Air is drawn in from the lungs into anterior air sacs, and exhaled through the trachea

40 Lungs of Birds Red = inhaled air

41 Lungs of Birds Blood runs 90° to the air flow Crosscurrent flow
Not as efficient as countercurrent, but greater capacity to extract O2 from the air than a mammalian lung Enables birds (which fly, by the way) to respire efficiently at altitudes of 6000 meters Bar-headed geese can fly over Mt. Everest (29,028 feet)

42 Mechanisms of Gas Exchange
Gas exchange is driven by differences in partial pressures Blood returning from circulation is depleted in O2 and has a partial O2 pressure (PO2) of ~40 mm Hg The gas mixture in the alveoli is ~105 mm Hg Because of the difference in pressures (Δp), oxygen moves into the blood


44 Mechanisms of Gas Exchange
The diaphragm is a sheet of muscle extending across the bottom of the ribcage The diaphragm separates the thoracic cavity from the abdominal cavity During inhalation, the diaphragm contracts, causing the diaphragm to lower and assume a more flattened shape This expands the volume of the thorax and lungs, produces negative pressure which draws air into the lungs



47 Mechanisms of Gas Exchange
If breathing is insufficient to maintain normal blood gas measurements (PCO2 & PO2), hypoventilation occurs If breathing is excessive, PCO2 is abnormally low, and hyperventilation occurs (why you should blow into a brown bag to stop hyperventilating) The maximum amount of air that can be exhaled forcefully is the vital capacity 4.6L in men; 3.1L in women

48 Mechanisms of Gas Exchange
Breathing is involuntary and is under nervous system control Neurons stimulate the diaphragm and external intercostal muscles to contract, causing inhalation O2 is transported by respiratory pigments Bound to hemoglobin inside red blood cells (all vertebrates, most inverts), or hemocyanin in the plasma (arthropods and some molluscs)

49 Mechanisms of Gas Exchange
Oxygen has a low solubility; blood plasma can only contain a maximum of 3mL O2 per liter However, whole blood contains ~200mL O2 per liter since most of the O2 in the blood is bound to hemoglobin Hemoglobin is a protein composed of 4 polypeptide chains and 4 heme groups In the center of each heme group is an atom of iron, which can bind to the O2 molecule

50 Hemoglobin

51 Hemoglobin Hemoglobin acquires O2 in the alveolar capillaries
O2-bound hemoglobin (oxyhemoglobin) appears bright red Hemoglobin without O2 (deoxyhemoglobin) appears dark red, but has a bluish hue in tissues Hemoglobin provides an oxygen reserve Only 1/5 of oxygen is released to muscles by oxyhemoglobin; the reminder serves as a reserve during physical exertion, and ensures enough O2 to maintain life for 4-5 minutes if breathing is interrupted or the heart stops

52 Mechanisms of Gas Exchange
CO2 is transported by hemoglobin (bound to protein portion) and dissolved in plasma and red blood cells as bicarbonate, HCO3+ Because CO2 binds to the protein portion and not to the heme group, it does not compete with O2 molecules, but it does, however, change the shape of hemoglobin, reducing its affinity for O2

53 Mechanisms of Gas Exchange
Removal of CO2 into the alveoli occurs because of the lower PCO2 of the gas mixture inside the alveoli Hemoglobin transports other dissolved gases, including carbon monoxide, CO Carbon monoxide binds strongly to the iron atom in the heme group preventing oxygen from binding with hemoglobin

54 Thank you for not smoking
Cancer smoking lung cancer correlation from NIH.svg Lung cancer is the #1 cancer killer Caused mainly by cigarette smoking

55 Gas Exchange in Plants More than 90% of the water taken up by the roots of a plant are lost to evaporation However, photosynthesis requires large amounts of CO2 from the atmosphere Plants must therefore balance their need to minimize water losses and the need to admit CO2 The stomata and cuticle have evolved in response to one or both of these requirements

56 Gas Exchange in Plants Transpiration (evaporation of water from the leaves) decreases at night, when the vapor pressure gradient between the leaf and the atmosphere is less Closing the stomata will reduce water loss; but limit CO2 uptake

57 Gas Exchange in Plants The stomata of plants are surrounded by two sausage-shaped guard cells Guard cells are distinctive because of their cell wall construction: thicker on the inside and thinner elsewhere This results in bulging out and bowing when they become turgid Guard cells regulate the opening and closing of stomata

58 Guard Cells & Stomata Turgor in guard cells results from the active uptake (requires ATP) of K+, Cl-, and malic acid (organic compound) As solute concentration increases, water potential decreases in the guard cells, and water enters osmotically Guard cells accumulate water, becoming turgid Opens stomata




62 Stomata open (turgid guard cells)
Stomata closed (flaccid guard cells)

63 Stomata & Guard Cells

64 Stomata and Guard Cells
Guard cells of most plants regularly become turgid in the day (stomata open), when photosynthesis occurs, and become flaccid at night (stomata closed), regardless of the availability of water Guard cells are also unique in that they possess chloroplasts (the only epidermal cells to do so) The active pumping of sucrose out of guard cells in the evening leads to a loss of turgor and stomata closing

65 Gas Exchange in Plants Transpiration rates increase with temperature and wind velocity because both conditions cause water molecules to evaporate more readily CO2 concentration, light and temperature can influence stomatal opening When CO2 concentrations are high, guard cells are triggered to decrease the opening (conserves H2O) Blue light triggers H+ transport, opening K+ channels and stimulating the opening of stomata (facilitates evaporative cooling)

66 Gas Exchange in Plants Stomata frequently close when the temperature exceeds 30-34°C (~90-95°F) Stomata also close when water conditions are unfavorable Under intense heat, stomata open when it is dark and temperature has dropped Some plants collect CO2 at night and utilize it in photosynthesis during the day Cacti take in CO2 at night, and store it in organic compounds, which are converted back to CO2 during the day (when stomata are closed to prevent evaporation)

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