Circulation and Respiration
How animals acquire oxygen and dispose of carbon dioxide, and how pumps and vessels distribute these and other substances through the body to meet metabolic demand.
Definition
Respiration in this physiological sense is the exchange of oxygen and carbon dioxide between an animal and its environment and their transport to and from tissues; circulation is the bulk movement of blood or haemolymph by a pump and vessel system that distributes gases, nutrients, wastes, hormones, and heat throughout the body.
Scope
This area covers the comparative physiology of gas exchange and internal transport: respiratory surfaces such as gills, lungs, and tracheae; the binding and carriage of oxygen by respiratory pigments; the structure and function of hearts and circulatory systems; and the regulation of breathing and blood gases including acid–base balance. It spans the physical principles of diffusion and convection and the diversity of solutions animals have evolved across aquatic, aerial, and terrestrial life. Coverage is comparative and mechanistic rather than clinical.
Sub-topics
Core questions
- How do respiratory surfaces maximise gas exchange across different media such as water and air?
- How do respiratory pigments load oxygen where it is plentiful and release it where it is needed?
- How are circulatory systems and hearts organised to move blood efficiently in animals of different sizes and lifestyles?
- How do animals sense and regulate their blood gases and maintain acid–base balance?
Key theories
- Cooperative oxygen binding and the sigmoid dissociation curve
- Respiratory pigments such as haemoglobin bind oxygen cooperatively, giving a sigmoid dissociation curve that promotes efficient loading at the respiratory surface and unloading in active tissues, with the position of the curve shifted by carbon dioxide, pH, and temperature.
- Convection–diffusion design of gas transport
- Effective gas exchange combines convective delivery of medium and blood to and from a thin respiratory surface with diffusion across it, and arrangements such as countercurrent flow in gills maximise the gradients that drive diffusion.
Mechanisms
Gas exchange depends on diffusion across thin, large-area respiratory surfaces kept supplied by ventilation of the external medium and perfusion of blood. Gills use countercurrent flow of water and blood to keep diffusion gradients high; lungs use tidal or, in birds, unidirectional flow; insects deliver oxygen directly to tissues through tracheae. Oxygen is carried mainly bound to respiratory pigments whose cooperative binding and sensitivity to CO2, pH, and temperature tune loading and unloading. Hearts generate pressure to drive blood through open or closed circulatory systems, and vascular resistance and capacitance distribute flow. Breathing and circulation are regulated by chemoreceptors that monitor O2, CO2, and pH, adjusting ventilation and cardiac output, while buffering and ion exchange maintain acid–base balance.
Clinical relevance
Comparative work on diving mammals, high-altitude species, and air-breathing fish illuminates the limits of human cardiorespiratory performance and informs research on hypoxia, exercise, and respiratory and cardiovascular function. This entry is educational and does not provide medical guidance.
History
August Krogh's studies of capillary function and gas exchange and Christian Bohr's discovery of the effect of carbon dioxide on oxygen binding established the foundations of respiratory physiology. Schmidt-Nielsen and others extended the field to the remarkable adaptations of desert, diving, and high-altitude animals, framing circulation and respiration as problems of design under physical constraints.
Key figures
- August Krogh
- Knut Schmidt-Nielsen
- Christian Bohr
- John B. West
Related topics
Seminal works
- schmidtnielsen1997
- hill2016
- westsd2012
Frequently asked questions
- Why is countercurrent flow important in fish gills?
- Running water and blood in opposite directions keeps an oxygen gradient along the whole gill surface, so blood can take up far more oxygen than if the two flowed together.
- What makes the oxygen dissociation curve S-shaped?
- Haemoglobin binds oxygen cooperatively, so binding one oxygen makes the next easier; this produces a sigmoid curve that favours full loading at the lungs or gills and ready unloading in active tissues.