Gas Exchange: Animals. Cellular Respiration All living things obtain the energy they need by metabolizing energy-rich compounds, such as carbohydrates

Gas Exchange: Animals

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)

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)

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

Fick’s Law of Diffusion

R = rate of diffusionD = diffusion constant (molecule specific)A = area over which diffusion occursp = pressure difference between two sidesd = distance over which diffusion occurs

R =D A p

d

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 p

dThe evolution of respiratory systems has involved changes in all of these factors

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


• 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


• 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




• 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

http://life.bio.sunysb.edu/marinebio/o2countercurrent.jpg

H20

Countercurrent gas exchange

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

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

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

Gills

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

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)

High O2

Low O2

Gas exchange in fish• Most cartilaginous fish swim constantly• Others must pump H2O across gills• Sand tiger sharks

and nurse sharksalternate betweenpumping and RAM- spiracles- 5 gills; 6-7 in more primitive sp.

http://www.shark-info.com/images/respiration.jpg

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

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

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

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)

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

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

Lungs

Esophagus

Air

Externalnostril

Buccal cavity

Nostrilsopen

Nostrilsclosed

a.

b.

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Air

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

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

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 0.5-1.5μm

R =D A p

d


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

Lungs of Mammals

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

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

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

http://people.eku.edu/ritchisong/birdrespiration.html

Lungs of Birds

Red = inhaled air

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)

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


• 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


• 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


• 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)


• 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

Hemoglobin

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


• 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


• 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

Thank you for not smoking• Lung cancer is

the #1 cancer killer

• Caused mainly by cigarette smoking

1900 1920 1940 1960 1980

Cancer smoking lung cancer correlation from NIH.svg

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


• 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


• 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

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

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

Stomata & Guard Cells

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


• 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)


• 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)

Documents

Gas Exchange: Animals. Cellular Respiration All living things obtain the energy they need by metabolizing energy-rich compounds, such as carbohydrates