Lecture Twenty Three - Gas exchange

Question 1

Why do animals respire?

Answer 1

Gas exchange across a respiratory surface is essential for all organisms.

• Uptake of oxygen.
• Removal of carbon dioxides.

Question 2

Explain air and water breathers.

Answer 2

Oxygen concentration is much greater in air.
However organisms who live in air must expend more energy to move in terrestrial environments.

Oxygen in water is at lower concentrations.
The warmer the water, the less oxygen there is dissolved in the water.
There is more oxygen in fresh water compared to salt water.

As altitude increases, air pressure decreases and the amount of oxygen per litre of air decreases.
Therefore it is more difficult to breath at higher air pressures.
Air pressure decreases as altitude increases as the weight of air on the organism is less (less distance of atmosphere above you).

Question 3

What is the Fick equation?

Answer 3

Q = D x A x ΔP/L 
Where: 
Q = amount of gas diffusing per unit time. 
D = diffusion constant. 
A = area of membrane. 
ΔP = gas pressure difference. 
L = thickness of membrane. 

In essence, an organism would be best off if A was large and L was small.

Question 4

What characteristics do animals without respiratory surfaces have?

Answer 4

Usually very small so that gases can just exchange over the outer surface of the animal.

Respiratory organs increase diffusive surface area.

In water dwelling animals, gills are a respiratory surface.
Gill area is proportional to activity level to supply oxygen requirement.
Tuna gill area is 20x that of toad fish (tuna is predatory).
Ram ventilation vs opercular pumping.

Ram ventilation:
The production of respiratory flow in some fish in which the mouth is opened during swimming, such that water flows through the mouth and across the gills. In fish which have a reduced or no ability to pump water buccally, such as mackerel and sharks, perpetual swimming is required to maintain ventilation.

Opercular pumping:
Sucking or blowing water through the mouth by means of lifting the operculum to create a suction mechanism. Used by some fishes to aerate their eggs.

Question 5

What is counter current flow?

Answer 5

Maintains concentration gradient.

Feature of many biological exchange systems.

Question 6

Explain different ways of respiration in air.

Answer 6

Skin - supplementary respiration in some amphibians ( trivial in mammals).
Gills - inefficient in air.
Tracheae - insects (limited transport capacity, rely on body composition, i.e. insect cannot be too big or else trachea would have to be very long and and animal wouldn’t be able to breath very well - think breathing through a hose on the bottom of a pool).
Lungs - nearly all vertebrates.
- More complex in higher vertebrates.
- Require energetic ventilation.
- Lung volume of mammals nearly isometric with body size - - (5% of body volume).
- 1cm cubed mammalian lung has 600cm squared of alveolar surface.
- Volume of lungs increase as pressure decreases.
- Gas exchange takes place only in the alveoli.
- Dead space - non respiratory surfaces.
- Oxygen dissolves in surfactant.
- Diffuses across epithelium into blood.
- Carbon dioxide in opposite direction.

Question 7

Describe there respiratory pigments.

Answer 7

Oxygen solubility is low. 
Increases oxygen capacity by about 40 times. 
Needed to supply metabolic need. 
Usually within specialised cell. 
Coloured by metal. 
Occur both in cells and in plasma. 
Haemoglobin widely distributed. 

Haemoglobin:
Protein with four identical porphyrin subunits.
Divalent iron (Fe 2+) bound to each subunit.
Reversibly binds oxygen for deliver to tissues.
~280,000,000 haemoglobin molecules in the body.
Binding of oxygen changes colour of molecule form blue (deoxygenate) to red (oxygenated).

Question 8

Explain blood cells.

Answer 8

Internalise respiratory pigments. 
Allow separation of chemical environment. 
Red blood cells (erythrocyte). 
- 4.5 -6.2 x 10^12 cells per litre of human blood. 
- In mammals round biconcave discs. 
- Oval in non mammals. 
- May or may not have a nucleus. 
- Will deform in capillaries. 

Oxygen dissociation:
Hb + O2 HbO2.
Binding or releasing of oxygen depending of oxygen concentration.
Sensitive to demands (bohr effect).
Dynamics of dissociation vary among animals.