Gas Exchange

Campbell Chapter 42

Pages 886 - 897

Gas Exchange in Animals

• Gas exchange = taking in molecular oxygen (O2) from the environment and disposing of carbon dioxide (CO2) to the environment.

Gas Exchange• Cellular respiration is the breakdown of

organic molecules to make ATP. A supply of oxygen is needed to convert stored organic energy into energy trapped in ATP.

• Carbon dioxide is a by-product of these processes and must be removed from the cell.

• There must be an exchange of gases: carbon dioxide leaving the cell, oxygen entering.

Air Supply

• The respiratory medium or source of oxygen is:

1. Terrestrial Animals = air

2. Aquatic Animals = water.

• Atmosphere is ~21% oxygen (O2)

• Bodies of water – variable oxygen content, but much less than in an equal amount of air.

Respiratory Surface

• Gases are exchanged with the environment at the respiratory surface.

• Gas movement is by diffusion.• Respiratory surfaces are usually thin and

have large areas as well as adaptations to facilitate the exchange.

• Gases are dissolved in water, so respiratory surfaces must be moist.

Diffusion Rate

• The net diffusion rate of a gas across a fluid membrane is – proportional to the difference in partial

pressure, – proportional to the area of the membrane and – inversely proportional to the thickness of the

membrane.

Fick’s Law

• The rate at which a substance can diffuse is given by Fick's law

Surface to Volume Ratio

• Rate of exchange of substances depends on the organism's surface area that is in contact with the surroundings.

• The ability to exchange substances depends on the surface area : volume ratio.

• As organisms get bigger, their volume and surface area both get bigger, but volume increases much more than surface area.

Unicellular Organisms

• Single-celled organisms exchange gases directly across their cell membrane.

• The slow diffusion rate of oxygen relative to carbon dioxide limits the size of single-celled organisms.

Simple Animals

• The cells of sponges, cnidarians, and flatworms are in direct contact with environment.

• Simple animals that lack specialized exchange surfaces have flattened, tubular, or thin shaped body plans, which are the most efficient for gas exchange. However, these simple animals are rather small in size.

Simple Animals

Respiratory Surfaces

• Some animals use their outer surfaces (skin) as gas exchange surfaces. (earthworms and some annelids)

• Arthropods, annelids, and fish use gills.

• Terrestrial vertebrates utilize internal lungs.

Outer Surface (Skin)

• Flatworms and annelids use their outer surfaces as gas exchange surfaces. Earthworms have a series of capillaries. Gas exchange occurs at capillaries located throughout the body as well as those in the respiratory surface.

• Amphibians use their skin as a respiratory surface. Frogs eliminate carbon dioxide 2.5 times as fast through their skin as they do through their lungs.

• Eels (a fish) obtain 60% of their oxygen through their skin.

• Humans exchange only 1% of their carbon dioxide through their skin.

Gills

• Gills have evolved many times in different animal groups, and the specific anatomy varies widely. But as a general rule, gills consist of fine sheets or filaments of tissue that extend outward from the body into the water

Fish Respiration

• Some fish ventilate their gills by swimming with mouth and gill slits open [e.g. sharks, which die of asphyxiation if immobilized].

• But many fish can respire while stationary, and do so by swallowing water through their mouths and forcibly expelling it through the gills.

Gills

• Gills are out-foldings of the body surface that are suspended in water.

• They increase the surface area for gas exchange.

• They are organized into a series of plates and may be internal (as in crabs and fish) or external to the body (as in some amphibians).

Gills

Who Has Gills?

• Gills are found in a variety of animal groups:

1. arthropods (including some terrestrial crustaceans)

2. annelids,

3. fish

4. amphibians.

Variety of Gills

Efficiency of Gills

• Gills are very efficient at removing oxygen from water: there is only about 1/20 the amount of oxygen present in water as in the same volume of air.

• Water flows over gills in one direction while blood flows in the opposite direction through gill capillaries. This countercurrent flow maximizes oxygen transfer.

How Gills Work

• Fish maximize gas exchange in their gills by a 'design principle' called countercurrent exchange.

• Countercurrent exchange requires that two fluids (in this case, the external water and the blood in the gills) flow past each other in opposite directions.

Counter-Current Exchange

1. When a fish swims, water moves over its gills from anterior to posterior.

2. Blood flow in the gill capillary bed is oriented from posterior to anterior. The blood picks up O2 from the external water (and loses CO2) as it flows through the gill capillaries.

3. This countercurrent arrangement insures that the most O2 -depleted blood (entering the gill) is confronted with the most O2 -depleted water (leaving the gill), and that the most O2 -rich blood (leaving the gill) contacts the most O2 -rich water (entering the gill). This arrangement maximizes O2 absorption.

Counter-Current Exchange

Terrestrial Respiratory Systems

• Many terrestrial animals have their respiratory surfaces inside the body and connected to the outside by a series of tubes.

• Insects, centipedes, and some mites and spiders have a tracheal respiratory system.

• Vertebrates have lungs.

Insect Tracheal Systems

• All insects are aerobic organisms - they must obtain oxygen (O2) from their environment in order to survive.

• The insect respiratory system is a complex network of tubes (tracheal system) that delivers O2-containing air to every cell of the body.

• Tracheae are the tubes that carry air directly to cells for gas exchange.

Tracheal System

• Air enters the insect's body through valve-like openings (spiracles) in the exoskeleton.   These are located laterally along the thorax and abdomen of most insects. 

• Air flow is regulated by small muscles that operate one or two flap-like valves within each spiracle -- contracting to close the spiracle, or relaxing to open it.

• After passing through a spiracle, air enters a longitudinal tracheal trunk, eventually diffusing throughout a complex, branching network of tracheal tube that subdivides into smaller and smaller tubes that reach every part of the body.  

• At the end of each tracheal branch, a special cell (the tracheole) provides a thin, moist interface for the exchange of gases between atmospheric air and a living cell.  

• Oxygen in the tracheal tube first dissolves in the liquid of the tracheole and then diffuses into the cytoplasm of an adjacent cell.   At the same time, carbon dioxide, produced as a waste product of cellular respiration, diffuses out of the cell and, eventually, out of the body through the tracheal system.

Insect Tracheal System

Human Respiratory System

Human Respiratory System

• This system includes the lungs, pathways connecting them to the outside environment, and structures in the chest involved with moving air in and out of the lungs.

• The main task of any respiratory system is to take in oxygen and remove carbon dioxide.

Lungs

• Lungs are ingrowths of the body wall and connect to the outside by as series of tubes and small openings.

• Lung breathing probably evolved about 400 million years ago.

• Lungs are not entirely the sole property of vertebrates, some terrestrial snails have a gas exchange structures similar to those in frogs.

• The lungs are large, lobed, paired organs in the chest (also known as the thoracic cavity).

• Thin sheets of epithelium (pleura) separate the inside of the chest cavity from the outer surface of the lungs.

• The bottom of the thoracic cavity is formed by the diaphragm.

Pathway of Air

• Air enters the body through the nose, is warmed, filtered, and passed through the nasal cavity.

• Air passes the pharynx (which has the epiglottis that prevents food from entering the trachea).

• The upper part of the trachea contains the larynx. The vocal cords are two bands of tissue that extend across the opening of the larynx.

• After passing the larynx, the air moves into the bronchi that carry air in and out of the lungs.

• Bronchi are reinforced to prevent their collapse and are lined with ciliated epithelium and mucus-producing cells.

• Bronchi branch into smaller and smaller tubes known as bronchioles.

• Bronchioles terminate in grape-like sac clusters known as alveoli.

• Alveoli are surrounded by a network of thin-walled capillaries. Only about 0.2 µm separate the alveoli from the capillaries due to the extremely thin walls of both structures.

Alveoli

• Only in the alveoli does actual gas exchange takes place.

• There are some 300 million alveoli in two adult lungs.

• These provide a surface area of some 160 m2 (almost equal to the singles area of a tennis court and 80 times the area of our skin!).

Alveoli

Diffusion of O2 and CO2

Alveoli: Designed for Rapid Gas Exchange

• After branching repeatedly the bronchioles enlarge into millions of alveolar sacs

• This arrangement produces an enormous surface are for gas exchange

• Each alveolus is surrounded by a net of capillaries

• The diffusion distance from gas in the alveoli to blood cells in the capillaries is very short

• Blood takes about 1 second to pass through the lung capillaries

• In this time the blood becomes nearly 100% saturated with oxygen and loses its excess CO2

Surfactants Prevent the Alveoli From Collapsing

• At air/water interfaces there is a high surface tension

• The high surface tension would cause the alveoli to collapse, but this is prevented by surfactants

• Surfactants are detergent-like phospholipids which accumulate at the air/water interface and lower the surface tension

• Reduced surfactant causes respiratory distress syndrome (seen in premature infants and some older persons)

Pigments• Respiratory pigments increase the oxygen-

carrying capacity of the blood. Humans have the red-colored pigment hemoglobin as their respiratory pigment.

• Hemoglobin increases the oxygen-carrying capacity of the blood between 65 and 70 times.

• Each red blood cell has about 250 million hemoglobin molecules, and each milliliter of blood contains 1.25 X 1015 hemoglobin molecules.

• Oxygen concentration in cells is low (when leaving the lungs blood is 97% saturated with oxygen), so oxygen diffuses from the blood to the cells when it reaches the capillaries.

Animation

• http://www.mdhs.unimelb.edu.au/bmu/examples/gasxlung/

• http://science.nhmccd.edu/biol/respiratory/alveoli.htm

• http://www.smm.org/heart/lungs/breathing.htm

• http://sprojects.mmi.mcgill.ca/resp/anatomy.swf

Ventilation

• Ventilation is the mechanics of breathing in and out.

• When you inhale, muscles in the chest wall contract, lifting the ribs and pulling them, outward. The diaphragm at this time moves downward enlarging the chest cavity. Reduced air pressure in the lungs causes air to enter the lungs.

• Exhaling reverses theses steps.

Negative Pressure Breathing

Diaphragm Action

Negative Pressure Animation

Lung Volume

• Tidal volume is the amount of air that is inhaled and exhaled in a normal breath.  

• Vital capacity is the maximum amount of air that can be inhaled and exhaled in a single breath.  

• Since the lungs hold more air than the vital capacity, the air that remains in the lungs is the residual volume.

Dead Space

• Only the air in the alveoli can exchange O2 and CO2 with the blood

• When you breath in the first 150 mL fills tubes which are outside of the alveoli (trachea, bronchi, bronchioles, etc.)

• This part of the tidal volume is called the anatomical dead space- it does not participate in gas exchange

• There is also a functional dead space- not all of the alveoli are perfused with blood; air in these alveoli doesn't exchange with the blood and is part of the dead space

Avian Respiration

• The avian respiratory system delivers oxygen from the air to the tissues and also removes carbon dioxide.

• In addition, the respiratory system plays an important role in thermoregulation (maintaining normal body temperature).

• The avian respiratory system is different from that of other vertebrates, with birds having relatively small lungs plus nine air sacs that play an important role in respiration (but are not directly involved in the exchange of gases).

• The air sacs permit a unidirectional flow of air through the lungs.

• Unidirectional flow means that air moving through bird lungs is largely 'fresh' air & has a higher oxygen content.

• In contrast, air flow is 'bidirectional' in mammals, moving back & forth into & out of the lungs. As a result, air coming into a mammal's  lungs is mixed with 'old' air (air that has been in the lungs for a while) & this 'mixed air' has less oxygen. So, in bird lungs, more oxygen is available to diffuse into the blood

Respiration

• http://www.wisc-online.com/objects/framz.asp?objID=AP2404

Credits

• All material for this PPT was found on various websites or is from Campbell Biology 6e.