Gas Exchange. 2 of 28© Boardworks Ltd 2008 Exchange surfaces All organisms require nutrients and...

Gas Exchange

2 of 28 © Boardworks Ltd 2008

Exchange surfaces

All organisms require nutrients and the ability to excrete waste. Many simple organisms, such as bacteria and sea anemones, can exchange substances directly across their external surfaces.

In mammals, gas exchange occurs in the lungs, and in particular the alveoli.

Larger organisms require specialized gas exchange and transport systems to transport substances such as oxygen and nutrients to their cells efficiently.

Crop photo

Fish exchange these substances across gills, while insects have openings called spiracles on their surfaces.

Adaptations for gas exchangeIssue: Requirements are

proportional to volume of organism however diffusion is proportional to surface area

Diffusion:

◦In large organisms the surface area to volume ratio is much less than in very small organisms

Larger organismsHave a small surface area to volume ratio, so

need adaptations to increase gas exchange:Ex:

◦gills for aquatic environments◦ lungs for terrestrial environments

Have: ◦ large surface area◦ thin (short diffusion distance)◦permeable surface◦moist◦good blood supply (carries gas away quickly,

maintaining the concentration difference)◦good ventilation (pumping mechanism-like lungs)

Small organismsthe surface area to volume ratio is so

large that diffusion through the body surface is sufficient to supply their needs. (as is the distance it needs to travel in the body)

ExampleAmoeba:

◦lives in fresh water allowing for easy diffusion of nutrients

◦Small body (single celled) so diffusion can supply requirements

Water Losslarge moist area for gaseous

exchange is a region of potential water loss for land animals

Strategies for gas exchange

Earthworms (annelids)

earthwormshave an increased efficiency of

gaseous exchange (sufficient for a slow moving animal)

are multicellular, terrestrial animals restricted to damp areas (for gas exchange)

long tube shape for high surface area

moist body surface (mucus) for diffusion of gases

Earthwormsuse their outer surfaces as gas

exchange surfaceshave a series of thin-walled blood

vessels known as capillaries

have a closed circulatory system (blood vessels- which is more efficient) and blood pigments (haemoglobin) to bind oxygen

gas exchange occurs at capillaries located throughout the body tissues as well as those in the respiratory surface

Insects Have a hard exoskeleton which is

not suitable for gas exchangehave evolved a different system of

gaseous exchange to other land animals (need lots of energy for rapid flight)

InsectsDo not use a transport system to

carry oxygenUse a branched, chitin-lined system

of tracheae with openings called spiracles (that can open and close)

insectsMain features:

◦Large surface area using a network of tubes

◦Small bodies so that diffusion from tubes to tissues is sufficient

◦Thin, fluid filled tracheoles to allow gases to dissolve and diffuse to/from tissues efficiently

◦Some species have rhythmical muscle contraction to assist the diffusion of gases (ventilation)

insectsCan control the rate of gas exchange

using lactic acidHigh respiration rate means more lactic acid

made and stored in tissuesThis causes fluid to move in by osmosis from

the fluid filled tracheolesGases can now diffuse faster from tracheoles

with less fluid in them.

Bony fishlarger and active animals so high

demand for oxygenuse gills with a large surface

extended by gill filaments with lamellae (ie highly folded)

gillsEach gill is composed of many

filaments that are each covered in many lamellae to increase surface area.

The lamellae contain blood capillaries, which have blood flowing in the opposite direction to the water.

The lamellae are thin, ensure that the diffusion distance between the blood, in the lamellae, and the water is short

mouth cavity (buccal cavity) and the chamber at the side (operculum cavity) help to increase ventilation over gills (like a pump)

Steps:1. Mouth opens2. Operculum (gill cover) closes3. Floor of buccal cavity lowers4. These all increases the volume inside,

thus lowering pressure5. Water is drawn in

Gill ventilation

http://www.youtube.com/watch?v=kf7vBjhjwec

http://www.youtube.com/watch?v=YLsmEhnYdM0&feature=related

Counter current flowThe lamellae contain blood

capillaries, which have blood flowing in the opposite direction to the water.

Figure 4

Countercurrent system

The blood flows through the lamellae in the opposite direction to the water. This is a countercurrent system. It ensures the maximum exchange possible occurs.

Counter current vs. Parallel FlowCounter flow allows continuous diffusion

of oxygen into the blood as there is always a concentration gradient across the gill lamella (plate) even when the blood is very saturated with oxygen

Reptiles and birdshave more efficient lungs than amphibiansribs assist ventilationBirds have air sacs to keep lungs always

inflated (like a bellows) that takes the dead air from the lungs during the next breathe to ensure fresh air goes into the lungs each time

Gills can also be externalExternal gills generally have a

higher surface area but are less protected

Amphibianslarval form (tadpole) develops in

water (uses gills) and undergoes metamorphosis into the adult form

inactive frog uses its moist skin as a respiratory surface but when active uses lungs

Terrestrial vertebrateshave adapted for exchange with

air, a less dense medium (air) so have internal lungs (gills don’t work in air)

internal lungs minimise loss of water and heat

The human respiratory system:

ventilation and exchange of gasesventilation involves creating

volume and pressure changes that allow a continuous exchange of gases inside the body, so maintaining concentration gradients

Structure of the lungs

Gas exchange in the alveoli

Maintaining the structure of the alveoli

During inhalation, the chest cavity increases in volume, lowering the pressure in the lungs to draw in fresh air.

Lung surfactant is a phospholipid that coats the surfaces of the lungs. Without it, the watery lining of the alveoli would create a surface tension, which would cause them to collapse. surfactant

alveoli

This decrease in pressure leads to a tendency for the lungs to collapse. Cartilage keeps the trachea and bronchi open, but the alveoli lack this structural support.

Keeping the airways clear

The walls of the trachea and bronchus contain goblet cells, which secrete mucus made of mucin. This traps micro-organisms and debris, helping to keep the airways clear.

The walls also contain ciliated epithelial cells, which are covered on one surface with cilia. These beat regularly to move micro-organisms and dust particles along with the mucus. They contain many mitochondria to provide energy for the beating cilia.

Structures of the human lung

Why do we breathe?

Fish manage this by keeping a continuous stream of oxygenated water moving over their gills.

Animals need to maintain a concentration gradient across their exchange surfaces so that oxygen will diffuse into the blood and carbon dioxide will diffuse out.

In animals such as mammals and birds, a concentration gradient is maintained in the alveoli by the mechanism of ventilation.

The mechanism of ventilation

The pleural cavity

Each of the lungs is enclosed in a double membrane known as the pleural membrane. The space between the two membranes is called the pleural cavity, and is filled with a small amount of pleural fluid.

This fluid lubricates the lungs. It also adheres to the outer walls of the lungs to the thoracic (chest) cavity by water cohesion, so that the lungs expand with the chest while breathing.

pleural membranes

Composition of inhaled/exhaled air

In one breathing cycle, the air in the lungs loses only some of its oxygen content. This is why mouth-to-mouth resuscitation can be effective.

908070605040302010

0N2 O2 CO2 H2O other

exhaled air

inhaled air

78% 78%

21%15%

0.04%4% <1% 3% <1% <1%

What’s the keyword?

Structures involved in gas exchange

Multiple-choice quiz

Comparing gas exchange surfaces

Plants rely entirely on diffusion for the exchange of

gases.Leaves are thin to shorten distances for

diffusion and have a large surface area and are permeated by air spaces

Leaves have a cuticle to prevent water loss which also reduces gaseous exchange.

The air spaces between mesophyll cells allow carbon dioxide and oxygen to diffuse to and from all the cells.

The cells are moist so gases can dissolve.

The presence of pores, stomata, allow water vapour and gases to pass through

Irregular arrangement of spongy mesophyll cells creates a large surface area for gaseous exchange

Cell wall is thin – short diffusion path

Guard cells change shape because of changes in turgor; in the light, water flows in by osmosis so the cells expand.

The inner wall is inelastic and thicker so the pairs of cells curve away from each other as water enters and the pore opens

Pores close due to the reverse process.

malate theorypotassium ions move from the

epidermal cells into the guard cells by active transport

This causes starch to change to malate (water soluble)

This creates a negative water potential in the guard cells.

Water moves in by osmosis.

Xerophytesmay open stomata at night instead

of during the day in order to conserve water

Heat/water lossLoss in large organisms than in

smallThis is because the organism has :

◦a low surface area : volume ◦longer diffusion pathways ◦longer distances in general ◦probably more insulation so it is harder

for the heat to escape.

◦http://www.bozemanscience.com/respiratory-system

Gas Exchange. 2 of 28© Boardworks Ltd 2008 Exchange surfaces All organisms require nutrients and...

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