Fish 'breathe' oxygen? But they don't have lungs?

Fish are unique little creatures, they require oxygen to live yet very few fish have lungs, so how do they actually ‘breathe’? Turns out, fish are quite interesting in their evolution to thrive within their aquatic landscape. The basis of their oxygen extraction is accomplished by their gills consisting of a dual-pump ventilation system. This system includes two pumps; the orobranchial pressure pump (mouth cavity region) and the parabranchial suction pump (gill cover). The orobranchial pump forces water to move over the gills and the parabranchial pump pulls the water through the branchial chambers. 

George Hughes’, the father of modern fish respiratory science, identified a mechanism which is divided into four stages during which the orobranchial pressure usually was greater than the parabranchial chamber indicating a near continuous flow of the ventilatory stream of water over the gills, guaranteeing near constant oxygen availability (Wegner & Graham, 2010). 

The gills are made of several components, the gill arches, gill filaments (primary lamellae), and then secondary lamellae. The arches and filaments provide structural support while the tiny secondary lamellae are rich in capillary beds and are the site of gas exchange. 

What is really unique, is that fish not only have the dual pump system, but they also rely on the counter-current flow to increase the efficiency substantially. As the water flows over the secondary lamellae, the blood within the vessels and capillary beds runs the opposite direction resulting in a counter-current. This proves to be a highly efficient method of oxygen extraction as it allows for continuous diffusion (Kumar et al., 2018). This is especially important because water dissolves less oxygen than air does, therefore it is more difficult to obtain the necessary oxygen. Due to this challenge, fish have evolved to be as successful as possible regarding oxygen extraction. The adaptation of the counter-currently flow is ideal, as it would be nearly useless if the blood flowed in the same direction as the water did over the gills.

Not only is the counter-current system successful, it also has very high surface area involving a very thin membrane called the water-blood barrier which allows for the diffusion of oxygen and CO2. The combination of increased surface area and the thin barrier makes fish gills very efficient at oxygen extraction and the overall evolution of fish captivating. 

References:

Kumar, P., Gandhi, P. S., & Majumder, M. (2018). Optimal morphometric factors responsible for enhanced gas exchange in fish gills. https://arxiv.org/pdf/1805.07744

Wegner, N. C., & Graham, J. B. (2010). George Hughes and the history of fish ventilation: From Du Verney to the present. Comparative Biochemistry and Physiology - A Molecular and Integrative Physiology, 157(1), 1–6. https://doi.org/10.1016/j.cbpa.2010.03.004