gas exchange in animals - monroe county school district

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Gas Exchange in Animals

37

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Concept 37.1 Fick’s Law of Diffusion Governs Respiratory Gas Exchange

Gas exchange is the uptake of oxygen from the environment and the discharge of carbon dioxide to the environment.

Can take place in air or water but diffusion is the only mechanism.

May use gills, skin/body surfaces, lungs, or a combination.


The pressure of a gas is the sum of the activity of its molecules.

If there is a mixture of gasses, the pressure of each is its partial pressure (Pi).

Partial pressure of O2

(PO2) at sea level is 159 mm Hg.

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Diffusion: particles will always move from an area of higher concentration to an area of lower concentration.

At equilibrium:

-  Gas molecules enter and leave at the same rate.

-  Pi of the gas in the solution = Pi of the gas in the air.

Review Ch. 5.2, p. 83-85.


Oxygen is easier to obtain from air than from water:

• O2 content (volume) of air is higher (up to 40x) than that of water, even with equivalent Pi

• O2 diffuses much faster (8000x) through air

• Less O2 per L in warm water vs. cold, less at high altitudes vs. sea level (both air and water)

• Water is thicker and denser than air—requires more energy to move across gas exchange surfaces

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Fick’s law of diffusion:

Q = DA P1– P2

L

Rate of diffusion

a constant – depends on diffusing substance, the

medium and temperature

cross-sectional area of respiratory

membrane.

diffusion distance across membrane

difference in partial pressure of gas on

either side of membrane

Concept 37.2 Respiratory Systems Have Evolved to Maximize Partial Pressure Gradients

Adaptations to maximize the exchange of O2 and CO2:

•  Increase surface area (A)

• Minimize diffusion path length (L)

• Minimize the diffusion that takes place in an aqueous medium (D)

• Maximize partial pressure difference (P1-P2)

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Maximizing partial pressure difference:

• Ventilation—active moving of the external medium over the gas exchange surfaces

• Perfusion—circulating blood over the gas exchange surfaces to transport CO2 to membrane and O2 away (and to tissues)


Many aquatic taxa have gills.

Gills are outfoldings of the body surface, suspended in water.

Total surface area often much greater than body surface.

Gills may be internal or external.

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Gills are ventilated by moving gills through water, or by moving water over gills.

•  Crayfish and lobster move water over gills with appendages.

•  Mussels and clams move water with cilia.

•  Squid/octopus pull in and eject water through their gills.


Fish move water over gills as they swim.

Water flow is unidirectional – enters mouth, passes through pharynx, flows over gills and exits body.

Gill filaments are covered with gill lamellae = lots of surface area and minimal path length for gas exchange.

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Fish use highly efficient countercurrent exchange - over 80% of oxygen in water passing over gills is extracted.

PO2 (mm Hg) in water

PO2 (mm Hg) in blood

150 120 90 60 30

110 80 20 50 140

PO2 (mm Hg) in water

PO2 (mm Hg) in blood

150 120 90 60 30

110 80 20 50 140

Oxygen-poor blood

Oxygen-rich blood

Lamellae

Blood flow through capillaries in lamellae

Water flow between lamellae

Oxygen-poor blood

Oxygen-rich blood


Making the transition from water to air:

•  Water loss – moist respiratory membrane required for oxygen – oxygen must be in fluid to dissolve.

•  Prevention of respiratory organ collapse (air is not buoyant)

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Amphibians and some other taxa can use cutaneous respiration.

Functions in both air and water, although skin must be kept moist.


Insects have a tracheal system throughout their bodies.

Spiracles in the abdomen open to allow gas exchange and close to limit water loss.

Spiracles open into tracheae that branch to tracheoles, which end in air capillaries right next to body cells. Body

cell

Air sac Tracheole

Body wall

Trachea

Air

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Most terrestrial vertebrates (and some others) use tidal ventilation in lungs.

Lungs must be in contact with the circulatory system to transport oxygen to body.

Concept 37.3 The Mammalian Lung Is Ventilated by Pressure Changes

Ventilation of the lungs can occur by:

•  positive pressure (air gulping)

•  negative pressure (expanding the lungs).

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Mammalian lungs are suspended inside a thoracic cavity.

The diaphragm is a sheet of muscle at the bottom of the cavity.

The pleural membrane covers each lung and lines the thoracic cavity.

The space between the membranes contains fluid to help them slide past each other during breathing.


Exhalation – diaphragm relaxes. Elastic lung tissues pull the diaphragm up and push air out of the airways.

Inhalation – diaphragm contracts, expanding the thoracic cavity and pulling on on the pleural membranes, causing lungs to expand.

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In tidal ventilation air flows in and out by the same path.

Lungs are never completely empty. The residual volume (RV) is the “stale” (low O2) air that remains in the lungs after exhaling.

Each inhalation mixes fresh air with stale air.


Birds solve this problem with air sacs that allow unidirectional flow of air over respiratory surfaces.

Anterior air sacs

Posterior air sacs Lungs

Air

Lungs

Air

1 mm

Trachea

Air tubes (parabronchi) in lung

EXHALATION Air sacs empty; lungs fill

INHALATION Air sacs fill

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In mammals, air enters through nostrils, flows through nasal cavity to pharynx, larynx, trachea and into two bronchi, to smaller and smaller bronchioles, and then into alveoli—the sites of gas exchange.


Capillaries surround and lie between the alveoli—diffusion path between blood and air is less than two micrometers.

Humans have about 300 million alveoli and surface area is about 70 sq. meters.

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Concept 37.4 Respiration Is under Negative Feedback Control by the Nervous System

Breathing is controlled in the medulla oblongata and the pons, in the brain stem.

Control circuits in medulla establish rhythm.

Neurons in pons regulate tempo.

Carotid and aortic bodies monitor O2 levels in blood leaving the heart.

Breathing control centers

Cerebrospinal fluid

Pons

Medulla oblongata

Carotid arteries

Aorta

Diaphragm Rib muscles

Concept 37.4 Respiration Is under Negative Feedback Control by the Nervous System

In mammals, the breathing rate is more sensitive to increases in CO2 than to falling levels of O2.

pH of major arteries is monitored to maintain appropriate blood CO2 concentration.

Ventilation increases rapidly with exercise, in anticipation of a rise in PCO2 (feedforward).

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Concept 37.5 Respiratory Gases Are Transported in the Blood

O2 is non-polar and does not dissolve in blood.

In most animals blood contains transport molecules that bind reversibly to O2.

Hemoglobin – iron – red blood. Found in vertebrates and most other invertebrates.

Hemocyanin – copper – blue blood. Found in arthropods and molluscs


Hemoglobin binds reversibly to oxygen and can drop off more at tissues where it is needed.

The affinity of hemoglobin for O2 is influenced by: •  PO2 of the blood.

•  Type of hemoglobin (α, β, λ)

•  pH (more oxygen released when pH is low)

•  BPG (2,3-biphosphoglyceric acid) - a metabolite of glycolysis – present at low blood oxygen, causes hemoglobin to release.

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Myoglobin has high affinity for oxygen, releases O2 when metabolic demands are high (in muscle cells; diving mammals have high concentrations)

Fetal hemoglobin has higher affinity relative to maternal.


Blood also transports CO2 away from the tissues.

Some stays in plasma, some bound to hemoglobin, most converted to bicarbonate ions.

In the lungs the reaction is reversed – CO2 diffuses from the blood into the alveoli and is exhaled.

CO2 + H2O <----> H2CO3 <----> H+ + HCO3-

bicarbonate ions carbonic acid

gas exchange in animals - monroe county school district

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