Physiology: Respiration


What is respiration?

O2, carbohydrates and sugars - IN

CO2, water and heat - OUT


External respiration: breathing movements which assure a continual supply of air or water across a respiratory surface.

Internal respiration: gas exchange between the cells of the animal body and the immediate cellular environment. 


External ventilation: simple diffusion in unicellular animals.

Internal respiartion: bulk transport of gas by a circulatory system in multicellular animals.


Water and Air as respiratory media: 

Why has animal evolution proliferated and accelerated in terrestrial environments?

Air has 20 times the O2 content of water.

To extract 1 litre of O2, 143kg of water needs to be ventilated or 0.0062kg of air. A 100,000 fold difference.

Life evolved in water 4.2 billion years ago.

Water and latterly air are the two respiratory media on earth.


What affects the availability of respiratory gases in water?

Temperature: increases in temperature decrease gaseous solubility or tension (ml l^-1) in water. Decrease in temperature, increase in gaseous solubility in water. 

A driver for the evolution of lungs.


Effects of temperature on solubility of respiratory gases in water.

Antarctic ice fish require no respiratory pigment. In arctic temperatures, solubility of respiratory gases is sufficiently high and metabolic rate sufficiently low to meet metabolic demand. 


Why is the total amount of CO2 diffusing across a membrane 25X greater than the amount of O2?

Solubility of gases in water and across membranes also varies according to individual molecular properties of gases.

Gas | Solubilty in water | Diffusion rate in air

CO2 | 1019 | 5.7

O2 | 34 | 6.6


Other properties of water as a respiratory medium.

Viscosity: influence of density of the respiratory medium on the work of breathing. 

Water is 50 times more viscous than…


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